Silicon – On - Insulator

(SOI)

SOI is a very attractive technology for large volume integrated circuit production and is particularly good for low – voltage, low power and high speed digital systems

Advantages of SOI technology over bulk technology

SOI wafers potentially offer:•Perfect transistor isolation•Higher packaging density•Reduced parasitic drain capacitance

Advantages of SOI technology over CMOS technology

• Excellent lateral and vertical isolation of active devices from substrate

eliminating inter-device leakage and latch-up in CMOS devices

Effective reduction of substrate coupling, allowing higher quality inductors with increased Q factor

Effective reduction of interference and cross talk between mixed signal devices

Reduced soft errors from radiation (electron – hole pair generation)

• Faster device operation due to reduced parasitic capacitance

• Lower power consumption due to lower operating voltages

• Reduced die area per function – reduced area required for lateral junction isolation, resulting from the absence of wells and the possibility of the possible direct contact of the source and drain diodes in the NMOS and PMOS transistors

• Performance improvement equivalent to the next technology node without scaling

• Potential to simplify device fabrication steps fewer mask and ion implantation steps, due to

the elimination of wellsLess complex lithography and etching required

to achieve next generation performance

SOI structure

SOI applications

This can be segmented into three categories depending on the BOX layer asThick BOX layer wafers Thin BOX layer wafers Ultra thin BOX layer wafers

Thick BOX layer wafers

Si layer thicker than one micron – 1000nm•Power switching devices•High speed bipolar devices•MEMSi.Military ii.Aerospaceiii.Industrial

Thin BOX layer wafers

• CMOS IC applicationsi. PDA / Hand setsii. Mainframesiii.Portable wireless devicesiv.Automotivev. Workstations

Ultra thin BOX layer wafers

• 0.1 micron CMOS fabricationi. High end PCii. Servers

SOI challenges• Cost• Continuity and thickness uniformity of the BOX

layer• Defects like voids and inclusions in BOX• SOI CMOS devices exhibit parasitic phenomenon• Floating body effects• Self heating effects - due to thermal insulation of

the device from the substrate by the BOX layer• Floating body leading to increased D-S leakage

currents

Types of SOI wafers

• PDSOI – silicon surface layer is thicker than the depth of the depletion region in the transistor’s channel

• FDSOI - silicon surface layer is equal to the depth of the depletion region in the transistor’s channel

• This also depends on the silicon layer thickness above the BOX layer and the doping concentration in the channel

• FDSOI – channel doping concentration is low enough such that the gate depletion region extends throughout the entire thickness

• Si surface layer thicker than 200nm – PDSOI• Si surface layer thickness reduced to about

100nm - FDSOI • The depletion layer of PDSOI does not reach

through the entire silicon channel / body region• PD material usually has a Si thickness greater

than 0.15μm• For FD devices, the SOI film is thinner than the

device depletion width• In FDSOI, there is no body region of the MOS

device that can be charged, hence no floation body

SOI fabrication

Separation by Implantation

Smart Cut

Implant with hydrogen and anneal

Oxidized, annealed

Flip implanted wafer onto substrate and bond the wafers

Split wafer along the stress fracture

Polish

Smart cut - unibond

Grind and etch back process

Epitaxial Layer transfer (Eltran)