We have friendly competition with researches at IBM. Low cost sheets of circuitry by roll to roll processes.
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Inorganic spin coat process readied for TFTs
By Chappell Brown
EE Times
March 17, 2004 (1:20 PM EST)
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HANCOCK, N.H. — A new spin-on process for inorganic semiconductor films developed at IBM Corp.'s Thomas J. Watson Research Center could usher in a new phase for electronics based on thin-film transistor (TFT) technology.
The novel aspect of the process is the use of a catalyst to facilitate the formation of very thin 50-nm films consisting of chalcogenide compounds that act as the channel for TFTs. High throughput spin-on processes have been developed for organic TFTs, but the mobilities of organics are inherently lower than inorganic semiconductors.
"This opens up an opportunity to apply the solution-based processes of organics to higher performance inorganics," said David Mitzi, the lead researcher on the project.
Organic semiconductors such as pentacene dissolve easily in solvents, making it easy to spin them onto a substrate at low temperature. Other low-cost high-throughput processes available to organic solutions are ink-jet printing and stamping.
While organic semiconductors dissolve easily at low temperature due to the weaker bonds between their atoms, they also have lower electronic performance. The chalcogenide films have electron mobilities ten times that of the best organics and generally are more robust electronically and structurally, Mitzi explained.
Inorganic semiconductor films can be produced by chemical vapor deposition or vacuum deposition, but require a high temperature substrate to form. Several years ago, Mitzi, working in the area of organic solution-based processes, began searching for a chemical means of dissolving inorganics.
"The strong bonds of inorganic compounds make it virtually impossible to dissolve them in solution, so we had to start looking for some indirect route," he explained. The first approach was to try and find an organic/inorganic combination that would dissolve in organic solvents. Chalcogenides are compounds built from tin, sulfur and selenium and have not been a major player in semiconductor technology. However, Mitzi said he is confident that the new chemistry of the chalcogenides can be reworked with a variety of catalysts and materials. The breakthrough came in the form of a two-stage inorganic process. An exotic and highly reactive organic compound called hydrazine creates a transitional compound with the tin sulfur and selenium. The bonds between these units are weak, just as with organics, so that they easily dissolve in water.
A solution of the hydrazine-mediated compounds is then spin coated on a substrate. The resulting film is heated to around 250° Celcius, at which point the hydrazine is forced out and the inorganics elements form into a chalcongenide.
Prototype TFTs were built with the films that showed mobilities of 10 cm2 per volt per second, which is more than ten times the figure for the best organic TFTs, Mitzi said.
The annealing temperature is low for standard semiconductor processes, but in the mid range for large area plastic substrates that might be used in flat panel displays.
"We are working on getting the processing temperature down to between 100 and 150° centigrade, which would allow deposition on flexible substrates in high-throughput roll-to-roll manufacturing systems," he said. |
