Abstract:
A gas distribution showerhead for uniformly distributing gas and undistorted radiant heat in an RTP chamber. A gas passageway exists within the showerhead and is suitable for connection to a source of gas. The gas passageway terminates in a plurality of gas ports on a surface of the showerhead. A plurality of energy passageways exist in the showerhead. The energy passageways are exclusive of both the gas passageway and the gas ports, and terminate on the surface of the showerhead. The energy passageways are preferably openings in the showerhead lined with reflectors, although any material which aids in the transmission of the energy may be used. Preferably a plurality of light sources are provided as the energy sources, and they are positioned to transmit light through the energy passageways and out of the surface of the showerhead.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 09/250,950, filed Feb. 16, 1999, now U.S. Pat. No. 6,064,800, which is a continuation of U.S. patent application Ser. No. 08/597,507, filed Feb. 2, 1996, now U.S. Pat. No. 5,892,886. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed generally to rapid thermal processing devices and, more particularly, to an apparatus used in the fabrication of solid state devices to uniformly disburse gases and distortion-free radiant energy. 
     2. Description of the Background 
     In the fabrication of semi-conductor wafers, the deposition of a film on a surface of the wafer is a common and necessary step. The film is typically a semi-conductor, a conductor, or a dielectric. It is well known in the prior art that film deposition occurs more readily on a hot surface than on a cold surface. As a result, it is necessary to heat the surface of the wafer to induce film deposition. Wafers are typically heated and processed either by conventional batch furnace processing or by rapid thermal processing (“RTP”). 
     RTP is an alternative to conventional batch furnace processing and is characterized by short processing times and rapid thermal rise and fall rates. An RTP process step typically lasts between several seconds and 15 minutes, with thermal rise rates typically between 100 and 500° C. per second, and reaching temperatures of 1200° C. 
     RTP has applications in the fabrication of very large scale integrated (“VLSI”) circuits and ultra large scale integrated (“ULSI”) circuits. In particular, RTP is used in the fabrication steps of thermal oxidation, thermal nitridation, dopant diffusion, thermal annealing, refractory metal silicide formation, and chemical vapor deposition (“CVD”). CVD may be used to form semi-conductive, conductive, and dielectric films. The design of RTP reactors is well known in the prior art, as disclosed, for example, in U.S. Pat. No. 5,446,825, issued to Moslehi et al., and U.S. Pat. No. 5,444,217, issued to Moore et al. An RTP reactor typically comprises a reaction chamber, a wafer handling system, a gas dispersion apparatus, a heat source, a temperature control system, and a gas control system. 
     The heat source is often high power lamps which drive chemical reactions in the reaction chamber and heat the wafer, thereby inducing film deposition on the surface of the wafer. The gas dispersion apparatus introduces gases into the reaction chamber so that chemical reactions can take place and films can be deposited on the surface of the wafer. Many types of gas dispersion apparatus are known, and one or more may be located below the wafer, to the side of the wafer, or above the wafer. 
     CVD process steps require both uniform gas distribution and uniform wafer temperature. If the gases are not distributed evenly over the surface of the wafer, the film will not be deposited evenly. That is in contrast to reactive processes, such as oxidation, which are not as sensitive to the distribution of the gases. That is because the gases in a reactive process are not deposited on the surface of a wafer, but rather react with the surface of the wafer, and therefore the process is self-limiting. 
     CVD process steps are also dependent on temperature, and if the surface of the wafer is not a uniform temperature, the film will not be deposited in a uniform manner. Furthermore, uneven heating of the wafer can cause slip dislocations, which are fractures in the crystal lattice, that may lead to a device failure. 
     One type of gas dispersion apparatus used for CVD process steps is known as a “showerhead.” Showerheads are located above the wafer, have a generally flat bottom surface with a plurality of gas ports therein, and provide for a generally uniform distribution of gas over the surface of the wafer. Showerheads are made from transparent materials which do not absorb light, such as quartz. 
     To provide uniform heat to the surface of the wafer, heating lamps are located above the showerhead and separated from the reaction chamber by a transparent window. The use of both single and multiple lamps is known, as disclosed in U.S. Pat. No. 5,444,217, issued to Moore et al. The energy generated by the lamps is intended to travel through both the transparent window and the transparent showerhead, and be absorbed by the surface of the wafer. 
