device fabrication example group:- 2. pn junction diode fabrication start:- the starting point is a...
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Device Fabrication Example
Group:- 2
pn junction Diode Fabrication
Start:-The starting point is a flat, damage-free , single-
crystal, Si wafer.Common dopants are boron for P-type layers and
phosphorus, antimony and Arsenic for N-type layers.
Assume the wafer is p-type, having been uniformly doped with boron during the formation of the
crystal.
1.Oxidation:-
The process of oxidation consists of growing a thin film of silicon dioxide on the surface of the silicon wafer.
It will serve as a diffusion barrier. The oxide thickness must be comfortably
greater than the projected masking thickness.
2.Lithography#1:-
This process performed to open “diffusion” holes in the oxide that will eventually become the positions of the pn junction diodes.
3.Phosphorus DiffusionAfter a proper clean-up the wafer is nest inserted
into a phosphorus pre-deposition Furnace.The formation of n+ -p junctions is surface regions
not protected by the oxide. (The + in n+ is indicate a very high doping.
4.Metallization(sputter Al)
Evaporation of Al yields a thin metal film over the entire surface of the wafer due to connect the
device to the ‘’outside world’’
5.Lithography #2:- It is performed to remove excess metal external to
the area of the diffused junction.
To produce commercial diodes, a diamond-edged saw would be used to cut the wafer into pieces containing a single device.
n-well process Fabrication Steps
Typically use p-type substrate for
nMOS transistorsRequires n-well for body of pMOS
transistors
OxidationBlank wafer covered with a layer of SiO2 using
oxidation
PhotoresistSpin on photoresist◦Photoresist is a light-sensitive organic
polymer◦Softens where exposed to light
p substrate
SiO2
Photoresist
LithographyExpose photoresist through n-well
mask
p substrate
SiO2
Photoresist
EtchEtch the uncovered oxide using HF
(Hydroflouric acid)
p substrate
SiO2
Photoresist
Strip PhotoresistEtch the remaining photoresist using a
mixture of acids
p substrate
SiO2
n-welln-well is formed using either diffusion
or ion implantation
n well
SiO2
Strip OxideStrip off remaining oxide using HF.
Subsequent steps use the same photolithography process
p substraten well
PolysiliconDeposit thin layer of oxide. Chemical Vapor Deposition (CVD) of silicon layer◦ Forms many small crystals called polysilicon◦Heavily doped to be good conductor
Thin gate oxidePolysilicon
p substraten well
Polysilicon PatterningUse same lithography process to pattern
polysilicon
Polysilicon
p substrate
Thin gate oxidePolysilicon
n well
Self-Aligned ProcessCover with oxide to define n diffusion
regions
p substraten well
N-diffusionPattern oxide using n+ active mask to
define n diffusion regions
n+ Diffusion
p substraten well
N-diffusion cont.Diffusion or ion implantation used to
create n diffusion regions
n wellp substrate
n+n+ n+
N-diffusion cont.Strip off the oxide to complete
patterning step
n wellp substrate
n+n+ n+
P-DiffusionSimilar set of steps form p+ diffusion regions for
pMOS source and drain and substrate contact
p+ Diffusion
p substraten well
n+n+ n+p+p+p+
ContactsCover chip with thick field oxide Etch oxide where contact cuts are needed
Contact
p substrate
Thick field oxide
n well
n+n+ n+p+p+p+
MetallizationSputter on aluminum over whole waferRemove excess metal leaving wires
Metal
p substrate
Metal
Thick field oxide
n well
n+n+ n+p+p+p+
Fabrication and Layout
Detailed Mask ViewsSix masks◦n-well◦Polysilicon◦n+ diffusion◦p+ diffusion◦Contact◦Metal
Slide 28
Metal
Polysilicon
Contact
n+ Diffusion
p+ Diffusion
n well
3D Structure
Thanks