Chapter 2 Manufacturing
Fabrication of integrated circuits starts with a thin slice of a silicon wafer, usually p-type silicon, 150 mm in diameter and 0.3 mm thick. On this wafer one can fabricate several hundred individual chips. Each chip may contain 10 to 20,000 components for a total of several million devices. The devices are resistors, diodes, capacitors and transistors of various types. The devices are fabricated by depositing controlled concentrations of dopants. Dopants are Group III and Group V elements that are introduced at concentration levels of parts per million. In most cases the depth to which the dopants are introduced into the wafer is less than a micron. These dimensions should give you the impression that the integrated chip is essentially a planar silicon crystal whose impurities or dopants are carefully created in specific small regions. In a sense, one can say fabricating an integrated circuit involves "planar chemical architecture".
A state-of-the-art silicon microchip contains 10 million devices within an area of about 1 sq. cm! The individual transistors are so small that one will need an electron microscope to see them. Figure 1 gives a scanning electron micrograph of a small area of an actual integrated circuit in which the line width is 0.5µm. Fig.2 shows a schematic cross-sectional view of a typical chip transistor. Materials typically used to make such a device are indicated on the figure. The thickness of the different layers is typically about 1 µm.
The size of peripheral devices or components used in electronic systems has also been decreasing steadily over the years. For example, the thin film magnetic head (Fig.3) which is used to read and write hard disks is about the size of the period at the end of this sentence. These heads are fabricated using techniques similar to that used for the microchip fabrication. The tiny spiral around the head, as shown in Fig.3, is a thin film of electrical conductor obtained by evaporation of a metal.
Great manufacturing challenges were faced and overcome in making these tiny high quality devices and components with high reliability and reproducibility. Scientists and engineers are at work now to fabricate 256 to 512 million transistors on a microchip. We are stepping into a dimensional region where the action of only a few thousand atoms control the function of a device.
Starting material for an integrated circuit is a silicon crystal in a wafer form. A wafer generally refers to thin flat silicon crystal; thickness is typically 300 µm and diameter is typically between 4 to 6 inches. Figure 4 shows a wafer being transferred for processing.
In the construction of integrated circuits, there are really two principle types of processing steps. First process type introduces desired chemical species into or on the wafer and the second type is used to remove undesired material from the wafer. In the process that introduce dopants (that is Group V and III elements), the principal methods used are ion implantation for deposition and diffusion for profiling the dopant. Other methods include oxidation, chemical and physical vapor deposition.
Processes used to remove material from the wafer surface are vapor phase etching, chemical etching and dissolution. We rely on a technique called lithography for the introduction of dopants at desired locations on the wafer and removal of undesired material.
Manufacture of microchips involves repeating the above two types of processing steps several times, until we fabricate a complete device, such as a transistor. As many as 200 steps may be needed to fabricate a chip which may have multiple layers. To get our final circuits, we need to wire up the arrays of components. The components are so small, their dimensions are in the micrometer range. We deposit a thin layer of aluminum film, by evaporative technique, on the surface of the wafer. Selective etching is done to remove unwanted aluminum films leaving a desired circuit pattern Finally a thin protecting layer, such as silicon dioxide, on the top of the circuits is introduced to protect them from the environment. The wafers are then diced into individual chips simply by a sawing operation. The chips are then packaged and mounted on a suitable platform, such as chip carriers or lead frames.
The manufacturing process is divided into five major segments, silicon, oxidation, lithography, ion implantation and etching.