Abstract:
A light detector having spaced electrodes preset by pins or a spacer within a sealed enclosure. The detector may have a MEMS structure that is separate from the sealing of the enclosure. Further, the detector may have a lens for the transmission of light onto the elements. The lens may be coated to affect the amount of light admitted into the enclosure. Light detectable by the sensor may be ultra-violet.

Description:
[0001]     The invention is related to U.S. patent application Ser. No. 10/957,376, filed Oct. 1, 2004, by Cole, and entitled “Small Gap Light Sensor”; and U.S. patent application Ser. No. 10/735,531, filed Dec. 12, 2003, by Cole et al., and entitled “Planar Ultra Violet Light Detector”; which are incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to sensors and particularly to ultra violet light (UV) detectors. More particularly, the invention relates to well and economically packaged UV detectors.  
       SUMMARY  
       [0003]     The invention is a low cost high reliability UV sensor having precisely placed electrodes, without sealing to the sensing structure, and an optional coating for reducing the level of radiation to the sensor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0004]      FIG. 1  is an illustrative sensor;  
         [0005]      FIG. 2  shows sensor electrode isolators of various geometrics;  
         [0006]      FIG. 3  shows two views of another illustrative sensor;  
         [0007]      FIG. 4  reveals package components for sensor;  
         [0008]      FIG. 5  is a top view of a package header for the sensor;  
         [0009]      FIGS. 6   a  and  6   b  are side views of the package header;  
         [0010]      FIG. 7  shows a grid assembly of the sensor;  
         [0011]      FIG. 8  shows an exploded view of a sensor assembly;  
         [0012]      FIG. 9  reveals a top view of the anode grid on the package header;  
         [0013]      FIG. 10   a  reveals the same view as  FIG. 9  except through a lid of the sensor package;  
         [0014]      FIG. 10   b  is a similar view as  FIG. 10   a  and shows a grid area of an older sensor;  
         [0015]      FIG. 11  shows a view of just a cathode situated on the package flange of the package header;  
         [0016]      FIG. 12  shows a view of just the anode situated on the flange of the package header;  
         [0017]      FIG. 13  is a perspective view of the cathode situated on a set of connecting pins of the header;  
         [0018]      FIG. 14  is a perspective view of the anode situated on another set of connecting pins of the header;  
         [0019]      FIG. 15  is a plan view of the anode grid;  
         [0020]      FIG. 16  is a plan view of the cathode;  
         [0021]      FIG. 17  shows another type of header having three pins through the header;  
         [0022]      FIG. 18  shows the basic internal parts for the sensor using the three pin header;  
         [0023]      FIG. 19  shows the header with the cathode and anode in placed on the three pin header;  
         [0024]      FIG. 20  shows the sensor with just one external pin connected to the anode;  
         [0025]      FIG. 21  reveals the hermetically enclosed sensor having the lid welded and sealed to the flange of the three pin header;  
         [0026]      FIGS. 22   a ,  22   b  and  22   c  show steps of fabrication for a sensor having MEMS elements;  
         [0027]      FIGS. 23   a ,  23   b  and  23   c  are cut away views of several MEMS types of sensors;  
         [0028]      FIG. 24  shows an assembled sensor in a package; and  
         [0029]      FIG. 25  shows the sensor having a radiation-affecting lens. 
     
    
     DETAILED DESCRIPTION  
       [0030]     Successful related art UV or flame detectors have been primarily based on highly specialized processes built around “vacuum” tube technology. The physics of such detectors are that the tube of each detector may have a cathode electrode such as tungsten or copper which is the surface from which optically excited electrons are originated, and an anode grid that lets light pass through it but is charged such that it will collect electrons generated by the breakdown instigated by the photoemission of an electron at the cathode surface. The tube may be filled with a neon/hydrogen (Ne/H 2 ) gas mixture to facilitate the breakdown nominally at about 100 Torr residual pressure. Several factors that appear to define and limit device yield and performance may include tube glass cleanliness, gas mixture, plate spacing and gas contamination. These potential causes of problems may be eliminated or minimized with the present invention.  
