EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie

Dry Etching

Lecture 2 Dry Etching I „

Agenda: Ê

DC Plasma – Plasma discharge zones – Paschen’s Law

Ê Ê Ê

RF Plasma High-density Plasmas DRIE – Microloading – Silicon grass

Reading: M. Madou, Chapter 2, pp. 77-107 Most figures in this presentation are adapted from M. Madou, Chapter 2 EEL6935 Advanced MEMS

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Plasmas

Glow Discharge Plasma „

Glow occurs when a DC voltage is applied between two electrodes in a gas Ê Ê Ê

Ê Ê Ê

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Low pressure (0.001~10 Torr) High voltage (~1kV) Electrons from cathode accelerated in the electric field ionize gas molecules and provide the plasma-sustaining current Energetic collisions create avalanche of ions and electrons Electrons move much faster than ions Neutral species greatly outnumber electrons and ions by 4 to 6 orders of magnitude

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Ions

Neutrals

Electrons

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Glow Discharge Plasma Ê

Ê

„

Average particle energy is given by =kBTe for electrons =kBTi for ions

Glow Discharge Plasma

Ions

Neutrals

Electrons

Ê

Dissociation

Typical values

Highly reactive radicals Ê

Ê

„

e- + Ar → Ar* + eAr* → Ar + hv Photons

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Reactive Plasma Etching

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e- + CF4 → CF3+ + F + 2e-

Excitation e - + F → F* + e F* → F + hv

Je = neq/4 Ji = niq/4 → is much greater than , so Je >> Ji ⇒ Permanent positive charge ⇒ electrons lost to the walls 2005 H. Xie

Ionization e- + Cl2 → 2Cl+ + 2e-

Effective current density

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e- + CF4 → CF3 + F + e-

e- + Cl2 → 2Cl + e-

: 1~10eV (hot) : 0.02~0.1eV (cold) Thus, Te>>Ti e.g., Ee~2eV; Ei~0.4eV Then, Te = 23,000 K! But Ti = 490 K Ê

Electron-Molecule Collisions

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Glow Discharge Plasma

Chemical etching Isotropic

Ê

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Color of light emission depends on gas, ionization energy, pressure and electric field

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Glow Discharge Plasma Ê Ê

Ê

Ê Ê

Ê

Special zones Aston dark space: low energy electrons Cathode glow: electrons gain sufficient energy to excite gas atoms Crookes dark space: electrons gain too much energy and luminescence is weak due to inefficient excitation Negative glow (brightest region): low electric field Faraday dark space: electrons slows down due to collisions and low electric field Positive column: quasineutral, low electric field, uniform; not important for etching or deposition

Breakdown voltage (V)

„

Paschen’s Law

„ „ „

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The breakdown voltage is a function of the product of the gas pressure and the gap distance, i.e., V = f(Pxd) The curves have minima. For large pxd, increasing pxd results in larger breakdown voltages. For small pxd, breakdown voltages increase with pxd decreasing. This is because when the pressure is too low or the distance is too small, most electrons reach the anode without any collisions. In air, the minimum breakdown voltage is 327 V.

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RF Plasmas

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RF Plasmas Capacitive coupling

RF voltage source

2VP ≈

(VRF ) p − p 2

− VDC

Vp: plasma potential VDC: self-bias VRF: applied RF signal „ „ „

Electrons oscillates between the electrodes with the AC voltage. No need for electron emission from cathode. Can sustain RF plasma at lower pressures than DC plasma. RF plasma allows etching of dielectrics as well as metals.

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Self-bias VDC: electrons move faster than ions and charge up the cathode (electrons cannot cross over the capacitor) to build up a negative potential.

(

•The maximum energy of positive ions striking the cathode is e VDC + VP •The maximum energy of positive ions striking the anode is 11

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eVP

)

~300eV ~20eV 12

RF Plasmas „

RF Plasmas Equivalent electrical circuit of RF plasma

Child-Langmuir equation for the ion-current density

„

V 3/ 2 Ji ∝ 2 d

VT = VDC + VP

where A is the area of each electrode; d is the thickness of the dark space.

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The RF voltage is split between the two capacitors in series, i.e.,

„

The ion-current densities on both the anode Ji(P) and cathode Ji(T) must be equal, i.e.,

VT CP = VP CT

VP 3 / 2 VT 3 / 2 = dP2 dT 2

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Combining the above three equations yields

VT A d A V  = P T = P T  VP AT d P AT  VP  13

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where AP is the area of anode; AT is

„

VT = VDC + VP

„

„

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VT  AP  =  VP  AT 

4

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High-Density Plasmas „

High etching rate requires high plasma densities (> 1011/cm3)

„

Higher pressures (more gas atoms) ⇒ higher plasma densities But smaller mean free path and thus less directionality

„

Better solution: Increase the number of collisions of each electron. But how to realize this?

the area of cathode. „

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RF Plasmas  A  VT = P  V P  AT 

A d

C∝

where V is the voltage drop across a dark space; d is the thickness of the dark space.

„

Each of the cathode and anode dark spaces behaves like a diode and can be modeled as a capacitor.

The above equation shows that the smaller electrode has greater voltage drop. Thus, for plasma etching where the substrate is placed on the cathode, the anode area must be larger than that of the cathode. This can be done by connecting the anode to the walls of the chamber. In practice, the exponent (i.e., 4) in the above equation is not a constant. Instead, it varies with the area ratio. Reducing VP by increasing the anode area will also help reduce the damage of the plasma to the chamber.

New plasma sources Electron Cyclotron Resonance (ECR) „ Inductively Coupled Plasma (ICP) „

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High-Density Plasmas „

High-Density Plasmas

Electron Cyclotron Resonance (ECR)

„

K K K F = qv × B

• Lorentz force

Inductively Coupled Plasma (ICP) Ê

• An electron in a static and uniform magnetic field will move in a circle.

Ê Ê

• Applying an alternating electrical field will result in a cycloid. The frequency of this cyclotron motion is given by eB

ω0 =

Ê

A 13.56-MHz RF signal applied to a coil (helical or planar) induces an alternating magnetic field Electron density can reach > 1012/cm3 An outer shield isolates RF field from surrounding equipment A slotted inner shield may be used. Planar Coil ICP

m

• This is called electron cyclotron resonance frequency. • When the frequency of the electric field is set to ωo, electron resonance occurs. • For the commonly used microwave frequency 2.45 GHz, the resonance condition is met when B = 875 G = 0.0875 T.

Cross-section view

Top view

• Electron density up to 1011 /cm3 EEL6935 Advanced MEMS

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Physical/Chemical Etching „

Physical/Chemical Etching

Two etching mechanisms

< 100 mTorr

Ê

Chemical etching

Ê

Physical etching (sputtering, ion milling)

SiF

Ar+

Higher pressure

Reactive Ion Etching (RIE)

Reactive Plasma Etching • Chemical • Fast • Isotropic • Highly selective • Less prone to radiation damage

Si

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Higher excitation energy

• Physical and chemical • Directional • Selective

Si Ar+

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Physical Sputtering • Physical momentum transfer • Directional • Poor selectivity • Radiation damage possible

100 mTorr range F

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Frequently Used Gases

Frequently Used Gases

, SF6 , SF6

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Etching Profiles

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Anisotropy • Energy-driven anisotropy

• Etch rate increases with increasing bias voltage • Undercut x is determined by the etch rate at zero bias Vx • The etch depth z ~ Vz, etch rate at a bias ⇒ x/z = Vx/Vz → Zero undercut if no etch at zero bias → Small undercut if very high bias

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Anisotropy

Some Simple Rules

• Inhibitor-driven anisotropy

1.

Fluorine-to-carbon ratio (F/C) – Fluorine → etching – Hydrocarbons → polymerization – Adding oxygen reduces polymer due to CO and CO2 formation but increases resist attack. NF3 or ClF3 may be used instead. – Adding hydrogen increases polymer due to HF formation

2.

Selective versus unselective dry etching – Higher polymerization rates typically lead to higher selectivity – Small additions of halogens significantly increase the selectivity of fluorine-based recipes

3.

Substrate bias – negative bias reduces the polymerization tendency

• Etch rate decreases with increasing hydrogen concentration • But undercut rate decreases even faster • This is because the formation of HF reduces F to C ratio and thus more polymer is formed. • But too much hydrogen will make the etching very slow.

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Bosch Process

Some Simple Rules Deep Reactive Ion Etch

4. and 5. Dry etching of III-V compounds – Group III halides (fluorides in particular) tend to be nonvolatile – Chlorine-based etchants are often used – And elevated substrate temperatures – Crystallographic etch patterns

• • •

Advanced Silicon Etch (ASE ) Inductively Coupled Plasma (ICP) Invented by Robert Bosch Corp.

¾ Simple, but very clever idea ¾ Huge impact to MEMS

6. Metal etching – Chlorocarbons and fluorocarbons – Chlorines are preferred for Al etching (AlF3 is not volatile)

Passivation

Si ICP etch „

„

Scallops EEL6935 Advanced MEMS

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Si ICP etch

Alternative etching and passivation Ê Sucessive SF6 silicon etch/CHF3 (or similar fluorinecarbon compound) deposition) Ê Sidewall passivation via ‘teflon-like’ compound Separate control of plasma generation and directionality Ê High density plasma Ê Tunable bias voltage 28

Bosch Process

Bosch Process

STS ICP Etcher

Alcatel 601E ICP Etcher

Other Deep Silicon ICP etcher providers: Alcatel, Plasma Therm

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Other Deep Silicon ICP etcher providers: STS, Plasma Therm

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Bosch Process

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Homework 1

Common Issues • Silicon Grass or Black Silicon • micromasking • Al2O3 contamination from mask and/or chamber walls • Native oxide or dusts • Redeposition of mask material

1.1

1.2

• Solutions: • Cleaning samples • Cleaning chambers • Good thermal contact • Ion energy (RF power, bias) 1.3 FEM simulation and 3D model (a) Design a cantilever beam with a resonator frequency of 1 MHz. (b) Build its 3D model using Coventorware and verify the resonant frequency.

• Microloading • RIE lag • Diffusion limited etching • For deep trench etches, increase SF6 flow rate. EEL6935 Advanced MEMS

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