A micromachining process for the fabrication of vibratory microgyroscopes from silicon wafers, and aspects of the microgyroscope design that are inextricably linked with the fabrication process, have been modified in an effort to increase production yields from perspectives of both quantity and quality. Prior to the modifications, the effective production yield of working microgyroscopes was limited to one or less per wafer. The modifications are part of a continuing effort to improve the design and increase production yields to more than 30 working microgyroscopes per wafer.
A discussion of pertinent aspects of the unmodified design and the unmodified fabrication process is prerequisite to a meaningful description of the modifications. The design of the microgyroscope package was not conducive to high yield and rapid testing of many microgyroscopes. One of the major impediments to high yield and testing was found to lie in vibration-isolation beams around the four edges of each microgyroscope, which beams were found to be unnecessary for achieving high resonance quality factors (Q values) characterizing the vibrations of petallike cantilevers. A Plug-and-Test Design enables the testing of many microgyroscopes in a short time.
The fabrication process included an 8- µm-deep plasma etch. The purpose of the etch was to create 8-µm vertical gaps, below which were to be placed large gold evaporated electrodes and sensing pads to drive and sense resonant vibrations of the "petals." The process also included a step in which bridges between dies were cut to separate the dies.
The etched areas must be kept clean and smooth (free of debris and spikes), because any object close to 8 µm high in those areas would stop the vibrations. However, it was found that after the etch, there remained some spikes with heights that were, variously, almost as high or as high as the etch depth. It also was found that the cutting of bridges created silicon debris, some of which lodged in the 8-µm gaps and some of which landed on top of the petals. The masses added to the petals by the debris altered resonance frequencies and/or Q values to unacceptable degrees. Hence, the spikes and the debris have been conjectured to cause most of the observed malfunctions of newly fabricated microgyroscopes.
Another pertinent aspect of the unmodified design and process was the fabrication of electrodes and the 8-µm capacitance gap on a 500-µm-thick wafer, and the fabrication of a 3-mmthick baseplate from another wafer. It was necessary to bond these wafers to each other in an assembly step that was later found to be superfluous in that it could be eliminated by a suitable modification of the design.
The modifications include a redesign of the microgyroscope package to eliminate the vibration-isolation beams while providing acceptably high Q values (4 × 104). The modified design includes a plug-in feature for quick testing (see figure). The plasma etch has been replaced by a wet etch, using a specially formulated KOH-based solution, that does not leave spikes. The design of the bridges has been modified to incorporate double notches, such that they can be cut without producing much debris, and a special suction tool resembling one used by a dentist has been developed to collect flying debris during cutting.
The superfluous assembly step has been eliminated by modifying the design so that all the functional parts previously fabricated on the 500-µm and 3-mm wafers are now fabricated entirely on 3-mm baseplate wafers only. In a previous approach to elimination of the superfluous step, KOH etches were made through 3-mm wafers, then metal patterns were formed by evaporating metals while using shadow masks (not standard practice). In the modified process, the metals are evaporated first (standard practice), then holes are ground by use of a diamond-tipped drill on an index table.
This work was done by Sam Y. Bae, Karl Y. Yee, and Dean Wiberg of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
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Refer to NPO-30341, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Modifications of Fabrication of Vibratory Microgyroscopes
(reference NPO-30341) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, detailing the modification of the fabrication process for vibratory microgyroscopes, specifically focusing on the Thin Baseline design. It is part of the NASA Tech Briefs series (NPO-30341) aimed at disseminating aerospace-related developments with potential wider applications.
The introduction outlines three designs considered for characterization: Thick Baseline, Thin Baseline, and Integrated. However, after the departure of Dr. Tang, the focus shifted solely to the Thin Baseline design. Initial testing of six individual gyroscopes from the first batch revealed a high failure rate, with only one gyro functioning correctly, achieving a performance of 16°/hr.
The document emphasizes the fabrication steps for the Thin Baseline design, which include the use of thick film lithography. This technique has been noted for its efficiency in shortening the fabrication process, particularly through the use of STS masks and 49% HF etching masks, which aid in step coverage during the lift-off process. However, the document also acknowledges various problems encountered during fabrication, although specific issues are not detailed in the provided excerpts.
In addition to fabrication techniques, the document discusses the current status of the project and outlines future plans, although specific details on these aspects are not included in the excerpts. The overall aim is to enhance the performance and reliability of microgyroscopes, which are critical components in various aerospace applications.
The Technical Support Package serves as a resource for those interested in the advancements in microgyroscope technology and provides contact information for further assistance through NASA's Scientific and Technical Information (STI) Program Office. It highlights NASA's commitment to sharing knowledge and fostering innovation in aerospace technology, while also clarifying that the information provided does not imply any government endorsement of specific products or manufacturers.
In summary, this document encapsulates the ongoing efforts to refine the fabrication of vibratory microgyroscopes, addressing both the challenges faced and the strategies employed to overcome them, ultimately contributing to advancements in aerospace technology.
