Peter D. Fuqua

Root cause determination of on-orbit degradation of the VIIRS rotating telescope assembly

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a sensor onboard the recently launched Suomi NPP spacecraft. Shortly after launch, VIIRS was found to exhibit a pronounced decrease in the optical throughput of several bands, with the near-infrared bands being more affected than those in the visible. The anomaly investigation team performed several experiments that concluded the primary source of degradation was throughput loss in the VIIRS rotating telescope assembly, likely caused by ultraviolet light illumination. This paper will discuss the laboratory investigation that determined the root cause of the telescope degradation to be UV photo-darkening of a tungsten oxide contaminant film that had been inadvertently deposited during the mirror manufacturing process. We will present data from experiments conducted on witness mirrors manufactured along with the telescope, as well as other mirrors of the same type that were not contaminated.

Corrosion characterization of durable silver coatings by electrochemical impedance spectroscopy and accelerated environmental testing

Highly reflective front-surface silver mirrors are needed for many optical applications. While various protective dielectric coating schemes have been developed, the long-term durability of Ag mirrors is still of great concern in the optics community for a variety of applications under harsh environments. The corrosion protection behavior of a SiNx-coated silver-mirror coating scheme was tested with electrochemical impedance spectroscopy (EIS) and accelerated environmental testing, including humidity and salt fog tests. The EIS data obtained were fitted with different equivalent circuit models. The results suggested that the 100A thick SiNx coating produced by rf magnetron sputtering was porous and acted as a leaky capacitor on the Ag film, whereas the addition of a NiCrNx interlayer as thin as 3A between SiNx and Ag films resulted in a much denser SiNx coating with a low-frequency impedance value of 2 orders of magnitude higher than that without the interlayer. Humidity and salt fog testing of different silver coatings showed similar results. The 100A SiNx/3A-NiCrNx/Ag coating exhibited excellent corrosion resistance against the corrosive environments used in this study.

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.

Fabrication, Characterization, and Thermal Failure Analysis of a Micro Hot Plate Chemical Sensor Substrate

This paper describes the fabrication and characterization of a micromachined, micro hot plate (MHP) sensor substrate for chemical sensor applications. The MHPs were fabricated using a silicon-processing foundry followed by a novel combination of laser and chemical etching of individual die. An IR microscope was used for thermal imaging of the MHP and shows that the device is very efficient (11.5°C/mW) and has a fast response time (30 ms). The performance of the MHP is compared to thermal measurements on a commercial sensor substrate; the MHP consumes almost 30 times less power. The MHPs also have very good performance as compared to other similar micromachined devices described in the literature. From efficiency measurements of devices in air and in vacuum, heat loss by conduction in air is determined to be ∼30% larger than conduction through the membrane. The measured thermal efficiency in vacuum and in air is consistent with expectations based on simple analytical models, the thermal conductivities of the membrane and air, and the MHP geometry. Data are presented on the high-temperature, thermal failure mode of the devices using scanning electron microscopy and energy dispersive X-ray analysis. At temperatures close to the Si-Au eutectic point, we find that the gold metallization fails due to interaction with underlying polysilicon layer.

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.

Control of stress in protected silver mirrors prepared by plasma beam sputtering

Plasma beam sputtering was used to deposit dielectric-protected silver mirrors that exhibited excellent durability and controlled stress. The durability of the mirrors was strongly dependent upon the presence of a very thin chromium adhesion layer between the silver layer and the dielectric overcoat. The stress of the five-layer mirror was balanced by controlling the compressive stress in the top dielectric layer, offsetting the net tensile stress of the combination of layers below.

Optical Properties and Corrosion Resistance of Durable Silver Coatings

There is a need in the optics community for highly reflective mirrors that do not degrade in harsh environments. While silver can be a highly efficient reflector of visible light, it suffers from poor resistance to corrosive atmospheric gases. Here, we investigate a durable silver coating scheme, in which silver is sputter deposited between very thin layers of nichrome nitride. Excellent corrosion resistance is achieved with a silicon nitride overcoat. Samples were exposed to a variety of atmospheres and it was found that the nichrome nitride dramatically increased corrosion resistance when compared to bare silver or silicon nitride coated silver. Similar results were found when electrochemical impedance spectroscopy was used to evaluate samples. The enhanced corrosion resistance is achieved even when the thickness of the nichrome nilTide adhesion layer is too thin to be considered continuous.

Simulated space environmental exposure of optical coatings for spacecraft solar rejection

Dielectric multilayers composed of niobium pentoxide and silicon dioxide, designed for broadband solar rejection, were exposed to a simulated space environment of ultraviolet light and low-energy (10-20-keV) electron radiation. Samples exhibited various degrees of exposure-induced absorption extending from the ultraviolet to the infrared. Processing variations were correlated to damage susceptibility, and methods were identified that produced parts that exhibited no degradation even though the same materials and coating design were used. Coatings prepared under energetic deposition conditions that provided the densest and most moisture-stable coatings exhibited the best stability to the exposure conditions used.

Observation and characterization of the Stierwalt effect in dielectric filters with model coating defects

Many applications of infrared technology require the use of narrow bandpass filters with excellent out-of-band rejection. Frequently, system designs require that the filter be placed in close proximity to a detector or focal plane array. More than twenty-five years ago Donald Stierwalt discovered that some filters that met out-of-band rejection specifications in a spectrophotometer did not meet specification when integrated into a focal plane assembly. In fact, he reported that proximal to the detector, one filter passed three orders of magnitude more out-of-band light. Since then, the Stierwalt Effect has become widely quoted and poorly studied. Many assume that it has to do with scatter in the film, but very little data has been reported. Here, we report the observation of the Stierwalt effect in an optical filter that was seeded with model defects. The seeding was done by depositing sparse 1μm polystyrene spheres upon a clean substrate before sputter depositing a simple band-stop filter. Light rejection from filters prepared in this way showed a strong dependence upon the distance between the film and the detector. Filters deposited without the spheres showed a much smaller effect.