Abstract:
The present invention generally relates to methods and apparatus for processing substrates. Embodiments of the invention include apparatuses for processing a substrate comprising a ceramic reflector plate, which may be optically transparent. The reflector plate may include a reflective coating and be part of a reflector plate assembly in which the reflector plate is assembled to a baseplate.

Description:
CROSS REFERENCE 
     This application claims the benefit of priority of application Ser. No. 61/371,792, filed on Aug. 9, 2010. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates generally to the field of semiconductor processing. More specifically, the invention pertains to a reflector plate used in a semiconductor thermal processing chamber, such as a rapid thermal processing chamber. 
     2. Background 
     Rapid thermal processing (RTP) is a process for annealing substrates during semiconductor fabrication. During this process, thermal radiation is used to rapidly heat a substrate in a controlled environment to a maximum temperature of over nine hundred degrees above room temperature. This maximum temperature is maintained for less than one second to several minutes, depending on the process. The substrate is then cooled to room temperature for further processing. The semiconductor fabrication process has several applications of RTP. Such applications include thermal oxidation (a substrate is heated in oxygen or a combination of oxygen and hydrogen which causes the silicon substrate to oxidize to form silicon dioxide); high temperature soak anneal (different gas mixtures such as nitrogen, ammonia, or oxygen are used); low temperature soak anneal (typically to anneal wafers deposited with metals); and spike anneal (primarily used in processes where the substrate needs to be exposed to high temperatures for a very short time). During a spike anneal, the substrate is rapidly heated to a maximum temperature sufficient to activate a dopant and cooled rapidly to end the activation process prior to substantial diffusion of the dopant. 
     High intensity tungsten or halogen lamps are used as the source of thermal radiation. A reflector plate (as shown in  FIG. 2  and described further below) aids in maintaining temperature uniformity as the reflector plate reflects heat radiation emitted from the wafer back toward the wafer. 
       FIG. 1  shows a side cross-section of an existing reflector plate  27 . As shown in  FIG. 1 , radiation pyrometer light pipes  42  protrude through an opening in the reflector plate  27  so that they have a clear view of the wafer, as best seen in  FIG. 2 . Existing reflector plates are made from aluminum. The pyrometer light pipes  42  are flush with the aluminum reflector plate  27  face on which there is a reflective coating (not shown) and which faces the wafer. Because the light pipes and reflective coating are exposed to the chamber environment, wafer byproduct material can deposit on both the light pipes and/or the reflective coating, which causes a drift in temperature measurement. This drift can occur quickly and drastically or in small increments over a long period of time. Furthermore, the reflective coating applied to the aluminum reflector plate is complex and difficult to manufacture (costly), has a maximum operating temperature limit of 150° C., and has been prone to peeling under certain process conditions. A quartz plate  60  is placed between the wafer and the reflector plate  27 , and the quartz plate  48  rests upon standoffs  64  affixed to the reflector plate  27 , leaving a gap  62 . The quartz plate  48  helps mitigate some of the problems mentioned above. However, there is still a need to minimize the issues discussed above with respect to existing reflector plates. 
     SUMMARY 
     Accordingly, one or more embodiments of the invention are directed to apparatus for processing a substrate having a front side and a back side. The apparatus comprises a process area within a chamber defined on one side by a window adjacent a radiant heat source located outside the process area and a reflector plate disposed opposite the heat source, the reflector plate comprising a body made from ceramic material and a reflective coating on a side of the reflector plate, and a plurality of apertures extending through at least the reflective coating. In one embodiment, the ceramic material comprises an optically transparent ceramic. In one embodiment, the optically transparent material is selected from alumina, silicon carbide, quartz, and sapphire. According to an embodiment, the side of the reflector plate has a first surface closest to the radiant heat source and a second surface furthest from the radiant heat source, the second surface having the coating thereon. 
