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.