Fabrication of Semiconductors

CONTENT OUTLINE

Introduction to Semiconductors

Applications of Semiconductors

Semiconductors Manufacturing Process

Wafer Manufacturing

Oxidation

Diffusion

Ion Implantation

INTRODUCTION TO SEMICONDUCTORS

Semiconductors are materials that have electrical conductivity between conductors such

as most metals and nonconductors or insulators like ceramics.

How much electricity a semiconductor can conduct depends on the material and its

mixture content.

Semiconductors can be insulators at low temperatures and conductors at high

temperatures.

As they are used in the fabrication of electronic devices, semiconductors play an

important role in our lives.

Semiconductors can be classified mainly into two types.

1. Intrinsic Semiconductors

2. Extrinsic Semiconductors

An intrinsic semiconductor material is very pure and possesses poor conductivity. It is a

single element not mixed with anything else.

On the other hand, extrinsic is a semiconductor material to which small amounts of

impurities are added in a process called doping which cause changes in the conductivity

of this material.

Current conduction in a semiconductor occurs via mobile or "free“ electrons and holes,

collectively known as charge carriers.

Doping a semiconductor such as silicon with a small amount of impurity atoms, such as

phosphorus or boron, greatly increases the number of free electrons or holes within the

semiconductor.

When a doped semiconductor contains excess holes it is called "p-type", and when it

contains excess free electrons it is known as "n-type", where p (positive for holes) or n

(negative for electrons) is the sign of the charge of the majority mobile charge carriers.

MATERIASL FOR SEMICONDUCTORS

Silicon (Si)

Germanium (Ge)

Gallium Arsenide (GaAs)

Other less common materials are also in use or under investigation.

Silicon carbide (SiC)

Various indium compounds (indium arsenide, indium antimonide, and indium

phosphide)

APPLICATION OF SEMICONDUCTORS

Used as building blocks of Logic gates

Sensors

Electronic IC

Diodes

Transistors

Microprocessor

Smart Power Devices

SEMICONDUCTORS MANUFACTURING PROCESS

Fundamental Processing Steps

1. Silicon Manufacturing (Wafer manufacturing)

2. Photolithography

3. Oxidation

4. Diffusion & Ion-implantation

Wafer Manufacturing

Wafer is manufactured from Silicon.

Silicon Can be Artificially Produced by Combining Silica and Carbon in Electric

Furnace.

It gives polycrystalline silicon.

Practical Integrated Circuits (IC) can only be Fabricated from Single-Crystal Silicon.

Czochralski Process is a Technique of Making Single-Crystal Silicon from Poly-

crystalline silicon.

In Czochralski Process, a Solid Seed Crystal is Rotated and Slowly Extracted from a

Pool of Molten Silicon.

It requires Careful Control to give Crystals desired Purity and Dimensions.

The Silicon Cylinder is Known as an

Ingot.

Typical Ingot is About 1 or 2 Meters in

Length.

Can be Sliced into Hundreds of Smaller

Circular Pieces Called Wafers.

Each Wafer Yields Hundreds or

Thousands of Integrated Circuits.

Cutting of Wafers from Ingot

Oxidation

SiO2 growth is a key process step in manufacturing all Si devices.

-Thick (1μm) oxides are used for field oxides (isolate devices from one another )

-Thin gate oxides (100 Å) control Electronic devices

-Sacrificial layers are grown and removed to clean up surfaces

The stability and ease of formation of SiO2 was one of the reasons that Si replaced Ge

as the semiconductor of choice.

The simplest method of producing an oxide layer consists of heating a silicon wafer

in an oxidizing atmosphere.

There are mainly two types of Oxides

1. Dry Oxide

2. Wet Oxide

1. Dry Oxide

- Pure dry oxygen is employed

Disadvantage

- Dry oxide grows very slowly.

Advantage

-Oxide layers are very uniform.

-Relatively few defects exist at the oxide-silicon interface (These defects interfere

with the proper operation of semiconductor devices)

2. Wet Oxide

- Steam is injected

Disadvantage

- Hydrogen atoms liberated by the decomposition of the water molecules produce

imperfections that may degrade the oxide quality.

Advantage

- Wet oxide grows fast.

- Useful to grow a thick layer of field oxide

Diffusion & Ion implantation

This is the process for adding impurities inside the silicon atoms for getting desired

properties , also known as doping.

WN-Junction Fabrication (Earliest method)

Process:

- Opposite polarity doping atoms are added to molten silicon during the Czochralski

process to create in-grown junctions in the ingot.

- Repeated counterdopings can produce multiple junctions within the crystal.

Disadvantages

- Inability to produce differently doped areas in different parts of the wafer.

- Repeated counterdopings degrade the electrical properties of the silicon.

Diffusion

- A uniformly doped ingot is sliced into wafers.

- An oxide film is then grown on the wafers.

- The film is patterned and etched using photolithography exposing specific sections

of the silicon.

- The wafers are then spun with an opposite polarity doping source adhering only to

the exposed areas.

- The wafers are then heated in a furnace (800-1250 deg.C) to drive the doping

atoms into the silicon.

Ion Implantation

- A particle accelerator is used to accelerate a doping atom so that it can penetrate a

silicon crystal to a depth of several microns

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