By Peter Wilson



The process of making silicon chips is as complex as the chips themselves. Each manufacturing plant, or "fab", may cost billions of dollars and is a triumph of engineering. But working inside these hi-tech plants can be a surreal experience, says Dr Peter Wilson of the University of Southampton. "Fab world" is like no other place on earth. Its pristine white walls, secure air locks, sterile air and ethereal yellow lighting makes it seem like you have arrived in the belly of an orbiting space station. I can still remember the first time I went there. It was set in classic "tumbleweed" territory - a small town in Arizona with just one road and the factory. The temperature was over 100 degrees outside, with dust everywhere, but when you crossed the threshold into the plant, the air-conditioning kicked in and you felt like you were in a different world. This is a common experience to anyone who works in the silicon manufacturing sector. The world outside and the fab world inside are on two different planes. The boundary can transcend geographic and political boundaries - it can become impossible to tell which country you are in, when everyone is wearing a mask, and is dressed head to foot in shapeless, white hooded-suits. 'Bunny men' Outside, we worry about dirt on our shoes and wipe our feet, or perhaps wipe some dust off our laptop screen. In fab world, we worry about a few atoms contaminating the environment. If dust falls on the delicate silicon wafers on which chips are printed it can render them useless. HOW A MICROCHIP IS MADE Microchips are built from wafers that consist of 99.9% pure silicon. The silicon is made from common beach sand. The chips are made in incredibly clean environments - the air is more than 1000 times purer than that of a hospital. The silicon wafers are produced by a specialist company and sent to the chip manufacturer for processing. The Silicon wafer consists of 4 layers. A mask that is created during the design phase defines the cicuit pattern. A mask is placed over the wafer, under a UV light. Patterns are repeatedly projected on the wafer, light can only reach the wafer through the openings. The UV light reacts with the photoresist to create the circuit patterns. Impurities are then implanted into areas of the wafer to alter the electrical properties of specific regions. This is called doping. Electrical contacts are formed by masking and etching the wafer to provide links between the different layers. Multiple layers of metal are applied to form the electrical connections between the chips's layers. The final chip is housed in a protective case that contains wires to connect it to the computer's circuit boards. BACK NEXT 1 of 11 Modern transistors - the tiny switches at the heart of these devices - are described in terms of the smallest feature sizes that can be made, such as a 45 nanometres, or 45 billionths of a meter. To put this in perspective, the average human hair will be between 20 and 100 micrometers across - over a thousand times larger - and a typical dust particle will be anything from 1 to 100 micrometres. Dust and contaminants must be kept out. The fab is a place for chips, not for people. As a result, only the pure and the clean are given permission to penetrate its' inner chambers. Anyone that enters must go through a strict set of procedures. All of the trappings of the outside world must be left behind, whether clothes, jewellery or even make-up. A series of ante-chambers serve as prep rooms where workers change into a series of gowns and gloves, collectively known as a "bunny suit". Sticky floors make sure that no one treads in any contaminants and an air shower before entry makes certain that any loose particles are stripped away. Skin flakes, lint, hair and anything else gets sucked into the grate in the floor. Pure products And then it's onwards into the hum of the clean rooms. Stark white walls reflect the yellow sodium lights from above and a constant breeze blows down from the ceiling taking any particles through the gridded floor. Fab world is an expensive place and, hence, it never stops

Peter Wilson

Sand to silicon chip In modern fabs, ultra high tech chips are manufactured in what are known as class 1 rooms that contain just one tiny particle per metre cubed. In contrast, a room where open heart surgery takes place may have as many as 20,000. Everything taken in either needs to be cleaned with alcohol or specially designed. Even the paper we use to take notes is designed from a special lint-free material. Inside, humans very rarely come into contact with the rainbow-streaked discs of reflective silicon on which the chips are cut. Instead, they are there to trouble shoot and monitor that everything goes correctly. The silicon wafers are handled on monorails that move above the fab floor and the processing is done by complex vacuum sealed robots. The wafers enter one end of the line costing a couple of hundred dollars and appear at the other - weeks later - patterned with billions of transistors and worth tens of thousands of pounds. The silicon itself is not made at the fab - the ultra pure ingots (up to 99.99999999% pure) are produced and cut by specialist companies and sold to the chip makers. The fab world's magic is creating the incredibly complex patterns of wires and circuitry on chips the size of a postage stamp time and time again That alchemy can cost billions of dollars. Non-Stop Each layer of a processor is constructed using a mask which is like a stencil, to highlight the areas to be deposited, etched or doped. Each plant can cost billions of dollars

Nano chip designer Doping involves adding impurities to the silicon to change its electrical characteristics - something which has to be done with astonishing precision. Each mask used to cost several thousand pounds but as the complexity of chips has increased, and the smallest possible feature size has reduced, the number and intricacy of these masks has increased. In addition, the size of individual features is now smaller than the wavelength of light that used to be used to pattern them, which means the use of some clever optics is required. The yellowish lights used inside the fab are to make sure that they do not interfere with this process. The result of all of this is that an individual silicon integrated circuit may require masks that cost hundreds of thousands of pounds, or perhaps even millions of pounds, to produce and machines that cost a similar amount. Fab world is an expensive place and, hence, it never stops. The plants churn out chips every single day of every year. So called giga-fabs may process more than 100,000 wafers every month, each containing hundreds of chips. Each one of the 10mm by 10mm silicon squares is a triumph of design. As a chip designer, the impact of the incredible complexity of fab world has led to an amazing transformation in what we can do on a single chip. The products of this strange and surreal place have burst out of its confines and have pervaded every facet of the outside world from computers and mobile phones to aircraft and microwave ovens. Yet, incredible as it is to visit, fab world is also a place that is blissful to leave. At the end of the day there's no better feeling than being able to rip off the itchy bunny suit, step outside into the searing heat and once again get dirty.

Dr Peter Wilson is Senior Lecturer in Electronics at the University of Southampton, School of Electronics and Computer Science.



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