Abstract:
A resistor layout and method of forming the resistor are described which achieves improved resistor characteristics, such as resistor stability and voltage coefficient of resistance. A resistor is formed from a conducting material such as doped silicon or polysilicon. The resistor has a rectangular first resistor element, a second resistor element, a third resistor element, a fourth resistor element, and a fifth resistor element. A layer of protective dielectric is then formed over the first, second, and third resistor elements leaving the fourth and fifth resistor elements exposed. The conducting material in the exposed fourth and fifth resistor elements is then changed to a silicide, such as titanium silicide or cobalt silicide, using a silicidation process. The higher conductivity silicide forms low resistance contacts between the second and fourth resistor elements and between the third and fifth resistor elements. The second and third resistor elements are wider than the first resistor element and provide a low resistance contacts to the first resistor element, which is the main resistor element. This provides low voltage coefficient of resistance thermal process stability for the resistor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    This invention relates to a layout and method to improve linearity and reduce voltage coefficient of resistance for resistors used in mixed-mode analog/digital applications.  
           [0003]    (2) Description of the Related Art  
           [0004]    U.S. Pat. No. 6,103,622 to Huang describes a silicide process for mixed-mode analog digital/devices.  
           [0005]    U.S. Pat. No. 5,924,011 to Huang describes a method for fabricating mixed analog/digital devices using a silicide process.  
           [0006]    U.S. Pat. No. 6,054,359 to Tsui et al. describes a method for fabricating high sheet resistance polysilicon resistors.  
           [0007]    U.S. Pat. No. 5,885,862 to Jao et al. describes a poly-load resistor for a static random access memory, SRAM, cell.  
           [0008]    A paper entitled “Characterization of Polysilicon Resistors in Sub-0.25 μm CMOS USLI Applications” by Wen-Chau Liu, Member IEEE, Kong-Beng Thei, Hung-Ming Chuang, Kun-Wei Lin, Chin-Chuan Cheng, Yen-Shih Ho, Chi-Wen Su, Shyh-Chyi Wong, Chih-Hsien Lin, and Carlos H. Diaz, IEEE Electron Device Letters, Vol. 22, No. 7, pages 318-320, July 2001 describes characterization of polysilicon resistors.  
         SUMMARY OF THE INVENTION  
         [0009]    High performance resistors are important devices in the design of mixed-mode analog/digital circuits. A number of parameters are of key importance for these resistors such as resistor linearity, insensitivity of resistance to thermal processing steps, and voltage coefficient of resistance (VCR).  
           [0010]    It is a principal objective of at least one embodiment of this invention to provide a method of forming a resistor having good linearity, thermal process stability, and low voltage coefficient of resistance (VCR).  
           [0011]    It is another principal objective of at least one embodiment of this invention to provide a resistor layout for a resistor having good linearity, thermal process stability, and low voltage coefficient of resistance (VCR).  
           [0012]    These objectives are achieved by first forming a resistor from a first conducting material such as doped polysilicon. The resistor has a rectangular first resistor element having a width, a length, a first end, and a second end; a second resistor element having a first edge and a second edge wherein the first edge of the second resistor element contacts the entire width of the first end of the first resistor element; a third resistor element having a first edge and a second edge wherein the first edge of the third resistor element contacts the entire width of the second end of the first resistor element; a fourth resistor element having a contact edge wherein the contact edge of the fourth resistor element contacts the entire the second edge of the second resistor element; and a fifth resistor element having a contact edge wherein the contact edge of the fifth resistor element contacts the entire the second edge of the third resistor element. A layer of protective dielectric is then formed over the first, second, and third resistor elements leaving the fourth and fifth resistor elements exposed.  
           [0013]    The first conducting material in the exposed fourth and fifth resistor elements is then changed to a second conducting material, which is a silicide, using a silicidation process. The second conducting material is a silicide such as titanium silicide. The second conducting material has a higher conductivity than the first conducting material. The higher conductivity second conducting material forms low resistance contacts between the second and fourth resistor elements and between the third and fifth resistor elements. The second and third resistor elements are wider than the first resistor element and provide a low resistance contacts to the first resistor element, which is the main resistor element. This provides low voltage coefficient of resistance and good resistor linearity.  
           [0014]    The protective dielectric over the first, second, and third resistor elements prevents the resistor from silicidation during subsequent process steps.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 shows a top view of one embodiment of the resistor of this invention before the protective dielectric layer has been formed.  
