Transcript Wafer Fabrication

Wafer Fabrication
CZ processing
o = Cs/Co increases as ingot grows
The dopant concentration is given
Ingot diameter varies inversely with pull by:
rate:
5
1 2mTm
IL=Io(1+Vs/Vo)ko
and
V pmax 
LN
3r
Cs = -dIL/dVs = Coko(1-f)(o-1)
L = latent heat of fusion
 = Stephan-Boltzman constant
C, I and V are concentration, number
of impurities and volume when
m = thermal conductivity at Tm
o: initial
N = density
Tm = melt temperature (1417 oC for Si)
L:
liquid and s: solid
Float Zone Processing
Temperature
Liquid
Solid
Cs
Co
CL
Concentration
In Float Zone refining, solid
concentration varies with initial
concentration as follows:
 ko  x


L
Cs ( x)  Co 1  1   o  exp



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:  = EaDT
 = stress, E = Young’s modulus, a = thermal expansion coefficient
(mm/m/oC)
Photolithography
MSE 630
Fall, 2008
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.
Resist Process Steps:
Spin Process Parameters:
•Viscosity
•Spin Speed
Step Coverage
Adhesion – surface chemistry
Typical Photoresist Problems
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
Methods of Pattern Definition
Radiation Sources:
• Photons
• Electrons
• X-ray
• Ions
Approaches:
• Shadow mask
• Direct write
Mask Controlled
Optical Lithography
Limitations in Optical Lithography:
The Diffraction Limit
K1 ~ 0.6-0.8 and
K2 ~ 0.5.
NA is the numerical
aperture number,
NA=n*sin(a)
where n=1 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
Example
Estimate the resolution and depth of focus of an
excimer laser stepper using KrF light source (l =
248 nm) and NA=0.6 Assume k1 = 0.75 and k2 =
0.5.
Solution:
R = k1*l/NA = 0.75(0.248/0.6) = 0.31 nm
DOF = ± k2*l/NA2 = ±0.5(0.248/(0.6)2) = ±0.34 mm
Shrinking device size drives need
for finer replication methods:
Direct Write
Electron Beam Direct
Write Performance
Details down to 20 nm
Alignment within 50 nm
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
Performance:
•On organic resist PMMA
~7 nm
•Inorganic resist
nm
~ 1-2
Limitations
Increasing the electron beam
energy (keV) or decreasing
the resist layer results in
broadening at the surface
X-ray lithography: wavelength l ~ 0.1 - 1 nm
Wet and Dry Etching
Wet Chemical Treatment
Substrate Cleaning
•Fuming HNO3
Many steps are
involved and repeated
in producing an
integrated circuit
including: resist
application and
removal, substrate
cleaning, and etching
•H2SO4/H2O2
•HCL/H2O2
•NH4OH/H2O2
Resist Technology
•Keytone Solvents
•Acetone
•Isopropanol (IPA)
Mask Removal
•Fuming HNO3
Wet Etching
•Strong Acids/Bases
Wet Etchants
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
Dry Etching Characteristics
High Resolution
Profile control
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
Dry Etching Chemicals and
Surfaces
Dry Etching Process Issues
Wet vs. Dry Etching
Thin Films and Diffusion
Thin Film Deposition Methods
Evaporation:
electron gun
resistance heating
electrically biased flux
Sputtering:
Ion beam
Plasma
Chemical Vapor Deposition (CVD)
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) lowpressure hot-wall system for
deposition of polycrystalline
silicon and amorphous films,
e.g. polysilicon and silicon
dioxide, respectively.
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.
Sputtering
Important processes in sputter deposition
Physical Vapor Deposition (PVD)
Schematic diagrams of PVD systems and
processes
Electroplating
Sub-micron features with high
aspect rations are easily achieved
via electroplating
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
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
Inspection – 25% of
fabrication time!
In-situ inspections
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,
Effective diffusion-time, (Dt)eff, is the sum
of the diffusivity and time at each step:
(Dt)eff= D1t1+D1t2(D2/D1)=D1t1+D2t2
Effective diffusivity is:
DAeff=Do+D-(n/ni)+D=)n/ni)2 for N-type
DeffA=Do+D+(p/ni)+D++(p/ni)2 for P-type
Values are tabulated, as in table 7.5
Diffusion Data
Example
Figure 7-17 Dopant surface
concentration vs. effective
conductivity for various substrate
concentrations, CB
Suggested exercises
Do Problem 2.1 in Silicon VLSI Technology
Look over example problem (7.3) and
examples on page 390 and 412.