Abstract:
A pedestal is provided for supporting wafer boats in a process chamber during semiconductor fabrication. The pedestal contains hollow spaces, such as within porous insulating plugs, and gases inside the pedestal may expand during semiconductor processing. The pedestal has an opening for exhausting gases out of its interior and into the process chamber. The opening is provided with a filter, in the form of a sintered ceramic or glass disc sealed within a tube covering the opening, to prevent the passage of particles which may be present inside the pedestal. By filtering the particles, the filter removes a source of contamination, thereby allowing for high quality process results on wafers processed in the process chamber.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to vertical furnaces for processing semiconductor substrates and, more particularly, to pedestals for supporting wafer boats in the vertical furnace. 
   BACKGROUND AND SUMMARY OF THE INVENTION 
   For high temperature processing using a wafer boat in a vertical furnace, a pedestal is commonly used at a lower end of the furnace to support the wafer boat and to provide a thermal insulation plug at the bottom of the furnace. Such a pedestal typically includes an insulating material inside a quartz envelope, with the quartz envelope supporting the wafer boat and the insulating material providing thermal insulation. 
   A furnace with such a pedestal is illustrated in  FIG. 1A . The furnace in its entirety is indicated by reference numeral  1  and is provided with a pedestal  90 . A process tube  10  includes a gas infeed tube  12  to feed gas into tube  10  at a top end thereof and the tube  10  is supported by flanges  14 . Gas is exhausted from a lower end of tube  10  by a gas exhaust tube, not shown. The cylindrical process tube  10  is surrounded by a cylindrical heating coil  20 , an insulation material  30  and an outer shell  40 . A metal doorplate  50  supports a stationary quartz door plate  60 , and includes a boat rotation bearing  70  in its center region. The bearing  70  supports a rotating quartz door plate  80 , which supports the pedestal  90 . The pedestal  90  supports the wafer boat  200 . The pedestal  90  and the wafer boat  200  can be inserted and removed from the furnace  1  with the aid of an elevation mechanism (not shown). The wafer boat  200  protrudes into a batch process chamber or reaction space  16 , i.e., the volume inside the furnace  1  in which process gases can interact with wafers (not shown) during semiconductor fabrication processes. It will be appreciated that in the illustrated furnace  1 , the process tube  10 , the pedestal  90  and the door plates together delimit the reaction space  16 . 
   The insulating material inside the pedestal  90  is typically a material with a high fraction of pores, such as ceramic foam or ceramic fibrous material. The material can be various materials, such as quartz foam, Al 2 O 3  with high porosity or a mixture of high porosity materials. Because of the high fraction of open pores and the large surface area of the insulating material, exposing the material to process gases is undesirable and the insulating material is typically placed in a quartz envelope  92 . However, upon loading the pedestal  90  into a hot furnace such as the furnace  1 , the pedestal  90  heats up and gases present in the quartz envelope  92  expand. With the process chamber at a temperature of, e.g., about 1000° C., and with the internal gas pressure inside the pedestal  90  at about atmospheric pressure at room temperature, the pedestal will likely explode due to an increase in the internal gas pressure caused by heating. 
   To prevent this explosion, one possible solution is to evacuate the pedestal during manufacturing. However, this is complicated and the pedestal might not remain gas tight during use, with possible disastrous consequences such as explosion if gas leaks into the pedestal and later expands. 
   Another possible solution is to provide a pressure relief opening in the quartz envelope, so that, upon expansion of the gas inside the pedestal, the gas can escape from the pedestal into the process chamber, so that the pressure inside the pedestal remains at about atmospheric pressure. The pressure relief opening can be provided at the lower side of the pedestal, close to the gas exhaust and relatively far from the wafers. Unfortunately, it appears that together with the expanding gas, many particles also escape from the interior of the pedestal into the process chamber. The particles are difficult to confine to the bottom of the pedestal and can eventually distribute throughout the process chamber. These particles can be detected on wafers processed in the process chamber, which is undesirable since the particles can degrade process results. Undesirably, the insulating material can serve as an infinite source of particles for contaminating wafers. 
   Accordingly, there is a need for furnace pedestals that are resistant to exploding and that are not sources of contaminating particles. 
   According to one aspect of the invention, the quartz envelope of the pedestal is provided with a pressure relief opening comprising a filter which is permeable for gas, allowing expanding gas to escape from the pedestal&#39;s interior, but which is not permeable for particles. 
   According to another aspect of the invention, a system for semiconductor processing is provided. The system comprises a batch process chamber and a wafer boat configured to be accommodated in the process chamber. A pedestal is provided for supporting the wafer boat in the process chamber. The pedestal comprises an envelope with an internal gas volume. A wall of the envelope comprises an exterior opening in gas communication with the internal volume. The exterior opening defines a flow path for gas exiting the internal volume. The pedestal further comprises a particulate filter disposed in the flow path. 
