Abstract:
A p-type polysilicon resistor formed in the inter-level dielectric layer contains an implanted diode. A positive voltage applied to the diode modulates the depletion region of the diode and changes the absolute resistance of the p-type polysilicon resistor. This modulation occurs not only horizontally, but also vertically. The fact that the tunable resistor is a p-type polysilicon resistor means that this structure can easily be integrated into the process since polysilicon is used as a gate material for basic CMOS processing.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention is directed generally toward a resistor and a method for making a resistor that is tunable to an absolute value. 
   2. Description of the Related Art 
   It is a well-known fact that in most processing technologies for manufacturing mixed-signal devices on silicon chips, resistors suffer from large process variation, with the actual resistance value varying from the predicted value by as much as 20%–30%. This is due in part to variations in the dopant profile and the resulting edge effects and to the fact that a fourth of a micron difference in the thickness of a salicide can alter the resistance by several orders of magnitude. There is currently no way to get around this problem; so analog designers make designs that do not rely on absolute values of resistance and rely more on matching of resistors. Even then, the resistor matching is best achieved with large area resistors, which wastes space on a die. This clearly limits the options for analog designs 
   One existing solution is to use a precision external resistor off chip, although this adds significantly to the cost. Another is to use large resistors in parallel, which helps with matching internal to the die. However, these large area internal resistors are expensive in the area consumed and in the time spent laying out the structure. They also require additional simulation time to confirm that the design will work with the expected large variation of the resistors, which is expensive in terms of time, both manpower and machine time. 
   It would be highly desirable to utilize a process that can produce a resistor with less variability. This solution, however, would require that the steps necessary to make the resistor fit in with the processing steps already used, to avoid large costs associated with such a process change. 
   SUMMARY OF THE INVENTION 
   A p-type polysilicon resistor formed in the inter-level dielectric layer contains an implanted diode. A positive voltage applied to the diode modulates the depletion region of the diode and changes the absolute resistance of the p-type polysilicon resistor. This modulation occurs not only horizontally, but also vertically. The fact that the tunable resistor is a p-type polysilicon resistor means that this structure can easily be integrated into the process since polysilicon is used as a gate material for basic CMOS (complementary metal oxide semiconductor) processing. 

   
     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: 
       FIGS. 1A–1D  depict the structure of a polysilicon resistor from various views, according to an exemplary embodiment of the invention. 
       FIG. 2A  depicts the structure of a polysilicon resistor in the same cross-section as  FIG. 1D , showing the depletion area that forms around the diodes according to an exemplary embodiment of the invention. 
       FIG. 2B  depicts a top-down view of a polysilicon resistor, showing several measurements of the resistor. 
       FIGS. 3A–3G  depict the formation of the formation of a polysilicon resistor in the same cross-section as  FIG. 1C , according to an exemplary embodiment of the invention. 
       FIG. 4  shows a flowchart of the method of forming a resistor according to an exemplary embodiment of the invention. 
       FIG. 5  depicts a block diagram of a chip on which the innovative resistor can be formed. 
   

   DETAILED DESCRIPTION 
   An embodiment of the invention will now be explained with reference to the figures. 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 represent cross sections of a portion of an integrated circuit during fabrication and are not drawn to scale, but instead are drawn so as to illustrate important features of the invention. 
   Structure of Resistor 
   With reference to  FIG. 1A , a top-down view of an embodiment of the invention is shown, after the implantation of the resistor, but prior to the formation of contacts. In this figure, the surface of resistor  100  forms a rectangle of polysilicon  102  that is surrounded by an oxide or other dielectric. Polysilicon  102  has been doped with a p-type dopant, with heavier areas of p-type doping  104  at each end of the rectangle where contacts will be formed. Near the midpoint of the rectangle are two areas  106  that have been doped with an n-type dopant. These two areas  106  will form diodes that can pinch off the current between the contact points  104 . 
   Turning now to  FIG. 1B , a cross-section along the length of the resistor at its midpoint is shown. In this cross-section, only the regions doped with p-type doping are visible, although contacts have been added to the drawing. Body  102  of resistor  100  is seen, with contact regions  104  at either end. Each of contact regions  104  is shown with a layer of salicide  108 , topped by a contact  110 . A cross-section taken at an offset from the longitudinal midline of resistor  100  is similarly shown in  FIG. 1C , except that this figure also shows n-type doped regions  106 , with their own layer of salicide  108  and contact  110 . Finally,  FIG. 1D  depicts an embodiment of the invention in a cross-section taken through the two n-doped regions  106 . 
