Abstract:
An improved nitrogen blanket distributor is provided for use in an atmospheric pressure chemical vapor deposition (CVD) apparatus of the type used for semiconductor fabrication. In the improved distributor, the nitrogen plenums are formed using angular components replacing the tubular components known in the art, thus considerably stiffening the distributor structure. Improved multi-layer lateral seals are also provided for preventing gas flow around the distributor assembly. Furthermore, improved stress relieving features are provided in the primary screen in order to reduce thermal cycling failures of that component.

Description:
FIELD OF THE INVENTION 
     This invention relates to improvements in semiconductor processing equipment, and in particular, in a preferred embodiment, to an improved nitrogen blanket distributor for an atmospheric pressure chemical vapor deposition (APCVD) apparatus. 
     BACKGROUND 
     Atmospheric pressure chemical vapor deposition (APCVD) is a semiconductor processing technique that is well known in the art. Several companies make APCVD apparatus for sale to semiconductor manufacturing companies. The present invention is an improvement to a prior art APCVD arrangement, as is described in detail in this specification. 
     U.S. Pat. No. 4,834,020 (Bartholomew, et al.) discloses a conveyorized atmospheric pressure chemical vapor deposition apparatus. This patent is assigned to Watkins-Johnson Company of Palo Alto, Calif. The apparatus described in this patent discloses use of nitrogen blanketing apparatus surrounding each deposition chamber to separate the ambient atmosphere from the deposition environment and to ensure that deposition chemicals do not escape from the immediate vicinity of the deposition chamber. Nitrogen distribution plenums are positioned on the entry side of each deposition chamber and on the exit side of each deposition chamber. 
     U.S. Pat. No. 5,304,398 (Krusell, et al.) also is assigned to Watkins-Johnson Company. This patent also discloses an atmospheric pressure chemical vapor deposition apparatus, which includes a gas injection assembly (indicated at reference  310  in FIG. 1 of the present specification, which is a reproduction of FIG. 3 of the Krusell, et al. patent) through which process gases and nitrogen are introduced onto the surface of wafer  315  that is processed by the apparatus. This is the type of system in which the preferred embodiment of the present invention is intended to be used. Referring to FIG. 1, the structures  12 ,  14   a  and  14   b  on either side of exhaust passages  336  and  338  are nitrogen distributors which provide a nitrogen blanket around the process area and the gas injection assembly  310 . The function of these nitrogen distributors is to provide a steady flow of nitrogen to the regions surrounding the process gas injection operation to remove excess or spent process gasses and to prevent introduction of foreign materials into the process area. The nitrogen that is fed through these distributors is removed from the process area via exhaust passages  336  and  338 , thus forming a continuous flow of inert gas surrounding the process area. The aforementioned patents are hereby incorporated by reference into this specification. 
     FIG. 2A shows the construction of a prior art nitrogen distributor assembly over which the present invention provides a number of improvements. Nitrogen distributor assembly  10  comprises primary nitrogen distributor  12  and secondary nitrogen distributors  14 . Each distributor  12 ,  14  comprises a solid shell  20  and a perforated screen  24  that form an enclosed space into which nitrogen may be introduced. Nitrogen supply tubes  16  are provided to convey nitrogen from an external nitrogen source to the nitrogen distributor assembly. Inside of nitrogen distributors  12  and  14 , these tubes  16  are perforated to release nitrogen within the distributors. Referring to the prior art primary distributor  12 , each nitrogen tube  16  enters the distributor and is bent at a 90° angle as shown at point  18  and then welded at selected points along its length to shell  20  of primary distributor  12  beneath screen  24 . A number of holes are drilled in nitrogen tube  16  proximate shell  20  in order to allow nitrogen to flow from the tube  16  into the enclosed space formed between shell  20  and screen  24  of primary nitrogen distributor  12 . Baffle  22  is constructed from a thin strip of metal and attached to tube  16  at one edge and to shell  20  at the other edge. Baffle  22  is provided in order to block the direct flow of nitrogen out of the perforations in tube  16 , to slow down the nitrogen flow and to allow it to evenly disperse within primary distributor  20  beneath primary screen  24 . A similar structure is employed in forming secondary nitrogen distributors  14  by using a corresponding tube having holes drilled therein positioned between a secondary shell a secondary stainless steel screen. 
     The prior art nitrogen distributor assembly shown in FIGS. 2A and 2B employs topographic features  26  in the primary screen  24  that are intended to control temperature-cycling related failures. Because these features exist only in the planar section of the primary screen  24  and do not extend around the curve that is formed by the screen as it passes around the lateral sides of primary distributor  12 , they tend to ensure that thermal-cycling failures occur primarily in the specific region of the features, rather than preventing such failures. 
