Method of controlling the film properties of a CVD-deposited silicon nitride film

Abstract

We have discovered that adding H2 to a precursor gas composition including SiH4, NH3, and N2 is effective at improving the wet etch rate and the wet etch rate uniformity across the substrate surface of a-SiNx:H films which are deposited on a substrate by PECVD. Wet etch rate is an indication of film density. Typically, the lower the wet etch rate, the denser the film. The addition of H2 to the SiH4 / NH3 / N2 precursor gas composition did not significantly increase the variation in deposited film thickness across the surface of the substrate. The a-SiNx:H films described herein are particularly useful as TFT gate dielectrics in the production of flat panel displays. The uniformity of the film across the substrate enables the production of flat panel displays having surface areas of 25,000 cm2 and larger.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention pertains to a method of controlling the film properties of a silicon nitride film deposited by PECVD (plasma-enhanced chemical vapor deposition) over a substrate having a large surface area, and to the film deposited by the method. In particular, the uniformity of the density of the silicon nitride film across the substrate surface is improved by controlling the film-forming precursors. [0003] 2. Brief Description of the Background Art [0004] Current interest in thin film transistor (TFT) arrays is particularly high because these devices are used in liquid crystal active matrix displays of the kind often employed for computer and television flat panels. The liquid crystal active matrix displays may also contain light emitting diodes for back lighting. Further, organic light emitting diodes (OLEDs) have been used for active matrix displays, and these organic light emitting diodes require TFTs fo...

AI Technical Summary

Benefits of technology

[0023] We have discovered that adding H2 to a precursor gas composition including SiH4, NH3, and N2 is effective in improving the wet etch rate and wet etch rate uniformity of a-SiNx:H films across a substrate surface upon which said films have been PECVD deposited. As mentioned above, wet etch rate is an indication of film density. Typically, the lower the wet etch rate, the denser the film. The addition of H2 to the SiH4 / NH3 / N2 precursor gas composition results in greater control over the density uniformity of PECVD deposited a-SiNx:H films than was previously achievable using prior art precursor gas compositions.

[0026] H2 is provided to the deposition chamber in an amount such that a component ratio of NH3: H2 in the plasma precursor gas composition ranges from about 1:2 to about 3:1; more typically, from about 1:1 to about 3:1. Adding too much H2 to the precursor gas composition may result in reduced film deposition rates and increased variation in film thickness uniformity.

Problems solved by technology

However, the a-Si:H TFTs with SiNx:H gate dielectric are said to have instability problems, such as the threshold voltage shift and the inverse subthreshold slope under a DC gate voltage bias.

Their instability problems are said to be caused by the high trap density in the SiNx: H film and the defects created at the a-Si:H / SiNx:H interface.

The authors claim that PECVD SiNx:H dielectric films are not useful as a gate insulator because they contain large amounts of bonded hydrogen (20%-40%) in the form of N—H and Si—H bonds.

As the size of flat panel displays increase, it becomes increasingly difficult to control the uniformity of the individual films produced across the increased surface area.

With respect to PECVD deposited silicon-nitride comprising films, which are used either as the gate dielectric layer or as the passivation dielectric layer, control of uniformity of the film across the substrate becomes particularly difficult when the PECVD is carried out in a process chamber having parallel-plate, capacitively coupled electrodes over about 1 m×1 m. At the higher RF power applications, the RF power appears to concentrate at the center of the electrode area, resulting in a dome-shaped thickness profile, and film properties are indicative of the non-uniform power distribution across the electrodes.

However, a plasma produced at high concentrations of N2 (where N2:SiH4 is greater than 2:1) in the precursor gas produces a particularly non-uniform plasma over a large surface area, for example, a substrate having dimensions larger than about 1000 mm×1000 mm (one square meter).

As discussed above, this higher hydrogen content leads to a higher threshold voltage requirement for the TFT, which is harmful to performance of the TFT.

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