A Study of Vacuum Packaging Methodsfor a Microfabricated Suspended Tube Reactor

by

Jeremy Chi-Hung Chou

S.B. Chemical Engineering, M.I.T., June 2000S.B. Electrical Engineering and Computer Science, M.I.T., June 2002

Submitted to the Department of Electrical Engineering and Computer Science

in Partial Fulfillment of the Requirements for the Degree of

Master of Engineering in Electrical Engineering and Computer Science

at the Massachusetts Institute of Technology

August 29, 2002

Copyright 2002 Jeremy Chi-Hung Chou. All rights reserved.

The author hereby grants to M.I.T. permission to reproduceand distribute publicly paper and electronic copies of this thesis

and to grant others the right to do so.

MASSACHUSETS INSTTUTOF TECHNOLOGY

JUL 3 0 2003

LIBRARIES

AuthorDeplifent lectrical Engineering and Computer Science

August 29, 2002

Certified byMartin A. Schnidt/Thesis Supervisor

Accepted byArthur C. Smith

Chairman, Department Committee on Graduate Theses

A Study of Vacuum Packaging Methodsfor a Microfabricated Suspended Tube Reactor

by

Jeremy Chi-Hung Chou

Submitted to the Department of Electrical Engineering and Computer Sciencein Partial Fulfillment of the Requirements for the Degree of

Master of Engineering in Electrical Engineering and Computer Scienceat the Massachusetts Institute of Technology

August 29, 2002

Abstract

A study of selected vacuum packaging methods for a microfabricated suspendedtube reactor is presented in this thesis. This project was motivated by the need tominimize heat loss from the suspended reaction tubes of our reactor, and vacuum is themost effective insulation method at the micro scale.

Vacuum packaging involves implementing a bonding method that can provide ahermetic seal, incorporating a getter material that can absorb residual gases inside thepackage after bonding, and designing a vacuum sensor with adequate sensitivity in therange of interest in order to confirm the final pressure inside the package.

The most suitable vacuum packaging methods for our reactor were selected basedon the results of literature search. For wafer bonding, the Vitta GPR- 10 glass frit waschosen because of its ability to flow at 550 C to conform to the roughness of bondingsurfaces. For gettering, the SAES St122 NEG (a mixture of Ti and Zr-V-Fe alloy) waschosen because of its compact size and high internal gettering surface. For vacuumsensing, thermal conductivity vacuum sensor was chosen because of its high sensitivity inthe range of interest (< 1 Torr).

To study these selected methods, a three-layer test structure with an integratedvacuum sensor has been designed, modeled, fabricated, and then calibrated. Sensorcalibration and die-level vacuum bonding apparatuses have been set up in the process.

Sensor calibration and test structure bonding results are reported. The Vitta GPR-10 glass frit seal failed to retain any vacuum. As a result, the SAES St122 NEG had noeffect because the high base pressure caused premature saturation.

Alternative bonding methods are presented at the end of the thesis for futurestudies on vacuum packaging.

Thesis Supervisor: Martin A. SchmidtTitle: Professor of Electrical Engineering & Computer Science

Director of Microsystems Technology Laboratories

3

4

Table of Contents

Abstract 3Table of Contents 5List of Figures 7List of Tables 7Acknowledgments 9Nomenclature 111. Introduction 132. Background Information 16

2.1. Wafer Bonding 162.2. Gettering 172.3. Vacuum Sensing 19

3. Project Approach 224. Selection of Vacuum Packaging Methods 24

4.1. Wafer Bonding 244.2. Gettering 254.3. Vacuum Sensing 28

5. Test Structure 295.1. Physical Features 295.2. Design Methodology 30

5.2.1. Top Layer: Electrical Contact 305.2.2. Middle Layer: Vacuum Sensor 315.2.3. Bottom Layer: Getter Room 365.2.4. Modeling Results 37

5.3. Fabrication Process 395.3.1. Top Layer 395.3.2. Middle Layer 405.3.3. Bottom Layer 41

6. Thermo Conductivity Vacuum Sensor Modeling 426.1. Thermal Conductivity 426.2. Temperature Coefficient of Resistance 436.3. Heat Generated by Resistor 436.4. Heat Dissipated by Resistor 446.5. Energy Balance 446.6. Modeling Results of the Suspended Tube Reactor 45

7. Vacuum Sensor Calibration 477.1. Apparatus Setup 477.2. Calibration Procedure 497.3. Calibration Curve 517.4. Comparison with Modeling Results 52

8. Die-Level Vacuum Bonding 558.1. Apparatus Setup 558.2. Bonding Procedure 588.3. Results 59

5

9. Sensor Resistor Failure Analysis9.1. Description9.2. Possible Causes9.3. Solutions

9.3.1. Tantalum Nitride (TaN) Diffusion Barrier9.3.2. Aluminum Oxide (A120 3) Diffusion Barrier9.3.3. Careful Temperature Control

10. Evaluation of Vacuum Package10.1. Summary10.2. Vacuum Sensor Measurements10.3. Leak Tests10.4. Vacuum Packaging Failure Analysis

11. Future Work12. Conclusion13. ReferencesAppendix A: Vacuum Sensor Modeling on Microsoft ExcelAppendix B: Test Structure Mask Drawings on AutoCADAppendix C: Test Structure Detailed Fabrication Process FlowAppendix D: Equipment ListAppendix E: Molecular Flow CalculationsAppendix F: Experimental Data on the Target Pressure

6

606060626262636464646667707374767882848586

Figure 1:Figure 2:Figure 3:

Figure 4:Figure 5:Figure 6:Figure 7:Figure 8:Figure 9:Figure 10:Figure 11:Figure 12:Figure 13:Figure 14:Figure 15:Figure 16:Figure 17:Figure 18:Figure 19:Figure 20:Figure 21:Figure 22:Figure 23:Figure 24:Figure 25:Figure 26:Figure 27:Figure 28:

Gettering mechanisms for some common gasesPhysical properties of Vitta GPR-10 glass transfer tapePhysical properties of SAES St122 getterParameters of the proposed sensor designModeling parameters of the suspended tube reactorTi/Pt/Ti resistance increase during bondingBonding parameters and vacuum sensor measurementsLeak test results

7

List of Figures

Top and side views of the suspended tube reactorLeaks created by surface roughness in the case of anodic bondingSAES St122 package and its pumping speed as a function of the absorbedquantityCross sectional view of the test structureTop and cross sectional views of the top layerTop, bottom, and cross sectional views of the middle layerThree heat transport modes for the sensor resistorDetailed diagram of the sensor resistorSpecifications of the sensor resistor and the membraneTop and cross sectional views of the bottom LayerModeling results of the proposed sensor designFabrication process for the top layerFabrication process for the middle layerFabrication process for the bottom layerModeling and experimental results for the suspended tube reactorVacuum sensor calibration setupUltra high vacuum chamber and its connectionsFour-point measurement setupGlass frit tape placement on a calibration sampleVacuum sensor calibration curves of three samplesDie-level vacuum bonding setupTop and cross sectional views of the bonding chuckDie-level vacuum bonding setupHeating cycle for the glass frit bondingPlacement of A120 3 diffusion barrier and glass fritVoids in Si-Pyrex glass frit bondVoids in Si-Si glass frit bondPossible implementation of solder bonding on test structure

142427

29303132363637383940414647484951545656575863686971

List of Tables

Table 1:Table 2:Table 3:Table 4:Table 5:Table 6:Table 7:Table 8:

1825273546606566

8

Acknowledgments

I would first like to thank Professor Martin Schmidt and Professor Klavs Jensenfor giving me the opportunity to do this research and for providing constant guidance.

Very important was also the guidance of Leonel Arana, who never hesitated tohelp me in lab and brainstorm with me.

I want to thank Ole Nielson for being a wonderful officemate and for answeringmy research and computer related questions.

Thanks to the other members of the Schmidt and Jensen groups who were alwaysvery supportive: Xue'en, Christine, Becky, Aleks, Cyril, Chelsey, and others.

9

10

Nomenclature

L LengthW WidthH HeightQ PowerA AreaJ Current Density

p ResistivityR Resistance91 Universal Gas ConstantV Voltagek Thermal ConductivityA Mean Free Path

c Mean Free VelocityCVm Constant Volume Molar Heat Capacity

N Molar ConcentrationK Boltzmann ConstantT TemperatureC- Collision Cross Section

M Molar MassP, Transition Pressure

D Distancea Thermal Coefficient of Resistances EmissivityK Stefan-Boltzmann Constant

11

12

1 Introduction

Power is a critical issue for all mobile electronics. While much work is being

done on minimizing the power consumption of circuits, on the power supply side, the

battery has been the only viable technology. Unfortunately, due to the fundamental

constraint on the energy density, battery technology does not seem to be able to keep up

with the power demand of increasingly complex electronics. As a feasible alternative to

the battery, a microfabricated suspended tube reactor has recently been developed in our

group using Micro Electro Mechanical System (MEMS) techniques. This reactor can be

used to produce electricity in different ways. One is to convert a chemical fuel into

hydrogen (e.g. through the ammonia cracking reaction), which can then be fed into a

micro fuel cell to generate electricity. Another is to create heat and light by combusting a

chemical fuel and feed the photons into a photovoltaic cell to generate electricity.'

Our reactor consists of two sets of free standing silicon nitride tubes connected to

one silicon heat exchange zone on one end as shown in Figure 1.1 An exothermic

reaction in one set of tubes can deliver heat through the heat exchange zone to an

endothermic reaction in the other set of tubes (e.g. a hydrogen-generating reaction). In

order to provide a maximum amount of heat to drive the reaction, it is critical to

minimize heat loss to the outside environment. Conductive heat loss occurs in two

modes as shown in Figure 1: one through the silicon nitride tubes, and the other through

the gases in the enclosure. While the heat loss through the tubes is minimized by making

the tube wall as thin as possible (i.e. 2 um), one effective method to prevent the heat loss

through the gases is to create a vacuum space surrounding the hot zone, or in other words,

to create a vacuum package for this reactor. Simply surrounding the tubes with insulation

13

material would not work because at such a small scale, the insulator would add enough

surface area to the tubes to actually increase the heat loss. Based on previous data

(presented in Appendix F), a vacuum of 40 mTorr or less is desirable in order to

minimize heat loss. At 40 mTorr or less, heat loss becomes independent of pressure for

the geometry of our reactor, and achieving a pressure lower than that will not further

decrease heat loss.

r ----- i r ----- Si heat exchange zone

Si slabs for heat recovery

SiN tubes

tube set I tube set 2

heat Conductionthru gaseC

capping chip

he t conduction ~~_~-~~- device chipthru nitride tunis c i h

| V capping chip

Figure 1. Top and side views of the suspended tube reactor, and heat loss modes.

