Feasibility of an Ag-alloy film as a thin-film transistor liquid-crystal display source/drain material

Journal of Electronic Materials, Jun 2002 by Jeong, C O, Roh, N S, Kim, S G, Park, H S, Et al

The Ag-alloy films have been investigated as source/drain materials applicable to thin-film transistor liquid-crystal displays (TFT-LCDs). The Ag-alloy consisting of 0.9at.%Pd, 1.7at.%Cu (designated APC) showed a resistivity that was lower than one-half that of AlNd. It also had good contact characteristics with amorphous Si (a-Si). In addition, the Ag/Si was stable after heating to above 700 deg C, requiring no diffusion barrier to prevent reaction between Ag and Si. Pure Ag films deposited on glass by DC magnetron sputtering showed severe hillock formation, hole growth, and agglomeration upon annealing in air. In comparison, the APC-alloy film exhibited improved resistance to agglomeration. Further, inverted-staggered back-channel-etch hydrogenated amorphous silicon (a-Si:H) TFTs using an APC-alloy film as a source/drain material had a threshold voltage of 4 V. A structure of single layers of gate-APC alloys and source/drain-APC alloys leads to lower costs and productivity improvements of large-area, high-resolution, active-matrix LCDs, such as 40-in. size panels through process simplification.

Key words: Metallization, Ag alloy, magetron sputtering, thin-film transistor, liquid-crystal display

INTRODUCTION

Because of the increased use of portable computers (PCs), the market for active-matrix liquid-crystal displays (AM-LCDs), which use amorphous silicon thin-film transistor (a-Si TFT) arrays, is growing very rapidly.1 In addition, the markets of large TFT-LCDs, as used in LCD televisions, are also expanding. However, several fundamental scaling problems are encountered in attempting to increase the display size.2 One problem is associated with gate metallization of the inverted-staggered TFTs of the active matrix.3 At the present time, refractory metals, such as tantalum/molybdenum (Ta/Mo), chromium (Cr),4 and a-tantalum (alpha-Ta)5 are used to ensure stable contacts during TFT fabrication. The resistivities of these materials are greater than 20 (mu)(omega)-cm, which makes them too resistive to be used in large-area, high-pixel-density displays.1 To meet requirements in terms of enlarging the size of the TFT-LCD panel and reducing the production cost, process simplification using new, low-resistivity materials must be achieved.6

Silver is the most attractive candidate for fabrication of large-area, high-resolution, AM-LCDs because it has low resistivity for reduction of the RC propagation delay. However, the most important technical problems that need to be addressed are poor adhesion to glass substrates and agglomeration upon annealing in air or oxygen ambient. To overcome these problems, the addition of another metal layer (e.g., Ti or Cr with Ag, Ti, and Al metallization 7,8 or Mg with Cu metallization9-11) has been explored.

In this paper, an Ag-alloy film containing 0.9at.%Pd and 1.7at.%Cu (designated APC) has been investigated as the gate electrode and the source/drain metal of an a-Si TFT in an AM-LCD. In addition, inverted-staggered back-channel-etch

Feasibility of an Ag-Alloy Film as a Thin-Film Transistor Liquid-Crystal Display Source/Drain Material

hydrogenated amorphous silicon (a-Si:H) TFTs, using APC as a source/drain material, have been demonstrated.

EXPERIMENTAL

The Ag-alloy films were deposited on glass by DC magnetron sputtering from an APC target that had a purity level of 99.99%. The resistivity was measured by the four-point probe method. After annealing, the surface morphology of the resulting films was examined using field-emission scanning electron microscopy (FESEM). Adhesion to the glass substrate was measured by a scratch test. Thermal stresses of Ag and APC films were measured using a wafer-bending beam apparatus, which is described in Ref. 12. The curvature of the sample was monitored optically during heating at the rate of 5 deg C/min. in an N^sub 2^-ambient chamber, and the stress was calculated from the curvature using Stoney's equation.13

An inverted-staggered a-Si:H TFT was fabricated. The AINd-alloy metal was initially deposited on a glass substrate by sputtering. The gate pattern was formed by lithography and wet etching in perchloric acid and ceric ammonium nitrate. Silicon nitride, undoped a-Si:H, and heavily doped a-Si:H(n'a-Si:H) films were deposited in a plasma-enhanced chemical-- vapor deposition reactor at 350 deg C without breaking the vacuum. The thickness of the SiN., a-Si:H, and n a-Si:H layers were 450 nm, 200 nm, and 50 nm, respectively. The 300-nm APC layers were then deposited on the n a-Si:H layer by DC magnetron sputtering, and the source/drain contacts were defined by photolithography.

Depth profiles of 300-nm APC and 150-nm Cr films that were deposited on a n a-Si/a-Si/SiNe^sub x^ multilayer were obtained by secondary ion-mass spectroscopy (SIMS). In addition, Auger electron spectroscopy (AES) analysis was carried out to investigate depth profiling of the elemental distribution in the films as a function of depth. High-resolution transmission electron microscopy (TEM) was also used to investigate the microstructural evolution and the accompanying grain-boundary precipitation of the alloying element, Cu. In order to analyze the segregation of Cu and Pd to the grain boundaries in APC films upon heating, distribution of Cu and Pd at the grain boundaries were measured by using energy dispersive spectroscopy (EDS), where the beam size was 2 nm.