Abstract:
A thermal reactor having a wafer chamber for containing at least one semiconductor wafer during processing. The thermal reactor contains a quartz window having an inward bow defining a concave outside surface.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to thermal reactors for processing semiconductor wafers, and more particularly to a reactor having a domed window with reduced stress at atmospheric and above atmospheric pressure processes.  
         BACKGROUND OF THE INVENTION  
         [0002]    Recent technological progress is closely identified with the increasing miniaturization of electronic circuits made possible by advances in semiconductor processing. Certain advanced processing techniques require exposing a semiconductor structure to a reactant gas under carefully controlled conditions. Examples of such processes include chemical vapor deposition and etching processes. Of particular concern is the uniformity of temperature and gas flow to ensure uniform results, e.g., deposition thickness, across a wafer.  
           [0003]    The process of depositing layers on a semiconductor wafer (or substrate) usually involves placing the substrate within a thermal reactor chamber and holding the wafer within a stream of a reactant gas flowing across the surface of a wafer. The thermal reactor is heated by external lamps which pass infra-red radiation into the reactor chamber through heating ports. The heating ports are covered by quartz windows that are transparent to the infra-red radiation.  
           [0004]    Prior art deposition processes involve the deposition of a reactant gas at ambient and subambient pressures. FIG. 1 illustrates a cross-sectional view of a thermal reactor  100  used for reduced pressure operations. Reactor  100  includes a chamber  102  for facilitating the flow of a process gas over the surface of a wafer. The housing includes a baseplate  104  having a gas inlet port  106  and a gas exhaust port  108 . An upper clamp ring  110  and a lower clamp ring  112  act to hold a quartz cover member  114  and a quartz lower member  116  in place, respectively. Cover member  114  generally includes a flange portion  118  and a central window portion  120 . Flange portion  118  is resiliently supported between baseplate  104  and clamp ring  110  by resilient o-rings  122 . Process gas is injected into chamber  102  through gas inlet port  106  which is connected to a gas source. Residual process gas and various waste products are continuously removed from the interior of chamber  102  through exhaust port  108 . A susceptor  124  holds the wafer in position during the semiconductor/layer deposition process. A susceptor support shaft  126  is coupled to susceptor  124  for positioning and rotating the wafer during the semiconductor fabrication process. Quartz central window portion  120  has an outward bow that forms a convex outside surface. The outward bow is curved enough to oppose the compressive force of the ambient pressure against the reduced internal pressure of chamber  102  during wafer processing. Heating lamps  128  and  130  provide infra-red radiant heat into the chamber through window portion  120  and quartz lower member  116  which are transparent to infra-red radiation.  
           [0005]    Wafer processing at ambient pressure is often desired because the deposition rate of the process gas is higher at ambient pressure than it is at a reduced pressure. Ambient pressure processing also allows the use of certain chemical species, for example, trichlorosilane, which has an undesirable effect of coating the chamber walls at reduced pressures.  
           [0006]    [0006]FIG. 2 illustrates a cross-sectional view of an ambient pressure thermal reactor  200 . As shown in FIG. 2, reactor  200  contains a flat quartz window  202  in lieu of the outwardly bowed window of the subambient pressure reactor of FIG. 1. Although the flat window provides a uniform reactant gas flow across the surface of the wafer, it cannot be used in processing applications wherein a differential pressure exists across the surface of the window. When subjected to chamber over pressure or under pressure situations the differential pressure across the flat window causes localized stresses to occur that subject the window to breakage. Another problem associated with the flat window design is that high internal tensile stresses resulting from temperature gradients within the window may result in breakage.  
           [0007]    One way to overcome these problems is to increase the wall thickness of the window. However, this produces an undesirable result in that the interior surface temperature of the quartz increases as the wall thickness increases. This increase in temperature can lead to deposition on the interior surface of the quartz window, which, in turn, reduces the radiant heat transfer through the window.  
         SUMMARY OF THE INVENTION  
         [0008]    A thermal reactor for processing a semiconductor wafer is disclosed. The thermal reactor vessel contains a cover member having a central quartz window portion having an inward bow defining a concave outside surface. The unique shape of the quartz window permits the operating pressure of the thermal reactor chamber to be maintained at a pressure greater than atmospheric pressure. The positive chamber pressure reduces the stress level in the heated cover member by compensating for the stress produced by the thermal expansion produced during heating of the thermal reactor. Thus, in accordance with the present invention the deposition of a layer onto the surface of a wafer may be achieved by mounting the wafer on a susceptor within the chamber and pressurizing the chamber above atmospheric pressure with a processing reactant gas. Once the chamber is pressurized, the wafer is heated by radiating heat through the quartz central window portion and a reactant gas is introduced into chamber to flow over the wafer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention is illustrated by way of example and is not limited by the figures of the accompanying drawings, in which like references indicate similar elements, and in which:  
         [0010]    [0010]FIG. 1 illustrates a typical cross-sectional view of a prior art subambient pressure thermal reactor.  
         [0011]    [0011]FIG. 2 illustrates a typical cross-sectional view of a prior art ambient pressure thermal reactor.  