     FIG. 2B shows a typical prior art version of a APCVD injection assembly including gas blanket distribution apparatus. A work piece, which is typically a semiconductor wafer  210 , may be conveyed past the operative assembly on conveyor belt  212 . Injector head  214  receives the reactant process gasses and furnishes them through injector nozzle  216 , which extends through a window formed in primary nitrogen distributor  218 , and onto the surface of work piece  210  as it passes beneath nozzle  216 . Secondary nitrogen distributors  220  are provided on each side of primary nitrogen distributor  218 . The sides of primary nitrogen distributor  218  and secondary nitrogen distributors  220  that face each other and that face toward the work piece comprise perforated screen that may be made of stainless steel, which allow nitrogen to flow from the interiors of primary distributor  218  and secondary distributors  220  and into the vicinity of work piece  210  and injector nozzle  216  such that excess and spent process gasses are diluted and carried by the nitrogen flow away from the process area and up through the exhaust channels provided between primary nitrogen distributor  218  and secondary nitrogen distributors  220 . The flow of nitrogen is generally shown by the arrows in FIG.  2 B. 
     Nitrogen supply tubes  222  are operatively connected to a source of nitrogen (or other selected blanket gas) to provide a steady flow of nitrogen into primary nitrogen distributor  218  and secondary nitrogen distributors  220 . Additional tubes  224  may be used to provide reactant gasses into injector head  214 . First, the secondary distributor seals used in prior art devices are subject to leaking and failure. This is due in part to the design of prior art sealing elements and in part to inadequate structural rigidity in the distributor assembly. Second, prior art devices are prone to cause “powdering,” which can introduce harmful particulate contaminants into the processing area. Third, the prior art nitrogen distributor assemblies are difficult to manufacture. Finally, the perforated screens in prior art devices are known to experience stress cracking due to differential rates of thermal expansion between distributor components. Each of these deficiencies is discussed further below. 
     In the atmospheric pressure chemical vapor deposition (APCVD) machines that are known to the applicant, the nitrogen blanketing apparatus that is employed has several deficiencies which are corrected by the present invention. 
     Secondary seals  226  may be provided on the outboard side of each secondary distributor  220 . Each secondary seal  226  provides a seal between secondary nitrogen distributor  220  and the wall of the enclosure into which this assembly is placed during operation, which is not shown in this illustration. (See FIG. 7) In the prior art, as shown in FIG. 2B, secondary seal  226  is a strip of flexible stainless steel, one edge of which is welded to the back of secondary nitrogen distributor  220 , and the other edge of which is bent away from secondary distributor  220  in order to contact the enclosure when the distributor assembly is inserted therein. One difficulty with this arrangement is that in the event that secondary distributor  220  bends or flexes during operation due to mechanical or thermal stresses, secondary seal  226  tends to buckle thereby lose its contact with the enclosure wall over a portion of its length, and a leak in the seal can therefore be formed. One deficiency in the prior art is that the secondary distributors are not adequately stiff to resist bending as a result of mechanical and thermal stresses applied during operation, and when such bending occurs the secondary seals can become incapable of closing the space between the back of the secondary distributor and the enclosure wall. 
     Furthermore, prior art seals have a spring rate that is relatively large compared to the stiffness of the secondary assemblies. Thus, when the distributor assembly is forced into its enclosure, a force is developed that distorts the straightness of the secondary distributors , non-linearly distorting the gap between primary and secondary distributors. This distortion in the gap width affects the uniformity of the process by introducing unpredictable variations in the shielding gas flow pattern. Another problem is that the prior art seals are non-conformal against irregularities in the walls of the enclosure they fit within. Some prior art seals consist of a thin strip of metal incorporating a single bend away from the body of the secondary shell. A serious problem with this design becomes apparent when installation into the enclosure is attempted. Because the strip opens outward into the enclosure, there is a tendency for the leading edge of this seal to be captured by irregularities in the sealing surface causing the thin metal to buckle or suffer an otherwise catastrophic failure. An alternative prior art seal design incorporates a reentrant bend along the distal edge of the seal. This bend makes the distributor easier to insert into its enclosure, but because of the reentrant bend, the sealing edge is more rigid and therefore even less conformal than the aforementioned design. 
