Transcript Etching - Virginia Tech

Chapter 10
ETCHING
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CONTENTS
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Introduction
Basic Concepts
– Wet etching
– Plasma etching
Manufacturing Methods
– Plasma etching conditions and issues
– Plasma etch methods for various films
Measurements Methods
Models and Simulations
Limits and Future Trends
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Introduction
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After a thin film is deposited, it is usually etched to
remove unwanted materials and leave only the desired
pattern on the wafer
The process is done many times(review flow chart of
Chapter 2)
An overview of the process is shown in Figure 10-1
In addition to deposited films, sometimes we also need
to etch the Si wafer to create trenches (especially in
MEMS)
The masking layer may be photoresist, SiO2 or Si3N4
The etch is usually done until another layer of a
different material is reached
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Introduction
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Introduction
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Etching can be done “wet” or “dry”
Wet etching
– uses liquid etchants
– Wafer is immersed in the liquid
– Process is mostly chemical
Wet etching is not used much in VLSI wafer
fab any more
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Introduction
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Dry etching
– Uses gas phase etchants in a plasma
– The process is a combination of chemical
and physical action
– Process is often called “plasma etching”
This is the normal process used in most VLSI
fab
The ideal etch produces vertical sidewalls as
shown in 10-1
In reality, the etch occurs both vertically and
laterally (Figure 10-2)
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Introduction
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Introduction
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Note that
– There is undercutting, non-vertical
sidewalls, and some etching of the Si
The photoresist may have rounded tops and
non-vertical sidewalls
The etch rate of the photoresist is not zero
and the mask is etched to some extent
This leads to more undercutting
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Introduction
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Etch selectivity is the ratio of the etch rates of
different materials in the process
If the etch rate of the mask and of the
underlying substrate is near zero, and the etch
rate of the film is high, we get high selectivity
This is the normally desired situation
If the etch rate of the mask or the substrate is
high, the selectivity is poor
Selectivities of 25 – 50 are reasonable
Materials usually have differing etch rates due
to chemical processes rather than physical
processes
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Introduction
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Etch directionality is a measure of the etch
rate in different directions (usually vertical
versus lateral)
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Introduction
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In isotropic etching, the etch rates are the
same in all directions
Perfectly anisotropic etching occurs in only one
direction
Etch directionality is often related to physical
processes, such as ion bombardment and
sputtering
In general, the more physical a process is, the
more anisotropic the etch is and the less
selective it is
Directionality is often desired in order to
maintain the lithographically defined features
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Introduction
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Note, however, that very anisotropic structures
can lead to step coverage problems in
subsequent steps
Selectivity is very desirable
– The etch rate of the material to be removed
should be fast compared to that of the
mask and of the substrate layer
It is hard to get good directionality and good
selectivity at the same time
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Introduction
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Other system requirements include
– Ease of transporting gases/liquids to the
wafer surface
– Ease of transporting reaction products away
from wafer surface
– Process must be reproducible, uniform,
safe, clean, cost effective, and have low
particulate production
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Basic Concepts
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We consider two processes
– “wet” etching
– “dry” etching
In the early days, wet etching was used
exclusively
It is well-established, simple, and inexpensive
The need for smaller feature sizes could only
be met with plasma etching
Plasma etching is used almost exclusively
today
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Basic Concepts
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The first wet etchants were simple chemicals
By immersing the wafer in these chemicals,
exposed areas could be etched and washed
away
Wet etches were developed for all step
For SiO2, HF was used.
