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After a thin film is deposited, it is usually etched to remove unwanted materials and leave the desired pattern on the wafer

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Title: MATERIALS IN MODERN COMMUNICATIONS SYSTEMS Subject: Elecronic materials lectures Author: Robert W. Hendricks Last modified by: Dr. Kathleen Meehan – PowerPoint PPT presentation

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Title: After a thin film is deposited, it is usually etched to remove unwanted materials and leave the desired pattern on the wafer


1
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.

2
Introduction
3
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

4
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

5
Introduction
6
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

7
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

8
Introduction
  • Etch directionality is a measure of the etch rate
    in different directions
  • Usually vertical versus lateral
  • Can be along particular crystalline planes

9
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

10
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

11
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

12
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

13
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

14
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

15
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

16
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

17
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

18
Basic Concepts
19
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

20
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

21
Basic Concepts
22
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

23
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

24
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?

25
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

26
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)

27
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

28
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)

29
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

30
Plasma Etching
31
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)

32
Plasma Etching
33
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

34
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

35
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

36
Plasma Etching
37
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

38
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

39
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

40
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

41
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

42
Chemical Etching
43
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

44
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

45
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

46
Chemical Etching
47
Chemical Etching
  • Since free radicals etch by chemically reacting
    with the material to be etched, the process can
    be highly selective

48
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

49
Physical Etching
50
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)

51
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

52
Ion-Enhanced Etching
53
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)

54
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

55
Ion-Enhanced Etching
56
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

57
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)

58
Ion-Enhanced Etching
59
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

60
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

61
Barrel Etchers
62
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

63
Parallel Plate Systems
  • Parallel plate systems are commonly used for
    etching thin films

64
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

65
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

66
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

67
High-Density Plasma Etching
68
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

69
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Summary
72
Summary
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