Sadayoshi Hotta

Plasma-Enhanced Chemical Vapor Deposition of Silicon Nitride

The optimum condition of plasma-enhanced chemical vapor deposition to deposite silicon nitride (SiNx) film and its application as a gate insulator of a-Si thin-film transistor (TFT) have been investigated. The internal stress of SiNx in the range of 4.3×109 dyn/cm2 tensile to 8.0×109 dyn/cm2 compressive is found to be controllable by changing the ratio of H2 and N2 in the source gases without affecting the optical band gap. Satisfactory TFT characteristics and high reliability are realized by using a gate insulator of SiNx having either stoichiometric or N-rich composition which shows the large optical band gap.

The Characteristics of Amorphous Silicon TFT and its Application in Liquid Crystal Display

An amorphous silicon TFT particularly suited for the full color liquid crystal display driver has been developed and reported here. Various fundamental factors involved in the a-Si TFT, such as the effects of structure, materials, and the method of fabrications were reviewed and investigated in terms of the field effect mobility, the threshold voltage and the reliabilities. An inverted-staggered TFT structure was employed for the purpose wherein the interface states between two layers was successfully lowered by employing the successive deposition procedures of SiNx gate insulator on the a-Si layer. Proper ohmic contacts and the blocking of hole injections were accomplished by forming a n + layer between the a-Si layer and the source/drain metal electrodes which consists of a double layer of Al/MoSi 2 in order to prevent the aluminum diffusion into the a-Si layer during the 300°C heat treatment at the succeeding fabrication processes. The degradation of display images due to the high intensity backlights was minimized by employing a light shielding layer and by making the thickness of a-Si layer 200 A against the direct sunlight of up to 100,000 luxes. Stable actual performances of TFT for more than 4000 hours at 80 °C were confirmed. The development of a color LCD TV driven by this TFT is also reported.

Process techniques of 15-inch full-color high-resolution liquid crystal displays addressed by a-Si thin film transistors

A 15 inch-diagonal-size full-color liquid crystal display (LCD) with 1152(×3)×900 pixels has been fabricated which enables a portable workstation with improved display performances. The process techniques used for this development are described, with special reference to metallization and dry etching. In multilevel metallization, Cr/Al interconnection is metallurgically undesirable. By contrast, the Cr/Ti/Al metal system provides excellent properties of contact resistivity and thermodynamical stability. Dry etching processes are developed for multilayered insulating films and metallization-related bilayers, namely SiO2/TaOx/SiNx/(i/n+)a-Si and a-Si/Ti, respectively. Fine patterning and easier stepcoverage of subsequently deposited layers are achieved.

Threshold Voltage Shifts in Amorphous Silicon Thin Film Transistors under Bias Stress

Through the time and temperature dependence measurements on threshold voltage shifts (Δ V T ) in amorphous silicon thin film transistors, it has been found that two separate instability mechanisms exist; within short stress time ranges Δ V τ increases as log t and this behavior corresponds to charge trapping in SiN. On the other hand, in long stress time ranges Δ V T increases as t t/4 and can be explained by time-dependent creation of trap in a-Si.

Generation of high-quality holograms with liquid-crystal SLM

Measure to evaluate holograms recorded with spatial light modulator(SLM), and optical systems to implement quantitative evaluation of the image quality are presented. We propose utilizing a pseudo-random diffuser with SLM to suppress speckle noise. High quality holograms have been obtained by using a high performance liquid-crystal display panel with a maximum contrast ratio as high as six hundred.