Patent Publication Number: US-2005124169-A1

Title: Truncated dummy plate for process furnace

Description:
FIELD OF THE INVENTION  
      The present invention relates to furnaces used in the fabrication of semiconductor integrated circuits on semiconductor wafer substrates. More particularly, the present invention relates to dummy plates having a modified shape for providing uniform gas flow and heating of wafers in a process chamber, particularly a LPCVD (low pressure chemical vapor deposition) furnace.  
     BACKGROUND OF THE INVENTION  
      The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.  
      In the semiconductor production industry, various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include the deposition of layers of different materials including metallization layers, passivation layers and insulation layers on the wafer substrate, as well as photoresist stripping and sidewall passivation polymer layer removal. In modern memory devices, for example, multiple layers of metal conductors are required for providing a multi-layer metal interconnection structure in defining a circuit on the wafer. A current drive in the semiconductor device industry is to produce semiconductors having an increasingly large density of integrated circuits which are ever-decreasing in size. These goals are achieved by scaling down the size of the circuit features in both the lateral and vertical dimensions. Vertical downscaling requires that the thickness of conductive and insulative films on the wafer be reduced by a degree which corresponds to shrinkage of the circuit features in the lateral dimension. Ultrathin device features will become increasingly essential for the fabrication of semiconductor integrated circuits in the burgeoning small/fast device technology.  
      Chemical vapor deposition (CVD) processes are widely used to form layers of materials on a semiconductor wafer. CVD processes include thermal deposition processes, in which a gas is reacted with the heated surface of a semiconductor wafer substrate, as well as plasma-enhanced CVD processes, in which a gas is subjected to electromagnetic energy in order to transform the gas into a more reactive plasma. Forming a plasma can lower the temperature required to deposit a layer on the wafer substrate, to increase the rate of layer deposition, or both. Other CVD processes include APCVD (atmospheric pressure chemical vapor deposition), and LPCVD (low pressure chemical vapor deposition). While APCVD systems have high equipment throughput, good uniformity and the capability to process large-diameter wafers, APCVD systems consume large quantities of process gas and often exhibit poor step coverage. Currently, LPCVD is used more often than APCVD because of its lower cost, higher production throughput and superior film properties. LPCVD is commonly used to deposit nitride, TEOS oxide and polysilicon films on wafer surfaces for front-end-of-line (FEOL) processes.  
      A typical conventional vertical LPCVD furnace is generally indicated by reference numeral  10  in  FIG. 1  and includes a base  12  on which is removably mounted a quartz tube  14 . The interior of the quartz tube  14  defines a reaction chamber  16  for processing of as many as 150 substrates  29  held by a wafer boat  24  that is supported on the base  12  and contained in the reaction chamber  16 . The wafer boat  24  may be a SiC (silicon carbide) wafer boat and, as shown in  FIG. 2 , typically includes a base plate  25  and a top plate  26  which are spanned by multiple vertical support rods  27 . The substrates  29  are supported in vertically-spaced relationship by slots (not shown) in the support rods  27 .  
      A gas inlet tube  18  may extend downwardly through the quartz tube  14  into the reaction chamber  16 , and a central gas inlet opening  20  may be provided in the top center of the quartz tube  14 , for distributing reaction gases into the reaction chamber  16 . A gas outlet  22  is provided typically in the base  12  for distributing exhaust gases from the reaction chamber  16 . The gas outlet  22  may be located on the opposite side of the wafer boat  24  with respect to the gas inlet tube  18  to facilitate a more uniform flow of the reaction gases throughout the reaction chamber  16 . Multiple circular dummy plates  31  may be provided in the bottom portion of the wafer boat  24  to further promote a uniform flow of the reaction gases  32 , particularly in the bottom portion of the reaction chamber  16  which is the closest to the gas outlet  22 , as shown in  FIG. 3 .  