Wet etches work through chemical processes
SiOa2 water
6HF 
H 2SiF6byproduct
 2H 2O
to produce
soluble
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Basic Concepts
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In some cases, the etch works by first
oxidizing the surface and then dissolving the
oxide
An etch for Si involves a mixture of nitric acid
and HF
The nitric acid (HNO3) decomposes to form
nitrogen dioxide (NO2)
Si  2NO2  2H2O  SiO 2  H2  2HNO2
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The SiO2 is removed by the previous reaction
The overall reaction is
Si  HNO3  6HF  H 2SiF6  HNO2  H 2O  H 2
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Basic Concepts
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Buffers are often added to keep the etchants
at maximum strength over use and time
Ammonium fluoride (NH4F) is often used with
HF to help prevent depletion of the F ions
This is called Basic Oxide Etch (BOE) or
Buffered HF (BHF)
The ammonium fluoride reduces the etch rate
of photoresist and helps eliminate the lifting of
the resist during oxide etching
Acetic acid (CH3COOH) is often added to the
nitric acid/HF Si etch to limit the dissociation
of the nitric acid
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Basic Concepts
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Wet etches can be very selective because they
depend on chemistry
The selectivity is given by
S
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r1
r2
Material “1” is the film being etched and
material”2” is either the mask or the material
below the film being etched
If S>>1, we say the etch has good selectivity
for material 1 over material 2
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Basic Concepts
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Most wet etches etch isotropically
The exception is an etch that depends on the
crystallographic orientation
Example—some etches etch <111> Si slower
than <100> Si
Etch bias is the amount of undercutting of the
mask
If we assume that the selectivity for the oxide
over both the mask and the substrate is
infinite, we can define the etch depth as “d”
and the bias as “b”
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Basic Concepts
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Basic Concepts
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We often deliberately build in some
overetching into the process
This is to account for the fact that
– the films are not perfectly uniform
– the etch is not perfectly uniform
The overetch time is usually calculated from
the known uncertainties in film thickness and
etch rates
It is important to be sure that no area is
under-etched; we can tolerate some overetching
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Basic Concepts
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This means that it is important to have as high
a selectivity as possible to eliminate etching of
the substrate
However, if the selectivity is too high, overetching may produce unwanted undercutting
If the etch rate of the mask is not zero, what
happens?
If m is the amount of mask removed, we get
unexpected lateral etching
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Basic Concepts
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Basic Concepts
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m is called “mask erosion”
For anisotropic etching, mask erosion should
not cause much of a problem if the mask is
perfectly vertical
Etching is usually neither perfectly anisotropic
nor perfectly isotropic
We can define the degree of anisotropy by
rlat
Af  1 
rvert
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Basic Concepts
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Isotropic etching has an Af = 0 while
anisotropic etching has Af = 1
There are several excellent examples in the
text that do simple calculations of these
effects
These examples should be studied carefully
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Example
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Consider the structure below
The oxide layer is 0.5 m. Equal structure widths and
spacings, Sf, are desired. The etch anisotropy is 0.8.
If the distance between the mask edges, x, is 0.35 m,
what structure spacings and widths are obtained?
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Example
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To obtain equal widths and spacings, Sf, the mask
width, Sm, must be larger to take into account the
anisotropic etching
Since
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where b is the bias on each side, and
Since
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Sm  S f  b
b
Af  1 
d
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Thus
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S m  S f  2 x f 1  A f 
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Example
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This result makes sense
– For isotropic etching, Af=0 and Sm is a maximum
– For perfectly anisotropic etching, Af=1 and Sm=Sf
and is a minimum
The distance between the mask edges (x) is the
minimum feature size that can be resolved
But
x  2S F  S m
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Substitution and rearranging yields (note typo in text)
x  S f  2 x f 1  Af 
S f  x  2 x f 1  Af 
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Example
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Substituting numbers for the problem
S f  0.35 m  20.5 m 1  0.8
 0.55 m
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This result shows that the structure size can approach
the minimum lithographic dimension only when the film
thickness gets very small OR as the anisotropy gets
near 1.0
Very thin films are not always practical
This means we need almost vertical etching
Wet etching cannot achieve the desired results
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Plasma Etching
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Plasma etching has (for the most part)
replaced wet etching
There are two reasons:
– Very reactive ion species are created in the
plasma that give rise to very active etching
– Plasma etching can be very anisotropic
(because the electric field directs the ions)
An early application of plasma etching (1970s)
was to etch Si3N4 (it etches very slowly in HF
and HF is not very selective between the
nitride and oxide)
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Plasma Etching
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Plasma systems can be designed so that either
reactive chemical components dominate or
ionic components dominate
Often, systems that mix the two are used
