Investigation on Crack Suppression by Thermal-Plasma-Jet Crystallization of Amorphous Silicon Films on Flexible Glass Substrate

Abstract

Introduction.Ultra-thin glass substrate is attracting much attention as flexible substrates for curved display applications. In fabrication of thin-film transistors (TFTs) on flexible glass, crystallization of amorphous Si (a-Si) is one of the key process technologies [1-3]. Rapid-thermal-annealing (RTA) in microsecond regime is necessary to form high crystallinity Si without thermal damage to glass. We have proposed the application of atmospheric pressure DC ark discharge thermal-plasma-jet (TPJ) to the crystallization of a-Si films [4]. Because of its simple structure and atmospheric pressure process, TPJ enables low cost crystallization compared with excimer laser annealing (ELA) technique. High speed lateral crystallization (HSLC) is induced by sweeping a molten region at a speed as high as 4000 mm/s. However, residual tensile stress is generated because of the densification of glass surface after RTA process. As a result, significant number of cracks are generated on the surface of glass substrate in cases of sever annealing conditions. In this study, we applied µ-TPJ crystallization of a-Si films on ultra-thin glass substrate and investigated the effect of glass thickness on residual stress and crack generation. Experimental.After RCA cleaning and HF treatment of 100-µm-thick flexible glass and 500-µm-thick conventional glass substrates, 100-nm-thick a-Si films were deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 250 °C. Dehydrogenation was carried at 450 °C in N2 ambient for 1 h. TPJ irradiation set up is shown in Fig. 1. µ-TPJ was generated by DC arc discharge under atmospheric pressure with supplying power (P) of 1.7 kW between electrodes. The Ar gas flow rate (f) was 1.0 L/min. µ-TPJ was generated by blowing out the thermal plasma through an orifice with its diameter (ϕ) of 600 µm. The distances between the plasma source and glass substrates (d) were fixed at 1.0 mm and the substrates were linearly moved by a motion stage in front of the µ-TPJ with scanning speed (v) of 2200 mm/s. For the measurement of temperature profile in the glass substrate, transient reflectivity during TPJ annealing was measured by irradiating bare conventional glass substrate with a He-Ne laser (632.8nm) from the back surface. Details of the non-contact temperature measurement are described in Ref [5]. Results and discussion.The microscope images of crystallized Si films on each substrate are shown in Fig. 2. Many cracks were observed in conventional glass substrate. On the other hand, no crack was observed in flexible glass substrate, although both glass substrates were annealed at the same time. Crystallization was conducted under various µ-TPJ irradiation conditions, and almost no cracks were found in flexible glass and it showed significant resistance against RTA. This result suggests that the tensile stress at glass surface is significantly reduced in the case of ultra-thin flexible glass. Previous study calculated residual stress profile in glass substrate on the basis of multilayered beam theory [6]. To obtain stress profile, we measured transient temperature profile during TPJ irradiation by optical probe (Fig .3). We assumed surface densification of glass when the temperature exceeded softening point (1258 K). We conducted residual stress analysis of glass substrate as the residual stress is shown in Fig. 4 [6]. Strong tensile stress is introduced in glass surface due to the shrinkage by RTA. At the same time, compressive stress is introduced inside of glass to compensate the tensile stress at the surface. It should be noted that the depth of stress compensation is in the range of ~400 µm as shown in Fig. 4, which is larger than the thickness of flexible glass. It is expected that the back side of thin glass substrate is free to relax the stress and plays a roll in suppression of residual stress in the surface of glass. Conclusions.We have crystallized a-Si films on flexible glass substrate by µ-TPJ without crack generation. Flexible glass substrate is useful to relieve surface tensile stress and as the result, crack generation is significantly suppressed. Acknowledgments. A part of this work was supported by Research Institute for Nanodevice and Bio Systems (RNBS) Hiroshima University, and JSPS KAKENHI Grant Number (B)(16H0433400).