Dawne Michelle Moffatt

Properties of Glass Substrates for Poly-Si Amlcd Technology

A major force for change in substrate requirements in the late 90s may well be the commercialization of poly-silicon thin film transistor (TFT) Active Matrix Liquid Crystal Display (AMLCDs) technology. The processes necessary for “poly-Si” occur at temperatures that are 150–300°C higher than the current amorphous-Si LCD processes. This impacts the thermal shrinkage and thermal gravimetric warp requirements of the glass, particularly as display resolutions tighten, as enabled by poly-Si. In addition, the expected integration of more components (e.g. chip-on-glass) impacts the requirements for the thermal expansion of the substrate. One approach for meeting the poly-Si demands for greater thermal-dimensional stability is to use glasses with higher temperature capability. A new glass, Code 1737, with the highest strain point commercially available at over 660°C, now enables poly-Si processing with acceptable sag and shrinkage after annealing. A logical goal for the next significant glass advancement would be to eliminate annealing altogether, but it is unclear what temperature capability is required. In this study, various glasses with strain points ranging from 600–800°C have been evaluated in terms of their density, thermal expansion, and thermal shrinkage following poly-Si thermal process simulations. It has been confirmed that the magnitude of shrinkage decreases with increasing strain point for glasses in this compositional family. In addition, future new insight into the effect of thermal expansion coefficient has been developed; the lower the thermal expansion coefficient (for a given strain point), the lower the magnitude of the shrinkage for a given strain point and high temperature thermal cycle. This is important new learning in the area of substrates for flat panel displays that will help in further design and development of glasses for future AMLCDs.

Flat Panel Display Substrates

The performance of advanced flat panel displays is intrinsically linked to critical properties of the substrate material. In the manufacture of active-matrix liquid crystal displays (AMLCDs) and some emissive displays, there are certain process steps that require extreme conditions such as strong chemical washes and temperatures in excess of 600°C. As a result, the glass substrate used in these displays must be able to withstand these environments without degradation of its properties. It has become apparent that the flat panel display (FPD) manufacturers will benefit from substrates with improved acid durability, higher temperature capability, and thermal expansion coefficients consistent with other display materials. This paper focuses on one of the less-understood features of the glass substrate: the expansion characteristics as a function of temperature. Thermal expansion is important as it affects the compatibility of the glass with display materials, which, in the case of AMLCDs and some silicon-microtip field emission displays (FED), require an expansion close to that of silicon. In addition, thermal breakage during processing is directly proportional to the expansion coefficient. This study focused on the thermal expansion characteristics of two different FPD substrate glasses. The first one is code 7059, manufactured by Corning Incorporated and currently the standard in AMLCDs. A new substrate composition, Corning code 1737, with enhanced durability, temperature capability, and expansion tuned to the AMLCD applications will also be discussed.