         [0031]     The present invention may use cathode and anode materials that are the same as those in other UV tubes. Many related art tubes may have issues with scrap and long term reliability due to a need for manual operations of attaching parallel anode and cathode surfaces. Precise spacing of these plates in an Ne/H2 environment sealed to about 100 Torr vacuum integrity, with about 350 volts between the plates, may provide a sensor which when provided additional energy provide by a proximal UV source (e.g., flame) receives sufficient energy for a discharge event which is seen as a current/voltage on the output pins. A precise but low cost location of the electrode surfaces in a sealed gas environment may be provided by the present invention. The TO package design may be used for vertical alignment of the electrodes to further reduce costs.  
         [0032]     The present sensor may leverage physical principles with advances in packaging. The sensor may use low cost vacuum sealed packages (e.g., TO5 or TO8) with UV transmissive windows. The sensor may use low cost ceramic, fused silica, glass and other substances of which precise thickness control (e.g., about 20 mils or 500 microns). Low cost approaches for attaching package pins to bond pads, foils or plated surfaces may be used. The elements for the sensor may be combined to provide a 500 micron geometry between the cathode and anode surfaces within a sealed gas environment improve performance of the UV (e.g., flame) sensor.  
         [0033]     The UV detector may incorporate a MEMS structure in which the mini-UV tube or package does not require sealing to the MEMS structure. Also, the UV detector may be designed to measure UV-C radiation in a proportional mode so as to be a radiometer.  
         [0034]      FIG. 1  is an illustrative example of a low cost high reliability UV light sensor  11 . A package  12  may be a TO5, TO8 or similar type, which is hermetically sealable. The package  12  may have a port  13  for pumping out air, for example. There may be about 100 Torr of Ne/H 2 . The package may include a UV transmissive lens or window  14  which may be made from silica or a special glass. An example of material for a lens or window for a present detector described in this application may include Schott™ (8337) glass. A cathode  15  may be a copper foil or thin plate held with mounting holes, tack, solder or braze. An anode  16  may be a mesh or screen stamped from a foil (or plated) with mounting holes, tack, solder or braze. Anode  16  may instead be a conductive sheet or layer (e.g., ITO) which is transmissive to the radiation of interest, such as UV light. There may be an isolator  17  situated between the cathode  15  and anode  16  for isolating the cathode and anode from each other. The isolator  17  may be made from ceramic or other low cost material having about a 500 micron thickness and with drilled mounting holes. It may another appropriate thickness, such as if the distance between the anode and cathode is less than 500 microns. Terminals  18  and  19  may be connected to the anode and cathode, respectively.  
         [0035]      FIG. 2  shows isolators  17  of different geometries, rectangle, sticks washer, disk, and so forth, which may be used in sensor  11  of  FIG. 1 .  
         [0036]      FIG. 3  shows another version  21  of the UV sensor. A cathode  15  may be plated on a low cost substrate  22 . An anode  16  may be plated on a 500 micron thick substrate  23 . Anode  16  and cathode  15  may be connected to terminals  18  and  19 , respectively.  
         [0037]      FIG. 4  reveals components of a TO8 package  31  for a low cost UV sensor. The dimensions indicated in the present description are illustrative examples, but could be of other values. The package may include a 0.75 inch tall Kovar™ lid  32  with a 0.535 inch inside diameter and a 0.005 inch thick wall. The lid may be stamped rather than machined. At one end of the lid  32  is an opening  33  for a sapphire window or a Schott™ glass window. The window may be brazed to the lid. A glass or ceramic insulator might be placed inside of the package  31 . Also, a TO8 Kovar™ header  34  with a 0.525 inch outside diameter flange is shown in  FIGS. 4 and 5 . The header may be stamped rather than machined. The lid may be projection welded to the header. The header  34  may have six 0.020 or 0.040 inch diameter pins at a distance of 0.380 inch between the pin centers. There are three pins  35  that are 0.024 inch lower in height than three pins  36  on the can side of the flange, as shown in  FIG. 6   a . This pin height difference may instead be another amount between 0.005 inch and 0.100 inch. The pin height difference may dictate the spacing between the anode and cathode. The pins  35  and  36  may nickel plated, at least the portion of the pins inside the enclosure of the lid. The pins may be insulated from the flange by a ceramic or similarly effective material  97  around the pins and between the pins and the flange. There may be a projection in the lid flange center (not shown). The pins  35  and  36  may be identified with, for example, a stamping or color, e.g., red for pins  35  and green for pins  36 .  
         [0038]     In  FIGS. 4 and 5 , dimension  38  may be about 0.40 inch. Dimensions  39  and  46  may be about 0.60 inch and 0.525 inch, respectively. Dimension  47 , which is the outside diameter of the pin  35  or  36  insulator, may be about 0.100 inch. The pin-to-pin centers at opposite sides of the flange may be about 0.380 inch, as indicated for dimension  48 .  