         [0016]    [0016]FIG. 2 shows a cross section view of the resistor of FIG. 1 taken along line  2 - 2 ′ of FIG. 1.  
         [0017]    [0017]FIG. 3 shows a top view of the embodiment of the resistor of this invention shown in FIG. 1 after the protective dielectric layer has been formed.  
         [0018]    [0018]FIG. 4 shows a cross section view of the resistor of FIG. 3 taken along line  4 - 4 ′ of FIG. 3.  
         [0019]    [0019]FIG. 5 shows a top view of another embodiment of the resistor of this invention before the protective dielectric layer has been formed.  
         [0020]    [0020]FIG. 6 shows a top view of the embodiment of the resistor of this invention shown in FIG. 5 after the protective dielectric layer has been formed.  
         [0021]    [0021]FIG. 7 shows a top view of another embodiment of the resistor of this invention before the protective dielectric layer has been formed.  
         [0022]    [0022]FIG. 8 shows a cross section view of the resistor of FIG. 7 taken along line  8 - 8 ′ of FIG. 7.  
         [0023]    [0023]FIG. 9 shows a top view of the embodiment of the resistor of this invention shown in FIG. 8 after the protective dielectric layer has been formed.  
         [0024]    [0024]FIG. 10 shows a cross section view of the resistor of FIG. 9 taken along line  10 - 10 ′ of FIG. 9.  
         [0025]    [0025]FIG. 11 shows a top view of another embodiment of the resistor of this invention after the protective dielectric layer has been formed.  
         [0026]    [0026]FIG. 12 shows resistance as a function of voltage for a P +  doped polysilicon resistor having the protective dielectric layer of this invention and a P +  doped polysilicon resistor, having the same doping level, without the protective dielectric layer.  
         [0027]    [0027]FIG. 13 shows resistance as a function of voltage for an N +  doped polysilicon resistor having the protective dielectric layer of this invention and an N +  doped polysilicon resistor, having the same doping level, without the protective dielectric layer.  
         [0028]    [0028]FIG. 14 shows the voltage coefficient of resistance as a function of voltage for a P +  doped polysilicon resistor having the protective dielectric layer of this invention and a P +  doped polysilicon resistor, having the same doping level, without the protective dielectric layer.  
         [0029]    [0029]FIG. 15 shows the voltage coefficient of resistance as a function of voltage for an N +  doped polysilicon resistor having the protective dielectric layer of this invention and an N +  doped polysilicon resistor, having the same doping level, without the protective dielectric layer.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Refer now to the drawings for a detailed description of the preferred embodiments of this invention. FIG. 1 shows a top view of a resistor having a first resistor element  120 , a second resistor element  130 , a third resistor element  170 , a fourth resistor element  150 , and a fifth resistor element  190 . The resistor is formed on a substrate  100 , such as a silicon substrate having devices formed therein. FIG. 2 shows a cross section of the resistor at this stage of fabrication taken along line  2 - 2 ′ of FIG. 1. The boundaries between the first  120  and second  130  resistor elements, the first  120  and third  170  resistor elements, the second  130  and fourth  150  resistor elements, and the third  170  and fifth  190  resistor elements are shown by dashed lines in FIGS. 1 and 2. The resistor is formed of a patterned layer of conducting material. The conducting material can be doped polysilicon doped with either N type impurities or P type impurities. As shown in FIG. 1, the first resistor element  120  is a rectangle having a length  20 , a width  22 , a first end  21 , and a second end  23 . The polysilicon is deposited, patterned, and doped using techniques well known to those skilled in the art.  
         [0031]    The resistance of the resistor is primarily determined by the resistance of the first resistor element  120 , as will be described in greater detail later. The resistance of the first resistor element is determined by the doping of the polysilicon, which determines the conductivity of the polysilicon, the length  20  of the first resistor element  120 , and width  22  of the first resistor element  120 .  
         [0032]    As shown in FIGS. 3 and 4, a layer of protective dielectric  140  is deposited and patterned to cover the first  120 , second  130 , and third  170  resistor elements. The fourth  150  and fifth  190  resistor elements are not covered by the protective dielectric  140 . The protective dielectric can be an oxide, such as silicon oxide, or silicon nitride deposited and patterned using techniques well known to those skilled in the art.  
