Part 2i State of the Art

8/8/2019 Part 2i State of the Art

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Overview of Part 2

Properties of Silicon

Structure

Doping

Overview of the Fabrication of a Silicon Based Device


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Silicon is a crystalline

material,

Properties of Silicon Semiconductor

with a tetra-

hedral unit cell.

Silicon has two types of charge carriers - electrons and holes. The

carrier concentration can be controlled by doping, or

electrostatically.

Each silicon atom has 10

core electrons (tightly

bound), and 4 valence

electrons (loosely bound).


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For simplicity we can consider a flattened model structure

At room temperature there are ~1 x 1010cm-3 free carriers (out of ~2

x 1023 cm-3)

Holes and electrons can

move around the lattice, or

recombine to form acomplete bond.

Due to thermal effects some

bonds are broken, givingmobile holes andelectrons.

+ -


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Donor dopants increase the number of conduction electrons

A donor atom, such as

phosphorus, has five

valence electrons, four ofwhich participate in

bonding, leaving one extra

electron that is easily

released for conduction.

The donor

site becomes positively charged (fixed charge).

Silicon doped with a donor is called n-type.


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Acceptor dopants increase the number of holes in the lattice

Silicon doped with an acceptor is called p-type.

An acceptor atom, such as

boron, has three valenceelectrons, and can therefore

easily accept an electron

from a neighbour, leaving a

free hole. The dopant has a

fixed negative charge.

B-


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Overall doping depends on the relative number of acceptors and

donors.

Silicon doped with donor andacceptor atoms is called

Counter Doped, and can

have multiple separate

regions of n- and p-typeconductivity

B- B-

P+


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The carriers distribution is also affected by electric fields

Between collisions with the

lattice the carriers are

accelerated in the directionof the electrostatic field.

B-

E

Combining the effects of doping and fields on the carrier concentration

and distribution, we can realise useful devices.


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Fabrication of a Device (MOSFET)

Silicon devices are built up in a series of layers, using processes

such as photo-lithography, etching and doping.

Oxide layer is grown on n-type silicon by heating to around

1000C in oxygen or steam.

A light sensitive resist is coated on the Si, and exposed to a

light pattern. The exposed resist is developed in a solvent

The unexposed areas of resist are used as a mask to protect

areas of the wafer from plasma etching, dopant implantation,

or metal coating.


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B+ B+ B+ B+ B+ B+ B+ B+ B+ B+

A Metal-Oxide-Semiconductor Field Effect Transistor is a fairly simpledevice, but still requires four separate lithography steps. The lithographic

step is probably the most important in microfabrication. Modern chips

frequently require 30 to 50 layers (each with multiple process steps)

Gate MetalContact

SiliconOxide

Silicon

Doped

Silicon


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Silicon devices are built up in

a series of layers, using

processes such as photo-

lithography, etching and

doping.

An oxide layer is grown on n-

type silicon by heating to

around 1000C in oxygen or

steam.


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Light sensitive material, known

as resist, is spin coated onto

the surface

Light is projected through a patterned mask onto the

resist.

A developing solvent is then

used to remove the exposedresist.


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The sample is etched with

plasma or acids to remove the

exposed oxide.

A 50 nm Gate Oxide layer isthermally grown.

The resist pattern is removed with

chemical stripper or oxygen

plasma


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A 50 nm layer ofpoly-silicon is

made by chemical vapour

deposition.

Another resist pattern isdefined, and developed.

The pattern is transferred

through both the poly-silicon

and the gate oxide by etching,

and the resist is stripped.


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Boron dopant atoms are ion

implanted,B+ B+ B+ B+ B+ B+ B+ B+ B+ B+

and then driven

in by heating to 950C in oxygen

Yet another photolitho-graphy,

etch and resist strip step is

used, to create contact cuts in

the oxide layer

Contact Cuts


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B+ B+ B+ B+ B+ B+ B+ B+ B+ B+Aluminium is evaporated on to

the surface.

And one last patterning, and

etching step to afford aMOSFET (metal-oxide-

semiconductor field effect

transistor)

It took four separate lithography steps to realise this fairly simpledevice. The lithographic step is probably the most important in

microfabrication. Modern chips frequently require 30 to 50 layers

(each with multiple process steps)

Part 2i State of the Art

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Transcript of Part 2i State of the Art