INTEGRATED CIRCUIT (IC) FABRICATION TECHNOLOGY

INTRODUCTIONAn Integrated circuits (ICs) are electronics.ICs form a branch of the microelectronics which originated in the late 1950s.At that time the military was interested in reducing the size of its electronic equipment to approx- 1/10th of its initial volume.

EVOLUTION OF ICsBefore the integrated circuit, assembly workers had to construct circuits by hand, soldering each component in place and connecting them with metal wires or they are assembled on PCB. These are discrete circuits.The discrete circuits suffer from two main disadvantages- Large space & Reliability.To meet the problems of space conservation & reliability, a new branch of electronics known as microelectronics was developed in the late 1950s. The microelectronics deals with microcircuits. One type of such circuit is IC.

Jack Kilby is probably most famous for his invention of the integrated circuit, for which he received the Nobel Prize in Physics in the year 2000. After his success with the integrated circuit Kilby stayed with Texas Instruments and, among other things, he led the team that invented the hand-held calculator.

Photo: Texas Instruments

Kilbys first chip Photo: Texas Instruments

What is an integrated circuit?

An integrated circuit is also known as IC, microcircuit, microchip, silicon chip, or chip . It is a miniature electronic circuit that consists of semiconductor devices, as well as passive components, that has been manufactured in the surface of a thin substrate of semiconductor material.

ADVANTAGES OF ICsSize- sub-micron vs. mm/cm. Systems are physically smaller, e.g.- cell phones.IC amplifiers provide better high frequency performance because of the reduced size of the components and of shorter lengths of connecting leads.The IC is much more reliable due to absence of solder joints.Low cost because a large no: of circuits or components cab be produced simultaneously on a small semi-conductor wafer.Speed & Power- Smaller size of IC components yield higher speed & lower power consumption due to smaller parasitic capacitances, resistances, inductances.

Silicon Wafer

DISADVANTAGES OF ICsThe inductors & transformers cannot be fabricated.Capacitors & Resistors are limited in maximum value.It is not possible to produce high power ICs ( greater than 10 W ).If any component of the IC goes out of order, the whole IC has to be replaced by new one.

ADVANCES IN ICsAmong the most advanced integrated circuits are the microprocessors or "cores", which control everything from computers to cellular phones to digital microwave ovens. Digital memory chips and ASICs are examples of other families of integrated circuits that are important to the modern information society. While the cost of designing and developing a complex integrated circuit is quite high, when spread across typically millions of production units the individual IC cost is minimized. The performance of ICs is high because the small size allows short traces which in turn allows low power logic (such as CMOS) to be used at fast switching speeds.

CLSSIFICATION OF ICsFrom fabrication point of view, ICs are-Monolithic ICsThick & Thin film ICsHybrid ICs

From functional point of view, ICs are-Liner ICsDigital ICs

Linear & Digital ICsCMOS DIP ICOpamp2-input NAND IC64-bit static RAM

Digital ICsDigital ICs operate at only a few defined levels or states, rather than over a continuous range of signal amplitudes. These devices are used in computers, computer networks, modems, and frequency counters. The fundamental building blocks of digital ICs are logic gates, which work with binary data, that is, signals that have only two different states, called low (logic 0) and high (logic 1).

Linear ICsLinear ICs have continuously variable output (theoretically capable of attaining an infinite number of states) that depends on the input signal level. As the term implies, the output signal level is a linear function of the input signal level. Ideally, when the instantaneous output is graphed against the instantaneous input, the plot appears as a straight line.

Linear ICs are used as audio-frequency (AF) and radio-frequency (RF) amplifiers. The operational amplifier (op-amp) is a common device in these applications. Others are-linear regulators, active filters, phase locked loop, oscilators.

Monolithic ICThe word monolithic is derived from the Greek mono, meaningsingle and litho, meaning stone. Thus monolithic circuit is built into a single stone orsingle crystal i.e. in monolithic ICs, all circuit components, (both active and passive) andtheir interconnections are formed into or on the top of a single chip of silicon.

Commercially available ICs of this type can be used as amplifiers, voltage regulators, crowbars, AM receivers, TV circuits, and computer circuits.

Thick & Thin film ICsThese devices are larger than monolithic ICs butsmaller than discrete circuits.

These ICs can be used when power requirement is comparatively higher.

Therefore, commercially available thin- and thick-filmcircuits are combination of integrated and discrete components.

The essential difference between the thin- and thick-film ICs is not their relative thickness but the method of deposition of film. Both have similar appearance, properties and general characteristics.

