Patent Publication Number: US-4485370-A

Title: Thin film bar resistor

Description:
TECHNICAL FIELD 
     This invention relates generally to resistors and more particularly to resistors comprising an insulative substrate and thin film elements thereon designed to introduce resistance into electric circuits. 
     BACKGROUND OF THE INVENTION 
     In numerous applications in the electrical art, the resistance needed in circuits are provided by what has become known as thin film resistors. A thin film resistor comprises an insulative substrate and portions of a thin metallic film (which may be a composite film) deposited on the substrate and designed to provide resistance of a preselected value in an electrical circuit. The thin film is deposited on the substrate by vacuum deposition, evaporation, sputtering or other process, and the film has a thickness which is less than one micron. A &#34;thin film&#34; is to be distinguished from &#34;thick films&#34; which have a separate technology, are usually formed by the deposition of pastes or the like on a substrate and have a thickness exceeding 10 microns. 
     The disclosure hereof regarding thin films is derived in part from the text Thin Film Technology authored by R. W. Berry, P. M. Hall and M. T. Harris and published in 1968 by D. Van Nostrand and Company (&#34;TFT&#34;). Such text and all the references cited therein are incorporated herein by reference. 
     The thin film elements of thin film resistors are often parts of a much larger thin film pattern produced by well known processes on a substrate to provide thin film integrated circuits each comprising many electric circuit components (such as resistors and capacitors) and conductive leads for coupling the circuit components together. The trend in the manufacture of such integrated circuits has been to miniaturize them insofar as possible in order to realize advantages such as cost and space saving and increased speed of operation of the circuits. The degree however to which such circuits and the electric circuit components thereon can be miniaturized is constrained by limitations in the techniques used to manufacture such circuits. In particular, when a thin film bar resistor is anodized to trim its resistance, size and shape limitations on the smallest mask window usable in the anodizing impose size and shape constraints on the thin film elements of the resistor, and those constraints have, in turn, heretofore limited the minimum resistance obtainable for anodized thin film bar resistors which have been miniaturized insofar as the techniques for manufacturing them will allow. By &#34;an anodized thin film bar resistor&#34; is meant herein a thin film resistor of which the resistance is primarily provided by a rectangular thin film body which has been anodized to trim its resistance. 
     SUMMARY OF THE INVENTION 
     Such limitation on the minimum resistance obtainable for anodized thin film bar resistors which have been so miniaturized is overcome according to the present invention in the following manner. A thin film bar resistor with a conventional structure comprises an insulative substrate and thin film elements thereon including a pair of spaced conductive resistor terminals, a rectangular resistance body disposed between the terminals so to be separated from them by guard margins and comprising a lower unanodized layer and an upper anodized layer, unanodized thin film strip regions occupying such margins and electrically coupling the terminals to opposite ends of the resistance body, and a pair of conductive leads respectively connected to the two terminals. That conventional structure thin film bar resistor is improved according to the invention by (a) providing by at least one of such leads a conductive circuit path which is spaced transversely outward on the substrate from a portion of the resistance body which is adjacent to that path and to the terminal to which such lead is connected, and which portion is on one of the transversely opposite sides of the body, and by (b) including in the resistor a supplementary unanodized thin film region deposited on the substrate and integrally joined to that side portion and to a length of such path to form an electrical bridge providing a current shunt around the mentioned terminal and its adjacent strip region. By virtue of the incorporation of such bridge in the bar resistor, its resistance as seen between its two leads is reduced to a value not heretofore obtainable in anodized thin film bar resistors which have been miniaturized as much as their manufacturing techniques will permit. While the invention is particularly applicable to anodized thin film bar resistors of such small size, it is also applicable to resistors of such kind of larger size. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a better understanding of the invention, reference is made to the following description of an exemplary embodiment thereof, and to the accompanying drawings wherein: 
