The Physical Structure (NMOS) Al SiO2 Field Oxide Gate oxide S n+ Polysilicon Gate Al SiO2 SiO2 D n+ L channel P Substrate Field Oxide contact Metal (S) n+ (G) L W n+ (D) Poly 1
3D Perspective 2
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Fabrication Process Crystal Growth Doping / Diffusion Deposition Patterning Lithography Oxidation Ion Implementation 4
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) 5
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 6
Fabrication: Wafering Finish ingot to precise diameter Mill flats Cut wafers by diamond saw: Typical thickness 0.5mm Polish to give optically flat surface 7
Fabrication: Oxidation Silicon Dioxide has several uses: - mask against implant or diffusion - device isolation - gate oxide -isolation between -layers Pump Quartz Tube Wafers O 2 or Water Vapor SiO2 could be thermally generated or through CVD Oxidation consumes silicon Wet or dry oxidation Quartz Carrier Resistance Heater 8
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: Temp: 1000 Dopant Gas wafers 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 Resistance Heater 9
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 10
Fabrication: Deposition Used to form thin film of Polysilicon, Silicon dioxide, Silicon Nitride, Al. Applications: Polysilicon, interlayer oxide, LOCOS, metal. Loader Pump 0.1-1 Torr Reactant Common technique: Low Pressure Chemical Vapor Deposition (CVD). SiO2 and Polysilicon deposition at 300 to 1000 o C. Aluminum deposition at lower temperature- different technique 11
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 In today s technology are cupper and its alloys. 12
Fabrication: Etching Wet Etching Etchants: hydrofluoric acid (HF), mixture of nitric acid and HF Good selectivity Problem: - under cut - acid waste disposal Plasma Reactive species Dry Etching Physical bombardment with atoms or ions good for small geometries. Various types exists such as: Planar Plasma Etching Reactive Ion Etching RF 13
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 14
Lithography: Mask making Optical Mask Technique 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 15
Fabrication: Lithography Printing Step & Repeat Printing 16
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 17
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 18
Patterning: Pwell mask 19
Patterning/ Printing SiO2 substrate 20
Fabrication Steps Inspect, measure Etch Post bake Strip resist Deposit or grow layer Develop, rinse, dry Printer align expose Pre-bake mask Apply PR 21
Fabrication Steps 22
3D Perspective 23
The Physical Structure (NMOS) Al SiO2 Field Oxide Gate oxide S n+ Polysilicon Gate Al SiO2 SiO2 D n+ L channel P Substrate Field Oxide contact Metal (S) n+ (G) L W n+ (D) Poly 24
Videos for Fabrication A very clear site showing each fabrication step http://www.virlab.virginia.edu/vl/mos_kit.htm/state/related 4 min wafer production https://www.youtube.com/watch?v=amgq1- HdElM&list=PL8InEUrivGYt2Fze1vXsdkHDPWBP7NTXw&index= 9 min video showing IC fabrication process https://www.youtube.com/watch?v=i8kxymmjdom A 10 minute presentation of Global Foundries IC manufacturing process. https://www.youtube.com/watch?v=qm67wbb5gmi&index=13&list=pl8ineurivgyt2fze1vxsdkhdpwbp7n TXw 3 min animation of IC fabrication https://www.youtube.com/watch?v=d9swnlzva8g A 4 min very nice presentation with animation of 3D IC manufacturing https://www.youtube.com/watch?v=yikmaqjsyp8 25