• PDF / 3,672,792 Bytes
• 29 Pages / 439.37 x 666.142 pts Page_size
• 86 Downloads / 287 Views

DOWNLOAD

REPORT

---

8.1 What is Silicon-on-Insulator? Silicon is by far the most widely used semiconductor material. It is abundant in the earth's crust and relatively easy to convert into a high-purity single crystal. Unlike some other semiconductor materials, silicon is stable when heated at high temperature, and a well-behaved insulating and passivating material, silicon dioxide, can readily be grown on it. The excellent electrical and chemical properties of thermally grown Si0 2 are probably the most important factor that has made silicon such a successful semiconductor material (concerning devices, see Chap. 18, by Risch). 8.1.1 General Properties of SOl MOS Transistors Classical silicon devices, such as metal-oxide-semiconductor (MaS) transistors, are made at the surface of silicon wafers that are 700-800 11m thick, but occupy less than the top first micrometer at the surface of the wafer. The remainder of the wafer serves as a mechanical support for the devices and sometimes gives rise to unwanted, parasitic interactions with the devices. In a silicon-on-insulator (Sal) wafer the devices are fabricated in a thin silicon layer. This silicon layer is single-crystal and sits on an insulating material, usually silicon dioxide. Typically the silicon layer thickness ranges from 10 nm to several micrometers, depending on the application, and the silicon dioxide layer thickness ranges between 50 nm and a micrometer. The whole structure rests on a mechanical substrate, typically silicon, although silicon films on glass or quartz substrates are preferred for some applications. The oxide layer between the active top silicon layer and the mechanical silicon substrate is called the buried oxide (BOX). Figure 8.1 shows an MaS transistor made on a silicon, a silicon-on-insulator, and a silicon on glass/quartz substrate. Typical thicknesses used in CMOS applications are indicated. MaS transistors made in an sal film offer several advantages. In bulk silicon technologies, devices are isolated from one another by reverse-biased PN junctions. As a result there is a capacitance (PN junction capacitance) between the source or the drain and the silicon substrate. The source and drain of sal devices, on the other hand, are fully isolated from the substrate by a dielectric material. This reduces significantly the capacitance of the source and drain,

P. Siffert et al. (eds.), Silicon © Springer-Verlag Berlin Heidelberg 2004

140

J.-P. Colinge

+700~m