1 High-performance top-gate a-Si:H TFTs for AMLCDs Chun-Sung Chiang (1) , Sandrine Martin (1) , Jeong-Yeop Nahm (1) and Jerzy Kanicki (1) ; Yasuhiro Ugai (2) , Teizo Yukawa (2) , and Shu Takeuchi (2) (1) : University of Michigan, Center for Display Technology and Manufacturing, Department of Electrical Engineering and Computer Science, Ann Arbor, MI 48109, USA. (2) : Hosiden and Philips Display Corp. 3-1, Takatsukadai, 4-chome, Nishi-ku, Kobe City, Hyogo, 651-22, Japan Abstract High-performance top-gate hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) structures have been fabricated over a large area from plasma-enhanced chemical vapor deposition (PECVD) materials. The electrical performances of the top-gate a-Si:H TFT (μ FE ≈0.75cm 2 /Vsec, V T ≈3.5V, S≈0.55V/dec) are comparable to the electrical performances observed for an inverted-staggered bottom-gate a-Si:H TFT. We have shown that the TFT field-effect mobility first increases with the a-Si:H thickness, and then decreases for thicker a-Si:H films. This change of the electrical performances can be associated either with the variation of a-Si:H microstructure with film thickness during the PECVD processes or a large density of TFT back interface states; it also involves the source/drain parasitic access resistances, especially for thick a-Si:H layers. Introduction Based on the order of thin-film deposition, there are two types of a-Si:H TFT structures: inverted- staggered bottom-gate and normal-staggered top- gate[1]. Today the top-gate TFT structure is not used by the majority of flat-panel display producers. It is generally believed that the top-gate a-Si:H TFTs have a much worse electrical performance (lower mobility and higher threshold voltage) than inverted-staggered bottom-gate a-Si:H TFTs [2]. In this paper, the experimental and simulated results of the influence of a-Si:H thickness on the TFT electrical performances clearly indicate that a high- performance top-gate a-Si:H TFT can be fabricated over a large-area from PECVD layers. Experiment Glass substrates covered with amorphous silicon oxide (a-SiO x :H) were used to fabricated the top-gate a-Si:H TFT structures. First a 400Å-thick indium-tin- oxide (ITO) layer was patterned as source and drain electrodes, and selective ohmic source/drain contacts were formed in-situ by P-treatment process of the ITO patterned electrodes in a PECVD system [3]. Following the P-treatment process, an intrinsic a-Si:H layer and hydrogenated amorphous silicon nitride (a-SiNx:H) gate insulator were deposited by the PECVD technique at 250°C. A 4500Å thick aluminum layer was used as the gate electrode. The amorphous silicon nitride thickness was not the same for all the samples; we, therefore, used the gate electrical field rather than the gate voltage: F V t G a SiN = - . The device parameters, such as the field-effect mobility (μ FE ) and the threshold voltage (V T ), were extracted from a-Si:H TFT I D -V G characteristics by least-square fitting to the MOSFET gradual channel approximation equation: I D =μ FE C i (W/L)(V G -V T )V D , where C i is the gate-insulator capacitance per unit, W is the channel width, L is the channel length, and V D is the applied drain voltage. The field-effect channel conductance activation energy (E A ) at different gate voltages was obtained from the slope of the Arrhenius ln(I D ) versus T -1 plot: I D ≈ I D0 exp(-E A /kT), where I D0 is a constant, k is the Boltzmann constant, and T is the absolute temperature. Results and discussion -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 10 -14 10 -13 10 -12 10 -11 10 -10 10 -9 10 -8 Electrical Field=V G /t SiN (MV/cm) V DS =0.1V W/L=50μm/100μm 120Å 350Å 900Å 1350Å 1850Å 2350Å I DS (A) Figure 1: I D -V G characteristics of the top-gate a-Si:H TFTs with different a-Si:H thicknesses. The values of the subthreshold slope (normalized to the a-SiN:H thickness) are shown in the inset. An example of the I D -V G transfer characteristics in the linear region (V DS =0.1V) is shown in Figure 1 for 0 1000 2000 0.02 0.04 0.06 0.08 S/t SiN (MVcm -1 /dec) t a-Si:H (Å) ISSN0098-0966X/98/2901-1014-$1.00 + .00 (c) 1998 SID ISSN0098-0966X/98/2901-$1.00 + .00 (c) 1998 SID