Device Fabrication Example

Group:- 2

pn junction Diode FabricationStart:-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 StepsTypically 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