By Wilson Dizard III
HANSCOM AFB, Mass.- Electronics manufacturing specialists at the U.S. Air Force Research Laboratory`s (AFRL) Sensor Directorate are working to apply a new technology for semiconductor fabrication to some of the most vexing problems facing designers of photonic devices, waveguides, lenses, and "vias" - which are circuits that pass through chips.
The heart of the new technology is a chemical technique for photo-etching p-type III-V semiconductors. The term p-type refers to the semiconductors` use of electronic "holes" to mimic the effect of positive electrical flows, in contrast to n-type semiconductors that use electrons to generate negative electrical flows.
The III-V semiconductors have the distinct advantage in optoelectronic systems of absorbing light directly, in contrast to chips made of silicon or gallium phosphide. That feature is termed "direct band gap," and is one of the key advantages of using indium phosphide, partly because it reduces the energy requirements of the system.
Kenneth Quinlan, a research chemist at AFRL, has developed a method of etching features on indium phosphide semiconductors using the chemical reduction of the material to metallic indium and phosphine. The method involves the use of a visible light helium-neon laser to trigger a reaction between the indium and nitric acid.
During the photochemical process, the indium phosphide acts as a cathode under the influence of the laser light while a corresponding oxidation reaction takes place at the anode to complete the circuit.
"Over the past years, different groups have tried to develop the kind of etches that Ken did develop," says Michael Alexander, chief of the electromagnetic materials branch of the AFRL Sensor Directorate. Quinlan says a similar approach had been attempted by AT&T`s Bell Laboratories unit on the problem of fabricating tiny lenses for fiber optic systems.
Industry experts are already applying the research work at AFRL to the problem of increasing the efficiency of photovoltaic cells. "Using this process, we expect to develop a semiconductor liquid junction solar cell with an efficiency of 15 percent," Quinlan says. "That compares with a maximum of 12 percent for existing solar cells."
Alexander explains that the photoelectrochemical etching process would enable the broad use of indium phosphide instead of gallium arsenide for III-V semiconductors. "As a result, you can use higher frequencies of up to 500 gigahertz."
"In gallium arsenide you can get the higher frequencies but you don`t get significant power," Alexander says. There are also indications that indium phosphide is more radiation-hard than gallium arsenide, Alexander adds, which holds promise for the production of various devices in space-based microwave and millimeter-wave applications.
"Because this method promises a more efficient way of etching vias on semiconductors, it promises to save a lot of real estate on chips," Alexander says. "The chips themselves can be more compact and the number of power leads can be reduced."
Another promising application of the technology is in making "mesas," or elevated areas, on semiconductors, Alexander says. "At this point it`s difficult to predict all the possible applications that people will pick up on."
Alexander continues, "In laying out the design of chips, one of the problems is how to bring in power supplies. They are always a problem because they have to go to all of the devices. The best solution is to put the power leads on the back side of the chip and bring them up through via holes."
AFRL officials say the indium phosphide etching technique holds promise for the fabrication of improved microelectromechanical systems, or MEMS, for microsensors that can detect pressure, temperature, and acceleration. MEMS devices also can be used in systems that detect chemical and biological warfare agents.
Formerly, p-type III-V semiconductors could only be fabricated with methods that were "time-consuming, complex, and not very efficient," Quinlan says. Alternative plasma etching methods require high temperatures that can cause defects in semiconductors, he says. "They use chlorine or fluorine gas, and use a lot of energy," he notes. Those dry etching methods also are as much as 25 times slower than the new technique.
There are other advantages of the new etching process, Alexander says. First, because it affects only p-type semiconductors, it does not etch other semiconductors in contact with them, so it can be used as an etch-stop technique.
Second, the technique uses no special chemicals or special instrumentation. "You could do this in a college chemistry laboratory," Alexander says. "The equipment is quite routine."
Third, the process can be conducted at room temperature. And fourth, because the method uses low-power lasers, experts keep environmental and safety issues to a minimum.
AFRL officials have submitted the etching process to the U.S. Patent Office and a patent on the technology is pending.
For further information on the photoelectrochemical (PEC) indium phosphide feature etching technique, contact Alexander by e-mail at alexande@maxwell. rl.plh.af.mil.
The electronic equipment pictured above is instrumental in performing the U.S. Air Force`s new photoelectrochemical etching of p-Type III-V semiconductors.
