Download Lecture 2: Dry Etching I

EEL6935 Advanced MEMS (Spring 2005)
Instructor: Dr. Huikai Xie
Dry Etching
Lecture 2
Dry Etching I
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Agenda:
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DC Plasma
– Plasma discharge zones
– Paschen’s Law
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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
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Plasmas
Glow Discharge Plasma
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Glow occurs when a DC
voltage is applied between
two electrodes in a gas
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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
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Average particle energy is
given by
<Ee>=kBTe for electrons
<Ei>=kBTi for ions
Glow Discharge Plasma
Ions
Neutrals
Electrons
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Dissociation
Typical values
Highly reactive radicals
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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<ve>/4
Ji = niq<vi>/4
→ <ve> is much greater than
<vi>, so Je >> Ji
⇒ Permanent positive charge
⇒ electrons lost to the walls
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Ionization
e- + Cl2 → 2Cl+ + 2e-
Effective current density
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e- + CF4 → CF3 + F + e-
e- + Cl2 → 2Cl + e-
<Ee>: 1~10eV (hot)
<Ei>: 0.02~0.1eV (cold)
Thus, Te>>Ti
e.g., Ee~2eV; Ei~0.4eV
Then, Te = 23,000 K!
But Ti = 490 K
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Electron-Molecule Collisions
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Glow Discharge Plasma
Chemical etching
Isotropic
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Color of light emission depends on gas, ionization energy, pressure and electric field
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Glow Discharge Plasma
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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)
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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
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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
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eVP
)
~300eV
~20eV
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RF Plasmas
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RF Plasmas
Equivalent electrical circuit of RF plasma
Child-Langmuir equation for the
ion-current density
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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.,
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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 
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where AP is the area of anode; AT is
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VT = VDC + VP
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VT  AP 
=

VP  AT 
4
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High-Density Plasmas
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High etching rate requires high plasma densities (> 1011/cm3)
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Higher pressures (more gas atoms) ⇒ higher plasma densities
But smaller mean free path and thus less directionality
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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.
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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
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High-Density Plasmas
Electron Cyclotron Resonance (ECR)
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K
K K
F = qv × B
• Lorentz force
Inductively Coupled Plasma (ICP)
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• An electron in a static and uniform magnetic
field will move in a circle.
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• Applying an alternating electrical field will
result in a cycloid. The frequency of this
cyclotron motion is given by
eB
ω0 =
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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
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Physical/Chemical Etching
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Physical/Chemical Etching
Two etching mechanisms
< 100 mTorr
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Chemical etching
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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
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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
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Scallops
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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
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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.
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