Problems in microelectronic circuit technology

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.