IL36479A - Electrical resistance element and its fabrication - Google Patents

Description

Electrical fabrication THE This invention relates generally to electrical resistance and more particularly to a resistive element therefor having a and a method of fabricating the The element Is particularly applicable In variable such as which are useful in affording desired adjustments of electrical Generally adjustment of cal resistance is accomplished in variable resistor units by ment of a wiper contact along a resistor In the miniaturized variable resistor units It Is preferable that the resistor track be a thin film deposit of material yielding uniform electrical properties A iron has been commonly used for such a but the Is subject to oxidation which gives rise to objectionable wear and in the operation of the variable resistor A partial solution has been afforded in the past by coating the resistor track with a thin overlay of a noble metal such as or as disclosed in Patent November metal overlays have been limited In application for a variety of For many of particularly are and the deposition process by thermal evaporation from a tungsten filament Is wasteful of gives poor ness as demonstrated by divergencies the ties of various resistive elements depostted the same the resistivity of the noble metals usually much lower than that of the resistor track on which they are For this the thickness of the noble metal overlays must be tremely to limit shunting of the resistor reduction of the resistance value by addition of the For thickness has been necessarily limited to less than Angstrom at which thickness the resistivity of the noble metal overlay Is likely to be between 8000 and 13000 ohms per In most such a resistivity practically cludes the use of an overlay the value the range of and for miniaturized resistors In the medium of to In other the tivity of the underlying resistor track than 1000 ohms per an overlay as low as 8000 ohms per an overlay as low as 8000 ohms per square Is not to overcome the foregoing and other difficulties the prior a new and Improved resistive element Is In accordance with the present Invention a thin overlay of a material covers the resistor track of a variable resistance The material may be indium titanium oxide or ferric The fabrication of the resistive In accordance the present Is accomplished a vacuum deposition apparatus where the individual resistive elements are arrayed suitable masks to expose their resistor tracks to a heat resistant The array of tive elements Is preheated to a temperature of about and subjected to a relatively high The material Is supplied In granular particles ranging in fineness between 320 mesh from a vibratory down a vertical feed In an even and steady flow to the which is electrically heated to the vaporization temperature of the Evaporation of the material occurs as the particles contact the The thickness of semi material may be deduced from measurements of the electrical resistance of the deposit upon a standard glass element having terminals and leads to the exterior of the deposition After the desired thickness has been the resistive elements are cooled to about at which point the vacuum deposition apparatus Is vented to the atmosphere and the resistive which then have a overlay accordance with the present may be removed The technological advantages of the invention are best pressed by comparing properties of Identically prepared trimmer potentiometer elements having no and those with germanium and silicon One of the properties which is a criterion in evaluating trimmers contact resistance variation often briefly referred to as Military specification prescribes ditions for Under these conditions It Is found that the noise for trimmers with germanium overlay is about that of trimmers with no The figure for those with overlay about that of trimmers with Another briefly referred to as the percentage change In between an initial urement and one made after 200 cycles of rotation of the movable The change observed trimmers with either germanium or silicon overlay less than half that found in those with no Temperature is of and here parison must be made between trimmers utilizing a noble metal as taught Patent and thos utilizing the present In of a noble metal overlay tends to give the composite resistance a strong temperature ficient of For some applications this may be but the overlays have a definite advantage where low values of temperature coefficient are The overlay material has a even temperature of resistance which aids in offsetting the normally positive coefficient of the basic resistor one of the objects of this Invention to provide a novel resistive element for a variable It is an object of this invention to provide an element giving reduced contact noise and wear and improved temperature stability In a variable Another object of invention is to provide an overlay Is practical for use on resistor tracks of relatively high resistance and of another object Is to provide an overlay of conductive material for a tract an adjustable resistance Also an object of invention is to provide a process of depositing material in an uniform layer upon a resistor Further and other and a more complete of the invention may be had by referring to the description and taken In conjunction with the accompanying For the purpose of the there Is shown in the drawings a form which is presently it being that this is not necessarily limited to the precise arrangement and Instrumentality I is a perspective view having a broken away showing a resistive element according to the present and Illustrating its application in a 2a through 2d are plan views showing the various steps depositing of thin film material on a resistive 3 a schematic perspective view illustrating a vacuum deposition apparatus for fabrication of resistive elements In accordance with the present 4 is a schematic block diagram setting forth the steps In fabricating the overlay of the present Referring now to the drawings in wherein like numerals indicate like there Is shown ly enclosed by dotted in a potentiometer which may be substantially the same as that shown in 3 of Patent For simplicity in some of the elements are or shown diagrammatical The potentiometer embodies a element 12 formed on a substrate 20 of flat figuration as providing a support surface A central opening 22 and a peripheral notch 23 are provided in substrate 20 for mating reception within a casing such as that closed In Patent The substrate 20 may be cated in blank form out of a variety of available insulating such as are afforded by Insulating for example the phenolic and epoxy a ceramic such as beryl quartz and the like are found to be erable materials for the substrate I icate glass and other high temperature insulating glass are highly The port surface 21 preferably Is smooth and regular for bonding of various films of materials as described A film track 30 of electrically resistive material bonded on the surface 21 about the periphery of substrate as The material of track 30 may be any of the commonly used resistor For a relatively thin film of chroml Iron alloy may be having a range of resistance of 500 ohms to ohms with a film thickness of to 100 Angstrom An composed of to to chromium and the remainder has been