Printed Metal Oxide TFTs
December 1,2011, Printed Electronics USA 2011, Santa Clara, CA—Warren Jackson from HP talked about fabricating large area metal oxide thin film transistors (MOTFT) using roll-to-roll imprint lithography. These developments can be applied to many areas including displays, packaging, and civil and military functions.
MOTFTs offer many advantages over amorphous silicon and poly-silicon. A-Si has low carrier mobility and low drive current, making the technology inadequate for large, ultra definition TVs and AMOLEDs. In addition, it has some stability problems. Poly-Si is a high complexity process that uses many masks. It has issues with uniformity and limited choice for substrates. It also has high cost and low throughput in manufacturing.
MOTFTs in comparison have a mobility of 20 cm2/v/sec and a high on current suitable for driving MOLEDs. It uses a low temperature process that allows for a wide range of substrates, and only need a small number of masks. The major display manufacturers are demonstrating large, high-performance displays using MOTFTs and are moving towards full commercialization.
Standard photolithography uses multiple masks that need alignment, resulting in lower throughput and higher costs for any process. Self-aligned printing can overcome these limitations by encoding multiple patterns and alignments into thickness modulations within the mask. The same mask is used to pattern all layers so there is no misalignment due to run-out or substrate distortions.
Unlike the typical IC process flow where the substrates go through many steps per layer, the HP process vacuum deposits the metals, dielectrics, and semiconductors in one step. Next the multiple mask levels are imprinted as a single 3-D structure, then the patterning is completed with various wet and dry processes. The final step is to remove gate dielectric to expose the gate lines.
The process has been demonstrated on flexible substrates and in a number of prototypes. They have developed a roll-to-roll e-ink process using the MOTFTs and a pixel current drive backplane for OLED and µOLED frontplanes.
The process still needs more developments before it is ready for commercialization. The etch for the metal oxides requires acid, but most flexible substrates and the metal electrodes dissolve in acid. Zinc is easy but tin is difficult, so materials engineering is needed for some of these steps. Dry etch is difficult because many of the materials are reactive. The materials stack for the active devices is still a science project as they try different metals and compounds for optimal processing and performance.