Fact sheet number: FS-2002-03-76-MSFC
One of the first materials science experiments on the International Space Station – the Solidification Using a Baffle in Sealed Ampoules (SUBSA) — will be conducted during Expedition Five inside the Microgravity Science Glovebox. The glovebox is the first dedicated facility delivered to the Station for microgravity physical science research, and this experiment will be the first one operated inside the glovebox.
The glovebox’s sealed work environment makes it an ideal place for the furnace that will be used to melt semiconductor crystals. Astronauts can change out samples and manipulate the experiment by inserting their hands into a pair of gloves that reach inside the sealed box.
Semiconductor crystals are used for many products that touch our everyday lives. They are found in computer chips, i.e., integrated circuits, and in a multitude of other electronic devices such as sensors for medical imaging equipment and detectors of nuclear radiation.
Materials scientists want to make better semiconductor crystals to be able to further reduce the size of high-tech devices. To control the opto-electronic properties of the crystals, a small amount of an impurity — named a dopant — has to be added to the pure semiconductor. Uniform distribution of the dopant in the semiconductor crystal is essential for production of opto-electronic devices.
For this investigation, tellurium and zinc are added to molten indium antimonide specimens that are then cooled to form a solid single crystal by a process called directional solidification. The lead scientists for this experiment, Dr. Aleksandar Ostrogorsky, selected this semiconductor for space processing because of its low melting point of around 512 degrees Celsius and because it is useful for creating models that apply to a variety of semiconductors.
On Earth, buoyancy forces continuously deform and move fluids in a complex manner, which is hard to depict mathematically. These fluids behave much differently in microgravity – the low-gravity environment created as the Space Station orbits Earth. The fluid motion is greatly reduced so that fluids are stagnant, resembling solids. The absence of complex motion greatly simplifies the analysis of processes occurring in the melt and the crystal. Therefore, microgravity is an ideal environment for studying solidification from the melt.
Consequently, in the past 30 years, numerous semiconductor crystal growth experiments have been conducted in space. Analysis of the space-produced crystals revealed that weak fluid motion existed in the melt during solidification. It remains unclear whether this motion was caused by residual microgravity or bubbles and de-wetting in which the melt separates from the container wall.
The goals of this experiment are to identify what causes the motion in melts processed inside space laboratories and to reduce the magnitude of the melt motion so that it does not interfere with semiconductor production.
These goals will be accomplished through a special ampoule and furnace design. Furthermore, a transparent furnace is being used so that the solidification of semiconductors in space will be visible for the first time. A video camera will send images to Earth that will allow scientists to observe de-wetting and undesirable bubbles.
In several ampoules, a disc-shaped baffle will minimize the effects of residual microgravity in the melt. On Earth, scientists demonstrated that the baffle drastically reduces fluid motion and thus yields more homogenous crystals. In microgravity, it is expected that the baffle will result in even less fluid motion in the melt, leading to better dopant distribution and improved semiconductor crystals.
The data obtained in this investigation will clarify the origin of inhomogeneities in space-produced crystals and will advance our ability to mathematically describe the complex process of crystal production, both on Earth and in space.
To prevent de-wetting, liquid encapsulation, a processing technique commonly used on Earth, will be tried for the first time in microgravity. In two ampoules, a thin layer of a chemically inert liquid called encapsulant will surround the semiconductor melt. Liquid encapsulation will prevent the semiconductor material from touching or sticking to the container wall.
Semiconductors expand or contract when they are heated and cooled. If the material is already touching the container wall and it expands, it causes stress in the crystal, creating defects. Even if the sample shrinks during cooling, defects are created as the material pulls away from the container wall. Scientists on the ground will watch the encapsulation process via video to see if it works in microgravity.
Space Shuttle Endeavor will transport the Microgravity Science Glovebox and this experiment to the Station during STS-111, ISS Flight UF2, set for spring 2002. After the Expedition Five crew installs the glovebox in the Destiny Laboratory and performs an initial setup and checkout of the facility, they will begin the glovebox experiments.
To install the furnace and its cameras, and other data-collection material, the crew will pull out the large, sealed work area on the glovebox. An astronaut will use ports on the sides and front of the glovebox to place the hardware inside. The thermal chamber, where samples are melted, will be located near the front for easy sample change-out with the gloves.
A camera collects real-time images of the samples as they are melting and resolidifying. These images are sent directly to the investigator on the ground working in a telescience center in the Microgravity Development Laboratory at the Marshall Center. From there, investigators can send commands to the experiment, changing temperatures, melt times and other variables that affect sample processing.
Other control and data equipment, including a laptop computer, will be on the right side of the glovebox. Space Acceleration Management System (SAMS-II) sensors will be placed inside the glovebox to measure accelerations — vibrations that can disrupt sample processing. These accelerations can cause fluid motions, such as bubbles, that result in poorly formed crystals.
Before flight, scientists will have loaded the 1.9-inch (4.8-centimeter) long, 1.2 centimeter (0.5 inch) diameter indium antimony crystals inside quartz glass tubes, called ampoules. Thermocouples outside each ampoule will measure temperature, and this information will be sent to scientists on the ground. A total of 10 to 12 samples will be processed during Expedition Five, with each sample processing run lasting 10 to 15 hours.
After the sample is placed in the furnace, the crew will use the glovebox laptop computer to start the experiment. For a typical run, it will take about two hours for the furnace to heat up from 70 degrees to 1,450 degrees Fahrenheit (21 degrees to 790 degrees Celsius). The sample material will slowly melt and resolidify. The crew will periodically monitor the sample and change out videotapes.
In the microgravity environment, convection and sedimentation are reduced, so fluids do not move and deform. Thus, space laboratories provide an ideal environment of studying solidification from the melt. In the past 30 years, numerous solidification crystal growth experiments have been conducted in reduced gravity. However, analysis of these crystals demonstrated that, in virtually all space-produced crystals, a weak melt motion interfered with the ideal “convection-free” process and degraded the crystal quality.
This investigation is expected to determine the mechanism causing fluid motion during production of semiconductors in space. It will provide insight into the role of the melt motion in production of semiconductor crystals, advancing our knowledge of the crystal growth process. This could lead to reduction of defects in semiconductor crystals produced on space and on Earth.
Two technologies will be tested for the first time in microgravity — the automatically moving baffle and liquid encapsulation. This investigation will allow scientists for the first time to watch semiconductor growth in microgravity as a sample is processed. They will be able to observe features, such as fluid motion, melting and bubble formation, which affect the final quality of the crystal. This data can be used in models to improve crystal growth on Earth and in space.
For more information on the this experiment, the Microgravity Science Glovebox and other Space Station investigations, please visit: