Transcript Document 7366320

Introduction
After a thin film is deposited, it is usually etched
to remove unwanted materials and leave the
desired pattern on the wafer
 Need to etch the Si wafer to create trenches for
trench capacitors, recessed gate transistors, and
a number of MEMS devices

– The masking layer may be photoresist, SiO2 or Si3N4
– The etch is usually done until another layer of a
different material is reached
 Monitoring systems to detect change in material type or a
selective etch may be used.
Introduction
Introduction


Etching can be done “wet” or “dry”
Wet etching
– uses liquid etchants
– Wafer is immersed in the liquid
– Process is mostly chemical

Rarely used in VLSI wafer fab today
– Particulates
– Isotropic nature of most wet etches
 Equal etch rate in all directions
– Hard to control etch rate and selectivity
– Difficult to integrate monitoring systems
Introduction

Dry etching
– Uses gas phase etchants in a plasma
 Combination of chemical and physical action
– Process is often called “plasma etching”

The ideal etch produces vertical sidewalls
– Anisotropic etching is usually desired, but
extremely difficult to achieve
 Etch rate depends significantly on direction
 Undercut changes linewidth of etch region
compared to originial linewidth of masking material
Introduction
Introduction

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
Introduction

Etch selectivity is the ratio of the etch
rates of different materials in the process
– High selectivity if the etch rate of the mask
and of the underlying substrate is near zero,
and the etch rate of the film is high
– Poor selectivity if the etch rate of the mask or
the substrate is high

Selectivities of 25 – 50 are desired
– Materials usually have differing etch rates due
to chemical processes rather than physical
processes
Introduction

Etch directionality is a measure of the etch rate
in different directions
– Usually vertical versus lateral
– Can be along particular crystalline planes
Introduction

Anisotropic etching or etch directionality is
often related to physical processes
– ion bombardment and sputtering
– Directionality is often desired in order to
maintain the lithographically defined features

However, perfect anisotropic etch can lead
to step coverage issues and other
problems during subsequent processing
steps
Introduction

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
Introduction

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
Basic Concepts

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
Basic Concepts
The first wet etchants were simple
chemicals
 By immersing the wafer in these
chemicals, exposed areas could be etched
and washed away
SiO 2 were
 6HF 
H 2SiF6  2Hfor
2 O all step
 Wet etches
developed
 For SiO2, HF was used.
 Wet etches work through chemical
processes to produce a water soluble
byproduct

Basic Concepts
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
Si  2NO
2H2O 3)SiO
 The nitric
acid
decomposes
2  (HNO
2  H2  2HNO2 to
form nitrogen dioxide (NO2)

Si  HNO3  6HF  H 2SiF6  HNO2  H 2O  H 2

The SiO2 is removed by the previous
Basic Concepts
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

Basic Concepts
Wet etches can be very selective because
they depend on chemistry
r1
 The selectivity is
by
S given

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

Basic Concepts
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

Basic Concepts
Basic Concepts
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 over
Basic Concepts
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,
over-etching 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

Basic Concepts
Basic Concepts
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
rlat
Af  1 
vert
 We can define the rdegree
of anisotropy by

Basic Concepts
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

Example

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.
Example
To obtain equal widths and spacings, Sf, the
mask width, Sm, must be larger to take into
account the anisotropic etching
Sm  S f  b
 Since

b on each side, and
where b isA fthe
bias
 1
d
 Since
S m  S f  2 x f 1  A f 
Example

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
x  2S F  S m
 But

x  S f  2 x f 1  Af 
S f  x  2 x f 1  Af 

Substitution and rearranging yields (note typo in
Example

Substituting numbers for the problem
S f  0.35 m  20.5 m 1  0.8
 0.55 m
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

Plasma Etching
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
Plasma Etching
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
Plasma Etching
Plasma Etching

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)
Plasma Etching
Plasma Etching
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

Plasma Etching
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

Plasma Etching





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
Plasma Etching
Plasma Etching

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

Plasma Etching Mechanisms

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

Plasma Etching Mechanisms
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

Chemical Etching
Chemical etching is done by free radicals
 Free radicals are neutral molecules that
have incomplete bonding (unpaired



e

CF

CF

F

e
electrons)
4
3
 For example

Both F and CF3 are free radicals
 Both are highly reactive

Chemical Etching
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)
4F  Si  SiF4
 The reaction below is such a reaction

Thus, we can etch Si with CF4
 There are often several more complex
intermediate states

Chemical Etching
Chemical Etching
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
Chemical Etching

The sticking coefficient is
Freacted
Sc 
Fincident
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

Chemical Etching
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

Chemical Etching
Chemical Etching

Since free radicals etch by chemically
reacting with the material to be etched,
the process can be highly selective
Physical Etching
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

Physical Etching
Physical Etching
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)

Ion-Enhanced Etching
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

Ion-Enhanced Etching
Ion-Enhanced Etching
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)

Ion-Enhanced Etching
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

Ion-Enhanced Etching
Ion-Enhanced Etching

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

Ion-Enhanced Etching
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
non-vertical walls (next slide)

Ion-Enhanced Etching
Types of Plasma Systems

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
Barrel Etchers







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
Barrel Etchers
Barrel Etchers
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

Parallel Plate Systems

Parallel plate systems are commonly used
for etching thin films
Parallel Plate Systems
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

Parallel Plate Systems
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

High-Density Plasma Etching
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

High-Density Plasma Etching
High-Density Plasma Etching
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

Summary
Summary