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+

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 D Perspective 2

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Fabrication Process • Crystal Growth • Doping / Diffusion • Deposition • Patterning •

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

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

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

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

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

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

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,

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

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

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

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

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

Fabrication: Lithography Step & Repeat Printing 16

Lithography: Mask making Electron Beam Technique u Main problem with optical technique: light diffraction

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

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: Pwell mask 19

Patterning/ Printing Si. O 2 substrate 20

Patterning/ Printing Si. O 2 substrate 20

Fabrication Steps Inspect, measure Post bake Etch Develop, rinse, dry Strip resist Printer align

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

Fabrication Steps 22

3 D Perspective 23

3 D Perspective 23

The Physical Structure (NMOS) Gate oxide Polysilicon Gate Al Si. O 2 S n+

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.

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