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Time of Flight SIMS Analysis of a-IGZO Thin Film Transistors Subjected to Bias Temperature Stress Fabricated on Different Glass Substrates and on Silicon

Gokul Prakash and Jerzy Kanicki

The use of thin film transistors has been constantly rising in flat panel display applications such as laptops, cellular phones, watches, televisions and other military high resolution liquid crystal displays (LCD). Substrate glass is an important element in the design of these thin film transistor (TFT) arrays. There are trends seen moving from soda lime towards alkali free glass to be able to produce electrically stable arrays. Some of the glasses used for TFTLCD applications include EAGLE2000, Corning 7059, Corning 1737 and AN100.

For innovative applications like electronically switchable transparent flat panel LCD and organic light emitting diode displays (OLED) active matrix arrays should be fabricated on window panes of buildings to control manufacturing costs. But the glass used for windows is generally soda lime glass. Different glass substrates need to be considered since this type of glass is one of the cheapest among variants of glass. However, this type of glass has a large concentration of alkali ions which could affect the performance of full color TFT LCD.

We have performed time of flight SIMS analysis on a-IGZO TFT fabricated on 2 different glass substrates - Asahi AS and PD200, both having different elemental compositions. When subjected to bias temperature stress, the TFT’s electrical response on the different glass substrates are very different and unique.

The substrate glass having high composition of sodium is shown to have higher degree of threshold voltage shifts. Our speculation is the electric field assisted diffusion of sodium, other alkaline earth metals like magnesium, strontium, beryllium and elements like aluminum might be responsible for the observed shifts in the thin film transistor transfer characteristics after positive and negative bias temperature stress.

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