The two elements we can now concentrate on, as by far the most important semiconductors, are silicon and germanium. Silicon is one of the most plentiful elements in the world, but occurs in chemical compounds such as sand (silica), from which it is difficult to extract pure silicon. The element can be isolated by the reduction of silica in an arc furnace. It then contains small quantities of calcium, iron, aluminium, boron and phosphorous as principle impurities. Alternatively, silicon can be prepared by the pyrolytic reduction of silicon tetrachloride and in this way the material can be obtained free from analytically detectable quantities of boron and phosphorus. Germanium is comparatively rare but it is rather easier to refine. It should perhaps be mentioned that the list of semiconductors given is not confined to elements; increasing attention is being paid to semiconductor compounds such as indium antimidine and other compounds of group III with group V elements.
Additional text 2.
All the components of the circuit must be fabricated in a crystal of silicon or on the surface of the crystal. Silicon is far from being ideal material for these functions and only modest values of resistance and capacitance can be achieved. Practical microelectronic inductors cannot be formed at all. On the other hand, silicon is a material without equal for the fabrication of transistors, and the abundance of these active components in microelectronic devices more than compensates for the shortcomings of the passive elements.
Additional text 3. Photoresists.
Photoresists are high-photosensitive materials used to generate etched patterns in substances. The close contact between resist and substrate required for strong adhesion can be inhibited by surface impurities or resist components. Surface contaminants can be dust, oil, abrorbed water or gases, dopant ions, or monolayers of previous resist coatings. Removal of obvious visible impurities such as grease, fingerprints, or dust can give an apparently clean surface, but contamination is often invisible. Weakly adsorbed layers of tobacco smoke, water vapour or nonstripped resist components may be present, even though difficult to detect. Condensing ones breath on the surface or placing the wafers on a cold plate can sometimes reveal an adsorbed pattern on unetched wafers after previous resist stripping.
Additional text 4. Made in Space.
In recent years active research has been going on in one of the fields of space industrialization space material study and production of new materials of better quality on board the spacecraft, ranging from semiconductors for microelectronics to unique and more efficient medicines for the treatment of quite a number of diseases.
Conditions on board a space vehicle orbiting the Earth drastically differ from those on its surface. All of these conditions can be simulated on Earth, except for one prolonged weightlessness. Many well-known physical processes proceed differently due to absence of weight, and many of the properties of the materials obtained in the zero-g (zero-gravity) conditions are much better pronounced as compared with those of the specimens produced on Earth. In case of melts of metals, glasses, or semiconductors, they can be cooled down to the solidification point even in space and then brought back to Earth. Such materials will possess quite unusual properties. Besides, various faults in semiconductors can be caused by convection, i.e. (that is) movements of gases or liquids caused by difference in temperature, and there is no gravitation convection in space.
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Additional text 5. Molecular Electronics.
Molecular electronics is a new concept of electronic systems. Basically, it seeks to integrate into a solid block of the material the functions performed by electronic circuits or even whole systems. Its goal is to rearrange the internal physical properties of the solid in such a way that phenomena occuring within or between molecules will perform a function ordinarily achieved through the use of an assembly of electronic components.
Molecular electronics is the most forward-looking of several modern approaches to the development of small, reliable, efficient electronic systems. Almost all of them attempt to perform the required electronic functions in solid semiconductor-type materials. Molecular electronics, however, is unique in its goal of going away with the traditional concept of circuit components. In addition to lowering size and weight, increasing reliability and reducing power requirements, molecular blocks could make it possible to execute tasks too complex to be performed today.
Additional text 6. Submicron Technology.
A number of technological changes in silicon processing must be expected with the development of submicron technology, i.e. (that is) electron-beam mask-making technology to produce ultra-complex devices based upon dimensions which can no longer be fabricated with the use of visible or near visible light. The second application of submicron technology is electron-beam direct-write lithography that is invaluable in making small batches of specific or semicustom ICs. Submicron technology refers to the fabrication of semiconductor devices with features having masked dimentions less than one micron. Normal IC technology uses mask dimentions of about five microns.
The need for submicron technology is based on the need for further improvements in microelectronic capabilities. Improvements in cost, speed, density and power consumption are being sought. Because of the smaller dimentions required, it is no longer possible to use conventional optical methods to define the surface of an integrated circuit. The present optical methods are reaching their limits. In place of light, in submicron technology X-rays and electron beams are used to pattern the surface of the wafer. The advantage of e-beam technology is that the wavelength of electrons is substantially less than the wavelength of light. The wavelength of the electron beam is so small that diffraction no longer defines the lithographic resolution, and very high resolution pattering can be obtained. X-rays have the advantage that they travel in a straight line. They do not require vacuum as do electrons, which may simplify production techniques.
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In the same manner as the electron microscope provided superior resolution over the optical microscope, electron beam technology is about to impact the integrated circuit industry. The use of submicron technology has the same effect as increasing the size of the silicon wafer. Since the devices are smaller, the number of devices per wafer is greater. Also, it is known that one of the basic measures of semiconductor performance is the number of good dice per wafer. A single particle can cause a defect that will result in the malfunction of a circuit. The larger the die, the greater the chance for a defect, and when the dice sizes are smaller the loss due to a die containing a material defect is smaller.