     It is well know in the prior art, however, that the temperature across a wafer is usually not uniform. One cause of nonuniform heating of a wafer is light distortion caused when light passes through the showerhead. As is well know in the prior art that film deposition occurs more readily on a hot surface than on a cold surface, and although quartz showerheads are very transparent, they still absorb some light and become hot. As a result, film depositions occur on showerheads, and these depositions absorb light, becoming hotter and inducing more film deposition. The result is a build up of film on the showerhead, which in turn blocks and distorts the light passing through the showerhead. This distortion causes uneven heating of the surface of the wafer and results in uneven film deposition, uneven film thickness, and can lead to defects in the wafer. In addition, film deposition on the showerhead may begin to flake, sending particulate matter into the reaction chamber and contaminating the wafer. To minimize those effects, the showerhead must be cleaned or replaced regularly. 
     Moving the showerhead away from the lamps, such as to the side of the reaction chamber, reduces the temperature of the showerhead but results in a less uniform distribution of gases, which is unacceptable in many applications. Although some other solutions of the light distortion problem have been proposed, such as the use of complex optics or special quartz showerheads, none of the solutions satisfactorily address the problem. Thus, a need exists for a device which both uniformly disperses gas and does not distort the light that is used for heating the surface of the wafer. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for uniformly distributing gas via a gas showerhead. A gas passageway is located within the showerhead, and it receives a source of gas and terminates in a plurality of gas ports on a surface of the showerhead. A plurality of energy passageways exist within the showerhead. The energy passageways are exclusive of both the gas passageway and the gas ports. The energy passageways terminate on the surface of the showerhead. The energy passageways are preferably openings in the showerhead lined with reflectors, although any material which aids in the transmission of the energy may be used. Preferably a plurality of light sources are provided as the energy sources, and they are positioned to transmit light through the energy passageways, preferably openings in the showerhead, and out of the surface of the showerhead. 
     The device of the present invention is preferably embodied in a semi-conductor processing apparatus, such as an RTP system. The apparatus comprises a reaction chamber and a wafer support platform located within the chamber and capable of supporting the wafer. A showerhead is located within the chamber and above the wafer support platform. The showerhead has a surface and a gas passageway for receiving a source of gas via an input connection port. The gas passageway terminates in a plurality of gas ports on the lower surface of the gas dispersion device. A plurality of energy passageways exist in the showerhead. The energy passageways are exclusive of both the gas passageways and gas ports, and they terminate on the surface of the showerhead. Preferably, a plurality of energy sources is positioned to direct energy through the energy passageways and towards the wafer support platform. A window is located between the light sources and the wafer support platform, and a vacuum pump for removing gases is connected to the chamber. 
     The present invention solves the problem of light distortion by providing a path for the light through the showerhead, separate from the path of the gases through the showerhead and without direct heating of the gas ports. By providing openings in the showerhead corresponding to the light sources, and by focusing the light generated by the light sources through the openings, the heating of the showerhead is significantly reduced. As a result of the reduced heating of the showerhead, as well as no direct heating of the gas ports, film growth on the showerhead is significantly reduced, thereby providing uniform heating and reduced contaminants in the chamber. Those and other advantages and benefits of the present invention will become apparent from the description of the preferred embodiments hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the present invention to be clearly understood and readily practiced, the present invention will be described in conjunction with the following figures, wherein: 
     FIG. 1 is a simplified schematic block diagram of an RTP reactor constructed according to the present invention; 
     FIG. 2 is a cross-sectional view of a preferred embodiment of a showerhead and lamp housing constructed according to the present invention; 
     FIG. 3 is a cross-sectional view or an alternative embodiment of a showerhead and lamp housing constructed according to the present invention; 
     FIG. 4 is a cross-sectional view of an alternative embodiment of a showerhead and lamp housing constructed according to the present invention; 
     FIG. 5 is a cross-sectional view of an alternative embodiment of a showerhead and lamp housing constructed according to the present invention; 
     FIG. 6 is a cross-sectional view of an alternative embodiment of a showerhead and lamp housing constructed according to the present invention; and 
     FIG. 7 is a cross-sectional view of an alternative embodiment of a showerhead and lamp housing constructed according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a simplified schematic block diagram illustrating a preferred embodiment of the present invention in the context of an RTP reactor  10 . It is to be understood that the reactor  10  has been simplified to illustrate only those aspects of the reactor  10  relevant for a clear understanding of the present invention, while eliminating, for the purposes of clarity, many of the elements found in a typical reactor  10 . Those of ordinary skill in the art will recognize that other elements are required, or at least desirable, to produce an operational reactor  10 . However, because such elements are well-known in the art, and because they do not relate to the design which is the subject of the present invention, a discussion of such elements is not provided herein. 
