William W. Hansen

Direct-write UV-laser microfabrication of 3D structures in lithium-aluminosilicate glass

The direct-write laser machining technique has been used to process a lithium-alumosilicate glass (FoturanTM) for an application which requires 3D patterned microstructures. Using two UV laser wavelengths (248 nm and 355 nm), microcavities and microstructures have been fabricated for the development of microthrusters for attitude and orbit control of a 1 kg class (10 cm diameter) nanosatellite. In addition, experiments have been conducted to define the processing window for the laser patterning technique. The results include a measure of the change in Foturan strength after a required program bake cycle plus HF etching rates as a function of the laser repetition rate for the two UV wavelengths.

A UV Direct-Write Approach for Formation of Embedded Structures in Photostructurable Glass-Ceramics

Photostructurable glass-ceramics are a promising class of materials for MEMS devices. Previous work micromachining these materials used conventional photolithography equipment and masking techniques; however, we use direct-write CAM tools and a pulsed UV laser micromachining station for rapid prototyping and enhanced depth control. We have already used this class of materials to build components for MEMS thrusters, including fuel tanks and nozzles: structures that would prove difficult to build by standard microfabrication techniques. A series of experiments was performed to characterize process parameters and establish the processing trade-offs in the laser exposure step. The hypothesis that there exists a critical dose of UV light for the growth of an etchable crystalline phase was tested by exposing the material to a fluence gradient for a variety of pulse train lengths, and then processing as usual. By measuring the dimensions of the etched region, we were able to determine the dose. We found that the dose is proportional to the square of the per-pulse fluence. This has allowed us to create not only embedded structures, but also stacked embedded structures. This also implies that we can embed tubes and tunnels with a single exposure inside a monolithic glass sample. We feel that this technique has promise for a number of applications, including microfluidics.

Fabrication of true 3D microstructures in glass/ceramic materials by pulsed UV laser volumetric exposure techniques

A pulsed UV laser based technique has been developed which permits the transfer, by direct-write exposure, of 3D image into a photosensitive glass/ceramic material. The exposed latent image volume is developed via temperature programmed bake process and then etched away using HF in solution. The height of the 3D microstructures is controlled by the initial laser wavelength used during the exposure and the time duration of the etching cycle. Using this technique we have fabricated large arrays of microstructures which have applications to microfluidics, microelectromechanical systems and optoelectronics. The resulting master copy can be used either as is or by use standard injection modeling techniques converted into a metallic or plastic copies. We present these results and others which have specific applications to miniature 1Kg class satellites - nanosatellites.

Effect of laser parameters on the exposure and selective etch rate in photostructurable glass

Photostructurable glass-ceramic materials have received significant attention due to their utility in aerospace engineering and micro technology. For example, the ability to fabricate structures in glass is important in the design and integration of micro scale electronic, optical and fluidic devices. Direct-write pulsed UV laser processing techniques have been utilized recently to create patterned 3D microstructures in a lithium-aluminosilicate glass. The direct-write microfabrication process involves the formation of an initial latent image in the glass via UV laser radiation. Thermal-induced ceramization is utilized to develop the latent image into a permanent image. Material removal and microstructure fabrication are then accomplished by preferential isotropic etching of the developed regions.

Development of a 100-gm-class inspector satellite using photostructurable glass/ceramic materials

A pulsed UV laser volumetric direct-write patterning technique has been used to fabricate the structural members and key fluidic distribution systems of a miniature 100 gm mass spacecraft called the Co-Orbital Satellite Assistant (COSA). A photostructurable glass ceramic material enables this photo-fabrication process. The COSA is a miniature space vehicle designed to assist its host ship by serving as a maneuverable external viewing platform. Using orbital dynamics simulation software, a minimum (Delta) V solution has been found that allows a COSA vehicle to eject from the host and maneuver into an observation orbit about the host vehicle. The result of the simulant show that a cold gas propulsion system can adequately support the mission given a total fuel volume of 5 cm3. A prototype COSA with dimensions of 50 X 50 X 50 mm has been fabricated and assembled for simulation experiments on an air table. The vehicle is fashioned out of 7 laser patterned wafers, electronics boards and a battery. The patterned wafers include an integrated 2-axis propulsion system, a fuel tank and a propellant distribution system. The electronics portion of the COSA vehicle includes a wireless communication system, 2 microcontrollers for system, 2 microcontrollers for system control and MEMS gyros for relative attitude determination. The COSA vehicle is designed to be mass producible and scalable.