found to be Resistance ranges of over ohms may be as well by the proper choice of resistance material and by appropriate dimensioning of the film track If cermet materials may be utilized to provide a relatively thick film resistance element for the track having a thickness appreciably above 2000 Angstrom Thus a relatively wide range of resistance say from ohms to 10 meg may be provided In different versions of variable resistor The track 30 may be bonded on to the surface 21 by the usual techniques presently common in the Terminal means are provided by the deposit of metallic terminal pads 42 at opposite ends of the track bordering the peripheral notch of pads 42 are a good electrical conductor such as having a good electrical contact with the track Electrical leads 24 and 25 are bonded to the terminals 42 and extend to the potentiometer end terminals A wiper or slider a portion of which is shown at is connected by a lead 28 to the potentiometer intermediate terminal An overlay film 50 of material coats the track 30 as The material of overlay 50 selected from a group consisting of Indium titanium oxide and ferric The preferred materials are germanium and The thickness of the overlay 50 not as was the case in the noble metal overlays of the prior since the resistivity of the material considerably higher than that of the resistor track thickness of the overlay 50 between 100 and Angstrom 2a through 2d illustrate the fabrication of a sistive element in accordance with present A blank substrate 20 is shown in The terminal pads 42 are first deposited thereon as illustrated in Then resistor track 30 is provided on the surface 21 as illustrated in And as shown in 2d the overlay 50 is provided over the resistor track 30 to provide a resistive element in accordance with the present A flash evaporation apparatus for the fabrication of sistive elements accordance with the present invention generally by the numeral 80 in the vacuum tion apparatus 80 consists of a base 81 and bell jar 82 closing a space which may be evacuated by the vacuum pump The apparatus 80 may be vented to the atmosphere through a vent port rods 85 are provided to position various elements within the Jar A mask array dome 90 is supported on the support rod 85 a distance above the base 81 and provides a parabolic or spherical port orientation for the removable trays 91 with respect to a point within the jar Depressions 92 in the trays are shaped for reception of the substrate blanks 20 shown in The depressions 92 openings therethrough to the resistor track 30 of each resistive element It is understood that preferably the shape of the depressions 92 counts for the peripheral notch so that the resistor tracks 30 may be properly oriented with respect to the in depressions A heater dome 95 is spaced above the mask array dome 90 on the support rods as Heater elements indicated by the dashed are provided on the underside of the heater dome 95 above each removable tray A power source 97 is connected by leads 99 to each heater element and in vides energy to heat the resistor trays 91 facing the inside of the heater dome understood that the heater element power source and leads 99 shown In 3 are merely simplified schematic and far more sophisticated means may be visualized by those skilled In the A 100 extends from a support rod outwardly over the heater dome The vibratory may be of the known type energized by electrical solenoids or motors to shake with regular A vertical feed pipe 102 having a funnel 103 at the top end extends through at the center of the mask array dome 90 and heater dome The funnel 103 engages within the opening through heater dome 95 to support the feed 102 In a vertical as The funnel and pipe extends freely through opening 104 in the mask array dome The funnel in 103 positioned directly beneath the vibratory A boat of a highly heat resistant material such as tantalum or molybdenum Is provided at a central with respect to the trays In the mask array dome the boat Is positioned at a point which is the focus of the or spherical support orientation of the trays Resistive heaters 107 are ted by power to a source of power and serve to heat the boat A bracket 115 extends from a support rod 85 as a mounting for a glass plate Spaced electrodes 119 are provided on the glass plate and leads extend therefrom outside the jar to a meter 120 for readings of resistance across the glass plate between electrodes The arrangement outlined Is monly referred to as a and its use is described at a later point in this The steps the process of the overlay 50 on a resistor track 30 are now described with reference to the apparatus shown in 3 and the block diagram shown in The Individual resistor elements each having a resistor tract 30 such as that shown are arrayed the movable trays 91 by them in the depressions 92 with surfaces face Only the tracks 30 are exposed through the depressions the remainder of surfaces 21 being masked The trays are placed the mask array dome 90 with the resistor tracks 30 exposed to a focus point at the boat The within the bell jar 82 is evacuated by the vacuum pump 83 to a pressure of about Current Is supplied from the source 97 through leads 99 to provide heat at heater elements 96 the array of masked elements are preheated to a temperature of about A supply of e such as provided the vibratory 100 in a granular form having particle fineness ranging between Vibration and gravity provide an even flow of Particles of the semi material flow from the 100 and down the feed pipe to drop on to the tantalum boat The heaters 107 are energized through power 109 by the source of power The tantalum boat 105 is heated to a temperature of about At this the e particles onto the boat are almost instantly so that the process is characterized as The vapor disperses in the direction of the arrows and the result Is a deposit on the surfaces of the resistor tracks At the same a coating of the material will be deposited upon the plate and with increasing thickness of the the resistance between the electrodes as served on the resistance measuring instrument will The glass monitor thus provides a means of the of the the point at which the desired thickness has been deposited on the tracks thickness of the overlay on the tracks is between 100 and 4000 Angstrom A typical deposit of germanium would be one having a room temperature tivity of 500 megohms per At this would be mately ohms per Such a deposit can be in about 2 When the desired thickness is the vibratory hopper heat elements 97 and heaters 107 are turned off and the array is allowed to cool to about Then the jar 82 may be vented to the atmosphere by vent The individual resistive elements may be removed from the apparatus 80 and have a overlay in accordance with the present While a particular embodiment of the invention has been illustrated and it be obvious to those skilled the art that changes and modifications may be made without departing from the invention In broader Deposit of the overlay by flash evaporation has been but suitable coatings within the purview of the invention can be laid down by thermal electron beam deposition from It is therefore the aim in appendant claims to cover such changes and modifications as followed the true spirit and scope of the insufficientOCRQuality

