Silicon is the raw material most often used in integrated circuit (IC)fabrication. It is the second most abundant substance on the earth. It isextracted from rocks and common beach sand and put through an exhaustivepurification process.
In this form, silicon is the purist industrial substancethat man produces, with impurities comprising less than one part in a billion.That is the equivalent of one tennis ball in a string of golf balls stretchingfrom the earth to the moon. Semiconductors are usually materials which haveenergy-band gaps smaller than 2eV. An important property of semiconductors isthe ability to change their resistivity over several orders of magnitude bydoping.
Semiconductors have electrical resistivities between 10-5 and 107 ohms.Semiconductors can be crystalline or amorphous. Elemental semiconductors aresimple-element semiconductor materials such as silicon or germanium. Silicon isthe most common semiconductor material used today. It is used for diodes,transistors, integrated circuits, memories, infrared detection and lenses,light-emitting diodes (LED), photosensors, strain gages, solar cells, chargetransfer devices, radiation detectors and a variety of other devices. Siliconbelongs to the group IV in the periodic table.
It is a gray brittle materialwith a diamond cubic structure. Silicon is conventionally doped with Phosphorus,Arsenic and Antimony and Boron, Aluminum, and Gallium acceptors. The energy gapof silicon is 1.1 eV.
This value permits the operation of silicon semiconductorsdevices at higher temperatures than germanium. Now I will give you some briefhistory of the evolution of electronics which will help you understand moreabout semiconductors and the silicon chip. In the early 1900's before integratedcircuits and silicon chips were invented, computers and radios were made withvacuum tubes. The vacuum tube was invented in 1906 by Dr.
Lee DeForest.Throughout the first half of the 20th century, vacuum tubes were used toconduct, modulate and amplify electrical signals. They made possible a varietyof new products including the radio and the computer. However vacuum tubes hadsome inherent problems.
They were bulky, delicate and expensive, consumed agreat deal of power, took time to warm up, got very hot, and eventually burnedout. The first digital computer contained 18,000 vacuum tubes, weighed 50 tins,and required 140 kilowatts of power. By the 1930's, researchers at the BellTelephone Laboratories were looking for a replacement for the vacuum tube. Theybegan studying the electrical properties of semiconductors which arenon-metallic substances, such as silicon, that are neither conductors ofelectricity, like metal, nor insulators like wood, but whose electricalproperties lie between these extremes. By 1947 the transistor was invented. TheBell Labs research team sought a way of directly altering the electricalproperties of semiconductor material.
They learned they could change and controlthese properties by "doping" the semiconductor, or infusing it withselected elements, heated to a gaseous phase. When the semiconductor was alsoheated, atoms from the gases would seep into it and modify its pure, crystalstructure by displacing some atoms. Because these dopant atoms had differentamount of electrons than the semiconductor atoms, they formed conductive paths.If the dopant atoms had more electrons than the semiconductor atoms, the dopedregions were called n-type to signify and excess of negative charge. Lesselectrons, or an excess of positive charge, created p-type regions.
By allowingthis dopant to take place in carefully delineated areas on the surface of thesemiconductor, p-type regions could be created within n-type regions, andvice-versa. The transistor was much smaller than the vacuum tube, did not getvery hot, and did not require a headed filament that would eventually burn out.Finally in 1958, integrated circuits were invented. By the mid 1950's, the firstcommercial transistors were being shipped. However research continued. Thescientist began to think that if one transistor could be built within one solidpiece of semiconductor material, why not multiple transistors or even an entirecircuit.
With in a few years this speculation became one solid piece ofmaterial. These integrated circuits(ICs) reduced the number of electricalinterconnections required in a piece of electronic equipment, thus increasingreliability and speed. In contrast, the first digital electronic computer builtwith 18,000 vacuum tubes and weighed 50 tons, cost about 1 million, required 140kilowatts of power, and occupied an entire room. Today, a complete computer,fabricated within a single piece of silicon the size of a child's fingernail,cost only about $10.00. Now I will tell you the method of how the integratedcircuits and the silicon chip is formed.
Before the IC is actually created alarge scale drawing, about 400 times larger than the actual size is created. Ittakes approximately one year to create an integrated circuit. Then they have tomake a mask. Depending on the level of complexity, an IC will require from 5 to18 different glass masks, or "work plates" to create the layers ofcircuit patterns that must be transferred to the surface of a silicon wafer.Mask-making begins with an electron-beam exposure system called MEBES.
MEBEStranslates the digitized data from the pattern generating tape into physicalform by shooting an intense beam of electrons at a chemically coated glassplate. The result is a precise rendering, in its exact size, of a single circuitlayer, often less than one-quarter inch square. Working with incredibleprecision , it can produce a line one- sixtieth the width of a human hair. Afterpurification, molten silicon is doped, to give it a specific electricalcharacteristic.
Then it is grown as a crystal into a cylindrical ingot. Adiamond saw is used to slice the ingot into thin, circular wafers which are thenpolished to a perfect mirror finish mechanically and chemically. At this pointIC fabrication is ready to begin. To begin the fabrication process, a siliconwafer (p-type, in this case) is loaded into a 1200 C furnace through which pureoxygen flows. The end result is an added layer of silicon dioxide (SiO2),"grown" on the surface of the wafer. The oxidized wafer is then coatedwith photoresist, a light-sensitive, honey-like emulsion.
In this case we use anegative resist that hardens when exposed to ultra-violet light. To transfer thefirst layer of circuit patterns, the appropriate glass mask is placed directlyover the wafer. In a machine much like a very precise photographic enlarger, anultraviolet light is projected through the mask. The dark pattern on the maskconceals the wafer beneath it, allowing the photoresist to stay soft; but in allother areas, where light passes through the clear glass, the photoresisthardens.
The wafer is then washed in a solvent that removes the soft photoresist,but leaves the hardened photoresist on the wafer. Where the photoresist wasremoved, the oxide layer is exposed. An etching bath removes this exposed oxide,as well as the remaining photoresist. What remains is a stencil of the maskpattern, in the form of minute channels of oxide and silicon. The wafer isplaced in a diffusion furnace which will be filled with gaseous compounds (alln- type dopants), for a process known as impurity doping. In the hot furnace,the dopant atoms enter the areas of exposed silicon, forming a pattern of n-typematerial.
An etching bath removes the remaining oxide, and a new layer ofsilicon (n-) is deposited onto the wafer. The first layer of the chip is nowcomplete, and the masking process begins again: a new layer of oxide is grown,the wafer is coated with photoresist, the second mask pattern is exposed to thewafer, and the oxide is etched away to reveal new diffusion areas. The processis repeated for every mask - as many as 18 - needed to create a particular IC.Of critical importance here is the precise alignment of each mask over the wafersurface. It is out of alignment more than a fraction of a micrometer(one-millionth of a meter), the entire wafer is useless. During the lastdiffusion a layer of oxide is again grown over the water.
Most of this oxidelayer is left on the wafer to serve as an electrical insulator, and only smallopenings are etched through the oxide to expose circuit contact areas. Tointerconnect these areas, a thin layer of metal (usually aluminum) is depositedover the entire surface. The metal dips down into the circuit contact areas,touching the silicon. Most of the surface metal is then etched away, leaving aninterconnection pattern between the circuit elements. The final layer is "vapox",or vapor-deposited-oxide, a glass-like material that protects the IC fromcontamination and damage.
It, too, is etched away, but only above the"bonding pads", the square aluminum areas to which wires will later beattached.
