introduction to ic technology iv ece -26the june

7/27/2019 Introduction to IC Technology IV ECE -26the June

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Basic VLSI Design

B LOKESHWARAssistant Professor

ECE DepartmentRVR & JC College of Engineering

Outlines

Introduction to IC Technology

Introduction to MOS Technology

MOSFETs ModesNMOS Fabrication

CMOS Fabrication

BiCMOS Fabrication

Introduction to IC technology:• It is a circuit where all discrete components

such as passive as well as active elements arefabricated on a single crystal chip

• Until 1950, electronic technology dominated by vacuum tube

• The first semiconductor chip held twotransistors each

Introduction to IC technologyAs on increasing the number of components(or

transistors) per IC the technology was developed

ULSI (Advances in VLSI)

Integrated Circuit Evolution

Year Technology Transistors/chip Examples1947 Invention of the

transistor 1 -

1950 DiscreteComponents

1 Tran and Diode

1961 SSI 10-100 Logic gates and flipflops

1966 MSI 100-1000 Counters andregisters

1971 LSI 1000-20000 8 bit mp, ROM,

RAM1980 VLSI 20000- 100000 16,32 bit mp

1990 USLI 100000 -1million Special purpose processors

2000 GSI More than 1 million Embedded system,

system on chip

VLSI(Very large scale integration)is the process of created integrated circuits by

combining thousands of transistors into a single chip.

VLSI begins in the 1970’s when complexsemiconductor and communication technologies were

being developed.

The microprocessor is a VLSI device.

Very large scale integration( Cont..)VLSI chips are widely used in various branches of engineering like

Digital signal processing.

Wireless LAN

Bluetooth

Bus interface via PCI,USB.

Commercial electronics: TV sets, DVD.

Computers and computer graphics.

Automobiles, toys.

Medicine: Hearing aids, implants for human body

Moore’s LawIn 1965, Gordon Moore, an industry pioneer,

predicted that the number of transistors on a chip

doubled every 18to 24 months.He also predict that semiconductor technology will

double its effectiveness every 18 months

Many other factors also grow exponentially those are – clock frequency

– processor performance

Moore’s Law(Cont..)Transistors count will be doubled for every 18

months

Introduction to MOS TechnologyWhatever the IC (it may be simple logic gate,

microprocessor), that we are seeing outside can be

manufactured using different MOS technologies.

The devices by which we can fabricate the different

types of ICs

These devices can be any type of the following:

Introduction to MOS Technology(Cont..)

Comparison of available Technologies

Introduction to MOS Technology(Cont..)To know about each type of MOS technology, we just

look about to know transistor, types of transistors.

Transistor:A transistor is a semiconductor device used to amplify and

switch electronic signals and power.

It is composed of a semiconductor material with at leastthree terminals for connection to an external circuit.

Today, some transistors are packaged individually, but many

more are found embedded in integrated circuits.

Types of Transistors:BJT:

Problems:• Low

resistance• Noise

Problems:

• gate must be reversebiased(decreasing theconductivity)

• Called depletion mode of transistor)

Channel width decreased

MOSFETHowever, there is a FET that can be operated to

enhance the width of the channel(Enhancement Mode)

Such FET is called MOSFET(depends on mode of operation.)

MO S F E T

En hancemen t

Mode MO S F E

MOSFETPrinciple :

is that the source-to-drain current(SD current) is

controlled by the gate voltage, or better, by the gateelectric field.

Heart: MOS Capacitor

MOS Capacitor

Figure 1: A Parallel Platecapacitor

Figure 2: A corresponding

MOS capacitor

Figure 3: The MOScapacitor withaccumulation of holes

• If the electric field penetrates the

semiconductor, the holes in the p-type

semiconductor will experience a force

toward the semiconductor interface

MOS Capacitor(Cont..)

Figure 1: Effect of +Vegate bias

Figure 2: Induced space charge

region due to moderate +veate bias

Figure 3: Due to larger+ve gate bias

MOS Capacitor(Cont..)When a larger positive voltage is applied to the gate,

the magnitude of the induced electric field increases.

Minority carrier electrons are attracted to the oxidesemiconductor interface, as shown in Figure

This region of minority carrier electrons is called an

electron inversion layer. The magnitude of the

charge in the inversion layer is a function of theapplied gate voltage.

MOSFET(Cont..)Therefore , apply the concepts of concepts of an inversion

layer charge in a MOS capacitor to create a transistor.

Such transistor is called Metal Oxide Semiconductor Field

Effect Transistor

Also called Insulated Gate FET

Depending upon the type of mode, it can be classified into

Depletion mode MOSFET

Enhancement Mode MOSFET

MOSFET(Cont..)Enhancement Mode:

The term enhancement mode means that a voltage must

be applied to the gate to create an inversion layer.For the MOS capacitor with a p-type substrate, a positive

gate voltage must be applied to create the electron inversion

layer.(nMOS E-MOSFET)

For the MOS capacitor with an n-type substrate, a negative

gate voltage must be applied to create the hole inversion

layer.(pMOS E-MOSFET)

MOSFET(Cont..)Depletion Mode:

The term depletion mode means that a channel exists

even at zero gate voltage.A negative gate voltage must be applied to the n-channel

depletion mode MOSFET to turn the device off.(much

behave like n channel FET)

NMOS D-MOSFET

PMOS D-MOSFET

MOSFET(Cont..)Channel is established due to the implant even when

Vgs = 0 and the channel can be cut off by applying a

negative voltage between the gate and source.

