Patent Publication Number: US-2022238516-A1

Title: Polysilicon resistor using reduced grain size polysilicon

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) to co-owned U.S. Provisional Patent Application Ser. No. 63/141,144, filed Jan. 25, 2021, entitled “POLYSILICON RESISTOR DRIFT AND 1/F NOISE REDUCTION USING REDUCED GRAIN SIZE POLYSILICON,” which is hereby incorporated by reference in its entirety herein. 
    
    
     TECHNICAL FIELD 
     This application relates generally to integrated circuits, and more particularly, but not exclusively, to examples of polysilicon resistors. 
     BACKGROUND 
     Polycrystalline silicon (polysilicon) resistors are an element of integrated circuits. Polysilicon, as opposed to crystalline silicon, does not have one crystalline structure, but rather includes randomly arranged multiple crystals. One way that integrated circuit designers can include resistors into integrated circuits is using polysilicon resistors. Polysilicon is generally used as a conductive interconnection between components in an integrated circuit and as elements of integrated circuit components, such as the gate electrode of field effect transistors. In this use, the polysilicon material is usually doped and may be clad with a conductive material such as tungsten silicide or molybdenum silicide to provide high conductivity. 
     In contrast, low doping is used in polysilicon resistors to maximize the polysilicon material&#39;s resistance. Even with this low resistance, polysilicon resistors are large by the standards of integrated circuit components. They are often configured in shapes such as serpentine or coils to increase the length between the ends of the resistor while minimizing the space occupied by the resistor. 
     In precision applications, such as analog circuits, polysilicon resistors are very carefully sized and doped to allow for precise operation of the circuit including the resistor. Sizing of the resistor can be controlled in the fabrication process, but variations in properties of the resistors can cause deviations within an integrated circuit, or between integrated circuits made on a same wafer or in a same lot of wafers. In particular, three characteristics are challenging to control: matching, drift, and 1/f noise. Matching is when resistors are manufactured to have a specific resistance (often to match other resistors) but vary from the precise design resistance. Drift is a change in resistance caused by use. This can be caused by heating and the effects of electron flow through the resistor. 1/f noise is a phenomenon where a resistor is affected by background radiation and/or noise generated in the resistor. Below a threshold frequency, this noise tends to increase with the inverse of the frequency of the noise, thus the name 1/f noise. 
     SUMMARY 
     In accordance with an example, a resistor includes a substrate having a non-conductive surface and a patterned polysilicon layer on the non-conductive surface, the patterned polysilicon layer including polycrystalline silicon wherein at least 90% of the grains in the polycrystalline silicon are 30 nm or smaller. The resistor also includes a first terminal in conductive contact with the patterned polysilicon layer and a second terminal in conductive contact with the polysilicon layer and spaced from the first contact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a layout view of an example polysilicon resistor. 
         FIGS. 2A-2I  (collectively “ FIG. 2 ”) are cut-away diagrams of an example process for forming an integrated circuit including an example polysilicon resistor. 
         FIGS. 3A and 3B  (collectively “ FIG. 3 ”) are a scanning electron micrograph and a grain size histogram, respectively, of polycrystalline silicon produced by an example process. 
         FIGS. 4A and 4B  (collectively “ FIG. 4 ”) are a scanning electron micrograph and a grain size histogram, respectively, of polycrystalline produced using another example process. 
         FIGS. 5A and 5B  (collectively “ FIG. 5 ”) is a plan view and cut-away view, respectively, of an example polysilicon resistor. 
         FIG. 6  is a histogram of an example experiment. 
         FIG. 7  is a histogram of another example experiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, corresponding numerals and symbols generally refer to corresponding parts unless otherwise indicated. The drawings are not necessarily drawn to scale. 
     In this description, the term “coupled” may include connections made with intervening elements, and additional elements and various connections may exist between any elements that are “coupled.” Also, as used herein, the terms “on” and “over” may include layers or other elements where intervening or additional elements are between an element and the element that it is “on” or “over.” 