     FIG. 1 is a plan view of an exemplary thin film bar resistor according to the invention; 
     FIG. 2 is a front elevation in cross section, not-to scale, taken as indicated by the arrows 2--2 in FIG. 1, of the FIG. 1 resistor; and 
     FIG. 3 is a side elevation in cross section, not-to scale, taken as indicated by the arrows 3--3 in FIG. 1, of the FIG. 1 resistor. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENT 
     By way of introduction and as explained in &#34;TFT,&#34; Section 7-1, a thin film on a substrate has a characteristic R s  which is known as &#34;sheet&#34; resistance, and which is equal to the specific resistivity in centimeter 3  of the thin film material, divided by the thickness in centimeter of the film. The sheet resistance R s  can be considered as the resistance offered by one square centimeter of the film to passage of electric current from one side to the other of the square. The characteristic has however a greater significance. When the thin film is in the shape of a rectangular (square or oblong) bar having a length l and a width w, the lengthwise resistance of such bar evidently will be R s  ×l/w. The quantity (l/w) is dimensionless (i.e., is a pure number) so that, as a consequence, rectangular thin film regions of the same thinness and of the same material and having the same ratio of length l to width w will have the same lengthwise resistance for any size whatever of those regions. If the width w equals the length l so that the ratio (l/w) equals 1.0, each of those regions will be square in shape, from which it follows that the sheet resistance R s  of the thin film can be considered not only to be a characteristic of the thin film as such (whatever its size or shape) but also the actual lengthwise resistance of a square region of any size provided by the thin film. For this reason, the value of R s  is often expressed as &#34;ohms per square.&#34; 
     An approach used in determining the lengthwise resistance of a rectangular thin film region is to, in effect, divide it into a number of squares of which the sides of each are equal to the width of the region, such width being sometimes referred to as the &#34;line width.&#34; Numerically this is done by dividing the length l of the region by its width w to arrive at a number n which represents the number of such squares, and which may be either greater or less than 1.0 in dependence on whether l is greater or less than w. Thus, for purposes of determining its resistance, any such region can be considered as being so many &#34;squares&#34; or a specified fraction of a &#34;square.&#34; For example, particular regions may be said to be &#34;2.0 squares&#34; or &#34;0.5 square.&#34; In the case of a thin film bar resistor comprising several rectangular regions in series, the number of squares in each such region is computed, and the individual &#34;square&#34; values of those regions are then added to yield a figure which is the value in &#34;squares&#34; of the whole resistor. 
     It follows from the foregoing that a way of determining the resistance value of a thin film bar resistor is to multiply the sheet resistance value R s  of the included thin film by the value of n which expresses the sum of the number of squares contained in each of the constituent rectangular film regions of the resistor which contribute to its resistance. That is, the resistance of such resistor is the product of an electrical factor, R s  (sheet resistance), and of a geometric factor, n (number of squares), determined by the shape but not by the size of the resistor. Evidently, for any given thin film, the factor R s  will be fixed in value. Accordingly, the only way to reduce the resistance value of any size bar resistor made of such thin film is to reduce the geometric factor n characterizing the resistor or, in other words, the &#34;square&#34; value resulting from the geometry of the resistor. Such square value is reduced as later explained in the thin film bar resistor which is shown in the figures hereof, and which resistor will now be described. 
     Referring to FIGS. 1-3, the reference numeral 10 designates a thin film bar resistor which may be a discrete resistor but which is more likely to be (and is considered herein as being) a part of a larger thin film integrated circuit primarily provided by a pattern formed in a thin film 11 constituted in some places in the pattern of a single film layer and in other places in the pattern of a plurality of thin film layers. Resistor 10 includes an insulative substrate 12 which supports all of the thin film pattern of such circuit but which, for convenience, is shown (FIG. 1) as being broken away so that only the portion of substrate 12 allocable to resistor 10 illustrated. Substrate 12 is preferably a glazed or unglazed alumina substrate. 
