Engineered Substrates
Why silicon is not fast enough for future microprocessors
Have you ever wondered why the first transistor was constructed from Germanium but all modern computer chips are manufactured from single crystal (100) orientation silicon? The reason is that silicon can grow a thermal oxide that has dielectric properties necessary for creating isolation areas in the transistors. However, germanium is a faster material which means that the speed of the signal carriers (electrons and holes) are higher and the overall device would operate more quickly if made from Germanium. Many other materials also have higher speeds than silicon and these include strained SiGe (silicon Germanium alloys), strained silicon, (110) orientation silicon, and even strained germanium.
So why don’t we utilize these materials for computer chips? The answer comes back around to several key evolutionary elements in semiconductor device fabrication. First, high quality dielectric layers in conjunction with the semi conducting materials are necessary to create a transistor. Silicon’s natural oxide was a low cost, high quality solution. Fortunately as time has advanced many synthetic oxide material are now readily available. Both low k (low dielectric strength, less insulating) and high k materials are developed for advanced electronic device fabrication. Secondly the growth and stability of some of the more advanced materials, e.g., strained layers, have required considerable development efforts to enable their use as commercial processes.
Decreasing transistor size has, until recently, allowed for microprocessor speed increases according to Moore’s law. However, as semiconductor manufacturing entered the sub-90 nm era, decreasing the feature size of the CMOS processor was unable to sustain progress according to Moore's Law. International forums like the International Electron Devices Meeting (IEDM), IEEE SOI Conferences, and the Symposium on VLSI Technology are venues where scientists have provided the proof of concept and underlying advantages of the solutions supported by the new materials as well as new architectures. The conclusion: “Silicon is not fast enough for our future microprocessor demands”.
Engineered substrates are a collective of man-made solutions that may provide materials solutions for the sub-90nm era. They are called “engineered” because they are not occurring naturally in the earth. Similar to the spin on dielectrics that can be used in place of native grown oxide layer, the engineered substrates in grown or manipulated in the lab by substrate suppliers. Growth chambers and layer transfer techniques are used to scientists to create starting materials that visually look much like a standard silicon substrate but have significantly enhanced performance properties.