     Without implied limitation, in example arrangements, the problems of drift, matching, and noise in polysilicon resistors are reduced by using polycrystalline silicon material where at least 90% the grain sizes of the material are less than 30 nm in diameter. In an example, a resistor includes a substrate having a non-conductive surface and a patterned polysilicon layer on the non-conductive surface, the patterned polysilicon layer including polycrystalline silicon wherein at least 90% of the grains in the polycrystalline silicon are 30 nm or smaller. The resistor also includes a first contact in conductive contact with the patterned polysilicon layer and a second contact in conductive contact with the polysilicon layer and spaced from the first contact. 
       FIG. 1  is a layout view of an example polysilicon resistor  100 . Substrate  104  is an insulating substrate. In an example, substrate  104  is a semiconductor substrate having an insulating layer on the surface that insulates polysilicon resistor  100  from the semiconductor material. In examples, the semiconductor material of substrate  104  may include silicon-on-insulator material, an epitaxial layer, and other configurations. Patterned polysilicon layer  102  includes first contact pad  106  at one end and second contact pad  108  at the opposite end of patterned polysilicon layer  102 . Patterned polysilicon layer  102  is lightly doped to provide high resistance. In this context, “lightly doped” means the polysilicon has a dopant concentration of about 1×10 15  atoms/cm′ or less, and/or has a resistivity p of about 300 Ω-cm or greater, “about” meaning±15%. The serpentine configuration is used to provide an extended length between first contact pad  106  and second contact pad  108  while consuming a minimum of area of an integrated circuit or other device in which polysilicon resistor  100  is included. 
       FIGS. 2A-2I  (collectively “ FIG. 2 ) are cut-away diagrams of an example process for forming an integrated circuit including an example polysilicon resistor.  FIG. 2A  shows two areas: the area for a transistor  202  and the area for a resistor  204 . These areas indicate where the transistor  202  and part of the resistor  204  are formed. In this example, transistor  202  is a field effect transistor. However, transistor  202  is merely one example of many components that can be fabricated in an integrated circuit with the resistor  204 . In addition, the resistor  204  can be a stand-alone resistor without any additional components. Substrate  205  may be a crystalline silicon epitaxial layer that serves as a crystalline silicon substrate in this example. Well  206  is formed in substrate  205  using a patterned implantation of dopant ions (not shown) followed by a thermal drive-in cycle. If transistor  202  is an n-channel transistor, well  206  is a p-type well in this example. First masking layer  208  is a patterned masking layer formed using photolithographic patterning and etching. The structure of  FIG. 2A  is subject to an oxidizing atmosphere, such as steam, at a high temperature, such as 500° C. to 800° C., to form field oxide  210  as shown in  FIG. 2B , which serves as a non-conductive layer. Field oxide  210  has a thickness in a range from 300 Å to 1,000 Å. Masking layer  208  is then removed and the structure is subjected to another oxidation to form gate oxide  211  to a thickness of 50 Å to 200 A, seen in  FIG. 2C . 
     Further in  FIG. 2C , a deposition of polycrystalline silicon using silane at a temperature at or less than 610° C., with a flow rate of at least 600 sccm at a pressure at or less than 200 mT (27 Pa) without a carrier gas, such as nitrogen, forms polycrystalline silicon layer  212 . Polycrystalline silicon layer  212  has small grains (crystals within the polycrystalline layer) with a size distribution having a peak at about 20 nm (e.g., as determined by converting the grain&#39;s area as seen in a transmission electron microscope (TEM) to an equivalent circle, wherein the diameter is the diameter of this circle), and where more than 90% of the grains have a grain size less than 30 nm. This specific deposition process is an example process that produces a layer having small grain sizes. Other processes can be substituted if they produce small grain sizes. Second masking layer  214  is deposited and patterned using photolithography on polycrystalline silicon layer  212 . An anisotropic plasma etch with a fluorine bearing plasma, such as carbon tetrafluoride, etches polycrystalline silicon layer  212  using second masking layer  214  as an etch mask. The resulting structure includes gate  218  and polysilicon resistor body  216  as shown in  FIG. 2D . In the example of  FIG. 2 , only a portion of polysilicon resistor body  216  is shown. In examples, the polysilicon resistor body extends to form a length of polysilicon material needed to provide the desired resistance in a coil or serpentine configuration, or any other configuration that provides a resistor structure. 
