Abstract:
The present invention provides a diffusion resistor that is formed in the substrate. A diffusion region is formed within the substrate that contains a first and second contact region. These contact regions extend downward from the surface of the substrate. A third contact is located within the diffusion region between the first and second contacts. This contact also extends downward from the surface of the substrate. These contacts are connected to metal layers. The first and second contacts form the two ends of the diffusion resistor. The third contact forms a Schottky diode such that application of a voltage to this contact forms a depletion region within the diffusion region. The depletion region changes in size depending on the voltage applied to the third contact to change the resistance of the depletion resistor.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates generally to an improved circuit system and in particular to a resistor. Still more particularly, the present invention relates to a high-precision voltage controlled diffusion resistor. 
   2. Description of the Related Art 
   A resistor is an electrical device that may convert energy into heat. The letter R is used to denote the resistance value of a resistor. With this device, two possible reference choices are present for the current and voltage at the terminals of the resistor. One is current in the direction of the voltage drop across the resistor and another is the current in the direction of voltage rise across the resistor. 
   Some existing problems with respect to resistors include transmission line impedance mismatching, the physical size required for diffusion resistors, and process variation in diffusion resistors. Currently, existing solutions for these problems include special Microwave Integrated Circuit (MIC) processes to make trimmed resistors. Further, high-precision discrete components are attached by soldering or bonding components to an integrated circuit (IC) or package. These currently used solutions are expensive with respect to the manufacturing of semiconductors. This type of process involves using a laser to trim the resistors. The resistance is measured and a laser is used to reduce the size of the resistor. This type of process requires much time and is expensive to perform on a per circuit basis. Further, these existing solutions are difficult to integrate into a silicon IC process because of the size of components and/or specialized manufacturing requirements needed to trim the devices. Further, discrete or trimmed components are not adjustable after the manufacture of a product. 
   Therefore, it would be advantageous to have an improved diffusion resistor that overcomes the problems of the existing solutions. 
   SUMMARY OF THE INVENTION 
   The present invention provides a diffusion resistor that is formed in the substrate. A diffusion region is formed within the substrate that contains a first and second contact region. These contact regions extend downward from the surface of the substrate. A third contact is located within the diffusion region between the first and second contacts. This contact also extends downward from the surface of the substrate. These contacts are connected to metal layers. The first and second contacts form the two ends of the diffusion resistor. The third contact forms a Schottky diode such that application of a voltage to this contact forms a depletion region within the diffusion region. The depletion region changes in size depending on the voltage applied to the third contact to change the resistance of the depletion resistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a cross-section of a voltage controlled diffusion resistor in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a cross-section of a non-salicided version of a diffusion resistor in accordance with a preferred embodiment of the present invention; 
       FIGS. 3A–3D  are diagrams illustrating cross-sections in processing steps for creating a voltage controlled diffusion resistor in accordance with the preferred embodiments of the present invention; and 
       FIG. 4  is a schematic diagram of a radio frequency (RF) driver or receiver circuit with RF feedback in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention provides for an improved diffusion resistor that is voltage controlled. The illustrative embodiments of the present invention take advantage of the fact that a Schottky diode is created when a metal comes into contact with a lightly doped semiconductor. The depletion layer that results in the metal to semiconductor contact may be used in conjunction with a voltage bias on a Schottky diode to reduce or increase the effective resistance of a diffusion resistor. 
   The structure of a high-precision voltage controlled diffusion resistor in the illustrative embodiments of the present invention includes a low mobility diffusion region with a positive contact at one end and a negative contact at the opposite end. A center tap is present consisting of the Schottky diode, formed by a metal to low mobility diffusion contact. The negative and positive contact regions are typical ohmic contacts. 
