Abstract:
A device with a plurality of structures with different resistance values is formed on a substrate. A polysilicon layer is formed upon the substrate. A silicon oxide layer is formed over the substrate. A hard masking layer is formed over the silicon oxide layer. The hard masking layer includes a full thickness portion and a thinner portion. The polysilicon layer below the full thickness portion is lightly doped forming a high resistance region. Below the thinner portion the polysilicon layer is heavily doped forming a low resistance region. However, in spite of the differences in resistance, the high resistance region and the low resistance region have the same thickness.

Description:
This is a division of patent application Ser. No. 09/073,950, filing date May 7, 1998, now U.S. Pat. No. 6,143,474 Method Of Fabricating Polysilicon Structures With Different Resistance Values For Gate Electrodes, Resistors And Capacitor Plates And The Devices Produced Thereby, assigned to the same assignee as the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to integrated circuit devices and more particularly to the process of manufacture of polysilicon structures with varying values of resistance and the devices produced by the process. 
     2. Description of Related Art 
     U.S. Pat. No. 5,705,418 of Liu for “Process for Fabricating Reduced-Thickness High-Resistance Load Resistors in Four-Transistor SRAM Devices” shows a method of forming polysilicon resistors where an oxide layer is used as an Ion Implantation (I/I) block. An oxidation resistant layer is formed and patterned for exposing regions of the polysilicon layer designated for the formation of the load resistors. An oxide layer is formed over the surface of the exposed portions of the polysilicon layer, so that the thickness of the designated regions of the polysilicon layer below the oxide layer is reduced. These designated regions are provided to form load resistors. The oxidation resistant layer is then removed. Then, impurity ions are implanted into exposed regions of the polysilicon layer, not covered by the oxide layer, which are designated for forming interconnectors for the memory cell unit. 
     U.S. Pat. No. 5,514,617 of Liu for “Method of Making a Variable Resistance Polysilicon Conductor for SRAM Devices” shows how to produce resistors using a patterning method with an I/I (Ion Implantation) process with step areas where variable doping results with higher resistance in the steeper areas than the flat areas, plus heavy doping formed in contact areas by doping through openings in a contact mask. 
     U.S. Pat. No. 4,643,777 of Maeda for “Method of Manufacturing a Semiconductor Device Comprising Resistors of High and Low Resistances” describes a method of forming resistors in portions of a polysilicon layer with portions covered with a mask and the other portions covered with a molybdenum film. Then the molybdenum film is subjected to a silicifying step. The result is that those regions of the polysilicon film located under the molybdenum film have a low resistance, while the regions of the polysilicon film covered by the mask have a high resistance value. 
     See U.S. Pat. No. 5,622,884 of Liu for “Method for Manufacturing a Semiconductor Memory Cell and a Polysilicon Load Resistor of the Semiconductor Memory Cell” describe a load resistor formed by depositing a polysilicon layer over an insulating layer. The polysilicon layer is ion implanted with dopant and is then masked and etched to form a high resistance load resistor. 
     In the past, in order to form polysilicon layers with a different resistance in an integrated circuit, the solution has been to modify the area and length of the polysilicon to meet the criteria required. However, that approach increases the cost of manufacturing due to the complex process. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention a device with a plurality of structures with different resistance values includes as follows: 
     a polysilicon layer upon a substrate, 
     a hard masking layer formed upon the polysilicon layer, 
     the hard masking layer including a full thickness portion and a lower thickness portion, and 
     the polysilicon layer having a high resistance beneath the full thickness portion and a low resistance beneath the low thickness portion. 
     The hard masking layer is composed of a material selected from the group consisting of silicon oxide and silicon nitride. 
     The full thickness portion is from about 0.3 μm to about 0.5 μm thick, and 
     the lower thickness portion is from about 0.01 μm to about 0.15 μm thick. 
     The polysilicon layer is formed over a silicon oxide layer selected from a field oxide layer and a gate oxide layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects and advantages of this invention are explained and described below with reference to the accompanying drawings, in which: 
     FIG. 1 shows a flow chart of processing steps in accordance with a first embodiment of this invention. 
     FIGS. 2A-2H illustrate a device in the process of manufacture which forms a single polysilicon layer PS into structures with different resistance values in accordance with the process of FIG.  1 . 
