It has slide of VLSI technology subject in a very conceptual manner.

1. Lithography &
Dielectric and
Polysilicon Film
Deposition

2. Contents
 Introduction
 Optical Lithography
 Electron Beam Lithography
 Photo Masks
 Wet Chemical Etching

3. Introduction
 Lithography, as used in the manufacture of ICs, is the
process of transferring geometric shapes on a mask to
the surface of a silicon wafer. These shapes make up the
parts of the circuit, such as gate electrodes, contact
windows, metal interconnections, and so on.

4. Introduction
 In the IC lithographic process, a photosensitive polymer film is
applied to the silicon wafer, dried, and then exposed with the proper
geometrical patterns through a photomask to ultraviolet (UV) light
or other radiation.
 After exposure, the wafer is soaked in a solution that develops the
images in the photosensitive material. Depending on the type of
polymer used, either exposed or non exposed areas of film are
removed in the developing process. The wafer is then placed in an
ambient that etches surface areas not protected by polymer patterns.
 Because the polymeric materials resist the etching process, they are
called resists; if light is used to expose the IC pattern, they are called
photoresists. Resists are made that are sensitive to UV light, electron
beams, x-rays, or ion beams.

5. Exposure tools do several jobs
 First, they rigidly hold the wafer and mask in place after
the mask pattern is aligned to a previous pattern already
processed into the wafer.--- aligners,
 Second, they provide a source of exposing radiation for
the resist.
 Some exposure tools, such as the e-beam machine,
provide a third function; they allow the silicon wafer to be
exposed directly without requiring a mask.

6. Exposure tool performance
parameters: Resolution,
Registration, and Throughput
 Resolution is defined in terms of the minimum feature
that can be repeatedly exposed and developed in at least
1 µm of resist.
 Registration is a measure of how closely successive mask
levels can be overlaid,
 Throughput is defined as the number of silicon wafers
that can be exposed per hour.

7. OPTICAL LITHOGRAPHY: Types

8. contact printing
 In contact printing, shown in Fig. 7a, a resist-coated
silicon wafer is brought into physical contact with the
glass photomask. The wafer is held on a vacuum chuck,
and the whole assembly rises until the wafer and mask
contact each other with a few kilograms of
force.(0.05atm-0.3atm)

9. contact printing
 To align the photomask pattern to a previously etched
silicon pattern, the mask and wafer are separated by
about 25 µm,
 A high- powered pair of objectives are brought in behind
the mask to view both the mask and wafer patterns at
two positions simultaneously.
 The objectives are connected to a split-field microscope
so that the right eye sees a spot on the right side of the
mask and wafer, and the left eye sees a spot on the left.

10. contact printing
 The mask and wafer are aligned by mechanically
translating and rotating the vacuum chuck assembly until
the patterns on the mask and wafer are aligned.
 At this point, the wafer is brought into contact with the
mask and reexamined for alignment
 When the expose button on the machine is pushed, the
split-field
 microscope is automatically withdrawn and a collimated
beam of UV light illuminates the entire mask for a fixed
exposure time.

11. Prons and cons
 The exposure intensity (in mW/cm^2-) at the wafer
surface times the exposure time (in seconds) gives the
exposure energy (mJ/cm^2), or dose, received by the
resist.
 Because of the intimate contact between resist and mask,
very high resolution is possible in contact printing.
Printing l-film features in 0.5 µm of positive resist is
relatively easy. The problem in contact printing is dirt. A
piece of dirt, such as a speck of Si dust, on the silicon
wafer can damage the mask surface when the mask is
forced into contact with the wafer.

12. proximity exposure method
 The proximity exposure method is very similar to contact
printing except that a small gap, 10 to 25µm wide, is
maintained between the wafer and mask during
exposure.
 This gap minimizes (but may not eliminate) mask
damage. Proximity printers operate in the Fresnel
diffraction region, where resolution is proportional to
{λg)^1/2. where λis the exposure wavelength and g is the
gap between the mask and the wafer.
 Approximately 2- to 4µm resolution is possible with
proximity printing.