         [0012]    [0012]FIG. 3 illustrates a cross-sectional view of a thermal reactor in accordance with one embodiment of the present invention.  
         [0013]    [0013]FIG. 4 illustrates a cross-sectional view of a thermal reactor in accordance with another embodiment of the present invention.  
         [0014]    [0014]FIG. 5 is a flow diagram of method for processing a semiconductor wafer in accordance with the present invention.  
         [0015]    [0015]FIG. 6 illustrates a cross-sectional view of a thermal reactor in accordance with yet another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]    A quartz window for a thermal reactor is disclosed. In the following description, numerous specific details are set forth, such as material types, dimensions, etc., in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known structures and processing steps have not been shown in particular detail in order to avoid unnecessarily obscuring the present invention.  
         [0017]    With reference to FIG. 3, a thermal reactor in accordance with one embodiment of the present invention is illustrated. Thermal reactor  300  is formed by a reactor vessel  302  defining a wafer reactor chamber  304 . Chamber  302  is defined, in part, by a cover member  306  mounted below an upper heating source  308 , and a lower member  310  mounted above a lower heating source  312 . Cover member  306  and lower member  310  are generally made of quartz. Heating sources  308  and  312  provide infra-red radiant heat into the chamber through members  306  and  310  which are transparent to infra-red radiation. The wafer cover member  306  includes a central window portion  314  and a peripheral flange portion  316  for supporting the central window portion. Central window portion  314  is typically made of a clear fused silica quartz whereas the flange portion is made of an opaque quartz. The flange is captured between a baseplate  318  and an upper clamp ring  320 . Clamp ring  320  is secured to baseplate  318  by a suitable clamping means such as locking bolts  322 . It is appreciated that cover member  316  may be made entirely of a single material, such as fused silica quartz. Moreover, it is to be understood that the present invention is not limited to the manner in which the cover member is attached to the reactor housing.  
         [0018]    Cover member  306  is resiliently supported by a cushioning material such as base sealing rings  324  that are positioned between baseplate  318  and flange  316 . Cover member  306  is further supported by clamp sealing o-rings  326  that are located between clamp ring  320  and flange  316 . The orings are preloaded by the locking bolts  322  to provide a double seal for preventing the processing gas within chamber  304  from escaping into the ambient atmosphere. Lower member  310  also has a window portion  328  and a flange portion  330  that is similarly mounted between baseplate  318  and a lower clamp ring  332  with locking bolts  334  and o-rings  336  and  338 .  
         [0019]    Process gas enters chamber  304  through a gas inlet port  340  and exits the chamber through an exit port  342 . The pressure of the gas within the chamber is maintained by metering the gas flow out of exit port  342 .  
         [0020]    A susceptor  344  is provided within chamber  304  for supporting a wafer  346 . Susceptor  344  includes a mounting shaft  348  that is coupled to a motor (not shown). In this manner, wafer  348  may be rotated during processing to permit a more uniform heating and deposition.  
         [0021]    As previously discussed, wafer processing at ambient pressure is often desired because the deposition rate of the process gas is higher at ambient pressure than it is at a reduced pressure. However, a problem associated with existing ambient pressure thermal reactors is that the flat window used in such reactors is susceptible to breakage when a chamber over pressure or under pressure situation occurs. The quartz material of window portion  314  is generally transparent to the direct radiation from the infra-red heat lamps that pass through the window into the chamber without significant absorption. However, some of the lower frequency energy re-radiated from the heated wafer and susceptor pass into the window quartz with significant absorption by the window material. These re-radiations generate heat within the window producing thermal expansion forces. The flat window of prior art thermal reactors are susceptible to breakage from the high internal tensile stresses that occur due to the thermal expansion of the window. As a result, the flat window is vulnerable to breakage particularly at points along the outer edge of the flange portion where a nick or chip may exist.  
         [0022]    A salient feature of the present invention lies in the construction of cover member  306 . In accordance with the present invention, the window portion  314  of cover member  306  has a slight inward bow forming a slightly concave outside surface. The inward bow configuration causes the stress within central window portion  314  to be transmitted into the flange portion  316 . The flange portion thus acts to resist the outward expansion of the domed cover member  306  due to a pressure differential across the cover and/or the thermal expansion due to heating of central window portion  314 . The inward bow configuration of the window more nearly approximates the flat window of conventional ambient pressure thermal reactors, thus resulting in a more desirable flow cross-section for the process gas.  
         [0023]    The diameter of window portion  314  may vary significantly from one thermal reactor to another. In one exemplary embodiment window portion  314  has a diameter of 17.5 inches. The radius of curvature of central window portion  314  is relatively large; typically 3 to 10 times that of the subambient pressure domed window of FIG. 1. In one embodiment, window portion  314  has a radius of curvature of 100 inches. Depending upon the specific application, the radius of curvature typically is in the range of 50 to 300 inches. The thickness of central window portion  314  is generally between of 0.1 to 0.2 inches. The thickness of flange portion  316  is in the range of 0.75 to 1.5 inches.  