     FIG. 3 shows a flow chart of processing steps in accordance with a second embodiment of this invention. 
     FIGS. 4A-4H illustrate a device in the process of manufacture which forms a single polysilicon layer into structures with different resistance values in accordance with the process of FIG.  3 . 
     FIG. 5 shows a flow chart of processing steps in accordance with a third embodiment of this invention. 
     FIGS. 6A-6H illustrate a device in the process of manufacture which forms a single polysilicon layer into structures with different resistance values in accordance with the process of FIG.  5 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The object of the method of this invention is to produce a set of high and low resistance polysilicon structures in an integrated circuit device. 
     There are numerous alternative sequences of processing steps which can be performed to produce the result seen in FIG.  1 G. 
     First Embodiment of the Invention 
     Referring to FIG.  1  and FIGS. 2A-2H, a method of forming a single polysilicon layer PS into structures with different resistance values, comprises the steps as follows: 
     (1) Referring to FIG. 1 in step  20 , and FIG. 2A an integrated circuit device  10  is made starting with a substrate  12 , such as a P-doped substrate. During this first step, the substrate  12  is coated with a silicon dioxide (SiO 2 ) layer OX 1  by a CVD process or a thermal process. 
     This layer can be a gate oxide layer for a gate electrode or can be a field oxide layer for a resistor. A capacitor plate can be formed over either a field oxide layer or a gate oxide layer. 
     (2) In step  21  deposit a polysilicon layer PS over the silicon dioxide layer OX 1  for a resistor or capacitor plate. 
     (3) In step  22 , form a hard mask layer  14  of silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) on the polysilicon layer PS with a thickness from about 0.3 μm to about 0.5 μm. 
     (4) Then, in step  23  define a first photoresist mask PR 1  over hard mask layer  14  to produce a surface area of the exposed region of the hard mask layer  14  by leaving portions of the hard mask layer exposed, i.e. without protection by first photoresist mask PR 1 . 
     (5) Referring to FIG. 2B, in step  24  of FIG. 1, etch away a substantial thickness of the hard mask layer  14  in the exposed area referred to hereinafter as the “low resistance” thin region  14 L which has been reduced to a predetermined thickness from about 0.01 μm to about 0.15 μm in the exposed area referred to hereinafter as thin region  14 L. 
     The remainder of hard mask layer  14  comprises the “high resistance” thick region  14 H of polysilicon layer  14  with its original thickness. 
     (6) Referring to FIG. 2C, in step  25 , remove the photoresist mask PR 1  leaving the thick region  14 H of hard mask layer  14 , as well as the thin region  14 L, exposed. 
     (7) Referring to FIG. 2D, in step  26  dope or ion implant an impurity DP into polysilicon layer PS through the hard mask thick region  14 H and the thin region  14 L of silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) to produce high resistance region PSH of layer PS under thick region  14 H and low resistance region PSL of layer PS under thin region  14 L. 
     The doping can be provided by ion implanting with a dose of N type or P type dopant from about 1 E 12 ions/cm 2  to about 1 E 15 ions/cm 2  at an energy from about 30 keV to about 120 keV. 
     After annealing the concentration of N-type or P-type dopant in the high resistance region PSH was from about 1 E 12 atoms/cm 3  to about 1 E 15 atoms/cm 3 . 
     The concentration of N-type or P-type dopant in the low resistance region PSL was from about 1 E 12 atoms/cm 3  to about 1 E 15 atoms/cm 3 . 
     (8) Referring to FIG. 2E, in step  27  form a second mask PR 2 /PR 2 ′ (formed of photoresist) for patterning the hard mask layer thick region  14 H and the hard mask thin region  14 L (silicon dioxide or silicon nitride) using photolithography and etching only. 
     (b  9 ) Referring to FIG. 2F, noting that the photoresist mask PR 2  is spaced away from the photoresist mask PR 2 ′, in step  28 , hard mask layer  14  was patterned into a separate full thickness hard mask  14 H′ under photoresist mask PR 2  which is spaced away from a separate thin hard mask  14 L′ under photoresist mask PR 2 ′. 
     (10) Then, referring to FIG. 2G, in step  29  the photoresist masks PR 2 /PR 2 ′ were stripped from the hard masks  14 H′ and  14 L′. 
