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

• 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

• Simultaneous creation of p-n junction over the
  entire surface of wafer
• Doesnt 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

• 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
Fabrication Steps P-well Process
Diffusion
P
P
Vin
Vo
P well

p
p
n n
p p

n
n
P well
Substrate n-type
21
Fabrication Steps P-well Process
VDD
Diffusion
P
P
Vin
Vo
P well

p
p
n n
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