V. Dry Etching, General Principles Advanced Dry Etching Techniques

Fall 2013Prof. Marc Madou

MSTB 120

Content

Dry etching: definition Pressure units and modes of gas

flow Plasmas or discharges

How to create a vacuum Plasmas: DC and AC Paschen curve

Dry etching mechanisms Dry etching types and equipment Etching profiles:

Sputtering Chemical Ion-enhanced Ion-enhanced inhibitor

Etching profiles in physical etching Faceting Ditching Redeposition

Comparing wet with Dry Etching

Dry etching: definition

Dry etching techniques are those that use plasmas (hot ionized gases) to drive chemical reactions or employ energetic ion beams to remove material. Dry-etching processes yield finer patterns than wet etching (surface tension !). These techniques also offers greater safety as large quantities of corrosive acids or bases are not required.

Within a dry etching reaction chamber the wafers lie directly in the plasma glow (also called a discharge), where reactive ions are accelerated towards the wafer (often biased). The ions are a species likely to attack the substrate material chemically with or without selectivity.

Pressure units

Energy Requirements Associated with Various Physical Processes.

Ion Energy (eV) Reaction<3 Physical adsorption4-10 Some surface sputtering10-5000 Sputtering10K-20 K Implantation

1 microbar = 0.1 Pa 1 in.H2O = 248.8 Pa1 µm Hg =0.1333 Pa 1 kPa = 1000 Pa1 N/m2 = 1 Pa 1 ft H2O = 2986 Pa1 mm H2O = 9.795 Pa 1 in. Hg = 3386 Pa1 mbar = 100 Pa 1 psi = 6895 Pa1 mmHg = 133.3 Pa 1 bar = 105 Pa1 torr = 133.3 Pa 1 atm = 101325 Pa

psi (1) in.H2O(2)

in.Hg (3) kPa millibar cm H2O(4)

mm Hg(5)

psi 1.000 27.680 2.036 6.8947 68.947 70.308 51.715in.H2O 3.612710-2 1.000 7.3554 10-2 0.2491 2.491 2.5400 1.8683in.Hg 0.4912 13.596 1.000 3.3864 33.864 34.532 25.400kPa 0.14504 4.0147 0.2953 1.000 10.000 10.1973 7.5006millibar 0.01450 0.40147 0.02953 0.100 1.000 1.01973 0.7500

6cm H2O 1.4223 10-2 0.3937 2.8958 10-2 0.09806 0.9806 1.000 0.7355mm Hg(torr)

1.9337 10-2 0.53525 3.9370 10-2 0.13332 1.3332 1.3595 1.000

(1) pounds per square inch(2) at 39 °F(3) at 32 °F(4) at 4 °C(5) at 0 °C

Definitions of Vacuum Regimes:1) Rough Vacuum: ~0.1- 760 torr (atmospheric pressure is 760 torr)2) Medium Vacuum:~ 0.1 to 10-4 torr3) High Vacuum: ~ 10 -8 to 10-4 torr4) Ultrahigh Vacuum: < 10 -8 torr 2 modes of gas flow:

Viscous Flow regime:gas density (pressure) is high enough, many moleculemolecule collisions occur and dominate the flow process (one molecule “pushes” another). Collisions with walls play a secondary role in limiting the gas flow.

Molecular flow regime: gas density (pressure) is very low, few molecule-molecule collisions occur and molecule- chamber wall collisions dominate the flow process (molecules are held back by walls). See further below for mathematical expressions for these two regimes.

Pressure units

How to create a vacuum

Visit on your own time http://et.nmsu.edu/ETCLASSES/vlsi/files/ARTICLE.HTM on vacuum pumping (take the quiz at the end).

The first thing that all basic systems have is a rough-pumping system. It is used to reduce the pressure from atmospheric pressure in the chamber to a lower pressure level that other low-pressure systems can use. Then there has to be a fine-pumping system that must be able to attain sufficient pumping speed to handle the outgassing from the work produced in the chamber of the vessel. There must also be vacuum gauges that determine the pressure at certain points of the system.

Pumps: Diffusion pumps operate from 10-4 Torr to 5x10-11 Torr. Diffusion

pumps operate by boiling a fluid, often hydrocarbon oil, and angling the dense vapor stream in a downward conical direction back into the pump boiler. Gas molecules from the system that enter the oil curtain are pushed toward the boiler by momentum transfer from the large fluid molecules.

