Optimal design of flexible silicon nano-membrane transistor with high performance

Abstract

In recent years, flexible electronics have gained a great interest and a flexible market is predicted to grow larger. As a result, many research groups focus on fabricating devices with flexible materials such as organic materials and 2D materials. However, devices with organic materials and 2D materials have limitation as they have low mobility, low on-off ratio, and high driving voltage. Since single crystalline silicon overwhelms the performance of these materials, some research groups work on flexible silicon device with nano or micro level thick silicon. In this work, flexible fully depleted silicon on insulator metal-oxide semiconductor field effect transistor (FDSOI MOSFET) with a single crystalline silicon was fabricated. Fabricated devices with various silicon thick-ness were analyzed. It was found that optimum thickness window for FDSOI is 15 nm to 40 nm, in order to avoid high access resistance issue and transition to partially depleted silicon on insulator (PDSOI). The device with best performance was obtained with silicon thickness of 26.8 nm. This device have effective mobility of $720.81 cm^{2}V^{-1}s^{-1}$, sub-threshold swing of 60.9 mV/dec and on-off ratio higher than $10^{7}$. Performance of this device overwhelms that of device with organic or 2D materials. An advanced transfer method which is simple, easy and cost-effective is proposed. This transfer method consists of wafer grinding before fabrication of device and bottom silicon wet etch by tetramethylammonium hydroxide (TMAH) solution after fabrication of device. With propose transfer method, fabricated devices were transferred on to polyimide (PI) tape and deeply analyzed through bending tests with the smallest bending ra-dius of 5 mm. Interestingly, device with Al gate was found to have much higher flexibility than that of device with poly silicon gate. From the electrical result and strain calculation it is concluded that gate material with low Young’s modulus, thinner thickness, low CTE with low deposition temperature is preferable for high flexibility. Devices with various silicon thicknesses were also transferred. From this experiment, it is concluded that thinner silicon thickness is preferable for high flexibility.