Claims (2)

1. A potentiometer comprising: - a resistance element which includes an insulating substrate providing a support surface, - an elongated film track of electrically resistive material bonded on said surface, - terminals at each end of said track for connecting to electric conductors, and - an overlay film of homogeneous non-particulate semi- conductive material coating said track, said material being selected from a group of semi- conductive materials such as germanium, silicon, indium antimonide, titanium oxide, ferric oxide or the like, and - an electrically conductive slider making contact with said film and movable along the length thereof, said slider having an electrically conductive connection therefrom to a third terminal,

2. A potentiometer in accordance with Claim 1 wherein the resistivity of said film track is in excess of 1000 ohms per square* 3· ¥he potentiometer of Claim 1 wherein said resistive material is an allow consisting essentially of nickel, chromium and iron* 4· The potentiometer of Claim 1 wherein said electrically resistive material is an alloy consisting essentially of 20 to 65j£ sickel, 15 to 75$ chromium and the remainder iron, said track having a thickness of between 100 to 4000 Angstrom units. 36479/2 13 5· The potentiometer of Claim 1 wherein said overlay film has a thickness of between 100 and 4000 Angstrom units. 6· The potentiometer of Claim l wherein said overlay film has a thickness such that the room temperature resistivity thereof is in excess of one hundred megohms per square. PARTNERS I3:CB