Contact Evolution in Micromechanical Switches · Contact Evolution in Micromechanical Switches an...

Contact Evolution in Micromechanical Switches

an experimental investigation using a contact test station

Lei Chen

Ph.D. DissertationPh.D. Committee: Dean Zavracky, Prof. Adams, Prof. McGruer

Boston, August 2007

Advisor: Nicol McGruerCo-Advisor: George Adams

MICROFABRICATION LABORATORY Lei Chen, August 07,07

MEMS SwitchesCompared to PIN or FET’s switches:

• Higher Linearity • Wider Frequency

Response Range

• Higher Isolation

• Lower Insertion Loss

• Lower Power Consumption

http://www.memagazine.org/backissues/oct03/features/littknow/littknow.html

Problems: Slow Response, Low Power Handling and Low Reliability


Switch Failure Modes• Adhesion prevents contact separation.• High resistance due to contamination.• Adhesion on large contact peel material off

substrate• Excessive material transfer, reshaping

– Contact damage– Material transfer prevents separation

Contamination

104 cycles

106 cycles

No cycles

Material Transfer

Object: understand dominant physical failure mechanismsin switch lifetime test


Outline• Experiment Setup and Cantilever Fabrication• Pull Off Force and Contact Evolution

– Separation Modes: Brittle and Ductile.– Rate Dependent Pull Off force– Force Evolution and Separation Modes– Size and Material Effects

• Resistance and Contact Evolution– Contamination buildup rates and their relation to

contact materials.


SPM Setup for Contact Study

Test Cantilever

Substrate

PL022

Gas Inlet

wedge


Pull Off Force Measurement

Signal Detected

Pull Off Force

Piezo Driving Signal

Piezo

Time

Loading Force

PD Response

Stiffness of the cantilever is 1~1.5x104N/m, with force measurement resolution of 10~15µN.


Fabrication Flow

1. SOI Wafer 6. Spin PR

7. Pattern PR

8. Reflow PR

9. Anisotropic Etching

13. Remove Al/Ti

10. Coat Al/Ti

12. ICP Silicon Etching

11. Pattern Al/Ti

2. Oxidation

3. Pattern Back

4. TMAH Etching

5. HF Release


Round Bump FabricationShipley 1818 Shipley 1818 The shape of the photo

resist is transferred to the silicon by using SF6/O2/Ar ICP silicon etching process.

Photo Resist Before Reflow Photo Resist After Reflow

O2:SF6:Ar=20:10:25

Silicon Bump

O2:SF6:Ar=15:10:25

Silicon Bump• Critical issues for

profile transfer:– Process Pressure– Biased Power– Gas Ratio


Contact Angles

32

hX h ZL

θ ⋅⎛ ⎞∆ = ⋅∆ = ∆⎜ ⎟⋅⎝ ⎠

:θ∆ Cantilever deflection angle

Horizontal movement comes along with vertical indentation

Single Wedges

Assume h=15µm and L=170µm, ∆Z=20nm will result in 2.6nm sliding


Sliding Cancellation

• Wedge 1 provides a contact angle

• Wedge 2 provides an actuation angle

⎥⎦

⎤⎢⎣

⎡∆

−∆⋅

⋅⋅

= )(23arctan

sub

sub

ZZ

Lh δϕ

Compensation angel is related to the indentation depth!


Outline• Experiment Setup and Fabrications• Pull Off Force and Contact Evolution

– Separation Modes: Brittle and Ductile.– Rate Dependent Pull Off force– Force Evolution and Separation Modes– Size and Material Effects

• Resistance and Contact Evolution– Contamination buildup rates and their relations to

contact materials.


Two Separation Modes

Brittle Separation: on separation, there is no plastic deformation before the rupture.

Ductile Separation: on separation, there is varying degree of plastic deformation during the rupture.


Physical Mechanisms of Separation Modes [1]

Dislocation Nucleation (Ductile)

Cleaving (Brittle)

:cleaveG Energy dissipation associated with cleaving surfaces

(=w, work of adhesion)Energy dissipation associated with dislocation nucleation

(crystalline, grain boundary, defects and material hardness):dislG

[1]: J.W.Kysar, Journal of the Mechanics and Physics of Solids, 51, 795-824, 2003MICROFABRICATION LABORATORY Lei Chen, August 07,07

Ductility of Gold • Au is FCC metal, with 111 slip planes and

<110> slip directions.• Assume a (100) crack intersected by (111) slip

plan, based on theoretical calculation [2]:– Critical surface energy needed for dislocation

nucleation is 1.27J/m2 (Pure Mode I)– Critical surface energy needed for dislocation

nucleation is 0.58J/m2(with 10% Mode II)

Surface energy of gold is 1.56J/m2, gold is an intrinsically ductile metal.

[2]: J.R. Rice, Journal of the Mechanics and Physics of Solids, 40, 239-271, 1992


Force-Displacement Separation Curve

The “plateau” region in the force-displacement curve is characteristic of ductile separation.

