Patent Publication Number: US-2015077216-A1

Title: High Voltage Resistor And Methods Of Fabrication

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates broadly to high voltage resistors and methods for fabricating the same. More particularly, this invention relates to a high voltage resistors which are useful in conjunction with high voltage power supplies such as may be used in conjunction with X-ray tubes, neutron generators, photo-multiplier tubes and the like, although the invention is not limited thereto. 
     2. State of the Art 
     High voltage resistors are well known in the art. A high voltage (HV) resistor is a resistor which is typically on the order of 100 mega-ohms (Mohms) or more. HV resistors on the order of giga-ohms (Gohms) are known in the art and are available from companies such as Vishay Intertechnology, Inc. of Malvern, Pa., Ohmcraft-Micropen Technologies Corporation of Honeoye Falls, N.Y., and Caddock Electronics of Riverside, Calif. 
     A standard high voltage resistor utilizes a ceramic substrate such as alumina on top of which is laid a film of resistive material in a serpentine or patterned fashion. The HV resistor may be arranged in a cylindrical or a planar fashion. Different techniques are known for laying the film down on the substrate. If a sputtering process is used, the resulting resistor is called a “thin film” resistor as the thickness of the film is controllable by the length of the sputtering process. If a screen and stencil printing process is utilized, the resulting resistor is called a “thick film” resistor. Typically, the film is selected from a ceramic-metal (cermet) material such as bismuth iridate (Bi2Ir2O7), ruthenium dioxide (RuO2), iridium dioxide (IrO2), etc. The material of the film, the thickness and width of the film, and the length of the path determine the resistance of the resistor. HV resistors typically operate at a voltage/inch ratio of 10 kV per inch. 
     HV resistors fail over time due to the effects of high electrical stress, high temperature, physical damage to the film, or a combination of factors. Sometimes surface tracking (charge movement) between loops of the path or the trapping of contaminants between loops of the path causes a short circuit to develop. 
     SUMMARY OF THE INVENTION 
     In accord with one embodiment of the invention, a high voltage resistor is provided and comprises a ceramic substrate having a surface and defining a groove, and a resistive film deposited in the groove such that the resistive film is recessed relative to said surface of said ceramic substrate. 
     According to one aspect of the invention, the resistive film is placed in the groove of the ceramic substrate by thick-film micro-pen technology. 
     According to one embodiment of the invention the ceramic substrate of the HV resistor is planar. According to another embodiment of the invention, the ceramic substrate is cylindrical. According to another embodiment, the ceramic substrate can take any desired shape. 
     According to a further embodiment of the invention, the ceramic substrate is made from alumina According to yet another embodiment of the invention, the resistive film is made from a flowable ceramic-metal (“cermet”) paste which is sintered or cured in place. 
     According to another aspect of the invention, where a flat ceramic substrate is utilized, the groove in the ceramic is laid out in a serpentine or winding format. 
     According to a further aspect of the invention, where a cylindrical substrate is utilized, the groove in the ceramic is laid out helically. 
     Objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is top view of a first embodiment of a high voltage resistor according to the invention. 
         FIG. 1   a  is a cross-sectional view through A-A of  FIG. 1 . 
         FIG. 1   b  is a cross-sectional view through B-B of  FIG. 1 . 
         FIG. 2  is a top view of a second embodiment of a high voltage resistor according to the invention. 
         FIG. 2   a  is a cross-sectional view through A-A of  FIG. 2 . 
