FOR years, manufacturers of electronics with flat-panel displays have dreamed of using plastic as a cheaper, more compact, more rugged, and far more lightweight alternative to glass. The Department of Defense is particularly interested in ultrathin yet flexible screens as standard equipment for the Pentagon's "information warrior" of the next century. With plastic displays, soldiers could hang satellite navigation system displays on their belts or keep electronic maps rolled up in a back pocket.|
The most advanced type of flat-panel displays, used in most portable computers, is active-matrix liquid-crystal displays. In this display, each of the million or so tiny screen pixels is controlled by thin-film transistors (TFTs) that act as tiny on/off electrical switches. By turning on and off dozens of times a second, the TFTs permit continuously changing images of words, pictures, and video.
Currently, TFTs for active-matrix displays are manufactured onto a rigid glass substrate in a process that involves baking glass sheets at temperatures of up to 600°ree;C. This conventional process is far hotter than any plastic can withstand without deforming and melting. But now a team of Lawrence Livermore researchers is showing how TFTs can be manufactured on top of thin, flexible plastic sheets instead of glass by keeping manufacturing temperatures at or below 100°ree;C.
The work was carried out by a group of electrical engineers, physicists, and materials scientists in the Device and Process Group in the Information Science and Technology Program of the Laser Programs Directorate. The research is part of a larger effort by Livermore scientists and their Department of Energy colleagues to apply laser-based processing techniques to current U.S. semiconductor production problems. The plastic substrate project, now in its third year, is funded by the Defense Advanced Research Projects Agency's High-Definition Systems Program, which sponsors development of new display concepts that address the issues of lighter weight, improved ruggedness, lower power, higher resolution, and easier use.
Laser Pulses Fast, Precise|
The novel Livermore transistor fabrication process combines well-established, low-temperature deposition techniques with excimer lasers that produce pulsed beams of ultraviolet light. These lasers are a much more powerful version of the instruments that are used in eyesight correction surgery to literally vaporize corneal tissue without damaging surrounding tissue. In fact, the lasers are so precise they can make precise notches in human hair that can be exactly and repeatedly duplicated. The Livermore team takes advantage of the laser's extreme precision and ultrafast operation to melt, crystallize, and dope (add impurities to) the silicon layers forming the TFTs at substrate temperatures lower than the melting temperature of plastic.
The Livermore team chose one of the most common plastics for the substrate: polyethyleneterephthalate (PET), more commonly known as polyester. Thin (175 micrometers), cheap, flexible, transparent, and rugged, PET is used for many other purposes, including the Mylar for viewgraphs. Standard 10-centimeter-diameter wafers are cut from 61-centimeter-wide rolls of PET. Onto these plastic circles are applied the materials fundamental to integrated circuits: an insulator (silicon dioxide), semiconductor (crystallized silicon or polysilicon), dopants of selected elements, and metal connectors.
The process begins with a thin layer of silicon dioxide deposited on the plastic wafer through a conventional process called plasma-enhanced chemical-vapor deposition that produces uniform films of molecules. Next, the team uses sputter deposition to apply an amorphous layer of silicon atoms to the substrate. Both of these layers are applied at a relatively cool temperature of about 100°ree;C to keep the plastic intact.
The excimer laser irradiates the amorphous silicon layer from 3 to 10 times at an ultraviolet (UV) wavelength of308 nanometers. Each pulse lasts only 35 nanoseconds (billionths of a second) while melting the amorphous silicon. The result (shown below) is a highly ordered, polycrystalline layer of silicon atoms some 40 nanometers thick. (This transformed silicon, typically called polysilicon, permits electrons to move more easily through its highly ordered lattices.)
Plastic Doesn't Melt|
During the melting process, the fleeting UV laser energy is absorbed mainly in the top 10 nanometers of the amorphous silicon layer before it diffuses downward into the plastic. That localization of the laser energy, together with the silicon dioxide layer that acts as a thermal barrier, keeps the plastic substrate from heating and melting.
Although the silicon layer melts at 1,400°ree;C, the plastic barely notices the heat from the deposited laser energy. The team's understanding of the physics and chemistry of the laser processing steps is aided by advanced simulation work done at Livermore.
The laser beam is adjusted to cover from 1 to 11 square millimeters at the wafer surface. Covering the entire wafer takes about one minute. In contrast, traditional processes require baking glass sheets in high-temperature furnaces for many hours.
The next steps are modified, lower-temperature versions of traditional semiconductor processing involving photolithography, which uses a sequence of photomasks. These masks act as photographic negatives do, allowing light to imprint a pattern on the wafer. The pattern defines the areas to be removed through etching, doped with impurities, and deposited with aluminum connectors.
The doping with boron and other elements is accomplished using another pulsed excimer laser in a technique also developed at Livermore. First, a thin layer of doping atoms is deposited using plasma-enhanced chemical-vapor deposition. Then repeated laser pulses drive the atoms deep into the polysilicon. (Doping allows the polysilicon, which is essentially an insulator, to conduct electricity by giving up or attracting electrons.)
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Key Words: active-matrix liquid-crystal display, amorphous silicon, Defense Advanced Research Projects Agency, enhanced chemical-vapor deposition, excimer laser, flat-panel display, liquid-crystal display, polysilicon, sputtering, thin-film transistor (TFT).
For further information contact Tom Sigmon (925) 422-6753 (firstname.lastname@example.org).