     In an example, a first ion implantation  217  ( FIG. 2D ) of boron ions having a flux density of 5×10 13  ions/cm 2  and an energy of 10 keV forms doped source/drain regions  220  after drive in, as shown in  FIG. 2E . An insulating layer, such as silicon nitride, is formed over the resistor body  216  and the gate  218  of  FIG. 2E  and then anisotropically etched to form sidewall layers  222 . An ion implantation of boron ions as second ion implantation  225  having a flux density of 3.66×10 15  ions/cm 2  and an energy of 4 keV forms source/drain regions  226  after drive-in as shown in  FIG. 2G . A short plasma dry etch then removes the exposed portions of gate oxide  211  as shown in  FIG. 2G . A silicide blocking layer  224  is formed and patterned as shown in  FIG. 2G . The structure of  FIG. 2G  is then exposed to a metal atom flux, e.g. by sputtering, including atoms that form a conductive layer, such as titanium silicide, molybdenum silicide, and/or cobalt silicide, thereby forming gate silicide  228 , first source/drain silicide  230 , second source/drain silicide  232  and resistor contact silicide  234  as shown in  FIG. 2H . Not shown is that the opposite end of polysilicon resistor body also is silicided in this step. The portion of polysilicon resistor body  216  not covered by resistor contact silicide  234  ( FIG. 2G ) is lightly doped. This may be achieved by counter doping second implantation  225 , including a small amount of dopant in the formation of polycrystalline silicon layer  212  ( FIG. 2C ), a light implantation of the structure of  FIG. 2H , or another procedure such as that shown in Montgomery, U.S. Pat. No. 9,991,120, issued Jun. 5, 2018, which is owned by applicant/assignee, and is hereby incorporated in its entirety herein by reference. 
     As shown in  FIG. 2I , an interlevel dielectric layer  236  is then formed on the structure of  FIG. 2H . First via  238 , second via  240 , third via  242 , and fourth via  244  are then formed by etching interlevel dielectric layer  236  using a patterned photolithographic mask and filling the openings formed by the etching with a conductive material such as tungsten and/or titanium.  FIG. 2  shows formation of the resistor  204  on an insulating layer, e.g. the field oxide  210 , with the via  238  used to couple the resistor to other components from above. In an alternative example, polycrystalline silicon layer  212  ( FIG. 2C ) may be formed over an opening in field oxide  210  and contacting a diffusion formed in substrate  205  that is part of an active component. 
     Polycrystalline silicon layer  212  in  FIG. 2C  is formed using an example process. This example process produces polycrystalline material having smaller grain sizes than a baseline process. Furthermore, the use of polycrystalline silicon with grain sizes having a diameter averaging 20 nm with approximately 90% or more of the grains having a diameter less than 30 nm provides performance advantages for resistors using this polycrystalline silicon, as further explained hereinbelow. 
       FIGS. 3A and 3B  (collectively “ FIG. 3 ”) are a scanning electron micrograph and a grain size histogram, respectively, of polycrystalline silicon produced by an example process, e.g. a baseline process. Polycrystalline silicon  300  was produced using a typical commercially available furnace. Substrates (not shown) were placed in the furnace. Silane gas with nitrogen was provided into the furnace chamber at a flow rate of 500 sccm, with a pressure of 300 mT (40 Pa) at 620° C. to produce a polycrystalline silicon layer having a thickness of 1,000 Å to 1,600 Å. The grain sizes produced by this process are shown in histogram  302  in  FIG. 3B . As shown in histogram  302 , the grain sizes have an approximately Gaussian distribution with a peak and average of about 100 nm in diameter and a 2σ width of about 100 nm. 
       FIGS. 4A and 4B  (collectively “ FIG. 4 ”) are a scanning electron micrograph and a grain size histogram, respectively, of polycrystalline produced using an example process like that used to produce polycrystalline silicon layer  212  ( FIG. 2 ). Polycrystalline silicon  400  was produce using another commercially available furnace. Substrates (not shown) were placed in the furnace. Silane gas without nitrogen was provided into the furnace chamber at flow rate of approximately 660 sccm, with a pressure of approximately 150 mT (20 Pa) at approximately 605° C. to produce a polycrystalline silicon layer having a thickness of 1,000 Å to 1,600 Å. In this context, “approximately” means±5%. The grain sizes produced by this process are shown in histogram  402  in  FIG. 4B . As shown in histogram  402 , the grain sizes have an approximately Gaussian distribution with a peak and average of about 20 nm in diameter and a 2σ width of about 15 nm, with greater than 90% of the grain sizes being less than 30 nm. 
