Patent Publication Number: US-7723200-B2

Title: Electrically tunable resistor and related methods

Description:
BACKGROUND OF THE DISCLOSURE 
   1. Technical Field 
   The disclosure relates generally to integrated circuit (IC) chips, and more particularly, to an electrically tunable resistor and related methods. 
   2. Background Art 
   Thin film resistors are used in the integrated circuit (IC) chip industry. The resistance R of a rectangular sheet of material of width W, height H and thickness d is determined by the film resistivity (rho) by: R=rho*W/(H*d). Any of the above mentioned parameters can be varied to vary the resistance R. 
   A number of challenges exist relative to controlling the resistance. First, if different resistance values in a circuit are desired, the geometric parameters width W, height H or thickness d, and/or the material resistivity rho, must be varied. Second, tolerance variations in material resistivity rho, width W, height H and thickness d will lead to variations in resistance. Third, new variations in the resistance values derived by changing the resistor geometry require new masks to make specialty IC chips. Finally, changes in the resistance from a variation in material resistivity rho require a change of the deposition material, which is costly and limited by available deposition materials. 
   One approach to adjust a resistance value includes trimming resistors to change their physical dimensions. SU 1020869 is an example of this approach in which a heating pulse current is applied to the resistor prior to laser trimming. 
   SUMMARY OF THE DISCLOSURE 
   An electrically tunable resistor and related methods are disclosed. In one embodiment, the resistor includes a first resistive layer, at least one second resistive layer, and an intermediate interdiffused layer of the first resistive layer and the at least one second resistive layer. One method may include providing a first plurality of layers of different materials surrounded by at least one insulating layer, and passing a current pulse through the first plurality of layers to affect a conductivity structure of the first plurality of layers in order to obtain a first predetermined resistance value for the resistor. 
   A first aspect of the disclosure provides a method of forming a resistor, the method comprising: providing a first plurality of layers of different materials surrounded by at least one insulating layer; and passing a current pulse through the first plurality of layers to affect a conductivity structure of the first plurality of layers in order to obtain a first predetermined resistance value for the resistor. 
   A second aspect of the disclosure provides a resistor comprising: a first resistive layer; at least one second resistive layer; and an intermediate interdiffused layer of the first resistive layer and the at least one second resistive layer. 
   A third aspect of the disclosure provides a method of forming a resistor, the method comprising: providing a first plurality of layers of different materials; first passing a current pulse through the first plurality of layers to affect a conductivity structure of the first plurality of layers in order to obtain a first predetermined resistance value for the resistor; measuring a resistance of the first plurality of layers; determining a calibration for tuning the resistance based on the measuring; providing at least one second plurality of layers of different materials substantially identical to the first plurality of layers; and second passing a current pulse through each of the at least one second plurality of layers to affect a conductivity structure of each second plurality of layers in order to obtain a second predetermined resistance value different from the first predetermined resistance value. 
   The illustrative aspects of the present disclosure are designed to solve the problems herein described and/or other problems not discussed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of this disclosure will be more readily understood from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings that depict various embodiments of the disclosure, in which: 
       FIG. 1  shows one embodiment of an electrically tunable resistor. 
       FIG. 2  shows one illustrative tuner for the electrically tunable resistor of  FIG. 1 . 
