Semiconducting materials engineering progress

I. In microelectronics, the steady reduction of IC feature1 sizes, accompanied by high current densities and increasing demands on electrical performance, has focused the attention of technologists on newer materials which exhibit2 characteristics such as low contact resistance, reduced vulnerability3 to elec-tromigration, and processibility4 at low temperatures.

Over the years, the device size has been reduced tremendously. Improvements available5 in materials technology have allowed integration of more and more devices on the same chip, resulting in increased area. According to the theory of scaling, the smaller dimensions of a MOS transistor should enhance6 its speed. As a first-order approximation, therefore, this should proportionally increase the circuit speed. Indeed, for smaller circuits it does happen. However, for large circuits, the time delays7 associated with the interconnections can play a significant8 role in determining9 the performance of the circuit.

As the minimum feature size is made smaller, the area of cross section of the interconnection also reduces. At the same time a higher integration level10 allows the chip area to increase, causing the lengths of the interconnections to increase. The net11 effect of this "scaling of interconnections" is reflected into an appreciable12 RC time delay. For a very large chip with extremely small geometries, the time delay associated with interconnections could become an appreciable portion of the total time delay, and hence the circuit performance could no longer be decided by device performance.

Thus, as the chip area is increased and other device-related13 dimensions are decreased the interconnection time delay becomes significant compared to the device time delay and dominates the chip performance. These are dominant factors limiting device performance.

Performance is the obvious goal of VLSI; reliability is a more subtle14 one. Therefore, new materials are required for VLSI interconnections.

The design15 of any machine or a device has always been limited by the materials available. The problem in question was that materials could be designed and tailored16 for any new structures.

Semiconductors are used in a wide variety of solid-state devices including transistors, integrated circuits, diodes, photodiodes and light-emitting diodes.

Several elements in and around group IV of the Periodic Table show intrinsic17 semiconductor properties but of these Ge and Si (and to a lesser extent Se) alone have shown chemical and electrical properties suitable18 for electronic devices operating near room temperature.

Germanium and silicon were the first semiconductor materials in common19 use.

A great contribution20 to the study of semiconductor physics has been made by the prominent Soviet scientist A.F.Yoffe. It was in 1930 when Academician A.Yoffe and his co-workers started a systematic research in the field of semiconductors.

The diffusion theory of rectification21 on the boundary of the two semiconductors was elaborated by B.I.Davydov, a Soviet physicist, in 1938. Experimental support of his theory was of great importance in the investigation of processes occurring22 in p-n junctions.

Right after World War II, physicists John Bardeen, Walter Brattain and William Shockley, and many other scientists, turned23 full time to semiconductor research. Research was centered on the two simplest semiconductors germanium and silicon.

Experiments lead to new theories. For example, William Shockley proposed an idea for a semiconductor amplifier24 that would critically test the theory. The actual device had far less amplification than predicted. John Bardeen suggested a revision theory that would explain why the device would not work and why previous experiments had not been accurately foretold by older theories. In new experiments designed to test tie new theory they discovered an entirely new physical phenomenon the transistor effect. In 1948 W.Shockley patented the junction transistor. Junction transistors are essentially solid-state devices having three layers of alternately25 negative or positive type semiconductor material.

The early history of modern semiconductor technology can be traced26 to December 1947 when J.Bardeen and W.H.Brattain observed transistor action through point contacts applied to poly-crystalline germanium. Germanium has become the material in common use. It was realized that transistor action occurred within the single grains27 of polycrystalline material.

G.K.Teal originally recognized28 the immense29 importance of single-crystal semiconductor materials as well as for providing the physical realization of the junction transistor. Teal reasoned3" in 1949, that potycrystalline germanium's uncontrolled resistances and electronic traps31 would affect32 transistor operations in uncontrolled ways. Additionally,33 he reasoned that polycrystalline, material would provide inconsistent product yields and thus be costly. He was the first to define chemical purity,34 high degree of crystal perfection35 and uniformity of structure as well as controlled chemical composition (i.e. donor or acceptor36 concentration) of the single-crystal material as an essential foundation for semiconductor products.

The next decade witnessed37 an ingermanium and the "universal" semiconductor material, silicon. Silicon gradually gained38 favour over germanium as the "universal" semiconductor material.

Silicon is to the electronics revolution what steel was to the Industrial Revolution.

II. Silicon has been the backbone () of the semiconductor industry since the inception of commercial39 transistors and other solid-state devices.

The dominant role of silicon as a material for microelectronic circuits is attributable40 in large part to theproperties of its oxide.

Silicon dioxide is a clear glass with a softening41 point higher than 1,400 degrees C. If a wafer42 of silicon is heated in an atmosphere of oxygen or water vapour,43 a film of silicon oxide forms on its surface. The film considered is hard and durable44 and adheres45 well. It makes an excellent insulator. The silicon dioxide is particularly importarit in the fabrication of integrated circuits because it can act as a mask46 for selective introduction of dopants.47

Silicon's larger band48 gap49 permitted50 device operation at higher temperatures (important for power devices) and thermal oxidation of silicon produced a non-water-soluble stable oxide (as compared to germanium's oxide) suitable forpassing p-n junctions, serving as an "impermeable51 diffusion mask" for common dopants, and as insulator coating52 for conductor overlayers.53

Oxygen concentration present influences many silicon wafer properties, such as wafer strength, resistance to thermal warping (), minorit carrier lifetime and instabilit in resistivity.

The presence of gen contributes to both beneficial and detrimental54 effects. The detrimental effects can be reduced if the oxygen is maintained55 at less than 38 ppms. Thus, the oxygen range56 of the wafer present should be controlled. The results achieved with silicon are great.

However, although the silicon wafer clearly is a fundamental ingradient in the fabrication of an integrated circuit, the silicon materials specification57 may not be critical element in doveloping a successful new 1C product strategy. If silicon material is to remain the semiconductor device material for the next ten years efforts must continue to reduce crystallographic defects, grown-up impurities introduced during device fabrication.

Large-scale integration (LSI) of devices has put great demands on electronic-grade single-crystal material. The semiconductor industry now requires high purity and minimum point-defects concentration in silicon in order to improve the component yield per silicon wafer. These requirements have become increasingly stringent58 as the technology changes from large-scale integration (LSI) to very large-scale integration (VLSI) and very high speed integrated circuits (VHSIC).

The yield (or circuit performance) of a device and the intrinsic and extrinsic materials properties of silicon are interdependent. The silicon wafer substrate must be practically defect-free when the active device density may be as high as 10s to 106 per chip.

To increase further the speed of semiconductor devices requires not only refinements59 in present designs and fabrication techniques, but also new materials that are inherently60 superior to materials presently being used, like germanium and silicon. New material under consideration is gallium arsenide.

Gallium arsenide has a much higher electron mobility than germanium and silicon. The opportunities61 present are as follows: it is potentially much faster; it has a larger band gap, permitting operation at higher temperatures; it is chemically and mechanically stable. Mobilities in this high-purity gallium arsenide are about twice those of germanium and four times those of silicon.

The potential of high-purity gallium arsenide was first explicit62 in a new gallium arsenide-germanium hetero-junction diode. The hetero-junction device has a potential for much faster switching than conventional63 p-n junction diodes. Its calculated switching time is on the order of a few picoseconds (trillions of a second).

However, the difficulty of producing gallium arsenide of sufficient64 purity has limited its application.

Yet, gallium arsenide is far from the end of the story. Any searching for an answer makes contributions. This is the way of developing better materials and devices.



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