   According to yet another aspect of the invention, a pedestal is provided for supporting an overlying wafer boat in a process chamber of a semiconductor processing furnace. The pedestal comprises an envelope defining walls of the pedestal and an exterior opening on a surface of the envelope. A particulate filter is configured to filter gas flow through the opening. 
   According to another aspect of the invention, a method for semiconductor processing is provided. The method comprises providing a wafer boat containing a plurality of semiconductor substrates. The wafer boat is supported on a hollow pedestal. Particles are filtered out from gas exiting an interior of the pedestal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from the Detailed Description of the Preferred Embodiments and from the appended drawings, which are meant to illustrate certain embodiments of and not to limit the invention, and wherein: 
       FIG. 1A  is a schematic, cross-sectional side view of a furnace provided with a boat placed on a pedestal, in accordance with the prior art; 
       FIG. 1B  is a schematic, cross-sectional side view of a furnace provided with a boat placed on a pedestal, in accordance with preferred embodiments of the invention; 
       FIG. 2  is a schematic, cross-sectional side view of a pedestal provided with a filter in a pressure relief opening in its envelope, in accordance with preferred embodiments of the invention; 
       FIG. 3  is a detailed schematic, cross-sectional side view of the filter and pedestal of  FIG. 2 , in accordance with preferred embodiments of the invention; 
       FIG. 4A  is a plot showing the number of detected airborne particles over time, in a furnace having a pedestal without a filter in its relief opening; and 
       FIG. 4B  is a plot showing the number of detected airborne particles over time, in a furnace having a pedestal provided with a filter in its relief opening, in accordance with preferred embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   According to preferred embodiments of the invention, a pedestal is provided with a relief opening, which in turn is provided with a particulate filter in the flow path of gas flowing through the opening. The filter is preferably substantially impermeable to particles in the pedestal&#39;s interior. The filter advantageously minimizes or eliminates the escape of particles out of the pedestal and into the reaction space of a furnace containing the pedestal. Thus, particulate contamination of semiconductor substrates, e.g., semiconductor wafers, processed in the furnace can be reduced, thereby allowing for higher quality process results, such as during processing at elevated temperatures, including processing at temperatures of, e.g., about 1000° C. or higher. 
   Reference will now be made to the Figures, wherein like numerals refer to like parts throughout. 
   With reference to  FIGS. 1B ,  2  and  3 , a pedestal  100  according to preferred embodiments of the invention is illustrated. Advantageously, the pedestal  100  can be directly substituted for existing pedestals, such as the pedestal  90  ( FIG. 1A ), for use in existing furnaces, such as the furnace  1 , as illustrated in  FIG. 1B . The pedestal  100  includes insulating material  110  disposed within an envelope  120 , preferably made of quartz. The insulating material  110  is preferably porous and can be similar to the insulating material  90  ( FIG. 1A ) described above, typically being a material with a high fraction of pores, such as quartz foam, Al 2 O 3  with high porosity or a mixture of high porosity materials. A recess  112  is made in the illustrated block of insulating material  110  to accommodate an exhaust tube  122 , which in the illustrated embodiment is a quartz cylinder and which is welded, with its cylinder axis perpendicular to the envelope  120  surface, to the inner surface of the bottom part of envelope  120 , as illustrated. A particulate filter  130 , which can be in the shape of a disc, is disposed inside the exhaust tube  122 . The filter disc  130  and the exhaust tube  122  are preferably dimensioned and shaped such that gas flowing out of the envelope  120  must flow through the filter disc  130 . Preferably, the outside diameter of filter disc  130  is equal to the inner diameter of the exhaust tube  122 , such that the disc  130  separates the inner space of exhaust tube  122  into two parts, so that gas communication between the two parts is only possible through filter disc  130  and the other walls of the envelope  120  are airtight. 
   The envelope  120  is also provided with a pressure relief channel  140  which provides an outlet for gas flowing through the filter disc  130 . The relief channel  140  has an interior opening  142  ( FIG. 3 ) within the exhaust tube  122  and on the interior surface of the envelope  120 . The interior opening  142  leads to a first vertical section  144 , a horizontal section  145 , and a second vertical section  146  of the relief channel  140 , leading to an exterior opening  148 . The filter disc  130  allows expanding gas to escape from the interior of the pedestal  120  through the channel  140  to the exterior of the pedestal  120 , while keeping particles in the interior of the pedestal  120  confined inside the envelope  120 . The exterior discharge opening  148  of the pressure relief channel  140  is preferably positioned at the bottom part of the pedestal  120 , so that gas exiting the channel  140  is discharged into the process chamber  16  ( FIG. 1B ) at a location remote from wafers being processed. 