     In one or more embodiments, the apertures are spaced to accommodate pyrometer probes. In one or more embodiments, the apertures extend only through the reflective coating. In one or more embodiments, the reflector plate is mounted to a baseplate to provide a reflector plate assembly. In one embodiment, the reflector plate and baseplate are spaced apart by less than about 5 mm. In other embodiments, the reflector plate and baseplate are in direct contact and are not spaced apart. In one or more embodiments, the reflector plate assembly includes standoffs to separate the reflector plate and baseplate in a spaced apart relationship. 
     In one embodiment, the reflective coating includes a plurality of dielectric layers. In one embodiment, the ceramic material includes a dopant to increase amount of heat absorbed by the reflector plate. In one or more embodiments, the dopant is selected from rare earth materials, OH and combinations thereof. In one or more embodiments, the baseplate includes a plurality of openings aligned with the apertures in the reflector plate. 
     In a second aspect embodiments of the invention pertain to a reflector plate assembly apparatus for a rapid thermal processing chamber comprising a baseplate having a plurality of openings therethrough to accommodate a pyrometer probe; and a reflector plate comprising a body made from ceramic material and a reflective coating on a side of the reflector plate, and a plurality of apertures extending through at least the reflective coating aligned with the openings through the baseplate, wherein the reflector plate is assembled to the baseplate such that the openings in the baseplate and the apertures in the reflector plate are aligned. In one embodiment, the baseplate includes a plurality of standoffs to maintain the reflector plate and baseplate in a spaced apart relationship. In one embodiment, the side with the coating faces the baseplate. In one or more embodiments, the ceramic material is optically transparent. In one or more embodiments, the ceramic material is selected from alumina, silicon carbide, quartz, and sapphire. In one or more embodiments, the ceramic material includes a dopant to increase amount of heat absorbed by the reflector plate, wherein the dopant is selected from rare earth materials, OH, and combinations thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a side-cross sectional view of a conventional rapid thermal processing chamber reflector plate assembly; 
         FIG. 2  illustrates a rapid thermal processing chamber; 
         FIG. 3  is perspective view of a reflector plate according to an embodiment of the invention; 
         FIG. 4  is a side cross-sectional view of a reflector plate assembly according to an embodiment of the invention; 
         FIG. 5  is a side cross-sectional view of a reflector plate assembly according to an embodiment of the invention; 
         FIG. 6  is a side cross-sectional view of a reflector plate assembly according to an embodiment of the invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways. 
       FIG. 2  schematically represents a rapid thermal processing chamber  10  that can include a reflector plate apparatus in accordance with embodiments of the invention. Peuse et al. describe further details of this type of reactor and its instrumentation in U.S. Pat. Nos. 5,848,842 and 6,179,466. A substrate or wafer  12 , for example, a semiconductor wafer such as a silicon wafer to be thermally processed is passed through the valve or access port  13  into the process area  18  of the chamber  10 . The wafer  12  is supported on its periphery by a substrate support shown in this embodiment as an annular edge ring  14 , which may have an annular sloping shelf  15  contacting the corner of the wafer  12 . Ballance et al. more completely describe the edge ring and its support function in U.S. Pat. No. 6,395,363. The wafer is oriented such that processed features  16  already formed in a front surface of the wafer  12  face upwardly, referenced to the downward gravitational field, toward a process area  18  defined on its upper side by a transparent quartz window  20 . The transparent quartz window  20  is located a substantial distance from the wafer  12  such that window has minimal effect on cooling of the substrate during processing. Typically, the distance between the wafer  12  and the window  20  is on the order of 20 mm. Contrary to the schematic illustration, the features  16  for the most part do not project substantial distances beyond the surface of the wafer  12  but constitute patterning within and near the plane of the surface. Lift pins  22  may be raised and lowered to support the back side of the wafer  12  when the wafer is handed between a paddle or robot blade (not shown) bringing the wafer into the chamber and onto the edge ring  14 . A radiant heating apparatus  24  is positioned above the window  20  to direct radiant energy toward the wafer  12  and thus to heat the wafer. In the reactor or processing chamber  10 , the radiant heating apparatus includes a large number, 409 being an exemplary number, of high-intensity tungsten-halogen lamps  26  positioned in respective reflective tubes  27  arranged in a hexagonal close-packed array above the window  20 . 