Displacement


Pull Off Force and Separation Modes

• In Brittle Mode( JKR Model) :

RwFpulloff ⋅⋅⋅= π23

HrFpulloff ⋅⋅≈ 2π

:R

:r:w

radius of curvaturework of adhesion

radius of ductile area:H hardness

:pulloffF pull off force• In Ductile Mode:

In brittle mode, magnitude of the pull off force is affected by surface conditions. Whereas, in ductile mode, magnitude of the pull off force is related to bulk properties of materials.


Rate Dependent in Brittle ModeMaximum Loading Force =200µN

• Gold contacts were tested in ambient condition (R.H=30~40%).

• In brittle mode, surface events affect magnitude of the pull off force.

• Longer time in contact, larger pull off force.


Rate Dependent in Ductile Mode

On ductile separation, higher loading and faster unloading can lead to larger pull off force.

• There is viscous effectduring ductile separation.


Atoms Flow in Nanostructure• For nanostructure, internal

cohesive bonding strength can be affected by the surface tension.

⎟⎠⎞

⎜⎝⎛ +−=

ldTT

mb

m 246

1 β

ldβ

mT

mbT

Melting temperature for nanostructure

Melting temperature for bulk material

Diameter (nm) Length (nm)

Material constant (1.12 for Au)

l

d

Liquid Drop Model:

Pulling out ductile tips

For Au, Tmb = 1337K,

Tm=1212K for d=10nm, l=20nm

Smaller melting temperature Tm

Lower activation energy for diffusion Ea

Larger diffusion coefficient D


Viscous Force Modeling nanotips as viscous liquid bridges during ductile separation:

vmpulloff FFF +=pulloffF

mF

vF: Capillary Force, surface tension, independent of rate

: Viscous Force, viscosity, depend on unloading velocity

:Pull Off Force during ductile separation.

a0=5nmD0=20nm

a0=15nmD0=20nm

a0=30nmD0=20nm

a0=50nmD0=20nm

Tm(K) 1212 1279 1295 1302

D(×10-15m2.s-1) 1.49 0.4 0.29 0.258

η(Pa.s)

Fm(nN)

Fv1(nN)@ 35µm/s

Fv2(nN) @0.05µm/s

a0=5nmD0=20nm

a0=15nmD0=20nm

a0=30nmD0=20nm

a0=50nmD0=20nm

Tm(K) 1212 1279 1295 1302

D(×10-15m2.s-1) 1.49 0.4 0.29 0.258

η


(Pa.s) 555 2043 2829 3223

Fm(nN)

Fv1(nN)@ 35µm/s

Fv2(nN) @0.05µm/s

a0=5nmD0=20nm

a0=15nmD0=20nm

a0=30nmD0=20nm

a0=50nmD0=20nm

Tm(K) 1212 1279 1295 1302

D(×10-15m2.s-1) 1.49 0.4 0.29 0.258

η(Pa.s) 555 2043 2829 3223

Fm(nN) 7.8 70 282 785

Fv1(nN)@ 35µm/s 26 461 2174 6444

Fv2(nN) @0.05µm/s 0.04 17 82 243

pulloffF

0D

02 a⋅

vUnloading Velocity

Evolution of Brittle Separation

After 10 cycles After 102 cycles After 103 cycles After 104 cycles

• For contact evolution in the brittle mode, the nominal contact area increase during the cycling test.

• The measured pull off force were the same for all four cases. (~100µN with the maximum loading force of 200µN).


Force and Ductile Separation

Cycle Number

1e+3 1e+4 1e+5 1e+6

Adh

esio

n (u

N)

40

60

80

100

120

140

160

Cycle Number

1e+3 1e+4 1e+5 1e+6

Adh

esio

n (u

N)

40

60

80

100

120

140

160

Cycling test leads to large area material transfer


Evolution of Ductile Separation (a)• Evolution of the ductile separation will cause

material transfer and lead to unstable pull off force.

After 10 Cycles After 20 Cycles After 30 CyclesDetect Ductile

The ductile separation is indicated by the plateau region in the force-displacement separation curve. It shows the pull off force increases after the ductile separation.


Evolution of Ductile Separation (b)

Pull off force decreases after the ductile separation.


Force, Modes and Evolution

Displacement (nm)

16 18 20 22 24

Forc

e ( µΝ)

-120-100-80-60-40-20

02040

3x104

8x104

The contact evolution start from a ductile separation, and evolve to a brittle separation.


Size Effects

• The tests are all for 200nm sputtered gold contacts with maximum loading force of 200µN.

• The large contact bump need large pull off force.

• Ductile separation have been observed on all four sizes.


Au-5%Ru Contact

• Hardness:– Au-5%Ru: 2.42GPa– Au: 1.04GPa

• Compared to Au contacts, 5%Ru alloy element can:– Reduce ductile

separation– Lower pull off force.