     Substrate 12 supports thereon the elements of thin film 11 which are included in resistor 10. Among those elements are a pair of longitudinally-spaced, transversely-extending resistor terminals 15a, 15b comprising (a) a pair of transverse strip regions 16a, 16b (FIG. 2) of valve metal deposited directly on substrate 12 and providing the primary film layer of film 11, and (b) electrically conductive film layer strips 17a, 17b deposited on regions 16a, 16b and constituted of a metallic material of much lower resistance than the mentioned valve metal. That is, strips 17a, 17b render terminals 15a, 15b electrically conductive in the sense of offering insignificant resistance as compared to the portions of film 11 included in resistor 10 and not overlain by such conductive film layers. 
     Preferably, the mentioned valve metal is tantalum nitride, and it is to be understood that the term &#34;tantalum nitride&#34; as used herein includes in its meaning oxygen doped tantalum nitride which is frequently referred to as &#34;tantalum oxynitride.&#34; If desired, however, the valve metal which provides the primary film layer of film 11 may be one of the other valve metals described in TFT, Section 5.4 and listed in TFT, FIG. 5-8. When the strip regions are 42 ohm per square tantalum nitride on an unglazed alumina substrate, they are about 900 Angstroms thick. The metallic material constituting the conductive film layer strips 17a, 17b is shown as being a homogenous layer, but in practice such strips are preferably multi-layer structures each comprising a titanium layer about 700 Angstroms thick on the tantalum nitride, a palladium layer about 2000 Angstroms thick on the titanium and a gold layer about 15,000 Angstroms thick on the palladium. Other examples of suitable conductive layer structures are given in TFT, Section 7.4. 
     Disposed between conductive resistor terminals 15a, 15b is a rectangular thin film resistance body 20 which is shown in FIG. 1 as being a square body with recesses for pattern registration errors. Body 20 is constituted before anodization of tantalum nitride about 900 Angstroms thick. The opposite longitudinal ends of body 20 are separated from terminals 15a, 15b by the guard margins 21a, 21b, the purpose of which is later explained. Such guard margins are occupied by transverse film layer strip regions 22a, 22b of unanodized tantalum nitride valve metal, about 900 Angstroms thick, which are integrally joined with body 20 and terminals 15a, to 15b electrically connect the former and latter together. 
     The terminals 15a, 15b have connected thereto a pair of conductive leads 25a, 25b of the same structure as such terminals in that leads 25a, 25b are constituted (FIG. 3) of underlying film layer regions 26a, 26b of tantalum nitride valve metal and of overlying conductive film layer strips 27a, 27b having the same composition as the strips 17 of the terminals 15a, 15b. Leads 25a, 25b continue on substrate 12 beyond the places where such substrate is shown broken away (FIG. 1) to connect the thin film bar resistor 10 to other parts of the thin film integrated circuit on such substrate. The leads 25a, 25b may be considered to be, structurally speaking, extension of the terminals 15a, 15b. 
     Returning to resistance body 20, in the manufacture of a thin film integrated circuit, it is a common practice to simultaneously trim the resistance of all the component resistors of such circuit to values preselected for such resistors. Such trimming is done by a process which increases the resistance of such resistors beyond their original resistance value. One such process is anodization which is, as described in TFT, Sections 5.4 and 7.3, accomplished by (a) masking the thin film circuit with a mask having windows registering with the resistance bodies of the resistors of the circuit, (b) introducing a liquid or semi-liquid electrolyte into these windows to make contact with these bodies, and (c) passing electric current through the electrolyte and through such bodies (connected as anodes) to produce an anodizing (i.e., oxidizing) of them. The anodizing progresses from the bodies&#39; top surfaces downward to reduce the thickness of their remaining unanodized metal until the resistance of the resistors has been increased to the value desired therefor, the anodizing current being thereupon discontinued. The foregoing explanation of anodization is highly simplified, and for further details on that technique, reference is made to the Sections of TFT set out above. 