Diffusion pump

Mechanical pump. Pump operation is based on bulk flow of gas; hence the pump works in the viscous flow regime. Used for obtaining "rough" vacuum (10-3 Torr), which is the lower limit of the viscous flow regime

Simplest plasma chamber is 2 parallel plate electrode set (anode and cathode) in a low pressure Argon filled chamber (e.g. 0.001 to 1 Torr). The two electrodes are positioned parallel to each other, with the top electrode and chamber walls electrically grounded while the lower electrode and substrate holder are connected through a dc-blocking capacitor and matching network to a 13.56 MHz F generator (AC plasma case)

Plasmas : DC and AC

Principle of mechanical operation:         (1) begin expansion cycle         (2) seal off expanded volume         (3) compress gas out exhaust

Apply 1.5 kV over 15 cm--field is 100-V/cm. Breakdown of Argon when electrons transfer a kinetic energy of 15.7 eV to the Argon gas.

These energetic collisions generate a second electron and a positive ion for each successful strike.

If the two electrons reenergize creating an avalanche of ions and electrons we get a glow or plasma.

At the start of a sustained gas breakdown a current starts flowing and the voltage drops to about 150 V.

To sustain a plasma, a mechanism must exist to generate additional free electrons after the plasma generating ones have been captured at the anode.

The additional electrons are formed by ions of sufficient energy striking the cathode (emitting secondary Auger electrons).

This continuous generation provides a steady supply of electrons and a stable plasma.

Plates too close: no ionizing collisions (not enough energy), too far too many inelastic collisons in which ions loose energy.

Plasmas: DC and AC

Plasmas : DC and AC

Plasma dark spaces: dark because the higher energy electrons cause ionization rather than light-generating excitation.

Plasma is always positive, this follows from kinetics: for a random velocity distribution the flux of ions and electrons upon a surface is given by:

where n is a density and <v> an average velocity. Ions are 4000 to 100,000 times more heavy than electrons so the average velocity of electrons is much larger. Electron flux to surrounding surfaces is larger resulting in a positive charge on the plasma.

Assymetry of voltage distribution: electrons move faster away from the cathode than positive ions are accelerated towards it larger space charge (also the dark space is larger at the cathode).

ji, e

=n

,i ev

,i e

4

Plasmas : DC and AC

The largest voltage drop is in front of the cathode where charged particles will experience their largest acceleration. The cathode gets etched the anode does not !! Substrates to be etched are laid down on the cathode.

Efficiency or ‘strength’ of a particular plasma is evaluated by the average electron energy (temperature) ion energy (temperature) electron density (e.g. 109 and 1012 cm-3) ion density (e.g. 108 to 1012 cm-3) neutral species density (e.g. 1015 to 10 16

cm -3) ion current (e.g. 1 to 10 mA/cm 2).

ve =kTe (e.g. 1−10eV)

v i =kTi (e.g. 0.04 eV)

Plasmas : DC and AC

The ratio between ionized species and neutral gas species is 10-6 to 10-

4.

Plasmas : DC and AC

An important quantity to describe a plasma is the ratio of electrical field over pressure (Equation I). With increasing fields the velocity of free electrons or ions increases (~E) but an increase in pressure decreases the electron or ion mean free path (~1/P).The mean free path () is given by Equation (II) where nv is the number of molecules per unit volume,

The number of molecules per unit volume, nv, can be determined from Avogadro's number and the ideal gas law, leading to Equation (III)

The bombarding flux of ions on the cathode is given by Equation (VI):

kTi , e

~

E

P

(I)

(II)

(Equation III)

ji

= qniμ

iE

(Equation IV)

Plasmas : DC and AC

Vdc =kTe

2eln

Temi

T ime

Emax =e Vdc +Vp( )=eVT

Emax =eVp

AC plasma’s for etching insulating surfaces. Capacitor makes voltage distribution assymetric in this case. A DC self bias results. Etching energy: Plasma energy: Self bias:

AC Plasma’s

Paschen curve

Paschen Curve in Air

V

BreakdownVoltage V

Pressure x distanceP X d ( mm Hg-mm)

At 1 atmosphere = 760 mm Hg

0

0

200

400

600

800

1000

1200

1400

10 20 30 402

2.6 µm 13. 16 µm

New Physics and Chemistry

Dry chemical etching mechanisms

Reactive species generation (1) Diffuse to the solid (2) Adsorption at the surface (3) Reaction at the surface (4) Reactive cluster desorption (5) Diffusion away from the surface

(6)

Continuous dry-etching spectrum low pressure <100 millitorr: physical

sputteringunselectivedirectional radiation damage

100 millitorr range:RIEphysical and chemicaldirectionalmore selective than sputtering

higher pressures:plasma etchingchemical (10-1000 times faster) --see

extreme example, gas phase etching with XeF2 (not really a plasma)

» isotropic

» more selective

» least damage

Dry chemical etching mechanisms

XeF2 Gas Phase Etching (high pressure, chemical only) no plasma (just pump) 10 µm/min no damage isotropic very selective (Si over Al,

photoresist, oxide and nitride) CMOS compatible

Dry chemical etching mechanisms: purely chemical

Dry chemical etching mechanisms: Physical-

chemical etching: Energy-driven anisotropy

Dry chemical etching mechanisms: Physical-chemical etching: Inhibitor-driven anisotropy

Dry etching types and equipment

Dry Etching

Glow dischargemethods-DiodeSet-up

Ion beam methods-Triode Set-ups

Plasmaetching

Reactiveion etchingsputtering

Sputteretching

Ionmilling

Reactiveion beametching

Physical etching

Ion beamassistedchem.etch

Reactive gasplasma

Reactive gasplasma

Inert gas

Inert gasion

Inert gasion

Reactive gasion beam

0.2 - 2 Torr 0.01-0.2

Torr

10-4-10-3 Torr

Low energybombard.

High energy

High energy

No reactiveneutrals

Reactive neutrals

Some reactive

Dry etching types and equipment

RIE chamber with load lock

Substrate holder for ion etching

Substrate holder for deposition

Anode

Cathode

Vacuum chamber wall

Matching network

13.56 MHzRF Generator, 1-2 kW

(RF electrode with target)

Ground shield

C A IB E R IB E IB E M IE M E R IE R IE B arre l

E tc h i n g

P E

P re s s u re

( T o rr)~10-4 ~10-4 ~10-4 10-3-10-2 10-3-10-2 10-3-

10-1

10-1-100 10-1-101

E t c h

M e c h an i s m

chem /

phys

chem/

phys

phys phys chem/

phys

chem/

phys

chem chem

S e l e c ti v i ty good good poor poor good good excel lent good

P ro f i l e anis or

i so

anis ani s ani s ani s iso or

ani s

iso iso or

ani s

Dry etching types and equipment

Reactive gas Inert gas

+ + + + + + + + + + + + + + Plasma source

RIBE CAIBEReactive gas

Vacuum pump

Substrate Substrate

Acronym/Technique Explanation

CAIBE Chemically assisted

ion beam etching

MERIE Magnetically enhanced

reactive ion etching

MIE Magnetically enhanced ion

etching

PE Plasma etching

RIBE Reactive ion beam

etching

RIE Reactive ion etching

Dry etching types and equipment

Dry etching types and equipment : RIBE vs. CAIBE

CAIBE is RIE in a triode system (e.g. 10, 000Å/min)

RIBE ion is reactive and etches

(e.g. 100Å/min)

Reactive gasInert gas+ + + + + + + + + + + + + + Plasma sourceRIBECAIBEReactive gasVacuum pumpSubstrateSubstrate

Etching profiles in dry etching

Sputtering: directional, physical. Chemical: non-directional

(diffusion). Ion-enhanced energetic:

directional. Ion-enhanced inhibitor: directional.

Etching profiles in physical etching

Faceting: angle of preferential etching

Ditching (trenching): sometimes caused by faceting

Redeposition: rotational stage might reduce this effect.

Comparing wet vs. dry etchingParameter Dry Etching Wet EtchingDirectionality Can be highly directional with

most materials (Aspect ratio of25 and above)

Only directional with singlecrystal materials (Aspectratio of 100 and above).

Production-lineautomation

Good Poor

Environmentalimpact

Low High

Masking filmadherence

Not as critical Very critical

Cost chemicals Low HighSelectivity Poor Can be very goodMaterials that can beetched

Only certain materials can beetched (not e.g. Fe, Ni, Co)

All

Radiation damage Can be severe NoneProcess scale-up Difficult EasyCleanliness Good under the right operational

conditionsGood to very good

CD control Very good (< 0.1µm) PoorEquipment cost Expensive InexpensiveSub micron features Applicable Not applicableTypical etch rate Slow (0.1 µm/min ) Fast (1 µm/min, anis. )Theory Very complex, not well

understoodBetter understood (Chapter4)

Operatingparameters

Many Few

Control of etch rate Good due to slow etching Difficult

Homework

1     How is a DC plasma created and how does an RF plasma differ?  Why is a plasma always positive with respect to the reactor vessel walls?  In which etching setup would you prefer to etch an insulator?  Is space positively charged?

2    Detail the different dry etching profiles available and how you obtain them.  3. Explain the DC breakdown voltage versus electrode distance curve

(Paschen’s law) and how it is relevant to dry etching.  How is miniaturization of an electrode set equivalent to creating a local vacuum?

4. Discuss the etch profiles in physical etching.  Also draw profiles exhibiting faceting, ditching, and redeposition. 

5. Design a process to fabricate a polyimide post 100µm high and 10 µm in diameter on a Si cantilever.  The Si cantilever must be able to move up and down over a couple of microns.