      During LPCVD processes carried out in the conventional furnace  10 , as many as 150 substrates  29  are processed in batches in order to maintain high wafer throughput. The substrates  29  in the upper sites (designated by the letter “U” in  FIG. 2 ) and the substrates  29  in the center sites (designated by the letter “C” in  FIG. 2 ) of the wafer boat  24  are substantially uniformly coated with deposition material, which forms films of uniform thickness, due to substantially uniform distribution of the reaction gases  32  along the surfaces of the substrates  29  in the upper sites “U” and the center sites “C”. However, at the lower sites “L”, the reaction gases  32  tend to flow in lesser volumes on the gas inlet tube  18  side than on the gas outlet  22  side of the reaction chamber  16 . Consequently, due to the proximity of the substrates  29  in the lower sites “L” in the wafer boat  24  to the gas outlet  22 , those substrates  29  tend to be coated with deposition material in various thicknesses along various regions on the surface of the substrate  29 , as shown in  FIG. 4 , with the heaviest-coated region  34  of each substrate  29  located on the side of the wafer boat  24  closest to the gas inlet tube  18  and the lightest-coated region  36  on the substrate  29  located on the side of the wafer boat  24  closest to the gas outlet  22 . A medium-coated region  35  is formed on the substrate  29  between the heaviest-coated region  34  and the lightest-coated region  36 . Due to this disparity in film thickness among the various regions on the substrate  29 , the L sites on the wafer boat  24  typically remain vacant during the LPCVD process. Consequently, each batch of substrates  29  typically contains only about 100 substrates, consisting of the substrates  29  in the U sites and the C sites, instead of the 150-wafer batch capacity. This reduces wafer throughput and processing efficiency.  
      It has been found that modifying the shape of the dummy plate to a truncated configuration provides a more uniform gas flow path and temperature profile within substrates located in the lower sites on the wafer boat. Consequently, the thickness uniformity of chemical vapor material deposited among the entire surface of the substrates in the lower sites of the wafer boat is substantially enhanced. Thickness uniformity on the wafers in the L-sites of the wafer boat have been improved from 4.2%, in the case of LPCVD processes which utilize the circular dummy plates, to 1.8%, in the case of LPCVD processes which utilize the truncated dummy plates of the present invention. CPK values for the U/C/L sites was improved from 1.9 to 3.3.  
      Accordingly, an object of the present invention is to provide a new and improved dummy plate for processing of substrates.  
      Another object of the present invention is to provide a new and improved dummy plate which facilitates enhanced thickness uniformity in film thickness among all regions on a substrate during CVD processes.  
      Still another object of the present invention is to provide a new and improved dummy plate which increases substrate throughput during semiconductor processing.  
      Yet another object of the present invention is to provide a new and improved dummy plate which promotes uniformity in process gas distribution among all regions on a substrate positioned in relatively close proximity to an exhaust gas outlet in a semiconductor processing furnace or chamber.  
      A still further object of the present invention is to provide a dummy plate which has a truncated configuration for the uniform distribution of process gases in a LPCVD chamber.  
      Yet another object of the present invention is to provide a method of promoting a substantially uniform flow of process gases along all regions on the surface of a substrate to facilitate formation of a film having a substantially uniform thickness among the regions on the substrate.  
      A still further object of the present invention is to provide a truncated dummy plate which is capable of increasing the batch size or number of substrates in a semiconductor fabrication process.  