     (11) Then, referring to FIG. 2H, in step  30  the thick hard mask  14 H′ and the thin hard mask  14 L′ (which is spaced away from the thick hard mask  14 H′) were used to pattern polysilicon regions PSH and PSL using highly selective etching to form the high resistance device RH from region PSH, under thick hard mask  14 H′ and the low resistance device RL from region PSL, under thin hard mask  14 L′. 
     Second Embodiment of the Invention 
     The method of the second embodiment of the invention is similar to the method of the first embodiment except that the steps are as shown in FIGS. 2A-2H with the doping step moved from FIG. 2D in the first embodiment to FIG. 6H in the first embodiment representing a switch in the sequence of processing of the doping step to nearer the end of the process. The method of the second embodiment is as follows: 
     (1) Referring to FIGS. 3 and 4A the device  10  described above is made by a different sequence of the above steps, starting with step  40  in FIG. 3 with reference to FIG.  4 A. The integrated circuit device  10  is made starting with a substrate  12 , such as a P-doped substrate. During this first step  40 , the substrate  12  is coated with a silicon dioxide (SiO 2 ) layer OX 1  by a CVD process or a thermal process. 
     This layer can be a gate oxide layer for a gate electrode or can be a field oxide layer for a resistor. A capacitor plate can be formed over either a field oxide layer or a gate oxide layer. 
     (2) In step  41  deposit a polysilicon layer PS over the silicon dioxide layer OX 1  for a resistor or capacitor plate. 
     (3) In step  42 , form a hard mask layer  14  of silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) on the polysilicon layer PS with a thickness from about 0.3 μm to about 0.5 μm. 
     (4) Then, in step  43  define a first photoresist mask PR 1  over layer  14  to produce a surface area of the exposed region of the hard mask layer  14  by leaving it without coverage by first photoresist mask PR 1 . 
     (5) Referring to FIG. 4B, in step  44  of FIG. 3, etch away a substantial thickness of the hard mask layer  14  in the exposed area referred to hereinafter as the “low resistance” thin region  14 L which has a reduced, predetermined thickness from about 0.01 μm to about 0.15 μm. 
     The remainder of hard mask layer  14  comprises the “high resistance”, thick region  14 H of polysilicon layer  14  with its original thickness. 
     (6) Referring to FIG. 4C, in step  45 , remove the photoresist mask PR 1  leaving the thick region  14 H of hard mask layer  14 , as well as thin region  14 L, exposed. 
     (7) Referring to FIG. 4D, in step  46 , form a second mask PR 2 /PR 2 ′ (formed of photoresist) for patterning the hard mask layer thick region  14 H and the hard mask thin region  14 L (silicon dioxide or silicon nitride) using photolithography and etching only. 
     (8) Referring to FIG. 4E, in step  47 , hard mask layer  14  has been patterned into separate thick hard mask  14 H′ under photoresist mask PR 2  and thin hard mask  14 L′ under photoresist mask PR 2 ′. 
     (9) Then, referring to FIG. 4F, in step  48  the photoresist masks PR 2 /PR 2 ′ were stripped from the thick and thin hard mask layers  14 H′ and  14 L′, respectively. 
     (10) Referring to FIG. 4G, in step  49  dope or ion implant an impurity DP into polysilicon layer PS through hard mask layer  14  of silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ). 
     The doping can be provided by ion implanting with a dose of N-type or P-type dopant from about 1 E 12 ions/cm 2  to about 1 E 15 ions/cm 2  at an energy from about 30 keV to about 120 keV. 
     After annealing the concentration of N-type or P-type dopant in the high resistance region RH was from about 1 E 12 atoms/cm 3  to about 1 E 15 atoms/cm 3 . The concentration of N-type or P-type dopant in the low resistance region RL was from about 1 E 12 atoms/cm 3  to about 1 E 15 atoms/cm 3 . 
     (11) Referring to FIG. 4H, use the thick hard mask  14 H′ under mask PR 2  and thin hard mask  14 L′ to pattern polysilicon using highly selective etching. 
