Patent Publication Number: US-2007113783-A1

Title: Band shield for substrate processing chamber

Description:
BACKGROUND  
      The present invention relates to a band shield for a substrate processing chamber.  
      In the fabrication of electronic circuits and displays, semiconductor, dielectric, and electrically conductors are formed on a substrate, such as for example, a semiconductor wafer, ceramic or glass substrate. The materials are formed for example, by chemical vapor deposition (CVD), physical vapor deposition (PVD), ion implantation, oxidation or nitridation processes. Thereafter, the deposited substrate materials are etched to form features such as gates, vias, contact holes and interconnect lines. In a typical process, the substrate is placed on a support in a process zone of a chamber and exposed to heat or gas plasma to deposit or etch material on the substrate. The chamber has enclosing walls and is pumped down with pumps, such as roughing and turbo molecular pumps.  
      A band shield  20 , as illustrated in  FIG. 1 , can also be used to protect the walls from erosion and also serve to receive process deposits from the process being conducted in the chamber. The band shield  20  is typically made from a ceramic material and is shaped to at least partially conform to the chamber walls. An exemplary prior art band shield  20  comprises a cylindrical sidewall  22  with a circumferential top flange  24  extending radially outward from the top end  26  of the sidewall  22  and a circumferential bottom flange  28  extending radially outward from the bottom end  30  of the sidewall  22 . The top flange  24  couples to an outer shield (not shown) in the substrate processing chamber and the bottom flange  28  rests on a ledge. The band shield  20  includes a frontside  32  with a slit  34  and a backside  36  opposing the frontside. The top end  26  of the wall  22  has a first sill  38  that extends around the frontside  32  of the wall and a second sill  40  that extend around the backside  35  of the wall  22 . The band shield  20  serves as a shield to receive process deposits, and thus, reduce the amount of process deposits formed on chamber walls.  
      However, in use, the conventional shield  20  has to be removed from the chamber and cleaned or replaced quite often. As process deposits accumulate on the sidewalls  22  and flanges  24 ,  28  of the shield  20 , after a period of time, it has to be removed from the chamber and cleaned or replaced. For example, in the deposition of aluminum by CVD, the shield  20  has to be typically replaced or cleaned after processing of 3000 to 5000 substrates. It is desirable to have an shield  20  which can last for a greater number of process cycles before needing to be cleaned or replaced, to reduce the frequency of preventive maintenance cycles which are needed to operate the chamber.  
      Another problem of the shield  20  is that it restricts the pumping flow efficiency of the process chamber in which it is used. The chamber (not shown) typically has a pumping channel or port around the substrate which connected via a throttle valve to the external roughing and turbomolecular pumps. However, because the band shield  20  is positioned in the gas flow path between the substrate and the pumping channel or port, the flanges  24 ,  28  often block or otherwise impede the flow of gas out of the chamber and into the pumping channel. For example, when using the shield  20 , the pressure in the chamber typically reaches about 5×10 −5  Torr after about 10 seconds of pump down. It is desirable to have a band shield that allows more efficient pump down to reach lower chamber pressures in a faster time.  
      Thus it is desirable to have a band shield capable of limiting formation of process deposits on the walls of a substrate processing chamber. It is also desirable for the shield to be used for a greater number of process cycles without requiring replacement or cleaning. It is further desirable for the shield not to excessively impede the flow of gas through the pumping channel of the chamber.  
     SUMMARY  
      A band shield for a substrate processing chamber has a cylindrical wall with a slit therethrough. A flange extends radially outward from a bottom end of the cylindrical wall. A casing extends radially outwardly from a top end of the cylindrical wall and wraps around the slit to join to the flange. At least a portion of the surfaces of the cylindrical wall, flange, and casing have a surface roughness average of less than about 16 micro inch, whereby less deposition occurs on these surfaces when they are exposed to the process environment in the substrate processing chamber.  
