Computer Chips

The building block of the computer revolution in the 1970s and the basis of today's multibillion-dollar computer industry is the computer chip. Usually about one-third of an inch square, these chips are the "brains" critical to many sophisticated and not-so-sophisticated devices such as calculators, appliances, televisions and radios. They also have great importance in areas of environmental monitoring, medicine, automobiles and other vehicles, and traffic control. They have greatly reduced the size and cost of most electronic products, while at the same time increasing their power and versatility.

Computer chips are integrated circuits called microprocessors built up from transistors and other components. Transistors are small binary electrical switches. Transistors have no moving parts and are switched between their two allowed states, "on" and "off", electronically. Microprocessors are fabricated on a single crystal of a semiconducting material such as silicon or germanium. A semiconductor is a substance whose ability to conduct electricity is between that of an insulator like rubber and a full conductor like copper. Often the conducting properties of a semiconductor can be varied by adding an impurity known as a dopant so that a semiconductor can be made to act like either an insulator or a conductor. A single chip's microscopically small components, including transistors, capacitors, and resistors, can number more than 1 million for an "ultra large-scale integration," or just 100 for a "small-scale integration."

There are two main kinds of chips. The first, called central processing unit (CPU) chips, give computers their information-processing power. An example of a CPU chip is Microsoft's Pentium III. Most personal computers (PCs) and small networks only require a single chip for their purposes, but larger systems link many chips together to support their greater demands. The second kind of chip, memory chips, come in many different configurations, ranging from those with only a few kilobytes of memory to those with 10 megabytes. The power of a computer's memory chip affects how fast the machine runs, because as the CPU processes instructions for the machine to carry out, it needs to store that information somewhere. The memory chip is that storage area, so the larger this space is, the more rapidly and efficiently the CPU can work.

Computer chips are manufactured in an industrial process that depends upon precision and extreme cleanliness, since trace contamination can ruin a complex chip's design. The process has analogy to children's stencils; masks are used to cover portions of a chip and desired patterns are formed on it.

The first step is to prepare a single crystal of an ultrapure material such as a silicon wafer that has a thin silicon dioxide surface. The wafer is then covered with a substance called a photoresist. A photoresist is a substance, which becomes soluble when exposed to ultraviolet (UV) light. The photoresist is in turn covered with a mask, or stencil, and then strong UV light is projected onto the assembly. Many different masks are used to control the end properties of the computer chip. Portions of the chip's photoresist that are not covered by the mask become soluble upon irradiation and those that are do not. The soluble photoresist is removed by washing with solvent and the resulting pattern can be etched onto the chip permanently by removal of the revealed silicon dioxide surface. Either chemicals ("wet etching") or corrosive plasma gases perform the etching in a vacuum chamber. The process of producing the patterns forming the basis for the chip's circuitry through the use of masks, chemicals, and UV light is known as photolithography.

Removal of the remaining photoresist (that which was not exposed to the UV light since part of the mask covered it) gives a wafer with a silicon dioxide surface with a controlled pattern etched out of it to reveal a polysilicon pattern. Another layer of polysilicon can then be applied and the etching procedure repeated to make a more elaborate assembly.

A "doping" procedure is used to controllably introduce impurities into the exposed polysilicon and change its crystalline structure. Ionized forms of the impurities are showered onto the chip and ions become implanted into the silicon. The portions of the chip still covered by the silicon dioxide surface are not affected in the doping process. Thus, the doping happens only where the silicon dioxide surface has been removed during the etching process. These impurities modulate the silicon's ability to conduct electricity (conductivity). Introduction of boron impurities results in electron-deficient or "positive" silicon; introduction of phosphorous impurities results in electron-rich or "negative" silicon.

The microprocessor is completed after multiple layering and further processing; "windows" are left in some layers so that connections can be made between surfaces as necessary. Metal is then deposited into the windows and more layers are formed. The resulting metal strips are the electrical connections between layers. A twenty-layered microprocessor is not uncommon.

The individual components of the microprocessor perform the functions of switching, amplifying, and detecting electricity running through the chip. Since microprocessors are rather fragile, they must be inserted into protective packaging. The completed chip then goes on a small plastic mount with gold wires linking its components to metal pins. These pins plug into the circuit board of the product for which the chip is intended.