microelectronics(redirected from thin film circuits)
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the field of electronics that deals with the development of functional microminiature integrated electronic assemblies, units, and devices. It originated in the early 1960’s as a result of the increasing size and complexity of functions of electronic apparatus and the more stringent reliability requirements. The use of several thousands, and even tens of thousands, of individually fabricated electron tubes, transistors, capacitors, resistors, transformers, and other elements in certain apparatus and connection of the leads by soldering or welding rendered apparatus unwieldy, labor-intensive in assembly, and insufficiently reliable; it also caused high power consumption. The search for methods of eliminating these disadvantages led to the appearance of new trends in the design of electronic apparatus, such as printed circuits, modules, micromodules, and integrated circuits (based on multiple processing methods).
Through the use of advances in solid-state physics, and particularly semiconductor physics, microelectronics is solving the problems mentioned above not merely by reducing the size of the electronic elements but by creating electronic structures (functional units and assemblies) that are structurally, technologically, and electrically integrated. In such structures a large number of microminiature elements and their electrical connections are structurally integrated according to the circuit in a common technological process. This process, which was made possible by the planar process of making semiconductor devices introduced in 1959, involves the use of an initial common blank (usually in the form of wafers of a semiconductor material) for a large number (100–2,000) of identical functional electronic assemblies, which pass simultaneously through a series of technological operations under identical conditions. Thus, each such assembly is produced as a result of step-by-step integrated multiple processing of many identical assemblies on the same wafer, rather than through the assembly of discrete elements. During the manufacturing process, the properties of various circuit elements and their combinations are imparted to certain parts of the semiconductor material, thus producing the overall assembly. This microminiature assembly, after being detached from the wafer and mounted in a housing, is called an integrated microcircuit, or integrated circuit. In this connection the concept of a circuit element itself is altered in microelectronics. In practical terms, the integrated circuit as an indivisible unit consisting of five or more elements becomes an element. An integrated circuit is characterized by the level of integration—that is, by the number of simpler elements in it.
In view of the exceptionally high accuracy in carrying out the technological processes and the large number of operations involved in the fabrication of microelectronic products, a variety of high-quality semiconductor and other materials, as well as precision equipment, is required. Monocrystalline silicon serves as the semiconductor base material. The equipment must provide for production of the elements within a dimensional tolerance of microns and fractions of a micron.
Several overlapping and complementary trends may be distinguished in microelectronics, depending on the design, technology, and physical principles used. Among the trends are integrated electronics, vacuum microelectronics, optical electronics, and functional electronics. Integrated electronics has achieved the highest level of development. Its appearance opened extensive possibilities for microminiaturization of radioelectronic devices and began the process of creating third-generation apparatus, using integrated circuits (first-generation apparatus used vacuum tubes; second-generation apparatus, semiconductor devices). The area of use of integrated circuits extends from computers and systems for space flight to household appliances. The rate of growth of integrated-circuit production is very high. More than 1 billion integrated circuits were manufactured worldwide in 1972.
Semiconductor integrated circuits were first produced on the basis of methods of multiple processing, by forming the required number of electronic elements and electrical connections inside one semiconductor crystal (1959–61). They are made mainly by means of planar-epitaxial technology, which was borrowed from the production of discrete semiconductor devices and differed only in the additional operations required for the electrical insulation of the individual elements on the semiconductor wafer and the connection of all elements in the crystal to form a functional assembly. Insulation is produced by creating around the element an area of semiconductor material with the opposite type of conductivity (thereby forming an insulating p-n junction) or a layer of a dielectric, such as silicon dioxide. The principal operations of planar-epitaxial technology are the mechanical and chemical processing of a semiconductor wafer, the epitaxial buildup on the wafer of a layer with the required electrophysical properties (type of conductivity, resistivity, and so on), photolithography, doping (by diffusion or ion implantation), and the deposition of metallic films for electrodes, connecting paths, and contact areas.
Photolithography is the most critical of the processing steps listed above. It provides for the selective processing of certain portions of the semiconductor wafer, such as the etching of a “window” in the oxide film on the wafer to permit the diffusion of impurities. A light-sensitive lacquer called a photoresist is used in this process. A film of the photoresist is deposited on the wafer and then is exposed to ultraviolet light through a photographic mask (“phototemplate”), which is a glass plate with a recurrent pattern formed by opaque and semitransparent areas (usually a layer of chromium) and pressed firmly to the wafer. After irradiation, the photoresist film undergoes selective etching, which causes reproduction of the patterns of the phototemplate on the wafer. The photoresist can also be irradiated by a contactless method, in which the pattern is projected onto the wafer. A promising method of exposing the wafer to the pattern is the electron-beam process (electronic photolithography).
