Silicon on insulator
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Silicon on insulator (SOI) is a layered structure consisting of a thin layer of silicon, from 50 nm to 100 µm, which is created on an insulating substrate, which is usually sapphire or a silicon with an insulating layer of silicon dioxide 80 nm thick on its surface. This process reduces the amount of electrical charge that the transistor has to move during a switching operation, increasing speed (up to 15%) and reducing switching energy (up to 20%) over CMOS-based chips. SOI chips cost more to produce and are generally used for high-end applications.
A popular SOI technology is Silicon on sapphire (SOS), used in special rad-hard applications in military and aerospace.
SOI differs from generic CMOS in that its silicon junction is above an electrical insulator. The advantage is that this insulator reduces the capacitance, meaning the transistor has less to charge-up before completing a switch, which results in reduced switching time.
SOI wafers are produced by one of two main methods:
- SIMOX - Separation by IMplantation of OXygen - uses an oxygen ion beam implantation process followed by high temperature annealing to create a buried SiO2 layer.
- Smartcut is another method that bonds the oxidized surface of two wafers together and then splits most of the top wafer away along a band of implanted hydrogen bubbles. The thin layer of silicon that is left behind is isolated from the substrate by what was originally the surface oxide layers.
As of 2005 the technology is being gradually adopted by electronics manufacturers. In particular, Sony, Nintendo, and Microsoft plan to use it in production of their next-generation consoles. It is important to note that the SOI based-processors that are going into these machines are all manuactured by IBM with the exception of the Sony Playstation. AMD is starting to use SOI in some of its new processors, and many Apple and high-end IBM server processors have used it for some time. Apple processors are manufactured by IBM and Freescale. It should be noted that while Sony, AMD, and Chartered manufacture their own SOI-based technologies, each of these companies is partnered with IBM.