Hybrid ICsA hybrid circuit serves as a component on a PCB in the same way as a monolithic integrated circuit; the difference between the two types of devices is in how they are constructed and manufactured. The advantage of hybrid circuits is that components which cannot be included in a monolithic IC can be used, e.g., capacitors of large value, wound components, crystals.

LEVEL OF INTEGRATIONSmall-scale integration (SSI)-3 to 30 gates/chip or 100 transistors/chip (e.g.-logic gates, flip-flops)

Medium-scale integration (MSI)- 30 to 300 gates/chip or 100 to 1000 transistors/chip (counters, multiplexers, adders)

Large-scale integration (LSI)- 300 to 3000 gates/chip or 1000-20,000 transistors/chip (8-bit up, ROM, RAM)

Very large-scale integration (VLSI)- more than 3000 gates/chip or 20,000-100,00,00 transistors/chip (16 & 32-bit up)

Ultra large-scale integration (ULSI)- more than 1 million (special processors- pentium processors, virtual reality machines, smart sensors.)

Basic Fabrication stepsLayer 1- Substrate upon which IC is built.(400um or 0.4mm)Layer 2- Epitaxial layer. All active & passive components are fabricated within this layer.(5 to 25um)Layer 3-Silicon dioxide i.e. insulation layer.(0.02-2um )Layer 4- Metallic layer of Al, used for obtaining interconnections b/w components.(~1um)

Logic Circuit Design / Layout DesignA 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 circuits operation.

IC Production in short

Clean RoomBut let us first take a look at the special place where integrated circuits are produced - the clean room.

Chip Production - in Detail Wafer PreparationEpitaxial GrowthInsulating layer- OxidationPhotolitography EtchingDiffusionMetallizationSeparating into chipsMounting & Packaging

Wafer FabricationA high-purity, single-crystal silicon called "99.999999999% (eleven-nine)" is grown from a seed to an ingot.The wafers are generally available in diameters of 150 mm, 200 mm, or 300 mm, and are mirror-polished and rinsed before shipment from the wafer manufacturer.

Epitaxial Growth

Insulation Layer & Photolithoraphy

Photoresist Coating A resin called "photoresist" is coated over the entire wafer. (~1m thick coating.) Photoresist is a special resin similar in behavior to photography films that changes properties when exposed to light.

Masking/ExposurePlaced over the photoresist-coated wafer, which is then irradiated to have the circuit diagram transcribed onto it. An irradiation device called the "stepper" is used to irradiate the wafer through the mask with ultraviolet (UV) light.

Patterning: DevelopmentThe photoresist chemically reacts and dissolves in the developing solution, only on the parts that were not masked during exposure (positive method). Development is performed with an alkaline developing solution. After the development, photoresist is left on the wafer surface in the shape of the mask pattern.

Diffusion

Aluminium Metalization

Dicing

In back end processing, a wafer completed in front end processing is cut into individual IC chips and encapsulated into packages.

MountingAfter the IC chips are cut apart, they are sealed into packages. The IC chips must first be attached to a platform called the "lead frame.

Characteristic SelectionThe packaged IC chips are tested and selected.

Printing and Lead FinishThe final step of IC chip manufacturing is the printing onto the package surface and the finishing of leads. After this step, the IC chips are complete.

COMPONENTS ON MONOLITHIC IC

Transistors

Diodes

APPLICATIONS OF ICsAn IC can function as an amplifier, oscillator, timer, counter, computer memory ( RAM, ROM, flash) or microprocessor, active filters, linear regulators. A particular IC is categorized as either linear (analog) or digital, depending on its intended application.

Integrated circuits are also found in almost every modern electrical device such as cars, television sets, CD players, cellular phones.

TYPES OF CHIPSDynamic Random Access Memory chips (DRAMs) - serve as the primary memory for computers Microprocessors (MPUs) - act as the brains of computers. Application Specific Integrated Circuits (ASICs) - are custom semiconductors designed for very specific functions Digital Signal Processors (DSPs) - process signals, such as image and sound signals or radar pulses. Programmable memory chips (EPROMs, EEPROMs, and Flash) - are used to perform functions that require programming on the chip.

REFERENCESElectronic Devices And Circuits By Allen MottersheadElectronic Devices And Circuits By Sanjeev GuptaLinear Integrated Circuits By D.Roy.Choudhury, Shail B.Jainhttp://google.com http://wikipedia.com