     The design and construction of RTP reactors is well known, and the present invention is applicable to any RTP reactor. The RTP reactor  10  of the present invention preferably comprises a cold wall reaction chamber  12  constructed of stainless steel. The bottom and sides of the reaction chamber  12  may be lined with quartz to protect the walls from film deposition during the processing steps. The walls of the reaction chamber  12  may be cooled by a circulating water jacket (not shown) in conjunction with a heat exchanger (not shown). The walls are maintained at or below 100° C., because higher temperatures may induce the deposition of films on the walls of the reaction chamber  12 . Such depositions are undesirable because they absorb energy and effect heat distribution within the reaction chamber  12 , causing temperature gradients which adversely affect the processing steps. Furthermore, depositions on walls may flake and produce particulates that can contaminate a wafer in the reaction chamber  12 . 
     A wafer support table  14  or the like is located near the bottom of the reaction chamber  12 , and is used for supporting a wafer  16 . The support table  14  is a flat surface, typically having three or more vertical support pins  15  with low thermal mass. 
     A wafer handling system  18  is adjacent to the reaction chamber  12 , and includes a wafer cassette  20  and a wafer handling robot  22 . The wafer cassette  20  holds a plurality of wafers, and the wafer handling robot  22  transports one wafer at a time from the wafer cassette  20  to the wafer support table  14 , and back again. A door  24  isolates the wafer handling system  18  from the reaction chamber  12  when the wafers are not being transported to and from the wafer support table  14 . 
     A showerhead  26  introduces gases into the reaction chamber  12 , and a plurality of light sources  28  heat the wafer  16 . The showerhead  26  and light sources  28  are disclosed in more detail below with respect to FIGS. 2,  3 , and  4 . For the purposes of this description, the invention will be described in terms of light sources  28 , although other sources of heating a wafer  16 , such as RF and microwave energy, are known and applicable to the present invention. 
     The showerhead  26  and light sources  28  are preferably integrated into the roof of the reaction chamber  12 . Although the showerhead  26  and light sources  28  may be suspended from the roof, as was done with showerheads in the prior art, integrating the showerhead  26  and the light sources  28  into the roof simplifies the cooling system which cools the light sources  28  and eliminates the need to cool or heavily insulate wires supplying electricity to the light sources  28 . The light sources  28  are cooled by one or more known cooling methods, such as liquid or gaseous cooling systems, with heat being dissipated through a heat exchanger  30 . 
     A source of gas  32  is connected to the showerhead  26  to provide the gases disbursed by the showerhead  26  within the reaction chamber  12 . More than one type of gas may be available from the source of gas  32 , and gases may be provided to the showerhead  26  individually or in combination. The gases may, for example, be used to deposit films on the wafer  16 , flush gases from the reaction chamber  12 , or cool the reaction chamber  12  and the wafer  16 . A power supply  34  is connected to the light sources  28  to provide electricity to power the light sources  28 . 
     Gases are removed from the reaction chamber  12 , and a vacuum may be created within the reaction chamber  12 , by a gas exhaust and vacuum system  36 , as is well known in the prior art. Also present is a wafer temperature sensor  38 , such as a pyrometer, which is used to measure the temperature of the wafer  16  through a window  40 . 
     A control computer  42  monitors and controls the various systems which make up the reactor  10 , such as the wafer handling robot  22 , the heat exchanger  30 , and the wafer temperature sensor  38 . Data indicating the temperature of the wafer  16  is generated by the wafer temperature sensor  38 , and is used by the control computer  42  to adjust the intensity of the light sources  28  so as to produce the desired wafer temperature. In addition, multiple wafer temperature sensors  38  may be used to sense the temperature of different regions of the wafer  16 . That data may be used by the control computer  42  to selectively adjust the intensity of some of the light sources  28  so as to compensate for uneven heating of the wafer  16 . The control computer  42  also controls when and what gases are provided to the showerhead  26 , as well as when gases are removed from the reaction chamber  12 , in a known manner. 
