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UNIT 2INTEGRATED CIRCUITFABRICATION PROCESS

(DOPING)

E5163 IC DESIGN

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LEARNING OUTCOMESAt the end of this topic, student should be able to:

List three method of doping.

Identify bipolar transistor and CMOS transistor area that can be

produced using each method.

Draw diagram showing the reaction using each method. Write equation for the reaction (if any) using each method.

Explain the process using each method.

Elaborate on the sources of dopant used.

Compare between the steps of predeposition and drive-in.

Discuss the advantages of ion implantation method.

Define epitaxial layer.

Explain how and why an epitaxial layer is produced.

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Doping

Doping is a process of chemical dopants are introduced into a silicon

substrate to form the electronic structures that make integrated

circuits useful.

It is used to form bases, emitters, and resistors in bipolar devices, aswell as drains and sources in MOS devices. It is also used to dope

polysilicon layers.

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Doping

Two types:

Diffusion / Thermal diffusion

the movement of a chemical species from an area of high

concentration to an area of lower concentration. Ion implant

process of depositing a chemical species into a substrate by directbombardment of the substrate with high-energy ions of the chemicalfor deposition.

Over the years, ion implant has steadily replaced thermaldiffusion for doping a material in wafer fabrication becauseof its many advantages. The greatest advantage of ionimplant over diffusion is its more precise control for

depositing dopant atoms into the substrate.

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Diffusion

Diffusion process performed in high temperature

furnace.

Two major ways:

Predeposition

a flux of impurities continuously arrives at the surface of the substrate

such that the concentration gradient of the impurity remains constant

at the surface of the substrate.

Drive-in

a thin layer of the impurity material is deposited on the substrate. In

this case, the impurity gradient at the surface of the substrate

decreases with time.

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Predeposition

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Drive-in

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Diffusion

Diffusion Furnace

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Ion Implantation

During ion implantation, impurity atoms are

vaporized and accelerated toward the siliconsubstrate.

These high-energy atoms enter the crystal lattice

and lose their energy by colliding with some silicon

atoms before finally coming to rest at some depth. The damage caused by atomic collisions during ion

implantation changes the electrical characteristics

of the target. Many target atoms are displaced,

creating deep electron and hole traps which

capture mobile carriers and increase

resistivity. Annealing is therefore needed to repair

the lattice damage and put dopant atoms in

substitutional sites where they can be electrically

active again.

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Ion Implanter

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