     SUMMARY OF THE INVENTION  
      In accordance with these and other objects and advantages, the present invention is generally directed to a truncated dummy plate which is particularly suitable for promoting substantially uniform flow of process gases among all regions on the surface of a substrate to facilitate deposition of a film having uniform thickness on the substrate. The truncated dummy plate has a generally circular shape with a flat edge provided in the curved edge of the dummy plate. At least two, and preferably, about three or four of the dummy plates are positioned in the sites on a wafer boat which are in relatively close proximity to a gas outlet in a process furnace typically during a LPCVD process carried out in the furnace. The flat or truncated edges of the dummy plates are disposed on the gas inlet side of the process chamber, with the round edges of the dummy plates disposed on the gas outlet side of the process chamber. The truncated shape of the dummy plates promotes a more uniform flow of the process gases over the surfaces of substrates positioned in the wafer boat in proximity to the gas outlet, resulting in deposition of material films having a substantially uniform thickness among all regions on the surfaces of the substrates.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  is a sectional view of a typical conventional LPCVD process furnace suitable for implementation of the present invention;  
       FIG. 2  is a front view of a typical conventional wafer boat loaded with a batch of multiple substrates and six conventional circular dummy plates;  
       FIG. 3  is a cross-sectional view, taken along section lines  3 - 3  in  FIG. 1 , of a conventional process furnace, illustrating use of the process furnace in conjunction with conventionally-shaped dummy plates;  
       FIG. 4  is a top view of a substrate deposited with a material film of variable thickness resulting from use of the conventionally-shaped dummy plates in an LPCVD furnace;  
       FIG. 5  is a top view of an illustrative embodiment of a truncated dummy plate of the present invention;  
       FIG. 5A  is a cross-sectional view, taken along section lines  5 A- 5 A in  FIG. 5 , of a truncated dummy plate of the present invention;  
       FIG. 6  is a front view of a conventional wafer boat shown holding multiple substrates and three truncated dummy plates of the present invention;  
       FIG. 7  is a sectional view of a typical conventional LPCVD process furnace in implementation of the present invention;  
       FIG. 8  is a cross-sectional view, taken along section lines  8 - 8  in  FIG. 7 , of a conventional process furnace, illustrating use of the process furnace in conjunction with the truncated dummy plates of the present invention; and  
       FIG. 9  is a cross-sectional view of a substrate after being subjected to a LPCVD process in implementation of the present invention.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention includes a truncated dummy plate which promotes substantially uniform flow of process gases among all regions on the surface of a substrate to facilitate deposition of a film having uniform thickness on the substrate. At least two, and preferably, about three of the dummy plates are positioned in the sites on a wafer boat which are in relatively close proximity to a gas outlet in a process chamber, typically a LPCVD furnace, during a LPCVD process carried out in the furnace. The truncated shape of the dummy plates promotes a more uniform flow of the process gases over the surfaces of substrates positioned in the wafer boat in proximity to the gas outlet, resulting in deposition of material films having a substantially uniform thickness among all regions on the surfaces of the substrates.  
      Referring to  FIG. 5 , a truncated dummy plate  61  of the present invention includes a plate body  68  having a curved edge  62  and a flat edge  63  which interrupts the curvature of the curved edge  62 . The plate body  68  includes a top surface  64  and a bottom surface  65 , as shown in  FIG. 5A . As shown in  FIG. 5 , in a preferred embodiment the maximum distance “A” between the curved edge  62  and the flat edge  63  is typically about 15 cm, whereas the maximum width “B” of the dummy plate  61 , as shown in  FIG. 5A , is typically about 20 cm. However, it is understood that the dummy plate  61  may have dimensions “A” and “B” which are larger or smaller than those described above, depending on the desired application of the dummy plates  61 . In the preferred embodiment of  FIG. 5 , the dummy plate  61  may be formed by fabrication of a circular plate and removing a portion  66  from the plate to define the flat edge  63 , in which case the removed portion  66  has a maximum width “C” of about 5 cm.  
      Referring next to  FIGS. 6-8 , in application of the dummy plates  61 , a wafer boat  54  is used to support multiple substrates  59  in a process chamber such as an LPCVD furnace  40  as an LPCVD process is carried out in the furnace  40 . The wafer boat  54  may be a conventional SiC (silicon carbide) wafer boat and typically includes a base plate  55  and a top plate  56  which are spanned by multiple vertical support rods  57 . The substrates  59  are supported in vertically-spaced relationship by slots (not shown) in the support rods  57 . However, it is understood that the wafer boat may have alternative designs as is known in the art.  