13. projection printing
 The third exposure method, projection printing, avoids
mask damage entirely. An image of the patterns on the
mask is projected onto the resist-coated wafer, which is
many centimeters away.
 To achieve high resolution, only a small portion of the
mask is imaged. This small image field is scanned or
stepped over the surface of the wafer. In scanning
projection printers, the mask and wafer are moved
synchronously.

14. Projection printing
 This technique achieves resolution of about 1.5-µm lines
and spaces. Projection printers that step the mask image
over the wafer surface are called direct-step-on wafer or
step-and-repeat systems. With these printers, the mask
contains the pattern of one large chip or a group of small
chips which are enlarged up to 10 x.

15. Projection Printing
 The image of this pattern, or reticle , is demagnified and
projected onto the wafer. After the exposure of one chip
site, the wafer is moved or stepped on an
interferometrically controlled XY table to the next chip
site, and the process is repeated. Step-and-repeat
reduction projection printers are capable of
approximately µm resolution.

16. ELECTRON BEAM LITHOGRAPHY
 Electron-beam fabrication of ICs offers several
advantages for lithographic pattern transfer: resist
geometries smaller than 1 µm can be generated, wafers
can be patterned directly without a mask, and the
technique can be highly automated.
 In addition, an electron beam has a much greater depth
of focus than an optical lithographic
 system. An electron beam can be used to detect features
on a silicon wafer.

17. ELECTRON BEAM LITHOGRAPHY
 The problem with e-beam lithographic machines is that
they are slow. Their throughput is approximately only five
wafers per hour at less than l-µm resolution.
 These throughputs do not economically compete with
optical machine throughputs of 40 wafers per hour at 1
.5- µm resolution.

18. ELECTRON BEAM LITHOGRAPHY

19. ELECTRON BEAM LITHOGRAPHY
 To write submicrometer patterns into a resist, the e-beam
must be focused to a diameter of 0.01 to 0.5 µm. The
current density in the focused spot should also be high,
to minimize resist exposure times.
 Most thermionic electron guns have current densities of a
few amperes per centimeter squared from a cathode that
is 10 to 100 µm
 in diameter.

20. ELECTRON BEAM LITHOGRAPHY
 Figure 12 gives a schematic of an e-beam lithography
machine.
 Since the beam scan is restricted by lens aberrations to
usually less than 1 cm, an interferometrically controlled
XY table is used to position the substrate to be patterned
under the e-beam.
 Registration to a previously defined pattern may be
accomplished at each chip site by scanning the e-beam
across reference marks etched in the substrate and
detecting the secondary and backscattered electrons.

21.  These signals are used to automatically position the
substrate under the beam. Alignment accuracy of ±0.2
µm (3σ) is reported.
 Electron-beam lithography machines are usually
designed for optimum performance in research and
development, in the production of photomasks, or in the
direct writing of silicon wafers.

22.  Machines used in research and development must
provide the smallest possible focused spot so that the
highest resolution can be obtained.
 Beam diameters as small as 5 A have been used to etch
13-A wide lines in NaCl crystals.
 A machine intended for the production of photomasks or
reticles with features of 2 to 4 µm can have a relatively
large beam diameter (0.25 to 1 fim) and modest
throughput.

23. a rule of thumb
 As a rule of thumb, the minimum device feature is about
4x the beam diameter, and the field that can be directly
accessed by the e-beam without XY stage motion is
about 2.000 x the minimum device feature.
 In other words, the smaller the device feature, the more
XY stage motion required. More stage motion, of course,
slows down production. Once again a tradeoff must be
made: smaller features for wafer throughput.

24. Photo resists

25. Resists may be either negative or
positive
 Negative resists become less soluble in developer when
they are exposed to radiation (as in Fig. 3),
 positive resists become more soluble after exposure.
 Optical Negative resist is a cyclized polyisoprene polymer
material combined with a photosensitive compound.
 Optical positive resist systems also contain a base resin
material and a photosensitizer, but are totally different from
negative resists in their response to exposure radiation.
 Positive resists exhibit higher resolution capability.
 Negative resists usually have poorer resolution capability

26. Electron Resists
 A radiation sensitive resist is one in which chemical or
physical changes are induced by ionizing radiation, which
allows the resist to be patterned.