     FIG. 1 depicts the results on resistor 10 of anodization. As shown in FIGS. 2 and 3, resistance body 20 comprises a lower layer 30 of unanodized tantalum nitride and an upper layer 31 of anodized tantalum nitride which is substantially non-conducting. Layer 30 is about 450 Angstroms thick so as to have a lesser thickness than the adjacent strip regions 22 of unanodized metal of about 900 Angstroms thickness. The (FIG. 1) dash-dash line 32 represents the outline of the window used during anodization to contain the electrolyte and to limit the area covered thereby to approximately that of body 20. Since there may be some spread of the electrolyte in contact with the thin film beyond the area defined by window 32 and there may be errors that occur in the registration of the electrolyte mask, guard margins such as margins 22 must be provided to prevent the electrolyte from reaching one or both of the resistor terminals 15a, 15b or reaching adjacent conductive leads 25a, 25b. The reason why such happening must be avoided is that the contact of the electrolyte with those conductive terminals would produce a &#34;short&#34; by-passing resistance body 20, and that body accordingly would not become anodized in response to passage of the mentioned current. In addition, it is necessary to assure complete coverage of the resistor body by providing an electrolyte window slightly larger than the width of the resistor body to allow for error in mask registration. 
     As so far described, resistor 10 is known to the art. 
     Coming now to the improvement according to the invention and matters relating thereto, it is often desirable, as has been pointed out, to miniaturize thin film bar resistors such as resistor 10 in order to miniaturize as a whole the integrated thin film circuit in which such resistors are contained and, thereby, to effect cost and space savings and increase the speed of operation of the circuit. Also, it is desirable that the circuit be so designed that the resistances of all its resistors be trimmed proportionally during anodization, and the range of resistor values available to the circuit designer be as great as possible. From the foregoing it follows that for the greatest miniaturization of adjustable resistors using anodization techniques, the resistor body should be contained within the smallest anodization mask window possible with practical manufacturing methods. When using a semi-liquid electrolyte, the smallest size of the width and length that defines the window is the least of such size that surface tension forces characterizing the anodizing electrolyte will not prevent it from passing through the window and contacting the resistance body, and be such that when the mask containing the window is removed, the electrolyte will remain deposited on the resistance body 20. In practice the minimum length and width dimensions of the anodizing window evidently are the same and form a square area. It follows that the smallest length of the body of a bar resistor is equal to the smallest practical anodizing window, and the length between resistor terminals 15a, 15b is equal to the length of the window 32 plus the guard margins 22a, 22b. Similarly, the width of the anodizing window together with required guard margins determines the minimum distance between adjacent thin film elements such as conductive leads 25a, 25b which traverse the lengthwise direction of resistor 10. As previously mentioned, the guard margins are required to prevent the electrolyte from shorting to the adjacent conductors. When, however, the object is to have the minimum resistance possible of the resistor 10 that has been miniaturized, the minimum width dimension required of the window to permit it to pass the electrolyte through it determines the maximum width of the body of a bar resistor and therefore the minimum resistance value attainable for a resistor that has been miniaturized insofar as the techniques for manufacturing them will allow. And as previously mentioned, an allowance in the width of the bar resistor should be provided to assure complete coverage of the resistor body by the electrolyte. This results in a bar resistor width slightly smaller than the width of the electrolyte window by an amount necessary to allow for mask registration errors. 
     Thus the geometry of resistor 10 as so far described obtains for an anodized thin film bar resistor the minimum resistance heretofore thought possible for a resistor of such kind which has been miniaturized insofar as manufacturing techniques will permit. The geometric factor of such minimum resistance resistor has a value which, taking into account not only the square value for the anodized body 20 but also the square values of un-anodized strip regions 22, is about 1.4 square. 
     Coming now to the improvement according to the invention, conductive lead 25a is laid out on substrate 12 (FIG. 1) to provide thereon a first circuit path 40a disposed on the substrate to be outward of body 20 in a first of the two opposite directions aligned with the transverse or width dimension of body 20, such first transverse direction being the &#34;upward&#34; direction in FIG. 1. More particularly, circuit path 40a is spaced outward in that transverse direction from a side portion 45a of body 20 which is adjacent to terminal 15a and is on that one of the transversely opposite sides of body 20 which is nearest to such circuit path. As shown, circuit path 40a is the form of an &#34;L&#34; having (a) a conductive transverse arm 41a integrally joined with and extending outward in the first transverse direction from terminal 15a, and (b) a conductive longitudinal arm 42a extending from an angular junction 43a with arms 41a longitudinally past side portion 45a. With circuit path 40a having such &#34;L&#34; configuration and being so disposed in relation to side portion 45a, the result is that arms 41a, 42a and that side portion bound three sides of a rectangular area 50a defined on substrate 12 by those elements, such arms and side portion enclosing the rectangular area 50a by a three sided enclosure which is continuous from one end to the other of the enclosure except for a gap 44a occurring between the transverse arm 41a and the side portion 45a. 
