Javaria Batool

2007-ag-153

(M-phil 3rd )

What is an IC …...??Integrated circuit, commonly referred to as an IC, is a microscopic array of

electronic circuits and components that has been diffused or implanted onto

the surface of a single crystal, or chip, of semiconducting material such as

silicon.

Where ICs are present…???

Microprocessors (MPUs) - act as the Brains of Computers.

Digital Signal Processors (DSPs) -process signals, such as image

Application Specific Integrated

Circuits (ASICs)

Types of ICs on basis of

structure

Thin and thick Film ICs

Monolithic ICs

Hybrid ICs

Thick and Thin film ICs…

Both have similar appearance, properties and general characteristics.

These ICs are form on the surface of the an insulating substrate such as glass or ceramic material.

Moreover only the resistor capacitor and inductor can be formed by using thick and thin film technique.

What is different between Thick & Thin film ICs

“The essential difference between thick and thick film ICs is not their relative thickness but the method of depositing the film”

We look

Same..

??

Thin film ICs fabrication Vacuum evaporation : Done in ultra high vacuum

Thin film of vaporized material is deposited on the glass or ceramic substrate .Target material is vaporized either by resistance heating or by electron beam

Gasses entrapment is negligible because of ultra high vacuum

Technique for depositing thin filmSputter deposition: In this method, the target material(cathode) is bombarded by

energetic ions to release atoms. These atoms are than condensed on the substrate to form the film. Sputtering, unlike evaporation is very well controlled and it is generally

applicable to all material metals, alloy, semiconductors and insulator

Thick film ICs fabrication

Silk screen printing technique :

Devices can be formed by using the masks called screens

Screens are placed on the substrate and inks (conducting or insulating materials pastes )are wiped across the screen

Screens are removed and formulation are fired at high temperature (about 600◦C) that bounded the inks permanently

with the substrate

Fabrication of different

components by thin and thick film technique

Resister and conductor are formed by varying the width and thickness of film and by using the material of different resistivity.

Capacitor are produced by an insulating oxide film between two conductive films.

Small inductor can be made by depositing a spiral formation of the film.

Transistor and diode can not formed so these are externally added and interconnected by wires.

Monolithic ICsMono is stand for “single”and lithic mean “layer”

All components and their interconnection are formed on a single wafer called semiconductor substrate

Fabrication of Monolithic ICsTwo methods for fabrication

Deposition method

All components are fabricated on n-type or p-type substrate

Epitaxial method In “Epitaxial method “a thin layer of n-type is grown over a p-type

substrate and than it used for fabrication of components. The Epitaxial technique is better than Diffusion technique as the

components characteristics are improved because of uniformity of doping in the epitaxial layer.

The thickness of epitaxial layer is about 15 micrometer

How impurities introduced within

waferThere are two method for the addition of impurities

into substrate

Thermal Diffusion

Ion implantation

Thermal Diffusion

The wafer is placed in a furnace at 1200◦c

introducing a gas containing impurity.

Impurity atoms diffused into the crystal because

their tendency for moving from high to low density

region.

Ion implantation Impurity ions is accelerated by using electric filed

Ion implantation provides precise control over the depth of dopant

Ion implantation can be done at relatively low temperature

Logic Circuit Design / Layout Design

A logic circuit diagram is drawn to determine the electronic circuit required for the requested function.

Once the logic circuit diagram is complete, simulations are performed multiple times to test the circuit’s operation.

Photomask Creation

The photo mask is a copy of the circuit pattern, drawn on a glass plate coated with a metallic film.

The glass plate lets light pass, but the metallic film does not.

Silicon ingot preparation Sand, especially quartz, has high percentages of silicon is the base

ingredient for semiconductor manufacturing. silicon is purified in multiple steps to finally reach semiconductor

manufacturing quality and p-type impurity is induced to change electric characteristics

A high-purity, single-crystal silicon called "99.999999999% is grown to form an ingot.

The ingot is doped with accepter impurity to form p-type substrate

Ingot Slicing

The silicon ingot is cut into thin slices (25mm in diameter & 200µm in thickness) called wafer.

wafer polishingThese silicon wafers being lapped and

polish to mirror finish serves as substrate to hundred of ICs.

