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IBM researchers develop low-cost method for making high-performance semiconductors

A major scientific milestone toward new low cost electronics

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Yorktown Heights, NY, USA - 18 Mar 2004: IBM scientists have achieved a significant scientific milestone toward creating very low-cost electronic circuits with record high performance.

A team of researchers at IBM's T.J. Watson Research Center (Yorktown Heights, NY.) recently developed a simple, low-cost process to make extraordinarily thin films of semiconducting materials that allows electrical charges to move through them about 10 times more easily than had been reported for all other similar approaches. Such an increase can enable a broad array of low-cost electronics and new pervasive-computing applications.

"These types of easily processed semiconducting films could eventually be used to make circuitry for very-low-cost or flexible displays, high-performance smart cards, sensors and solar cells or for flexible electronics coated onto a wide variety of molded or plastic shapes," said the IBM Research team leader, David Mitzi.

Mitzi's team reported their findings in the March 18 issue of the technical journal, Nature.

While advances in high-performance microelectronics are typically aimed at making ever-smaller features in intricate and precisely defined patterns on perfectly formed silicon crystals -- a task that is becoming increasingly difficult as the feature dimensions decrease and the cost of manufacturing equipment increases -- researchers are also seeking extremely low-cost methods for making massive numbers of relatively simple electronic devices for use in many potential applications that are not now practical due to their cost.

Spin coating is one of the simplest and cheapest of such techniques: Several drops of a liquid solution are simply placed onto a spinning platter in a high-tech version of a carnival paint spinner. Centripetal forces then spread the liquid to a uniform thickness over the entire surface. The film's thickness is usually determined by the solution's viscosity (its resistance to flow) and the rate and duration of spinning. The liquid is then cured into a solid thin film upon which transistors and other various electronic devices can be made.

Until now, the only semiconducting materials that could be made using spin coating had limited usefulness due to their low charge "mobility" -- a measure of how fast electronic circuits made with a semiconductor can operate. Better semiconductors could not be dissolved in any liquid that would result in a thin film that retained the desired mobility. Mitzi's team developed a way to dissolve such higher-mobility materials in a liquid that could be used in a spin-coating process, leaving a very uniformly-controlled film. Moreover, in a transistor made on the films, the materials exhibited 10 times the charge mobility of any previously spin-coated seniconductor.

Further refinements of the process will be explored, but IBM researchers believe this technique will significantly accelerate progress toward the widespread use of thin-film electronics made by the family of fast, inexpensive, high-throughput "solution processes," such as spin coating, printing, stamping, nanoimprinting, inkjet printing and dipping. Applications for solution-processed electronics include: advanced displays, flexible devices, high-function smart cards and RFID tags, photovoltaic solar cells and phase-change solid-state memories. Semiconductors are increasingly being put to use in a wide array of applications. IBM advances in basic materials and manufacturing techniques, such as the spin-coating process outlined here, hold the potential of lower costs and broader availability in products that touch people's lives every day.

Technical details
To dissolve the semiconducting material, Mitzi combined a very strong solvent -- hydrazine, a molecule made up of two nitrogen and two hydrogen atoms -- with equal numbers of chalcogen** atoms and semiconducting metal chalcogenide molecules (e.g. sulfur and tin sulfide, respectively). Although it is well known that hydrazine is generally not a good solvent for metal chalcogenides, the presence of the extra chalcogen atoms both improves the solubility and enables control over the film's final composition and grain structure. Heating the resulting film causes both the hydrazine and extra sulfur to dissociate and evaporate, leaving just a very thin layer of solid metal chalcogenide with a uniform thickness as small as about 5 nanometers. When Mitzi's team optimized the molecular proportions, spin-coating conditions and heat/annealing procedures, the films exhibited charge mobility approaching that of polycrystalline silicon and 10 times that of any previously spin-coated material and of amorphous silicon.

Mitzi's next step is to reduce or replace the use of hydrazine, a highly energetic molecule also used as rocket fuel, with a more benign but still effective solvent.

"Now that I understand how this new process dissolves the metal chalcogenide, I'm confident that new solvents can be substituted," Mitzi said.

** Chalcogen (say: CAL-ko-gin) refers to four chemical elements in one column of the periodic table -- oxygen, sulfur, selenium and tellurium. The term is derived from two Greek words meaning "ore formers" and was proposed in the early 1930s by a German chemist. Compounds of these elements are called chalcogenides (say: cal-CAW-gin-ides). Some metal chalcogenides (such as cadmium sulfide, tin selenide and zinc telluride) are well-known high-performance semiconductors.

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