       FIGS. 5A and 5B  (collectively “ FIG. 5 ”) is a plan view and cut-away view, respectively, of an example polysilicon resistor  500 . Polysilicon resistor  500  includes body  502 , first terminal  504  at one end of body  502  and second terminal  506  at an opposing end of body  502 . First terminal  504  includes silicide layer  508 , e.g., titanium silicide, and the polycrystalline silicon in first terminal  504  is silicided. Therefore, first terminal  504  is sufficiently conductive that it contributes very little to the resistance of polysilicon resistor  500 . Similarly, second terminal  506  includes titanium silicide layer  510  and the polycrystalline silicon in second terminal  506  is also silicided. Therefore, second terminal  506  also contributes very little to the resistance of polysilicon resistor  500 . Thus, the resistance of polysilicon resistor  500  is determined by the width W, length L, and thickness T (shown in  FIG. 5B ) of body  502 , along with the sheet resistance (R S ) of the material of body  502 . The formula for determining the resistance of polysilicon resistor  500  is given by Equation 1: 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       R 
                       S 
                     
                     ⁢ 
                     
                       L 
                       W 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Width W, length L and thickness T can be controlled with good precision during manufacturing. In addition, the sheet resistance R S , equal to the resistivity ρ divided by thickness T, can be well controlled. However, for high precision circuits, critical variances must be minimized. Surprisingly, the use of small-grain polycrystalline silicon in resistors provides improvement in at least three important characteristics of polysilicon resistors: drift, matching and 1/f noise. 
     Drift is a change in the resistance value of the resistor over time and use of the resistor.  FIG. 6  is a normal probability plot  600  of experimental data collected to determine the drift of experimentally fabricated resistors using high sheet resistance (HSR) polycrystalline silicon. HSR polycrystalline silicon has a low effective doping of approximately 5×10 15  atoms/cm 3  to 1×10 16  atoms/cm′ to provide a high sheet resistance of about 600 Ω/square. With one wafer, including many dies, the polycrystalline silicon layer used to fabricate the resistors was the process described hereinabove with regard to  FIG. 3 . With another wafer, the polycrystalline silicon used to fabricate the resistors was the process described herein above with regard to  FIG. 4 . Each wafer was subjected to a temperature of 150° C. for 168 hours to cause drift in the resistor values. The resistance values of the experimental resistors were measured before and after this furnace cycle. The vertical axis corresponds to the percentile deviation of the resistance from the median resistance after the resistors had been subjected to the furnace induced drift. The horizontal axis is the percentage increase in resistance. Data set  602  is the data points for the resistors fabricated using the process of  FIG. 4 . Data set  604  is the data points for the resistors fabricated using the process of  FIG. 3 . The percentage values on the right of the chart indicates the percentage of data point less than the indicated line. For example, the 50% line is the median line and half of the data points are less than this value. For the 99.9% line, nearly all of the data points are less than this value.  FIG. 6  shows that for each percentile, the increased resistance (drift) is less for data set  602  than data set  604 . The median value was an increase of 0.82 for the resistors fabricated using the process of  FIG. 3 . The median value was an increase of 0.75 for the resistors fabricated using the process of  FIG. 4 . Therefore, using polycrystalline silicon material produced by the process of  FIG. 4  reduced drift in HSR material by 8.5%. 
       FIG. 7  is a normal probability plot of another example experiment like that of  FIG. 6  showing the results of the same experiment described above with regard to  FIG. 6  using very high sheet resistance (VSR). VSR polycrystalline silicon has a very low effective doping of about 2×10 15  atoms/cm′ to provide a very high sheet resistance of about 720 Ω/square. In the plot  700 , data set  702  includes the results from the resistors fabricated using the process of  FIG. 4 . Data set  704  includes the results from the resistors fabricated using the process of  FIG. 3 .  FIG. 7  shows that for each percentile, the increased resistance (drift) is less for data set  702  than for the data set line  704 . The median value was an increase of 0.75 for the resistors fabricated using the process of  FIG. 3 . The median value was an increase of 0.68 for the resistors fabricated using the process of  FIG. 4 . Therefore, using polycrystalline silicon material produced by the process of  FIG. 4  reduced drift in VSR material by 9.3%. 
