Abstract:
A high stability, high resistance metal film resistor having layered metallic films deposited and annealed such that one layer has a positive TCR and a negative TCR Slope, while a second layer has a negative TCR and a positive TCR Slope, thereby yielding a resistive film having TCR and a TCR Slope approaching zero.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to metal film resistors and in particular to resistors having two or more layers of a metallic film deposited on an insulative substrate, wherein at least two different metallic compositions are deposited alternately in the sequence of layers. Alternating metallic compositions in a layered resistive film structure provides a technique for controlling the TCR and the TCR Slope of the resistive film. 
     2. Description of the Prior Art 
     Metal film resistors are typically made by single target sputtering of a metallic alloy composition on an insulative substrate and subjecting the resulting sputtered substrate to a heat treatment in air at approximately 300° C. Typically either a ceramic core or a ceramic chip is utilized as the substrate. The resistive films used are typically alloys of nickel and chrome with some other metals used in lesser percentages. Sputtered or evaporated NiCr alloys are widely used as deposited resistive film. 
     The desired TCR is obtained by heat treating the resistive film. The range of time and temperature for the heat treatment is usually a function of the desired temperature coefficient of resistance (TCR) of the resistor. During the heat treatment there is a growth of crystals in the bulk of the resistive film applied to the substrate; the larger the crystals, the more positive the TCR will be. However, during heat treatment crystals on the surface of the metal film break down and surface oxidation takes place, causing the TCR to be less positive in that area. With the addition of a heat treatment to the process of making resistors, the net effect is that for most resistors the TCR will be positive because crystal growth is promoted in the bulk of the metal film. To prevent the TCR from becoming too positive, contaminants can be introduced into the sputtering process. Reactive sputtering can be used concurrently for TCR control. However, only TCR is controlled thereby, not TCR Slope. 
     One problem with prior art metal film systems for resistor applications is that the TCR Slope cannot be controlled. Controlling the TCR Slope enables one to produce a resistor whose operation is more independent of temperature and is therefore more stable. Ideally, a TCR of 0 (zero) and a TCR Slope of 0 (zero) is desirable. To control to the TCR Slope and thereby obtain a TCR approaching 0 (zero) over a wide range of factors, a layering of metallic films of differing material composition has been found to be effective. The present invention is directed to a layered metal film resistor having significantly higher stability than prior art metal film resistors and having a significantly higher resistance in ohms per square than prior art metal film resistors. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide a high stability, high resistance metal film resistor with a sheet resistance of 2000 to 15000 ohms per square. 
     A further object of the invention is to privide a resistive film system which yields much higher resistances than previous resistive films, while exhibiting good temperature characteristics and high stability. 
     A further object of the invention is to provide high resistance, high stability resistors to be made on much smaller substrates than were previously possible. 
     The objects of the invention are achieved by depositing one layer of each of two different conductive films on an insulating substrate. A first layer of metal silicides, such as chromium-silicon (CrSi), is reactively deposited by sputtering in an argon and nitrogen mixture. This layer is annealed at 500° C. in air for sixteen hours. A second layer of a metal alloy, such as a nickel-chromium-aluminum alloy (NiCrAl), is deposited by sputtering coextensively over the first layer. This layer, together with the first layer, is then annealed at 300° C. in air for sixteen hours. 
     The chromium-silicon under-layer has a positive temperature coefficient of resistance with a negative TCR Slope. The nickel-chromium-aluminum over-layer has a negative temperature coefficient of resistance with a positive TCR Slope. The combined effect of the two layers is a TCR near 0 (zero) and a TCR Slope of 0 (zero). This resistive material system allows high resistance, high stability resistors to be made on much smaller substrates than were previously possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a cross-sectional view of a layered metal film resistor according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention provides a high stability metal film with a sheet resistance of 2000 to 15000 ohms per square by using a layered resistive material system in which the metals or alloys of each layer have complimentary temperature characteristics which offset one another in the film processing. A resistive material film having good temperature characteristics, high resistance and high stability can be achieved through a material system which allows control of the temperature coefficient of resistance (TCR) (the first derivative of resistance with respect to temperature), and the temperature coefficient of resistance Slope (TCR Slope) (the second derivative of resistance with respect to temperature). In this invention, control over the TCR and TCR Slope is achieved through the use of a layered film system. The first or under-layer is selected to have a positive TCR with a negative TCR Slope. The second or over-layer is selected to have a negative TCR with a positive TCR Slope. The combined effect of the layers is that the resistive film will have a near 0 (zero) TCR and a TCR Slope of 0 (zero). 
     A preferred embodiment of a metal film resistor 10 is illustrated in cross-section in the FIGURE. Resistor 10 has an insulative substrate 12, an under-layer 14 of a first conductive film and an over-layer 16 of a second conductive film. 
     In the preferred embodiment, two metallic layers are used on an insulative substrate, each layer being a conductive film having a material composition differing from the other layer in TCR and TCR Slope. 
     A first layer 14 of metal silicides, such as chromium-silicon (CrSi), is reactively deposited on insulative substrate 12 by sputtering in an argon and nitrogen mixture. This layer is annealed at 500° C. for sixteen (16) hours in air. 
     A second layer 16 of a metal alloy, such as a nickel-chromium-aluminum alloy (NiCrAl), is deposited coextensively over said first layer 14 by sputtering in argon. The second layer 16, together with the first layer 14, is annealed at approximately 300° C. for sixteen (16) hours in air. 
     The CrSi under-layer 14 has a positive TCR with a negative TCR Slope. The NiCrAl over-layer 16 has a negative TCR with a positive TCR Slope. The combined effect of the two layers is to provide a resistive film on a substrate 12 having a TCR near 0 (zero) and a TCR Slope of 0 (zero). 
     After the conventional steps of laser trimming to adjust resistance value and tolerance and the addition of terminations, the resulting product is a resistor having high stability and high resistance in ohms per square. 
     The layered film of this invention may be deposited by other methods such as thermal evaporation, ion beam deposition, chemical vapor deposition, or ARC vapor deposition. 
     The substrate 12 may be any of various materials such as ceramic, glass, sapphire or other insulative material suitable for the deposition method used. The substrate 12 may be flat or cylindrical. 
     Other metal silicides and metal alloys may be utilized. The alternatives must compliment each other in TCR and TCR Slope. 
     For the preferred embodiment, test results of three batches of ten units of finished resistors indicate the following. 
     
         ______________________________________    TCR    TCR @    Slope  85° C.                     Ohms/Sq.  Resistance______________________________________CrSi under-layer      -19.2    29.5      5517    3476ΩBoth layers      -2.6     -1.2      3938    2481ΩCrSi under-layer      -19.0    22.7      11914   7506ΩBoth layers      4.7      2.9       7830    4933ΩCrSi under-layer      -19.3    38.3      7538    4749ΩBoth layers______________________________________ 
    
     When resistance is plotted against temperature, the following equation explains this effect. ##EQU1## 
     The second layer 16 may also be reactively sputtered in argon and nitrogen.