Fabricating High-Performance Silicon Thin-Film Transistor by Meniscus Force Mediated Layer Transfer Technique

INTRODUCTION. If such a Silicon (Si) metal-oxide-semiconductor field-effect transistor (MOSFET) technology, namely, high-performance single crystalline silicon (c-Si) MOSFETs formed locally at the required position at low temperature (150°C or less) on flexible substrates, were established, so-called “flexible electronics” could be further advanced. Moreover, this technology could also be applied to system integration, namely, simultaneously mounting different devices such as integrated circuits and sensors[1]. We have proposed a novel layer transfer of the (100)-oriented c-Si thin film to a flexible substrate at very low process temperature[2]. In this study, by applying above-mentioned technology, the performance of flexible MOSFETs fabricated by using a c-Si film transferred by meniscus force was verified experimentally.

EXPERIMENTAL. The SOI layer [p-type Si(100), 10-30 W∙cm] was patterned to form a 3-µm-wide and 5-µm-long dog-bone shape with 20×20 µm² square regions at both ends. This pattern is applicable to MOSFET fabrication to form a source, a channel, and a drain. To form supporting columns, holes with size of 2 × 2 µm² were made at intervals of 3 µm in the square regions. The 300 nm-thick buried-oxide (BOX) layer was isotropically etched by 33% hydrofluoric acid at room temperature for 90 seconds by using the patterned SOI layer as a mask for the midair cavity. As a result of the etching of the BOX layer below the patterned SOI layer, the narrow SiO₂ columns can be formed underneath both end regions of the film. Figure 1 schematically shows the meniscus force(F=πR²γ[(1/R)-(2cosθ_E/H)]+2πR²γsinθ_E) mediated layer transfer (MLT) of SOI layer to a flexible substrate[2]. The starting and counter PET substrates were in close face-to-face contact with the filling water, and these substrates were heated on a hot plate at 80°C.

RESULTS AND DISCUSSION. When the starting SOI wafer and the PET substrate were separated, it was confirmed that the original form and position of the SOI layer were completely maintained after the film transfer. The electron-backscatter-diffraction (EBSD) pattern map confirms that a (100)-oriented c-Si thin film was successfully formed on PET substrate at 80°C. Here, to confirm the adaptability of this layer transfer technique to thin-film devices, MOSFET were fabricated on a PET substrate. The source and drain regions were implanted with phosphorus ions at a dosage of 1×10¹⁵ cm^-2. To control the threshold voltage, boron ions were injected (at a dosage of 1×10¹¹ cm^-2) into the channel region. After impurities in the source, channel, and drain regions were activated by heating at 1000 ºC in nitrogen ambient for 10 minutes, the BOX layer was isotropically etched by 33% hydrofluoric acid at room temperature for 90 seconds by using the patterned SOI layer as a mask for the midair cavity. As a result of the etching of the BOX layer below the patterned SOI layer, the narrow SiO₂ columns can be formed underneath both end regions of the film. Here, the key process is the thermal oxidation of the SOI layer on the midair cavity. An 11-nm-thick gate insulator was formed on a SOI wafer temperature at 1000 ºC in dry oxygen ambient for four minutes[3]. By thermally oxidizing the SOI layer, the SOI layer can be covered on both surfaces with SiO₂. After that, the forming gas annealing was performed at 400 ºC in a 3%-hydrogen/nitrogen mixture for 30 min. The thermal oxidation and subsequent hydrogen anneal ensure a good MOS interface, and the SiO₂ layer works as a “blocking” layer that blocks contamination from PET. After the SOI layer was transferred to the PET substrate by MLT technique, a 200-nm-thick SiO₂ film was deposited at 130 ºC by remote plasma chemical vapor deposition (RPCVD). After the contact holes are opened, the gate, source, and drain electrodes are formed by aluminum evaporation and wet etching. The MOSFET dimensions are length (L) of 3.7 µm and width (W) of 3 µm, and the maximum temperature of the fabrication process is 130ºC. Figure 2 shows I_d-V_g characteristics of the MOSFETs fabricated on a transferred Si film. The MOSFET dimensions are L = 3.7 µm and W = 3 µm. The MOSFET showed a mobility of 343 cm²V^-1s^-1.

ACKNOWLEDGEMENTS.A part of this work was supported by Research Institute for Nanodevice and Bio Systems, Hiroshima University and Funding Program for Next Generation World-Leading Researchers (NEXT Program) from the Japan Society for the Promotion of Science (JSPS). This work was supported by JSPS KAKENHI Grant Number 252156.

REFERENCES. [1] T. Fukushima, et. al., IEDM  Tech. Dig., 1 (2008). [2] K. Sakaike el al., Jpn. J. Appl. Phys. 53, 018004 (2014). [3] K. Sakaike el al., Appl. Phys. Lett., 103, 233510  (2014).