Dean V. Wiberg

A wafer-scale membrane transfer Process for the fabrication of optical quality, large continuous membranes

This paper describes a new fabrication technique developed for the construction of large area mirror membranes via the transfer of wafer-scale continuous membranes from one substrate to another. Using this technique, wafer-scale silicon mirror membranes have been successfully transferred without the use of sacrificial layers such as adhesives or polymers. This transfer technique has also been applied to the fabrication and transfer of 1 /spl mu/m thick corrugated membrane actuators. These membrane actuators consist of several concentric-ring-type corrugations constructed within a polysilicon membrane. A typical polysilicon actuator membrane with an electrode gap of 1.5 /spl mu/m, fabricated using the wafer-scale transfer technique, shows a vertical deflection of 0.4 /spl mu/m at 55 V. The mirror membranes are constructed from single-crystal silicon, 10 cm in diameter, and have been successfully transferred in their entirety. Using a white-light interferometer, the measured average peak-to-valley surface figure error for the transferred single-crystal silicon mirror membranes is approximately 9 nm as measured over a 1 mm/sup 2/ membrane area. The wafer-scale membrane transfer technique demonstrated in this paper has the following benefits over previously reported transfer techniques: 1) No postassembly release process to remove sacrificial polymers is required. 2) The bonded interface is completely isolated from any acid, etchant, or solvent during the transfer process, ensuring a clean and uniform membrane surface. 3) Our technique is capable of transferring large, continuous membranes onto substrates.

High performance MEMS micro-gyroscope

This paper reports on JPLs on-going research into MEMS gyroscopes. [1-4] This paper will describe the gyroscopes fabrication- methods, a new 8-electrode layout developed to improve performance and performance statistics of a batch of six gyroscopes (of the 8- electrode design) recently rate tested. Previously in our group, T. Tang and R. Gutierrez presented the results of their extensive use of ethylene diamine pyrocatechol (EDP) to deep-etch the inertial- sensitive r4esonators and post-supporting structures in a 4- electrode gyroscope design. Today, JPL is utilizing an in-house STS DRIE, replacing the old wet-etching steps. This has demonstrated superior precision in machining symmetry of the resonators, thus significantly reducing native rocking mode frequency splits. A performance test of six gyros has shown an average, un-tuned, frequency split of 0.4% (11Hz split for rocking modes at 2.7KHz). The new JPL MEMS gyroscope has a unique 8-electrode layout, whose large electrodes can provide significant electrostatic softening of the resonators springs. This allows matching of the Coriolis sensitive rocking modal frequencies to be improved from the native 0.4% to an average tuned frequency split of 0.02%. In separate tests, electrostatic tuning in the 8-electrode design has demonstrated the ability to match frequency-splits to within 10mHz, thus ensuring full degeneracy in even a very high Q device. In addition, a newly selected ceramic package-substrate has improved the devices dampening loses such that a mean Q of 28,000 was achieved in the six gyroscope tested. These Qs ere measured via the ring-down time method. The improved fabrication development and other modifications described have led to the JPLs MEMS gyroscope achieving an average bias instability (Allan variance 1/f floor estimate) of 11degree/hr with best in the group being 2degree/hr. In an independent test, Honeywell Inc. reported one of our MEMS gyroscopes as achieving 1degree/hr bias instability flicker floor estimate measured at constant temperature.

Taguchi optimization for the processing of Epon SU-8 resist

Using the Taguchi technique an optimization experiment was performed to characterize the processing of Epon SU-8 negative photoresist. This photoresist has proven to be very sensitive to process variations and difficult to use. The Taguchi method reveals output sensitivity to variations in control factors. Using five processing parameters as control factors, experiments were performed and results were evaluated by comparison to ideal output characteristics. Analyzing the results, a proposed fabrication process was derived from optimizing the control factors for the best overall mean across all the outputs. The experiment was repeated for three film thicknesses: 50 /spl mu/m, 100 /spl mu/m,and 220 /spl mu/m. The desired output characteristics were straight sidewall profile, fine line and space resolution, and adhesion to substrate. Tables show optimized processing parameters for these three film thicknesses.

LIGA-fabricated two-dimensional quadrupole array and scroll pump for miniature gas chromatograph/mass spectrometer

A 3X3 array of hyperboloid quadrupole mass filters with a 3 mm pole length was fabricated using the LIGA (LIthographic Galvanoformung and Abformung) process. Electrical connectivity and spatial orientation are established by bonding the pole array to a low temperature co-fired ceramic (LTCC) substrate. A miniature scroll pump for vacuum pumping with a scroll height of 3 mm was also fabricated using the LIGA process. New LIGA fabrication steps (e.g. expose and developed freestanding PMMA, compression bonding of electroplating base and PMMA, low-stress electroplated films) have been developed to fabricate ultra thick PMMA molds with high aspect ratios (70:1) and high precision. Computational analysis was performed to estimate the miniature scroll pump performance characteristics.

A new wafer-level membrane transfer technique for MEMS deformable mirrors

This paper describes a new technique for transferring an entire wafer-level silicon membrane from one substrate to another. A 1 /spl mu/m thick silicon membrane, 100 mm in diameter, has been successfully transferred without using adhesives or polymers (i.e. wax, epoxy, or photoresist). Smaller or larger diameter membranes can also be transferred using this technique. The fabricated actuator membrane with an electrode gap of 1.50 /spl mu/m, shows a vertical deflection of 0.37 /spl mu/m at 55 V. The proposed technique has the following benefits over those previously reported: (1) No post-assembly release process (e.g. using HF) is required, and no wax, photoresist, or epoxy is used for the transfer purpose; (2) The bonded interface is completely isolated from any acid, etchant, or solvent, which ensures a clean and flat membrane surface; (3) It offers the capability of transferring wafer-level membranes over deformable actuators.

