Chapter 10 ETCHING

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Chapter 10ETCHING
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CONTENTS

• 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

• 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

• 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

• 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

• 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

• 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

• Etch directionality is a measure of the etch rate
  in different directions (usually vertical versus
  lateral)

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Introduction

• 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

• 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

• 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

• 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

• 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 to
  produce a water soluble byproduct

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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
• The nitric acid (HNO3) decomposes to form
  nitrogen dioxide (NO2)
• The SiO2 is removed by the previous reaction
• The overall reaction is

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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
• 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

• Wet etches can be very selective because they
  depend on chemistry
• The selectivity is given by
• Material 1 is the film being etched and
  material2 is either the mask or the material
  below the film being etched
• If Sgtgt1, we say the etch has good selectivity for
  material 1 over material 2

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Basic Concepts

• Most wet etches etch isotropically
• The exception is an etch that depends on the
  crystallographic orientation
• Examplesome etches etch lt111gt Si slower than
  lt100gt 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

• 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-etching

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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

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Basic Concepts
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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
• We can define the degree of anisotropy by

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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

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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.
• If the distance between the mask edges, x, is
  0.35 ?m, what structure spacings and widths are
  obtained?

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Example

• To obtain equal widths and spacings, Sf, the mask
  width, Sm, must be larger to take into account
  the anisotropic etching
• Sincewhere b is the bias on each side, and
• Since
• Thus

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Example

• This result makes sense
• For isotropic etching, Af0 and Sm is a maximum
• For perfectly anisotropic etching, Af1 and SmSf
  and is a minimum
• The distance between the mask edges (x) is the
  minimum feature size that can be resolved
• But
• Substitution and rearranging yields (note typo in
  text)

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Example

• Substituting numbers for the problem
• 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

• 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

• 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

• 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

• 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

• 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
  light-emitting electron-ion collisions

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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
• Free radicals

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Plasma Etching
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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 electrons

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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

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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 ion-enhanced
  etching

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Chemical Etching

• Chemical etching is done by free radicals
• Free radicals are neutral molecules that have
  incomplete bonding (unpaired electrons)
• For example
• 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

• 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
• 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

• 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
  n1 is isotropic

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Chemical Etching

• The sticking coefficient is
• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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)

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Ion-Enhanced Etching
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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

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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 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

• 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

• Parallel plate systems are commonly used for
  etching thin films

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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 primarily chemical
  etching

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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 can
  become even more directional

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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 without generating a
  large sheath bias

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High-Density Plasma Etching
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High-Density Plasma Etching

• These systems still generate high ion fluxes and
  etch rates even though they operate at much lower
  pressures (110 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