      A method of forming the band shield comprises forming a cylinder of a ceramic material and machining the cylinder to form the cylindrical wall with the slit, the flange extending radially outward from the bottom end of the cylindrical wall, and the casing extending radially outwardly from the top end of the cylindrical wall. The surfaces of the cylindrical wall, flange, and casing are polished to have the surface roughness average of less than about 16 micro inch.  
    
    
     DRAWINGS  
      These features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, which illustrate examples of the invention. However, it is to be understood that each of the features can be used in the invention in general, not merely in the context of the particular drawings, and the invention includes any combination of these features, where:  
       FIG. 1  (PRIOR ART) is a perspective view of a prior art band shield for a substrate processing chamber;  
       FIG. 2A  is a perspective view of an exemplary embodiment of a band shield according to the present invention;  
       FIG. 2B  is a top plan view of the band shield of  FIG. 2A ;  
       FIG. 2C  is a side elevation view of the band shield of  FIG. 2A ;  
       FIG. 2D  is a front elevation view of the band shield of  FIG. 2A ;  
       FIG. 3  is a schematic sectional view of an exemplary embodiment of a processing apparatus comprising a chamber having the band shield; and  
       FIG. 4  is a schematic partial sectional view of an exemplary embodiment of a CVD plasma process chamber containing the band shield.  
    
    
     DESCRIPTION  
      An exemplary embodiment of a band shield  50  suitable for a substrate processing chamber is illustrated in  FIGS. 2A  to  2 D. The band shield  50  comprises a cylindrical wall  52  that is shaped and sized to surround the substrate held in the chamber. The cylindrical wall  52  is typically a right cylindrical shape that is substantially vertical or perpendicular to the plane of the substrate processed in the chamber and with a central axis of symmetry  53 . However, the cylindrical wall  52  can also have a rectangular or square shaped cross-section to surround a substrate such as a display panel. While an exemplary version of the band shield  50  is illustrated, other versions that would be apparent to those of ordinary skill in the art are also within the scope of the present invention; thus, the present invention should not be limited to the illustrative embodiments described herein.  
      The cylindrical wall  52  has a midsection with a slit  54  that is typically an elongated oval hole having a diameter sized to pass a substrate, such as a circular semiconductor wafer, though the slit  54 . In use, the slit  54  is positioned adjacent to a wafer loading slit  54  in the outer sidewall of the chamber so that a wafer can be passed from a transfer chamber through the slit  54  to rest on the substrate support in the chamber. For a wafer that is 300 mm in diameter, the width of the slit  54  is sized about 25% larger, for example, from about 360 to about 390 mm. The height of the slit is typically from about 30 to about 40 mm.  
      A flange  56  extends radially outward from a bottom end  58  of the cylindrical wall  52 . The flange  56  is provided to support the band shield  50  in a process chamber. Generally, the flange  56  extends radially outward and substantially perpendicularly to the cylindrical wall  52 . The flange  56  can also have notches  57  to align, secure, or serve as a pass-through in the chamber. Typically, the flange  56  extends around substantially the entire circumference of the cylindrical wall  52 .  
      A casing  60  extends radially outwardly from a top end  62  of the cylindrical wall  52 . The casing  60  wraps around the slit  54  in the midsection of the cylindrical wall  52  and is joined to at least a portion of the flange  56 . The casing  60  is provided to enclose the slit  54  to surround the slit  54  from the surrounding chamber walls. The casing  60  is shaped as an oval frame that extends around the slit  54 . The casing  60  comprises a top sill  62  and curved side walls  64  which are joined to a front frame  66 , and closing the slit  54 .  
      Prior art band shields  20 , as shown in  FIG. 1 , included a second sill  40  that extended around the backside  36  of the cylindrical wall  22 , opposing the frontside  32  with the slit  34 . The second sill  40  was determined to be the cause of obstruction of the exhaust port and pump down system resulting in longer pump-down times for the chamber. Advantageously, the present band shield  50 , as shown in  FIG. 2A , is absent the second sill  40  about the exhaust port, and instead, in the band shield  50 , the cylindrical wall  52  ends in a vertical wall  68 . This provides significantly improved pump-down efficiency because the shield  50  lacks the obstruction of the second sill  40 . Whereas chambers having the conventional shield  20  required  60  seconds to pump down to a vacuum level of 5×10 −5  Torr, chambers that include the present version of the band shield  50  had pump-down times of about 10 seconds, to reach the same pressure. This was an unexpected result and significant improvement of 6 times better pump down efficiency which was surprising and unexpected.  