   

   It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , one embodiment includes a method of forming a resistor  100 . First, a plurality of layers  102  of different materials is provided. As shown, plurality of layers  102  includes a first resistive layer  104  and at least one second resistive layer  106 . Any number of second resistive layer(s)  106  may be employed. Resistive layers  104 ,  106  may include any now known or later developed resistive material. However, in one embodiment, resistive layers  104 ,  106  each include a conductive material such as a metal. Where metals are used, resistive layers  104 ,  106  of, for example, iron (Fe) and cobalt (Co), respectively, may be employed. First resistive layer  104  may include a first metal material and at least one second resistive layer  106  may include at least one other different metal material. Other materials may also be employed including but not limited to chromium (Cr), copper (Cu), silver (Au), gold (Ag), aluminum (Al), ruthenium (Ru), platinum (PI), tantalum (Ta), molybdenum (Mb), tungsten (W) and nickel (Ni). Alloys of the metals could also be used, such as 80 permalloy (80:20 Ni:Fe). It is best to choose metals which will mix and interdiffuse with one another during tuning to form a (new) interdiffused layer  108  as well as having different enough resistivities so that the resistance of the fully mixed metals is different enough from the resistance of the two initial parallel sheets of the metals. The metals chosen should undergo interdiffusion prior to electromigration for the range of possible pulse durations and power levels which could be reasonably used to tune the system resistance. Note the positioning of each layer is only illustrative. Also, note that interdiffused layer  108  could expand to include the physical extant of layers  104  and  106  after the tuning process is complete. 
   Resistive layers  104  and  106  are positioned between insulating layers  110  and  116 . That is, insulating layers  110 ,  116  are adjacent to resistive layers  104 ,  106 , respectively. In one embodiment, insulating layer  110 ,  116  are diffusion barrier layers to ensure interdiffusion of resistive layers  104 ,  106  during the tuning pulsing. In another embodiment, insulating layers  110 ,  116  do not prevent diffusion, i.e., they are chosen so that diffusion into those layers is preferential during the tuning pulsing. Note, they could also be chosen to be opposite in terms of their diffusivity. Insulating layers  110 ,  116  serve to electrically isolate resistive layers  104 ,  106 , and in one embodiment, to serve as a diffusion barrier to contain the total thickness of resistive layers  104 ,  106 . Examples of insulating layers  110 ,  116  include alumina, silica and low dielectric constant materials. For electrical isolation, the materials for insulating layers  110 ,  116  are chosen to have a high electrical resistivity relative to resistive layers  104 ,  106 . For a diffusion barrier, insulator layers  110 ,  116  are chosen such that resistive layers  104 ,  106  have a low solubility in them, such that insulator layers  110 ,  116  are stable at the temperatures and pulse durations used to tune resistor  100 . 
   Next, in order to electrically tune resistor  100 , a tuner  120  passes a current pulse through plurality of layers  102  to affect a conductivity structure of plurality of layers  102  in order to obtain a first predetermined resistance value R 1  for resistor  100 . That is, tuner  120  can alter the resistance of plurality of layers  102  (i.e., resistor  100 ) by passing current pulse(s) through layers  102  to heat the stack to a desired temperature for a desired pulse duration using an appropriate current level. The desired change in resistance dictates the power level, duration and number of the current pulse(s). 
   As shown in  FIG. 1 , the conductivity structure change may result in an interdiffused layer  108  of first resistive layer  104  and the at least one second resistive layer  106 . For example, where iron (Fe) and cobalt (Co) are used in resistive layers  104 ,  106 , an iron-cobalt alloy interdiffused layer  108  results. The materials in plurality of layers  102  are chosen such that with the proper amount and duration of current pulse (power applied), interdiffusion occurs in layers  102 , resulting in a stable alloy rather than electromigration. In any event, the final resistance values ranges from the parallel resistance of the separate layers  104 ,  106 ,  108  to the resistance for a complete alloy. Again, note that depending on the materials chosen and the power applied, layers  104  and  106  may have been more or less enveloped by layer  108 . 
   Tuner  120  may further be employed to pass a plurality of current pulses through plurality of layers  102 , and measure a resistance change ΔR in plurality of layers  102  after each pulse or group of pulses. Each current pulse may have a different voltage and/or duration. In any event, based on the measurements, a calibration for tuning the resistance can be determined, e.g., for each type of plurality of layers and/or for each layer. For example, layers  102  can be subjected to increasing current level pulses of controlled levels. The resistance change after each current pulse can be measured and based on the change in resistance. The next current pulse can either be of a slightly higher voltage or at the same voltage. Current pulse(s) are applied to layers  102  until the desired resistance is achieved. 
