Manufacturing an Integrated Circuit
By Craig R. Barrett
The fundamental device of the digital world is the integrated circuit, a small square of silicon containing millions of transistors. It is probably the most complex of man-made products. Although it looks flat, it is in fact a three-dimensional structure made by painstakingly building up on the silicon base several microscopically thin layers of materials that both insulate and conduct electricity. Assembled according to a pattern carefully worked out in advance, these layers form the transistors, which function as switches controlling the flow of electricity through the circuit, which is also known as a chip. 'On' and 'off' switches manipulate the binary code that is at the core of what a computer does.
Building a chip typically requires several hundred manufacturing steps that take weeks to complete. Each step must be executed perfectly if the chip is to work. The conditions are demanding. For example, because a speck of dust can ruin a chip, the manufacturing has to be done in a 'clean room' containing less than one submicron particle of dust per cubic foot of air (in contrast, the average living room has between 100,000 and one million particles per cubic foot of air). Much of the equipment needed for making chips embodies the highest of high technology, with the result that chip factories—which cost between $1 billion and $2 billion for a state-of-the-art facility—are among the costliest of manufacturing plants.
A basic technology of chipmaking is the 'planar' process devised in 1957 by Jean Hoerni of Fairchild Semiconductor. It provided a means of creating a layered structure on the silicon base of a chip. This technology was pivotal in Robert N. Noyce's development of the integrated circuit in 1958. (Noyce later became co-founder with Gordon E. Moore of Intel Corporation, the company that invented the microprocessor and has become the world's leading supplier of semiconductor chips.…) Bridging the gap between the transistor and the integrated circuit, the planar technology opened the way to the manufacturing process that now produces chips. The hundreds of individual steps in that process can be grouped into a few basic operations.
Chip Design
The first operation is the design of the chip. When tens of millions of transistors are to be built on a square of silicon about the size of a child's fingernail, the placing and interconnections of the transistors must be meticulously worked out. Each transistor must be designed for its intended function, and groups of transistors are combined to create circuit elements such as inverters, adders and decoders. The designer must also take into account the intended purpose of the chip. A processor chip carries out instructions in a computer, and a memory chip stores data. The two types of chips differ somewhat in structure. Because of the complexity of today's chips, the design work is done by computer, although engineers often print out an enlarged diagram of a chip's structure to examine it in detail.
The base material for building an integrated circuit is a silicon crystal. Silicon, the most abundant element on the earth except for oxygen, is the principal ingredient of beach sand. Silicon is a natural semiconductor, which means that it can be altered to be either an insulator or a conductor. Insulators, such as glass, block the passage of electricity; conductors, such as copper, let electricity pass through. To make a silicon crystal, raw silicon obtained from quartz rock is treated with chemicals that remove contaminants until what remains is almost 100 percent silicon. This purified silicon is melted and then formed into cylindrical single crystals called ingots. The ingots are sliced into wafers about 0.725 millimeter (0.03 inch) thick. In a step called planarization they are polished with a slurry until they have a flawless, mirror-smooth surface. At present, most of the wafers are 200 millimeters (eight inches) in diameter, but the industry is moving toward achieving a standard diameter of 300 millimeters (12 inches) by 1999. Because a single wafer yields hundreds of chips, bigger wafers mean that more chips can be made at one time, holding down the cost per chip.
The First Layers
With the wafer prepared, the process of building the chip's circuitry begins. Making the transistors and their interconnections entails several different basic steps that are repeated many times. The most complex chips made today consist of 20 or more layers and may require several hundred separate processing steps to build them up one by one.
The first layer is silicon dioxide, which does not conduct electricity and therefore serves as an insulator. It is created by putting the wafers into a diffusion furnace —essentially an oven at high temperature where a thin layer of oxide is grown on the wafer surface.
Removed from the furnace, the wafer is now ready for its first patterning, or photolithographic, step. A coating of a fairly viscous polymeric liquid called photoresist, which becomes soluble when it is exposed to ultraviolet light, is applied to the surface. A spigot deposits a precise amount of photoresist on the wafer surface. Then the wafer is spun so that centrifugal force spreads the liquid over the surface at an even thickness. This operation takes place on every layer that is modified by a photolithographic procedure called masking, described in the next step.