         [0033]    Next a silicidation process, well known to those skilled in the art, is carried out which converts the conducting material in the fourth  150  and fifth  190  resistor elements to a silicide. In this example the conducting material of polysilicon in the fourth  150  and fifth  190  resistor elements is converted to a silicide such as titanium silicide, cobalt silicide, or the like. As those skilled in the art will readily recognize the silicidation process is usually part of the process for forming contacts in other regions of the substrate  100 . The protective dielectric  140  protects the first  120 , second  130 , and third  170  resistor elements from the silicidation process so that the first conducting material remains unchanged and the conductivity of the conducting material forming the first  120 , second  130 , and third  170  resistor elements remains unchanged. The protective dielectric  140  also protects the conducting material forming the first  120 , second  130 , and third  170  resistor elements from subsequent process steps so that the conductivity of the conducting material in these regions is not changed. Contacts  24  to the resistor can be formed in the fourth  150  and fifth  190  resistor elements using methods well known to those skilled in the art.  
         [0034]    The conductivity of the silicide in the fourth  150  and fifth  190  resistor elements is substantially greater than the conductivity of the conducting material in the first  120 , second  130 , and third  170  resistor elements. The resistance of the interface  18  between the second resistor element  130  and the interface  16  between the third  170  and fifth  190  resistor elements is low compared to the resistance of the first resistor element  120  because the conducting material forming the fourth  150  and fifth  190  resistor elements has been converted to a silicide. The second  130  and third  170  resistor elements are designed to be wide relative to the width  22  of the first  120  resistor element so their resistance will be small compared to the first  120  resistor element.  
         [0035]    The resistance, R, of the resistor can be expressed as R=R 1 +2 R 2 +2 R 3 +2 R 4 +R 5 . In this equation R 1  is the resistance of the first  120  resistor element, R 2  is the resistance of the contacts  24  to the fourth  150  and fifth  190  resistor elements, R 3  is the resistance of the fourth  150  and fifth  190  resistor elements, R 4  is the resistance of interfaces,  18  and  16 , between the second  130  and fourth  150  resistor elements and between the third  170  and fifth  190  resistor elements, and R 5  is the resistance of the second  130  and third  170  resistor elements. Of these resistances R 2 , R 3 , R 4 , and R 5  are all quite small with respect to R 1 , and the resistance, R, of the resistor is very nearly equal to R 1 . This makes it possible to accurately adjust the resistance of the resistor by controlling the doping of the polysilicon, the length  20  of the first resistor element  120 , and the width  22  of the first resistor element  120 .  
         [0036]    Another embodiment of the resistor layout of this invention is shown in FIGS. 5 and 6. As shown in FIGS. 5 and 6 dummy resistor elements  26  can be formed on either side of the first resistor element  120 . The dummy resistor elements  26  can be used to compensate for proximity effects when the dimensions of the first resistor element  120  are very small. FIG. 5 shows the resistor before the protective dielectric layer  140  is formed. FIG. 6 shows the resistor after the protective dielectric layer  140  is formed.  
         [0037]    [0037]FIGS. 7-11 show another embodiment of the resistor layout and method of this invention. FIG. 7 shows the top view of a resistor and FIG. 8 a cross section taken along line  8 - 8 ′ of FIG. 7. As in the preceding embodiments, the resistor has a first resistor element  32 , a second resistor element  33 , a third resistor element  37 , a fourth resistor element  35 , and a fifth resistor element  39 . In this embodiment, as can be seen in FIG. 8, the resistor is formed within the substrate  30  and at the top surface of the substrate. In this embodiment the first  32 , second  33 , third  37 , fourth  35 , and fifth  39  resistor elements can be formed by a patterned deposition of impurities in a silicon substrate  30  using techniques well known to those skilled in the art. In this embodiment the first  32 , second  33 , third  37 , fourth  35 , and fifth  39  resistor elements can be formed by deposition of N or P type impurities in a silicon substrate  30 .  
         [0038]    As shown in FIGS. 9 and 10, a layer of protective dielectric  34  is deposited and patterned to cover the first  32 , second  33 , and third  37  resistor elements. The fourth  35  and fifth  39  resistor elements are not covered by the protective dielectric  34 . The protective dielectric can be an oxide such as silicon oxide deposited and patterned using techniques well known to those skilled in the art.  
