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Wafer Fabrication

Wafer Fabrication. CZ processing. k 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) ( k o-1). Ingot diameter varies inversely with pull rate:. L = latent heat of fusion N = density

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Wafer Fabrication

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  1. Wafer Fabrication

  2. CZ processing ko = 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)(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

  3. Float Zone Processing Liquid Temperature Solid Cs Co CL Concentration In Float Zone refining, solid concentration varies with initial concentration as follows:

  4. 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 = Young’s modulus, a = thermal expansion coefficient (mm/m/oC)

  5. Photolithography MSE 630 Fall, 2008

  6. 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.

  7. Resist Process Steps: • Spin Process Parameters: • Viscosity • Spin Speed • Step Coverage • Adhesion – surface chemistry

  8. Typical Photoresist Problems

  9. 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

  10. Methods of Pattern Definition • Radiation Sources: • Photons • Electrons • X-ray • Ions • Approaches: • Shadow mask • Direct write

  11. Mask Controlled Optical Lithography

  12. 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

  13. 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

  14. Shrinking device size drives need for finer replication methods:

  15. Direct Write

  16. Electron Beam Direct Write Performance Details down to 20 nm Alignment within 50 nm

  17. 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

  18. Limitations Increasing the electron beam energy (keV) or decreasing the resist layer results in broadening at the surface

  19. X-ray lithography: wavelength l ~ 0.1 - 1 nm

  20. Wet and Dry Etching

  21. 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

  22. Wet Etchants

  23. 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

  24. Dry Etching Characteristics High Resolution Profile control

  25. 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

  26. Dry Etching Chemicals and Surfaces

  27. Dry Etching Process Issues

  28. Wet vs. Dry Etching

  29. Thin Films and Diffusion

  30. Thin Film Deposition Methods Evaporation: electron gun resistance heating electrically biased flux Sputtering: Ion beam Plasma Chemical Vapor Deposition (CVD)

  31. 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.

  32. 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.

  33. Sputtering Important processes in sputter deposition

  34. Physical Vapor Deposition (PVD) Schematic diagrams of PVD systems and processes

  35. Electroplating Sub-micron features with high aspect rations are easily achieved via electroplating

  36. 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

  37. 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

  38. Inspection – 25% of fabrication time!

  39. In-situ inspections

  40. 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,

  41. 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

  42. Diffusion Data

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