MICROFABRICATION - Lasers sculpt nanosatellite parts

In their quest to develop nanosatellites, researchers at the Center for Micro technology at the Aerospace Corp. (Los Angeles, CA) are using lasers to machine critical components for micro thrusters. Unlike an ordinary satellite, which typically has a mass of hundreds of kilograms, costs hundreds of millions of dollars, and takes months to build, the nanosatellite will have a mass of 1-1000 g and be inexpensively produced in large quantities using technologies such as microlithography.

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In their quest to develop nanosatellites, researchers at the Center for Micro technology at the Aerospace Corp. (Los Angeles, CA) are using lasers to machine critical components for micro thrusters. Unlike an ordinary satellite, which typically has a mass of hundreds of kilograms, costs hundreds of millions of dollars, and takes months to build, the nanosatellite will have a mass of 1-1000 g and be inexpensively produced in large quantities using technologies such as microlithography.

As conceived by its inventor, senior scientist Siegfried Janson, such a satellite will be deployed in space in arrays of hundreds for sensing and monitoring purposes.

The researchers are developing several different kinds of microthrusters, each with different properties and each built in different ways. The resulting fabrication techniques can be applied to the construction of other sorts of micro devices. One sort of thruster, called a cold gas microthruster, consists of a pressurized cold-gas feed, a valve, and a nozzle that is a miniature version of a standard rocket nozzle (see Fig. 1). The re searchers construct this device out of Foturan, a photosensitive lithium alumosilicate glass-ceramic material made by Schott Glassworks (Mainz, Germany).

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FIGURE 1. Cold-gas nanosatellite thruster fabricated from Foturan is 1 mm in length and has a throat diameter of 100 µm. Subsonic gas flow becomes supersonic at the throat.
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Upon exposure to ultra violet (UV) light and a subsequent heat treatment, Foturan undergoes a chemical change that allows the exposed areas to be preferentially etched away with hydrofluoric acid. Using an x-y-z micropositioner and any one of various UV lasers, the researchers are able to write a three-dimensional pattern in the glass at a resolution of better than 20 µm. Lasers used include a 1-kHz pulsed Nd:YAG with harmonically generated outputs at 266 and 355 nm, and a 248-nm-emitting krypton-fluoride excimer laser. According to Henry Helvajian, senior scientist, Foturan absorbs 266-nm light within a 100-µm depth, while 355-nm light passes through a 1-mm thickness. "However, there appears to be a nonlinear absorption process at 355 nm that could be an advantage for creating a localized exposure," he adds. "We're working on understanding the physics." He notes that the researchers are looking for a suitable tunable UV laser.

Tiny cooling fins, resonant mechanical devices, and fluid-flow channels also can be constructed using this technique. By focusing a laser deep inside the substrate, channels can be formed that are completely enclosed except for their ends. Helvajian describes the fabrication of ceramic-based microelectromechanical systems (MEMS) such as linear piston valves and other structures in which a contained component is free to move.

A second propulsion device developed at the Aerospace Corp., in collaboration with TRW Inc. (Redondo Beach, CA) and the California Institute of Technology (Pasadena, CA), is called a digital single-shot thruster (see Fig. 2). This device-actually an array of thrusters-is made of a layer of laser-patterned Foturan between two lithographically patterned silicon (Si) wafers. Each thruster's Si nozzle is sealed at its throat by a thin silicon nitride diaphragm. Below the nozzle is a propellant-filled hole etched into the Foturan and capped on its other side by Si that contains a heating element to set off the propellant. In use, the individual thrusters are fired one at a time as needed.

A third type of thruster in development consists of a sheet of Foturan coated with a conductor on each face and containing a laser-written hole or holes. A voltage potential across the electrodes propels ions through the hole, resulting in a microscopic ion engine for modest attitude-control requirements or, in arrays, a macroscopic engine to control larger satellites.

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FIGURE 2. Digital single-shot thruster containing 19 microthrusters is wire-bonded in a silicon-die chip-holder package (left). The silicon nozzles are sealed with silicon nitride membranes 0.15 µm thick. When the thruster is set off, its propellant burns completely in milliseconds (right).
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Direct-write seeding

The researchers also are developing a laser direct-write metal-seeding technique to create metal patterns. In this technique, a substrate to be patterned is immersed in a solution containing nickel and palladium and then direct-written by a galvanometer-steered laser beam. The result is a patterned metallic seed film 1-10 nm thick that then grows to 0.5 µm thick when placed in a batch-processing solution. Helvajian describes electrostatic actuators as one type of device constructed by this technique. "The key to very sophisticated structures," he notes, "is to be able to put metal down where you want it."

The throughput of direct-write seeding is currently limited by the relatively low output power of the lasers used for writing. According to Helvajian, the first high-speed direct-write approach will be carried out using the free-electron laser (FEL) at Thomas Jefferson National Laboratory (Newport News, VA). Now producing 1-ps pulses of 5-µm light at a 37-MHz repetition rate, resulting in an average power of 710 W, the lab's FEL will be modified with the intent of producing 1 kW of average power at a 210-nm wavelength.

Nanosatellites will contain propulsion, attitude control, and communications systems in a sandwich-like structure of Si wafers, multichip modules, Foturan, and other layers. They are intended to be launched into space as clusters and deployed as flocks of individually maneuverable units suitable for such uses as low-earth-orbiting global communications, earth monitoring, and spacecraft and space-station monitoring. For example, deep-space probes could carry a few camera-containing nano satellites and send one or more of them out for an inspection if things go wrong. One real-life turn of events occurred when the Galileo spacecraft ran into trouble on its way to Jupiter. Through an extensive analysis of data, it was eventually determined that the spacecraft's large antenna did not open. "The amount of money spent to determine what happened [to Galileo] was exorbitant," says Helvajian. "If we'd had a picture, money would have been saved."

A microtechnology testbed being assembled at the Aerospace Corp. and intended for flight on the STS-93 Shuttle mission will include MEMS accelerators, gyros, and chemical sensors; some of the devices were made by the Aerospace Corp., says Helvajian. In addition, a digital single-shot thruster array is to be tested on a sounding rocket. A disposable nanosatellite called the Untethered Flying Observer (UFO) will eventually fly aboard the Space Shuttle and the International Space Station. Made partially of plastic and containing a camera, the UFO can be used to examine the shuttle for missing tiles. In the long term, says Helvajian, a nanosatellite could carry tools to a space-walking astronaut or even contain enough on-board intelligence to do a bit of repair work itself.

John Wallace

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