   A temperature versus power of the combined initial resistance of resistor  100  can be determined by the following means. First, the resistance versus temperature of the initial resistor  100  can be measured in an oven. Second, the temperature rise (ΔT) versus power (P in ) at a fixed pulse duration (τ) is given by the thermal conductance κ r , (κ r (τ)=P in /ΔT(τ)) and can be determined for fixed pulse durations by measuring the voltage across, and the current into, resistor  100  at low power levels where the sensor is not damaged. The power is given by the current times the voltage, and the resistance is given by the voltage divided by the current. The temperature rise is then determined by the measured resistance change, which is here assumed to increase linearly with temperature. The temperature of resistor  100  is then determined by assuming a linear relationship between resistor  100  resistance and the temperature rise: R r (ΔT(τ))=R r (0)*(1+αΔT(τ)), where R r  is the resistance, ΔT is temperature rise, τ is duration and α is a thermal coefficient of resistance. The thermal coefficient of resistance (α) is measured in an oven. When applying a high-current pulse, the temperature rise is then determined by: ΔT(τ)=(P in /κ r (τ)), where ΔT is temperature rise, τ is duration, P in  is power, and κ r  is the thermal conductance. These equations can be used to accurately predict the current/voltage requirements for tuning resistor  100 . Based on this, the temperature of resistor  100  versus power can also be determined from low voltage pulses. 
   Based on this information, any number of other substantially identical (e.g., substantially similar dimensions and/or materials) plurality of layers  130  can also be tuned. In this setting, plurality of layers  102  may act as a test section  127  within an integrated circuit (IC) chip  125 . That is, one set of layers  102  may act as sacrificial parts to quantify the behavior of layers  102  so the appropriate current pulses for a given resistance change can be pre-determined. As a result, at least one second plurality of layers  130  substantially identical to plurality of layers  102  may be provided in IC chip  125  to provide resistor(s)  100 . Tuner  120  can then be used to pass a current pulse through each of the at least one second plurality of layers  130  to affect a conductivity structure of each second plurality of layers  130  in order to obtain a second predetermined resistance value different from the first predetermined resistance value for plurality of layers  102 . 
   The above-described process may occur during manufacture or in-the-field after manufacture. For example, tuner  120  may be employed to alter the resistance of any resistor  100  by passing a current pulse having a known voltage through resistor  100  to affect a known change in the resistance of resistor  100 . In-situ resistance tuning enables the manufacturer to loosen the tolerances during deposition of plurality of layers  102 ,  130 . It also allows the manufacturer to use a single deposition process with subsequent tuning of specific resistors to the desired values. Neighboring resistors made with the same materials can be tuned to different values. Based on this structure, resistor networks may be employed in which series and/or parallel combinations are used to achieve, first, a gross value of resistance and then electrically tuned to the final value. Note, a multiplexer (not shown) may be necessary to reconfigure the resistor network before invoking the electrical fine tuning. Furthermore, tuning during the life of the IC chip is now possible, further enabling autonomic or self-correcting computing. 
   A resistor  100  according to one embodiment may include first resistive layer  104 , at least one second resistive layer  106  surrounded by at least one insulating layer  110 ,  116 , and intermediate interdiffused layer  108  of first resistive layer  104  and at least one second resistive layer  106 . Further, resistor  100  may include tuner  120  for passing a current pulse through plurality of layers  102  to affect a conductivity structure of resistive layer  104  and second resistive layer(s)  106  in order to obtain a first predetermined resistance value for resistor  100 . As indicated above, tuner  120  may be operable after manufacture of resistor  100 . 
   Referring to  FIG. 2 , one illustrative tuner  120  is shown. The simplest embodiment of tuner  120  is just a single tuner and application circuit  132   a . In all cases, the tuning supply could be either internal or external to the chip. 
