Semiconductors are widely used in microelectronic technology. A semiconductor is a material having a resistivity in the range between conductors and insulators and having a negative temperature coefficient. In a conductor current is known to be carried by electrons that are free to flow through the lattice of the substance; in an insulator all the electrons are tightly bound to atoms or molecules and none are available to serve as a carrier of electric charge; whereas in a semiconductor free charge carriers are not ordinarily present, but they can be generated when a certain voltage is applied. At voltages below this critical voltage the semiconductor material acts as an insulator. The conductivity increases not only with temperature but is also affected very considerably by the presence of impurities in the crystal lattice. It should be noted that intrinsic semiconductors such as a crystal of pure silicon are poor conductors of electricity. To improve the conductivity of the semiconductor crystal the impurities known as dopants are added to the crystal to produce a special type of conductivity (p-type or n-type) in a certain area. For a p-type diffusion the most generally used dopant is boron while phosphorus and arsenic are used for n-type diffusion. Most semiconductor devices are known to be made by doping, i.e. (that is) by introducing controlled numbers of impurity atoms into a crystal. The dopants are diffused into semiconductor crystals at high temperatures when the crystals are surrounded by vapour containing atoms of the desired dopant.
Out of several elements of the Periodic Table with semiconductor properties only germanium(Ge) and silicon (Si) (and to a lesser extent Se) show chemical and electrical properties suitable for electronic devices operating at room temperature. Germanium and silicon were the first semiconductor materials in common use. A small piece of polycrystalline germanium was used in assembling the first transistor. However, it was soon recognised that polycrystalline materials had uncontrolled resistances, and it could affect transistor operations in uncontrolled ways. Therefore, single-crystal materials became essential for semiconductor products.
Silicon gradually replaced germanium and became the universal semiconductor material. The dominant role of silicon as a material for microelectronic circuits is largely attributable to the properties of its oxide. Silicon dioxide is a clear glass with a softening point higher than 1.400 degrees C. If a wafer of silicon is heated in an atmosphere of oxygen or water vapour, a film of silicon oxide is formed on its surface. Its durable, adheres well and makes an excellent insulator. The silicon dioxide is particularly important in the fabrication of integrated circuits because it can act as a mask for selective introduction of dopants.
Processing a silicon substrate is a complicated technological process that has to be performed to well-defined specifications. At each diffusion step in which n-type or p-type regions are to be created in certain areas, the adjacent areas are protected by a surface layer of silicon dioxide, which effectively blocks the passage of impurity atoms. This protective layer is created by exposing the silicon wafer to an oxidizing atmosphere at high temperatures. The silicon dioxide is then etched away in conformity with the n-type and p-type regions to be made. To define the microscopic regions that are to be exposed to diffusion at various stages of the process, extremely precise photographic procedures have been developed. The surface of the silicon dioxide is coated with a photosensitive organic compound called photoresist that polymerizes wherever it is struck by ultraviolet radiation and that can be dissolved and washed away everywhere else. A mask is manufactured, the pattern of which defines the area to be etched and is transparent where the oxide is to be retained. By the use of this high-resolution photographic mask the desired configurations can be transferred to the coated wafer. In areas where the mask prevented the ultraviolet radiation from reaching the organic coating the coating is later removed, and an etching acid can then attack the silicon dioxide layer and leave the underlying silicon exposed to diffusion. The photoresist under the transparent area of the mask being subjected to the light becomes polymerized and is not affected by the acid developer which is subsequently used to dissolve the unexposed resist. After the surface has been cleaned, the chip is ready for the first diffusion process. Due to the masking technique in practice many devices are manufactured at the same time on a single sheet of silicon and are later separated with a diamond stylus and broken into individual chips.
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Thermal oxidation of silicon produces a non-water-soluble stable oxide (as compared to germaniums oxide). Its suitable for passing p-n junctions and can serve as insulator coating for conductor overlayers. Oxidation is widely used to create insulating areas, the cost of the process is low, and several hundred wafers can be oxidized simultaneously in a single operation. However, the presence of oxygen influences many silicon wafer properties such as wafer strength, resistance to thermal warping and instability in resistivity. It can have harmful effects, and oxygen concentration should therefore be controlled.
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Very large scale integration (VLSI) of devices has put great demands on electronic-grade single-crystal materials. The semiconductor industry now requires high purity silicon with minimum defects. The silicon wafer substrate must be practically defect-free when the active device density is as high as 10 to 10 per chip. To increase further the speed of semiconductor devices requires not only improvements in present designs and fabrication techniques, but also new materials that would be superior to materials presently being used, like germanium and silicon. The circuit speeds in some advanced computer equipment are now approaching the theoretical limits of silicon, and for many years scientists have been experimenting with faster-working alternative materials.