   The filter  130  is able to withstand process temperatures in the process chamber  16  ( FIG. 1B ) and preferably has a relatively high temperature resistance. Suitable materials for forming the filter disc  130  are sintered ceramic or glass materials in which the sintering is performed such that the material is porous and has an open pore structure that allows gas to pass through the porous material. The porous filter disc  130  can be, e.g., a porous filter disc with controlled pore size made of sintered glass powder, commercially available from Heraeus Quartzglas of Kleinostheim, Germany. In contrast to the isolating material  110  inside the pedestal  100 , the porous filter discs  130  have a high purity and high integrity, so that they do not readily emit particles into a contacting fluid but rather filter the particles out. In addition, another advantageous property of these porous filter discs is that they can be joined relatively easily to clear fused quartz, such as that forming the exhaust tube  122 , by welding. 
   Filters formed from sintered glass powder are suitable for various applications, depending on their pore size. For example, filter discs with fine pore sizes, below about 100 μm, can be used to filter or capture very small materials (pores of this size can be useful in analytical chemistry), and filters with larger pore sizes, about 100-550 μm are used for many liquid and gas distribution and filtration applications. Preferably, for use in the pedestals  100 , the filters  130  have a pore size rating of about 30 μm or less, more preferably, about 20 μm or less and, more preferably, about 16 μm or less. Typically, the rated pore size is substantially the maximum size of the pores in a filter, although it will be understood that there may be some variation in the pore size, with, e.g., a negligible number of pores having a size exceeding the rated pore size. 
   Preferably, the porosity of the filter disc  130  is chosen based upon the sizes of the particles present inside the pedestal  100 . The pores are preferably of a similar size or smaller than the sizes of the particles for which filtration is desired. It has been found that good results can be obtained with a filter disc # 4 , having a maximum pore size of about 10-16 μm. This is the filter disc with the smallest pore size currently available from Heraeus Quartzglas. Although this pore size is still larger than the dimensions of some particles, it will be appreciated that these particles do not easily pass the porous filter disc if the filter disc is sufficiently thick, so that the path of the particles out of the porous filter is long. Preferably, the pore size is about 10 or more times, more preferably 100 times or more, smaller than the thickness of the porous filter disc. 
   In the exemplary embodiment shown in  FIGS. 2 and 3 , the filter disc  130  has a diameter of about 35 mm and a thickness of about 4 mm. It will be clear to a person of skill in the art that different dimensions and designs are possible, e.g., the filter disc  130  and the mating exhaust tube  122  are not necessarily cylindrical but can have any other shape as long as they allow the filter  130  to effectively filter particles from the interior of the pedestal  100 . For example, the filter disc  130  and/or the exhaust tube  122  can be in the shape of a square or rectangle. Preferably, whatever the shape, the filter disc  130  is plate-like. In other embodiments, a part of the envelope  120  or the entire envelope  120  can be made of a porous material having a high integrity, e.g., the entire bottom of the pedestal  100  can be made of the porous material, allowing the bottom to act as a filter. In yet other embodiments, a part of the quartz material of the envelope  120  can be replaced by a porous filter disc  130 , such that the porous filter disc separates the interior of the pedestal  120  from the exterior ambient and gas transport is only allowed through the porous filter disc  130  or other shaped filter plate. It will also be appreciated that multiple filter plates can be provided in the pedestal  100  and/or multiple surfaces of the pedestal  100  can be provided with porous material which functions as a filter. 
   The effect of the filter disc  130  can be seen in  FIGS. 4A and 4B . The presence of air borne particles having a size of 0.3 μm or more was measured in a quartz exhaust channel connected to the process tube of a vertical furnace. A valve downstream in the exhaust channel was repeatedly switched, resulting in repeated pressure fluctuations. A pressure reduction resulted in escape of gas from the interior of a pedestal, and a pressure increase resulted in gas streaming into the pedestal. Air borne particles were measured continuously at regular time intervals. The results for a conventional pedestal with a pressure relief opening are shown in  FIG. 4A  and the result for a pedestal provided with a filter disc in the pressure relief opening, in accordance with the preferred embodiments, are shown in  FIG. 4B . 
   Advantageously, it can be observed that the filter is effective in reducing the number of particles.  FIG. 4B  shows the presence of few particles, especially in comparison to  FIG. 4A . The high counts at t=800 minutes in  FIG. 4B  can be attributed to the movement of the pedestal and boat at that time (movement of the pedestal and the boat out and then back into the process chamber). Thus, the particles measured at that time are likely not particles which escaped from the interior of the pedestal. At other times, the boat and pedestal remained in a stationary position inside the process chamber, with the door to the chamber closed. The difference in performance is even more pronounced when it is taken into account that the conventional pedestal had been already in use for months whereas the pedestal having a filter was new. It will be appreciated that the particle counts of a conventional pedestal are typically much higher when new and can be expected to decrease over time. Thus, the particle levels shown in  FIG. 4A  would be higher, if the pedestal used in those tests were as new as the pedestal used in the tests for  FIG. 4B . Nevertheless, it can seen that the pedestal with the filter allows for a dramatic reduction in the number of particles present. 
   It will be appreciated that various modifications, omissions and additions may be made to the methods and structures described above without departing from the scope of the invention. All such modifications and changes are intended to fall within the scope of the invention, as defined by the appended claims.