Monday, November 5, 2001		On Hardware

Small is powerful if it has VLSI chip
Shobha Jindal

PARADOXICALLY in this big world, small is trendy. The keyword is miniaturisation. The world of mobile phones, pen cameras, watches, laptop and palm-top computers is getting hi-powered in delivering the output and is getting smaller in size. Such miniaturisation revolves around an electronic silicon chip called the VLSI (Very Large Scale Integrated) Chip.

Sand that has been refined to produce 99.9999 per cent of pure silicon is the raw material used for this chip. Silicon has a unique property to change the flow of electricity passing through it by a process called doping. This means that it can act both as a good conductor as well as an insulator. Therefore it is also known as semiconductor.

Broadly speaking, the process of VLSI chip development can be divided into VLSI chip design and VLSI chip fabrication. Whereas the VLSI chip fabrication is highly sophisticated, expensive and largely automated, it is the VLSI chip design that holds the key to the success of the VLSI chip.

The process of the VLSI chip design involves extensive use of EDA (Electronic Design Automation) software on computers to design circuits from an idea or a concept.

VLSI chip design is an extremely important function in the whole process of VLSI chip fabrication. It is more so because of the formidable cost of VLSI chip fabrication. Whereas the fabrication is a highly automated process and works in batch mode to produce devices in volumes, the chip design is manual process.

The process of fabrication is a highly complex process carried out under extremely stringent parameters of temperature, humidity, cleanliness and infrastructure stability. When you are dealing with 0.6 or 0.8 micron, even a speck of dust or a little vibration can cause havoc with the process. Although the complete VLSI chip fabrication process is highly complex in nature, one can attempt to divide this in following basic steps:

Step 1: Silicon wafer is heated in an oxygen rich furnace so that a layer of silicon oxide is formed on the top surface of wafer.

Step 2: A thin coating of photographic emulsion is applied over the oxidised surface of the wafer. This is followed by a dry and bake stage so that the photographic emulsion also known as photo resist hardens.

Step 3: At this stage the working photomask is generated from the VLSI Design.

Through a chain of process, optical reticle is generated that is further processed to generate working photomasks. These masks are used in wafer fabrication for selective etching of silicon wafer. Photomask is aligned with the wafer and is exposed to light so that photo resist surface of the wafer gets selectively exposed and developed as per the photo mask design.

Step 4: A highly sophisticated chemical process selectively etches the surface of the wafer leaving behind the parts that are masked by the resist material. When the required etch depth is achieved the wafer is stripped of the resist and washed clean. This process is repeated for successive masking layers.

Step 5: The complete Integrated Circuit is made up of the transistors. Now the transistors have to be formed on the silicon wafer. A process known as doping achieves this. By doping the conduction or resistance, silicon wafer is modified so that transistors can be formed.

Step 6: In order to produce a functional circuit, the transistors formed by doping are to be interconnected. This is achieved by depositing fine wire traces made from aluminum or copper to connect up the transistors in a correct way.

Step 7: The processed wafer goes for testing as per the test program where each die on a wafer is probed and marked as good or bad die.

Step 8: The wafer is scribed and broken into dice. Good dice go for packaging.

Step 9: Good dice are packaged as per the packaging requirement.

The VLSI chip design is a highly rewarding global career. This has resulted in a large number of Indian VLSI chip design houses catering to the global market and so also many a manufacturing companies the world over has set up a VLSI chip design centre in India of their own or through joint ventures.

In India, although there are a large number of VLSI Design houses, there are only two VLSI fabrication facilities that is Semiconductor Complex Limited at SAS Nagar, Punjab, and ITI, Bangalore.

Other leading names are IBM Global Services, Lucent Technologies, Philips Semiconductors, Texas Instruments, Motorola India, Cypress Semiconductors, HCL Technologies, ST Microelectronics, GE India, Bi Square Consultants DCM Data Systems, Artech Information Systems, Alliance Semiconductors, Analog Devices, Core El Logic Systems, Sage Design Systems, Infineon Technologies, Wipro Limited and Centillium India Pvt Ltd.