     Another important characteristic for precision circuitry is resistor matching. This means that resistors formed to the same size have matching resistance values, or that proportionally sized resistors have proportional resistance. This is important for accurate operation of analog circuitry. In a simple example, if two resistors are made to the same size, they may be used in a voltage divider. That is, the resistors are connected in series with one end connected to a measured voltage and the other end connected to a reference potential. With perfect matching the voltage between the two resistors is half of the difference between the measured voltage and the reference voltage. However, if the resistors do not match, the voltage between the two resistors will be higher or lower than the desired voltage. A voltage divider is only a simple example where matching is important. With precision analog circuits there are many instances where matching performance of resistors limits the accuracy of the circuit including those resistors. 
     Adjacent resistors in the experimental lots discussed above with regard to drift were measured to determine how well these resistors matched. For HSR polycrystalline material, the average mismatch was 1.68% for polycrystalline silicon material formed using the process described hereinabove with regard to  FIG. 3 . The average mismatch for the HSR material formed using the process described herein above with regard to  FIG. 4  was 1.55%. Therefore, the resistors formed using the process of  FIG. 4  provided 7.7% improved matching. For VSR polycrystalline material, the average mismatch was 1.59% form polycrystalline silicon material formed using the process described hereinabove with regard to  FIG. 3 . The average mismatch for the VSR material formed using the process described herein above with regard to  FIG. 4  was 1.47%. Therefore, the VSR resistors formed using the process of  FIG. 4  provided 7.8% improved matching. These measurements demonstrate the surprising result that resistors formed using the process described hereinabove with regard to  FIG. 4  provide improved matching characteristics. 
     Another important characteristic is the 1/f noise characteristic. Below a frequency point known as the 1/f corner, background noise begins rising in a near-linear fashion (on a logarithmic scale) from the 1/f corner to near zero frequency (See for example Kiely, “Understanding and Eliminating 1/f Noise,” Analog Dialogue 51-05, May 2017, which is incorporated herein by reference in its entirety). In this area, the amplitude of the noise is inversely proportional to the frequency, thus the name 1/f noise. The sources of 1/f noise are a source of considerable debate. However, it is not debatable that semiconductor manufacturers have devoted considerable resources to minimizing 1/f noise. Minimizing noise of any kind is important in precision analog circuitry 
     The experimental lots discussed above with regard to drift were measured to determine the 1/f noise characteristics. For each resistor measured, a bias current of 1.66 mA was applied. The voltage and current through the resistor are measured at a frequency of 100 Hz (chosen for convenience of measurement). Using Fourier analysis of the signal across the resistor, a power density in watts is determined for the noise at that frequency. That total 1/f noise power is divided by the length L and width W of the resistor to provide a normalized measure of watts/μm 2 . For HSR polycrystalline material, the average noise power density was 9.2×10 −11  watts/μm 2  using the process described hereinabove with regard to  FIG. 3 . The average noise power density for the HSR material formed using the process described herein above with regard to  FIG. 4  was 8.7 10 −11  watts/μm 2 . Therefore, the resistors formed using the process of  FIG. 4  provided 5.4% lower 1/f noise. For VSR polycrystalline material, the average mismatch was 1.16×10 −10  watts/μm 2  for polycrystalline silicon material formed using the process described hereinabove with regard to  FIG. 3 . The average mismatch for the VSR material formed using the process described herein above with regard to  FIG. 4  was 1.12×10 −10  watts/μm 2 . Therefore, the resistors formed using the process of  FIG. 4  provided 4.1% lower 1/f noise. These measurements demonstrate the surprising result that resistors formed using the process described hereinabove with regard to  FIG. 4  provide improved 1/f noise characteristics. 
     Modifications are possible in the described examples, and other examples are possible, within the scope of the claims.