     The description just given of the elements 40a-45a and 50a applies with equal force to their counterpart elements 40b-45b and 50b which are shown in FIG. 1 as being in the lower right hand corner of resistor 10, and which are preferably (but not necessarily) included as parts of the resistor along with elements 40a-45a and 50a. It is to be noted that circuit path 40b is spaced from body 20 in the second of the mentioned transverse directions, such second direction being downward in FIG. 1. 
     Returning to area 50a, occupying that area is a thin film region 55a (FIGS. 1 and 3) about 900 Angstroms thick of unanodized tantalum nitride valve metal deposited on substrate 12. Region 55a (FIG. 3) may conveniently be integrally joined with, and an extension of, guard margin strip region 22a, and region 55a is, in any event, integrally joined (FIG. 3) with (a) the unanodized tantalum nitride thin film layer 30 underlying the side portion 45a (FIG. 1) of body 20 and (b) the unanodized thin film tantalum nitride layer 26a (FIG. 3) underlying longitudinal arm 42a. Preferably region 55a is also integrally joined with the unanodized tantalum nitride thin film layer underlying transverse arm 41a to thereby be electrically connected with that conductive arm over its length. 
     Region 55a, 55b thus forms an electrical bridge extending between side portion 45a and a length of circuit path 40a to provide a current shunt for resistor 10 around its terminal 15a and its guard margin strip region 22a. The presence of that shunt eliminates a significant amount of the resistance of resistor 10 which is seen between its leads and is introduced in part into the resistor by the presence therein of its strip region 22a. Thus, the bridge region 55a is alone adopted to lower the geometric factor of the resistance of resistor 10 below 1.4 square. 
     Preferably, however, the resistance lowering effect of bridge region 55a is supplemented by providing the matching bridge region 55b which further lowers the resistance of resistor 10. While either of regions 55a, 55b may have any desired longitudinal dimension so long as that region provides a current shunt around its adjacent guard margin strip region, the resistance reducing effect of each of such regions becomes larger with increase in its longitudinal dimension. 
     With resistor 10 having the geometry shown in FIG. 1 so that each of the side regions 45a, 45b which bound the bridge regions 55a, 55b has a length only slightly less than one-half of the entire length of body 20, the geometric factor for the resistance of resistor 10 (as seen between its leads 25a, 25b is reduced from 1.4 square to 1.18 square or to about 84% of the value which such factor has when resistor 10 is conventional in its geometry. Resistors of the same form factor as shown in FIG. 1 but utilizing only one side termination extending 80% of the length have been produced that yielded 0.9 square. A practical value of approximately 0.6 square can be achieved by the extension of the side terminations on both sides of the resistor body shown in FIG. 1 for 80% of the entire lengths of such sides. Hence, the invention hereof permits a substantial lowering beyond that heretofore thought possible in the resistance of anodized thin film bar resistors which have been miniaturized as much as their manufacturing techniques permit, thus extending the lower range of values available to circuit designers. 
     As a detail of design of resistor, the length and width dimensions of its body 20 may each conveniently be 1500 microns with the other dimensions of such resistor being to scale in FIG. 1, such dimensions being not limitative of the invention. 
     The above-described embodiment being exemplary only, it will be appreciated that additions thereto, omissions therefrom and modifications thereof can be made without departing from the spirit of the invention, and that, accordingly, the invention is not to be considered as limited save as is consonant with the recitals of the following claims.