The purpose of this project is to evaluate the feasibility of selected vacuum

packaging methods, including a wafer bonding method that can provide a hermetic seal

over a rough surface in vacuum (i.e. 100 nm thick roughness), a gettering method that can

best absorb residual gases inside the package after bonding, and a vacuum sensing

14

method that can provide adequate sensitivity in the range of interest in order to confirm

the final pressure inside the package. Based on the results of literature search, one wafer

bonding method, one gettering method, and one vacuum sensing method are selected for

investigation. The selection is based on the suitability of each method for our specific

suspended tube reactor. After actually implementing these methods, the feasibility of

each method can be determined.

15

2 Background

2.1 Wafer Bonding

A variety of hermetic wafer bonding methods are available. Anodic bonding is a

method to bond silicon to glass. A voltage of about 1000 V is applied across the silicon

and the glass (with the glass held at the negative potential) while raising the temperature

to 300-500 C. In this condition, the positive Na+ ions in the glass move towards the

cathode and the negative 0- ions move towards the silicon-glass interface. The resulting

electric field pulls the two materials together and forms silicon dioxide at the interface to

make a strong bond.2

Silicon fusion bonding is a method to bond silicon to silicon. Before bonding,

OH groups are attached to the bonding surfaces through a hydration step. Then, they are

brought into contact at a temperature of about 1000 C. Bonding occurs according to the

mechanism: Si-OH + Si-OH - Si-O-Si + H20. No electric field is required.3

Trade-offs exist between these two bonding methods. Anodic bonding requires a

high electrical voltage, which may damage any potential electronics already on the wafer.

Also, the difference between the thermal expansion coefficients of silicon and glass could

result in thermal stress. In contrast, silicon fusion bonding requires no electrical voltage

and has no chance for thermal stress (because no other material is used other than

silicon). However, anodic bonding offers higher tolerance of bonding surface roughness

and requires a lower bonding temperature than silicon fusion bonding.

In addition to anodic bonding and silicon fusion bonding, there are other bonding

methods available that require an intermediate layer as a glue. For example, glass frit

bonding utilizes glass powder suspended in an organic binder. It is first applied onto the

16

bonding surfaces, followed by burning off the binder at about 300 C. Then, they are

pressed together and heated to about 600 C to sinter the glass to form a Si-O-Si bond. No

electric field is required.4

In solder bonding, wettable metal pads need to be first deposited over the non-

wettable silicon surface. This wettable metal usually comprises three layers: an adhesion

metal (e.g. chromium or titanium), a barrier metal (e.g. copper or platinum), and a

sacrificial metal (e.g. gold). The barrier metal is required to prevent the solder from

dissolving the adhesion metal. Then, solder (e.g. tin/lead, indium, gold) is deposited over

the wettable metal. After contacting the two surfaces and heating to the melting point of

the solder (i.e. 250-350 C), the solder begins to flow and bonds the two surfaces.5

Eutectic bonding typically utilizes gold as a glue layer. As the temperature rises,

gold gradually diffuses into silicon. When the eutectic composition is reached (97%

silicon and 3% gold), the melting point of the silicon-gold system decreases to the

minimum value of 363 C. At this temperature, liquid alloy of silicon-gold forms and

bonds the two surfaces.4

Thermal compression bonding also relies on gold as a glue layer. In this case,

gold is deposited onto each of the two surfaces with titanium as an adhesion layer. The

two gold surfaces are then brought into contact while applying about 20 psi pressure and

heating to about 300 C (this temperature even lower than the eutectic temperature).

Bonding occurs between the two gold layers. 3

2.2 Gettering

Getters are highly reactive metals or metal alloys that chemically react with gas

molecules and pump away the products by adsorption and/or absorption. These gas

17

molecules may have originated from the silicon surface and from the bonding process.

Because getters work by chemical reaction, they are unable to remove inert gases (e.g.

He, Ne, and Ar).6

Getters provide a clean surface for the gas molecules to collide on. After

collision, gas molecules stick on the surface, react with the getter, and then either remain

on the getter surface or diffuse into the getter bulk. The gettering mechanisms for some

of the common gases are shown in Table 1 .6

Table 1. Gettering mechanisms for some common gases. (g) stands for the gaseous state,and (a) stands for the adsorbed state. G stands for getter.

Gas Reaction MechanismCO CO (g) 4 CO (a) + G 4 G-C + G-OC02 CO2 (g) - CO (a) + G - G-C + 2 G-ON2 N2 (g) 4 N2 (a) + G + 2 G-NNO NO (g) 4 NO (a) + G 4 G-O + G-NH2 H2 (g) 4 2 H (a) 4 2 H (bulk)

H20 H20 (g) 4 2 H (a) + G-O+ 2 H (bulk) + G-O

There are two ways to create clean getter surface. In the case of evaporable

getters, the getter is evaporated and a thin layer is deposited on the inner surface of an

enclosure. Another thin layer is to be deposited periodically to replenish the clean

surface. In the case of non-evaporable getters, instead of relying on the inner surface of

an enclosure, the getter has a porous internal structure to provide the reaction surface.

Heat induces the adsorbed gas molecules on the surface to diffuse into the getter bulk to

leave the surface clean.7

One common evaporable getter is titanium due to its high reactivity with many

gases. It evaporates at about 1500 C. Due to its high reactivity, the titanium getter can

18

only operate in a vacuum of 10-3 Torr or less. If it is exposed in atmosphere, it will

quickly oxidize and lose its gettering capability. 6

Non-evaporable getters (NEG) are mostly metal alloys (e.g. Zr(84%)-Al(12%)

and Zr(70%)-Al(25%)-Fe(5%)). They have high porosity (50-70%) so they can rely on

their own internal surface area for reaction. In order to activate the getter, it needs to be

heated to 500-800 C so an oxide passivation layer on the surface can diffuse into the

bulk. NEGs prefer to operate in a heated environment because heat helps the adsorbed

gas molecules to diffuse into the bulk and free up the surface; however, they can still

work at room temperature with a lower gettering speed. As the getter becomes saturated,

its gettering speed decreases accordingly. When the pumping capacity is eventually

reached, the whole getter will need to be replaced.6

2.3 Vacuum Sensing

Once the suspended tube reactor is packaged in vacuum, it is important to

accurately measure the pressure inside the package in order to confirm the achieved

vacuum and to monitor for leaks and outgassing. Many vacuum sensing methods are

available. The simplest method is to create a membrane on the package and optically

measure its deflection, which is directly proportional to the pressure difference across the

membrane. The amount of deflection follows Equation 1:

W = P(a 2 - r2)2 , where D = Et3 Equation 164D 12(1 - v2

where W is the amount of deflection, P is the pressure load, a is the radius of membrane,

r is the radial distance from center of membrane, D is the flexural rigidity of the

membrane, E is the elastic modulus, and v is the Poisson's ratio.8 When the membrane

19

deflection is zero, the pressure inside the package equals the pressure outside the

package. Adjusting the external pressure until the membrane becomes flat is a simple

method to know the pressure inside.

In the capacitive method, two metal plates are placed close to each other, one

fixed and the other on a flexible membrane. The capacitance between the two plates is

inversely proportional to the distance between them, which in turn depends on the

deflection of the membrane, which in turn depends on the pressure difference across the

membrane. Therefore, capacitance corresponds to pressure. Capacitance measurement

can be more accurate than the optical measurement of membrane deflection.9

The basic setup of a thermal conductivity vacuum sensor consists of a resistor

placed on a membrane inside an enclosure. The resistor generates heat when a voltage is

applied across it. The balancing heat loss from the resistor occurs in three modes:

conduction through the gases in the enclosure, conduction through the membrane which

the resistor is sitting on, and radiation. While the latter two modes are independent of the

pressure, heat conduction through the gases is directly proportional to the thermal

conductivity of the gases, which is in turn directly proportional to the pressure within a

certain pressure range. Thermal conductivity does not become dependent on the pressure

until the mean free path of the gas molecules becomes limited by the geometry of the

enclosure housing the resistor.10

A thermal conductivity vacuum sensor can measure in two ways. In the constant

voltage mode, the pressure can be determined by measuring the resistance when a

constant voltage is applied across the resistor. The relationship between the pressure and

the resistance is that an increase in pressure will increase the thermal conductivity, which

20

will in turn increase the heat loss from the resistor, which will in turn decrease the

temperature of the resistor, which will in turn decrease its resistance. In the constant

temperature mode, the pressure can be determined by measuring the amount of power

required to maintain the resistor at a constant temperature. The relationship between the

pressure and the power is that an increase in pressure will increase the heat loss from the

resistor, which will in turn increase the power necessary to maintain the given

temperature.

21

3 Project Approach

This project investigates the feasibility of selected vacuum packaging methods. It

focuses on three components of vacuum packaging. The first is an effective wafer

bonding method. In order to create an enclosed package, the suspended tube reactor

needs to be bonded to a layer on the top and a layer on the bottom. Furthermore, in order

to have vacuum inside the package, this bonding process needs to take place in vacuum

and the resulting bond needs to be hermetic to prevent air leak after it is taken out of the

vacuum environment. The second component is an effective getter material. A getter

helps maintain a low pressure by pumping away the residual gases in the package through

chemical reaction. Getters may not always be required for vacuum packaging. However,

for the micro-scale MEMS devices where the surface-to-volume ratio is relatively high,

such as our suspended tube reactor, a getter is usually required to counter the outgassing

from the internal surface of the package. The third is an effective vacuum sensor which

has an adequate sensitivity in the targeted pressure range of the package. A vacuum

sensor is not necessarily required in the actual finished package; however, in the process

of creating such a package, it is needed to confirm the achieved vacuum level.

The first part of the project is to select the most suitable wafer bonding, gettering,

and pressure sensing methods for our reactor through literature search. The factors being

considered in these selections are presented in Section 4. The goal is to evaluate the

vacuum package achieved by using these selected methods.

The second part of the project, presented in Section 5, is to design and fabricate a

test structure to experiment with these selected vacuum packaging methods. The actual

suspended tube reactor can not be used for this project because its supply is very limited

22

due to its complicated fabrication process. This simpler test structure basically includes a

vacuum sensor (to test the selected vacuum sensing method), a getter room (to test the

selected getter material), and a bonding surface topography similar to that of a real

suspended tube reactor (to test the selected wafer bonding method). The assumption is

that if this test structure can achieve a vacuum package, then the same methods can be

transferred to a real suspended tube reactor.

The third part of this project, presented in Section 6, is to model the thermal

conductivity vacuum sensor. This model, based on an energy balance between the heat

generated and the heat lost from the sensor resistor, helps design a sensor that has

adequate sensitivity between 0 and 1 Torr, the targeted pressure range of the package.

The fourth part of the project, presented in Section 7, is to calibrate the vacuum

sensor and compare the experimental results to the modeling results. The resulting

calibration curve is relied upon when the sensor is later used to determine the vacuum

level inside the package.

