Abstract:
An apparatus that includes a reflector having a mirrored surface facing down, a glass structure located beneath the reflector, a susceptor within the glass structure having a surface facing up that is capable of holding a part to be processed, and one or more radiant heat sources directed at and located beneath the glass structure.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to thermal processing a semiconductor wafer. More particularly, the present invention relates to a method and apparatus for emissivity independent heating of the wafer during the processing. 
     BACKGROUND OF THE INVENTION 
     Apparatus to fabricate a thin film using such techniques as chemical vapor deposition (CVD) or atomic layer deposition (ALD) are routinely used in the manufacture of semiconductor wafers. In a film forming apparatus, a reproducible temperature distribution across the wafer is extremely important for film uniformity. 
     The most common epitaxial (epi) film deposition reactors used in modern silicon (Si) technology are similar in design. A quartz reaction chamber contains the wafer support, the susceptor, which is rotated to improve the deposition uniformity across the wafer. Only one wafer is processed at a time. Process and carrier gases flow over the wafer in a laminar mode and parallel to the wafer surface. The wafer is heated by tungsten-halogen lamps located underneath and above the reaction chamber, radiating through the quartz and directly heating the wafer and susceptor. The lamps and the quartz walls of the chamber are air-cooled to protect the lamps and to prevent the risk of Si depositing on the reactor walls. The wafer is loaded and unloaded fully automatically and the reaction chamber is separated from the ambient by load locks and a wafer transfer chamber. 
     The susceptor can be a graphite disc with a SiC coating to smooth out local temperature variations from the radiant heat source. At temperatures in the 500-900° C. range, the growth of epi Si (silicon) and SiGe (silicon germanium) is very temperature sensitive and epi growth is often effected on patterned wafers in selective or in blanket growth mode. 
     A patterned wafer (having circuitry and devices) can have a different emissivity characteristic than a blanket wafer. In addition, a pattern on a wafer can have a different emissivity characteristic than a different pattern on another wafer. Heat can be emitted from the wafer with the first pattern different than with the different pattern and the temperature distribution of wafers with different patterns can vary which can vary the deposition rate and characteristics of films being placed on the wafer. Since a varying thermal profile on a wafer can affect reaction rates, the varying thermal profile can determine a film deposition rate. As a result, a change in wafer patterns can require a re-tuning of the process to confirm that correct heating of the wafer is accomplished. In addition to film deposition processes, any process involving thermal treatment such as bake, anneal, etc. can be affected by the wafer emissivity. 
     Often in thermal processing reactors, such as in Epi CVD apparatuses, the substrate is heated from both the device side and the non-device side. Using this dual side heating approach, the temperature distribution across the wafer is very sensitive to the emissivity of the surface onto which a film is deposited. As a result, the deposition rate will be different at different locations on the wafer. The deposition rate also varies between wafers having different patterns on their front surface due to the different emissivity of these patterns. Moreover, the deposition rate can change during the deposition itself because the species which are being deposited can change the wafer&#39;s emissivity. In addition to the dependence of the deposition rate on emissivity, the chemical composition of the deposited film will also be emissivity sensitive because incorporation of species into grown films can be temperature dependent. 
     SUMMARY OF THE INVENTION 
     An apparatus for heating and monitoring a wafer that reduces dependence of the temperature distribution along the wafer, to wafer emissivity, is disclosed. The apparatus provides a susceptor capable of holding a wafer that is placed in a process chamber within quartz domes. An array of lamps placed outside the quartz domes heat a susceptor backside. A reflector is placed outside the quartz domes to have a mirrored surface face the wafer device side and reflect heat back onto the wafer. The shape of the reflector is optimized to provide the best temperature uniformity. The chamber is designed to restrict light from the lamps from leaking around the susceptor to heat the wafer directly. An optical thermometer may be placed above the reflector to read a wafer device side temperature through a hole in the reflector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
     FIG. 1 is an illustration of a backside heating chamber for emissivity independent thermal processing. 
     FIG. 2 is an illustration of the backside heating chamber with a flat upper dome. 
     FIG. 3A is an illustration of the backside heating chamber with a ribbed flat upper dome. 
     FIG. 3B is an illustration of a top view of the ribbed flat upper dome. 
     FIG. 4 is an illustration of a cluster tool system. 
