Liquid-Si Technology for High-Speed Circuits on Flexible Substrates

Abstract

Recently, flexible, wearable and disposable electronics have attracted a lot of attention. Printing enables low-cost fabrication of circuits on flexible substrates. Printed organic and metal oxide thin-film transistors (TFTs) have been researched intensively due to the ease of solution-processing. But their carrier mobility and reliability are inferior to conventional CMOS transistors fabricated with crystalline Si. Printed Si TFTs have also been reported, including amorphous Si and poly-crystalline Si TFTs. Both techniques are based on a precursor of liquid-Si solution. The high temperature required for forming Si film and the low mobility due to randomly positioned grain boundaries inside the channel region are limitations for fabricating high-speed circuits on flexible substrates. In this thesis single-grain Si TFTs with high performance produced at a low temperature (< 350 °C) from a printed liquid-Si solution on a flexible substrate is presented. Applications may include display drivers, flexible memories, printed RFID tags and other high-speed circuits on flexible substrates. Liquid Si is the mixture of a cyclopentasilane (CPS) monomer, UV-polymerized CPS and solvent. It can be spin coated on top of a substrate. Under thermal treatment, the solvent is evaporated, Si-H bonds are broken, and an amorphous Si film is formed. After the film is thermally annealed at 650 °C for dehydrogenation, it is crystallized by a XeCl excimer laser (308 nm) to make location-controlled single grains, using the ?-Czochralski crystallization method. Top-gated Si TFTs are fabricated with the channel inside a grain, and self-alignment source/drain doping by ion implantation is employed in the process. In Chapter 3, the fabrication process is discussed in detail. Due to the absence of grain boundaries in the channel region, the TFTs show carrier mobilities of 423 cm2/Vs for electrons and 118 cm2/Vs for holes, which are higher than those of organic-, metal oxide-, a-Si- or poly-Si TFTs. NMOS TFTs show stable behavior under gate and drain stress, and negligible hysteresis effect. On the other hand, PMOS TFTs show trap generation and carrier injection from the gate. To meet the temperature requirements for fabrication on flexible substrates, a low-temperature (