Analysis of a Two Wrap Meso Scale Scroll Pump

The scroll pump is an interesting positive displacement pump. One scroll in the form of an Archimedes spiral moves with respect to another, similarly shaped stationary scroll, forming a peristaltic pumping action. The moving scroll traces an orbital path but is maintained at a constant angular orientation. Pockets of gas are forced along the fixed scroll from its periphery, eventually reaching the center where the gas is discharged. A model of a multi‐wrap scroll pump was created and applied to predict pumping performance. Meso‐scale scroll pumps have been proposed for use as roughing pumps in mobile, sampling mass spectrometer systems. The main objective of the present analysis is to obtain estimates of a scroll pump’s performance, taking into account the effect of manufacturing tolerances, in order to determine if the meso scale scroll pump will meet the necessarily small power and volume requirements associated with mobile, sampling mass spectrometer systems. The analysis involves developing the govern...

Design and fabrication of electrostatic actuators with corrugated membranes for MEMS deformable mirror in space

A novel Microelectromechanical Systems (MEMS) deformable mirror (DM) technology for large, light weight, segmented space telescopes is being proposed. This technology is reported to provide an unprecedented imaging capability in a visible and near infrared spectral range. The MEMS-DM proposed in this paper consists of a continuous membrane mirror supported by electrostatic actuators with pixel-to-pixel spacing as small as 200 micrometer. An array of 4 X 4 electrostatic actuators for the DM has been successfully fabricated by a new membrane transfer technique. The fabricated actuator membrane has been characterized by using an optical surface profiler. The actuator shows a vertical deflection of 0.37 micrometer at 55 V. This device can also address requirements for smaller size and high resolution applications involving optical transmission through aberrating mediums such as imaging and optical communications through atmospheres, high resolution biometric retina signatures through the eye and endoscopic investigation of tissues and organs.

Fabricating subcollimating grids for an x-ray solar imaging spectrometer using LIGA techniques

We are fabricating sub-collimating X-ray grids that are to be used in an instrument for the High Energy Solar Spectroscopic Imager (HESSI), a proposed NASA mission. The HESSI instrument consists of twelve rotating pairs of high aspect ratio, high Z grids, each pair of which is separated by 1.7 meters and backed by a single Ge detector. The pitch for these grid pairs ranges from 34 micrometers to 317 micrometers with the grid slit openings being 60% of the pitch. For maximum grid X-ray absorbing with minimum loss of the solar image, the grid thickness-to-grid-slit ratio must be approximately 50:1, resulting in grid thicknesses of 1 to 10 millimeters. For our proof-of-concept grids we are implementing a design in which a 34 micrometers pitch, free-standing PMMA grid is fabricated with 20 micrometers wide slits and an 800 micrometers thickness. Stiffeners that run perpendicular to the grid are placed every 500 micrometers . After exposure and developing, metal, ideally gold, is electrodeposited into the free-standing PMMA grid slits. The PMMA is not removed and the metal in the slits acts as the X-ray absorber grid while the PMMA holds the individual metal pieces in place, the PMMA being nearly transparent to the X-rays coming from the sun. For optimum imaging performance, the root-mean-square pitch of the two grids of each pair must match to within 1 part in 10000 and simultaneous exposures of stacked sheets of PMMA have insured that this requirement is met.

MEMS technology at NASA's Jet Propulsion Laboratory

The MEMS Technology Group is part of the Microdevices Laboratory (MDL) at the Jet Propulsion Laboratory (JPL). The group pursues the development of a wide range of advanced MEMS technologies that are primarily applicable to NASAs robotic as well as manned exploration missions. Thus these technologies are ideally suited for the demanding requirements of space missions namely, low mass, low power consumption and high reliability, without significant loss of capability. End-to-end development of these technologies is conducted at the MDL, a 38,000 sq. ft. facility with approximately 5500 sq. ft. each of cleanroom (class 10 - 100,000) and characterization laboratory space. MDL facilities include computer design and simulation tools, optical and electron-beam lithography, thin film deposition equipment, dry and wet etching facilities including Deep Reactive Ion Etching, device assembly and testing facilities. Following the fabrication of the device prototypes, reliability testing of these devices is conducted at the state-of-the-art Failure Analysis Laboratory at JPL.

Fine Pitch Grids for an X-Ray Solar Imaging Spectrometer Fabricated by Optical Lithography and XeF2 Etching

We have developed fine pitch, sub-collimating X-ray grids for an instrument in the High Energy Solar Spectroscopic Imager (HESSI), a proposed NASA mission. In addition to high- energy X-rays, the instrument requires collimation of photons with energies of less than 4 keV such that free-standing grids are required that have no material between the grid slats. We have fabricated 25 micrometer thick gold grids that can collimate photons from visible light up to 30 keV X-rays. They are 55 millimeters in diameter and have 200 micrometer thick silicon support structures. The fabrication process starts with 200 micrometer thick 3 inch wafers onto which a 50 angstrom chrome, 300 angstrom gold electroplating strike is e-beam evaporated. A 25 micrometer thick optical resist is deposited on the wafers using a low spin rate. The resist is exposed and developed and an oxygen plasma clean is performed to fully strip resist residue from the strike. 25 micrometers of gold is then plated in the resist mold, resulting in a gold grid with photoresist between each gold slat. The wafer is turned over and a 50 micrometer dry resist is patterned such that it has a array of 1 by 4 millimeter openings to the silicon. The silicon is etched through to the chrome/gold strike using a xenon difluoride etching process. Both types of photoresist are removed with acetone followed by a piranha clean and the chrome/gold strike is removed with a hydrochloric acid and hydrogen peroxide chrome etch which also slowly etches gold.