Improvement of PECVD-SiNx for TFT Gate Insulator by Controlling Ion Bomberdment Energy
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ABSTRACT It has been revealed that ion energy and ion flux density play an essentially critical role in SiNx deposition process of PECVD in TFT-LCD production. Ion energy and ion flux density bombarding onto substrate surface are known to be extracted from waveform of RF applied to an electrode. Using this method, authors investigated film quality of SiNx formed in the conventional parallel plate PECVD equipment. When N 2+H2 or N 2+Ar is employed as a carrier gas in source gas(SiH 4+NH 3), authors have defined normalized ion flux density as ion flux density divided by deposited SiNx molecule which must be increased to obtain high quality SiNx film while ion energy is suppressed at low level as not giving damages on the film surface. This technique has made it possible to securely form SiNx film (2500 A) featuring dielectric breakdown field intensity of 8.5MV/cm at 250°C on a glass substrate with Cr gate interconnects of 1000A having vertical step structure. One of the important factors to improve film quality of SiNx deposited in PECVD is to increase ion flux density while keeping ion energy low enough to protect growing surface against any damages. Using this technique inverse-staggered TFT-array featuring field effect mobility of 0.96 •/V's has been demonstrated which gate insulator SiNx,non-doped a-Si:H and aSi:H(n+) were formed continuously at the same substrate temperature of 2500C.
INTRODUCTION P.G.LeComber et al.( Dundee University)proved the followings in their experiment in 1975811 5 - a- SilH, which is formed with the RF glow discharge method using Sill4 gas and H2 gas, is PN controllable. - Thin film transistor (TFT) with MIS structure is fabricated where SiNx formed by adding NH 3 gas to S11l4 is employed as a gate insulator. - A practical flat panel display (FPD) can be realized when two-dimensional matrix of this device is arrays as switching element to drive liquid crystal. The Japanese LSI manufacturers have been taking initiatives in developing technologies to make FPD using TFT-LCD commercially practical. It is partially because TFT manufacturing technology, a key technology, has a lot in common with LSI manufacturing technology. Now every effort is currently focused on reducing costs of components and materials and on introducing volume production to obtain scale of economy. Meanwhile configuration of theTFT device which is currently available in the market is designed merely to improve yield. As a result, its configuration becomes increasingly redundant : layer of interconnects increases in number, insulators increases in number and thickness, and TFT features increasingly complex structure. [Fig. 1] The most important basic properties based on material properties and interface properties, such as electron mobility in a-Si:H, however, remain almost unchanged from what was presented by P.G.LeComber et al. Though this current approach leads to some improvement at the initial stage, it consequently increases the number of manufacturing process steps, size of equipment, and items to be controlled.
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