High-Rate Glass Etching Process for Transferring Polycrystalline Silicon Thin-Film Transistors to Flexible Substrates

Abstract

We report on a process technology that makes possible the transfer of polycrystalline silicon thin-film transistor (poly-Si TFT) arrays from an original rigid glass substrate to another flexible plastic substrate. The transfer technology is characterized by its high-rate glass etching process, using a newly developed apparatus. After a description of the transfer sequence free from adhesive contamination, we present experimental observations for highrate glass etching in hydrofluoric (HF) and hydrochloric (HCl) acid solution mixtures. The etching apparatus provides jets of the solution mixtures to the glass surface in order to achieve good circulation of the solutions in the bath, as well as to remove etch products effectively from the surface. We successfully achieved etch rates as high as 6 m/min with the etched surfaces almost as smooth as the original glass. In order to gain insight into the chemical mechanism of the etching, we developed a simplified kinetic etching model based on a Langmuir isotherm. The model and experimental etch-rate data are generally in good agreement, indicating that the basic modeling approach captures much of the essential chemistry for the high-rate glass etching. The transfer technology allows us to obtain TFT flexible substrates with good electrical characteristics and flexibility even after an annealing process at as high as 150 C. These results demonstrate that the transfer technology is a promising candidate for achieving entirely new lightweight electronic devices such as flexible displays and radio frequency identification tags based on TFT flexible substrates.Index Terms-Flexible substrate, glass etching, kinetic etching model, polycrystalline silicon thin-film transistor, transfer process.

Transfer Processes for Thermally Stable Large-Size TFT Flexible Substrates

“…We numerically solved the following stress-strain equation to estimate the deformation of the materials and the principal stress at 150 C (1) where and denote the stress and strain matrices, respectively.…”

“…In order to obtain TFT flexible substrates, transfer technologies have been developed [1]- [5], in which TFT arrays are fabricated on a rigid glass substrate and then transferred onto a flexible plastic substrate using permanent adhesives. Recent studies on flexible TFT technologies have been motivated by two major applications, namely, flexible displays and electronic papers [6]- [8].…”

“…We previously reported on a process technology for transferring polycrystalline silicon thin-film transistor (poly-Si TFT) arrays fabricated on a glass substrate (150 mm 150 mm) to a plastic film using a permanent adhesive [1]. The transfer technology is characterized by a high-rate glass etching process that allows us to produce etched surfaces almost as smooth as the original glass.…”

“…We have used glass substrates as large as 300 mm 350 mm in the experiments. The sequence has been described in detail in our previous paper [1]. The TFT glass substrate was thinned to several tens of micrometers from the back by applying a jet flow of an HF and HCl mixture solution (b).…”

“…The TFT glass substrate was thinned to several tens of micrometers from the back by applying a jet flow of an HF and HCl mixture solution (b). Using a solution with proper mixture ratios makes it possible to obtain the etched surface almost as smooth as the original glass [1], [11]- [13]. The adhesion of the protective film was reduced to zero after heating it to approximately 110 C for 90 s. This allowed the TFT flexible substrate to be peeled off easily (d).…”