SOS

Silicon on sapphire (SOS) is a hetero-epitaxial process for integrated circuit manufacturing that consists of a thin layer (typically thinner than 0.6 micrometres) of silicon grown on a sapphire (Al₂O₃) wafer. SOS is part of the Silicon on Insulator (SOI) family of CMOS technologies. SOS is primarily used in aerospace and military applications because of its inherent resistance to radiation. Typically, high-purity artificially grown sapphire crystals are used. 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. SOS has seen little commercial use to date because of difficulties in fabricating the very small transistors used in modern high-density applications. This drawback is because the SOS process results in the formation of dislocations, twinning and stacking faults from crystal lattice disparities between the sapphire and silicon. Additionally, there is some aluminium, a p-type dopant, contamination from the substrate in the silicon closest to the interface.

Silicon on Sapphire Circuits and Systems

A Silicon on Sapphire microchip designed by e-Lab [1]

Silicon on Insulator (SOI) and Silicon on Sapphire (SOS) technologies have recently attracted more and more interest in the development of next-generation high-performance VLSI circuits and systems. The absence of latch-up, the reduced parasitic capacitance, the transparency of the substrate, the isolation and multi-threshold devices are just a few of the advantages of this technology.

The physical differences in the fabrication of the SOS devices and the insulating substrate make SOS MOSFETs quite different from a bulk process. While the characteristics of the devices are not significantly different from the devices in a bulk process, the insulating substrate, the floating body and the different thermal properties of the sapphire give rise to characteristics that have to be fully mastered to allow for high-precision design of circuits and systems [2].

The advantages of the SOS technology allowed research groups as Yale e-Lab to fabricate a variety of SOS circuits and system that benefit from the technology and advance the state-of-the-art in:

• analog-to-digital converters (a nano-Watts prototype was produced by Yale e-Lab)[3][4]
• monolithic digital isolation buffers [5]
• SOS-CMOS image sensor arrays (one of the first standard CMOS image sensor arrays capable of transducing light simultaneously from both sides of the die was produced by Yale e-Lab)[6]
• patch-clamp amplifiers [7]
• energy harvesting devices [8]
• three-dimensional (3D) integration with no galvanic connections
• charge pumps [9]
• temperature sensors [10]

Substrate Analysis - SOS Structure

TEM Cross-Section of epitaxially grown silicon and sapphire substrate taken by Semiconductor Insights

The application of epitaxial growth of silicon on sapphire substrates for fabricating MOS devices involves a silicon purification process that mitigates crystal defects which result from a mismatch between sapphire and silicon lattices. The Peregrine PE42612 SP4T switch is formed on an SOS substrate where the final thickness of silicon is approximately 95nm. Silicon is recessed in regions outside the polysilicon gate stack by poly oxidation and further recessed by the sidewall spacer formation process to a thickness of approximately 78nm.

See also

• Silicon on Insulator
• Radiation hardening
• e-Lab
• Semiconductor Insights

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