The fifth part of the project, presented in Section 8, is to perform die-level

vacuum bonding experiments and to evaluate the effectiveness of the package.

Finally, the last part of the project, presented in Section 9, 10, and 11, is to

analyze and evaluate the selected vacuum packaging methods and to suggest improved

techniques for future studies.

23

4 Vacuum Packaging Method Selection

4.1 Wafer Bonding

The suspended tube reactor has 400 nm thick platinum resistors on the top

surface. This poses a challenge to achieving a hermetic seal because it is difficult to seal

the area where the edge of the resistor meets the substrate as shown in Figure 2 in the

case of anodic bonding. Anodic bonding and silicon fusion bonding will not work

because of their low tolerance of surface roughness (about 50 nm and 6 nm,

respectively).3 Eutectic bonding and thermal compression bonding are possible

candidates, but they also require a very flat bonding surface. Glass frit bonding has a

much higher tolerance for surface roughness due to its flow characteristic. At the melting

temperature of the glass, softened glass can flow everywhere and conform to the surface

topography. Gooch has achieved 9 mTorr pressure with glass frit bonding." Solder has

the same flow characteristic as glass frit at its melting temperature; however, solder is

electrically conductive while glass frit is insulating, so special care needs to be taken to

prevent the solder from short-circuiting the platinum resistor

leaks

pyrex glassmetal

Si substrate

Figure 2. Leaks created by surface roughness in the case of anodic bonding.

24

The glass frit material chosen for this project is the GPR-10 Glass Transfer Tape

manufactured by Vitta Corporation. This glass frit comes in the form of a tape with one

adhesive side. It is specifically designed for glazing and sealing silicon wafers. It may

also be used for glazing, sealing, and joining other materials having a coefficient of

expansion of approximately 6.5 x 10-6. GPR-10 is a lead-zinc-borosilicate type glass

thermally matched to silicon so that the fired coatings do not have stress or cracks. The

thickness of the tape is 25 um before sintering and 40 um after sintering. This increase in

thickness (and volume) is caused by the expansion of the organic binder as it is heated up

and evaporates from the interior of the glass frit during sintering. The increased volume

is believed to be made up of voids. Physical properties are summarized in Table 2.

Table 2. Physical properties of Vitta GPR-10 glass transfer tape.Glass Family Lead-Zinc-BorosilicateGlass Type VitreousAnnealing Point ~ 425 CSoftening Point ~510 CWorking Temperature 552 - 600 CCoefficient of Thermal Expansion 6.5 x 10-6 in/in CDielectric Constant, 1 megacycle, 25 C 12

4.2 Gettering

Non-evaporable getters (NEG) have many advantages over evaporable getters,

especially when applied in the MEMS devices.6, 7 Their porous structure supplies their

own reactive surface instead of relying on an external surface, which a MEMS device

usually does not have much of. NEGs obtain clean surface by making the adsorbed

molecules diffuse into the bulk instead of by the rather troublesome evaporation process

in the case of evaporable getters. The absence of evaporation also means a lower

25

operating temperature. A mixture of titanium (Ti) and zirconium (Zr)-vanadium (V)-iron

(Fe) alloy is popular due to its low activation temperature of about 450 C. Esashi has

achieved a vacuum level of 105 Torr with Ti and Zr-V-Fe. 1 2

The getter chosen for this project is St122 manufactured by SAES Corporation, a

mixture of Ti and Zr-V-Fe. A key goal in the development of St122 was flexible

manufacture. Strips may be produced as either single or double coated and can be

designed to fit in constrained volumes. Possible substrate materials include nichrome,

nicrofer, titanium, nickel, moly, stainless steel, and zirconium. A process involving

screen printing and sintering of the getter material results in a high-porosity, low-

particulating, and mechanically strong structure. Successful application in MEMS

applications requires a good activation of the getter to maximize use of the available

active mass. Typical processes would consist of prebaking the package before bonding in

order to reduce the amount of gases present on the surface that could outgas. When the

package is ready for final sealing the getter should be activated first in order to gain

maximum performance before sealing. The getter activation process consists of

supplying enough heat energy for the protective passivation layer on the outer surface to

diffuse into the bulk, thereby exposing chemically active getter alloy. This process is

typically 10 minutes at 450 C to 500 C. Physical properties of St122 are summarized in

Table 3. Like all other NEGs, the pumping speed of St122 is a function of the type of gas

and the amount already absorbed as shown in Figure 3.

26

Table 3. Physical properties of SAES St122 getter.

Emissivity 0.6-0.65

Density 4.7 g/cm 3

Apparent Density 2 g/cm3 (due to high porosity 55%-65%)

Mass of Getter Material 20 mg/cm2 for a 1 00um thick layer

Thermal Properties N/A

Figure 3. SAES St122 package (with six 1 mm x 1 mm pellets shown in picture) and its

pumping speed as a function of the absorbed quantity.

27

Pumping Speed Vs. Sorbed Quantity

EIC

U

10

10 -Coating Thickness: 100 um

E Sorption Temperature: 25 CSorption Pressure: 3e-5 Torr

0.01 0.1 1 10 100

Sorbed Quantity (cc.torr/cm2 of getter coating)

4.3 Vacuum Sensing

Both the optical method and the capacitive method rely on the deflection of a thin

silicon membrane caused by the pressure difference across it. One problem with this

approach is that even when the pressure difference across the membrane is zero, there

may still be some membrane deflection due to residual stress, and it is difficult to

quantify the residual deflection. In addition, these membrane-based methods have a poor

sensitivity in a vacuum of 1 Torr or lower because such a small change in pressure can

not cause enough membrane deflection to be detected.

A thermal conductivity vacuum sensor seems the most appropriate for this

project. The actual suspended tube reactor has resistors over the bonding surface. In

order to allow the test structure to accurately simulate the bonding surface of the reactor,

it is a good idea to have the same resistors on the bonding surface of the test structure as

well. This consideration naturally leads to a sensor based on thermal conductivity

because it would also need to have resistors. In addition, such a sensor has been shown

to have good sensitivity below 1 Torr, which is the targeted range of this project.

28

5 Test Structure

5.1 Physical Features

The three-silicon-layer test structure, whose cross section is shown in Figure 4, is

a vehicle on which bonding experiments are carried out in this project. Its outer

dimensions are 10 mm (L) x 8 mm (W) x 1.5 mm (H). The top layer includes four

electrical contact holes to the resistor and a cavity in which the pressure is measured.

The middle layer contains two cavities etched from the backside in potassium hydroxide

(KOH): one is used to release a stoichiometric silicon nitride (Si 3N4) membrane and the

other is used to equilibrate the pressures across the membrane. A three-layer metal

structure, comprised of titanium-platinum-titanium on the first generation devices and

tantalum-platinum-tantalum on the second generation devices, sits on top of the

membrane and serves as the heating resistor of the vacuum sensor. The bottom layer

includes a deep cavity for placing the getters.

electricalcontact heating resistor

Si

Si3N4

Si glass frit

getter room

Si3N4 membrane pressure equilibration hole

Figure 4. Cross sectional view of the test structure.

29

5.2 Design Methodology

5.2.1 Top Layer: Electrical Contact

The top layer, shown in Figure 5, includes a 15 um deep cavity to provide the

space in which the pressure is measured. In other words, this space provides the sensor

resistor with a conduction path through the gases. The 15 um depth is chosen based on

the modeling results to have the optimal sensor sensitivity. The 2.2 mm x 5.45 mm

cavity is just large enough to cover the membrane area on the middle layer so that the

sensor resistor would not be in contact with the top layer. The four electrical contact

holes are in preparation for the four-point measurement. Each hole has a 1.2 mm

diameter so a standard probe tip can fit in.

1.2 mm mdiamete

2.2

15 urn deep cavity through holesthrough through

TI~u~e~TJT SSP wafer

525 urn thick

Figure 5. Top and cross sectional views of the top layer.

30

5.2.2 Middle Layer: Vacuum Sensor

The middle layer, whose top view is shown in Figure 6 , includes the heating

resistor of the vacuum sensor. Four contact pads are placed in the far corners to

maximize the area of flat surface for bonding.

1.4 mm

1.8 mn contactpad

resistor

KOH etchedthrough holes

contactpad

contactpad

DSP wafer450 urn thick

Figure 6. Top, bottom, and cross sectional views of the middle layer.

31

To maximize sensor sensitivity, the heat loss through the gases should be

maximized with respect to the heat losses through radiation and through the substrate by

conduction as depicted in Figure 7. The reason is that only the heat transport through the

gases is a function of the pressure while the other two heat transport modes are not

affected by the pressure. For a given amount of pressure change, the larger change in

heat loss is observed, the more sensitive the sensor becomes. To meet this goal, the

majority of the resistance sits on top of the silicon nitride membrane so that most of the

heat loss is conducted through the gases above and beneath the membrane instead of the

substrate. At the same time, the silicon nitride membrane is kept as thin as possible to

minimize the heat conduction along the membrane.

conduction Oniaif

thru. gases

conductionthru substrate

Figure 7. Three heat transport modes for the sensor resistor.

The following are some general guidelines for the design of the vacuum sensor

and the membrane:

1. The surface area of the membrane needs to be large enough to carry the required

resistor, but at the same time it needs to be minimized to decrease thermal radiation.

32

2. While maintaining the structural integrity of the membrane, the thickness and the area

of the membrane should be minimized and maximized, respectively, in order to

minimize the heat transport along the membrane.

3. The distance between the resistor and the top enclosure wall needs to be minimized in

order to maximize heat transport through the gases given the constraint that the

resistor can not be in contact with the wall.

4. The applied voltage and the base resistance dictate the current and the temperature of

the resistor. The current needs to be high enough to be accurately measured but not

higher than what the electron migration limit allows. Also, the resistor temperature

should remain low to minimize the radiation effects.

5. The resistance of the metal leading to the membrane from the contact pads should be

minimized. It is important to keep as much of the resistance on the membrane as

possible.

6. It is important to perform four-point measurement to minimize the noise effect of the

contact resistance of the pad and the probe. Therefore, four contact pads and four

wires leading to the membrane are required.

7. The resistor material should be chosen to have a large and linear Temperature

Coefficient of Resistance (TCR).

The specifications of the proposed vacuum sensor design are summarized in

Table 4 and graphically presented in Figure 8 and Figure 9. The composition and the

thickness of the resistor on the test structure are set be identical to those of the resistor on

the actual suspended tube reactor. The resistor composition is Ti (10 nm)/Pt (100 nm)/Ti

(10 nm) on the first generation devices, and Ta (10 nm)/Pt (400 nm)/Ta (20 nm) on the

33

second generation devices. The reason for this change is that the Ta/Pt/Ta structure

provides a much longer resistor lifetime at high temperatures (i.e. -1000 C). The vacuum

sensor design presented in this section is based on the Ti/Pt/Ti resistor structure. The

second generation vacuum sensor has the identical design except for the thickness of the

resistor. The width of the resistor is set to be 100 um to balance between

manufacturability and the membrane size. An increase in the metal line width will make

it easier to fabricate but will make the membrane area larger and thus easier to break.