         [0039]      FIG. 6   a  shows a side view of flange  34 . Dimension  41  may be 0.240 inch±0.0005 inch. Dimension  42  may be about 0.12 inch. Dimensions  43  and  44  may be about 0.060 inch and 0.25 inch, respectively. Tube  37  may have an outside diameter  45  of about 0.125 inch and a wall thickness of about 0.008 inch. The pins  35  and  36  may be supercut to about 0.001 inch in height tolerance and about 90 percent flat on top. The pins  35  and  36  may be electrodeless nickel plated Kovar™ pins.  FIG. 6   b  shows the side view of  FIG. 6   a  except only two pins  35  and  36  protrude through the flange to the bottom side for cathode and anode connection purposes.  
         [0040]      FIG. 7  shows a TO8 package  31  grid assembly pieces.  FIG. 8  shows an exploded view of package  31  showing final dimensions when assembled. The height  49  from the top of lid  32  to the top of the outside edge of the flange  34  may be about 0.77 inch. Dimension  51  from the top of lid  32  to the bottom of a pin  35  may be about 1.10 inch. Dimension  52  from the top of lid  32  to the bottom of tube  37  may be about 1.4 inches before a pinch off of the tube.  
         [0041]      FIG. 9  reveals a new anode grid  56  area from looking down on the flange  34 .  FIG. 10   a  reveals the same view except through the lid  32 .  FIG. 10   b  is a similar view of an old grid  56  area  52 .  FIG. 11  shows a view of only a cathode  55  situated on flange  34  with finger-like portions  53  making contact with lower pins  35 .  FIG. 12  shows a view of only the anode grid  56  situated on flange  34  and having round surface portions  54  making contact with higher pins  36 . There may be about 0.020 inch between the cathode  55  and anode  56 . The distance between the high voltage and ground may be greater than 0.030 inch.  
         [0042]      FIG. 13  is a perspective view of the cathode  55  with portions  53  situated on the pins  35  of the flange  34 .  FIG. 14  is a perspective view of the grid  16  with portions  54  situated on pins  36  of the flange  34 . Portions  53  may be for making electrical contact between cathode  55  and pins  35 . Portions  54  may be for making electrical contact between anode  56  and pins  36 .  
         [0043]      FIGS. 15 and 16  show dimensions of the tungsten electrodes  56  and  55 , respectively. The electrodes may each have a thickness of about 0.05 inch. Grid  56  may have an outside diameter of dimension  57 . The outside ridge of anode  56  may have a width dimension  58  of about 0.010 inch. The dimension  48  may be a diameter or distance between centers of portions  54  (or between centers of opposing pins  35  or  36  above). Holes  59  proximate to pins  35  may have an outside diameter of about 0.058 inch and an inside diameter of about 0.046 inch. Portions  54  may have an outside diameter of about 0.040 inch. The screen of grid  56  may have square portions  59  of grid and have border pieces with a width of about 0.005±0.0005 inch and a thickness of about 0.004±0.0007 inch. The centers of the square portions  59  may be about 0.025±0.0005 inch apart.  
         [0044]     An outside dimension  61  of cathode  55  in  FIG. 16  may be about 0.42 inch. Openings  62  for pins  36  may have a cut at a radius of about 0.050 inch. The edges of opening  62  at the outside of cathode  55  may have a radius of about 0.005 inch. Portion  53  may have an outside center shape of a radius of about 0.020 inch. The other cuts about portion  53  on the inside and outside edges may have a radius of about 0.005 inch.  
         [0045]     The steps of assembling sensor  31  may include cleaning the cathode  55  and anode  56 . Then the cathode  55  may be spot-welded to pins  35  and the anode  56  may be spot-welded to pins  36 . The lid  32  may projection welded to the TO8 flange  34 . It may instead be attached in another manner. The assembly  31  may be attached to a manifold to be baked at 400 degrees C. H 2  plus gas fill at  25 ′ may be provided. Then tube  37  may be pinched off. Assembly  31  may be removed from the manifold and tested.  
         [0046]     Another aspect of the present invention may include not having to seal to the MEMS device of the UV sensor for the hermetic seal of the package. The sensor may be regarded as a mini-UV tube which has a hermetic package used to contain Ne/H 2  gas. The MEMS structure mounted in the package may provide the required spacings and electrodes but does not necessarily need to provide the basis for a seal.  