Materials EffectsAu Ru Rh Pt Au5%Ru Au10%Ru Au10%Pt

3.99 2.79

Maximum Pull Off Force (µN)

200 <50 50 50 50~75 50 75

Surface Damage Severe None Minimal Minimal Medium Medium Medium

Hardness (GPa) 1.04 15.3 9.75 5.39 2.42

Materials are prepared in Air Force Research Laboratory. The pull off force value are measured with the maximum loading of 200µN. Tests are performed between the same material pair contact tests.

• For hard material, pull off force is low and surface damage is minimal


Outline• Experiment Setup and Fabrications• Pull Off Force and Contact Evolution

– Separation Modes: brittle and ductile.– Rate Dependent Pull Off force– Force Evolution and Separation Modes– Size and Material Effects

• Resistance and Contact Evolution– Contamination buildup rates and their relations to

contact materials.


Resistance Measurement• Contact resistance measured with 1mA current source and a 2.1V

compliance voltage. • Measured resistance includes the sheet resistance component for

the cantilever and contamination film between the contacts.

Test layout for contact resistance measurement


Resistivity and Hardness of Au Alloys

* Data were measured in Air Force Research Laboratory by Dr. Kevin Leedy.

•Ru, Pt, and Rh show higher hardness than Au.

•There is an increase in contact resistance and decrease in hardness of Pt, Rh and Ru by alloying with Au.

•Ideal contact materials: moderate hardness, low resistivity.MICROFABRICATION LABORATORY Lei Chen, August 07,07

Resistance Evolution Test

Cycle Number1e+3 1e+4 1e+5 1e+6 1e+7

Res

ista

nce

(ohm

)

0

5

10

15

20

Au Rh

RuPt

Test conditions: Contact Force, 200µN; Identical contacting materials, ~300nm thick sputtered thin film; Cold switching in room air


Au and Au Alloys


Res

ista

nce

(ohm

)

0

5

10

15

20

90%Au,10%Pt

30%Au,70%Ru70%Au,30%Ru

30%Au,70%Rh70%Au,30%Rh

50%Au,50%Pt

Au


Ru and Au-Ru Alloys


Res

ista

nce(

ohm

)

0

5

10

15

20

Ru

95%Au,5%Ru

80%Au,20%Ru

30%Au,70%Ru

Au


Surface Reactivity [3]

Building-up contamination around Ru bump.

• Pt, Ru, Rh are transition metals with unfilled d-band.

• Au is a noble metal with filled d-band.

• Transition metals show strong surface reactivity.

[3] B.Hammer and J.K.Norskov, Nature, 376, 238-240, 1995.


EDX SpectrumAu-30%Rh

A

B

C

A:

B:

• Contaminants are carbon based materials.

C:


Au-5%Ru and Au-10%Ru• Cold Switching for 106 cycles

Au-5%Ru Au-10%Ru


Conclusion• Developed practical approaches for contact

evolution study: such as monitoring rate-dependent pull off force, force-displacement curve.

• Gained some understanding about ductile separation and contact evolution.

• Material intrinsic properties are important for contact reliability: such as hardness, electron structure, resistivity, melting point.


Future Research• Plasma Cleaning• Current, rate effects on

contact evolution.

• New test structures

Structures were designed by Jim Guo

• Contact materials


AcknowledgementThis research has been supported by Northeastern University, and

DARPA under its HERMIT program through research contract F33615-03-1-7002

• I would like to thank Prof. McGruer and Prof.Adams for their guidance and support;

• I am grateful to Dean Zavracky for serving as my Ph.D. committee member;

• I wish to thank Dr. Kevin Leedy in Air Force Research Lab for collaboration.

• Also express my gratitude to the rest of faculty, staff, and colleagues in MFL group.


Questions?


Inert Gas Control

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⋅⋅=

2/10 )4(flow

pathab

A

VL

D

TerfcCC

:T Thickness of gas flow :pathL Length of gas flow

:flowV Gas flow rate :abD Diffusion coefficient

:0C Concentration in the air

:AC Concentration in the contact area

The set-up can keep the organic vapor concentration at the contact area five orders of magnitude lower than the concentration in the air


Force Measurement

cPD ZLd⋅

=∆3

ccc ZkF ⋅=

Fc: contact force.

kc: stiffness of cantilever

Zc: deflection of the cantilever

∆PD: shift of the laser spot on PD.

L: length of the cantilever

d: the distance between the cantilever and the PD

Stiffness of the cantilever is 1~1.5x104N/m, with force measurement resolution of 10~15µN.


Test Method• Testing method: cantilever integrated with

contact bump + laser beam– Convenient to vary the contact shapes and contact

materials; – Easy to control the contact force and the actuation

methods; – In-situ measure the pull off force and contact

resistance;– Easy to inspect contact area.


Rate Dependent at Brittle ModeMaximum Loading Force =200µN

• Gold contacts were tested in the ambient condition (R.H=40%).

• Magnitude of the pull off force affected by the time in contact and the time between contacts.


RuO2 Contact Tests

The film with high resistivity shows slow contamination rate. ρRuO2=80µΩ.cm with different film thickness

ρRuO2=167µΩ.cm 50Ǻ 200Ǻ

400Ǻ 2500Ǻ

With film thickness of 2500Ǻ


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