    
    
     DETAILED DESCRIPTION 
     An apparatus for thermal processing wafers is described. The apparatus provides heating to the backside of the wafer with a mirrored surface to reflect back heat escaping the opposite side of the wafer. Wafer backside (non-device side) heating only reduces the effects of the water device side emissivity on wafer heating. This apparatus reduces the dependence of wafer emissivity on film deposition as compared to apparatus with direct radiant heating to both sides of the wafer. Such thermal processing may be the epitaxial deposition of various coatings such as, for example, silicon, silicon-germanium, and silicon-germanium-carbon films. The deposition may be accomplished through one of several methods for film deposition such as chemical vapor deposition or atomic layer deposition. Other processes where this invention can be used include but not limited to are silicon oxide, silicon nitride, polycrystalline deposition, and more broadly, any temperature-dependent treatment or process. 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical, and other changes may be made without departing from the scope of the present invention. 
     The present invention can be implemented by an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose process chamber, selectively activated or reconfigured to achieve the required purposes. 
     It is to be understood that various terms and techniques are used by those knowledgeable in the art to describe communications, protocols, applications, implementations, mechanisms, etc. 
     FIG. 1 illustrates a backside heating chamber for emissivity independent thermal processes in which the techniques described may be applied. In one embodiment, the backside heating chamber for emissivity independent thermal processing (process chamber)  100  includes an array of radiant heating lamps  102  for heating a backside  104  of a susceptor  106 . A wafer  108  (not to scale) can be brought into the process chamber  100  and positioned onto the susceptor  106  through a loading port  103 . Pin lifts  105 , passing through holes in the susceptor  106 , can raise up to accept the wafer  108  and then translate down to position the wafer  108  device side up  116  on a front side  110  of the susceptor  106 . The susceptor  106  can be located inside the process chamber  100  and within an upper dome  128  and a lower dome  114  where the domes  128  and  114  can be made from clear glass such as a quartz. One or more lamps, such as an array of lamps  102 , can be located outside and under the lower dome  114 . The loading port  103  can include one or more rings or partial rings to act as a liner  112  that can line the edge of the susceptor  106  to minimize or prevent leakage of heat from the lamps  102  to the wafer front (device) side  116 . The liners  112  can be made from a non-light conducting material such as an opaque quartz. By using a liner  112  of opaque quartz, most of the heating energy reaching the wafer  108  is conducted through the susceptor 106  rather than by leakage from the lamps  102  around the susceptor  106  to the wafer front side  116 . Since heat transfer through the susceptor  106  to the wafer  108  is conductive and therefore emissivity independent, deposition of films onto the wafer  108  are therefore emissivity independent. 
     As a result of backside heating of the wafer  102  from the susceptor  106 , the use of an optical thermometer  118  for temperature measurements on wafer front (device) sides  116  can be performed. This temperature measurement by the optical thermometer  118  can be done on wafer device sides  116  having an unknown emissivity since heating the wafer front side  116  in this manner is emissivity independent. As a result, the optical thermometer  118  can only sense radiation from the hot wafer  108  that conducts from the susceptor  106 , with minimal background radiation from the lamps  102  directly reaching the wafer front side  116  or the optical thermometer  118 . 
     The reflector  122  can be placed outside the upper dome  128  to reflect infrared light that is radiating off the wafer  108  back onto the wafer  108 . Due to the reflected infrared light, the efficiency of the heating will be improved by containing heat that could otherwise escape the system  100 . A further aspect is that with the heat radiating off the wafer, continually reflected back onto the wafer, a frequency distribution of this heat will approach a near black body radiation of the wafer  108 . As a result, any direct light leaking from below the wafer to reflect onto the wafer front side  116  can only contribute a small percentage of the total heat to the wafer, and therefore emissivity effects on the front surface  116  resulting from this leakage, will be minimized. 
     By reducing the dependence of the wafer heating on the wafer emissivity, the use of the optical thermometer  118  to read a surface temperature of the wafer device side  116  can therefore become effective. The effectiveness of the optical thermometer  118  has resulted since the reduced percentage of the light due to the lamps decreases the percentage of “parasitic” signal in the optical thermometer. In addition, the optical thermometer  118  may not have to be recalibrated when a wafer circuit design (pattern) is changed since wafer heating is emissivity independent with a low error resulting from leakage. 
