In the increasingly competitive global aerospace market of the 1990s, American manufacturers are intensifying their research on electronic products and components to gain economic advantage. A key area of this effort is research on advanced semiconducting materials. In the interest of U.S. competitiveness, NASA is providing industry a technological boost with a materials research program of exceptional potential.
The majority of electronics components in use today are made of the semiconductor silicon. However, there are many other semiconductors with much higher predicted performance than silicon, for example, gallium arsenide and aluminum gallium arsenide. Devices made from these materials could provide higher speeds and superior optical response, consume less power, and operate at higher temperatures, thereby offering important advances in superspeed computers, lasers, communications and infrared systems, and many other high technology microelectronics applications.
The problem is that current materials available do not have the requisite structure and high enough quality to attain predicted performance levels. To improve their quality, materials can be grown as crystalline thin films in a vacuum chamber - but this technique, known as epitaxy, is limited by vacuum conditions in Earth-based chambers. A key to improving semiconductor materials, therefore, is to improve the vacuum environment for thin film growth.
That is the purpose of the Wake Shield Facility (WSF) program being conducted by the Space Vacuum Epitaxy Center (SVEC) of the University of Houston, one of NASA's Centers for the Commercial Development of Space, in cooperation with a consortium of industrial partners led by Space Industries, Inc. (Sll), Houston, Texas. The aim of the program is to demonstrate that low Earth orbit (LEO) offers an "ultravacuum" for growing electronics materials of significantly higher quality than can be produced on Earth, and that these materials can be processed in situ, pointing the way toward future orbital manufacturing facilities producing increasingly sophisticated materials.
The WSF is an orbital laboratory that makes space epitaxial research possible. Carried in the payload bay of the Space Shuttle Orbiter, it is a two-part system consisting of a support unit known as the Shuttle Cross Bay Carrier and a free-flying spacecraft. Released from the Orbiter, the WSF Free Flyer becomes a separate orbiting vehicle (below), pushing the few atoms present in the moderate LEO vacuum out of the way and creating a wake in which there are virtually no atoms. The wake ultravacuum is 1,000 to 10,000 times better than those in the best Earth vacuum chambers. Additionally, the wake region exhibits a nearly infinite pumping speed, removing the excess materials that contaminate Earth chambers. Built by SII and the SVEC team, the 8,100 pound WSF package takes up one quarter of the Orbiter's payload bay, mounted horizontally with the Carrier on the bottom and the 12-foot-diameter disc-shaped Free Flyer atop it, connected by a latch system. The 4,350-pound Free Flyer has 60 kilowatt hours of energy, stored in silver-zinc batteries, to power the thin film growing cells, the substrate heaters, process controllers and a sophisticated array of instruments for characterizing the vacuum.
The Shuttle Cross Bay Carrier remains in the Orbiter. It contains equipment to support the Free Flyer's independent operation, including an innovative communication system that routes commands and data in such a way that, from an operations standpoint, the Free Flyer appears to remain in the bay. This greatly simplifies communications and gives ground crews and Orbiter crews equal access to command and telemetry data.
On a typical mission, the Orbiter's remote manipulator arm grasps the unlatched Free Flyer, moves it outside the payload bay and releases it. Using its own propulsion, the Free Flyer moves to a point 30-40 miles behind the Orbiter for about 50 hours of epitaxial thin film growing free of Shuttle contamination. The film growing cells are located on the wake side, or "clean" side of the disc; the "ram" side houses an avionics platform with 65 square feet of area for other experiments. On completion of the film growth experiment, the Free Flyer is recaptured by the Orbiter crew and, still in the grasp of the manipulator arm, it conducts additional research related to the near- Orbiter environment. Ultimately, the Free Flyer is returned to mate with the Carrier in the payload bay for return to Earth.
The WSF program contemplates four proof of concept missions. The first, WSF-l, was flown in February 1994. On that occasion, the Free Flyer remained attached to the manipulator arm and water vapor outgassing from the Orbiter contaminated the vacuum to a degree. Nonetheless, the mission was largely successful: WSF-l generated a high wake vacuum and the epitaxy apparatus proved itself by growing five gallium arsenide single crystal thin films, the first ever grown in space.
In July 1995, at Spinoff publication time, the WSF-2 was being readied for flight aboard the Space Shuttle. The mission plan called for growth of gallium arsenide and aluminum gallium arsenide films to demonstrate high electron mobility and advanced crystalline structures. The Free Flyer was to be deployed some 30 miles from the Orbiter, well out of contamination range, to form and characterize the first ultravacuum in LEO. Above, WSF-2 is pictured undergoing prelaunch check- out at Kennedy Space Center.
Two more flights are planned. WSF-3, targeted for 1996, is expected to show increased capability in the number and types of thin films grown, and in command and control of the growth process through a ground - based operations center. The program will advance another major step with WSF-4 in 1998; the WSF-4 Free Flyer will be equipped with solar panels for greater power and it will be capable of processing up to 300 wafers.