TFT materials and processes
July 11, 2012, FlexTech Alliance, San Francisco—Douglas Keszler from Oregon State University talked about the various materials and processes needed to produce metal oxide thin film transistors. He identifying the processes and requirements for both deposited and solution-based production.
Materials for MOTFT require cross disciplinary work in chemistry, electronics, and fabrication to define and build devices with suitable performance for the application. Materials and processing affect the device performance, and even decisions for passivation can affect the illumination degradation.
Making a MOTFT requires a flat surface dielectric. A solution-based design would start with some materials in a solution that is applied and soft baked, resulting in a deposited solid. They've managed to synthesize soluble molecules and have achieved 0.2 nm flatness. When converted to an active device, the resulting transistors have equivalent characteristics as those produced by deposition or sputtering.
A water-based indium aluminum zinc oxide (IAZO) is stable compared with IGZO, and exhibits similar electrical characteristics. The biggest problem is removing the last water trapped protons. One alternative is to make the material at 300 to 350° C. to drive out the water. If the process can go to 600° C., most of the active materials use a barrier transition layer to keep the gate materials from boiling off.
Another issue is negative bias illumination stress, which causes shifts and kinks in the materials with exposure to light and negative bias. They characterized a number of different device geometries, and found all were stable under the negative bias illumination stress test.
The theory for NBIS is that chemically absorbed gas depletes the channel and sets the turn on voltage near zero. NBIS changes the internal absorption mechanisms and causes changes in the voltage thresholds. They've experimented with a ZTSO (zinc, tantalum, sulfur, oxide) passivation layer which results in approximately the same bandgap.
They are performing more investigations into other materials, to see what the trade-offs are with other combinations of materials. The biggest problem they face is that the rest of the ecosystem for the process and ongoing studies does not exist. For example, they use capillary deposition with equipment used for LCD color filters. They've tried experiments with mists and various deposition techniques.
One of their goals is to push performance in the devices. The target for mobility is greater than 100 cm² per volt per second. They are looking into metal-insulator-metal topologies and also looking into other materials to reduce the oxygen in the compounds.