         [0047]      FIG. 17  shows another type of header  92  having three pins  93  through the header. The pins may have height of 0.024 inch above a header plane  94  of the header  92 . However, the height of the pins  93  may be between 0.005 inch and 0.100 inch. Each of the pins  92  may be insulated from the header  92  with a ring or circular barrier  97  of ceramic (or another insulator material) between the respective pin and header. The pins and the header may be fabricated from Kovar™ or equivalent material. The pins  93  may be nickel plated. The dimensions of header  92  may be similar to the dimensions of header  34  described herein. It may also have a tube  37  for gas addition or removal and sealing.  
         [0048]      FIG. 18  shows the basic internal parts for the sensor using the three pin header  92 . The parts include a tungsten cathode  95  and a nickel or tungsten anode  96 . The cathode and anode may be fabricated from other appropriate materials. The cathode  95  may be placed on and attached to the plane surface  94  of header  92 . The attachment of the cathode to the header may be a weld. The cathode  95  may have cutout areas  98  with clearances for the pins  93  and a cutout area  104  for the opening of tube  37 . The header body  92  in this version may be at the same voltage potential as the cathode. However, to isolate the cathode potential from the header, the center portion of the header floor or plane  94  may be isolated from the outside part of the header  92  with a ceramic ring (or other insulation) with the cathode  95  welded to the inner metal part. Provisions may be made for a connection with the cathode  95  outside of the sensor body. The anode  96  may be placed on the pins  93  which maintain the appropriate distance from the cathode  95 . Anode  96  may have a cutout to make room for an opening of the tube  37 . An electrical connection to the anode  96  may be made through one of the pins  93 .  
         [0049]      FIG. 19  shows the header  92  with the cathode  95  and anode  96  in placed on the header  92 . Placed on a flange  101  of header  92  may be the lid  32  which is described herein and shown in  FIG. 21  as enclosed sensor  102 . The lid  32  may have a window  103  at the top. The lid  32  may be projection welded to the header at the flange. If the cathode and header are electrically connected, the lid  32  may act as a cathode loop. Gas may be added to the enclosure of sensor  102 , formed by the lid  32  and header  92 , in an appropriate quantity, mixture and pressure as for similar sensors described herein. After the gas placement in the enclosure, the tube  37  may be closed or pinched off to hermetically seal the sensor enclosure.  
         [0050]     The sensor of  FIG. 20  may have only one external pin  93  connected to the anode  96 . The other pins  93  may be placed and insulated from the header in the same manner as several pins  35  and  36  in the left side of header  34  in  FIG. 6   b.    
         [0051]      FIG. 21  reveals the hermetically enclosed sensor  102  having the lid  32  welded and sealed to the flange  101  of header  92 . Lens  103  may be transmissive to the light or radiation which the sensor  102  is designed to detect.  
         [0052]      FIGS. 22   a ,  22   b ,  22   c  and  23   a  illustrate a fabrication and assembly of the UV sensor with a MEMS structure. A silicon substrate  65  and a Pyrex™ or silicon substrate  66  are obtained in  FIG. 22   a . In  FIG. 22   b , an anode grid  67  may be formed of an electroplated or mesh copper  69  which may be regarded as the anode, or formed with alternative approaches, such as a sheet or layer  87  of light transmissive (e.g., UV) conductive material on silicon wafer  65 . A tungsten layer or electrode  68 , such as a cathode, may be deposited or formed on substrate  66 . A portion of the silicon substrate  65  under grid  67  of anode  69  may be etched away with an RIE or plasma etch to provide a free standing grid  67  over an etched-out space  71 . Wafers  65  and  66  may be bonded together by fritting or anodic bonding  72  as shown in  FIG. 22   c . The distance between the anode  69  and the cathode  68  may be between 0.005 and 0.100 inch. An example distance may be 0.020 inch.  
