Silicon Material Properties for VLSI Circuitry

Publisher Summary This chapter defines VLSI as a circuit composed of at least 10 5 active electronic gates (AEGs) per die. VLSI structures will provide for high-speed—low-delay-time—circuitry that consumes little power in the smallest possible area of a silicon wafer. The active components of the VLSI circuit will likely be concentrated in a region limited to the micrometer of silicon nearest the wafer surface. Within this micrometer, the VLSI structure may consist of multiple layers of p–n junctions separated in depth by as little as 1000 A. These shallow junctions will require steep dopant gradients to ensure high-speed circuit performance. The active components will likely be laterally separated by regions of selectively oxidized silicon to enhance device-to-device isolation. The fabrication of VLSI circuit structures with these features may require the extensive use of several processing techniques only now coming to the forefront. The availability of high-quality polycrystalline silicon is the starting point for high-performance VLSI circuitry. Unwanted impurities in the starting raw polycrystalline silicon can directly contribute to VLSI circuit degradation and failure. The starting raw materials for polycrystalline silicon are quartzite and carbon. Carbon, a common impurity in silicon, is introduced into the refining process in the form of coal, coke, and wood chips. The process for transforming semiconductor-grade polycrystalline silicon into single-crystal ingots can introduce a number of factors that may degrade the performance of VLSI circuits, such as dopant in-homogeneities, excess vacancies or silicon self-interstitials, unwanted impurities, and grown-in crystal imperfections.