Value of Liquid Precursors and Low Temperature Depositions
Low temperature silicon deposition processes are needed for creating devices on a host of alternative substrates such as glass, polymers, or certain metal foils. When considering a commercial fabrication process, the thermal budget – temperature and time – are critical factors, and attempts to decrease the reaction temperatures and residence times in the reactor are highly sought. Additionally, the deposition processes must be high yielding in terms of conversion of the silicon precursor, as well as the growth rate. An added benefit of low temperatures is the possibility of roll-to-roll processing, which would dramatically increase throughput and allow for device complexity and function to be fully realized. Traditional silicon precursors such as monosilane, SiH4, are poor candidates to deliver devices with the attributes mentioned above. Sluggish growth of films and other architectures below 400 °C limit the use of that material. Other higher order silanes such as disilane, trisilane, tetrasilane, and neopentasilane are also limited and often deliver films of relatively poor quality. Cyclohexasilane, on the other hand, has demonstrated high rate growth of silicon thin films and nanowires at temperatures below 300 °C. While trisilane, tetrasilane, and neopentasilane are also liquids at room temperature, only cyclohexasilane (CHS) is well suited to liquid phase deposition. CHS is unique in that it has light absorption above 250 nm, meaning that it can be used for UV photooligermization – significantly expanding the scope of fabrication methods that can be used to make silicon thin films or other nanostructures. Furthermore, the decomposition temperatures (temperature at which conversion to silicon devices or reactive precursors) of the other silanes is higher than the boiling point of the material. This results in evaporation instead of film growth, limiting the yields. CHS does not have that limitation. Solution-based processes, such as those utilizing CHS, may also have much lower processing costs and can use equipment with lower capital costs. These methods open up a variety of highly scalable techniques such as ink jet printing and spin coating, which are not possible with SiH4. Thus we see CHS as a material with tremendous promise.