         [0024]    The operating strength of quartz is in the range of 5,000 to 14,000 pounds per square inch (p.s.i.). It is desirable to limit the internal tensile stress of the quartz window to 2,000 p.s.i. in order to provide a safety factor for variations in material quality. The unique shape of cover member  306  permits the operating pressure of chamber  304  to be maintained at a pressure slightly greater than atmospheric pressure. The positive chamber pressure actually reduces the stress level in the heated cover member  306  by compensates for the stress produced by the thermal expansion produced during heating. Thus, in accordance with the present invention the deposition of a layer onto the surface of a wafer  346  may be achieved by mounting the wafer on susceptor  344  and pressurizing the chamber above atmospheric pressure with a processing reactant gas. In one embodiment the chamber pressure is maintained at approximately 3 p.s.i.g. It is to be understood, however, that the present invention is not limited to any one elevated chamber operating pressure. The chamber operating pressure will generally range from 1 to 10 p.s.i.g. Pressurization of chamber  304  is achieved by restricting the flow of gas exiting the chamber. The gas used to initially pressurize chamber  304  may be a non-reactive gas, such as helium, or may comprise the processing reactant gas. The reactant gas may comprise any of a number of gases, such as, for example, hydrogen or a hydrogen/deposition species mixture. The deposition species may include trichlorosilane, dichlorosilane, silane, or any of a variety of dopant species. As shown in FIG. 4, a throttle valve  402  may be positioned in the processing gas exhaust piping  404  to restrict the flow of gas. An orifice, or other restricting means may also be used to restrict the flow of gas in order to create a back pressure to pressurize chamber  304 . Once the chamber is pressurized, wafer  346  is heated by radiating heat through central window portion  314 , and a reactant gas is introduced into chamber  304  to flow over wafer  346 . FIG. 5 is a flow diagram of the process. In an alternative embodiment, the chamber pressure is pressurized and heated simultaneously.  
         [0025]    By operating the thermal reactor at a positive pressure higher growth rates are achieved due to the greater gas density in the chamber. An additional benefit of operating the thermal reactor at a positive pressure is that the chamber pressure can controlled to a predetermined value which improves process repeatability and uniformity. In addition, by operating at a positive pressure, the thermal reactor chamber can be leak checked more accurately than an ambient pressure thermal reactor.  
         [0026]    It is important to note that the present invention is not limited to applications wherein a positive pressure is established and maintained within the processing chamber. The thermal reactor of the present invention may also be used for ambient pressure processing During ambient pressure processing, the inward bow of the chamber window acts to inhibit cracking or breaking of the window during over pressure situations.  
         [0027]    [0027]FIG. 6 illustrates a thermal reactor  500  in another embodiment of the invention. Thermal reactor  500  is formed by a reactor vessel  502  defining a wafer reactor chamber  504 . Chamber  502  is defined, in part, by an upper cover member  506  mounted below an upper heating source  508 , and a lower cover member  510  mounted above a lower heating source  512 . Cover members  506  and  510  are generally made of quartz. Heating sources  508  and  512  provide infra-red radiant heat into the chamber through members  506  and  510  which are transparent to infra-red radiation. Each of cover members  506  and  510  includes a central window portion  514  and  515  and a peripheral flange portion  516  and  517 , respectively. Central window portions  514  and  515  are typically made of a clear fused silica quartz whereas the flange portions  516  and  517  are made of an opaque quartz. Upper flange portion  516  is captured between a baseplate  518  and an upper clamp ring  520 . Clamp ring  520  is secured to baseplate  518  by a suitable clamping means such as locking bolts  522 . Upper cover member  506  is resiliently supported by a cushioning material such as base sealing rings  524  that are positioned between baseplate  518  and flange  516 . Cover member  506  is further supported by clamp sealing o-rings  526  that are located between clamp ring  520  and flange  516 . The o-rings are preloaded by the locking bolts  522  to provide a double seal for preventing the processing gas within chamber  504  from escaping into the ambient atmosphere. Lower cover member  510  is similarly mounted between baseplate  518  and a lower clamp ring  532  with locking bolts  534  and o-rings  536  and  538 .  
         [0028]    Process gas enters chamber  504  through a gas inlet port  540  and exits the chamber through an exit port  542 . The pressure of the gas within the chamber is typically maintained by metering the gas flow out of exit port  542 .  
         [0029]    A susceptor  544  is provided within chamber  504  for supporting a wafer  546 . Susceptor  544  includes a mounting fixture  545  that supports the susceptor within the chamber. Although FIG. 6 shows the susceptor being fixedly supported within the chamber, it is appreciated that the susceptor may be rotatably mounted to permit rotation of the susceptor during processing operations.  
         [0030]    It is appreciated that the methods and apparatus of the present invention may be used for multiple wafer processing and single wafer processing. It is further understood that the relative dimensions, geometric shapes, materials and process techniques set forth within the specification are exemplary of the disclosed embodiments only. Whereas many alterations and modifications to the present invention will no doubt become apparent to a person ordinarily skilled in the art having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be limiting. Therefore, reference to the details of the illustrated diagrams is not intended to limit the scope of the claims which themselves recite only those features regarded as essential to the invention.