The Physical Structure NMOS Gate oxide Polysilicon Gate
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The Physical Structure (NMOS) Gate oxide Polysilicon Gate Al Si. O 2 S n+ Field Oxide D n+ channel Field Oxide L P Substrate contact Metal (G) (S) n+ L n+ (D) W Poly 1
3 D Perspective 2
3
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) Si. O 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 u. Basic idea: dip seed crystal into liquid pool u. Slowly pull out at a rate of 0. 5 mm/min ucontrolled amount of impurities added to melt u. Speed of rotation and pulling rate determine diameter of the ingot u. Ingot- 1 to 2 meter long u. Diameter: 4”, 6”, 8” 6
Fabrication: Wafering u Finish ingot to precise diameter u Mill “ flats” u Cut wafers by diamond saw: Typical thickness 0. 5 mm u Polish to give optically flat surface 7
Fabrication: Oxidation u. Silicon Dioxide has several uses: - mask against implant or diffusion - device isolation - gate oxide -isolation between Quartz Tube -layers Pump u Si. O 2 could be thermally generated or through CVD u. Oxidation consumes silicon u. Wet or dry oxidation Wafers O 2 or Water Vapor Quartz Carrier Resistance Heater 8
Fabrication: Diffusion u Simultaneous creation of p-n junction over the entire surface of wafer u Doesn’t offer precise control u Good for heavy doping, deep junctions u Two steps: Temp: 1000 wafers Dopant Gas Pre-deposition Dopant mixed with inert gas introduced in Resistance Heater to a furnace at 1000 o. C. Atoms diffuse in a thin layer of Si surface Drive-in Wafers heated without dopant 9
Fabrication: Ion Implantation Precise control of dopant u Good for shallow junctions and threshold adjust u Dopant gas ionized and accelerated u Ions strike silicon surface at high speed u Depth of lodging is determined by accelerating field u 10
Fabrication: Deposition u Used to form thin film of Polysilicon, Silicon dioxide, Silicon Nitride, Al. u Applications: Polysilicon, interlayer oxide, LOCOS, metal. Pump Loader 0. 1 -1 Torr Reactant u Common technique: Low Pressure Chemical Vapor Deposition (CVD). u Si. O 2 and Polysilicon deposition at 300 to 1000 o. C. Aluminum deposition at lower temperature- different technique u 11
Fabrication: Metallization u Standard material is Aluminum u Low contact resistance to p-type and n-type u When deposited on Si. O 2, Al 2 O 3 is formed: good adhesive u All wafer covered with Al u Deposition techniques: Vacuum Evaporation Electron Beam Evaporation RF Sputtering Other materials used in conjunction with or replacement to Al u. In today’s technology are cupper and its alloys. u 12
Fabrication: Etching Wet Etching u Etchants: hydrofluoric acid (HF), mixture of nitric acid and HF u Good selectivity u Problem: - under cut - acid waste disposal Plasma Dry Etching u Physical bombardment with atoms or ions u good for small geometries. u Various types exists such as: Planar Plasma Etching Reactive Ion Etching Reactive species RF 13
Fabrication: Lithography Mask making u Most critical part of lithography is conversion from layout to master mask u 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. u Masks uses photographic emulsion or hard surface u Two types: dark field or clear field u 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 Step & Repeat Printing 16
Lithography: Mask making Electron Beam Technique u Main problem with optical technique: light diffraction System resembles a scanning electron microscope + beam blanking and computer controlled deflection u 17
Patterning/ Printing Process of transferring mask features to surface of the silicon wafer. u Optical or Electron-beam u u 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 Si. O 2 substrate 20
Fabrication Steps Inspect, measure Post bake Etch Develop, rinse, dry Strip resist Printer align expose mask Deposit or grow layer Pre-bake Apply PR 21
Fabrication Steps 22
3 D Perspective 23
The Physical Structure (NMOS) Gate oxide Polysilicon Gate Al Si. O 2 S n+ Field Oxide D n+ channel Field Oxide L P Substrate contact Metal (G) (S) n+ L n+ (D) W 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=AMg. Q 1 Hd. El. M&list=PL 8 In. EUriv. GYt 2 Fze 1 v. Xsdk. HDPWBP 7 NTXw&index= 9 min video showing IC fabrication process https: //www. youtube. com/watch? v=i 8 kxymmjdo. M A 10 minute presentation of Global Foundries IC manufacturing process. https: //www. youtube. com/watch? v=qm 67 wb. B 5 Gm. I&index=13&list=PL 8 In. EUriv. GYt 2 Fze 1 v. Xsdk. HDPWBP 7 N TXw 3 min animation of IC fabrication https: //www. youtube. com/watch? v=d 9 SWNLZv. A 8 g A 4 min very nice presentation with animation of 3 D IC manufacturing https: //www. youtube. com/watch? v=YIk. Ma. QJSy. P 8 25
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