     It is desirable to control the temperature across the wafer  12  to a closely defined temperature uniform across the wafer  12 . In this regard, a reflector plate  28  extending parallel to and over an area greater than the wafer  12  and facing the back side of the wafer  12 . The reflector plate  28  efficiently reflects heat radiation emitted from the wafer  12  back toward the wafer  12 . The spacing between the wafer  12  and the reflector plate  28  can be within the range of 3 to 9 mm, and the aspect ratio of the width to the thickness of the cavity is advantageously greater than 20. The reflector plate  28 , which, as noted above, is made from aluminum and includes a gold coating or multi-layer dielectric interference mirror, effectively forms a black-body cavity at the back of the wafer  12  that tends to distribute heat from warmer portions of the wafer  12  to cooler portions. In other embodiments, for example, as disclosed in U.S. Pat. Nos. 6,839,507 and 7,041,931, the reflector plate  28  may have a more irregular surface or have a black or other colored surface. The reflector plate  28  can be supported on a water-cooled base  53  made of metal to heat sink excess radiation from the wafer, especially during cool down. Accordingly, the process area  18  of the processing chamber has at least two substantially parallel walls, of which a first is a window  20 , made of a material being transparent to radiation such as quartz, and a second wall/base  53  substantially parallel to the first wall which is made of metal and is significantly not transparent. 
     The lamps  26  are divided into zones arranged generally ring-like about the central axis  34 . Control circuitry varies the voltage delivered to the lamps  26  in the different zones to thereby tailor the radial distribution of radiant energy. Dynamic control of the zoned heating is affected by, one or a plurality of pyrometers  40  coupled through one or more optical light pipes  42  positioned to face the back side of the wafer  12  through apertures in the reflector  28  to measure the temperature across a radius of the rotating wafer  12 . The light pipes  42  may be formed of various structures including sapphire, metal, and silica fiber. A computerized controller  44  receives the outputs of the pyrometers  40  and accordingly controls the voltages supplied to the different rings of lamps  26  to thereby dynamically control the radiant heating intensity and pattern during the processing. Pyrometers generally measure light intensity in a narrow wavelength bandwidth of, for example, 40 nm in a range between about 700 to 1000 nm. The controller  44  or other instrumentation converts the light intensity to a temperature through the well known Planck distribution of the spectral distribution of light intensity radiating from a black-body held at that temperature. 
     The chamber shown in  FIG. 2  allows the wafer  12  support to be easily levitated at different vertical positions inside the chamber to permit control of the substrate&#39;s thermal exposure. It will be understood that the configuration shown in  FIG. 2  is not intended to be limiting. In particular, the invention is not limited to configurations in which the heat source or lamps are directed at one side or surface of the substrate and the pyrometers are directed at the opposite side of the wafer. 
     As noted above, wafer temperature in the process area of a processing chamber is commonly measured by radiation pyrometry. While radiation pyrometry can be highly accurate, radiation which is within the radiation pyrometer bandwidth and which originates from the heating source may interfere with the interpretation of the pyrometer signal if this radiation is detected by the pyrometer. In Applied Materials&#39; RTP systems this minimized by the process kit and by the wafer itself. The process kit couples the wafer with the rotation system. It may include a support cylinder which is shown as  30  in  FIG. 2 . It may also include a support ring which is not shown in the Figures but it may be used in certain processing chamber configurations). Such a support ring is basically an auxiliary edge ring which supports the edge ring, which is shown as  14  in  FIG. 2 . 