         [0039]    Next a silicidation process, well known to those skilled in the art, is carried out which converts the conducting material in the fourth  35  and fifth  39  resistor elements to a silicide. In this example with the conducting material of silicon the conducting material in the fourth  35  and fifth  39  resistor elements can be converted to a silicide such as titanium silicide, cobalt silicide, or the like. As those skilled in the art will readily recognize the silicidation process is usually part of the process for forming contacts in other regions of the substrate  30 . The protective dielectric  34  protects the first  32 , second  33 , and third  37  resistor elements from the silicidation process so that the first conducting material remains unchanged and the conductivity of the conducting material forming the first  32 , second  33 , and third  37  resistor elements remains unchanged. The protective dielectric  34  also protects the conducting material forming the first  32 , second  33 , and third  37  resistor elements from subsequent process steps so that the conductivity of the conducting material in these regions is not changed. Contacts  34  to the resistor can be formed in the fourth  35  and fifth  39  resistor elements using methods well known to those skilled in the art.  
         [0040]    The conductivity of the silicide in the fourth  35  and fifth  39  resistor elements is substantially greater than the conductivity of the conducting material in the first  32 , second  33 , and third  37  resistor elements. The resistance of the interface  38  between the second resistor element  33  and the interface  36  between the third  37  and fifth  39  resistor elements is low compared to the resistance of the first resistor element  32  because the conducting material forming the fourth  35  and fifth  39  resistor elements has been converted to a silicide. The second  33  and third  37  resistor elements are designed to be wide relative to the width  42  of the first  32  resistor element so their resistance will be small compared to the first  32  resistor element.  
         [0041]    The resistance, R, of the resistor can be expressed as R=R 1 +2 R 2 +2 R 3 +2 R 4 +R 5 . In this equation R 1  is the resistance of the first  32  resistor element, R 2  is the resistance of the contacts  44  to the fourth  35  and fifth  39  resistor elements, R 3  is the resistance of the fourth  35  and fifth  39  resistor elements, R 4  is the resistance of interfaces,  38  and  36 , between the second  33  and fourth  35  resistor elements and between the third  37  and fifth  39  resistor elements, and R 5  is the resistance of the second  33  and third  37  resistor elements. Of these resistances R 2 , R 3 , R 4 , and R 5  are all quite small with respect to R 1 , and the resistance, R, of the resistor is very nearly equal to R 1 . This makes it possible to accurately adjust the resistance of the resistor by controlling the doping of the silicon, the length  40  of the first resistor element  32 , and the width  42  of the first resistor element. In addition to providing the ability to accurately design the resistance of the resistor, the protective dielectric keeps the resistance stable throughout subsequent processing. The design and methods of this invention provides a resistor having a low voltage coefficient of resistance (VCR).  
         [0042]    Another embodiment of the resistor layout of this invention is shown in FIG. 11. As shown in FIG. 11 dummy resistor elements  46  can be formed on either side of the first resistor element  32 . The dummy resistor elements  46  can be used to compensate for proximity effects when the dimensions of the first resistor element  32  are very small. FIG. 6 shows the resistor with dummy resistor elements  46  after the protective dielectric layer  34  has been formed.  
         [0043]    The improvement of resistor characteristics due to the protective dielectric layer of this invention is shown in FIGS. 12-15. FIG. 12 shows a first curve  70  and a second curve  72 . The first curve  70  shows resistance as a function of voltage for a P+ doped polysilicon resistor having the protective dielectric layer of this invention. The second curve  72  shows resistance as a function of voltage for a P+ doped polysilicon resistor having the same doping level but without the protective dielectric layer.  
         [0044]    [0044]FIG. 13 shows a third curve  71  and a fourth curve  73 . The third curve  71  shows resistance as a function of voltage for an N +  doped polysilicon resistor having the protective dielectric layer of this invention. The fourth curve  73  shows resistance as a function of voltage for an N +  doped polysilicon resistor having the same doping level but without the protective dielectric layer.  
         [0045]    [0045]FIG. 14 shows a fifth curve  74  and a sixth curve  76 . The fifth curve  74  shows the voltage coefficient of resistance as a function of voltage for a P +  doped polysilicon resistor having the protective dielectric layer of this invention. The sixth curve  76  shows the voltage coefficient of resistance as a function of voltage for a P +  doped polysilicon resistor having the same doping level but without the protective dielectric layer.  
         [0046]    [0046]FIG. 15 shows a seventh curve  75  and a eighth curve  77 . The seventh curve  75  shows the voltage coefficient of resistance as a function of voltage for an N +  doped polysilicon resistor having the protective dielectric layer of this invention. The eighth curve  77  shows the voltage coefficient of resistance as a function of voltage for an N +  doped polysilicon resistor having the same doping level but without the protective dielectric layer.  
         [0047]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.