Background information: The birth of a semiconductor device

An integrated circuit is usually considered to be a small but very sophisticated device, capable of implementing an electronic function. An integrated circuit, in reality, is made up of two major parts: a tiny and very fragile silicon "chip" which implements electronic functions, and a "package" which is intended to protect the internal silicon chip and to provide the user with a practical way of handling the component itself.

The manufacturing phase of an integrated circuit can be split into two parts. The first, diffusion, is the extremely complicated and intricate process of "photoprinting" the silicon chip; the second, assembly, is the highly precise and automated process used to package the silicon before final testing of the integrated device takes place. Those two phases are commonly known as "front-end" and "back-end".

Initially, the silicon chip forms part of a very thin, round silicon slice: the wafer. Wafer diameters are typically 125mm, 150 mm or 200mm (5-, 6- or 8-inch). However raw pure silicon has a single electrical property: it is an isolating material. So some of the features of raw silicon have to be altered, by means of well controlled processes, in order to have a less "pure" form in the wafer. This is obtained by "doping" the silicon. Dopants (or doping atoms) are purposedly inserted in the silicon lattice, hence changing the features of the material in predefined areas and generating all the electronic building blocks (transistors, resistros, capacitors) which are present in electronic circuits.

Many different dopants are used to achieve these desired features: e.g. Phosphorous and Boron. The wafer is inserted into a furnace. Doping gases are then introduced which impregnate the silicon surface. The wafer will have been previously "masked" in some areas, in the same way one "masks out" or protects the windscreens of a car before painting the body. This part of the manufacturing process is commonly called diffusion.

One has to remember that, even if the process is somewhat similar to the painting of a car-body, in the case of a silicon chip the dimensions are measured in microns, i.e. one thousandth of a millimeter. This is the reason why one of the most commonly used masking methods calls for the photographic imprint of the mask itself onto the silicon surface.

The photomasking and diffusion process is repeated many times, using different masks and different dopants (or doping agents) at different temperatures in order to achieve all the impurity levels needed to produce the requested characteristics of the silicon chip.

Just to have an idea of how complicated this process is, one should bear in mind that the temperature in a diffusion furnace is over 1200ºC and is controlled to within plus or minus half a degree over the complete length of the furnace (about 1m) containing the "wafer shuttles" (capable of holding 25-50 wafers).

Diffusion takes place in an extremely clean environment, where air cleanliness is one million times better than the air we normally breath in a city, or some orders of magnitude better than the air in a heart transplant operating theater.

All these processes are part of the manufacturing phase of the chip itself. Silicon chips are grouped on a silicon wafer, in the same way postage stamps are printed on a single sheet of paper before being separated from each other.

Once the single electronic elements have been diffused on the silicon, and have been separated as previously mentioned, they have to be interconnected by means of wires, which enable electronic signals to be fed through the chip. On a surface smaller than a baby's nail we now have thousands (or millions) of electronic components, all of them interconnected and capable of implementing a subset of a complex electronic function. At this stage the device is completely functional, but it would be impossible to use it without some sort of supporting system. Any external variation of pressure would alter its behavior, any shock cause failure.

Wires thinner than a human hair are required to interconnect different chips. These are among the reasons why a chip is then mounted in a ceramic or plastic package. The package not only protects the chip from external shocks, but also makes the whole device of a more easy-to-handle size. At the end of the packaging process, the integrated circuit is tested by a computer. It verifies the functionality of the device performing thousands of electrical tests, by means of special microprobes. Only those products which pass all the tests are then marked with a code which is used by the manufacturer and the user to identify the function of the microcomponent. The integrated circuits are then shipped to their final destination.

For further information, please contact :
Maria Grazia Prestini,
Corporate Press Relations Manager
Agrate Brianza, Italy
Saint Genis-Pouilly, France
Tel : +39 39 6035901 Tel : +33 4 50402532
Fax : +39 39 6036094 Fax : +33 4 50402860
B657H
September 1995
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