   Another embodiment would include tuning and application circuits  132   a - 132   d  and a decode circuitry  140 . This example is for on-chip tuning and uses decode circuitry  140  to allow each resistor  100  to be individually tuned. Although simple logic gates are shown and would correctly function as shown, other schemes, could be used that would potentially save space or write time. Transistor T 2  is a decoding activation/isolation transistor, and transistors T 3  and T 4  are activation/isolation transistors from an application circuit  142 . One implementation would be for a single set of decoders to be used to select resistor  100  being tuned and to provide the appropriate tuning voltage level and pulses externally. 
   Another implementation would be to use additional decoder circuitry  140 , once a given resistor  100  has been selected, to build the appropriate signal internally. In this case, shown as a fine current tuning circuit  150 , one or more transistors T 6 -T 8 , would be placed in parallel with a decode transistor T 5 , and used with a fine current tuning decoder  140   a  to apply the required signal level. Each transistor could have differing load resistors R 2 -R 5 , to apply the appropriate pulse controlled by the second decode network, i.e., fine current tuning decoder  140   a.    
   Each general application circuit  142  type would need to be considered for the best tuning circuit, although most would likely be very similar. Application circuit  142  is shown on the other side of activation/isolation transistors T 3  and T 4  when those transistors are used, but would be directly connected to resistor  100  when they are not used. Activation/isolation transistors T 3  and T 4  are optional isolation transistors and are off during the tuning current pulse. Activation/isolation transistors T 3  and T 4  might not be necessary if application circuit  142  has an input impedance that is sufficiently large so that application circuit  142  is not damaged by the tuning voltage pulse. Activation/isolation transistor T 2  and decode transistor T 5  between resistor R 2  and ground is also potentially optional depending on application circuit  142 . 
   One illustrative operation of tuner  120  would be as follows: First, activation/isolation transistors T 3  and T 4  would be turned off to isolate resistor  100  from the application circuit  142 . Second, fine current tuning circuitry  150  would be activated, uniquely turning on decode transistor T 5 . Finally, decode circuitry  140  would turn on activation/isolation transistors T 1  and T 2  the appropriate length of time to provide the tuning current pulse. Also note that activation/isolation transistor T 2  could be operated independently of activation/isolation transistor T 1 . One case would be for activation/isolation transistor T 1  to be turned on and then use activation/isolation transistor T 2  to control the pulse duration. 
   The above-described circuit and the detailed operations are only meant as an example of an implementation where most of the control is at the chip level. Depending on the circuit and conditions, several turn on and turn off schemes could be envisioned even for this simple circuit. 
   In principle this device could be used at the end of manufacturing process of a first metal layer (M 1 ), at final wafer test, at module final test, or in the field with a system designed for autonomic computing. Note that in the first two cases, the tuning of each resistor  100  might be best accomplished by addressing each resistor with an external tuning supply by means of wafer probing. For the latter two cases, some on-chip decoding would likely be necessary. 
   Real-time tunable resistor  100  also allows design of a system capable of detecting when a given critical parameter is reaching its failure point. In this case, tuner  120  may implement the above-described method to take corrective action such that: 1) the IC chip or a portion thereof is taken out of service, 2) the tuning activation/isolation transistors are activated, 3) the appropriate decode circuitry  140  and/or fine current tuning circuitry  150  is activated, and 4) resistor  100  is then tuned to extend the operating life of that critical component. Although for some technology mechanisms, other means are available to extend the life due to reliability, an electrically tunable resistor  100  provides the ability and opportunity to change the operating point of transistors even for those mechanisms which cannot be directly healed and hence provide relief during the operation in the field. 
   Although one illustrative tuner  120  has been shown, it is understood that a large variety of other tuners may be employed and are considered within the scope of the disclosure. 
   The structures and methods as described above are used in the fabrication of integrated circuit chips. For example, resistor  100  may be used in a tape head, magnetic random access memory (MRAM), etc. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
   The foregoing description of various aspects of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the disclosure as defined by the accompanying claims.