The role of materials science in the evolution of microelectronics
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Introduction It is no surprise that every leap in human civilization is identified with a material: Stone Age, Bronze Age, Iron Age. The current era is labeled the Information Age, but the designation of Silicon Age might be more appropriate. Indeed, without some of the crucial developments in the synthesis and processing of silicon and related materials, the Information Age might not have materialized. The existence of semiconductors has been recognized since the time of Michael Faraday in the middle of the 19th century. However, the concept of semiconduction could not be tested because of the lack of availability of high-purity semiconductors. With the advent of zone refining in the mid-1940s, it became possible to purify germanium and silicon to very high levels. This enabled scientists from Bell Telephone Laboratories to demonstrate transistor action in 1948, for which they received the Nobel Prize in Physics in 1956. In the information technology industry, the principal driving forces for large-scale, very-large-scale, and now ultra-largescale integration (ULSI) are higher device speeds and reduced costs per chip. These objectives were accomplished by reducing device dimensions and increasing wafer diameters so that the yield of chips per wafer was increased. Because thermalgradient-induced stresses are the major source of dislocations in as-grown crystals,1 large-diameter silicon crystals must be grown under precisely tailored thermal gradients. This avoids
the multiplication of defect clusters formed by the condensation of point defects during cooling from the melt.2 These objectives were successfully achieved, and macroscopically dislocation-free silicon crystals as large as 30 cm in diameter are now routinely grown by the Czochralski (CZ) process. The resulting crystals are highly perfect, a crowning achievement of materials engineering. The as-grown crystals are subsequently sliced into wafers and then polished to remove slicing damage. The polished wafers serve as the platform for the fabrication of ULSI circuits, which entails many steps. Figure 1 shows a schematic of the well-known metal oxide semiconductor field-effect transistor (MOSFET), one of the building blocks of integrated circuits. In addition, the circuits consist of resistors and inductors, and the various components in a chip are selectively connected to each other by metal interconnects and insulated from each other by dielectrics. In this article, we review some of the critical challenges that were faced in the realization of ULSI circuits, the siliconbased chips that have driven the information revolution. These challenges included an understanding of electronic properties of defects, growth of highly perfect silicon crystals, development of highly stable dielectrics, localized doping of source and drain regions, and improvement of contacts and interconnects. Lithography is crucial as well for creating ever-finer lateral features, but it is beyond the scope of this article.
S. Mahajan, Department of Chemical Engineering and Materials, Univ
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