      At least a portion of the surfaces of the band shield  50 , such as the cylindrical wall  52 , flange  56 , or casing  60 , are exposed to the environment inside the chamber  100 . Exposure to the process environment can include exposure to energized gases, such as plasma, formed in the chamber  100 . The exposed surfaces can be treated to reduce their surface activity, and consequently, reduce process deposition on these surfaces. Such surface treatment can include polishing, sanding, bead blasting and the like. In one version, the exposed surfaces of the band shield  50  are treated to have predefined surface characteristics comprising a low surface roughness average. The surface roughness average is the mean of the absolute values of the displacements from the mean line of the peaks and valleys of the roughness features along the exposed surface. The roughness average can be determined by a profilometer that passes a needle over the surface to generate a trace of the fluctuations of the height of the asperities on the surface, by a scanning electron microscope that uses an electron beam reflected from the surface to generate an image of the surface, or by other surface measurement methods. For example, the band shield  50  can be cut into coupons and measurements made for each of the coupons to determine their surface characteristics. These measurements are then averaged to determine the surface roughness average. To measure properties of the surface such as roughness average, skewness, or other characteristics, the international standard ANSI/ASME B.46.1-1995 specifying appropriate cut-off lengths and evaluation lengths, can be used. In one version, the surface is treated to have a surface roughness average of less than about 50 microinch (˜1.3 micrometers; or even less that about 20 micronch (˜0.5 micrometers), or even less than about 16 microinch (˜0.4 micrometers). These surface roughness average limitations were found to significantly reduce process deposition on the shield surfaces when exposed to the process environment in the substrate processing chamber.  
      The band shield  50  is made from a dielectric material into the desired shape and then surface treated to achieve the desired surface roughness average levels. In one embodiment, the dielectric is made of a material that is permeable to RF energy, such as to be substantially transparent to RF energy from a plasma generator. For example, the dielectric may be a ceramic material, such as quartz or aluminum oxide. The shield  50  can be made by molding ceramic powder into the desired shape, for example, by cold isostatic pressing. In the cold isostatic pressing process, ceramic powder is combined with a liquid binding agent such as the organic binding agent polyvinyl alcohol. The mixture is placed in a rubber bag of an isostatic pressing device and a pressure is uniformly applied on the walls of the bag to compact the mixture to form a ceramic structure having the desired shape. The pressure can be applied, for example, by immersing the flexible container in water, and also by other methods of providing pressure. The molded ceramic preform can be made cylindrical or ring-like, using a hollow tube. The molded ceramic preform can be further shaped by machining the preform to provide the desired size. The shaped ceramic preform is then sintered to form a sintered ceramic. For example, aluminum oxide can be sintered at a temperature of from about 1300° C. to about 1800° C. in a duration of from about 48 to about 96 hours, typically at a pressure of about 1 atm. The sintered ceramic material can be further shaped, for example by at least one of machining, polishing, laser drilling, and other methods, to provide the desired ceramic structure.  
      The surface of the ceramic component is then bead blasted using beads comprising a grit of aluminum oxide having a mesh size selected to suitably grit blast the component surface, such as for example, a grit of aluminum oxide particles having a mesh size of 36. Grit blasting is used to roughen the surface. Thereafter, the surfaces are polished with a diamond pad to have a roughness average of less than about 16 microinches. This is much less than prior art shields which typically roughened to average surface roughness values of from about 150 microinches (˜3 micrometers) to about 450 microinches (˜18 micrometers). Lowering the surface roughness by a factor of greater than  4  was found to significantly and unexpectedly improve the life of the band shield. The resulting ceramic structure is cleaned to remove impurities and loose particles by blowing clean dry air or nitrogen gas across the surface, and then immersing the component in a solution of HNO 3  and/or HCl, then further cleaned by an ultrasonic rinse in distilled water. The component is then heated in an oven to bake out any residues from the cleaning process at a temperature of at least about 100° C.  