     Third Embodiment of the Invention 
     The method of the third embodiment of the invention is similar to the method of the second embodiment except that the steps are as shown in FIGS. 6A-6H with FIGS. 6G and 6H representing a reversal of the processing of the doping step. The method of the third embodiment is as follows: 
     (1) Referring to FIGS. 5 and 6A, the device  10  described above is made by a different sequence of the above steps, starting with step  60  in FIG. 3 with reference to FIG.  6 A. The integrated circuit device  10  is made starting with a substrate  12 , such as a P-doped substrate. During this first step  60 , the substrate  12  is coated with a silicon dioxide (SiO 2 ) layer OX 1  by a CVD process or a thermal process. 
     This layer can be a gate oxide layer for a gate electrode or can be a field oxide layer for a resistor. A capacitor plate can be formed over either a field oxide layer or a gate oxide layer. 
     (2) In step  41  deposit a polysilicon layer PS over the silicon dioxide layer OX 1  for a resistor or capacitor plate. 
     (3) In step  62 , form a hard mask layer  14  of silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) on the polysilicon layer PS with a thickness from about 0.3 μm to about 0.5 μm. 
     (4) Then, in step  63  define a first photoresist mask PR 1  over layer  14  to produce a surface area of the exposed region of the hard mask layer  14  by leaving it without coverage by first photoresist mask PR 1 . 
     (5) Referring to FIG. 6B, in step  64  of FIG. 5, etch away a substantial thickness of the hard mask layer  14  leaving an exposed area referred to hereinafter as the “low resistance”, thin region  14 L which has a reduced, predetermined thickness, from about 0.01 μm to about 0.15 μm. 
     The remainder of hard mask layer  14  comprises the thick, “high resistance”, thick region  14 H of polysilicon layer  14  with its original thickness. 
     (6) Referring to FIG. 6C, in step  65 , remove the photoresist mask PR 1  leaving the thick region  14 H of hard mask layer  14 , as well as thin region  14 L, exposed. 
     (7) Referring to FIG. 6D, in step  66 , form a second mask PR 2 /PR 2 ′ (formed of photoresist) for patterning the hard mask layer thick region  14 H and the hard mask thin region  14 L (silicon dioxide or silicon nitride) using photolithography and etching only. 
     (8) Referring to FIG. 6E, in step  67 , hard mask layer  14  has been patterned into a separate thick hard mask  14 H′ under photoresist mask PR 2  and into a separate thin hard mask  14 L′ under photoresist mask PR 2 ′. 
     (9) Then, referring to FIG. 6F, in step  68  the photoresist masks PR 2 /PR 2 ′ were stripped from both the thick and the thin hard mask layers  14 H′ and  14 L′, respectively. 
     At this point the process diverges from FIGS. 4G and 4H to the steps as illustrated by FIGS. 6G and 6H. 
     (10) Referring to FIG. 6G, in step  69 , use the thick hard mask  14 H′ under mask PR 2  and thin hard mask  14 L′ to pattern the polysilicon layer PS into a high resistance device RH and low resistance device RL using highly selective etching. 
     (11) Referring to FIG. 6H, in step  70 , dope or ion implant an impurity DP into polysilicon layer PS through hard mask layer  14  of silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ). 
     The doping of the polysilicon layer can be provided by ion implanting with a dose of N-type or P-type dopant from about 1 E 12 ions/cm 2  to about 1 E 15 ions/cm 2  at an energy from about 30 keV to about 120 keV. 
     After annealing the concentration of N-type or P-type dopant in the high resistance region RH was from about 1 E 12 atoms/cm 3  to about 1 E 15 atoms/cm 3 . The concentration of N-type or P-type dopant in the low resistance region RL was from about 1 E 12 atoms/cm 3  to about 1 E 15 atoms/cm 3 . 
     In summary, the three embodiments described above provide as follows: 
     1. Doping polysilicon after etching the hard mask pattern to variable thicknesses. 
     2. Etching the hard mask pattern to variable thicknesses followed by patterning the hard mask and then patterning the polysilicon and doping after using the hard mask in step 10. 
     3. Etching the hard mask pattern to variable thicknesses followed by patterning the hard mask and patterning the polysilicon followed by doping of the polysilicon in step 11. 
     While this invention has been described in terms of the above specific embodiment(s), those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims, i.e. that changes can be made in form and detail, without departing from the spirit and scope of the invention. Accordingly all such changes come within the purview of the present invention and the invention encompasses the subject matter of the claims which follow.