Wafer Fabrication

CZ processing Ingot diameter varies inversely with pull rate: L = latent heat of fusion N = density  = Stephan-Boltzman constant  m = thermal conductivity at T m T m = melt temperature (1417 o C for Si)  o = C s /C o increases as ingot grows The dopant concentration is given by: I L =I o (1+V s /V o ) ko and C s = -dI L /dV s = C o k o (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 In Float Zone refining, solid concentration varies with initial concentration as follows: Concentration Liquid Temperature Solid CoCo CsCs CLCL

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: n v = n o exp(-E v /kT) Thermal stresses cause dislocations. Thermal stress is:  = E  T  = stress, E = Young’s modulus,  = thermal expansion coefficient (  m/m/ o C) O and C are also defects with concentrations of cm -3 and 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, TgPattern Stability Molecular WeightResolution Substrate atomic number ZProximity effects Chemical compositionEtch 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

K 1 ~ and K 2 ~ 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: K 1 /NA Depth of Focus: K 2 /NA 2 from microns to 50 nm

Example Estimate the resolution and depth of focus of an excimer laser stepper using KrF light source ( = 248 nm) and NA=0.6 Assume k 1 = 0.75 and k 2 = 0.5. Solution: R = k 1 * /NA = 0.75(0.248/0.6) = 0.31 nm DOF = ± k 2 * /NA 2 = ±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 nmAlignment 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 ~ nm

Wet and Dry Etching

Wet Chemical Treatment Substrate Cleaning Fuming HNO 3 H 2 SO 4 /H 2 O 2 HCL/H 2 O 2 NH 4 OH/H 2 O 2 Resist Technology Keytone Solvents Acetone Isopropanol (IPA) Mask Removal Fuming HNO 3 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)

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: D A eff =D o +D - (n/n i )+D = )n/n i ) 2 for N-type D eff A =D o +D + (p/n i )+D ++ (p/n i ) 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 = D 1 t 1 +D 1 t 2 (D 2 /D 1 )=D 1 t 1 +D 2 t 2

Diffusion Data

Example Figure 7-17 Dopant surface concentration vs. effective conductivity for various substrate concentrations, C B

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