Advantage of polishing that it remove scratches that come during cutting

Epitaxial growth On the high p-type substrate

an n-type silicon layer (about 15µm thick) is grown by placing the wafer in a furnace at 1200◦c

Introducing a gas containing donor impurity.

The thickness of the epitaxial layer is depends on the temperature and time used for thermal diffusion.

Epitaxial layer ultimately become the collector for a transistor or an element for diode or a capacitor.

Oxidation The layer of SiO2 is grown over

the epitaxial layer

Si +O2 SiO2

The wafer expose to an oxygen atmosphere at about 1000◦C

Thickness of layer depends upon the temperature and the time for which wafer exposed to thermal oxidation .

Photolithography

When a sample of silicon is covered with silicon dioxide,the oxide layer act as barrier to the diffusion ofimpurities.

p-n junction can thus formed in selected region by firstcovering the sample with layer of oxide and thanremoval of oxide layer from selected region (to diffusionof impurity)

The selective removal of oxide in the desired area isperformed by the photolithography .

Step#1: photoresist coating The wafer is coated with

thin layer of light sensitive material that is photo-resist

The photo resist applying to the centre of wafer.

The wafer accelerate rapidly to the speed 3000 RPM that form thin layer of photo resist over the surface

Step#2: Mask AlignmentThe photoresist coated wafer is aligned

with the mask by placing the wafer in mask aligner

Step#3: Exposure to UV light

The wafer surface with mask exposed to ultra violet light.

The photo-resist below the transparent region of photo-mask polymerized

Step#4:DevelopementThe wafer is developed (by using chemical

trichloroethylene) to remove un-polymerized photo-resists.

After the development, photoresist is left on the wafer surface in the shape of the mask pattern.

Photoresist

Polymerized Photoresist

Step#5: Etching "Etching" refers to the physical or chemical etching

of oxide films.

Uncovered SiO2 is etched by solution of HF.

SiO2

Polymerized Photoresist

Step#6:Photoresist StrippingThe photoresist remaining on the wafer surface

is no longer necessary after etching is complete.

The polymerized photoresist is stripped by using the H2SO4

Window for diffusion of impurity

Isolation diffusionAfter the etching in desired area is doped with p-type impurity that has following advantages

By p-type diffusion the n-type epitaxial layer is isolated into islands on which transistor or some other component is fabricated.

The P+ result in improve isolation between the different components that form on same chip

Base formation During the base formation process

a new layer of SiO2 is formed over the wafer

The new pattern of opening creating depending upon the circuit need.

In this opening p-type impurity is diffused this act as base region for an transistor and as well as resister or anode for diode or junction capacitor.

Emitter diffusion

For emitter diffusion again a layer of SiO2 is formed over entire surface again new pattern of opening creating depending upon the circuit need .

In the opening in desired region n-type impurity is diffused this act as the emitter for the transistor .

Metallization

Metallization is done and providing bonding pads around the circumference of the chip for the later connection of wires.

Al is used for metallization

Al is deposited by vacuum evaporation technique and etched for the desired region by using photolithography.

Circuit probingEach IC on the wafer is

checked electrically for the proper performance probes .

Faulty chips are marked and discard after the wafer has been scribed and broken down into the individual chips.

Scribing and separating into the

chips

After the circuit probing ,wafer is broken into the individual chips containing the integrated circuits. For this purpose, wafer s are first scribed with a diamond tipped tool and than separated into the single chip.

Mounting and packingThe individual chip is

very small and brittle. Hence it is soldered on a gold plate header trough witch leads are already connected

Summary of Fabrication

Limitation to film & monolithic ICs

Transistor and diode can not formed by using film technique

Inductor cannot formed by using monolithic technique

Hybrid ICs Such circuits are formed either by

inter-connecting a number of individual chips or by combination of monolithic and film ICs.

In such IC the transistors & diode are first formed within the silicon wafer (using monolithic technique).

which is subsequently covered with an insulating layer such as SiO2 .

Film technique is than employed to form the resister,inductor) components on the SiO2 surface.

Connections are made from film to monolithic through the windows cut into the SiO2 layer.

Thanks

Any question..??