Fabrication Process

1. Fabrication Process • Crystal Growth • Doping • Deposition • Patterning • Lithography • Oxidation • Ion Implementation

2. Fabrication- CMOS Process Starting Material Preparation 1. Produce Metallurgical Grade Silicon (MGS) SiO2 (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)

3. 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”

4. Fabrication: Wafering • Finish ingot to precise diameter • Mill “ flats” • Cut wafers by diamond saw: Typical thickness 0.5mm • Polish to give optically flat surface

5. Fabrication: Oxidation Quartz Tube O or Water 2 Pump Wafers Vapor Quartz Carrier Resistance Heater • Silicon Dioxide has several uses: - mask against implant or diffusion - device isolation - gate oxide - isolation between layers • SiO2 could be thermally generated • or through CVD • Oxidation consumes silicon • Wet or dry oxidation

6. Fabrication: Diffusion Temp: 1000 • 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 oC. • Atoms diffuse in a thin layer of Si surface • Drive-in • Wafers heated without dopant wafers Dopant Gas Resistance Heater

7. 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

8. Fabrication: Deposition Loader 0.1 -1 Torr Pump • 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). • SiO2 and Polysilicon deposition at 300 to 1000 oC. • Aluminum deposition at lower temperature- different technique Reactant

9. Fabrication: Metallization • Standard material is Aluminum • Low contact resistance to p-type and n-type • When deposited on SiO2, Al2O3 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

10. 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 Reactive species RF

11. 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

12. 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

13. Fabrication: Lithography Step & Repeat Printing Printing

14. 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

15. 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

16. Patterning: Pwell mask

17. Patterning/ Printing SiO2 substrate

18. Fabrication Steps Inspect, measure Post bake Etch Develop, rinse, dry Strip resist mask Printer align expose Deposit or grow layer Pre-bake Apply PR

19. Fabrication Steps

20. VDD Fabrication Steps: P-well Process Diffusion P+ P+ Vin Vo P well p+ n+ n+ p+ p+ p+ n+ n+ P well Substrate n-type

21. Fabrication Steps: P-well Process VDD Diffusion P+ P+ Vin Vo P well p+ n+ n+ p+ p+ p+ n+ n+ P well Substrate n-type

22. Fabrication Steps n+ n+ p+ p+ P well n+ n+ p+ p+ P well Substrate n-type

23. Fabrication Steps Oxidation oxide Substrate n-type Patterning of P-well mask Substrate n-type

24. Fabrication Steps Diffusion: p dopant, Removal of Oxide P-well Si3N4 Deposit Silicon Nitride P-well

25. Fabrication Steps Patterning: Diffusion (active) mask P-well substrate FOX FOX FOX Oxidation substrate