Enhancement Mode Transistor

p-diffusion

n-diffusion

Poly silicon

p-Substrate

n-Substrate

depletion

Symbols:

NMOS E-Mode Transistor

Figure: nMOS Enhancement modetransistor

• Lightly doped p-type

substrate• Tow heavily doped N+

regions are diffused• Thin Sio2 is grown over the

entire surface

Operation:1. When Vgs < Vtn no inversion layer no conduction

2. When Vgs > Vtn electron inversion layer

but no conduction since Vds=0

5. When Vgs > Vtnelectron inversion layer conductiontakes place

but the transistor in non saturation region since Vds< Vgs-

6. When Vgs > Vtn

electron inversion layer

conductiontakes place

but the transistor in saturation region since Vds >Vgs-Vtn

Where Vgs-Vtn= Vds(sat)

Operation :

Operation(Cont..)

Characteristics:

Figure: VI Characteristics of n channel E-

MOSFET

Silicon wafer Fabrication

n-MOS fabrication process

1) Process is carried out on a thin wafer (75 to150 mm dia, .4mm thick)

Doped with boron impurity concentration of 1015/cm3 to make substrate

Substrate

n-MOS fabrication process

2) A layer of SiO2 grown all over the surface(1µm thick) to protect.

SubstrateSiO2

RVR & JC College of Engineering- B

Photolithography

3) Then the oxidized wafer is covered withPhoto resist.

SubstrateSiO2

Photo-Resist

Photolithography

4) Now the wafer is exposed to UV Lightthrough a photo mask to define regions.

SubstrateSiO2

Photo-Resist

Photo-Mas

Photolithography

5.1) Now oxide which is unprotected fromphotoresist is etched away.

SubstrateSiO2

Photolithography

5.2) The rest of the photo resist is removed. Then the further fabrication process iscarried out, say doping.

SubstrateSiO2 SiO2

6) Thin layer of SiO2(0.1µm) grown and thenpolysilicon is deposited on top to form gatestructur.

6) Polysilicon layer consists of heavily doped

polysilicon depositd by CVD.

Substrate

SiO2 Thinox

Polysilicon on thin oxid

Poly Si

Metallization

7) Use mask to remove exposed area intowhich n-type impurities(phosphorus) are tobe diffused to form source and drain.

Substrate

Thinox

Poly Si

Metallization

8) Again thinox is grown all over the surfaceand is then masked to expose selectedareas of gate, drain and source to makeContact holes (cut)

Substrate

SiO2 Thinox

Poly Si

Metallization

9) Finally, whole chip is metal deposited overits surface for required interconnectionpattern.

Substrate

SiO2 Thinox

Poly Si

nMOS FabricationStep 1:

Step 2:

Step 3:

nMOS FabricationStep 4:

Step 5:

nMOS Fabrication

CMOS fabricationWhen we need to fabricate both nMOS and pMOS

transistors on the same substrate we need to followdifferent processes.

The three different processes are , P-well process ,N-well process and Twin tub process.

CMOS fabricationTo accommodate both pMOS and nMOS devices, special

regions must be created in which the we place semiconductor

is opposite to the substrate type.

These regions are called wells or tubes

A p-well is created in the p-type substrate, alternatively an n-

well is created in the n-type substrate

CMOS fabricationIn the simple n-well CMOS fabrication technology

The nMOS transistor is crated in the p-type substrate and

The pMOS transistor is crated in the n-well which is built in into the P-type

substrate

Similarly, in the p-well CMOS fabrication technology

The pMOS transistor is crated in the n-type substrate and

The nMOS transistor is crated in the p-well which is built in into the p-type

substrate

N-well process- Flow chart

Main step in a typical n-well process

Formation of n-well regions

Define nMOS and pMOS active areas

Field and gate oxidations (thinox)

Form and pattern polysilicon

p+ diffusion

n+ diffusion

Contact cuts

Deposit and pattern metallization

Over glass with cuts for bonding pads

Main step in a typical n well process

Poly silicon patterning

P-diffusion

Metallization

Mask sequence.

Mask 1: Defines the areas in which the deep p-well

diffusion takes place. Mask 2: Defines the thin oxide region (where the thick

oxide is to be removed or stripped and thin oxide grown) Mask 3: It’s used to pattern the polysilicon layer which is

deposited after thin oxide.

P-well process

Mask 4: A p+ mask (anded with mask 2) to define areas

where p-diffusion is to take place.

Mask 5: We are using the –ve form of mask 4 (p+ mask) It

defines where n-diffusion is to take place.

Mask 6: Contact cuts are defined using this mask.

Mask 7: The metal layer pattern is defined by this mask.

Mask 8: An overall passivation (overglass) is now applied and italso defines openings for accessing pads.

n-Well CMOS

Twin-tub COMSn-type material and then we create both n-well and p-

well region.

Bi-CMOSDriving capability of MOS transistors is less

Bi-CMOS technology capable to drive large

capacitive loads.

1. The npn transistor is formed an n-well & the additional

p+ base region is located in the well to form the p-base

region of the transistor.

2. The second additional layer, the buried n+ subcollector (BCCD) is added to reduce the n-well (collector)

resistance & thus improve the quality of the bipolar

transistor .

Bi-CMOS

Comparison between CMOS and

pBipolar technologies

Low static power dissipation

High input impedance

Scalable threshold voltage

High noise margin

High packing density

High delay sensitivity to load

Low output drive current

Low gm

Bidirectional capability

A near ideal switching device

High power dissipation

Low input impedance

Low voltage swing logic

Low packing density

Low delay sensitivity to load

High output drive current

High gm

Essentially unidirectional

CMOS Bipolar technologies

cost versus delay graph

CMOS for logic

BiCMOS for I/O

ECL for critical highspeed parts

introduction to ic technology iv ece -26the june

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