In the production of semiconductor integrated circuits the photolithographic process must be repeated to reproduce various superimposed patterns on the wafer. A set of seven or eight phototemplates is usually used. The design and production of the phototemplates require particularly high accuracy and the maintenance of airtight cleanliness conditions in the production shops (no more than three to five dust particles about 0.5 micron in size per liter of air): to produce hundreds of elements having dimensions measured in microns for hundreds of identical integrated circuits that are fabricated simultaneously on a single wafer, the phototemplates must ensure the reproducibility of the dimensions from one pattern to another and their mutual coincidence. Consequently, complicated precision equipment, such as coordinatographs with computer program control to draw the original pattern magnified hundreds of times and photographic dies of various designs to reduce and duplicate the original pattern, is used in the design and production of phototemplates.
To form the structures of the elements in the original semiconductor wafer, the areas prepared in the photolithographic stage are doped with impurities. The main method of doping is diffusion, in which the silicon wafer is exposed for a certain time to vapors of the impurity at a temperature of 1100°-1200°C. The accuracy with which the temperature is maintained, the steadiness of the impurity concentration near the surface, and the duration of the process control the distribution of the impurity with respect to the thickness of the wafer and, correspondingly, the parameters of the element being formed. In addition to diffusion, doping may be done by ion implantation (bombarding the wafer with ionized impurity atoms), a new technological trend that supplements and partially replaces diffusion. Semiconductor integrated circuits have a high level of integration (up to 10,000 elements, and sometimes more, in one crystal).
The advancement of the technology for producing active elements (diodes and transistors) on wafers of semiconductor material through the transition to multiple processing has stimulated the development of printed circuitry and film technology to produce passive microminiature components (resistors and capacitors) that formed the basis for the development of film integrated circuits. Such circuits are usually purely passive, because the deposition of monocrystalline semiconductor films to form active elements does not provide the required quality. The base for a film integrated circuit is a dielectric substrate (for example, a ceramic). The technology of production of such circuits is divided into the thick-film type, in which layers of conducting, resistive, and dielecric pastes are applied in thicknesses of 1–25 microns (μ), and the thin-film type, in which films up to 1 JUL thick are sputtered in a vacuum through metal stencils, or sputtering is used in combination with subsequent photolithographic processing.
A film integrated circuit with unencapsulated individual semiconductor devices (diodes and transistors) and unencapsulated semiconductor integrated circuits mounted on it is known as a hybrid integrated circuit. Its passive portion may be multilayered, in the form of a set of ceramic substrates with layers of film circuit elements. Sintering of the substrates produces a monolith with a multilayered arrangement of electrically interconnected passive elements. The unencapsulated active elements are mounted on the upper surface of the monolith.
In addition to semiconductor and film integrated circuits, a combined type is being produced. The active elements are manufactured in the body of the semiconductor substrate by planar-epitaxial technology, and the passive elements and electrical connections are applied to the surface of the monolithic structure in the form of thin films. The level of integration of combined integrated circuits approaches that of semiconductor circuits.
Multichip integrated circuits are also produced with a high level of integration, in which several semiconductor integrated-circuit crystals are combined on a dielectric substrate with film connections in a very complicated electronic device. Its functional purpose may correspond to that of an individual unit or even a system, such as a desk calculator.
The combination of the film technology of producing passive elements with the use of microminiature electrovacuum devices as the active elements has resulted in the appearance of vacuum integrated circuits and a new field, vacuum microelectronics. A vacuum integrated circuit may be made in the form of a film integrated circuit with microminiature electrovacuum devices mounted on it or in the form of apparatus having all its components in a vacuum. Unlike semiconductor integrated circuits, vacuum circuits have greater resistance to the effects of cosmic radiation; their packing density is up to 20–30 elements per cm3.
All types of integrated circuits are divided into two large groups, depending on their functional attributes—digital (logic) and linear circuits. Digital circuits are designed to operate in logic units, particularly in digital computers. All other integrated circuits belong to the linear class and are designed mainly for the linear processing (amplification, modulation, detection, and so on) of electrical signals, although they may also include such nonlinear elements as sine-wave generators and frequency converters.
Microelectronics is developing in two main directions: an increase in the level of integration and the packing density in integrated circuits, which has become traditional, and the search for new physical principles and phenomena for the development of electronic devices that perform the functions of a circuit or even a system. The first trend has resulted in levels of integration characterized by many thousands of elements in one circuit housing, with micron and submicron dimensions of certain elements. The second trend may make it possible to abandon further increases in the level of integration (because of structural complexity), as well as to reduce the dissipation of power and increase the speed-of-response of the apparatus. In general, this new trend is called functional microelectronics, or the electronics of combined media, using such phenomena as solid-state optical phenomena (optical electronics), the interaction of an electron flux with acoustic waves in solids (acoustic electronics), the properties of superconductors, and the properties of magnetic materials and semiconductors in magnetic semiconductors (magnetoelectronics).
REFERENCESIntegral’nye skhemy. Moscow, 1970. (Translated from English.)
Mikroelektronika: Sb. St., issues 1–5. Moscow, 1967–72.
A. A. VASENKOV and I. E. EFIMOV