Introduction To

Microelectronics

Ravi Dadsena

Microelectronics & Integrated CircuitsMicroelectronics-

• It is defined as that area of technology associated with and applied to the realization of electronic systems made of extremely small electronic parts or elements.

• The term microelectronics is normally associated with integrated circuits (IC).

• As the name suggest ue related to the study and manufacturing(fabrication) of very small electronic design & components. These device are made of semiconductor materials.Microelectronicsinclude active(transistors & diodes) and passive(R,L,C) components.

Integrated Ckt-

• It is an electronic assembly built in such a way that all the components in the ckt are fabricated on a single container called the chip.(For interconnection metallization is used)

• Silicon is generally used for IC fabrication.

• Temperature range Si-150◦C,Ge-100◦C,low leakage current,abundance.

Major process used in IC fabrication

Classification of ICS

a) Based on application :

• Linear ICS : These are discrete circuits which need external components like resistor and capacitor for satisfactory performance apart from the input.Theywork on analog signals rather than digital signals.eg-op-amp,ADC,DAC,Timer(555).

• Digital ICS : They require only digital input and not any other component for operation.eg-all ics likelogic gates,adders,comparaters etc.

b) Based on fabrication techniques :

• Monolithic ICS : All the active and passive components of a circuit can be fabricated on a single piece of slicon.(Metallization is used for interconnection). In this ic fabrication process is complex as this is used for fabrication of large value of components.It involves many different fabrication techniques such as oxidation,ion implantation,deposition of new material etc.eg-uchip,computerchip,silicon chip,ic.

• Hybrid ICS : Separate components parts are attached to a ceramic substrate and interconnected by means of either metallization pattern or wire bonds.

The only difference between monolithic & hybrid is their manufacturing (fabrication) and construction.

Based on technology :

a) BJT – BIPOLAR

MOSFET – NMOS (N-Channel MOS)

PMOS (P-Channel MOS)

CMOS (Complimentary MOS)

BICMOS

Based on device count :

Nomenclature Active Device Typical function

SSI 1-100 gates,opamps

MSI 100-1000 Registers,filters etc

LSI 1000-100000 Up,a/d

VLSI 10*5 to 10*6 Memories,computers,signal processors

ULSI 10*6 Ultra large scale integration(used in japan)

IC Production ProcessFor fabrication of any ICS i.e. monolithic or hybrid various process are :

a) NMOS Process

b) CMOS Process

c) Bipolar Process

Processing Steps :

1) Crystal preparation : The substrate of ICS is generaliy a single crystal of silicon of high purity which is lightly doped with impurities of n-type for a p-type material and vice versa.

2) Masking(pattern generation) : Masks are produced by phtolithographic process. In this process photoresist is applied to entire surface of wafer.Its physical characterstics can be changed by exposure to light.In some methods laser beam or E-beam may be used for developing the mask on the surface of wafer.2 types of photoresist are there negative and positive photoresist.

3) Deposition : Films of various materials must be applied to the wafer during processing,eg insulators,resistive films,conductive films,n and p type semiconductor materials,dopants.For deposition we use CVD(chemical vapourdeposition) or PVD(physical vapour deposition).

4) Etching : Now by etching we remove the unwanted material from the substrate by using photoresist and masks with the help of UV light.

1) Diffusion : It refers to controlled forced migration of impurities into the substrate of adjacent material.Diffusion will occur between any silicon that has different densities of impurity with impurity tending to diffuse from high conc. area to low conc. area.Deposition and implantattion are different diffusion techniques.

2) Oxidation : For producing SiO2 layer oxidation process is carried out so that oxide is grown on the surface of substrate.This serves as a very good insulator.Oxidation process consumes silicon.2 common approaches are oxidation and dry oxidation.

3) Epitaxy (carried out only in bipolar process) : This involves growing a single crystal film on the silicon surface.This is done by CVD.

4) Conductors and resistors : For interconnection of components on an IC aluminium or other metals are generally used.

5) Packaging & testing : After processing,the ICS are tested and packaged.

