Wafer Fabrication
CZ processing
Ingot diameter varies inversely with pull rate:
r
T
LNV mmp 3
21 5
max
L = latent heat of fusion
N = density
= Stephan-Boltzman constant
m = thermal conductivity at Tm
Tm = melt temperature (1417 oC for Si)
o = Cs/Co increases as ingot grows
The dopant concentration is given by:
IL=Io(1+Vs/Vo)ko and
Cs = -dIL/dVs = Coko(1-f)(o-1)
C, I and V are concentration, number of impurities and volume when
o: initial L: liquid and s: solid
Float Zone Processing
L
xk
oos
o
CxC exp11)(
In Float Zone refining, solid concentration varies with initial concentration as follows:
Concentration
Liquid
Tem
pera
ture
Solid
CoCs CL
Typical defects in crystalsTypical 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)
Thermal stresses cause dislocations. Thermal stress is: = ET
= stress, E = Young’s modulus, = thermal expansion coefficient (m/m/oC)
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
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.
Spin Process Parameters:
•Viscosity
•Spin Speed
Step Coverage
Adhesion – surface chemistry
Resist Process Steps:
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 ControlledOptical Lithography
K1 ~ 0.6-0.8 and
K2 ~ 0.5.
NA is the numerical aperture number, NA=n*sin()
where n=1 and is the angle formed by the point light source and the aperture width
Limitations in Optical Lithography:
The Diffraction Limit
Resolution: K1/NA
Depth of Focus: K2/NA2
from microns to 50 nm
ExampleEstimate the resolution and depth of focus of an
excimer laser stepper using KrF light source ( = 248 nm) and NA=0.6 Assume k1 = 0.75 and k2 = 0.5.
Solution:
R = k1*/NA = 0.75(0.248/0.6) = 0.31 nm
DOF = ± k2*/NA2 = ±0.5(0.248/(0.6)2) = ±0.34 m
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
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
Limitations
Increasing the electron beam energy (keV) or decreasing the resist layer results in broadening at the surface
X-ray lithography: wavelength ~ 0.1 - 1 nm
Wet and Dry Etching
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
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) low-pressure 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 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
Effective diffusion-time, (Dt)eff, is the sum of the diffusivity and time at each step:
(Dt)eff= D1t1+D1t2(D2/D1)=D1t1+D2t2
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.
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