      A band shield  50  according to the present invention may be used in a processing apparatus  100  having a chamber  110  that defines a process zone  112  capable of enclosing a substrate  114 , an exemplary embodiment of which is shown in  FIG. 3 . The apparatus  100  can be, for example, a CVD chamber from Applied Materials, Inc., of Santa Clara, Calif. The apparatus  100  can be a stand-alone chamber or can be mounted on a platform, such as the ENDURA or CENTURA platform also from Applied Materials, to be part of a larger processing system that includes multiple chambers. The apparatus  100  can be adapted to deposit a metal and/or metal nitride layer by thermal or plasma enhanced CVD processes, including aluminum, cobalt, copper, molybdenum, niobium, titanium, tantalum, tungsten and some of their nitrides or other compounds.  
      A substrate support  120  in the process zone  112  of the chamber  110  supports a substrate  114  which is inserted into the chamber through a slit  116  by a robot  118  for processing. A gas distributor  126  provides precursor gases to the apparatus  100  which are energized in the chamber  110  to deposit a layer on the substrate  114 . An annular pumping channel  128  around the substrate leads to an exhaust port  130  which is connected to an external exhaust pump  132  to evacuate the gases from the chamber  110 . A throttle valve  134  along the conduit  136  and between the port  130  and the pump  132  is used to control the gas pressure in the chamber  110 . A gas energizer  140  is provided to energize the process gas provided in the chamber  110 . A controller  150  is used to control operation of the chamber components, such as the support  120 , gas distributor  126 , exhaust pumps  132 , and gas energizer  140 . The controller  150  comprises a general purpose computer with a CPU, such as a Pentium™ processor, Intel Corporation, Santa Clara, Calif., with appropriate program code written in a computer readable language, such as Pascal, and compiled appropriately.  
      A more detailed view of an exemplary embodiment of a chamber  110  is provided in  FIG. 4 . The chamber  110  comprises a lid assembly  160  at an upper end of the chamber  110  having a radial axis  164  of symmetry. While the lid assembly  160  shown is substantially disc-shaped the invention is not limited to a particular shape, and parallelograms and other shapes are contemplated. The lid assembly  160  comprises a number of components stacked on top of one another including a lid rim  162 , an isolator ring  170 , and lower plate  174 , and an upper plate  180 . The upper plate  180  which in combination with the lower plate  174  defines channels  182  which allow heating or cooling of the lid assembly  160  when a fluid is passed therethrough, such as deionized water. The upper plate  180  (also known as a temperature control plate, gas-feed cover plate, backing plate or waterbox), is preferably made of aluminum or an aluminum alloy, and rests on the isolator ring  170  and acts to support the lid assembly  160 . The plate  180  further includes a centrally located process gas inlet  184  adapted to deliver process gas to a showerhead  182 . Although not shown, the process gas inlet  184  is coupled to one or more upstream gas sources and/or other gas delivery components, such as gas mixers, to form the gas distributor  126 . A blocker plate  190 , is preferably made of an aluminum alloy, and includes passageways  194  to disperse the gases flowing from the gas inlet  184  to a cavity  193  above a showerhead  196 , from which it passes to the process zone  112  via plurality of holes  199  formed in the showerhead  196 . The gas energizer  140  comprises a power supply  198  coupled to the lid assembly  160  to provide electrical power to the lid assembly to energize the process gas during substrate processing.  
      The band shield  50  surrounds the substrate  114  and is positioned in the chamber so that its flange  56  rests on a vertical inside wall  200  of the chamber  110 . The slit  54  of the band shield  50  is sealable and is sized to allow a robot blade (not shown) to transfer substrates into and out of the apparatus  100 . The band shield  50  is spaced from the substrate support  120 .  