IC Fabrication Process

Advantages of ICS over discrete components• Fabrication process is simple.

• Low power consumption.

• Production will be in bulk quantity so that cost is getting reduced.

• More no of components will be fabricated in a single chip of IC.So size will be reduced.

• Heat dissipation is reduced.

• More reliable.

• Space is less.

• Weight is less.

• Cost of production of electronic ckts due to batch production is reduced.

Disadvantages :

• Inductors and transformers cannot be fabricated.

• Capacitors with values more than 30pf cannot be fabricated on chip.

• Cannot withstand rough handling and excessive heat.(If we increase the current, heat dissipation will be increased).

Bipolar and MOS Technology1) Bipolar device : The basic active device used is BJT. The speed is very high with

bipolar process.

2) MOS Device :

• NMOS : primarily used in many applications, as mobility of p-type material is poorer than n-type.Also it has excellent density and good performance.

• PMOS : It is dual of NMOS. The basic active device is the p-channel MOSFET.

• CMOS : Complementary MOS : It comprises both PMOS & NMOS.CMOS offers improvement in power dissipation and performance when compared with NMOS.

• BiCMOS : It is a combination of bipolar CMOS transistors Therefore in BiCMOStechnology we can have the advantages of both bipolar & CMOS technology.Thisseems to be effective in speeding up VLSI ckts.

1) Bipolar device : The basic active device are NPN,PNP and BJT,UJT.

• Unipolar junction transistor(UJT) : Current conduction is due to only one type of charge carriers i.e.majority charge carriers.

• Bipolar junction transistors : Current conduction is due to both the type of charge carriers i.e. holes & electrons are majority and minority charge carriers.

NPN & PNP TransistorsTransistor can raise the strength of a weak signal hence it is used for amplification.Itconsist of 2 PN junctionwith junctions formed by sandwiching either p-type or n-type semiconductor layers a pair of opposite types.

MOSFETMOSFET,gate is isolated from channel so –ve and +ve VGS can be applied.InMOSFET gate is insulated from its conducting channel by an ultra thin metal oxide insulating film (SiO2).Hence it is called as IGFET.

a) Depletion Enhancement MOSFET

b) Enhancement only MOSFET

Enhancement only MOSFET : It operates only in enhancement mode and has no deplition mode.It has no channel between drain and source.

Working • When Vgs=0 or Vgs<Vt then Id=0 i.e. drain current is 0.

• When Vds is applied or Vgs>Vt then Id flows i.e. drain current starts flowinf and it is equal to

Id=K(Vgs-Vt)*2

K=design or conductivity parameter.

Transfer characterstics :

Fabrication of Active Elements(Transistors & diodes)

Fabrication of transistor :

Transistor manufacturing process

Diodes manufacturing process

Fabrication of Passive Elements(Resistor ,Capacitors & Inductors)

1. Silicon wafer (substrate) preparation2. Epitaxial growth3. Oxidation4. Photolithography5. Diffusion6. Ion implantation7. Isolation technique8. Metallization9. Assembly processing & packagingResistor :

• A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element.

• The current through a resistor is in direct proportion to the voltage across the resistor's terminals. This relationship is represented by Ohm’s law (I=V/R)

• I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms.

Resistance of a material

IC Resistor

Capacitors (Farads)• Capacitor is a basic storage device to store electrical charges and release it as it is

required by the circuit. In a simple form it is made of two conductive plates(Electrodes) and an insulating media (Dielectrics) which separate the electrodes.

• The charges (Q) on the capacitor plates depend on the voltage (V)and the capacitance value (C) and is as follows:

Q=C.V

n-Si

SiO2

n-SiGROW 4000 A OXIDE ON N-TYPE SILICON MATERIAL

PATTERN AND ETCH OXIDE<THEN ION IMPLANT 50 KeV BORON TO FROM P+ REGION

Thin SiO2Al

n-Si n-SiGROW THIN OXIDE (2000 A) PATTERN AND ETCH OXIDE TO FROM CONTACT

DEPOSIT 5000A ALUMINUM PATTERN AND ETCH ALUMINUM

Inductors (henry)• An inductor (also choke, coil or reactor) is a passive two terminal electrical

component that stores energy in its magnetic field. For comparison,a capacitor stores energy in an electric field, and a resistor does not storeenergy but rather dissipates energy as heat.