27. electron resist
 The electron resist sensitivity S for positive and negative
resists are defined as the electron dose required per
centimeter squared to ensure complete positive resist
development or to correspond to a 50% remaining
thickness in the case of negative resist.
 the positive resist PMMA is about three orders of
magnitude less sensitive than the negative resist COP
and would therefore require an exposure time about
1000 times longer to form useful resist images.

28. Photo resist Materials
 Positive photoresist
 DNQ-Novolac photoresist
 One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is
based on a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde
resin). DNQ inhibits the dissolution of the novolac resin, but upon exposure to light, the dissolution
rate increases even beyond that of pure novolac. The mechanism by which unexposed DNQ inhibits
novolac dissolution is not well understood, but is believed to be related to hydrogen bonding (or more
exactly diazocoupling in the unexposed region). DNQ-novolac resists are developed by dissolution
in a basic solution (usually 0.26N tetramethylammonium hydroxide (TMAH) in water).
 Negative photoresist
 Epoxy-based polymer
 One very common negative photoresist is based on epoxy-based polymer. The common product
name is SU-8 photoresist, and it was originally invented by IBM, but is now sold by Microchem and
Gersteltec. One unique property of SU-8 is that it is very difficult to strip. As such, it is often used
in applications where a permanent resist pattern (one that is not strippable, and can even be used in
harsh temperature and pressure environments) is needed for a device.[15] Mechanism of epoxy-based
polymer is shown in 1.2.3 SU-8.

29. Photo resist Materials
 Off-stoichiometry thiolenes(OSTE) polymer
 In 2016, OSTE Polymers were shown to possess a unique photolithography
mechanism, based on diffusion-induced monomer depletion, which enables
high photo structuring accuracy. The OSTE polymer material was originally
invented at the KTH Royal Institute of Technology, but is now sold by
Mercene Labs. Whereas the material has properties similar to those of SU8,
OSTE has the specific advantage that it contains reactive surface
molecules, which make this material attractive for microfluidic or biomedical
applications.
 Hydrogen Silsesquioxane (HSQ)
 HSQ is a common negative resist for e-beam, but also useful for
photolithography. Originally invented by Dow Corning (1970),[16] and now
produced (2017) by Applied Quantum Materials Inc. (AQM). Unlike other
negative resists, HSQ is inorganic and metal-free. Therefore, exposed HSQ
provides a low dielectric constant (low-k) Si rich oxide. A comparative study
against other photoresists was reported in 2015 (Dow Corning HSQ)

30. Photoresist chemicals
 Photoresist chemicals are organic compounds whose
properties change when exposed to ultraviolet light.
Numerous such chemicals prevalent in the market are
polymethyl methacrylate (PMMA), polymethyl glutarimide
(PMGI), and phenol-formaldehyde resin (DNQ).
 Positive photoresists are able to maintain their size and
pattern as the photoresist developer solvent doesn't
permeate the areas that have not been exposed to the UV
light. With negative resists, both the UV exposed and
unexposed areas are permeated by the solvent, which can
lead to pattern distortions.

31.  Examples of positive resist include:
 MCC PMMA Series (e-beam)
 DuPont S1800 Series (g-Line)
 DuPont SPR-220 (i-Line)
 MRT ma-P1200 Series (broadband)
 Examples of negative resist include:
 MCC SU-8 Series (i-Line)
 MCC KMPR® Series (i-Line)
 DuPont UVN-30 (DUV)
 MRT ma-N 1400 Series (i-Line)
 MRT ma-N 2400 Series (DUV)

32. photomask
 A photomask is an opaque plate with holes or
transparencies that allow light to shine through in a
defined pattern.
 Masks are used to produce a pattern on a substrate,
normally a thin slice of silicon known as a wafer in the
case of chip manufacturing.
 Several masks are used in turn, each one reproducing a
layer of the completed design, and together they are
known as a mask set

33. photomask
 Lithographic photomasks are typically transparent Fused
silica blanks covered with a pattern defined with a
chrome metal absorbing film. Photomasks are used at
wavelengths of 365nm, 248 nm, and 193 nm. Photomasks
have also been developed for other forms of radiation
such as 157 nm, 13.5 nm (EUV), X-ray, Electrons, and ions;
but these require entirely new materials for the substrate
and the pattern film.