         FIG. 3  is a perspective view of a third embodiment of a high voltage resistor according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning to  FIGS. 1 ,  1   a , and  1   b , a first embodiment of the invention is seen. A high voltage resistor  100  includes a substrate  110  and a resistive film  120 . The substrate  110  is shown to have a top surface  112 , side surfaces  113   a ,  113   b , a bottom surface  114 , and a groove  116  which is defined in the substrate  110 . The groove  116  is shown to be serpentine or winding with curved areas and straight areas, although it may be laid out different fashions with only straight areas or only curved areas. The groove  116  is defined by side wall(s)  116   a , and a bottom wall  116   b . At each of the ends of the groove  116 , the substrate defines well areas  118   a ,  118   b . Each well area may include one or more extensions  119   a ,  119   b  which extends to the side surface of the substrate. The resistive film  120  is shown located inside the groove and in contact with the side walls  116   a  and the bottom wall  116   b  and recessed below the top surface  112  of the substrate  110 . Conductive pads  130   a ,  130   b  are shown laid down inside of the wells  118   a ,  118   b . The conductive pads are in electrical contact with the ends of the resistive film  120 . The conductive pads are preferably recessed below the top surface  112  of the substrate, but, if desired may extend up to or beyond the top surface of the substrate. Electrical contact may be made to the pads  130   a ,  130   b  either via the sides  113   a ,  113   b  of the substrate through the extensions  119   a ,  119   b  of the wells  118   a ,  118   b , or to the top of the pads. 
     The substrate  110  is preferably made from a relatively non-conductive ceramic material such as alumina. Other ceramic materials such as zirconia may be utilized. The substrate may be etched, subject to a laser cut, or otherwise treated according to well known techniques in order to form the groove. 
     The resistive film  120  is preferably selected from a ceramic-metal (cermet) material such as bismuth iridate (Bi2Ir2O7), ruthenium -oxide (RuO2), iridium-oxide (IrO2), depending upon the desired resistivity of the resistor, although other materials (cermet or otherwise) can be utilized. According to one embodiment of the invention, the resistive film is laid down as a flowable paste and cured in place, e.g., by sintering. 
     According to one aspect of the invention, the ceramic substrate  110  may be of any desired thickness. Typical substrate thicknesses are in the range of 0.5 mm to 5 mm. According to another aspect of the invention, the groove is typically at least 200 microns wide, and preferably less than 500 microns wide, although other widths may be utilized depending on the resistive film width. According to a further aspect of the invention, the groove is preferably at least 5 microns deep, and more preferably at least 20 microns deep, although other depths may be utilized. Regardless of depth, the resistive film is preferably recessed at least 5 microns from the top surface  112  of the substrate  110 . By recessing the film relative to the top surface  112  of the substrate, it is believed that, all other factors being constant, the resulting HV resistor will have a longer effective life than prior art HV resistors where the resistive films are laid on the top surface of the substrate. Alternatively, the resulting HV resistor may be able to be used in higher voltage situations than prior art HV resistors, or may provide the same desired resistance with a smaller footprint than prior art HV resistors. 
     According to a preferred embodiment of the invention, the resistive film  120  is laid down in the groove of  116  of the substrate  110  utilizing a direct writing technique. One direct writing technique utilizes a micro-pen having a nozzle through which a flowable paste is deposited. (See, “Cai, Zhixiang et al., “Laser sintering of thick-film PTC thermistor paste deposited by micro-pen direct-write technology”:  Microelectronic Engineering  86, pages 10-15 (2009). After laying the paste into the groove the paste is cured by subjecting the entire substrate to a high temperature in order to sinter the paste in place. Alternatively, a laser may be used to sinter the paste in the groove. 
     According to other embodiments of the invention, the resistive film is laid down in the groove using other desired techniques known in the art. 
