Fabrication of a Flexible CMOS Image Sensor Integrating Crystalline Selenium Photoconversion Film and Thinned FDSOI Circuit

In a smart society based on the surrounding sensing and monitoring infrastructure, high-performance flexible sensing devices that adapt to various shapes and object movements are necessary. Although flexible sensors using organic and inorganic thin-film transistors have previously been developed to automatically collect and visualize large information around us, a further performance and reliability enhancement of the devices for practical use is required. Here, we demonstrate the flexible silicon (Si)-based complementary metal-oxide semiconductor (CMOS) image sensors with high performance and flexibility by transferring the ultra-thinned CMOS circuits and photoconversion films onto flexible substrates. Combining CMOS circuits on a fully depleted silicon on insulator (FDSOI) substrates [1], which allows very thin device layers, and a crystalline selenium (c-Se) photoconversion film with its outstanding light absorption properties [2] achieves high-performance and excellent flexibility. In this presentation, we discussed substrate thinning and transfer technology, as well as the deposition process of c-Se-based photoconversion film, confirming the mechanical flexibility and operation of thinned CMOS circuits.<br/><br/>The CMOS circuit is formed on the FDSOI wafer with a handle layer, device except for handle layer, and CMOS active layer with thicknesses of 725 µm, 6 µm, and 50 nm, respectively. After dicing into chips and bonding to the temporary fixing substrates, we first ground the handle layer to a thickness of 30 µm mechanically and then removed it using xenon fluoride etching, where the Si is selectively etched and the reaction stopped at the surface of the buried oxide layer. Next, we bonded the device to a 50-µm-thick flexible polyethylene terephthalate (PET) substrate with an adhesive film and removed the temporary fixing substrate after the ultraviolet irradiation. Finally, the photoconversion film, which comprises a 20-nm-thick wide-bandgap n-type gallium oxide layer, 300-nm-thick p-type c-Se absorption layer, and 30-nm-thick indium tin oxide transparent electrode is directly provided on the CMOS circuit. A c-Se was fabricated by annealing amorphous Se at 200 degrees, whereas tellurium was used as a nucleation layer to prevent Se film from peeling. Consequently, we successfully produced a 6-μm-thick FDSOI device with a 350-nm-thick c-Se-based photoconversion film on a flexible PET substrate.<br/><br/>We compared the characteristics of the drain current-gate voltage of the transferred CMOS circuit with that of the bulk (before transferred) ones, where the gate length is 2 μm. The measurement results reported no significant changes in the characteristics of the devices before and after the transfer, thereby indicating that the thinning and transfer process does not adversely affect the circuit operation. Furthermore, the flexural rigidity (<i>D</i>) of an object is expressed by the following equation [3]:<br/><br/><i>D=E</i>×<i>t³/ 12(1-ν²)</i><br/><br/>where <i>E</i>, <i>t,</i> and <i>ν</i> are the Young's modulus, thickness, and Poisson's ratio, respectively. FDSOI, which is mostly composed of silicon dioxide, has a smaller Young's modulus and Poisson's ratio than Si. Additionally, the thickness of the device, which comprises the photoconversion film and the FDSOI circuit, can be made extremely thin, thereby allowing the device flexibility. Also, the fabricated device can be wrapped around a cylindrical glass rod with a curvature of 6 mm without cracking, confirming its mechanical flexibility.<br/><br/>Conclusively, we developed a fabrication process for a flexible CMOS image sensor that integrates a c-Se-based photoconversion film and a thinned FDSOI circuit. Since this technology enables the flexibility of various Si devices, it can open up new fields of application in image sensors and several high-performance flexible Si devices.<br/><br/>[1] M. Goto et al., IEDM Tech. Dig., 4.2 (2014)<br/>[2] S. Imura et al., Sci. Rep. 10, 21888 (2020)<br/>[3] L. D. Landau et al., Theory of elasticity: volume 7, Elsevier (1986)