The current density of the resistor is set to be 2 x 109 A/m 2 while the electron migration

limit of platinum is 1010 A/m 2 . Power generated at room temperature is set to be 0.05 W

to balance between the resistor length and the signal power. When more power is

generated, the sensor signal becomes stronger, but the resistor must become longer to

maintain the pre-determined current density and thus more prone to fracture. Length is

calculated to be 11.8 mm from Equation 2:

L = Q EquatiorWHJ 2 p

where L, W, and H are the length, width, and thickness of the resistor, Q is the power

generated by the resistor, J is the current density through the resistor, and p is the

resistivity of platinum. The resistance is calculated to be 125 ohm from Equation 3:

R= PL EquatiorWT

The amount of voltage must applied is calculated to be 2.5 V from Equation 4:

V = RE Equation 4

The length of conduction through the membrane is set to be 100 um to balance between

the undesirable heat transport and membrane size. When this length increases, the heat

34

2

3

transport through the membrane decreases, but the membrane must become larger and

thus weaker. The length of conduction through the gases is set to be 55 um (15 um cavity

depth + 40 urn glass frit thickness) to create a reasonable buffer space between the

resistor and the top enclosure wall while maintaining an adequate sensitivity.

Table 4. Parameters of the proposed sensor design.Platinum (Pt)

Resistivity 1.06 x 10~7 Q-mElectron Migration Limit 1010 A/m2

Temperature Coefficient of Resistance 0.26%/KSilicon Nitride (Si 3N4)

Thermal Conductivity @ Room Temperature 11.9 W/K-mResistor Design

Thickness 100 nmLength 11.8 mmWidth 100 umResistance @ Room Temperature 125 QCurrent Density 2 x 10 9 Aim2

Power Dissipated 0.05 WMembrane Design

Thickness 100 nmLength 1.4 mmWidth 1.4 mmArea 1.96 mm2

Conduction through Gases- Length 55 umConduction through Membrane- Length 100 urnConduction through Membrane- Area 476 um 2

OthersApplied Voltage 2.5 VSubstrate Temperature 300 K

35

1.2 mm

20U

0.8 1.2mm mm

0.22 mm

Figure 8. Detailed diagram of the sensor resistor.

cross section view top view

substrate

< 1.4mm >

55 um

100 nm100 nm

edge of membraneSiN 4 Menirase 100 n

Figure 9. Specifications of the sensor resistor and the membrane.

5.2.3 Bottom Layer: Getter Room

The bottom layer, shown in Figure 10, has a deep cavity of about 200 um to store

the getters. Each SAES St122 getter pellet is roughly 1 mm x 1 mm, so this 2.2 mm x

5.45 mm cavity is able to fit multiple getter pellets side by side.

36

2.2 mm

200 umn deen

200 urn deep SSP wafer

2 0 0 u m d5 2 5 u rn th ic k

Figure 10. Top view of the bottom Layer.

5.2.4 Modeling Results

The modeling results of this vacuum sensor design indicate its ability to measure

pressure between 1 mTorr and 1 Torr with a resistance span of 120 ohm and a current

span of 8.1 mA in the constant voltage mode (2.5 V), as shown in the top two graphs of

Figure 11. At pressures below 1 mtorr, the temperature of the resistor is high enough to

make radiation dominate the heat dissipation from the resistor, thus causing the pressure

dependence of the resistance to diminish. At pressures above 1 torr, the thermal

conductivity of the gases becomes decreasingly dependent on the pressure as further

explained in Section 6.1.

The thermal conductivity vacuum sensor can also be modeled in the constant

temperature mode. Figure 11 shows the power vs. pressure graph at 87 C. The sensor

measures pressure between 1 mTorr and 1 Torr with a power span of about 20 mW.

37

Resistance Vs. Pressure @ 2.5 V

Figure 11. Modeling results of the proposed sensor design.

38

200 -

150 -

100-

50 -

0

0.01 0.1 1 10 100 1000 10000

P [mtorr]

15 -

5-

Current Vs. Pressure @ 2.5 V

0.01 0.1 1 10 100 1000 10000

P [mtorr]

Power Vs. Pressure @ 87 C

30 - - - - - - -

25-

20 -E

15-

10 -

5-

00 200 400 600 800 1000 1200

P [mTorr]

.......... I ....... I ....... .... ........... ..

5.3

5.3.1

Fabrication Process

Top Layer

The fabrication process for the top layer is shown in Figure 12. 500 nm thick of

silicon dioxide (SiO2) is first grown on a single side polished wafer (525 um thick) in the

tube furnace. Then, the oxide on the bottom side is patterned and the silicon is etched 15

um in a Deep Reactive Ion Etcher (DRIE), which specializes in etching cavities with high

aspect ratios. Then, the oxide on the top side is again patterned and the silicon is etched

through. Because these holes need to be etched completely through, the wafer must be

mounted on another wafer in order to protect the etcher. 10 um thick AZP-4620 positive

photoresist is used for the DRIE.

am amaummm am

77 1 77

6.

7.

8.

9.

D silicon

silicon dioxide

photoresist

Figure 12. Fabrication process for the top layer.

39

1.

2.

3.

4.

5.

5.3.2 Middle Layer

The fabrication process for the middle layer is shown in Figure 13. 100 nm thick

stoichiometric silicon nitride (Si 3N4) is first deposited on a 450 nm thick double side

polished wafer. Then, the nitride on the bottom side is patterned with 1 um thick OCG-

825 positive photoresist and etched through in a RF magnetically coupled etcher. Then,

the nitride on the top side is patterned for metal deposition. 10 nm Ti/100 nm Pt/10 nm

Ti (first generation) or 10 nm Ta/400 nm Pt/20 nm Ta (second generation) is deposited on

the wafer. The unwanted metal is then removed in a lift-off process. 1.5 um thick

negative image reversal photoresist is used for the lift-off process. Finally, the cavities

are etched through from the back side, stopping on the nitride layer, in 20% KOH

solution at 80 C. Because KOH attacks Pt easily, the front side of the wafer is protected

in a sealed chuck while the back side is being etched.

I I 5.

6.

7.

D silicon * silicon nitride

Figure 13. Fabrication process for the middle

8.

h r11metal photoresist

laye

40

1.

2.

3.

4.

* as 7 .. 1.ann

r.

5.3.3 Bottom Layer

The fabrication process for the bottom layer is shown in Figure 14. 500 nm thick

silicon dioxide (SiO 2) is first grown on a 525 um thick single side polished wafer in the

tube furnace. Then, the oxide on the top side is patterned and the silicon is etched 200

um in the DRIE.

1.

2.

3.

4.

5.

6.

E] silicon E silicon dioxide

Figure 14. Fabrication process for the bottom layer.

photoresist

41

Lj~ IS=

5170 =17

77T,

Thermal Conductivity Vacuum Sensor Modeling

Thermal Conductivity

The thermal conductivity of a gas can be expressed as

1 -k = 3 A c C,, N

3Equation 5

where A, c, Cv,, , and N are the mean free path, the mean velocity, the constant-

volume molar heat capacity, and the molar concentration of the gas molecules,

respectively. 13 The mean free path can be expressed as

A = Equation 6

where K is the Boltzmann constant (1.381 x 10-23 J/K), and T , a, P are the

temperature, the collision cross section, and the pressure of the gas molecules,

respectively.13 The mean velocity can be expressed as

- 891T Equation 7;TM

where 91 is the universal gas constant (8.3145 J/K-mol) and M is the molar mass of the

gas molecules.1 3 The molar concentration can be expressed as

Equation 8N =91T

As a result, the overall expression for the thermal conductivity is

k 5 K91T Equation 93V V - M

At high pressures, the thermal conductivity is independent of the pressure as

shown in Equation 9 because the pressure terms in the mean free path expression and the

42

6

6.1

molar concentration expression cancel each other. However, as the pressure decreases,

the mean free path will increase until it eventually becomes limited by the geometry of

the enclosure. The transition pressure at which this phenomenon occurs can be expressed

as

P =KT Equation 10

where D is the geometry size (i.e. the distance between the resistor and the enclosure

wall). 13 As the pressure decreases past P, the mean free path gradually reaches the

saturation value D , and the thermal conductivity gradually becomes dependent on the

pressure. To model this pressure dependence, the thermal conductivity is expressed as' 4

1+P

Equation 11

6.2 Temperature Coefficient of Resistance

The resistance value of a resistor is directly proportional to the temperature. The

relationship can be expressed as

R R = a(T - T') Equation 12

where a is a material property called the temperature coefficient of resistance (TCR).1 5

A high a value is desirable because it helps achieve better sensor sensitivity. For

example, platinum has a TCR of 0.26%/K.1 5

6.3 Heat Generation by Resistor

When a voltage is applied across a resistor, the amount of energy generated can

be expressed as

43

Equation 13

Heat Dissipation from Resistor

Heat transport by conduction is governed by the Fourier Law:

S . = kAT - ToQConduction r L

Equation 14

where k is the thermal conductivity of the medium, T,. is the resistor temperature, T is

the ambient temperature at the wall, and A and L are the area and the length of the

conduction path (i.e. the path between the resistor and the wall), respectively. 16

Heat transport by radiation follows the equation

QRadiation ~ KA(T 4 _T4) Equation 15

where e is the emissivity of the material, and K is the Stefan-Boltzmann constant (5.67

x 10-8 W/m 2 _K 4). 16

6.5 Energy Balance

The model is based on the following energy balance:

QGeneration C Qconduction-gases + QConduction-membrane + QRadiation Equation 16

Heat conduction takes two paths: one through the gases in the enclosure (Qconduction-gases),

and the other through the membrane (QConduction-membrane). Heat radiation comes from only

the heated resistor (QRadiation). Expanding out each of the Q terms, the energy balance

becomes:

v2 T -T T - T-- = kairA m, +kmAe, + CmemKAm(T,- Tr 4)R Lmem-cs Lmem

Equation 17

44

6.4

QGenerationV 2

R

where Ame, is the membrane surface area, Amem-cs is the membrane cross sectional area, k

is the thermal conductivity, T, is the resistor temperature, T, is the ambient temperature,

Leavity is the distance from the resistor to the wall, Lmem is the distance from the resistor to

the edge of the membrane, e,,, is the membrane emissivity, and K is the Stefan-

Boltzmann constant.

This model makes the following assumptions:

1. Heat conduction and radiation are one-dimensional.

2. Air has a constant thermal conductivity independent of the temperature (neglecting

the temperature gradient between the resistor and the wall).