     FIG. 2 shows a cross-sectional view of a preferred embodiment of the showerhead  26  and light sources  28 . The showerhead  26  may be constructed from a transparent material, such as quartz, sapphire, calcium fluoride, magnesium fluoride or aluminum fluoride. The showerhead  26 , however, may also be constructed from non-transparent materials, such as stainless steel or aluminum, since transparency of the showerhead  26  is not required, as described below. The showerhead  26  has an upper surface  46  and a lower surface  48 , and includes a gas passageway  50  which connects the source of gas  32  (shown in FIG. 1) with a plurality of gas output ports  52  on the lower surface  48  of the showerhead  26 . The gas ports  52  may be shaped and distributed in any manner which promotes the uniform distribution of gas, such as concentric circles and rectangular patterns. The showerhead  26  will typically contain between 50 and 1,000 gas ports  52 , with the size of each port preferably being between 0.5 millimeters and 50 millimeters. The gas flow through the showerhead  26  is typically between 100 and 5,000 standard cubic centimeters per minute (“sccm”). 
     The showerhead  26  also has a plurality of openings  54  therein, extending from the upper surface  46  to the lower surface  48 , and being defined by inner surfaces  56  of the showerhead  26 . The openings  54  are preferably cylindrical in shape, and include reflectors  58  on the inner surfaces  56 . The reflectors  58  may be of any design, such as a two-piece design comprising a heat resistant base, made from aluminum or stainless steel, and a reflective coating, made from gold, chromium, silver, or nickel. The reflectors  58  may also be a polished surface of the showerhead  26 , for example, when the showerhead  26  is constructed of a metallic material such as stainless steel or aluminum. The purpose of the reflectors  58  is to direct light through the openings  54 , while absorbing as little light as possible. 
     Also shown in FIG. 2 is a plurality of light sources  28 . The light sources  28  are preferably quartz-halogen lamps, each rated between 500 and 2000 Watts. There are preferably between 20 and 500 light sources  28 , and around the light sources  28  are parabolic reflectors  60 . The reflectors  60  redirect the light from the light sources  28  in a downward direction and through the openings  54  in the showerhead  26 . The reflectors  60  may be of any design, such as the two-piece design described above with respect to the reflectors  58  in the openings  54 . 
     A housing  62  is in good thermal contact with the light sources  28  and reflectors  60 , and fixes their position within lamp openings  61 , which are formed in the housing  62 . The housing  62  includes a coolant passageway  64  which is adjacent to the reflectors  60  and the light sources  28 . The coolant passageway  64  houses a liquid coolant, such as water, which flows through the housing  62 , carrying away heat generated by the light sources  28 . Satisfactory results are achieved with flow rates between 1 and 3 gallons of water per minute. The water may be cooled, for example, in a heat exchanger  30  (shown in FIG.  1 ), or it may be a non-recirculating system, wherein cool water constantly enters the system and hot water is discarded. 
     Optionally, the housing  62  may include an air passageway  66  for forced air cooling of the housing  62 . Air cooling is preferably used to supplement the liquid cooling system because it usually does not provide sufficient cooling when used alone. The forced air may be supplied by a blower or a fan, and it is preferably provided at a flow rate of between 1,000 and 10,000 sccm. The air may be cooled through a heat exchanger  30  or an air conditioner (not shown). Alternatively, like the liquid coolant system, the present invention may utilize a non-recirculating system wherein cool air is taken in and hot air is discarded. 
     The housing  62  is preferably constructed from heat-resistant material, such as stainless steel or aluminum. The housing  62  and the showerhead  26  are separated from each other by a window  68 . The window  68  isolates the light sources  28  from the reaction chamber  12 . The window is made from a transparent material, such as quartz, sapphire, calcium fluoride, magnesium fluoride and aluminum fluoride, and is preferably between 1.0 and 50 mm thick. 
     Seals  70  may be provided between the showerhead  26  and the window  68 , and between the housing  62  and the window  68 . The seals  70  are used because satisfactory sealing usually cannot be achieved with the materials typically used to form a showerhead  26 . The use of such seals in RTP chambers and other applications is well known in the art. The seals  70  provide an air tight fit between the showerhead  26 , the window  68 , and the housing  62 . Due to the combination of high temperatures and corrosive materials often used in RTP chambers, high temperature, corrosive-resistant seals may be used. In the preferred embodiment, however, the chamber  12  is adequately cooled so that lower cost seals, such as those sold under the name “Viton”, may be used. Seals may be obtained, for example, from Eriks/West, located at 14600 Interurban Avenue S., Seattle, Wash. 98168-4651. 
