Abstract:
A sensor having a light detector with a small gap between the cathode and anode to enable a high pressure cavity resulting in a long lifetime of the detector due to insignificant sputtering from the cathode and subsequent minimal burying of the noble gas in the cavity. The detector may be made with MEMS technology and its techniques. The sensor may contain an array of light detectors. Some of the detectors may be UV detectors.

Description:
BACKGROUND  
       [0001]     The invention relates to sensors and particularly to sensors having MEMS structures. More particularly, the invention relates to light sensors having MEMS structures.  
       SUMMARY  
       [0002]     The present invention may be a multi-wafer tube-based light sensor having an exceptionally small gap for discharge and a significantly high pressure in the cavity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0003]      FIG. 1  is a side cross-section view of the sensor;  
         [0004]      FIG. 2  is a top cross-section view of the sensor; and  
         [0005]      FIG. 3  is a graph of the sensor cavity breakdown voltage versus the product of the pressure of the cavity and the distance of the cathode and anode spacing. 
     
    
     DESCRIPTION  
       [0006]     The present sensor  10  is a MEMS (micro electro mechanical systems) device fabricated as a light detector. This detector may be used for the detection of infrared (IR), visible and ultraviolet (UV) light, depending on the materials used in its structure. The illustrative example described here may be a UV light detector. It may be used for flame detection and other applications having UV attributes. Related art UV detection tubes are bulky, fragile, and have limited lifetimes. The present MEMS detector may be miniaturized, robust, and have a long lifetime. The present MEMS detector may cost significantly less to build than the related art detection tubes. The present detector may be built with MEMS assembly techniques. The invention may be regarded as a tube type device despite its containment in a MEMS solid state enclosure. It may have other applications besides UV detection.  
         [0007]     A typical UV sensor tube may have relatively large volume, e.g., 9000 cubic millimeters (mm 3 ). One of the concerns is the lifetime of the respective tube which may be limited due to the consumption of neon gas in the tube. The consumption of neon may be due to the sputtering of a cathode material which buries the neon. Such tube may operate with a spacing of about 500 microns between the anode and cathode in a cavity having a pressure of about 100 Torr. The discharge gas composition may include neon with about 15 percent of composition being H 2 . Other noble gases besides neon may be used. The addition of H 2  may reduce the metastables which are secondary discharges that may occur during and/or after a primary discharge between the cathode and anode in the cavity.  
         [0008]     The cathode-to-anode spacing may be made around 125 microns in the present device. Fabrication with MEMS technology makes such a small gap possible. Minimum gaps of other tubes in the art may be about four times larger. Relative to the other UV tubes, the internal cavity pressure may be raised about four times from, for instance, 100 Torr to about 400 Torr. The four-fold pressure increase and the four-fold reduction in the cathode-to-anode gap keep the tube cavity conditions at a Paschen point  25  of the same breakdown or discharge voltage, as shown in the graph of  FIG. 3 . It may be advisable to have a design that keeps the point within 20 percent of the original Paschen point. The sputter rate of a cathode in a device may have a relationship of 1/P 5  where P is pressure of the gas in the cavity. This relationship may vary among different structures of the device. However, for an illustrative example, if the pressure is increased by approximately four times, the sputtering rate of the cathode material may be reduced to 1/1024 of the rate under the same pressure. Further, increasing volume of the cavity makes room for more neon and consequently may proportionally increase the lifetime of the tube.  
         [0009]      FIG. 1  shows a cross-section view of the MEMS tube  10 . The base or substrate  11  may be fabricated from a fused silica. On the fused silica base  11  may be a frit  12  formed as a seal between a fused silica spacer  13  and base  11 . A top  14  may have an anode  15  formed on a bottom surface of the top  14 . Anode  15  may be situated on a peripheral seal  16  which in turn is situated on the spacer  13 . This seal  16  may effectively hold top layer  14  in place, though with the thin anode layer  15  in between, to form a cavity  26  between a cathode  18  and anode  15  of the tube  10 . The anode may be a metal grid with openings so that light  30  may enter through the fused silica top layer  14  and the anode  15  into the cavity. Or anode  15  may be a material that is conductive, and is transparent or transmissive relative to light  30  to be detected by sensor  10 . Deposited or formed on at the center of the substrate or base  11  of tube  10  may be the cathode  18 . Cathode  18  may have a distance  19  from anode  15  which may be about 125 microns. Other tube  10  designs may result in other magnitudes for distance  19 . A thickness  20  of cathode  18  composed of tungsten may be about one micron. Thickness  20  may be varied for other cathode materials. Cathode materials may include tungsten, copper, nickel, gold, silver, nickel-iron, barium oxide, cesium, hafnium, molybdenum, and the like. The seal  16  between anode  15  and spacer  13  may be a Eutectic gold/silicon seal, or seal  16  may be of some appropriate insulative material.  
