11. RF CMOS Circuits on Flexible, Application-Specific Substrates
Category: Physically Flexible Electronics
Paper 15.7 – Application-Oriented Performance of RF CMOS Technologies on Flexible Substrates; Justine Philippe et al, IEMN/STMicroelectronics/CEA LETI Minatec

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Although physically flexible circuitry would enable innovative wearable, biomedical, security and other products, flexible circuits so far have demonstrated only limited performance. That’s because high-performance CMOS devices are fabricated using harsh high-temperature processes that damage most flexible materials. A team led by France’s Institut d’Electronique de Microélectronique et de Nanotechnologie, though, has developed what they call an ultimate thinning and transfer-bonding (UTTB) process which they used to build radio-frequency CMOS circuits on a variety of flexible substrates: polyimide plastic film, glass, and stainless steel. First they built RF CMOS circuits on an SOI substrate, then they thinned it to 30µm by completely removing the backside. The circuits were then transferred to the various substrates using a laminating process. For plastic and glass substrates, the circuitry was attached by laminating it using a dry polymer film and rollers. For stainless steel substrates, a 400nm–thick indium layer was first deposited, and then the circuits were laminated to it in a similar manner. The small-signal performance of these devices wasn’t significantly degraded from what it had been on the original substrate, and unwanted harmonics were actually reduced. The researchers say their UTTB technique can be adapted to meet application-specific requirements for ultra-mechanical flexibility, heat dissipation and transparency.

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