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Laser Machines Microthrusters

Photonics Spectra
May 1997
R. Winn Hardin

LOS ANGELES -- As part of a program to create a mass-producible nanosatellite, scientists at the Aerospace Corp. are developing batch material processing techniques that use UV lasers to create complicated 3-D micro- and mesoscale structures in photosensitive glass.
Groups around the world have used lasers to create microstructures on metal and other materials. However, heat and waste material deposition common to long-pulse ablation have confounded microscale manufacturing. Many researchers look to femtosecond-pulse lasers as the answer to nonablative material processing, but high costs have kept these lasers off the factory floor.

Laser sculpting
A team of scientists led by Henry Helvajian at the Aerospace Corp. is developing a technique called embedded interface fusion that uses cheaper, nanosecond pulses of infrared light to produce microstructures on Foturan, a lithium-alumosilicate glass produced by Schott of Mainz Weisenau, Germany.
Imagine the laser as an artist's chisel. By tuning the laser's wavelength, the artist changes the length of the cutting tool because a material seems more or less transparent to a beam, depending on the wavelength (100 percent absorption at 248 nm down to 10 or 20 percent at 355 nm). Helvajian's team can produce these wavelengths with more cost-efficient lasers such as the Continuum frequency-upconverted Nd:YAG-pumped dye lasers and high-repetition-rate waveguide excimer lasers from Potomac Photonics.
These lasers cause a material metamorphosis inside Foturan. Similar techniques have been used in biomedical research. They either focus a beam inside the material or through a multiple photon process. This is important in creating structures such as fuel channels, and embedding them in a material when ablative methods would cause damage to surfaces and embedded processing.
The process doesn't end with the laser treatment. By carefully adjusting the power, the Foturan material is not ablated, but exposed, much like a photographic plate but in three dimensions. After laser exposure, the material is placed in an oven for about four hours, during which time the treated material turns brown or black. After the heat treatment, the wafer is slipped into a high-frequency etching bath, where the altered material is not ablated, but etched 20 to 30 times faster than the surrounding material. Small holes are drilled into the material to allow the high-frequency bath to reach the altered Foturan.
Helvajian can also make unique "chisel" shapes to create various 3-D shapes, measuring on the order of tens to thousands of microns, with a single pass of the laser. This direct-write technique will be crucial to mass producing microthrusters for nanosatellites, such as hourglass nozzles for compressed air and ridged, spiral- shaped arrays of field emitters for electric microthrusters. This technique is unique, Helvajian said, because it creates 3-D microstructures without the use of multiple masks or laser passes. Future experiments will develop techniques using other glass and ceramic materials.

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