3. Heat convection is negligible because air is relatively stationary inside the enclosure.

4. The membrane is isothermal and has the same temperature as the resistor itself.

5. The enclosure wall is at room temperature.

6.6 Modeling Results for the Suspended Tube Reactor

The resistance vs. pressure relationship of the actual suspended tube reactor is

generated by this model and compared with previous experimental data in order to verify

the accuracy of the model. The suspended tube reactor has free-standing silicon nitride

tubes with resistors sitting on top of them. These resistors are used to heat up the

reactants inside the tubes in order to initiate a chemical reaction. Therefore, the reactor

itself can function as a thermal conductivity vacuum sensor. This model generated a

resistance vs. pressure graph at a constant voltage input of 10 V, shown in Figure 15,

based on the parameters of the reactor summarized in Table 5. The modeling results do

not exactly agree with the previously collected experimental data,I which show that the

resistance reaches a maximum value of 2400 ohm at 40 mTorr. The modeling results

45

show that the resistance reaches a maximum value of 1700 ohm at 1 mTorr. The

discrepancies are likely the result of the assumptions made in the model, explained in

Section 6.5, and the approximations of the geometries of the heat conduction and

radiation paths, presented in Table 5.

Table 5. Modeling parameters of the suspended tube reactor.RO of Resistor 900 QTCR of Platinum 0.26%/KThermal Conductivity of SiN @ Room Temperature 1.9 W/K-mApplied Voltage lOVSubstrate Temperature 300 KSurface Area of Resistor 6.03 mm 2

Cross Sectional Area of SiN tubes 9536 um2

Conduction Length through Gases 1 mmConduction Length trough SiN Tubes 3020 um

Figure 15. Modeling and experimental results for the suspended tube reactor.

46

Resistance Vs. Pressure @ Constant 10 V

2.62.4 - ..

2.2

- 2.00 1.8

1.6

1.4

1.2

1.0

1.E-06 1.E-04 1.E-02 1.E+00

- exp -+- model Pressure rorr]

Vacuum Sensor Calibration

7.1 Apparatus Setup

The calibration apparatus is shown in Figure 16 and Figure 17. A turbo pump in

series with a mechanical roughing pump (Leybold Trivac B D25B) is used to pump down

the ultra high vacuum chamber. A pressure gauge (KJL-205 Thermocouple Controller

with KJL-6000 Thermocouple Tube) and a gas flow valve are connected to the

feedthroughs. The sample being calibrated is placed into the chamber through a

feedthrough on a loader. This loader is basically a flat piece of stainless steel attached to

a CF2.75" flange with four through wires for electrical connection. To provide a four

point measurement, a volt meter, a amp meter, and a power supply are connected as

shown in Figure 17.

devicerotary gas loaderflow valve

UHVchamber

Figure 16. Ultra high vacuum chamber and its connections.

47

7

CF 2.75"OD flange

device under calibration

rotary gasvacuum gauge flow valve

UHV chamber

oil trap

mechanical roughing pump

Figure 17. Vacuum sensor calibration setup.

The purpose of the four-point measurement, shown in Figure 18, is to minimize

the effect of the contact resistance of the leads. Because the current through the volt

meter is negligible compared to the current through the power supply, the current and the

voltage across Rheater are assumed to be I and V, respectively. Then, the value of Rheater is

simply - and the power generation by the resistor is V x LI

48

Rontact Rontactamp

+ meter

V vRheater Ipower

- supplyRontact Rontact

Figure 18. Four-point measurement setup.

7.2 Calibration Procedure

As described in Section 6, heat transport becomes dependent on the pressure once

the mean free path of the gas particles becomes equivalent to the size of the enclosure.

As the pressure decreases, the heat transport from the resistor also decreases, and the

power required to keep the resistor at a given temperature becomes less. This is called

the constant-temperature calibration.

The reason that the constant-temperature calibration is chosen over the constant-

voltage calibration is to eliminate the effect of different resistance values. The resistors

may have different resistance values even if they are fabricated on the same wafer. After

bonding at 550 C in vacuum, the difference in resistance between samples could become

up to 25% due to annealing and the resistance increase caused by silicon diffusion from

the glass frit. In the constant-voltage calibration, when the same voltage is applied across

two different resistors, different amounts of power would be generated and the resistors

would be heated to different temperatures. As a result, they would have different

resistance values even though they are under the same pressure, rendering the constant-

voltage calibration not very accurate.

On the other hand, in the constant-temperature calibration, two different resistors

49

are heated to the same elevated temperature under a known pressure. As long as the

thermal pathways of the two sensors are identical, the amounts of power needed to

maintain the same temperature should be identical, and thus the possible difference in

their base resistances no longer has an effect. At a given pressure, two resistors with

identical thermal pathways must require the same amount of power to sustain the same

temperature.

During calibration, a target temperature T is first chosen by balancing between the

current density and the sensor sensitivity. This target temperature should be set as high

as possible to maximize the range of power inputs across a given pressure range (and thus

maximizing the sensor sensitivity); however, at the same time the current can not exceed

the electron migration limit to keep up with the target temperature under atmospheric

pressure, in which condition heat loss is the greatest and thus the most current needs to

flow through the resistor to keep up with the target temperature. After the target

temperature T is determined, the corresponding target resistance R for each sample is

calculated through Equation 12 given the a (temperature coefficient of resistance) of the

resistor material and the base resistance R, of each sample. a of platinum (i.e. 0.26%/K)

is obtained from literature15 , and R0 is obtained by four-point measurement with a

minimal voltage to prevent any temperature increase. Now, the sample is placed into the

vacuum chamber. At each pressure level, the voltage supply is adjusted until - equals

the target resistance R, and then V x I will give the power input.

R - R" = a(T - T) Equation 12RO c

To prepare samples for calibration, the three layers of the test structure are bonded

50

in ambient pressure with glass frit. The glass frit is only applied onto the edges of the die

to achieve a partial bond as shown in Figure 19, so the gases can still go in and out of the

package. The bonding process consists of a 60-minute ramp-up to 575 C, followed by a

15 minute soak, followed by a 60-minute ramp-down to the room temperature.

glass frit

Figure 19. Glass frit placement on a calibration sample.

7.3 Calibration Curves

The calibrations curves for three samples, shown in Figure 20, line up very

closely below 500 mTorr; however, they begin to diverge slightly at 1 Torr, indicating

possible discrepancies in their thermal pathways. It is difficult to keep the thermal

pathways of different samples identical due to processing constraints. These three

samples span a power range of more than 8 mW from 2 mTorr to 500 mTorr, generating

a sensitivity of 16 uW/mTorr. The KJL-205/6000 vacuum gauge has a 2% error for the

pressure readings. The multimeter used to measure V and Ihas a 0.03% error in voltage

measurement and 0.15% error in current measurement, resulting in a 0.16% error for the

power readings.

51

7.4 Comparison with Modeling Results

The constant-temperature power vs. pressure relationship generated by the model,

shown in Figure 20, shows a curve that has a higher slope (i.e. 30 uW/mTorr) than that of

the experimental data (i.e. 16 uW/mTorr), but the shapes of the curves are similar. They

are linear in the low pressure range and gradually level off starting around 500 mTorr.

The model over-estimates the heat loss from the resistor, resulting in a curve with

a higher slope. The difference is certainly contributed by many approximations and

assumptions made in this simple model. One major approximation was the conditions of

heat conduction and radiation. Conduction was approximated by an one-dimensional

model (i.e. the Fourier Law) even though in reality it occurs in three dimensions. The

conduction lengths were taken from the edge of the resister cluster to the edge of the

membrane and to the top capping layer. The conduction areas through the membrane and

through the gases were approximated by the perimeter of the resistor cluster times the

thickness of the membrane and the top surface area of the resistor, respectively. The

resistor cluster was assumed to have an uniform temperature throughout, and the

substrate was assumed to have the room temperature even though it is most likely to

heated up slightly by the resistor also. Air in the structure was assumed to have a

constant thermal conductivity independent of the temperature, neglecting the temperature

gradient between the resistor and the enclosure wall. Convection was assumed to be

negligible because air is relatively stationary inside the enclosure. The membrane was

assumed to be isothermal and have the same temperature as the resistor itself. All these

approximations and assumptions contributed to the discrepancies between the modeling

and the experimental results.

52

Two parameters in the model may have caused this over-estimation of heat loss.

One is the final thickness of the glass frit after sintering, which is supposed to be 40 um

according to the manufacturer's data. If this thickness was actually higher when the

calibration samples were bonded, resulting in a longer conduction path, then the resistor

would lose less heat in reality than the prediction of the model. The other parameter is

the value of the temperature coefficient of resistance (TCR) of platinum, which is

supposed to be 0.26%/K according to previous experimental data.1 5 If this value is

actually higher in reality, then the model would have assumed a higher resistor

temperature and thus over-estimated the heat loss. Other reported values for the TCR of

platinum go as high as 0.38%/K.'"

53

Vacuum Sensor Calibration @Constant Temperature -87C

20-18-16 -14 -12 -

10 -

8 - xC6- x x

4 - x * g I2-

0 100 200 300 400 500

P (mTorr)

+ exp1 * exp2 A exp3 x mod

Vacuum Sensor Calibration @Constant Temperature -87C

14-

12-

10 -IH

8 -

6-

4- -2-

0 -

0 200 400 600 800 1000

P (mTorr)

+ exp1 m exp2 A exp3

Figure 20. Vacuum sensor calibration curves of three samples (expI, exp2, and exp3).

Modeling results (mod).

54

Die-Level Vacuum Bonding

8.1 Apparatus Setup

The die-level vacuum bonding apparatus is shown in Figure 21 and Figue 23. A

glass bell jar (Kurt J Lesker BJ1 2X18) sits on top of a stainless steel feedthrough collar

(Kurt J Lesker FTC 12-1-8-0) with a rubber gasket in between, which in turn sits on top

of a stainless steel base plate (Kurt J Lesker BASEPLATE12) with a rubber o-ring in

between. A mechanical roughing pump (Leybold Trivac B D25B) is connected to one

feedthrough to provide vacuum inside the bell jar. A pressure gauge (Kurt J Lesker 205

Thermocouple Controller + Kurt J Lesker 6000 Thermocouple Tube) is connected to

another feedthrough to monitor the pressure inside the bell jar.

A special stainless steel bonding chuck, shown in Figure 22, is made to align the

three-layer stack of the test structure. This two-piece chuck has pre-drilled holes for

cartridge heaters and thermocouple. The sample being bonded is placed inside the cavity

of the bottom piece and sandwiched by the top piece. Cartridge heaters (Omega

Engineering CSH-10 1100/120) are used to heat up the glass fit to its softening point of

550 C to initiate bonding. One or two stainless steel blocks (each 4.5 cm x 4 cm x 3 cm,

430 g) placed on top of the chuck are used to provide pressure (13.2 kPa with one, 26.4

kPa with two) to the bonding interface. A thermocouple (K type) is used to monitor the

temperature inside the chuck. The cartridge heaters and the thermocouple are connected

to the outside environment through feedthroughs.

Power to the cartridge heaters is provided by a variable-output transformer. This

power supply is controlled by a temperature controller (Omega Engineering CNi32)

through a solid state relay (Omega Engineering SSR240DC25). The controller regulates

55

8

the temperature by sending high (5 V) and low (0 V) signals to the relay to close and

open the circuit based on the thermocouple input.

glass bell jar

stainlesssteel block

bonding chuck

thermocouple

feedthrough

TCinput

controlwires

temperaturecontroller

Figure 21. Die-level vacuum bonding setup.

8.1 mm

10.1

1.2 mmdeep/

1/16" OD

25.2 mm

29.2mm

Figure 22. Top and cross sectional views of the bonding chuck.

56

silicainsulation

cartridgeheaters

uImvacuigauge

solidstaterelay

IIpowersupply

powersupply

Livanaccontroller

o o (

Fr-I

I I 1 1, 1 1 -, , I I - 7M7

feedthroughcollar

thermocouple_

cartridgeheaterwires

glassbell jar

variaccontroller

-- temperaturecontroller

mechanicalvacuum

pump

stainlesssteelblock

bondingchuck

silicainsulatingboard

Figure 23. Die-level vacuum bonding setup.

57

8.2 Bonding Procedure

First, the Vitta GPR-10 glass frit is taped onto the bonding surface of the top and

the bottom layers of the test structure. These two pieces are then heated up to 500 C for

15 minutes in air to burn off the organic binder (pre-sintering). After cooling down, the

glass frit covering the contact holes and the cavities are scraped off with a needle. A thin

channel in the glass frit can be created with a razor blade to provide an exit way for the

gases. The channel can be sealed once the glass frit flows at 550 C. However, this

channel is optional because air can exit through the gaps between two unbonded chips.

Next, all three layers of the test structure are placed into the cavity of the lower

half of the bonding chuck. The top half is then stacked on top, followed by one or two

stainless steel blocks. This whole assembly is placed onto an insulating silica board

inside the glass bell jar. Cartridge heaters and a thermocouple are inserted into the

appropriate holes on the bonding chuck.

The power output transformer and the temperature controller are then turned on.

Bonding follows the heating cycle shown in Figure 24. A 60 minute soak at 175-225 C is

intended to degas the silicon substrate, the glass frit, and the getter before bonding. Then,

the temperature is increased to 560-575 C to soften the glass frit for bonding.

560-575C

175-225

room temp-X room emp

20 mins 60 mins 40 mins 15 mins 60 mins

Figure 24. Heating cycle for the glass frit bonding.

58

8.3 Results

The resulting bond was very robust. The three-layer stack could not be separated

by a sharp razor blade. The alignment of the three layers was slightly off due to the

inexact fit of the dies in the cavity of the bonding chuck. Some glass fit over-flowed to

the sides of the stack and to cover some area under the contact holes, but electrical

conduction was not hindered.

In the first several bonding trials, the resistance of the metal line increased

dramatically (into the high kilo-ohm and low mega-ohm range) after the heating cycle.

The suspected causes were: (1) diffusion of silicon from the glass frit into the metal line

to form nonconductive silicide, (2) metal fracture induced by the thermal mismatch

between the metal and the glass frit. This problem was later solved with a ~300 nm thick

aluminum oxide (A120 3) diffusion barrier deposited on top of the metal. More details

about this problem and its solutions are presented in Section 9.

59

9 Vacuum Sensor Resistor Failure Analysis

9.1 Description

The resistance of both the Ti/Pt/Ti and Ta/Pt/Ta three-layer metal lines increased

dramatically after the test structure went through the heating cycle shown in Figure 24 in

sub 10 mTorr vacuum for glass frit bonding. The exact final resistances were not

consistent among samples, but they all fell between high kilo ohm and low mega ohm

range. This major resistance increase was only observed when the test structure was

heated enough (i.e. above 550 C) to completely soften the glass frit to make a robust

bond. If the temperature was not high enough, then the resistance would only increase to

at most 1 kilo ohm, but at the same time bonding would not occur.

9.2 Possible Causes

Experiments were done to determine whether the combination of vacuum and

glass frit caused this problem. Samples with Ti/Pt/Ti metal lines were bonded either with

or without glass frit in either vacuum or air. The final resistance of each metal line was

measured and presented in Table 6. According to these data, the Vitta GPR-10 glass frit

combined with vacuum increased the resistance the most by far. Glass frit in air also

increased the resistance somewhat. Heating alone in either air or vacuum had no effect.

Table 6. Ti/Pt/Ti resistance increase during bonding.10 mTorr Vacuum Air

With glass frit 200 ohm + -1 mega ohm 200 ohm 4 -600 ohm

Without glass frit 200 ohm -200 ohm 200 ohm 4 -200 ohm

60

The possible causes include: (1) silicon diffusion into the platinum metal line to

form nonconductive platinum silicide, and (2) physical fracturing of the metal line during

the heating cycle in vacuum.

The Vitta GPR-10 glass frit consists of zinc, lead, and borosilicate. Because the

glass frit was in direct contact with the metal during high-temperature bonding, silicon in

the glass frit could diffuse into the platinum layer through the grain boundaries of the

titanium layer to form nonconductive platinum silicide. This hypothesis was partially

confirmed by the Auger Electron Microscopy on some metal samples after the glass frit

was scraped off after bonding. The microscopy results did show some silicon signal in

the platinum layer, but due to the rough metal surface caused by scraping, these results

were not very reliable.

Thermal mismatch between the glass frit (thermal expansion coefficient = 6.5 x

10-6) and the three-layer metal line (thermal expansion coefficients of Ti = 9.2 x 10-6 and

of Pt = 8.9 x 10-6) could fracture the metal during temperature ramp down because the

glass frit was already tightly bonded to the metal at the time of temperature ramp down.

The stainless steel weight used to facilitate bonding also added an extra strain on the

metal.

Silicon diffusion is believed to be the major cause of the resistance increase based

on the following observations:

1. Thermal mismatch alone can not explain why the resistance did not increase as much

when the sample was bonded in air as in vacuum. If the thermal mismatch alone was

the problem, then the resistances should have increased to the same level both in

vacuum and in air.

61

2. Masahiro has shown that the diffusion of titanium in gold films increased with

lowering vacuum.1 7 This finding can explain the observation that the resistance

increased much more in vacuum than in air.

9.3 Solutions

9.3.1 Tantalum Nitride (TaN) Diffusion Barrier

One possible solution to deter diffusion is to deposit a layer of TaN diffusion

barrier on top of the middle (vacuum sensor) dies by reactive sputtering. The

composition of TaN can be controlled by adjusting the supply of nitrogen gas during

sputtering. The resulting TaN will become less conductive and a better diffusion barrier

with more nitrogen gas present. It is important to have a nonconductive diffusion barrier

in this case in order to prevent short circuiting the various metal lines on the vacuum

sensor die.

10 sccm nitrogen and 40 sccm argon were supplied into the sputterer to make a

~300 nm thick TaN layer. However, the resistance of the resulting TaN turned out to be

comparable to that of the metal line, which was not acceptable. In addition, several

bonding trials with the TaN-coated sensor dies produced resistance values from 20K to

30K ohm, which was still too high for the sensor to function. This indicated the poor

performance of this specific TaN composition as a diffusion barrier.

9.3.2 Aluminum Oxide (A120 3) Diffusion Barrier

~300 nm thick A120 3 was deposited onto the vacuum sensor dies with a shadow

mask by electron beam evaporation, shown in Figure 25. The positive results of the six

bonding trials shown in Table 7 indicated the ability of A12 0 3 to stop diffusion.

62

* ~ A120 3

glass frit

Figure 25. Placement of A120 3 diffusion barrier and glass frit.

9.3.3 Careful Temperature Control

Even though thermal mismatch is unlikely the main cause of the resistance

increase, careful temperature control can minimize the possibility of metal fracturing.

When the temperature decreases slowly enough, the glass frit molecules can re-orient

with the titanium and platinum molecules to lower the stress, and thus lower the

possibility of metal fracturing.

63

10

10.1

Evaluation of Vacuum Package

Summary

Six samples of the second generation test structure (i.e. 10 nm thick Ta/400 nm

thick Pt/20 nm thick Ta metal structure with 300 nm thick A120 3 diffusion barrier on top)

were bonded in 10 mTorr vacuum. No gas exit channel was created in the glass frit.

Based on the vacuum sensor measurements and the leak tests, no vacuum was retained in

any of the samples after leaving the vacuum environment.

The SAES St122 NEG had no effect on the vacuum level due to the premature

saturation caused by a high base pressure inside the test structure (i.e. 1 ATM). Getters

typically need to operate at a base pressure of 10-2 Torr or lower to prevent premature

saturation.

10.2 Vacuum Sensor Measurements

The vacuum sensor measurements of the six samples within one hour after

leaving the vacuum environment are summarized in Table 7, where V and I, are used to

calculate the base resistance R, = V,/10 initially, Target is the target temperature, Rtarget is

the target resistance calculated with Equation 12, V and I are measurements taken to

calculate R = V/I (to match Rtarget), and Q is the required power input to maintain Ttarget.

These measurements can not pinpoint the exact pressures inside the samples

because the sensor has zero resolution above roughly 50 Torr, which is a characteristic of

all thermal conductivity based vacuum sensors. In addition, because the calibration

curves of different sensor samples exhibit discrepancies of up to 18% in the high pressure

range, it is difficult to correlate these high sensor readings to the actual pressures.

64

Table 7. Bonding parameters and vacuum sensor measurement for each sample.Measurement taken within one hour after each sample left vacuum environment.

Sample #1 #2 #3 #4 #5 #6

Prebake Time 60 60 0 0 0 0[min]

Prebake 150-190 150-185 n/a n/a n/a n/aTemp[C]

Bonding 15 15 15 15 15 15Time[min]

Bonding 550-560 550-562 560-565 555-560 553-562 550-570Temp[C]

# Weight 1 1 1 1 1 2BlocksGetter? No Yes No Yes No No

Rbeore 116 116 113 110 112 115[ohm]Rafer 322 146 192 165 190 187[ohm]

VO 20.2 18.6 25.2 54.6 34.0 13.2[mV_]

10 0.067 0.144 0.142 0.379 0.196 0.080[mA]

R) 301.5 129.0 177.3 144.2 173.4 165.0[ohm]Ttarget 87 87 87 87 87 87

[C]TCR 0.26 0.26 0.26 0.26 0.26 0.26[%/C]Rtarget 349.7 149.7 205.7 167.3 201.3 191.4[ohm]

V 6.94 3.67 4.43 4.15 4.41 4.11

[V]1 19.86 24.56 21.76 24.78 21.9 21.5

IPA]R 349.6 149.6 203.5 167.3 201.3 191.6

[ohm}_]Q '137.8 90.2 96.3 102.7 96.0 88.3

[niW]P ~50-760 -50-760 -50-760 ~50-760 -50-760 ~50-760

[Torr]

65

10.3 Leak Tests

This leak test involves placing the bonded sample into a vacuum chamber,

pumping down the chamber, and then taking a measurement with the vacuum sensor. If

the sensor reading indicates a lower pressure than before, then that means some gases

have exited the structure through the leaks in the glass frit. If the pressure stays the same

as before, than that means the seal is hermetic and the high pressure inside the structure is

probably caused by the outgassing from the internal surfaces and the glass frit itself.