     In one embodiment, the reflector  122  can be made of a metal such as aluminum or stainless steel. The aluminum can have machined channels  124  to carry a flow of a fluid  126  such as water for cooling the reflector  122 . In addition, the efficiency of the reflection can be improved by coating a reflector area with a highly reflective coating such as with gold. The reflector  122  can have a hole  120  through a location such as the center of the reflector  122 , through which to sense a temperature of the wafer  108  with the optical thermometer  118 . In one embodiment, the susceptor  106  can be manufactured from a material such as graphite and coated with silicon carbide. The susceptor  106  can be supported by struts  130  and a central shaft  132  that can move the wafer  108  in an up and down direction  134  during wafer  108  loading and unloading. 
     In one embodiment (FIG.  1 ), the upper dome  128  of the process chamber  110  is curved. The degree of curvature and the thickness of the quartz glass material of the upper dome  128  can be dependent on the pressure differential acting on the sides of the upper dome  128 . In this embodiment, the exterior pressure is one atmosphere and the pressure during processing within the upper dome  128  and the lower dome  114  is approximately 0.1-700 Torr. As a result, the quartz glass thickness of the upper dome 1.28 can be approximately 0.12 inch and the radius of curvature approximately 15.0 inches. 
     In an alternate embodiment as shown in FIG. 2, the pressures on both sides of the quartz dome  230  and  232  can be kept approximately the same. With no structural concern, the upper dome  228  can be flat and the reflector  222  can be placed closer to the wafer  208  to improve efficiency. To ensure equal pressure acting on both sides of the upper dome  228 , the volumes on either side  230  and  232  of the upper dome  228 , can be connected to each other. If the volumes  230  and  232  on either side of the upper dome  228  are not connected, a pressure control system (not shown) could be in place to ensure closeness of pressures in the two volumes  230  and  232  to not break the upper dome  228 . 
     In another alternate embodiment shown in FIGS. 3A &amp; 3B, the upper dome  328  of the process chamber  300  is reinforced with ribbing  330 . Where there is a significant pressure differential acting on the upper dome  328 , the upper dome  328  can be made stronger by using these stiffening ribs  330 . As a result of ribbing  330 , the upper dome  328  can still be substantially flat where facing the wafer  308 . FIG. 3A further illustrates a susceptor  306  that is centerless meaning there is a hole  334  in the susceptor  306  and the wafer  308  can contact the susceptor  306  at the edges of the hole  334 . The centerless susceptor  306  can allow radiation from the lamps  302  can strike a backside  332  of the wafer  308  directly, and heat can propagate through the thickness of the wafer  308  to heat the wafer front side  316 . Only one form of ribbing  330  is shown, however, it should be appreciated that a variety of ribbing designs are possible to meet pressure differential considerations. Also, a variety of chamber shapes are possible other than the circular geometry shown in FIGS. 1-3. To improve the ability of the upper dome  328  to withstand a pressure differential, rectangular or elliptical chamber shapes are also possible. 
     FIG. 4 is an illustration of a cluster tool system. The cluster tool  400 , such as an Epi Centura, can contain multiple backside heating chambers  402  and  402 ′ with wafers  403  robotically fed  407  to and from the chambers  402  and  402 ′ from cartridges  401  and  401 ′. The backside heating chambers  402  and  402 ′ can all perform a similar function, such as epitaxial deposition, or each perform a different function. Shown in FIG. 4 is a configuration of a system for low temperature epi deposition, which has one EpiClean chamber  404  for pre-epitaxial cleaning and three deposition chambers  402  and  402 ′. Each chamber  402 ,  402 ′ and  404  can have an optical thermometer  406  or  406 ′. The thermometers  406  and  406 ′ can be controlled individually or all together by a single control unit with multiple channels  408 . Since the direct lamp radiation above the wafer is minimized, the parasitic signal in the optical thermometers will be minimized, too. The optical thermometers  406  and  406 ′ do not have to be re-calibrated with each change of wafer size and/or circuit pattern, therefore reducing process cycle times. 
     Backside heating only, achieves immunity to emissivity in temperature dependent processes. This allows reproducibility of thermal processes regardless of the wafer circuit design (pattern) or intrinsic film emissivity. The time spent for tuning a process can be shortened due to this feature. The reactor (process chamber) can be more compact due to the absence of an upper lamp array. Finally, this apparatus will allow a direct probe of the wafer temperature of patterned wafers with unknown emissivity. 
     Thus, an apparatus for heating a wafer that is emissivity independent through wafer backside heating, reflecting wafer radiant heat back onto the wafer, and minimizing the effects of lamp heat leakage has been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.