Anomalous Mobility Improvement in Ultra-Low-Temperature Polycrystalline-Silicon Thin-Film Transistors on Flexible Substrate after Laser Lift-Off Process
Authors: Wei Yin-Chang , Lee I-Che , Cheng Huang-Chung 
Published: February 2020

This work investigates an improvement in anomalous mobility in high-performance ultra-low-temperature polycrystalline-silicon (ULTPS) thin-film transistors (TFTs) on flexible substrate after laser lift-off process. The experiment results reveal that the improvement was observed in p-type TFTs but not observed in n-type ones. It was contributed to the boron atom has more kinetic energy than phosphorous one during laser lift-off process. Consequently, the effect of slight boron movement is to reduce the effective p-TFT channel length.Electronic devices fabricated on flexible substrates have drawn a lot of attention to recent researches in these years. Transistors on flexible substrates were the hearts of flexible technologies. Many literatures reported the implementing methods including carbon-nano-tubes transistors, amorphous silicon TFTs, amorphous IGZO TFTs, and etc.. Some of them suffered from either high temperature process or poor transistor characteristics. And the most important, some of them suffered from single polarity transistors, such only n-type transistors. A very attractive technology for flexible electronics was using ULTPS TFTs fabricated direct on flexible substrate on carrier glass [1]. It could not only realize large-size manufacturing process but also is compatible with current CMOS LTPS TFTs circuits. Since the ULTPS TFTs on flexible substrates needs to be lift-off from carrier glass, the electrical characteristics between before and after lift-off process should be studied. However, there were lacks of study of anomalous mobility improvement in p-channel ULTPS TFTs after laser lift-off process.

In this experiment, the complementary ULTPS TFTs, i.e. n- and p- channel devices, were both fabricated on flexible material coated on carrier glass then lift-off by laser irradiation. The brief fabrication process was as followings. First, the carrier glass was coated by a layer of polyimide film. After curing, the complementary TFTs were fabricated under ultra-low temperature process by fine tune the recipes. Finally, to implement flexible electronic applications, the ULTPS TFTs were lift-off from the carrier glass by virtue of excimer laser irradiation. The laser irradiation energy densities were varied to investigate the lift-off process effects on the electrical performance of ULTPS TFTs, as shown in Fig1(a)-(c). One sample, named high-energy laser lift-off (HELLO), was irradiated at 320 mJ/cm2. The other sample, named low-energy laser lift off (LELLO), was irradiated at 300 mJ/cm2. And the device dimension was W/L=20um/4.5um for both n- and p- channel TFTs.

As fabricated, the HELLO TFTs have been demonstrated the field-effect electron and hole mobility of 82 and 85 cm2/V-s, respectively, while the corresponding characteristics were 78 and 86 cm2/V-s for the LELLO TFTs. Both of them had the similar performance on carrier glass. After laser lift-off process, it was observed that the electron mobility were 81 and 77 cm2/V-s for HELLO and LELLO devices, accordingly. The hole mobility for LELLO TFTs was 88 cm2/V-s. In the contrast, that for HELLO TFTs was improved to 92 cm2/V-s. It could be seen that the electron mobility was not improved for both HELLO and LELLO devices. And no improvement in hole mobility was observed in LELLO devices, but on the contrary an anomalous hole mobility improvement was found in HELLO devices. In the meanwhile, no significant changes were observed in the other electrical parameters, such as threshold voltage and subthreshold swing[2].

The anomalous field-effect hole mobility improvement may be resulted from the reduction of interface trap density state and the shrinkage of channel. Since the subthreshold swing reflected the interface trap density state between gate insulator and channel and no changes were found, that interpreted that the interesting improvement was not caused by the reduction of interface trap density. On the other hand, since only the HELLO p-channel exhibited the interesting anomalous improvement, it is believed that the boron atom is lighter than phosphorous and diffused during laser lift-off process. Therefore, the boron diffusion occurred only when the laser lift-off energy density. Consequently, the source and drain dopants, namely boron, were slight diffusing into the channel to reduce effective channel length.

In conclusion, we investigated the anomalous improvement in p-channel ULTPS TFTs after the high-energy laser-lift-off process. The hole mobility was improved from 85cm2/V-s to 92cm2/V-s. It is believed that these anomalous phenomena may be contributed to boron diffusion caused reduction of channel length. The ULTSP TFTs are promising to future flexible electronic circuits.

Reference

 [1] ISDRS 2009, December 9-11, 2009, College Park, MD, USA

 [2] IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 11, NOVEMBER 2010

Figure 1