A novel micromachined incandescent light source has been designed and fabricated to operate at temperatures exceeding 2,500 K. The high-temperature, tungsten filament-based source has a high-brightness, broad spectral band emission. The monolithic design allows for ease of incorporation with on-chip electronics as well as with fiber optics. Previously micromachined incandescent lamps contained, variously, tungsten or polycrystalline silicon filaments that glowed more dimly because they could only be operated at temperatures between 900 and 1,200 °C. The present devices can be used either in a single (discrete device) or two-dimensional array format for miniature spectroscopic instruments and for automotive dashboard displays.
A prototype light source was successfully operated at 2,500 K and has a spectral output closely resembling a simulated blackbody source at the same temperature (see Figure 1). The fabrication process is outlined schematically in Figure 2. The source is based on a modular design and consists of three separately micromachined chips that are subsequently bonded together. The three chips consist of one that includes the tungsten filament, one that includes a reflective mirror, and one that includes an encapsulating transmission window. Each chip starts out as part of a Si wafer that is coated with an insulating layer of SiN.
To form the mirror chip, a highly reflective metal is deposited on one face of the wafer. Then a metal ring for subsequent wafer-to-wafer bonding is deposited on the mirror surface. The processing of the filament chip is more complex. First, gold contact pads for the tungsten filament are deposited on one surface of the wafer. An electrically insulating layer of SiO2 is deposited over the contact pads and the rest of this face. Metal rings for subsequent wafer-to-wafer bonding are deposited on both the SiO2 layer and the opposite SiN face of the wafer. A central hole destined to become the evacuated light source chamber is patterned and etched through the Si and SiN layers. The SiO2 layer is patterned and etched to expose the contact pads. The tungsten filament is installed, either by bonding a commercially available thin tungsten wire onto the contact pads, or by attaching a micromachined tungsten filament or by the deposition and patterning of tungsten thin films. Following installation of the filament, the remaining layer of SiO2 extending within the lamp chamber is etched away. Processing of the window chip begins with the deposition of a bonding ring on face of the wafer. The window is patterned on the bonding-ring face and then the SiN and Si layers are etched away in the window area, leaving only a window of SiN on the face opposite the bonding ring. Finally, the window, filament, and mirror wafers are bonded under vacuum or a controlled atmosphere.
This work was done by Thomas George and Eric Jones of Caltech for NASA's Jet Propulsion Laboratory, in collaboration with Margaret Tuma of NASA's Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers 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
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