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Another semiconductor material considered by scientists is a compound called gallium arsenide (GaAs). It has a much higher electron mobility than germanium and silicon, it is potentially much faster, permits operation at higher temperatures and is chemically and mechanically stable. The relatively high electron mobility (carrier mobility) allows GaAs to be used for high-speed applications, and its high resistivity allows easier isolation between different areas of the crystal. The potential of high-purity gallium arsenide was first shown in a gallium arsenide-germanium hetero-junction diode. The hetero-junction device has a potential for much faster switching than conventional p-n junction diodes. GaAs junction field effect transistors (GaAs MESFET = metal-Shottkey field effect transistors) have been used in microwave devices which led to the development of low noise GaAs MESFET microwave amplifiers as well as for high speed logic circuits. GaAs chips work several times faster than the speediest of todays silicon-based counterparts. However, the difficulty of producing gallium arsenide of sufficient purity has limited its application.
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Apart from conventional oxidation and diffusion, techniques presently used in manufacturing integrated circuits include a film technique, a high pressure oxidation method, plasma etching, submicron technology, ion implantation and laser annealing.
Even before the invention of the transistor the electronic industry had studied the properties of thin films of metallic and insulating materials. A film technique enables a variety of films of resistive, insulating or constructive materials to be laid down onto a suitable substrate. In the fabrication of a typical large-scale integrated circuit there are more thin-film steps than diffusion and oxidation steps, and therefore thin-film technology is probably more critical to the overall performance of the circuit than the oxidation and diffusion are. The major techniques used for the deposition of thin films are evaporation and sputtering. Most often, thin-film deposition on a ceramic substrate is done in a vacuum chamber by evaporating or sputtering conductive, resistive, or dielectric material on a carefully cleaned substrate. The vacuum prevents oxidation and allows the molecules to get to the target with minimum collisions with gas molecules.
Efforts to implement low-temperature processes into silicon device fabrication led to the growing use of high pressure oxidation technology. The lower temperature aspect of high pressure oxidation has a great potential impact in the high density world of submicron VLSI.
Reactive gas plasma etching is also reported to play an important role in manufacturing devices requiring fine-line lithography. This technology is being applied to the deposition and removal of selected materials during the manufacture of semiconductor devices. A glow discharge is used to generate reactive species from relatively inert molecular gases. These reactive gases combine chemically with certain solid materials to form volatile compounds which are then removed by a vacuum pumping system. Plasma etching has important advantages in the terms of cost, cleanliness, fine-line resolution, and potential for production line automation.
Integrated circuit technology is evolving so rapidly that even a period as short as six months can produce a significant change.
The manufacturing process consists in forming the sequence of layers precisely in accordance with the plan of the circuit designer. Nowadays computers are used to simulate the operations of the designed circuit. Computer simulation is less expensive and more accurate than assembling a bread-board circuit made up of discrete circuit elements.
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Materials used in producing microelectronic devices greatly affect the device performance. For instance, there is demand for materials that exhibit such characteristics as low contact resistance, reduced vulnerability to electromigration, and processibility at low temperatures.
The circuit speed is increased with the reduction of IC feature sizes. At the same time a higher integration level allows the chip area to increase, causing the lengths of the interconnections to increase. For a very large chip with extremely small components, the time delay associated with the interconnections can be considerable and can play a significant role in determining the performance of the circuit. Therefore, severe requirements are imposed on VLSI interconnections materials.
The main requirements for interconnection materials and contacts are high electrical conductivity (= conductance), low ohmic contact resistance, good electromigration resistance, stability when in contact with silicon oxide, high temperature stability, corrosion and oxidation resistance and strong adhesion characteristics. The basic demand is conductivity because it can substantially improve the resistances and delay times of the electrical interconnections used for VLIC structures.
Historically, metals like aluminium and gold were used as interconnection materials. Later they were replaced by polysilicon. Polysilicon has been extensively used to form gate electrodes and interconnections. Refractory metals such as tungsten (W), molybdenum (Mo), titanium (Ti), and tantalum (Ta) and their silicides are receiving increased attention as a replacement / compliment of polysilicon. Silicides of W, Mo and Ta have reasonably good compatibility with the IC fabrication technology. They have fairly high conductivity and they can withstand all of the chemicals normally encountered during the fabrication process.