34. photomask
 A Set of photo mask, each defining a pattern layer in IC
fabrication, is fed into a photolithography Stepper
or Scanner, and individually selected for exposure.
In double patterning techniques, a photomask would
correspond to a subset of the layer pattern.

35. photomask
 In photolithography for the mass
production of integrated circuit devices, the more correct
term is usually photoreticle or simply reticle. In the case
of a photomask, there is a one-to-one correspondence
between the mask pattern and the wafer pattern. This was
the standard for the 1:1 mask aligners that were
succeeded by steppers and scanners with reduction
optics.[1]

36. photomask
 As used in steppers and scanners, the reticle commonly
contains only one layer of the chip. (However, some
photolithography fabrications utilize reticles with more
than one layer patterned onto the same mask). The
pattern is projected and shrunk by four or five times onto
the wafer surface. To achieve complete wafer coverage,
the wafer is repeatedly "stepped" from position to
position under the optical column until full exposure is
achieved

37. Photomask

38. Photo chemical Etching
 Several anisotropic wet etchants are available for silicon, all of them hot
aqueous caustics. For instance, potassium hydroxide (KOH) displays an
etch rate selectivity 400 times higher in <100> crystal directions than in
<111> directions.
 EDP (an aqueous solution of ethylene diamine and pyrocatechol),
displays a <100>/<111> selectivity of 17X, does not etch silicon dioxide
as KOH does, and also displays high selectivity between lightly doped and
heavily boron-doped (p-type) silicon.
 Use of these etchants on wafers that already contain CMOS integrated
circuits requires protecting the circuitry.
 KOH may introduce mobile potassium ions into silicon dioxide, and EDP is
highly corrosive and carcinogenic, so care is required in their use.
 Tetramethylammonium hydroxide (TMAH) presents a safer alternative
than EDP, with a 37X selectivity between {100} and {111} planes in silicon.

39. Wet Chemical etching

40. DEPOSITION PROCESSES
 1 Reactions

41. DEPOSITION PROCESSES
 2 Equipment

42. DEPOSITION PROCESSES
 3 Safety
 Many of the gases used to deposit films are hazardous.
The safety problems are more severe for low-pressure
depositions because the processes often use
concentrated gases.
 The hazardous gases fall into four general classes:
poisonous; pyrophoric, flammable,or explosive; corrosive;
and dangerous combinations of gases.
 Many of the flammable gases react with air to form solid
products.

43. POLYSILICON
 Polysilicon is used as the gate electrode in MOS devices.
It is also used for highvalue
resistors, diffusion sources to form shallow junctions,
conductors, and to ensure ohmic contact to crystalline
silicon. The polysilicon is deposited by pyrolyzing silane
between 600 and 650°C in a low-pressure reactor (Fig. 1a).

44. 1 atm=760 torr=0.9869atm =1 bar=
100000 Pa
 Two low-pressure processes are common for depositing
polysilicon. One uses 100% silane at a pressure of 25 to
130 Pa (0.2 to 1 .0 Torr). The other process is performed at
the same total pressure but uses 20 to 30% silane diluted
in nitrogen.
 Both processes deposit polysilicon on 100 to 200 wafers
per run with thickness uniformities within 5%. The
deposition rates are 100 to 200 Angstrom/min.

45. Deposition Variables
 Temperature, pressure, silane concentration, and dopant
concentration are important process variables in the
deposition of polysilicon;
 wafer spacing and load size have only minor effects.

50. Structure
 The structure of polysilicon is strongly influenced by
dopants or impurities, deposition temperature, and post-
deposition heat cycles. Polysilicon deposited below 575°C
is amorphous with no detectable structure. Polysilicon
deposited above 625°C is polycrystalline and has a
columnar structure. Crystallization and grain growth
occur when either amorphous or columnar polysilicon is
heated.

51. Structure
 The deposition temperature at which the transition from
amorphous to columnar structure occurs is well defined
but depends on many variables, such as deposition rate,
partial pressure of hydrogen, total pressure, presence of
dopants, and presence of impurities (O, N, or C). The
transition temperature is between 575 and 625°C for
depositions in an LPCVD reactor.'

