SOI PROCESS

• silicon on insulator is also an advance semiconductor technology where silicon is replaced as a substrate material.

• Rather than using silicon as the substrate material, technologists have sought to use an insulating substrate to improve process characteristics such as speed and latch-up susceptibility

• The SOI CMOS technology allows the creation of independent, completely isolated nMOS and pMOS transistors virtually side-by side on an insulating substrate (for example: sapphire).

• The main advantages of this technology are the higher integration density (because of the absence of well regions), complete avoidance of the latch-up problem, and lower parasitic capacitances compared to the conventional n-well or twin-tub CMOS processes.

WHY SAPPHIRE• The silicon is usually deposited by the decomposition of silane

gas (SiH4) on heated sapphire substrates. • The advantage of sapphire is that it is an excellent

electrical insulator, preventing stray currents caused by radiation from spreading to nearby circuit elements

STEPS• The basic SOI process can be explain as follows• A thin film 7-8 micrometer of very lightly doped n type si is

grown over an insulator. Sapphire or sio2 is a commonly used insulator.

• Anisotropic etching is required to etch away the silicon except where a diffusion area will be needed.

Step 1.Sapphire as an insulator

Step2: place lightly doped silicon (n-) over sapphire.

After the etching process,

• Step3: create P Island and N Island over the lightly doped silicon

Step3ap type island is formed by boron implantation by masking silicon n substrate type

Step3bn type island is formed by phosphorous implantation by masking p type island

Step-4• Polysilicon formation

NMOS fabricationBy masking the n type island phosphorous impurities are added to make NMOS transistor

PMOS FabricationBy masking the p type island boron impurities are added to make NMOS transistor

After this it undergoes the process of metallization