Nanosatellites and MEMS fabrication by laser microprocessing

By definition Nanosatellites are space systems that can weigh 1010 kg and can perform unique missions (e.g. global cloud cover monitoring, store-and-forward communications) acting either in constellation of distributed sensor-nodes or in a many-satellite platoon that flies in formation. The Aerospace Corporation has been exploring the application of microelectronics fabrication and advanced packaging technology to the development of a mass-producible nanosatellite. Particular attention is being directed at M3 (Micromachining/MEMS/Microsystems) technology which appears to be important in the integration and manufacturing of these satellites. Laser direct-write processing techniques are being applied for rapid prototyping and to specific 3D fabrication steps where conventional microelectronics fabrication techniques fall short. In particular, a laser based technique has been developed that combines the rapid prototyping aspects of direct-write and the low cost/process uniformity aspects of batch processing. This technique has been used to develop various fluidic components and a microthruster subsystem in a photostructurable glass/ceramic material.

Microengineered cold gas thruster system for a co-orbiting satellite assistant (COSA)

Miniaturization technologies such as Micro-Electro-Mechanical Systems (MEMS) have been used to fabricate a prototype 100-gm class cold gas propulsion system suitable for use on a Co-Orbiting Satellite Assistant (COSA). The propulsion system is fabricated from bonded layers of photostructurable glass (Foturan glass; the design is based on fabricating integrated modular parts. Thus, the propulsion system is mass producible, expandable, expendable (low unit cost), and highly integrated.

Laser post-processing of Inconel 625 made by selective laser melting

The effect of laser remelting of surfaces of as-built Selective Laser Melted (SLM) Inconel 625 was evaluated for its potential to improve the surface roughness of SLM parts. Many alloys made by SLM have properties similar to their wrought counterparts, but surface roughness of SLM-made parts is much higher than found in standard machine shop operations. This has implications for mechanical properties of SLM materials, such as a large debit in fatigue properties, and in applications of SLM, where surface roughness can alter fluid flow characteristics. Because complexity and netshape fabrication are fundamental advantages of Additive Manufacturing (AM), post-processing by mechanical means to reduce surface roughness detracts from the potential utility of AM. Use of a laser to improve surface roughness by targeted remelting or annealing offers the possibility of in-situ surface polishing of AM surfaces- the same laser used to melt the powder could be amplitude modulated to smooth the part during the build. The effects of remelting the surfaces of SLM Inconel 625 were demonstrated using a CW fiber laser (IPG: 1064 nm, 2-50 W) that is amplitude modulated with a pulse profile to induce remelting without spallation or ablation. The process achieved uniform depth of melting and improved surface roughness. The results show that with an appropriate pulse profile that meters the heat-load, surface features such as partially sintered powder particles and surface connected porosity can be mitigated via a secondary remelting/annealing event.

Reduction in ignition energy for single-shot microthrusters using pulsedlaser excitation

An experiment has been conducted to compare the ignition energy of an existing digital thruster design between a pulsed electrical and laser excitations. A 355nm Nd-YAG pulsed laser is used to ignite the stored lead styphnate propellant charge. Given the device design, roughly 800 μJ is necessary to ignite a 180 μg charge volume with a 90% probability of ignition. This energy value is considered an upper limit. Under equivalent conditions, roughly 2.4mJ of electrical energy is required to ignite the same volume. The digital thruster concept is one approach to provide a valveless, slap-on propulsion capability for small (1kg mass class) and large satellites (1000kg mass class) to help maintain attitude or control the damping of low frequency oscillations in extended surfaces.

Micromachined reference samples for particle counting

Three micromachined reference sample prototypes for particle counting have been fabricated by using dark ceramic spots in a transparent glass wafer to simulate particles on a surface. The direct write approach permits the spots to be positioned at random locations within an indicated area of the sample with sizes and numbers that are consistent with a distribution of particles. The goal of this work is to provide a path to creating a set of particle counting and sizing samples that can be used to establish the accuracy and precision of different measurements.