52. Structure
 The average diameter of the column, that is, the columnar
grains, can be measured by TEM surface replication. The
diameter, which depends on film thickness, is typically
between 0.03 and 0.3 µm and is often reported as grain
size.

53. Structure
 Polysilicon deposited at 600 to 650°C has a {1 10}-
preferred orientation.
 At higher deposition temperatures the {100} orientation
predominates, but the structure contains significant
contributions from other orientations, such as {1 10}, {1 1
1}, {31 1},and {331}.
 Dopants and impurities, as well as temperature, also
influence the preferred orientation.

54. Doping Polysilicon
 Polysilicon can be doped by diffusion, implantation, or
the addition of dopant gases during deposition.
 The resistivity of implanted polysilicon depends primarily
on implant dose, annealing temperature, and annealing
time.
 Polysilicon films that are doped during deposition by
adding phosphine, arsine, or diborane have resistivities
that are strong functions of deposition temperature,
dopant concentration, and annealing temperature

55. Oxidation of Polysilicon
 Polysilicon is usually oxidized in dry oxygen at
temperatures between 900 and 1000°C to form an
insulator between the doped-polysilicon gate and other
conducting layers. Under these conditions, oxidation is
controlled by surface reactions.
 The silicon dioxide grown on polysilicon has lower
breakdown fields, higher leakage currents, and higher
stress than oxides grown on single-crystal silicon.

56. Properties of Polysilicon
 The chemical and physical properties of polysilicon often
depend on the film structure (amorphous or crystalline)
or on the dopant concentration. The etch rate of
polysilicon in a plasma and its thermal oxidation rate
depend on the dopant concentration.
 Polysilicon which is heavily phosphorus-doped etches
and oxidizes at higher rates than undoped or lightly
doped polysilicon. The reaction rates for oxidation and
etching are determined by the free carrier concentration
at the doped-polysilicon surface.

57. Properties of Polysilicon
 Polysilicon' s optical properties depend on its structure. The
imaginary part of the dielectric function is particularly
structure-sensitive.
 Crystalline polysilicon has sharp maxima in the dielectric
function near 2950 and 3650 A (4.2 and 3.4 eV).
 Amorphous polysilicon has a broad maximum without sharp
structure.
 In addition,
 amorphous polysilicon has a higher refractive index
throughout the visible region than crystalline polysilicon.

58. SILICON DIOXIDE
 Silicon dioxide films can be deposited with or without dopants.
 Undoped silicon dioxide is used as an insulating layer between
multilevel metallizations.
 Phosphorus-doped silicon dioxide is used as an insulator between
metal layers, as a final passivation over devices, and as a gettering
source.
 Oxides doped with phosphorus, arsenic, or boron are occasionally
used as diffusion sources.
 Oxides used as insulators between conducting layers are deposited,
densifiedby annealing, and plasma-etched to open windows.
 Phosphorus-doped oxides used for passivation are deposited at
temperatures lower than 500°C, and areas for bonding are opened
by etching

59. Deposition Methods
 Several deposition methods are used to produce silicon
dioxide.
 They are characterized by different chemical reactions,
reactors, and temperatures.
 Films deposited at low temperatures, lower than 500°C,
are formed by reacting silane, dopant, and oxygen.
 The chemical reactions for phosphorus-doped oxides are

60. chemical reactions for phosphorus-
doped oxide
•The deposition can be carried out at atmospheric
pressure in a continuous reactor (Fig. lb) or at reduced
pressure in an LPCVD reactor (Fig. la).
•The main advantage of silane oxygen reactions is the low
deposition temperature, which allows films to be
deposited over aluminum metallization.
•The main disadvantages of silane-oxygen
reactions are poor step coverage and particles
caused by loosely adhering deposits on the

61.  Silicon dioxide is also deposited at 650 to 750°C in an LPCVD
reactor by decomposing tetraethoxysilane, Si(OC2H5)4.This
compound, also called tetraethyl orthosilicate and
abbreviated TEOS, is vaporized from a liquids. The overall
reaction is
 The advantages of TEOS deposition are excellent uniformity,
conformal step coverage,and good film properties.
 The disadvantages are the high-temperature and liquid
source requirements source.