Masking
A mask is the device through which ultraviolet light shines to define the circuit pattern on each layer of a chip. Because the pattern is intricate and must be positioned precisely on the chip, the arrangement of opaque and transparent spaces on a mask must be done carefully during a chip's design stage.
The mask image is transferred to the wafer using a computer-controlled machine known as a stepper. It has a sophisticated lens system to reduce the pattern on the mask to the microscopic dimensions of the chip's circuitry, requiring resolution as small as 0.25 micron. The wafer is held in place on a positioning table below the lens system. Ultraviolet light from an arc lamp or a laser shines through the clear spaces of the mask's intricate pattern onto the photoresist layer of a single chip. The stepper table then moves the wafer the precise distance required to position another chip under the light. On each chip, the parts of the photoresist layer that were struck by the light become soluble and can be developed, much like photographic film, using organic solvents. Once the photoresist is patterned, the wafer is ready for etching.
Etching
During this step, photoresist remaining on the surface protects parts of the underlying layer from being removed by the acids or reactive gases used to etch the pattern on the surface of the wafer. After etching is complete, the protective layer of photoresist is removed to reveal electrically conducting or electrically insulating segments in the pattern determined by the mask. Each additional layer put on the chip has a distinctive pattern of this kind.
Adding Layers
Further masking and etching steps deposit patterns of additional materials on the chip. These materials include polysilicon as well as various oxides and metal conductors such as aluminum and tungsten. To prevent the formation of undesired compounds during subsequent steps, other materials known as diffusion barriers can also be added. On each layer of material, masking and etching create a unique pattern of conducting and nonconducting areas. Together these patterns aligned on top of one another form the chip's circuitry in a three-dimensional structure. But the circuitry needs fine-tuning to work properly. The tuning is provided by doping.
Doping
Doping deliberately adds chemical impurities, such as boron or arsenic, to parts of the silicon wafer to alter the way the silicon in each doped area conducts electricity. Machines called ion implanters are often used to inject these impurities into the chip.
In electrical terms, silicon can be either n-type or p-type, depending on the impurity added. The atoms in the doping material in n-type silicon have an extra electron that is free to move. Some of the doping atoms in p-type silicon are short an electron and so constitute what is called a hole. Where the two types adjoin, the extra electrons can flow from the n-type to the p-type to fill the holes.
This flow of electrons does not continue indefinitely. Eventually the positively charged ions left behind on the n-type side and the negatively charged ions on the p-type side together create an electrical force that prevents any further net flow of electrons from the n-type to the p-type region.
The material at the base of the chip is p-type silicon. One of the etching steps in the manufacture of a chip removes parts of the polysilicon and silicon dioxide layers put on the pure silicon base earlier, thus laying bare two strips of p-type silicon. Separating them is a strip that still bears its layer of conducting polysilicon; it is the transistor's 'gate.' The doping material now applied to the two strips of p-type silicon transforms them into n-type silicon. A positive charge applied to the gate attracts electrons below the gate in the transistor's silicon base. These electrons create a channel between one n-type strip (the source) and the other (the drain). If a positive voltage is applied to the drain, current will flow from source to drain. In this mode, the transistor is 'on.' A negative charge at the gate depletes the channel of electrons, thereby preventing the flow of current between source and drain. Now the transistor is 'off.' It is by means of switching on and off that a transistor represents the arrays of 1 and 0 that constitute the binary code, the language of computers.
Done many times in many layers, these operations provide the chip with its multitude of transistors. But just as provision must be made to run electrical wires and plumbing pipes between floors of a building, provision must be made in chips for interconnecting the transistors so they form an integrated circuit.
Interconnections This final step begins with further masking and etching operations that open a thin layer of electrical contacts between layers of the chip. Then aluminum is deposited and patterned using photolithography to create a form of wiring that links all the chip's transistors. Aluminum is chosen for this application because it makes good electrical contact with silicon and also bonds well to silicon dioxide.
This step completes the processing of the wafer. Now the individual chips are tested to ensure that all their electrical connections work using tiny electrical probes. Next, a machine called a dicer cuts up the wafer into individual chips, and the good chips are separated from the bad. The good chips—usually most of the wafer's crop—are mounted onto packaging units with metal leads. Wire bonders then attach these metal leads to the chips. The electrical contacts between the chip's surface and the leads are made with tiny gold or aluminum wires about 0.025 millimeter (0.001 inch) in diameter. Once the packaging process is complete, the finished chips are sent to do their digital work.
