I. The manufacture of silicon microcircuits consists of a number of carefully controlled processes, all of which have to be performed to well-defined specifications.
Processing a "wafer" of silicon, a substrate on which the microelectronic circuits are made, is not a simple technological process.
In order to understand how transistors and other circuit elements can be made from silicon, it is necessary to consider the physical nature of semiconductor materials.
In a conductor current is known to be carried by electrons that are free to flow through the lattice1 of the substance.2
In an insulator all the electrons are tightly3 bound to atoms or molecules and hence4 none are available to serve as a carrier of electric charge.
The situation in a semiconductor is intermediate5 between the two: free charge carriers are not ordinarily present, but they can be generated with a modest expenditure6 of energy.
Semiconductors are similar to insulators in that they have their lower bands completely filled.7 The semiconductor will conduct if more than a certain voltage is applied. At voltages in excess of this critical voltage, the electrons are raised from the top8 of the band 1 (the valence band) to the bottom9 of band 2 (the conducting band). Below10 this critical voltage, the semiconductor material acts as an insulator. Semiconductors such as that described above are called intrinsic semiconductors — they are pure materials (for example silicon or germanium). It should be noted that a crystal of pure silicon is a poor11 conductor of electricity. Thus,12 conductivity poses13 a problem.
Several other requirements are imposed on materials. The basic demand appears to be conductivity because it can substantially improve14 the resistance and delay times for VLSI. The improvement of conductivity has been made in several ways. Most semiconductor devices are known to be made by introducing controlled numbers of impurity atoms into a crystal, the process called doping.
Two independent lines of development are considered to lead to microscopic technique that produced the present integrated circuits. One involved the semiconductor technology; the other is a film technology.
Let us consider the former15 one first. Tp improve the semiconductor crystal the impurities known as dopants are added to the silicon to produce a special type of conductivity, characterized by either positive (p -type) charge carriers or negative (n -type) ones. The dopants are diffused16 into semiconductor crystals at high temperature. In the furnace the crystals are surrounded by vapour containing atoms of the desired dopant. These atoms enter the crystal by substituting17 for the semiconductor atoms at regular sites18 in the crystal lattice and move into the interior19 of the crystal by jumping from one site to an adjacent20 vacancy.21
Silicon crystals may be doped with different elements. Suppose silicon is doped with boron. Each atom inserted22 in the silicon lattice creates a deficiency23 of one electron, a state that is called а hole. A hole also remains associated with an impurity atom under ordinary circumstances24 but can become mobile in response to an applied voltage. The hole is not a real particle, of course, but merely25 the absence of an electron at a position where one would be found in a pure lattice of silicon atoms. Nevertheless26 the hole has a positive, electric charge and can carry electric current. The hole moves through the lattice in much the same way that the bubble27 moves through a liquid medium. An adjacent atom transfers28 an electron to the impurity atom, "filling" the hole then but creating a new one in its own cloud of electrons; the process is then repeated, so that the hole is passed along from atom to atom.
Silicon doped with phosphorus or another pentavalent element is called an n -type semiconductor. Doping with boron or another trivalent element gives rise to а р -type semiconductor.
Impurities may be introduced by the diffusion process. At each diffusion step29 in which n -type or p -type regions are to be created in certain areas, the adjacent areas are protected30 by surface layer of silicon dioxide, which effectively blocks the passage of impurity atoms. This protective layer is created very simply by exposing31 the silicon wafer at high temperature to an oxidizing atmosphere. The silicon dioxide is then etched32 away in conformity, with a sequence33 of masks that accurately delineates34 multiplicity35 of n -type and p -type regions.
To define the microscopic regions that are exposed to diffusion in various stages36 of the process, extremely precise37 photolithographic procedures38 have been developed. The surface of the silicon dioxide is coated with a photosensitive organic compound that polymerizes wherever it is struck by ultraviolet radiation and that can be dissolved39 and washed away everywhere else. By the use of a high-resolution photographic mask the desired configurations can thus be transferred to the coated wafer. In areas, where the mask prevents40 the ultraviolet radiation from reaching the organic coating the coating is relmoved. An etching acid41 can then attack the silicon dioxide layer and leave the underlying silicon exposed to diffusion.
A transistor can be made by adding a third doped region to a diode so that, for example, а р -type region is said to be sandwiched between two n -type regions. One of the n -doped areas is called the emitter and the other, the collector; the p -region between them is the base.
The transistor described is called an npn transistor. There may be рпр transistors. The terms42 are likelу to denote43 the sequence of doped regions in the silicon.
The first transistor structures were formed by alloying44 or diffusion in bulk45 single-crystal Ge or Si, but with the development of "planar technology" in the early 1960s the possibility of forming high frequency transistors and integrated circuits using epitaxial semiconductor films was realized.