62.  Silicon dioxide is also deposited at temperatures near
900°C and at reduced pressure by reacting dichlorosilane
with nitrous oxide.
Doping is achieved by adding small amounts of the
dopant hydrides (phosphine, arsine, or diborane) during
the deposition.
Dopant concentrations are reported by weight percent
(wt. %), atom percent (at.%), or mole percent (mol %).

63.  Glass with lower phosphorus concentrations will not
soften and flow, and higher concentrations react slowly
with atmospheric moisture to form acid products, which
corrode the aluminum metallization

64. Deposition Variables
 The deposition of silicon dioxide depends on the same
variables that are important for polysilicon, that is,
temperature, pressure, reactant concentration, and
presence of dopants.
 In addition, other variables, such as wafer spacing and
total gas flow, are important for some silicon dioxide
depositions.

65.  In Figure 10 ,The relation has been explained by assuming
surface-catalyzed reactions.
 At high concentrations the oxygen adsorbs on the surface
and blocks further silane reactions.
 When phosphine is added to the reaction, the rate rapidly
decreases and then slowly increases. This deposition
behavior may also be attributable to surface adsorption
effect
 The reaction between silane and oxygen at reduced pressure
follows similar trends. The activation energy is very low, less
than 0.4 eV (10 kcal/mole).
 The deposition of silicon dioxide by decomposing TEOS
occurs at temperatures between 650 and 750°C.

66. Fig. 10 The deposition rate of silicon dioxide at atmospheric pressure for
different oxygen concentrations.

67. deposition rate as a function of
temperature
 Figure 1 1 shows deposition rate as a function of
temperature for the TEOS decomposition.
 activation energy for the TEOS reaction is about 1.9 eV
(45 kcal/mole). which decreases to 1.4 eV (32 kcal/mole)
when phosphorus doping compounds are present.

68. Fig. 11 Arrhenius plots for the low-pressure
deposition of SiO2.

69. Properties of Silicon Dioxide

70.  Thickness Film thickness can be measured by a stylus
instrument, reflectance spectroscopy ellipsometry, or a prism
coupler.
 Automated instruments, suitable for routine use, are
available for all these techniques.
 Composition : Silicon dioxide deposited at low temperatures,
(400— 500°C) contains hydrogen
 The bonded hydrogen can be observed by infrared
spectroscopy.
 Phosphorus concentrations in doped silicon dioxide can be
measured by infrared absorption, neutron activation, x-ray
emission spectroscopy, sheet resistance of diffused layers,
etch-rate variation, the refractive index, or an electron
microprobe.

71. Refractive index and stress
 The refractive index of silicon dioxide is 1.458 at a
wavelength of 0.6328 µm. Deposited oxides with
refractive indices above 1.46 are usually silicon-rich.

72. SILICON NITRIDE
 Silicon nitride is chemically deposited by reacting silane
and ammonia at atmospheric pressure at temperatures
between 700 and 900°C or by reacting dichlorosilane
and ammonia at reduced pressure at temperatures
between 700 and 800°.
 The reduced-pressure technique has the advantage of
very good uniformity and high wafer throughput.

73. Deposition Variables
 Silicon nitride depositions at reduced pressure are
controlled by temperature, total pressure, reactant
concentrations, and temperature gradients in the furnace.
 The temperature dependence of deposition rate is similar
to that of polysilicon.
 activation energy for the silicon nitride deposition is
about 1.8 eV (41 kcal/mole).
 The deposition rate increases with increasing total
pressure or dichlorosilane partial pressure, and decreases
with an increasing ammonia to dichlorosilane ratio.

74. Properties of Silicon Nitride
 Silicon nitride, chemically deposited at temperatures
between 700 and 900°C, is an amorphous dielectric
containing up to 8 at. % hydrogen.
 Silicon nitride has a refractive index of 2.01 and an etch
rate in buffered hydrofluoric acid of less than 10 A/min.
 The resistivity of silicon nitride at room temperature is
about 10 ^6 Ohm-cm.
 Silicon nitride is an excellent barrier to sodium diffusion

75. Properties of Silicon Nitride

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