     The array of lamps  26  is sometimes referred to as the lamphead. However, other radiant heating apparatus may be substituted. Generally, these involve resistive heating to quickly ramp up the temperature of the radiant source. Examples of suitable lamps include mercury vapor lamps having an envelope of glass or silica surrounding a filament and flash lamps which comprise an envelope of glass or silica surrounding a gas such as xenon, which provides a heat source when the gas is energized. As used herein, the term lamp is intended to cover lamps including an envelope that surrounds a heat source. The “heat source” of a lamp refers to a material or element that can increase the temperature of the substrate, for example, a filament or gas that can be energized. 
     As used herein, rapid thermal processing or RTP refers an apparatus or a process capable of uniformly heating a wafer at rates of about 50° C./second and higher, for example, at rates of 100° to 150° C./second, and 200° to 400° C./second. Typical ramp-down (cooling) rates in RTP chambers are in the range of 80-150° C./second. Some processes performed in RTP chambers require variations in temperature across the substrate of less than a few degrees Celsius. Thus, an RTP chamber must include a lamp or other suitable heating system and heating system control capable of heating at rate of up to 100° to 150° C./second, and 200° to 400° C./second distinguishing rapid thermal processing chambers from other types of thermal chambers that do not have a heating system and heating control system capable of rapidly heating at these rates. In accordance with a further aspect of the present invention embodiments of the present invention may be applied also to flash annealing. As used herein flash annealing refers to annealing a sample in less than 5 seconds, specifically, less than 1 second, and in some embodiments, milliseconds. 
     One way of improving temperature uniformity includes supporting the edge ring  14  on a rotatable cylinder  30  that is magnetically coupled to a rotatable flange  32  positioned outside the chamber. A rotor (not shown) rotates the flange  32  and hence rotates the wafer about its center  34 , which is also the centerline of the generally symmetric chamber. 
     Another way of improving the uniformity divides the lamps  26  into zones arranged generally ring-like about the central axis  34 . Control circuitry varies the voltage delivered to the lamps  26  in the different zones to thereby tailor the radial distribution of radiant energy. Dynamic control of the zoned heating is affected by, one or a plurality of pyrometers  40  coupled through one or more optical light pipes  42  positioned to face the back side of the wafer  12  through apertures in the reflector  28  to measure the temperature across a radius of the rotating wafer  12 . The light pipes  42  may be formed of various structures including sapphire, metal, and silica fiber. A computerized controller  44  receives the outputs of the pyrometers  40  and accordingly controls the voltages supplied to the different rings of lamps  26  to thereby dynamically control the radiant heating intensity and pattern during the processing. Pyrometers generally measure light intensity in a narrow wavelength bandwidth of, for example, 40 nm in a range between about 700 to 1000 nm. The controller  44  or other instrumentation converts the light intensity to a temperature through the well known Planck distribution of the spectral distribution of light intensity radiating from a black-body held at that temperature. 
     The chamber shown in  FIG. 2  allows the wafer  12  support to be easily levitated at different vertical positions inside the chamber to permit control of the substrate&#39;s thermal exposure. It will be understood that the configuration shown in  FIG. 1  is not intended to be limiting. In particular, the invention is not limited to configurations in which the heat source or lamps are directed at one side or surface of the substrate and the pyrometers are directed at the opposite side of the wafer. 
     As noted above, wafer temperature in the process area of a processing chamber is commonly measured by radiation pyrometry. While radiation pyrometry can be highly accurate, radiation which is within the radiation pyrometer bandwidth and which originates from the heating source may interfere with the interpretation of the pyrometer signal if this radiation is detected by the pyrometer. In Applied Materials&#39; RTP systems this minimized by the process kit and by the wafer itself. The process kit couples the wafer with the rotation system. It may include a support cylinder which is shown as  30  in  FIG. 1 . It may also include a support ring which is not shown in the Figures but it may be used in certain processing chamber configurations). Such a support ring is basically an auxiliary edge ring which supports the edge ring, which is shown as  14  in  FIG. 2 . 