   Depending on the particular embodiment, a salicided region may be used at the contact interface while in another illustrative embodiment, only a metal contact is present. The resistance is made variable in these depicted examples through providing an ability to tune the resistor through a voltage controlled contact (VCC). This contact is located at about center of the resistor structure in these examples. Since the contact acts as a Schottky diode, a depletion layer is created at the VCC interface, which partially depletes the thickness of the diffusion resistor by a selected distance. The total conduction thickness may be changed by altering the depletion thickness through biasing the VCC contact. Through changing the depletion thickness, the total conduction thickness may be changed. As a result, an increase or decrease in effective resistance in the structure is created depending on the particular voltage applied to the VCC contact. In this manner, an ability to vary the resistance of the diffusion resistor through a voltage bias is accomplished. 
   The illustrative embodiments of the present invention provide for a reduction in physical resistor size. The reduction in the conduction thickness provides for creating a resistor of a higher value in the same space as a diffusion resistor without the use of a Schottky contact. This advantage is accomplished in the depicted examples because the effective conduction thickness is reduced by “Xd”. A basic diffusion resistor has the same conduction thickness of “t”. In contrast, the diffusion resistor of the present invention using a VCC has a conduction thickness of “t-Xd”. In these examples, the VCC may be tied to ground and an increase in the effective resistance still exists. 
   The processes, steps, and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as necessary for an understanding of the present invention. The figures below represent cross sections of a portion of an integrated circuit during fabrication and are not drawn to scale, but are drawn so as to illustrate the important features of the invention. 
   With reference now to the figures and in particular with reference to  FIG. 1 , a cross-section of a voltage controlled diffusion resistor is depicted in accordance with a preferred embodiment of the present invention. In this example, diffusion resistor  100  is formed within region  102  in substrate  104 . Substrate  104  is a p-substrate or an insulator in these examples. Typically, a p-substrate is used in a CMOS process and an insulator is used in a silicon on insulator process (SOI). Region  102  is an n− diffusion region in these examples. Further, diffusion resistor  100  is surrounded by shallow trench isolation (STI) region  106 . As illustrated, n+ contact region  108  and n+ contact region  110  are formed within diffusion region  102 . Contact  112  and contact  114  are formed on salicided regions  113  and  115 . Salicided region  113  is formed on n+ contact region  108 , and salicided region  115  is formed on n+ contact region  110 . These contacts are standard ohmic contacts formed by metal layers. Contact  112  in this example is a positive terminal for diffusion resistor  100 , while contact  114  forms a minus terminal for diffusion resistor  100 . Contact  116  is a voltage control contact (VCC) for a Schottky diode. In this example, contact  116  is formed over salicided region  118 . Depending on the voltage bias applied to contact  116 , depletion region  120  is formed and may grow or shrink. 
   As voltage is applied to contact  116 , depletion region  120  grows in size. In particular, x d  represents the depth of depletion region  120 . This value increases as voltage is applied to contact  116 . In this example, t represents the thickness of region  102  and also represents the conductivity. The overall conductivity is t−x d  in which the conductivity decreases as x d  increases with the size of depletion region  120 . 
   Turning next to  FIG. 2 , a non-salicided version of a diffusion resistor is depicted in accordance with a preferred embodiment of the present invention. Diffusion resistor  200  is essentially identical to diffusion resistor  100  in  FIG. 1 . As can be seen, diffusion region  202  is formed within substrate  204  and surrounded by STI region  206 . Within depletion region  202  are n+ contact region  208  and n+ contact region  210 . Contact  212  and contact  214  are formed over salicided regions  213  and  215 , which are formed over n+ contact regions  208  and  210 . Further, contact  216  is formed on region  202 . In this example, however, a salicided region is absent. As with diffusion resistor  100  in  FIG. 1 , depletion region  218  is formed and may increase or decrease in size depending on the voltage bias applied to contact  216 . 