     A second embodiment of the invention is seen in  FIGS. 2 and 2   a , where a high voltage resistor  200  includes a substrate  210  and a resistive film  220 . The substrate  210  is shown to have a top surface  212 , side surfaces  213   a ,  213   b , a bottom surface  214 , and a groove  216  which is defined in the substrate  210 . The groove  216  is shown to be winding in a maze-like manner with only straight areas, although it may be laid out different fashions with only curved areas or in a serpentine manner. The groove  216  is defined by side wall(s)  216   a , and a bottom wall  216   b . At each of the ends of the groove  216 , the substrate defines well areas  218   a ,  218   b . Each well area may include one or more extensions  219   a ,  219   b  which extend to the side surface of the substrate. The resistive film  220  is shown located inside the groove  216 , recessed from the top surface  212  of the substrate  210 , in contact with the bottom wall  216   b  of the groove, but spaced from the side walls  216   a  of the groove. Conductive pads  130   a ,  130   b  are shown laid down inside of the wells  218   a ,  218   b . The conductive pads are in electrical contact with the ends of the resistive film  220 . The conductive pads are preferably recessed below the top surface  212  of the substrate, but, if desired may extend up to or beyond the top surface of the substrate. Electrical contact may be made to the pads  230   a ,  230   b  either via the sides  213   a ,  213   b  of the substrate through the extensions  219   a ,  219   b  of the wells  218   a ,  218   b , or to the top of the pads. 
     By recessing the film  220  relative to the top surface  212  of the substrate, and by locating the film  220  in the groove  216  but spaced from the side walls  216   a , it is believed that, all other factors being constant, the resulting HV resistor will have a longer effective life than prior art HV resistors. Alternatively, the resulting HV resistor may be able to be used in higher voltage situations, or may provide the same desired resistance with a smaller footprint than prior art HV resistors. 
     It is noted that aspects of the HV resistor  200  such as substrate material, film material, groove width and depth, and mechanisms for laying down the film material in the groove may be as described above with respect to HV resistor  100 . 
     A third embodiment of the invention is seen in  FIGS. 3 ,  3   a  and  3   b  where an HV resistor  300  is provided. HV resistor  300  includes a substrate  310  and a resistive film  320 . The substrate  310  is shown to be cylindrical with an outer surface  312 , end surfaces  314   a ,  314   b  and a groove  316  which is defined in the substrate  310 . The groove  316  is shown to be a helical groove, although it could be arranged to be serpentine or winding as in the first two embodiments. The groove  316  is defined by side wall(s)  316   a , and a bottom wall  316   b . At each of the ends of the groove  316 , the substrate defines well areas  318   a  (only one shown). Each well area may include one or more extensions  319   a  (only one shown) which extend to the end surface  314   a ,  314   b  of the substrate. The resistive film  320  is shown located inside the groove  316 , recessed from the outer surface  312  of the substrate  310 , in contact with the bottom wall  316   b  of the groove and the side walls  316   a  of the groove. If desired, the film  320  could be spaced from the side walls  316   a  of the groove. Conductive pads  330   a  (only one shown) are shown laid down inside of the wells  318   a . The conductive pads are in electrical contact with the ends of the resistive film  320 . The conductive pads are preferably recessed below the outer surface  312  of the substrate, but, if desired may extend up to or beyond the outer surface of the substrate. Electrical contact may be made to the pads  330   a  either via the ends  314   a ,  314   b  of the substrate through the extensions  319   a  of the wells  318   a , or to the top of the pads. 
     By recessing the film  320  relative to the outer surface  312  of the substrate it is believed that, all other factors being constant, the resulting HV resistor will have a longer effective life than prior art HV resistors. Alternatively, the resulting HV resistor may be able to be used in higher voltage situations, or may provide the same desired resistance with a smaller footprint than prior art HV resistors. 
     It is noted that aspects of the HV resistor  300  such as substrate material, film material, groove width and depth, and mechanisms for laying down the film material in the groove may be as described above with respect to HV resistor  100 . 
     The HV resistors  100 ,  200 ,  300  may be used in conjunction with high voltage and/or high temperature applications such as high voltage power supplies for X-ray tubes, neutron generators, photo-multiplier tubes and the like, although the invention is not limited thereto. 
     There have been described and illustrated herein several embodiments of a While particular geometries have been described for the ceramic substrate and groove, it will be appreciated that other geometries could be utilized. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.