– The etch rate of the mixed system may be
much faster than the sum of the individual
etch rates
A basic plasma system is shown in the next
slide
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Plasma Etching
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Plasma Etching
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Features of this system
– Low gas pressure (1mtorr – 1 torr)
– High electric field ionizes some of the gas
(produces positive ions and free electrons)
– Energy is supplied by 13.56 MHz RF
generator
– A bias develops between the plasma and
the electrodes because the electrons are
much more mobile than the ions (the
plasma is biased positive with respect to the
electrodes)
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Plasma Etching
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Plasma Etching
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If the area of the electrodes is the same
(symmetric system) we get the solid curve of
10-8
The sheaths are the regions near each
electrode where the voltage drops occur (the
dark regions of the plasma)
The sheaths form to slow down the electron
loss so that it equals the ion loss per RF cycle
In this case, the average RF current is zero
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Plasma Etching
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The heavy ions respond to the average voltage
The light electrons respond to the
instantaneous voltage
The electrons cross the sheath only during a
short period in the cycle when the sheath
thickness is minimum
During most of the cycle, most of the
electrons are turned back at the sheath edge
The sheaths are thus deficient in electrons
They are thus dark because of a lack of lightemitting electron-ion collisions
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Plasma Etching
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For etching photoresist, we use O2
For other materials we use species containing halides
such as Cl2, CF4, and HBr
Sometimes H2, O2, and Ar may be added
The high-energy electrons cause a variety of reactions
The plasma contains
– free electrons
– ionized molecules
– neutral molecules
– ionized fragments
– Free radicals
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Plasma Etching
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Plasma Etching
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In CF4 plasmas, there are
– Free electrons
– CF4
– CF3
– CF3+
–F
CF and F are free radicals and are very
reactive
Typically, there will be 1015 /cc neutral species
and 108-1012 /cc ions and electrons
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Plasma Etching Mechanisms
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The main species involved in etching are
– Reactive neutral chemical species
– Ions
The reactive neutral species (free radicals in
many cases) are primarily responsible for the
chemical component
The ions are responsible for the physical
component
The two can work independently or
synergistically
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Plasma Etching Mechanisms
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When the reactive neutral species act alone,
we have chemical etching
Ions acting by themselves give physical
etching
When they work together, we have ionenhanced etching
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Chemical Etching
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Chemical etching is done by free radicals
Free radicals are neutral molecules that have
incomplete bonding (unpaired electrons)
For example
e   CF4  CF3  F  e 
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Both F and CF3 are free radicals
Both are highly reactive
F wants 8 electrons rather than 7 and reacts
quickly to find a shared electron
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Chemical Etching
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The idea is to get the free radical to react with
the material to be etched (Si, SiO2)
The byproduct should be gaseous so that it
can be transported away (next slide)
The reaction below is such a reaction
4F  Si  SiF4
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Thus, we can etch Si with CF4
There are often several more complex
intermediate states
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Chemical Etching
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Chemical Etching
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Gas additives can be used to increase the
production of the reactive species (O2 in CF4)
The chemical component of plasma etching
occurs isotropically
This is because
– The arrival angles of the species is isotropic
– There is a low sticking coefficient (which is
more important)
The arrival angle follows what we did in
deposition and there is a cosn dependence
where n=1 is isotropic
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Chemical Etching
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The sticking coefficient is
Freacted
Sc 
Fincident
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A high sticking coefficient means that the
reaction takes place the first time the ion
strikes the surface
For lower sticking coefficients, the ion can
leave the surface (usually at random angles)
and strikes the surface somewhere else
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Chemical Etching
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One would guess that the sticking coefficient
for reactive ions is high
However, there are often complex reactions
chained together. This complexity often means
low sticking coefficients
Sc for O2/CF4 on Si is about 0.01
This additional “bouncing around” of the ions
leads to isotropic etching
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Chemical Etching
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Chemical Etching
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Since free radicals etch by chemically reacting
with the material to be etched, the process
can be highly selective
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Physical Etching
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Due to the voltage drop between the plasma
and the electrodes and the resulting electric
field across the sheaths, positive ions are
accelerated towards each electrode
The wafers are on one electrode
Therefore, ionic species (Cl+ or Ar+) will be
accelerated towards the wafer surface
These ions striking the surface result in the
physical process
The process is much more directional because
the ions follow the field lines
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Physical Etching
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Physical Etching
