Fabrication Process Crystal Growth Doping Deposition Patterning Lithography Oxidation Ion Implementation 1
Fabrication- CMOS Process Starting Material Preparation 1. Produce Metallurgical Grade Silicon (MGS) SiO 2 (sand) + C in Arc Furnace Si- liquid 98% pure 2. Produce Electronic Grade Silicon (EGS) HCl + Si (MGS) Successive purification by distillation Chemical Vapor Deposition (CVD) 2
Fabrication: Crystal Growth Czochralski Method Basic idea: dip seed crystal into liquid pool Slowly pull out at a rate of 0.5mm/min controlled amount of impurities added to melt Speed of rotation and pulling rate determine diameter of the ingot Ingot- 1to 2 meter long Diameter: 4, 6, 8 3
Fabrication: Wafering Finish ingot to precise diameter Mill flats Cut wafers by diamond saw: Typical thickness 0.5mm Polish to give optically flat surface 4
Fabrication: Oxidation Silicon Dioxide has several uses: - mask against implant Pump or diffusion - device isolation - gate oxide - isolation between layers Quartz Tube Wafers Quartz Carrier O 2 or Water Vapor SiO2 could be thermally generated or through CVD Oxidation consumes silicon Resistance Heater Wet or dry oxidation 5
Fabrication: Diffusion Simultaneous creation of p-n junction over the entire surface of wafer Doesn t offer precise control Good for heavy doping, deep junctions Two steps: Pre-deposition Dopant mixed with inert gas introduced in to a furnace at 1000 o C. Atoms diffuse in a thin layer of Si surface Drive-in Wafers heated without dopant Dopant Gas Temp: 1000 Resistance Heater wafers 6
Fabrication: Ion Implantation Precise control of dopant Good for shallow junctions and threshold adjust Dopant gas ionized and accelerated Ions strike silicon surface at high speed Depth of lodging is determined by accelerating field 7
Fabrication: Deposition Used to form thin film of Polysilicon, Silicon dioxide, Silicon Nitride, Al. Applications: Polysilicon, interlayer oxide, LOCOS, metal. Common technique: Low Pressure Chemical Vapor Deposition (CVD). Pump 0.1-1 Torr Loader Reactant SiO2 and Polysilicon deposition at 300 to 1000 o C. Aluminum deposition at lower temperature- different technique 8
Fabrication: Metallization Standard material is Aluminum Low contact resistance to p-type and n-type When deposited on SiO 2, Al 2 O 3 is formed: good adhesive All wafer covered with Al Deposition techniques: Vacuum Evaporation Electron Beam Evaporation RF Sputtering Other materials used in conjunction with or replacement to Al 9
Fabrication: Etching Wet Etching Etchants: hydrofluoric acid (HF), mixture of nitric acid and HF Good selectivity Problem: - under cut - acid waste disposal Dry Etching Physical bombardment with atoms or ions good for small geometries. Various types exists such as: Planar Plasma Etching Reactive Ion Etching Plasma RF Reactive species 10
Fabrication: Lithography Mask making Most critical part of lithography is conversion from layout to master mask Masking plate has opaque geometrical shapes corresponding to the area on the wafer surface where certain photochemical reactions have to be prevented or taken place. Masks uses photographic emulsion or hard surface Two types: dark field or clear field Maskmaking: optical or e-beam 11
Lithography: Mask Optical Mask Technique making 1. Prepare Reticle Use projection like system: -Precise movable stage -Aperture of precisely rectangular size and angular orientation -Computer controlled UV light source directed to photographic plate After flashing, plate is developed yielding reticle 12
Fabrication: Lithography Printing Step & Repeat Printing 13
Lithography: Mask making Electron Beam Technique Main problem with optical technique: light diffraction System resembles a scanning electron microscope + beam blanking and computer controlled deflection 14
Patterning/ Printing Process of transferring mask features to surface of the silicon wafer. Optical or Electron-beam Photo-resist material (negative or positive):synthetic rubber or polymer upon exposure to light becomes insoluble ( negative ) or volatile (positive) Developer: typically organic solvant-e.g. Xylen A common step in many processes is the creation and selective removal of Silicon Dioxide 15
Patterning: Pwell mask 16
Patterning/ Printing SiO2 substrate 17
Fabrication Steps Inspect, measure Etch Post bake Strip resist Develop, rinse, dry Printer align expose mask Deposit or grow layer Pre-bake Apply PR 18
Fabrication Steps 19
Fabrication Steps: P- well Process VDD P+ Diffusion P well Vin P+ Vo n+ n+ p+ p+ p+ Substrate n-type n+ n+ P well 20
Fabrication Steps: P- well Process VDD P+ Diffusion P well Vin P+ Vo n+ n+ p+ p+ p+ Substrate n-type n+ n+ P well 21
Fabrication Steps n+ n+ P well p+ p+ n+ P well n+ p+ p+ Substrate n-type 22
Fabrication Steps Oxidation oxide Substrate n-type Patterning of P-well mask Substrate n-type 23
Fabrication Steps Diffusion: p dopant, Removal of Oxide P-well Si 3 N 4 Deposit Silicon Nitride P-well 24
Fabrication Steps Patterning: Diffusion (active) mask P-well substrate Oxidation FOX FOX FOX substrate 25