     As can be seen in FIG. 2, the light sources  28  in the housing  62  generate light which is directed by the reflectors  60  through the window  68 , and through the openings  54  in the showerhead  26 . The reflectors  58  along the inner surfaces  56  of the openings  54  further direct the light downward towards the wafer  16 . In that way, little or no light passes through the showerhead  26 . As a result, heating of the showerhead  26  is substantially reduced, resulting in substantially less film deposition. Furthermore, even if film deposition occurs on the showerhead  26 , the light does not travel through the showerhead  26  and so it will not be distorted by film depositions thereon. 
     FIG. 3 shows a cross-sectional view of an alternative embodiment of the present invention. As can be seen in FIG. 3, a portion of the housing  62  including the light sources  28  is positioned within the openings  54 . This design eliminates the need for the reflectors  58  on the inner surfaces  56  of the showerhead  26 . Seals  71  are provided between the windows  68  and the showerhead  26 , and between the housing  62  and the showerhead  26 . 
     Alternatively, as shown in FIG. 4, the window  68  may be located on the lower surface  48  of the showerhead  26 . In that embodiment, holes  72  are provided in the window  68  so that gas may leave the gas ports  52  and enter the reaction chamber  12 . Corresponding holes  72  in the seals  70  between the window  68  and the showerhead  26  are also be provided. The seals  71  between the showerhead  26  and the housing  62  are optional, and are usually not needed to obtain a good seal. 
     FIG. 5 shows an embodiment wherein the showerhead  26  and windows  68  are a single piece, eliminating the need for seals  70  and  71 . In that embodiment the openings  54 , which contain a portion of the housing  62 , do not extend entirely through the showerhead  26 . The window  68 , of course, must be transparent to the energy source  28 . 
     FIG. 6 shows an embodiment wherein the window  68  includes a protective portion  74  which engages the showerhead  26 . Although the protective portion  74  usually will not form an airtight seal with the showerhead  26 , it protects the seal  70  from a substantial amount of corrosive gases which are often present in an RTP reactor  10 , and which can shorten the useful life of the seal  70 . In addition, the vertical seals  71  between the showerhead  26  and the housing  62 , shown in FIGS. 3 and 4, are preferably not used because the protective portion  74  and the seal  70  together provide sufficient sealing between the window  68  and the showerhead  26 . 
     FIG. 7 shows an embodiment wherein two showerheads  26  are integrated around a housing  62 . Electrical wires  76  carrying electricity to the light sources  28  are preferably located within the air passageway  66  to keep them cool. That embodiment may be used in a device to process wafers both above and below the showerheads  26  and may share a common gas passageway  50 . 
     Combinations of the disclosed embodiments are possible. For example, the design shown in FIG. 2 will also function with the window  68  located within the openings  54 , or on the lower surface  48  of the showerhead  26 , as described with respect to FIGS. 3 and 4. The design shown in FIG. 2, however, is preferred because it does not require multiple windows  68 , as in the embodiment shown in FIG. 3, and it does not require holes  72  in the window  68  and in the seal  70 , as in the embodiment shown in FIG.  4 . 
     It is contemplated that the openings  54  may include materials, or be evacuated, to aid in the efficient transmission of radiant energy. For example, in an embodiment which uses light as the radiant energy, the openings  54  may be sealed and a vacuum created therein, to reduce the scattering of light by the ambient gases, and so that the light is more focused when it exits the showerhead  26 . Liquid and solid materials may also be used. Likewise, the lamp opening  61  may also include a material, or be evacuated, to optimize the radiant energy transmission. As a result, the showerhead  26  does not need to contain an opening, but only a passageway for the transmission of radiant energy. 
     Those of ordinary skill in the art will recognize that many more modifications and variations of the present invention may be implemented. For example, at least some of the benefits of the present invention can be realized by having the showerhead  26  below the wafer  16 , or with some light sources  28  providing light through openings  54 , while other light sources  28  are providing light in a more conventional manner through the showerhead  26 . In addition, other types of energy sources may be used to heat the wafer  16  and the gases in the reaction chamber  12 , the locations of the window  68  and the light sources  28  may be modified, the shapes and locations of the coolant, air, and gas passageways  64 ,  66 , and  50  may be modified, and the openings  54  in the showerhead  26  may be modified. Furthermore, the invention may be used in the fabrication of devices other than semiconductor wafers, such as flat panel displays. The foregoing description and the following claims are intended to cover all such modifications and variations.