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This means n is very large in the cosn
distribution
But, because the process is more physical
than chemical, the selectivity will not be as
good as in the more chemical processes
We also assume that the ion only strikes the
surface once (which implies that the sticking
coefficient is near 1)
Ions can also etch by physical sputtering
(Chapter 9)
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Ion-Enhanced Etching
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The ions and the reactive neutral species do
not always act independently (the observed
etch rate is not the sum of the two
independent etch rates)
The classic example is etching of Si with XeF2
and Ar+ ions are introduced
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Ion-Enhanced Etching
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Ion-Enhanced Etching
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The shape of the etch profiles are interesting
The profiles are not the linear sum of the
profiles from the two processes
The profile is much more like the physical etch
alone (c)
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Ion-Enhanced Etching
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If the chemical component is increased, the
vertical etching is increased, but not the
lateral etching
The etch rate is also increased
The mechanisms for these effects are poorly
understood
Whatever the mechanism, the enhancement
only occurs where the ions hit the surface
Since the ions strike normal to the surface,
the enhancement is in this direction
This increases the directionality
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Ion-Enhanced Etching
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Ion-Enhanced Etching
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Possible models include
– Enhancement of the etch reaction
– Inhibitor removal
The reaction takes place only where the ions
strike the surface
Since the ions strike normal to the surface,
removal is thus only at the bottom of the well
It is assumed that etching by radicals
(chemical etching) is negligible
Note that even under these assumptions, the
side walls may not be perfectly vertical
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Ion-Enhanced Etching
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Note that an inhibitor can be removed on the
bottom, but not on the sidewalls
If inhibitors are deliberately deposited, we can
make very anisotropic etches
If the inhibitor formation rate is large
compared to the etch rate, one can get nonvertical walls (next slide)
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Ion-Enhanced Etching
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Types of Plasma Systems
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Several different types of plasma systems and
modes of operation have been developed
– Barrel etchers
– Parallel plate systems (plasma mode)
– Parallel plate systems (reactive ion mode)
– High density plasma systems
– Sputter etching and ion milling
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Barrel Etchers
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Barrel etchers were one of the earliest types of systems
VT has a small one
Here, the electrodes are curved and wrap around the
quartz tube
The system is evacuated and then back-filled with the
etch gas
The plasma is kept away from the wafers by a
perforated metal shield
Reactant species (F) diffuse through the shield to the
wafers
Because the ions and plasma are kept away from the
wafers, and the wafers do not sit on either electrode,
there is NO ion bombardment and the etching is purely
chemical
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Barrel Etchers
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Barrel Etchers
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Because the etches are purely chemical, they
can be very selective (but is almost isotropic)
The etching uniformity is not very good
The systems are very simple and throughput
can be high
They are used only for non-critical steps due
to the non-uniformity
They are great for photoresist stripping
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Parallel Plate Systems
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Parallel plate systems are commonly used for
etching thin films
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Parallel Plate Systems
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This system is very similar to a PECVD system
(Chapter 9) except that we use etch gases
instead of deposition gases
These systems are much more uniform across
the wafer than the barrel etcher
The wafers are bombarded with ions due to
the voltage drop (Figure 10-8)
If the plates are symmetric (same size) the
physical component of the etch is found to be
rather small and one gets primarily chemical
etching
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Parallel Plate Systems
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By increasing the energy of the ions
(increasing the voltage) the physical
component can be increased
This can be done by decreasing the size of the
electrode on which the wafers sit and
changing which electrode is grounded
In this configuration, we get the reactive ion
etching (RIE) mode of operation
Here, we get both chemical and physical
etching
By lowering the gas pressure, the system can
become even more directional
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High-Density Plasma Etching
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This system is becoming more popular
These systems separate the plasma density
and the ion energy by using a second
excitation source to control the bias voltage of
the wafer electrode
A different type of source for the plasma is
used instead of the usual capacitively coupled
RF source
It is non-capacitively coupled and generates a
very high plasma density without generating a
large sheath bias
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High-Density Plasma Etching
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High-Density Plasma Etching
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These systems still generate high ion fluxes
and etch rates even though they operate at
much lower pressures (1—10 mtorr) because
of the higher plasma density
Etching in these systems is like RIE etching
with a very large physical component
combined with a chemical component
involving reactive neutrals
They thus give reasonable selectivity
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Summary
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Summary
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