         [0010]     The cathode  18  material may be selected to provide a long wavelength limit of the detector  10  spectral response. The cathode material may photo emit electrons below a certain wavelength (i.e., a photo emission threshold). The window (i.e., top  14  and anode  15 ) of the detector  10  may provide the short wavelength limit of the detector spectral response. However, the window may also have a filter that limits some of the long wavelength radiation or light impinging the detector. Thus, the kinds of materials used for the top  14 , anode  15  and cathode  18  may be selected to determine the spectral response of the detector.  
         [0011]     A trench  17  may be formed around the cathode  18  to add more cavity volume to cavity  26 . A result of the trench  17  may be an island or mesa-like structure that supports the cathode  18 . A bridge  27  may be formed across the trench  17 . On bridge  27  may be a conductor  28  connecting cathode  18  to the periphery of base or substrate  11  for connection purposes outside of the cavity of tube  10 . The peripheral seal  12  may be situated over or formed across conductor  28 . On the top of conductor  28  may be formed a thin glass or other insulative coating  29  from cathode  18  to the seal  12  to hinder possible shorting from the cathode  18  with anode  14  inside the cavity due to a possible accumulation of metal sputter from the cathode  18  during the operational lifetime of the device  10 . Spacer  13 , situated on seal  12 , may likewise have electrical insulative properties. Thus, cathode  18  and anode  15  may be connected externally outside of the cavity of tube along with keeping the cavity hermetically sealed.  
         [0012]     Relative to the cathode  18 , at standard pressure of 100 Torr, 25 microns of copper may provide an adequate lifetime, for example, 10,000 hours. For that lifetime, only about one micron of tungsten may be sufficient. Tungsten may be regarded as sputtering less material than nickel, under the same cavity and electrical conditions, by a factor of about 20. Copper may sputter more than nickel. It is fair to conclude that copper sputters about 25 times greater than tungsten. The sputter rate at a higher pressure may be reduced by up to R n , where “R” is the ratio of the pressure increase and “n” is power of R, and as applied with the above-noted relationship, 4 5 =1024. Thus, the needed thickness for the tungsten cathode may be less than one micron.  
         [0013]     Various factors may play a part in the material and thickness of the cathode. For instance, if the sputter rate is reduced by about 1000 times, then neon burial may be reduced by about 1000 times due to the four-fold increase of the cavity pressure to about 400 Torr. Thus, for a similar lifetime of the tube, which is dependent on the presence of the neon, the required volume for the neon (or other noble gas) may be about 1000 times less than the volume of the typical related art tube.  
         [0014]     In the case where a typical UV tube may have a volume of about 9000 mm 3  at a pressure of 100 Torr and an anode-to-cathode distance of about 500 microns, the normal lifetime of such tube may be about 10,000 hours. For the new and present tube  10 , having an increase of pressure to 400 Torr and a decrease of distance or gap between the anode and cathode to about 125 microns, the volume of the tube may be reduced by a factor of 1000 down to 9 mm 3  for a similar lifetime in view of the above-noted information.  
         [0015]      FIGS. 1 and 2  show the layout of device  10  relative to cavity  26 . Without the trench or channel  17 , the cavity for the noble gas would have dimensions of approximately 5 mm×5 mm×0.125 mm, resulting in a volume of about 3 mm 3 . Trench  17  may add more volume to a total tube cavity volume. For instance, looking at the Figures, one may note the dimensions  21 ,  22 ,  23  and  24  to be 1, 1, 3 and 6 millimeters (mm), respectively. Thus, trench  17  could add about 16 mm 3 , i.e., ((5 mm×5 mm)−(3 mm×3 mm))×1 mm≈16 mm 3 . The resultant volume of the tube cavity may be about 19 mm 3 .  FIGS. 1 and 2  are not necessarily drawn to scale.  
         [0016]     Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.