Low Hydrogen Concentration Silicon Nitride as a Gate Dielectric of TFTs for Flexible Display Application

  • PDF / 109,100 Bytes
  • 6 Pages / 612 x 792 pts (letter) Page_size
  • 56 Downloads / 142 Views

DOWNLOAD

REPORT


A22.6.1

Low Hydrogen Concentration Silicon Nitride as a Gate Dielectric of TFTs for Flexible Display Application Joong Hyun Park, Chang Yeon Kim, Kwang Sub Shin, Sang Geun Park and Min Koo Han School of Electrical Engineering and Computer Science (#50), Seoul National University San 56-1, Shinlim-Dong, Kwanak-Gu, Seoul, 151-742, Korea ABSTRACT We have proposed low hydrogen concentration (CH) silicon nitride (SiNX) as a dielectric for flexible display application. The fabrication temperature on plastic substrate is limited below Tg (glass transition temperature, typically 130~180 oC) and it was reported that CH in thin film is strongly depends on fabrication temperature. As the fabrication temperature is decreasing, hydrogen concentration is increasing. SiNX deposited in ultra low temperature (< 150 oC) has high CH which is porous, low density. Our experimental results using SiH4, He, N2 gas mixture shows that in the SiNX CH is less than 15 at.%. Breakdown voltage of proposed SiNX dielectric is 5 MV/cm. In the wet etch rate test using a nitride etching solution, He dilution is more dense than NH3 dilution. This process approach is useful for flexible display application. INTRODUCTION Hydrogenated amorphous silicon thin film transistor (a-Si:H TFT) on flexible plastic substrate is attractive for flexible display which is foldable, unbreakable and light-weight [1-2]. Silicon nitride (SiNX) is widely used in flexible display as a gate dielectric and passivation layer due to high breakdown voltage, protecting oxygen exposure and compatible with conventional chemical vapor deposition (CVD) process [3]. The fabrication temperature on plastic substrate is limited below ultra low temperature (< 150 oC). As the fabrication temperature is decreasing, SiNX as a gate dielectric has worse characteristics than high temperature. Especially, hydrogen concentration (CH) is strongly influenced on fabrication temperature, and SiNX fabricated at 150 o C has a high CH. It makes SiNX becomes porous, low density and weak bonding [4]. A typically CH of SiNX deposited using conventional plasma enhanced CVD (PECVD) and SiH4, NH3, N2 mixture is 25~40 at.% [5]. In order to ensure a stable TFT, hydrogen in dielectric must be eliminated or minimized. As the fabrication temperature is decreasing, the energy of hydrogen removal is decreasing. It leads more hydrogen incorporation and reduction of film density. Because the energy of hydrogen

A22.6.2

removal is insufficient on ultra low temperature, the optimization of dielectric characteristics is necessary to obtain high reliability. One way to decrease CH is use He gas instead of NH3. The most of hydrogen in the SiNX is bonded to nitrogen suggests that hydrogen is carried into the material attached to N in the precursor state [4]. In addition, it was reported that He dilution results in the increase of film density assisted by mechanical stress transition [6]. The purpose of our work is to obtain a low CH SiNX. We used inductive coupled plasma CVD (ICP-CVD) at 150 oC with SiH4, N2, He, instead