•Basic raw material – wafer – disk of silicon
•Dia varies from 75mm to 230mm
•Thickness - less than 1mm (0.25mm – 1mm)
•Wafer are cut using diamond blades from ingots of single silicon crystal pulled from a melt of polycrystalline silicon
•Quartz crucible surrounded by graphite radiator temp ~~1425 C.
•Atmosphere above is Argon or Helium
•Impurities added determine the electrical characteristics
•Crystal orientation determined by seed
•Growth rates - 30 – 180 mm /hr
•One face is polished to a flat , scratch free mirror finish
•By heating the wafers in an oxidizing atmosphere such as oxygen or water vapour.
»Wet oxidation – atmosphere contains water vapour 900ºC to 1000ºC , rapid process.
»Dry oxidation: atmosphere contains pure oxygen. 1200ºC
•SiO2 layer grows equally in both directions
Epitaxy, Deposition, Ion – implantation & Diffusion
•● To generate silicon that contains varying
portions of donor or acceptor impurities.
•Epitaxy: Grow a single crystal film on the silicon
surface, by subjecting silicon wafer to elevated
temp and exposing to dopants.
•Deposition : Evaporate dopant material onto the
silicon surface followed by a thermal cycle (to drive
the impurity from the Si surface into the bulk)
•
•Ion implantation : Subject the Si substrate to
highly energized donor
or acceptor atoms. The injected impurities will
travel below the surface of
the Si, forming regions with varying doping
concentration.
•Diffusion : Occurs at temp. > 800ºc between Si
and dopants.
Impurities will diffuse from areas of high
concentration to area
of low concentration.
Construction of devices depends on ability to control the above processes.
Note: =>It's important once the doped areas have been put in place,
to keep the remaining process steps at as low a temperature
as possible.
● Impurities: Boron : acceptors
Arsenic, phosphorous : donors
● Amount is controlled by
1. energy and time of “Ion implantation”.
2. Time and temperature of “deposition” and “diffusion”.
● Where deposition depends on masks
- Common materials used as masks include:
1. photoresist
2. polysilicon (polycrystalline silicon)
3. silicon dioxide (SiO2)
4.silicon Nitride (Si3N4) (SiN)
Function of the mask : form a barrier against doping impurities
selective diffusion - steps
1. patterning "windows" in a mask material on the surface of the wafer
2. subjecting exposed areas to a dopant source.
3.remove any undesired mask materials.
● SiO2 is removed using an etching technique.
A. Use a acid resistant coating (photoresist)(PR), which can be polymerized by ultraviolet (UV) light.
B. The polymerized areas may be removed with an organic solvent.
C. Etching of exposed SiO2 then may proceed.
•A-B-C "Positive resist“
•"Negative resist" = unexposed PR is dissolved by the solvent.
•*Note diffraction around the edges of the mask patterns (= 0.8µm)
•
Alternative approach : high cost, precise -Electron beam lithography (EBL)Line widths of 0.5µm with good definition are achievable
Main adv:
1.Patterns are derived directly from digital data.
2.No intermediate H/W images such as masks.
3.Different patterns may be accommodated in diff sections without any difficulty.
4.Changes can be implemented quickly.
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