      The LPCVD furnace  40  typically includes a base  42  on which is removably mounted a quartz tube  44 . The interior of the quartz tube  44  defines a reaction chamber  46  for processing of as many as 150 substrates  59  held by the wafer boat  54  as the wafer boat  54  is supported on the base  42  and contained in the reaction chamber  46 . A gas inlet tube  48  may extend downwardly through the quartz tube  44  into the reaction chamber  46 , and a central gas inlet opening  50  may be provided in the top center of the quartz tube  44 , for distribution of reaction gases  67  into the reaction chamber  46 . A gas outlet  52  is provided typically in the base  42  for distributing the reaction gases  67  from the reaction chamber  46 . The gas outlet  52  is typically located on the opposite side of the wafer boat  54  with respect to the gas inlet tube  48  to facilitate a more uniform flow of the reaction gases  67  throughout the reaction chamber  46 . It is understood that the foregoing description of the LPCVD furnace  40  is illustrative only and is not intended to limit the scope of the invention. The present invention is equally suitable for use with substrate processing chambers or furnaces having designs which depart from that heretofore described with respect to the LPCVD furnace  40 .  
      In accordance with the present invention, at least two, and preferably, three of the truncated dummy plates  61  of the present invention are positioned typically in the bottom portion, or lower sites “L”, as shown in  FIG. 6 , of the wafer boat  54 , which bottom portion is usually the portion of the wafer boat  54  that is in closest proximity to the gas outlet  52 , as shown in  FIG. 7 . The dummy plates  61  are inserted in the slots (not shown) provided in the support rods  57  of the wafer boat  54 . As shown in  FIG. 8 , the flat or truncated edge  63  of each truncated dummy plate  61  faces the gas inlet tube  48  side of the reaction chamber  46 , whereas the curved edge  62  of each truncated dummy plate  61  faces the gas outlet  52  side of the reaction chamber  46 . Multiple substrates  59  to be processed in the LPCVD furnace  40  are loaded in the upper sites “U”, the center sites “C” and most of the lower sites “L” of the wafer boat  54 . After the wafer boat  54 , loaded with the substrates  59  and the truncated dummy plates  61 , is placed in the reaction chamber  46  of the furnace  40 , the LPCVD process is carried out typically using standard LPCVD process parameters by introducing reaction gases  67  into the reaction chamber  46  through the gas inlet tube  48  and the gas inlet opening  50 .  
      During the LPCVD process carried out in the reaction chamber  46 , the reaction gases  67  flow in a substantially uniform path over the surfaces of the substrates  59  in the upper sites “U” and the substrates  59  in the center sites “C” of the wafer boat  54 . Consequently, the substrates  59  in the upper sites “U” and in the center sites “C” are substantially uniformly coated with deposition material, which forms films of uniform thickness on the surfaces of those substrates  59 . As the reaction gases  67  traverse the reaction chamber  46  to the gas outlet  52 , a substantial volume of the reaction gases  67  tends to bypass the surfaces of the substrates  59  in the lower sites “L”. However, the truncated shape of the truncated dummy plates  61  facilitates flow of a greater volume of the reaction gases  59  across the surface of the substrates  61  positioned in the lower sites “L” of the wafer boat  54 , and this promotes a substantially uniform flow of the reaction gases  67  across the surfaces of those substrates  61 , resulting in deposition of a material film  60  which is substantially uniform in thickness across the entire surface of each substrate  61 , as shown in  FIG. 9 . Consequently, prior to the LPCVD process, the maximum batch number of substrates  59  may be loaded into the wafer boat  54 , wherein each batch may contain as many as  150  of the substrates  54 . This substantially enhances throughput and processing efficiency of the substrates  54 .  
      Referring again to  FIG. 6 , it is understood that the truncated dummy plates  61  may alternatively be positioned in the upper sites “U” of the wafer boat  54 , as shown in phantom, or alternatively, in the center sites “C” of the wafer boat  54 , in the event that the design of the particular process chamber or furnace being used for substrate processing causes a higher volume of flow of the process gases on one side of the reaction chamber than on the other side of the reaction chamber in these areas of the chamber or furnace.  
      While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.