     According to a first aspect of the invention, an improved reflector plate  28  and reflector plate assembly is provided.  FIG. 3  shows a perspective view of a reflector plate  28  of the type used in the apparatus shown in  FIG. 2 , including openings to allow lift pins  22  to protrude through the reflector plate  28  top side  29 . 
     In one or more embodiments, the reflector plate  28  body is made from an optically transparent material such as quartz, sapphire or transparent YAG. The reflector plate also includes a plurality of light pipe apertures  31  for accommodating the pyrometer light pipes  42  as shown in  FIG. 1 . 
     A reflector plate assembly  25  is shown in  FIG. 4  according to an embodiment of the invention. It is noted that the openings to accommodate the lift pins shown in  FIG. 3  are not shown in  FIG. 4 . The reflector plate assembly comprises a baseplate  19 , which in one embodiment is made from a suitable metal such as stainless steel. The baseplate  19  can be affixed to the chamber bottom, for example, by bolts, screws or other suitable fasteners to the chamber base  53  shown in  FIG. 2 . The baseplate  19  has openings through which pyrometer lightpipes  42  can pass through. The openings in the baseplate are aligned with the apertures in the reflector plate to accommodate the pyrometer lightpipes. The reflector plate assembly  25  further comprises a reflector plate  28 , the body of which is made from a ceramic, including but not limited alumina, silicon carbide, quartz, sapphire. This ceramic may or may not be optically transparent depending on the selected embodiment. With through-holes allowing the pyrometer light pipes to pass through the ceramic, the coating can be deposited on the first surface and the ceramic does not need to be optically transparent. According to one embodiment, if the coating is deposited on the second surface, then the ceramic should be optically transparent. A first surface reflector will results in a cooler reflector plate, which may or may not be desirable. A second surface reflector will result in a warmer reflector plate, and the coating is also better shielded from any process by products becoming embedded or deposited on top of the coating. 
     The reflector plate  28  has a diameter similar to the baseplate  19  diameter. The reflector plate  28  is mounted above the baseplate  19 . One suitable way of mounting the reflector plate  28  is to place the reflector plate  28  on standoffs  33  such that the reflector plate  28  and the baseplate  19  are spaced apart to provide a gap  44 , of less than about 5 mm. In one or more embodiments, the reflector plate  28  and the baseplate  19  are spaced apart by less than about 1 mm, and in other embodiments, there is no spacing between the reflector plate  28  and the baseplate  19 . In the configuration shown, the standoffs  33  are positioned and contained by the baseplate but locate and support the reflector plate  28 . The reflector plate  28  may further include a reflective coating  35  applied to either side or both sides of the reflector  28 . Accordingly, the reflector plate assembly  25  includes a reflector plate  28 , which is resting on standoffs  33 , which are constrained by the baseplate  19 , through which the pyrometer light pipes  42  pass, which are bolted to the chamber bottom  53 . In some embodiments, lift pins  46  pass through the standoffs  33 . By making the body of the reflector plate  28  of an optically transparent ceramic such as quartz or sapphire the reflective coating can be placed on the backside  37  of the reflector plate (away from the wafer  12 ). In this orientation, the coated backside  37  is not exposed directly to wafer processing byproducts, and therefore, the coating is less likely to experience peeling. In addition, the pyrometer probes or light pipes  42  which are shown as extending into the coating, but by may flush with the coating  35  are also not directly exposed to the byproducts. As such, the reflector plate, which is optically transparent, will absorb more radiative energy and will get hotter. 