         [0053]     The resulting UV tube MEMS structure  73  may be mounted with a cathode  68  on the floor of, for example, a TO8 or TO5 header  74 , within a hermetic package  75  as shown in  FIG. 23   c . The package  75  may be back-filled with an appropriate gas mixture (e.g., 100 Torr with Ne/H 2 ) in volume  82  and sealed off with either a weld seal (e.g., projection) or a solder seal  89 . The pressure may be regarded dependent on the spacing and/or volume of the enclosure and could be between 80 and 120 Torr. The pressure and the cathode-to-anode gap  91  may keep the tube cavity conditions at a Paschen point of the same breakdown or discharge voltage. It may be advisable to have a design that keeps the point within 20 percent of the original Paschen point. The enclosure may involve a projection weld for a seal or an Au:Sn 380 degree seal. The seal  89  may constitute attaching a lid  76  to header  74 . The top of the lid may have a hermetic silica UV transmissive lens  77 . The lens  77  may be brazed to an enclosure  76 . Pin  78  of header  74  may be wire-bonded to anode  69  and pin  79  may be wire-bonded to cathode  68 . Pins  78  and  79  may be electrically isolated from header  74  with an insulative material  86 , such as for example a ceramic, around the respective pins. The resultant UV sensor may look like sensor  81  of  FIG. 24 .  
         [0054]     The sensor design may provide for a larger gas volume  82  which may mean less impart from Ne or H 2  sputter removal. The open anode grid  67  over volume  71  may eliminate coating of the anode  69  UV window. The two wafers  65  and  66  may need to be frit (insulator) attached (not necessarily sealed). Silicon technology may be used for forming holes rather than ultrasonic drilling. The critical components are MEMS. The complexity and cost are lower because of the elimination of a third wafer, dual seals, ultrasonic hole drilling in three wafers, and seal metals. The sensor design is scalable to different sizes. The MEMS part may be separated from the gas reservoir.  
         [0055]      FIG. 23   b  reveals a version of the MEMS sensor that has two sealed volumes  82  and  71 . A light transmissive layer  87  may be situated on anode  69  to seal the volume  71  from volume  82 . Also, situated on substrate  66  may be a layer  88  which forms a support for cathode  68 . The raised nature of cathode  68  on layer  88  and its spacing from an inside wall of the opening in substrate  65  may prevent or minimize a build-up of sputtered material on the wall of volume  71  over time which might eventually short the anode  69  and cathode  68  to each other. Volume  82  may be a buffer or reservoir containing an H 2  and Ar (or Xe or Ne) mixture at about 760 Torr (i.e., one atmosphere). The amount of H 2  in the mixture may be between 10 and 30 percent. The second sealed volume  82  may have a pressure between about 600 and 900 Torr. Volume  71  may have an H 2  and Ne mixture at about 100 Torr. The amount of H 2  may be between 10 and 20 percent.  
         [0056]      FIG. 25  reveals a sensor  83  that is a radiometer version of the UV tube. One may measure UV-C radiation levels as a way to determine the radiation level either for CATOX (catalytic oxidation) like filters or other machines that put out UV radiation. A UV tube&#39;s spectral response may be determined by the photocathode. A typical UV tube with a tungsten cathode may be sensitive to light below 280 nanometers (nm). UV-C radiation is radiation emitted below 280 nm. A planar window  84  of the UV light sensor may be coated with a film  85  to provide partial occlusion, absorption or adsorption of UV light and/or other radiation or to provide a tailoring of the frequency versus sensitivity profile of the sensor. This coating or film of the window or lens may be applicable to any of the light sensors disclosed in the present description.  
         [0057]     In addition to having a cathode which can detect UV-C radiation, it may be significant that the UV tube having that cathode operates in a proportional (or non-saturated) mode to be a radiometer. If the UV tube is operated at a high frequency (with a voltage below breakdown without UV but above breakdown with UV), under DC UV illumination, a discharge should occur on every pulse every time that the voltage swings high. This may be regarded as a saturation mode. If the UV tube window  84  is coated with and absorbing film  85  to reduce the radiation intensity level such that a pulse does not occur on every cycle but on a proportional number of cycles, then the UV tube sensor may be operated as a radiometer. The number of pulses in a time period may be proportional to the UV-C signal level. The density of the absorber is such to put the tube in the proper radiation range. If the cathode of the sensor  83  emits an electron on every second cycle, then the count rate may be ½ of the maximum. If the signal is two times smaller, then the count rate may drop to ¼ of the maximum. The sensor may be designed (i.e., electrodes, spacing of electrodes, gas mixture, window, optical coating, configuration, and so forth) for various applications such measuring biological effect and so on.  
         [0058]     In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense.  
         [0059]     Although the invention has been described with respect to at least one illustrative example, 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.