The success of silicon in microelectronics is believed to be largely attributed to excellent properties of SiO2 interface46 and ease of thermal oxidation of silicon.
The recent years have seen corisiderable interest in the subject of oxygen and its precipitates47 in silicon. It has now been established48 that their, presence can have a variety of effects, harmful49 as well as beneficial. Oxygen concentration is knowr to influence many silicon wafer properties, such as wafer strength, resistance to thermal warping, minority carrier lifetime, and instability in resistivity. Oxidation is widely used to create insulating areas. However many phenomena happen not to be understood at present.
An important aspect of the oxidation process fails low cost. Several hundred wafers can be oxidized simultaneously in a single operation.
Reactive gas plasma technology is reported to be presently in wide-spread use in the semiconductor industry. This technology is being applied to the deposition and removal50 of selected materials during the manufacture of semiconductor devices.
Contributing greatly to the manufacturing technique is a unique crystal forming method known as epitaxial growth.
Epitaxial growth in combination with oxide masking and diffusion has given the device designer extremely flexible tools51 for making an almost limitless variety of structures.
After 1964 epitaxial growth remains an important technique in semiconductor device fabrication and the demand for improved device yield per slice,52 still higher device operating frequencies and more sophisticated53 device structures has needed continuing innovation54 and development.
Advances55 in silicon crystal growth technology have encouraged advances in the automation of crystal growing equipment. Crystal pulling56 equipment now available uses computer software to control all the growing parameters. Preprogrammed process changes are used to tailor crystal characteristics.
II. Let us see what a film technique is like.
Even before the invention of the transistor the electronic industry had studied the properties of thin film of metallic and insulating materials. Such films range in thickness from a fraction of a micron, or less than a wavelength of light, to several microns.
The techniques for the deposition57 of thin films are numerous and include the following methods: evaporation, sputtering,58 anodization, radiation, induced "cracking" or polymerization, chemical reduction thermal reduction of oxidation and electrophoresis. The first three are the major techniques used in integrated thin film circuit construction and are also applicable to silicon integrated circuitry and device work. These methods singly or in combination enable59 a variety of resistive, insulating and constructive materials to be laid down onto a suitable substrate.
The two most important processes for the deposition of thin films are chemical-vapour deposition and evaporation. The film technology has proved to provide precise dimensions.
In the fabrication of a typical large-scale integrated circuit there are more thin-film steps than diffusion steps. Therefore thin-film technology is probably more critical to the overall yield and performance of the circuit than the diffusion and oxidation steps are. A thin film happens even to be employed to select the areas on a wafer that are to be oxidized.
For VLSI structures several other requirements are imposed on interconnection materials by the fabrication technology.
The deposition of layers is followed by shaping operations, such as etching, to form the required outlines.60 Alternatively, the film can be deposited through a mask onto the substrate to define the outlines directly. In this way many identical thin-film devices can be made on a single sheet of material, which then are cut apart to yield individual devices.
Plasma etching, which is expected to play an important role in manufacture of semiconductor and other devices requiring fine-line lithography, involves the use of a glow discharge to generate reactive species61 from relatively inert molecular gases. These reactive species combine chemically with certain solid materials to form volatile62 compounds which are then removed by vacuum pumping system.
This plasma-etching process has been shown to have important advantages in terms of cost, cleanliness, fine-line resolution, and potential for production line automation.
Additionally, the inside of a wafer-fabrication must be extremely clean and orderly: a single particle happens to cause a defect that will result in the malfunction of a circuit. The larger the die,63 the greater the chance for a defect.
The structure of an integrated circuit is sure to be complex both in the topology of its surface and in its internal composition. Each element of such a device has intricale64 three-dimensional architecture that must be reproduced exactly in every circuit. The structure is made up of many layers, each of which is a detailed pattern. Some of the layers lie within the silicon wafer and others are stacked65 on the, top. The manufacturing process consists in forming the sequence of layers precisely in accordance with the plan of the circuit designer.
Nowadays much of the procedure by which ICs are transformed from the conception of the circuit designer to a physical reality is done with the aid66 of computers. In the first stage of the development of new microelectronic circuits the designers themselves used to work at specifying the functional characteristics of the device. They also selected the processing steps that will be required to manufacture it. The process was difficult and not always exact. A computer can simulate67 the operations of the circuit. Besides, computer simulation is less expensive than assembling a "bread-board" (макет) circuit made up of discrete circuit elements; it is also more accurate.
The layout is known to specify the pattern of each layer of the IC. The goal of the layout is to achieve the desired function of each circuit in the smallest possible space. At present much of the preliminary (предварительный) work is done with the aid of computers. The final layout is also made with that of a computer.
Increasing interest in submicron layer now poses new problems. New developments in materials are believed to be due68 to new manufacturing forms and vice versa.
Integrated circuit technology is evolving so rapid that even a period as short as six months can produce a significant change.