     In the embodiment shown, there are regions on the backside  37  of the reflector plate  28  where no coating is applied, so that the pyrometers still have clear line of sight through the optically transparent reflector plate  28  to the wafer. These uncoated regions may be flush with or within 1 mm of the coated surface. In the embodiment shown, the uncoated regions form blind holes or blind openings. In alternative embodiments, the pyrometer openings may be in the form of apertures  41 , or through holes  148  as shown in  FIG. 5 . In one or more embodiments shown in  FIG. 6 , blind holes  248  are provided so that the pyrometer light pipes  42  can be designed to be nominally flush with the reflective coating, but allowing for vertical variation due to tolerance stack-ups in the assembly. Furthermore, blind holes offer a convenient way to accurately locate masks that can be used during the coating process to ensure those regions remain uncoated. In one or more embodiments, voids in the coating can be created either by masking before coating deposition or coating removal processes such as laser ablation which selectively removes coating in the areas only where the pyrometers need line of sight into the processing environment. In one or more embodiments, coating can be applied to the top side  29  or first surface, and openings or apertures may partially or fully extend through the coating on the top side  29 . 
     In the configuration of the reflector plate assembly  25  shown with the coating  35  on the backside  35  of the reflector plate  28 , the rate of wafer byproduct deposition on the reflector plate  28  is decreased, thus extending what may be referred to as the Mean Wafer Between Clean (MWBC). The MWBC refers to the mean number of wafers processed before cleaning must be performed on the reflector plate  28  to reduce or remove byproduct deposition the on the reflector plate  28 . The optically transparent reflector plate  28  stays hotter with the reflective coating facing the baseplate surface (i.e. the backside  37  or second surface) because light must past through the optically transparent reflector plate body to the reflective coating and then back through the optically transparent reflector plate body on its way out. The amount of heat that is retained during this operation can be tuned with dopants (such as rare earth elements) or with higher OH concentrations in the reflective coating  35 . If the optically transparent reflector plate  28  has a higher OH content, it will absorb more radiated energy. 
     In an alternative embodiment, the body of the reflector plate  28  is made from a non-transparent ceramic. In this embodiment, if the reflective coating  35  is placed on the top side  29  of the reflector plate  28 , the light would be reflected off the coating. This may aid in helping to allow the reflector plate  28  to remain cooler by avoiding uneven heating the wafer early in the heating process. A ceramic such as quartz or sapphire has a low coefficient of thermal expansion over a large temperature range (for example in the range of about 22° C. to about 800° C.). Accordingly, reflective coatings can be more easily applied compared to aluminum reflector plates. The coatings tend to adhere more readily to ceramic materials such as quartz than to aluminum or stainless steel. According to one or more embodiments, the reflective coating has a maximum operating temperatures around 400° C. (compared to about 200° C. for a reflective coating on an aluminum reflector plate). Again, because of the higher operating temperature capability, wafer byproduct deposition can be reduced or nearly eliminated. 
     The reflective coating  35  can be any variety of materials. Processes and providers of services to provide windows with thin layers of reflective layer for reflection in specified range of wavelengths are known. One provider of such coating services is for instance JDS Uniphase. Materials that can be used in a reflective coating  35  may be alternating layers of, in general, any combination of high index and low index dielectric materials which are substantially transparent to most of the radiation emitted from the heating source, such as titania-silica or tantala-silica. In one embodiment, the reflective layer is made up of SiO 2  and Ta 2 O 5  layers. In another embodiment, the reflective layer is made up of SiO 2  and TiO 2 . In a specific embodiment, the outermost layer comprises SiO 2 . 
     In one embodiment, the layers may include multiple (thin) layers of optically transparent materials with different refractive indices, which are sometimes referred to as dielectric mirrors. A multilayer dielectric mirror may work as a reflective filter, wherein radiation is reflected. Radiation may be reflected selectively dependent among other elements on the wavelength of the radiation, the angle of incidence of the radiation, properties of the applied dielectric material including the refractive index of the applied dielectric material, the thickness of each layer, the number of layers a different thickness, and arrangement of layers. 
     Reference throughout this specification to “one embodiment,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. For example, while the present invention has been described with respect to a particular type of heating lamp, other variants are possible. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.