As shown in Table 8, the leak tests confirmed that none of the six samples

achieved a hermetic seal. However, the rate of gas movement across the seal was

significantly slowed down. For example, when the leak test was conducted on Sample #5

within 1 hour after pump down, the pressure inside the structure remained at 3.5 Torr

while the pressure in the chamber was only 50 mTorr. Roughly one day after pump

down, the internal pressure reached 94 mTorr while the external pressure was 10 mTorr.

Table 8a. Leak test results (Measured within 1 hour after pump down).Sample #1 #2 #3 #4 #5 #6

Chamber 25 25 20 50 50 30Pressure mTorr mTorr mTorr mTorr mTorr mTorr

Sensor 3.09 1.82 1.97 3.38 22.45 2.35Reading mW mW mW mW mW mW

Pressure 118 ±2.4 52 ±1.0 60 ±1.2 135 ±2.7 3.5 ±1 82 ±1.6Inside mTorr mTorr mTorr mTorr Torr mTorr

Structure

66

Table 8b. Leak test results (Measured 1 day after pump down).Sample #1 #2 #3 #4 #5 #6

Chamber 8 10 8 10 10 8Pressure mTorr mTorr mTorr mTorr mTorr mTorr

Sensor 2.19 1.64 1.31 1.58 2.67 1.65Reading mW mW mW mW mW mW

Pressure 73 ± 1.5 44 ±0.9 29 ±0.6 40 ±0.8 94 ±1.9 45 ±0.9Inside mTorr mTorr mTorr mTorr mTorr mTorr

Structure

10.4 Vacuum Seal Failure Analysis

Based on the vacuum sensor measurements and the leak tests, the Vitta GPR-10

glass frit can not achieve a hermetic seal with the current bonding procedure (Section

8.2). In order to visually examine the glass frit bond, transparent pyrex glass was bonded

to silicon with the same procedure. Also, bonded test structures were broken apart to

examine the bond between silicon and silicon.

While the glass frit bond was very strong (i.e. passing the razor blade test), voids

could be seen within the bonded area between silicon and pyrex, shown in Figure 26, and

between silicon and silicon, shown in Figure 27. These voids could potentially provide

an air leak path.

The major contributor to the formation of these voids is believed to be the

expansion of the organic binder as it is heated up and evaporates from the interior of the

glass frit during pre-sintering. This is evidenced by the increase in thickness from 25 um

to 40 um, and this extra volume created must be made up with voids. Apparently these

voids did not completely collapse when the glass frit melted and flowed during bonding.

67

Another contributor to the formation of these voids might be particle

contamination. The glass flit is 40 um thick, and any dust particle on the bonding surface

of that size may cause a void. This problem can be reduced by moving the bonding setup

into a clean room.

sensor resistor

cavityno glass frit

glass fritbonded areawith voids

Figure 26. Voids in Si-Pyrex glass frit bond.

68

glass frit bondw/ voids

Figure 27. Voids in Si-Si glass frit bond.

69

11 Future Work

The Vitta GPR- 10 glass frit can not achieve a hermetic seal with the current

bonding procedure. The suspected cause is the voids formed in the glass frit by the

organic binder. Some bonding parameters can be changed to try to reduce the voids.

First, the glass frit can be pre-sintered at a higher temperature for a longer time to

completely burn off the binder before bonding. Second, the bonding temperature and

time can be increased to make sure that the glass frit is completely melted during

bonding, which can help compress the voids. Third, contact pressure onto the bonding

surface can be increased (by adding more weight on top) to help compress the voids and

form a tighter bond.

Even if the Vitta GPR-10 can not achieve a hermetic seal after adjusting the

bonding parameters, other glass frit materials are still available for future studies. An

attractive alternative would be a continuous glass preform without any binder. This way

the void problem may be completely avoided.

A different bonding method worth an investigation is the solder bonding. Solder

(e.g. tin, lead, indium, gold) has the same flow characteristics as the glass frit, making it

ideal for bonding over rough surfaces, and the resulting metallic bond is known to

provide a strong seal. H. Tilmans, M. Van de Peer, and E. Beyne have successfully

created a vacuum package with solder bonding.5 However, for the current test structure

and the actual suspended tube reactor, an insulating layer (e.g. silicon dioxide, silicon

nitride, tantalum nitride, etc.) must be deposited on top of the resistor line, as shown in

Figure 28, to prevent short circuit by the electrically conductive solder. Also, the

70

- urn

relatively low melting point of a solder would limit the operating temperature of the

reactor.

insulating layer (e.g. SiO 2)

solder ring (e.g. Sn/Pb) on top ofwettable metal (e.g. Au)

Figure 28. Possible implementation of solder bonding on test structure.

In solder bonding, wettable metal pads need to be first deposited over the non-

wettable silicon surface. This wettable metal usually comprises three layers: an adhesion

metal (e.g. chromium or titanium), a barrier metal (e.g. copper or platinum), and a

sacrificial metal (e.g. gold). The barrier metal is required to prevent the solder from

dissolving the adhesion metal. Then, solder (e.g. tin/lead, indium, gold) is deposited over

the wettable metal. After contacting the two surfaces and heating to the melting point of

the solder (i.e. 250-350 C), the solder begins to flow and bonds the two surfaces.5

After working out a robust bonding method on the test structure, it will be

important to apply the same technique to packaging the actual suspended tube reactor.

The amount of getter required may be different since they may have different surface

area-to-volume ratios. Also, if another material is found to have a longer life time than

71

platinum under high temperatures, then the replacement will potentially alter the resistor

thickness, and whether the glass frit or the solder can still form a hermetic seal will need

to be investigated once again.

72

12 Conclusion

The purpose of this thesis project is to study the feasibility of selected vacuum

packaging methods, including a wafer bonding method that can provide a hermetic seal

over a rough surface in vacuum (i.e. -400 nm thick roughness), a gettering method that

can best absorb residual gases inside the package after bonding, and a vacuum sensing

method that can provide adequate sensitivity in the range of interest in order to confirm

the final pressure inside the package. Based on the results of literature search, a three-

layer test structure is built to experiment with glass frit bonding (Vitta GPR- 10 glass frit),

non-evaporable getter (SAES St122 NEG, mixture of Ti and Zr-V-Fe), and thermal

conductivity vacuum sensor. The main conclusion from this study is that the Vitta GPR-

10 glass frit can not achieve a hermetic seal with the current bonding procedure. The

vacuum sensor measurements, along with the leak tests, indicated atmospheric pressure

inside the test structure. And because the pressure inside the structure was too high, the

SAES St122 NEG quickly saturated and failed. Getters typically need to operate at a

base pressure of 102 Torr or lower, and any pressure higher than that would result in

premature saturation. For future work, various bonding parameters for the GPR- 10 can

be adjusted to try to make a better seal, and other bonding methods, such as solder

bonding, can be examined. The effectiveness of the SAES St122 NEG can only be

known when a robust hermetic seal is achieved to maintain a reasonable base pressure

inside the test structure.

73

13 References

1 K. Jensen, "Integrated Chemical Fuel Microprocessor for Power Generation inMEMS Applications," Quarterly Report for 2/1/2000-4/31/2001, MIT, 2001.

2 N. Maluf, An Introduction to Microelectromechanical Systems Engineering, Boston,MA: Artec House, 2000.

3 M. Schmidt, "Wafer-to-Wafer Bonding for Microstructure Formation," Proceedingsof the IEEE, Volume 86, Number 8, 1998.

4 W. Ko, J. Suminto, and G. Yeh, "Bonding Techniques for Microsensors," inMicromachining and Micropackaging of Transducers, C. Fung, P. Cheung, W. Ko,and D. Fleming, Amsterdam, The Netherlands: Elsevier, 1985.

5 H. Tilmans, M. Van de Peer, and E. Beyne, "The Indent Refloww Sealing Technique-A Method for the Fabrication of Sealed Cavities for MEMS Devices," Journal ofMicroelectromechanical Systems, Volume 9, Issue 2, 2000.

6 D. Hoffman, B. Singh, and J. Thomas, Handbook of Vacuum Science andTechnology, San Diego, CA: Academic Press, 1998.

7 T. Giorgi, B. Ferrario, and B. Storey, "An Updated Review of Getters and Gettering,"Journal of Vacuum Science and Technology A, Volume 3, Issue 2, 1985.

8 L. Parameswaran, "Silicon Pressure Sensor Using Wafer Bonding Technology," MSThesis, MIT, 1993.

9 M. Esashi, S. Sugiyama, K. Ikeda, Y. Wang, and H. Miyashita, "Vacuum-SealedSilicon Micromachined Pressure Sensors," Proceedings of the IEEE, Volume 86,Number 8, 1998.

10 A. Robinson, P. Haswell, and R. Lawson, "A Thermal Conductivity MicrostructuralPressure Sensor Fabricated in Standard Complementary Metal OxideSemiconductor," Review of Scientific Instruments, Volume 63, Issue 3, 1992.

11 R. Gooch, T. Schimert, W. McCardel, and B. Ritchey, "Wafer-Level VacuumPackaging for MEMS," Journal of Vacuum Science and Technology A, Volume 17,Issue 4, 1999.

12 H. Henmi, S. Shoji, Y. Shoji, K. Yoshimi, and M. Esashi, "Vacuum Packaging forMicrosensors by Glass-Silicon Anodic Bonding," Sensors and Actuators A, Volume43, 1994.

13 P. Atkins, Physical Chemistry, New York, NY: Freeman, 1991.

74

14 0. Paul, 0. Brand, R. Lenggenhager, and H. Baltes, "Vacuum Gauging withComplementary Metal-Oxide-Semiconductor Microsensors," Journal of VacuumScience and Technology A, Volume 13, Issue 3, 1995.

15 S. Firebaugh, "Investimation of Materials for Use in High-Temperature, Thin-FilmHeaters and Temperature Sensors," MS Thesis, MIT, 1997.

16 F. Incropera and D. DeWitt, Fundamentals ofHeat and Mass Transfer, New York,NY: Wiley, 1996.

17 K. Masahiro and S. Noboru, "Effects of Temperature, Thickness and Atmosphere onMixing in Au-Ti Bilayer Thin Films," 1993.