      The annular pumping channel  128  has sides generally defined by the band shield  50 , liners  202 ,  204 , and the isolator ring  170 , with a choke aperture  208  being formed between the isolator ring  170  and the band shield  50 . The isolator ring  170  comprises a monolithic ring-like structure manufactured of ceramic. The liner  202  is at the side of the pumping channel  160  facing the lid rim  162  and conforms to its shape. Both the liners  202  and  204  are maintained at an electrically floating potential during processing of a substrate  114 . The liners  202 ,  204  are preferably made of metal, such as aluminum, and are bead blasted to increase the adhesion of any process deposits formed thereon, to reduce flaking of the deposited material which can otherwise result in contamination of the chamber  110 . Optionally, the band shield  50 , and the liners  202 ,  204  are assembled and sized as a process kit. The band shield  50  is annular having a diameter d 1  and is disposed about the center of support  120 . The liner  202  is also annular in the shape of a band extending axially along the centerline of the support  120  and with a diameter d 2  greater than d 1 . The liner  204  is also annular and forms a ring-shape about the substrate  114 .  
      In use, the support  120  is moved to a lowered receiving position, and the robot  118  with a substrate  114  thereon is moved through the outer slit  116  in the chamber wall, through the annular slit  54  in the band shield  56 , and to a position directly above the support  120 . The substrate is then held by the prongs  210  of the support  120  and the robot  118  is retracted from the apparatus  100 . Process gas is then supplied to the lid assembly  160  by the gas distributor  126 , and the gas enters the process gas inlet  184  to be distributed into the chamber through the passageways  194  in the blocker plate  190  and then through the plurality of holes  199  formed in the showerhead  196  where it is delivered to the process zone  112 .  
      Upon delivery to the process zone  112 , the gas contacts the substrate  114  which is maintained at an elevated temperature corresponding to the disassociation temperature of the process gas, for example, between about 100° C. and about 450° C., or even from about 250° C. to about 450° C. The substrate  114  is heated by the support  120  which has a heater, such as resistance heating elements in the support  120 . The process gas is introduced into the chamber  110  and typically maintained at a pressure of from about 100 mTorr to about 20 Torr. Thereby, a metal and/or metal nitride layer is conformally deposited on the substrate  114  via a CVD process. The disassociation process is a thermal process not usually relying upon plasma excitation of the precursor gas; however, a plasma can also be formed during the deposition process or post deposition to remove impurities by applying power to the RF source  130  to form a plasma from the process gas. Unreacted gas and gaseous byproducts are then exhausted from the apparatus  100  under the influence of the negative pressure provided by a vacuum pump  255 . Accordingly, the gas flows through the choke aperture  208  over the top wall  68  of the shield  50  into the pumping channel  160 .  
      A band shield  50  having a surface finish and shape according to the present invention provides significant advantages over conventional band shields  20 . For example, the band shield  50  reduces the deposition of precursor gases and vapors sputtered material onto the shield surfaces. Thus, the band shield  50  exhibits longer operational lifetimes between cleaning cycles than a conventional shield  20 . The lifetime of the band shield  50  is prolonged because the band shield  50  accumulates much less deposits on its surfaces, and thus, does not have to be removed or cleaned as often as the conventional shield  20 . Furthermore, the present band shield  50  has a vertical wall  68  without a sill extending from the wall as in the prior art shield  20 , provides substantially improved pump-down time over the prior art shield  20 . This occurs because removal of the blockage caused by the sill of prior art designs, increases in chamber pumping conductance and thereby, improves pump down performance.  
      While the present invention has been described in considerable detail with reference to certain preferred versions, many other versions should be apparent to those of ordinary skill in the art. For example, other shapes and configurations of the shield  50  should be apparent to those of ordinary skill in the art. In addition, the shield  50  may be used in other types of chambers, such as for example, PVD, ion implantation, RTD or other chambers. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.