Wafer Fabrication

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Wafer Fabrication
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CZ processing
ko Cs/Co increases as ingot grows The dopant
concentration is given by ILIo(1Vs/Vo)ko and C
s -dIL/dVs Coko(1-f)(ko-1)
Ingot diameter varies inversely with pull rate
L latent heat of fusion N density s
Stephan-Boltzman constant km thermal
conductivity at Tm Tm melt temperature (1417 oC
for Si)
C, I and V are concentration, number of
impurities and volume when o initial L
liquid and s solid
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Float Zone Processing
Liquid
Temperature
Solid
Concentration
Co
Cs
CL
In Float Zone refining, solid concentration
varies with initial concentration as follows
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Typical defects in crystals
Typical defects are Point defects vacancies
interstitials Line defects dislocations Volume
defects stacking faults, precipitates
The equilibrium number of vacancies varies with
temperature nv noexp(-Ev/kT)
O and C are also defects with concentrations of
1017-1018 cm-3 and 1015-1016 cm-3 Other
impurities are in the ppb range
Thermal stresses cause dislocations. Thermal
stress is s EaDT s stress, E Youngs
modulus, a thermal expansion coefficient
(mm/m/oC)
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Photolithography
MSE 630 Fall, 2008
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The way patterns are defined on thin films is
called Lithography. If light is used to transfer
patterns from a mask on to a wafer, then this
special kind of lithography is called
photolithography.
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Resist Process Steps

• Spin Process Parameters
• Viscosity
• Spin Speed
• Step Coverage
• Adhesion surface chemistry

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Typical Photoresist Problems
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Factors Affecting Resist

• Materials
• Glass Transition Temperature, Tg Pattern
  Stability
• Molecular Weight Resolution
• Substrate atomic number Z Proximity effects
• Chemical composition Etch resistance, adhesion
• Process
• Development (strength, time temperature)
• Baking time
• Post-treatment scum removal, stripping native
  oxide

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Methods of Pattern Definition

• Radiation Sources
• Photons
• Electrons
• X-ray
• Ions

• Approaches
• Shadow mask
• Direct write

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Mask Controlled Optical Lithography
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Limitations in Optical Lithography The
Diffraction Limit
K1 0.6-0.8 and K2 0.5. NA is the numerical
aperture number, NAnsin(a) where n1 and a is
the angle formed by the point light source and
the aperture width
Resolution K1l/NA Depth of Focus K2l/NA2 from
microns to 50 nm
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Example

• Estimate the resolution and depth of focus of an
  excimer laser stepper using KrF light source (l
  248 nm) and NA0.6 Assume k1 0.75 and k2
  0.5.
• Solution
• R k1l/NA 0.75(0.248/0.6) 0.31 nm
• DOF k2l/NA2 0.5(0.248/(0.6)2) 0.34 mm

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Shrinking device size drives need for finer
replication methods
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Direct Write
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Electron Beam Direct Write Performance
Details down to 20 nm
Alignment within 50 nm
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Limitations to e-Beam Lithography

• Resolution factors
• Affected by beam quality ability to focus on
  surface (1 nm)
• Blurred by secondary electrons (lateral range a
  few nm

• Resolution factors
• Affected by beam quality ability to focus on
  surface (1 nm)
• Blurred by secondary electrons (lateral range a
  few nm

• Performance
• On organic resist PMMA 7 nm
• Inorganic resist 1-2 nm

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Limitations
Increasing the electron beam energy (keV) or
decreasing the resist layer results in broadening
at the surface
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X-ray lithography wavelength l 0.1 - 1 nm
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Wet and Dry Etching
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Wet Chemical Treatment

• Substrate Cleaning
• Fuming HNO3
• H2SO4/H2O2
• HCL/H2O2
• NH4OH/H2O2
• Resist Technology
• Keytone Solvents
• Acetone
• Isopropanol (IPA)
• Mask Removal
• Fuming HNO3
• Wet Etching
• Strong Acids/Bases

Many steps are involved and repeated in producing
an integrated circuit including resist
application and removal, substrate cleaning, and
etching
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Wet Etchants
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Etching Challenges
Actual etch profiles that can occur. (a) Lateral
etching under mask (b) rounded photoresist which
is further eroded during etching, leading to even
more lateral etching. (b) also illustrates etch
selectivity
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Dry Etching Characteristics
High Resolution
Profile control
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Reactive Ion Etch (RIE)
A diagram of a common RIE setup. An RIE
consists of two electrodes (1 and 4) that create
an electric field (3) meant to accelerate ions
(2) toward the surface of the samples (5). Ion
species react with substrate, and remove material
by sputtering and chemical reaction
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Dry Etching Chemicals and Surfaces
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Dry Etching Process Issues
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Wet vs. Dry Etching
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Thin Films and Diffusion
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Thin Film Deposition Methods
Evaporation electron gun resistance
heating electrically biased flux Sputtering Io
n beam Plasma Chemical Vapor Deposition (CVD)
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Chemical Vapor Deposition (CVD)
Steps involved in the CVD process. Gas species
(1) is attracted to the surface (2), where it
reacts to form surface compounds (3,4) and gases
(5) which rejoin the gas stream (6,7)
CVD systems (a) atmospheric cold-wall system for
deposition of epitaxial silicon, (b) low-pressure
hot-wall system for deposition of polycrystalline
silicon and amorphous films, e.g. polysilicon and
silicon dioxide, respectively.
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Typical thin-film problems
Step coverage of metal over nonplanar topography.
(a) shows conformal step coveage, with constant
thickness on horizontal and vertical surfaces,
(b) shows poor step coverage.
Thin film filling issues. (a) good metal filling
of a via or contact hole in a dielectric layer.
(b) shows silicon dioxide dielectric filling the
space between metal lines, with poor filling
leading to void formation, and (c) shows poor
filling of the bottom of a via hole with a
barrier or contact metal.
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Sputtering
Important processes in sputter deposition
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Physical Vapor Deposition (PVD)
Schematic diagrams of PVD systems and processes
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Electroplating
Sub-micron features with high aspect rations are
easily achieved via electroplating
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Thin Film Deposition Issues

• Surface Coverage
• Surface diffusion
• Flux directionality
• Film Morphology
• Temperature
• Ion Treatment
• Stress
• Thermal
• Growth Induced
• Adhesion
• Compatibility to film substrate
• Stress
• Beneficial role of few nm of Ti, Cr, or NiCr

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Step Coverage
Directional evaporation leaves sides of
structures uncovered
Temperature and gas pressure change the mean free
path of the atoms, thus influencing coverage
profiles
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Inspection 25 of fabrication time!
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In-situ inspections
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Diffusion is not constant across cross section,
and continues with every subsequent
high-temperature step hence, we use charts as
below to calculate surface concentrations, Cs,
from average conductivity,
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Effective diffusion-time, (Dt)eff, is the sum of
the diffusivity and time at each step (Dt)eff
D1t1D1t2(D2/D1)D1t1D2t2
Effective diffusivity is DAeffDoD-(n/ni)D)n/n
i)2 for N-type DeffADoD(p/ni)D(p/ni)2 for
P-type Values are tabulated, as in table 7.5
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Diffusion Data
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Example
Figure 7-17 Dopant surface concentration vs.
effective conductivity for various substrate
concentrations, CB
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Suggested exercises

• Do Problem 2.1 in Silicon VLSI Technology
• Look over example problem (7.3) and examples on
  page 390 and 412.