18 Nanonics Imaging Ltd (http://www.nanonics.co.il/cont/probes/wired.html).

75

Appendix A: Vacuum Sensor Modeling on Microsoft Excel

Thermal Conductivity of Air

pressure

boltzmann constant

stefan-boltzmann constant

collision cross section

molar mass

gas constant

approximate air temp

conduction distance

transition pressure

thermal conductivity (1 ATM & 300 K)

thermal conductivity

Thermal Conductivity of SiN

@ room temp

Pt Resistor

base resistance

base temperature

TCR (0.26%/K)

number of squares

power

current density

resistivity

width

thickness

length

top surface area

mTorr

1.381 E-23 J/K

5.67E-08 W/m2-K4

4.2E-1 9 m2

0.029 kg/mol

8.3145 J/K-mol

300 K

55 um

126.82 Pa

0.03 W/K-m

W/K-m

1.9 W/K-m

125 ohm

300 K

0.33 ohm/K

118

0.05 W

2E+09 A/m2

1.06E-07 ohm-m

100 nm

100 nm

11.8 mrn

1.42E-06 m2

Membrane and Resistor Geometry

membrane heater area Al

edge length

membrane conduction area A2

top gap distance LI

membrane conduction length L2

applied voltage V

substrate temp Ts

middle wafer thickness Li

nitride layer thickness

P [mtorr]

0.0001

1.42 mm2

1.19 mm

475.83 um2

55 um

100 urn

2.5 V

300 K

400 um

100 nm

T [K] R [ohm] Qg [mW] Qair [mW] QSiN [mW] Qrad [mW] I [mA]

714 261 24 0 4 20 10

76

Results

0.001

0.010.1

1

10

100

1000

10000100000

714

714

714

711

690

552

386

353

350

261

261

261

260

253

208

153

143

141

24

24

24

24

25 4

30

41

44

44

4 18 10

Statistics

current range = 8.11 mA

resistance range = 119.80 ohm

max temp = 440.73 C

77

0

0

0

0

4

4

4

4

20 10

20 Ic

20 1C

20 1C

21

39

43

43

2

1

0

0

7 12

1 16

I 18

I 18

Appendix B: Test Structure Mask Drawings on AutoCAD

Mask 1: Top Layer, Front Side - Sensor CavityBottom Layer, Front Side - Getter Room

78

Mask 2: Top Layer, Back Side - Contact Holes

O 00 o00 0 0

00, 0 0

0 01 -00 '00 010 -0.via

o 00 00 00' 00o 00 00 o " Orp

o 0 0 00(- 010 4 0 9

0 C 00 0 0 00 00 0

0 00 0 0 00 06 I 00 0

0 00 .0 o 00 0 - 00 0, 0 00 0

0 0:0 00

0 00

00 0 c 0O 10 o0 0 0

00 0 @ 00, 0 0 00, 00 0

0 00 00 00 00 00

0 00 00 00 00 00 00

o oo o6-t 00

0 Q0 0@

00. 001 00

o00 pGQO

00 00 0.

00 00 0,

00, 0 0

0I 0

0 00 00 '00. 00 00

c& 9 00 0

0.0,d

79

00 , 0

Mask 3: Middle Layer, Top Side - Sensor Resistor and Contact Pads

I-' ~L ww:[LWL

Li iif L IW

HHI+HH77 F'UHU U H LFHI

HH7

80

Mask 4: Middle Layer, Back Side - KOH Etch and Pressure Equalization Holes

I..

LiLi

...

El

E

DLi

Li

EEIi]0.Li

Li

Li

LiLI

L

Li

Li

El

l

ID

IIILi

LI

Li

LiLi

E

L]

EL

DLEL

EL

Li

LiLi

0ILi

LiL]

LiLi

L

[11

l

E]

E]

l

Li

LiLi

l

L-I

E

LiLi

LiLi

LiLi

LiLi

LiLi

K]F.

LiLi

Li

Li

E]Li

LiLi

LiLi

LI]Li

LiLi

LiLI

Li

Li

Li

Li

Li

LI]

Li

Li

Liii

L]Li

LiLi

LiLi

LILi

L..i

E

Li

81

Appendix C: Test Structure Detailed Fabrication Process Flow

Starting Material:

TOP: 1 single-side polished wafer

MIDDLE: 1 double-side polished wafer

BOTTOM: 1 single-side polished wafer

TOP WAFER

RCA clean

Grow SiO2 (0.5 urn thick)

BACKSIDE PROCESSING

HMDS

Coat photoresist on backside (AZP4620, positive, 10 um thick)

Prebake photoresist (60 mins at 90 C)

Expose backside- cavity

Develop photoresist

Postbake photoresist (30 mins at 90 C)

Remove SiO2 from patterned area on backside in BOE

STS etch backside (15 um)

Remove photoresist in Piranha

FRONTSIDE PROCESSING

HMDS

Coat photoresist on front side (AZP4620, positive, 10 um thick)

Prebake photoresist (60 mins at 90 C)

Expose frontside- heater contact holes

Develop photoresist

Postbake photoresist (30 mins at 90 C)

Remove SiO2 from patterned area in BOE

Mount to handle wafer (bull eye pattern)

Prebake photoresist (30 mins at 90 C)

STS etch through wafer from frontside (525 um)

Dismount handle wafer in acetone over night

Remove photoresist in Piranha

MIDDLE WAFER

RCA clean

Deposit stoichiometric Si3N4 (100 nm thick)

BACKSIDE PROCESSING

HMDS

Coat photoresist on backside (OCG 825, positive, 1 um thick)

Prebake photoresist (30 mins at 90 C)

Expose backside- membrane & pressure equilibration hole

HMDS

COATER

PREBAKE OVEN

EV1

PHOTOWET-L

PREBAKE OVEN

ACIDHOOD

STS

ACIDHOOD

HMDS

COATER

PREBAKE OVEN

EV1

PHOTOWET-L

PREBAKE OVEN

ACIDHOOD

COATER

PREBAKE OVEN

STS

ACIDHOOD

ACIDHOOD

RCA

TUBE A5

HMDS

COATER

PREBAKE OVEN

EV1

82

RCA

TUBE A3

ICLICL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

ICLICL

TRL

TRL

TRL

TRL

Develop photoresist

Postbake photoresist (30 mins at 120 C)

Pattern Si3N4

Remove photoresist in Piranha

PHOTOWET-L

POSTBAKE OVEN

AME5000

ACIDHOOD

FRONTSIDE PROCESSING

HMDS

Coat photoresist on frontside (image reversal, negative, 1.5 um thick-2200 rpm spin)

Prebake photoresist (30 mins at 90 C)

Expose frontside- metal heater (1.5 sec)

Place wafers on hotplate (90 sec)

Flood Expose (1 min)

Develop photoresist

UVOzone cleanAsh

Deposit 10 nm Ta/400 nm Pt/20 nm Ta

Lift-off

Nanostrip

HMDS

COATER

PREBAKE OVEN

EV1

POSTBAKE OVEN

EV1

PHOTOWET-L

UVOZONEASHER

E-BEAM

PHOTOWET-R

ACIDHOOD

KOH etch through wafer from backside in sealed holder, stopping on frontside Si3N4 layer KOH

Post KOH clean in Nanostrip & first half of RCA ACIDHOOD

Anneal metal (60 mins at 650 C) TUBE B1

BOTTOM WAFER

No processing needed

BACKSIDE PROCESSING

FRONTSIDE PROCESSING

HMDS

Coat photoresist on frontside (AZP4620, positive, 10 um thick)

Prebake photoresist (60 mins at 90C)

Expose backside- cavity

Develop photoresist

Postbake photoresist (30 mins at 90C)

Remove SiO2 from patterned area on frontside in BOE

STS etch backside (225 um)

Remove photoresist in Piranha

HMDS

COATER

PREBAKE OVEN

EV1

PHOTOWET-L

PREBAKE OVEN

ACIDHOOD

STS

ACIDHOOD

83

TRL

TRL

ICLTRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

SGL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

TRL

Appendix D: Equipment List

Kurt J Lesker Co Glass Bell Jar BJ12X181515 Worthington Avenue Feedthrough Collar FTC-12-1-8-0Clairton, PA 15025 Base Plate BASEPLATE12www.lesker.com Vacuum Gauge KJL205+KJL6000

Omega Engineering Cartridge Heater CSH-101100/1201 Omega Drive Temperature Controller CNi-32Stamford, CT 06907 Solid State Relay SSR240DC25www.omega.com

Leybold Vacuum Products Inc Mechanical Roughing Pump Trivac B D25B5700 Mellon RoadExport, PA 15632www.buyvacuum.com

Vitta Corp Glass Frit Tape GPR-107 Trowbridge DriveBethel, CT 06801www.vitta.com

SAES Non Evaporable Getter St1221122 E Cheyenne Mt BlvdColorado Springs, CO 80906www.saesgetters.com

84

Appendix E: Molecular Flow Calculations

The purpose of these calculations is to estimate the amount of time the gas moleculeswould need in order to exit the test structure through a thin channel carved in the glassfrit during the bonding process. The gas exit time was thought to be a concern because ina vacuum environment where the mean free path of the gas molecules is on the order ofthe enclosure size, gas flow becomes characterized by probablistic gas-wall collisions.

The Knudsen's Number (Kn) is defined by:

Kn = A = mean free pathd channel diameter

Molecular flow region is defined by having the Knudsen Number greater than 1.

Throughput (Q) is defined by:d dQ -PV = V -P (if cons tant volume)dt dt

Conductance of a channel (C) is defined by:

C= Q (at constant temperatur)

For air at room temperature, C[L/s] = 11.6 a A[cm2], where a is characteristic of thechannel geometry. The length and the diameter of the gas exit channel are 2 mm and 40um, respectively. The volume of the enclosure is 3 x 10-6 liter.

V = 3x10-6L]

L 0.02cm-- = =50 --> a = 0.025

D 0.00004cm

A = r(0.002) 2 = 1.26 x 10~5[cm 2]

C = 11.6(0.025)(1.26 x 10 5 ) = 3.6 x 10-6[]

d dQ =C(P - P) = -PV= V P (at cons tan t volume)

dt dt760 Torr TJ dP =J-dt0.04 Torr O 0

760 _3.6 x10-6in = T (assu ming PO = 0)

0.04 3 x 10- 6

T = 8.2[s]

The gas molecules inside the enclosure should be able to exit in less than 10 seconds.

85

Experimental Data on the Target Pressure

2.8

15 V2.6 -

E. 1*- 40 mTorr05 2.4 -

CO)-j 2.2-

2.0 -CUa)

1.8-

1.6 -

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01

Ambient Pressure (Torr)

The above graph shows that a vacuum of 40 mTorr or less is desirable in order tominimize heat loss. At 40 mTorr or less, heat loss becomes independent of pressure for

the geometry of the suspended tube reactor, and achieving a pressure lower than that will

not further decrease heat loss.

86

Appendix F: