memsii lecture 2 dry etching i - university of florida lecture 2...eel6935 advanced mems 2005 h. xie...

EEL6935 Advanced MEMS 2005 H. Xie 1

Lecture 2Dry Etching I

Agenda:DC Plasma

– Plasma discharge zones– Paschen’s Law

RF PlasmaHigh-density PlasmasDRIE

– Microloading– Silicon grass

1/7/2005

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

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


Dry Etching


Plasmas


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

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 currentEnergetic collisions create avalanche of ions and electronsElectrons move much faster than ionsNeutral species greatly outnumber electrons and ions by 4 to 6 orders of magnitude

Glow Discharge Plasma

Electrons Ions Neutrals


Average particle energy is given by

<Ee>=kBTe for electrons<Ei>=kBTi for ions

Typical values<Ee>: 1~10eV (hot)<Ei>: 0.02~0.1eV (cold)Thus, Te>>Ti

e.g., Ee~2eV; Ei~0.4eVThen, Te = 23,000 K!But Ti = 490 K

Effective current densityJe = neq<ve>/4Ji = niq<vi>/4→ <ve> is much greater than

<vi>, so Je >> Ji

⇒ Permanent positive charge⇒ electrons lost to the walls


Electrons Ions Neutrals


Electron-Molecule Collisions


Dissociation

e- + Cl2 → 2Cl + e- e- + CF4 → CF3 + F + e-

Highly reactive radicals

Ionization

e- + Cl2 → 2Cl+ + 2e- e- + CF4 → CF3+ + F + 2e-

Excitation

e- + F → F* + e-

F* → F + hve- + Ar → Ar* + e-

Ar* → Ar + hv

Photons


Reactive Plasma Etching

Chemical etchingIsotropic


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



Special zones


Aston dark space: low energy electronsCathode glow: electrons gain sufficient energy to excite gas atomsCrookes dark space: electrons gain too much energy and luminescence is weak due to inefficient excitationNegative glow (brightest region): low electric fieldFaraday dark space: electrons slows down due to collisions and low electric fieldPositive column: quasi-neutral, low electric field, uniform; not important for etching or deposition


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.

Paschen’s Law

Bre

akdo

wn

volta

ge (V

)


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.

RF Plasmas

RF voltage source


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.

RF Plasmas

Capacitive coupling

Vp: plasma potential

VDC: self-bias

VRF: applied RF signal

( )2

2RF p p

P DC

VV V−≈ −

•The maximum energy of positive ions striking the cathode is

•The maximum energy of positive ions striking the anode is

( )DC Pe V V+ ~300eV

PeV ~20eV


Child-Langmuir equation for the ion-current density

RF Plasmas

T DC PV V V= +

3/ 2

2iVJd

∝

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

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

3 / 2 3 / 2

2 2P T

P T

V Vd d

=


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

RF Plasmas

Equivalent electrical circuit of RF plasma

ACd

∝

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

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

T P

P T

V CV C

=

Combining the above three equations yields

3 / 4

T P T P T

P T P T P

V A d A VV A d A V

= =

4

T P

P T

V AV A

=


RF Plasmas

T DC PV V V= +

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.

4

T P

P T

V AV A

=

where AP is the area of anode; AT is the area of cathode.


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?

New plasma sourcesElectron Cyclotron Resonance (ECR)Inductively Coupled Plasma (ICP)


• Lorentz force

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

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

• This is called electron cyclotron resonancefrequency.

• 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.

• Electron density up to 1011 /cm3

High-Density PlasmasElectron Cyclotron Resonance (ECR)

0eBm

ω =

F qv B= ×


High-Density PlasmasInductively Coupled Plasma (ICP)

A 13.56-MHz RF signal applied to a coil (helical or planar) induces an alternating magnetic fieldElectron density can reach > 1012/cm3

An outer shield isolates RF field from surrounding equipmentA slotted inner shield may be used.

Cross-section view Top view

Planar Coil ICP


Physical/Chemical Etching

Two etching mechanisms

Chemical etching

Physical etching (sputtering, ion milling)

F SiF

Si

Ar+ Si

Ar+


Physical/Chemical EtchingPhysical Sputtering

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

Reactive Ion Etching (RIE)

• Physical and chemical• Directional • Selective

Reactive Plasma Etching

• Chemical• Fast• Isotropic• Highly selective• Less prone to radiation damage

< 100 mTorr

100 mTorr range

Higher pressure

Higher excitation energy


Frequently Used Gases

, SF6

, SF6


Frequently Used Gases


Etching Profiles


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


Anisotropy

• Inhibitor-driven anisotropy

• 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.


Some Simple Rules

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


Some Simple Rules

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

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


Deep Reactive Ion Etch

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

Simple, but very clever ideaHuge impact to MEMS

Bosch Process

Alternative etching and passivationSucessive SF6 silicon etch/CHF3 (or similar fluorine-carbon compound) deposition)Sidewall passivation via ‘teflon-like’ compound

Separate control of plasma generation and directionalityHigh density plasmaTunable bias voltage

Si ICP etch Passivation Si ICP etch

Scallops


Bosch Process

STS ICP Etcher

Other Deep Silicon ICP etcher providers: Alcatel, Plasma Therm


Bosch Process

Alcatel 601E ICP Etcher

Other Deep Silicon ICP etcher providers: STS, Plasma Therm


Bosch Process

Common Issues• Silicon Grass or Black Silicon

• micromasking• Al2O3 contamination from mask and/or chamber walls• Native oxide or dusts• Redeposition of mask material

• Solutions:• Cleaning samples• Cleaning chambers• Good thermal contact• Ion energy (RF power, bias)

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


Homework 1

1.1

1.2

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.

memsii lecture 2 dry etching i - university of florida lecture 2...eel6935 advanced mems 2005 h. xie...

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