• Any conductor has inductance. An inductor is typically made of a wire or otherconductor wound into a coil, to increase the magnetic field.

Uncertainties in IC Fabrication• The precision of transistors and passive components fabricated using IC

technology is surprisingly, poor.

Sources of variations:

• Ion impant dose varies from point to point over the wafer and from wafer towafer.

• Thicknesses of layers after annealing vary due to temperature variations acrossthe wafer.

• Widths of regions vary systematically due to imperfect wafer flatness (leadingto focus problems) and randomly due to raggedness in the photoresist edgesafter development.

PACKAGING TECHNIQUESOnce the IC has been produced, it requires a housing that will protect it from damage. This damage could result from moisture, dirt, heat, radiation, or other sources. The housing protects the device and aids in its handling and connection into the system in which the IC is used.The three most common types of packages are TRANSISTOR-OUTLINE (TO) PACKAGE, FLAT PACK, DUAL INLINE PACKAGE (DIP).

DUAL INLINE PACKAGE : The dual inline package (DIP) was designed primarily to overcome the difficulties associated with handling and inserting packages into mounting boards. DIPs are easily inserted by hand or machine and require no spreaders, spacers, insulators, or lead-forming tools. Standard hand tools and soldering irons can be used to field-service the devices. Plastic DIPs are finding wide use in commercial applications, and a number of military systems are incorporating ceramic DIPS.

Transitor-Outline Package :The transistor-outline (TO) package was developed from early experience with transistors. It was a reliable package that only required increasing the number of leads to make it useful for ICs. Leads normally number between 2 and 12, with 10 being the most common for IC applications. Once the IC has been attached to the header, bonding wires are used to attach the IC to the leads. The cover provides the necessary protection for the device. You can easily see that the handling of an IC without packaging would be difficult for a technician.

Flat packMany types of IC flat packs are being produced in various sizes and materials. These packages are available in square, rectangular, oval, and circular configurations with 10 to 60 external leads. They may be made of metal, ceramic, epoxy, glass, or combinations of those materials.

SSI,MSI,LSI & VLSI• SSI : Small Scale Integration

• MSI : Medium Scale Integration

• LSI : Large Scale Integration

• VLSI : Very Large Scale Integration

VLSIVery Large Scale Integration

• design/manufacturing of extremely small, complex circuitry using modified semiconductor material

• integrated circuit (IC) may contain millions of transistors, each a few mm in size

• applications wide ranging: most electronic logic devices.

Origins of VLSI• Much development motivated by WWII need for improved electronics,

especially for radar

• 1940 - Russell Ohl (Bell Laboratories) - first pn junction

• 1948 - Shockley, Bardeen, Brattain (Bell Laboratories) - first transistor

• 1956 Nobel Physics Prize

• Late 1950s - purification of Si advances to acceptable levels for use in electronics

• 1958 - Seymour Cray (Control Data Corporation) - first transistorized computer -CDC 1604.

• 1959 - Jack St. Claire Kilby (Texas Instruments) - first integrated circuit - 10 components on 9 mm2

• 1959 - Robert Norton Noyce (founder, Fairchild Semiconductor) - improved integrated circuit.

• 1968 - Noyce, Gordon E. Moore found Intel.

• 1971 - Ted Hoff (Intel) - first microprocessor (4004) - 2300 transistors on 9 mm2

• Since then - continued improvement in technology has allowed for increased performance as predicted by Moore’s Law.

Three Dimensional VLSI• The fabrication of a single integrated circuit whose functional parts (transistors,

etc) extend in three dimensions.

• The vertical orientation of several bare integrated circuits in a single package.

Advantages of VLSI :

• Low power dissipation.

• Area consumption is less.

• Power consumption is less.

• High speed of operation.