   Tuning the Resistor 
   Turning now to  FIG. 2A , the action of the resistor will be discussed. In the completed circuit, contacts  210  are connected to form diodes. When a positive voltage is applied to contacts  210 , a depletion region  212  forms around each of regions  206 , with the size of depletion regions  212  growing as a greater voltage is applied. As the depletion regions grow larger, the resistance of the device increases and current through the device decreases. Thus the value of the resistor can be adjusted after manufacture. The ability to adjust a resistance in an analog circuit has many advantages, such as tunable voltage or current references, impedance matching, as well as reducing or shutting off current draw in a mixed signal block for a sleep mode. In an exemplary embodiment of the invention, a feedback circuit is utilized to maintain a stable, reliable value. 
     FIG. 2B  depicts four important dimensions of the resistor: L 1 , L 2 , and L 3 , which define the length of the resistor, and together with its doping level determines the resistor&#39;s value, and W, the distance between the two diodes, which defines the effective width of the resistor. These dimensions can be optimized depending on specific design criteria. 
   Formation of Resistor 
   With reference to  FIGS. 3A–3G , the formation of an embodiment of the inventive resistor will be discussed; a flowchart of the method is referenced in  FIG. 4 . When the process of forming the resistors begins, n-well  302  has been formed in the substrate (not specifically shown). Shallow trench isolation is used to etch a trench and to fill the trench with oxide  304  (step  405 ), as seen in  FIG. 3A . Preferably, this oxide layer will have a thickness of 0.4microns, with a range of 0.32–0.48 microns. A layer of polysilicon  306  is deposited over the chip (step  410 ). This layer has a thickness in the range of 120–300 nm, with a preferred embodiment having a thickness of 180 nm. A mask of resist RST is created over polysilicon  306 , leaving exposed those areas that are to receive a p-type implant. In the implant step (step  420 ), the concentration of dopant should be approximately 10 15  cm 3  or greater. The resulting p-type polysilicon  306 ′ is shown in  FIG. 3B  and will form the body of the resistor. Resist RST is removed and a new mask RST is formed, followed by an etch step that removes the unwanted polysilicon around the resistor-to-be (step  420 ), as seen in  FIG. 3C . Following this step, the existing resist is removed. A new layer of resist is deposited and patterned to expose only the two contacts regions  308  for the resistor. This is followed by the implantation of additional p-type dopant into regions  308 , to reach a concentration of 10 18 –10 20  (step  420 ). If the previous doping step for the entire resistor created a concentration of at least 10 18 , then additional doping into the contact areas is not always necessary, so step  420  is considered optional, depending on the specific requirements of the resistor. The completion of this step is shown in  FIG. 3D . 
   Again, the existing resist layer RST is removed from the chip and a new resist RST is deposited and patterned to expose the regions  310  that will become diodes. This region  310  is then implanted with n-type dopants, as shown in  FIG. 3E , to achieve a concentration of 10 18 –10 20  cm 3  (step  430 ). In the various implantation steps, the thickness of the implant can be in the range of 90–300 nm, with a preferred embodiment having a thickness of 180 nm. The current resist is again removed and the implantation of the resistor and diodes is complete. Next, another resist RST is deposited and patterned to expose both contact areas  308  for the resistor and contact areas  310  for the diodes. A layer of a refractory metal, such as cobalt or titanium  312 , is deposited over the chip to a thickness in the range of 1–2 nm, with a preferred embodiment having a thickness of 1.5 nm. The chip is next subjected to a rapid thermal annealing (RTA) (step  435 ). During this process, the refractory metal will react with polysilicon to form a silicide  312 ′ at any point where the two materials contact each other, such as overlying regions  308 ,  310 . In regions where the refractory metal overlies resist or other materials, no reaction will occur, as shown in  FIG. 3F . Because this reaction is self-aligning, the resultant silicide is also referred to as a salicide, or  s elf- a ligned si licide . The resist is then removed, which also removes the un-reacted refractory metal  312 . Following creation of the salicide, an inter-level dielectric ILD is deposited over the chip in a thickness of 200–500 nm, with a preferred thickness of 280 nm; typically the inter-level dielectric is planarized by chemical-mechanical planarization (CMP) (step  440 ). The final step is to form contacts for the resistor and diodes (step  445 ), as seen in  FIG. 3G . To create these contacts, vias are etched in the interlevel dielectric layer, and then filled with a metal  314  and planarized. This completes the processing of the innovative resistor, although further steps will be taken to complete the other layers of wiring for the circuit. 
     FIG. 5  depicts a block diagram of a chip on which the innovative resistor can be formed. In this figure, chip  500  includes analog-to-digital circuitry  502 , reference  504 , clock squarer  506 , sample and hold  508 , reference buffer  510 , and contact pads  512 , which will be connected to input/output pins. 
   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. For example, the oxide that insulates the resistor from the substrate can be a grown oxide, rather than a deposited oxide. Also, if the resistor were to be used in a poly-poly process, the resistor could be formed entirely surrounded by the inter-level dielectric (ILD) layer. 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. It is noted that the resistor could, for example, be formed with the opposite dopings, although this would require that a negative voltage supply be available.