   Turning now to  FIGS. 3A–3D , diagrams illustrating cross-sections in processing steps for creating a voltage controlled diffusion resistor are depicted in accordance with the preferred embodiments of the present invention. In  FIG. 3A , substrate  300  is a p-substrate or may be an insulator. Oxide regions  302  and  304  have been formed around n-diffusion region  306 . Further, resist sections  308  and  310  have been placed on the surface of the device. In this example, the cross-section represents the manufacturing of the n-diffusion resistor at a point after shallow trench isolation formation has occurred. An n-type dopant is implanted into the device. In this example, the dopant may be, for example, phosphorous or arsenic. The implant is performed to result in a low concentration of n-type dopants. These dopants in these examples have a concentration of about 1×10 15  per cm 3 . The doping profile of n-diffusion region  306  may be tuned in these examples to reduce parasitic capacitance. 
   In  FIG. 3B , photo resist sections  311 ,  312 , and  314  have been formed over the device. This formation of these sections is typically formed by laying a photo resist layer and removing sections through selected development of the resist layer. Next, an n-type dopant is implanted into the device to form n+ contact region  316  and n+ contact region  318 . This implant step is performed to result in a high concentration of n-type dopants in the contact regions. Typically, the concentration may range from 1×10 18  per cm 3  to 1×10 20  per cm 3 . 
   In  FIG. 3C , resist regions  320  and  322  are formed on the device. Thereafter, salicided regions  324 ,  326 , and  328  are formed. These regions are formed by deposition of a refractory metal followed by a rapid thermal anneal process. In the depicted examples, formation of these regions may be blocked depending on the particular implementation or processing used. By avoiding the creation of these salicided regions, the effect of the Schottky diode is enhanced. However, blocking formation of these regions complicates the typical processing of the device. Therefore, depending on the particular implementation, the salicided regions may remain. A salicided contact region is more common in CMOS devices. These regions are used to increase the effect of resistance of the device. The refractory metal used for salicided regions  324 ,  326 , and  328  is typically titanium or cobalt. 
   In  FIG. 3D , interlayer dielectric regions  330 ,  332 ,  334 , and  336  are formed. These regions are formed by creating a single dielectric layer and then selectively etching the layer to form contact regions. The deposition of a refractory metal into the contact regions forms contacts  338 ,  340 , and  342 . In these examples, the length “l” and the thickness “t” are designed to be minimized to maximize the effect of the voltage effect on the resistor. These contacts are typically formed using tungsten. 
   Turning now to  FIG. 4 , a schematic diagram of a Rf driver or receiver circuit with RF feedback is depicted in accordance with a preferred embodiment of the present invention. In these examples, the RF feedback employs a variable resistor, such as the variable resistor in the illustrated examples. In this example, circuit  400  includes current source  402 , transistor  404 , resistor  406 , and resistor  408 . In these examples, resistor  406  is a Rd resistor connecting transistor  404  to ground. Current source  402  has one end connected to transistor  404  and another end connected to voltage source VDD. Further, transistor  404  and current source  402  are connected to Vout. Vin is connected to the gate of transistor  404  and resistor  408 . In these examples, resistor  408  is a variable diffusion resistor as illustrated in the depicted examples. 
   Thus, the present invention in the illustrated examples provides for an adjustable or tunable resistance value in a diffusion resistor. The absolute value of the resistor in these examples may be modified with a voltage bias on the metal contact of the Schottky diode. By changing the voltage bias, the thickness of the resistor may be increased or decreased. With this feature, impedance matching adjustment for radio frequency (Rf) driver/receiver circuits may be made. The voltage controlled diffusion resistor in the illustrated examples allows for adjustment of the resistor value Rin for a receiver application or Rout for a driver application to match the transmission line impedance. In this manner, unwanted voltage reflections and signal loss are reduced or eliminated. 
   Further, adjustments to resistance allow for a bias current adjustment for mixed signal circuits. Also, the reduction in the size of the resistor is accomplished by reducing the resistor thickness. Additionally, resistance values may be self-adjusting through various circuit design techniques, such as implementing a feedback circuit with the resistor of the present invention. Further, the variable resistance value may be adjusted to compensate for process variations to provide for uniform resistance. Also, the variable resistance may be adjusted to a very high resistance to put an analog circuit in a low current or low power sleep mode. 
   The description of the preferred embodiment of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.