     A phenomenon referred to as “powdering” is known to occur in prior art nitrogen blanket injection assemblies. This occurs when the flow of nitrogen and process gasses becomes restricted and silicon or some other solid material becomes deposited upon surfaces within that apparatus. Those deposits can break loose and form particulate impurities which can adversely affect the semiconductor manufacturing process. In addition to providing a shielding atmosphere for the process, the nitrogen passing through the perforations in the screens serves to prevent such reaction products from building up on the outside of the distributor assemblies. Some prior art nitrogen distributors have areas of the screen that are blocked so that nitrogen cannot flow therethrough. For example, some designs include an overlying piece of the screen material around each tube penetration that in effect blocks many of the perforations in those areas. A similar problem is caused by a feature sometimes added in an attempt to stiffen the secondary assembly, wherein the edge of the secondary shell secondary is bent back upon itself. This forms a solid surface that lies beneath the perforated screen material and eliminates any possible gas flow. These features are believed to be largely responsible for producing the powdering effect noted in the prior art. 
     Another deficiency in the prior art nitrogen distribution assemblies is that they are not conducive to mass production manufacturing techniques, primarily because the nitrogen supply tubes are an integral part of the plenum assembly. Once they are welded to the primary and secondary shells early in the manufacturing process, the supply tubes become cumbersome impediments during the remaining stages of production. It would be preferable to be able to install the nitrogen supply tubes as a final step in the manufacturing process. 
     Yet another deficiency in the prior art is that the secondary distributors are relatively long, thin assemblies which may be prone to bending or flexing during use. This is undesirable because it is important to provide a reliable seal between the outboard side of the secondary distributors and the enclosure into which the assemblies are installed. It has been noted that the prior art secondary distributors are not stiff enough to prevent flexing due to mechanical forces, primarily the forces exerted by prior art seal structures. Furthermore, the seals that are used on the outboard sides of the prior art secondary distributors are not adequate, when combined with the flexibility of the secondary distributors to provide a reliable seal during use of the apparatus. 
     Both the primary nitrogen distributor and the secondary nitrogen distributors in the prior art have outer surfaces made of stainless steel screen material. In use, nitrogen flows from the nitrogen distribution apparatus within the nitrogen distributors and outward through the screen material into the regions surrounding the wafers being processed. During operation of these machines, the temperature of the processing chamber can fluctuate between room temperature and 600°. This temperature cycling results in flexing or “oil canning” of the perforated screen material, due to differences in the rate of thermal expansion between the screen and the shell to which it is attached. Repeated stressing of the screen material in this way results in a common mode of failure where cracks form in the surface of the screen. These cracks interrupt the desired nitrogen flow patterns and can cause undesired turbulence in the vicinity of the cracks, decreasing the effectiveness of the nitrogen blanket and the APCVD process. Some prior art distributors have features  26  formed in the screens that serve to localize stress, but the known approaches for providing stress relief are insufficient to cure the “oil canning” problem. 
     It is therefore desirable to provide a nitrogen blanket distribution apparatus for an atmospheric pressure chemical vapor deposition process for semiconductor processing which reduces powdering, improves manufacturability, provides stress relief for the screen material, provides improved stiffness of secondary nitrogen blanket distributors, and provides improved sealing between the secondary distributors and the walls of the enclosure in which the nitrogen blanket distributor assembly is located. 
     SUMMARY OF THE INVENTION 
     In presently preferred embodiments, the present invention provides a nitrogen blanket distributor assembly for the existing type of chemical vapor deposition equipment described above, which reduces powdering, improves manufacturability, provides stress relief for the screen material, improves the stiffness of the secondary nitrogen distributors, and provides improved seals on the outboard sides of the secondary distributors so as to provide an improved apparatus for use in existing and future chemical vapor deposition machinery. 
     The present invention improves manufacturability by removing the perforated tube from the distributors, replacing it with a plenum member formed of sheet metal, which may be stainless steel, which is then welded to the base plate of the injection assembly. Holes may be drilled in the plenum member in order to provide for escape of nitrogen therefrom. Openings are provided in the plenum member for placement of nitrogen feed tubes later in the manufacture process. The assembly formed by the combination of plenum member and base plate welded together provides improved stiffness of the assembly over that seen in the prior art, both in the primary distributor and in the secondary distributors. Furthermore, because the nitrogen supply tubes can be installed at the end of the manufacturing process, distributors according to this invention can be produced more accurately, consistently and efficiently without cumbersome tubes getting in the way. 
     In preferred embodiments of the present invention, the secondary seals are also improved by providing a two-layer, overlapping, segmented arrangement, which provides a conformal seal with less force applied to the secondary distributors, and which prevents the seals from becoming distorted or buckled and thereby leaking if there is any bending of the secondary distributors or enclosures due to thermal or mechanical stresses applied thereto. 
     An improved nitrogen distribution screen arrangement is also disclosed for use in preferred embodiments of the present invention. In particular, three-dimensional stress relieving features are formed into the distribution screen in order to permit the screen to absorb the applied stresses without “oil canning” and eventually failing. 
     In one aspect, the present invention provides a gas blanket distributor for use in a reaction chamber, said distributor comprising a primary distributor having a window for passage of an injection nozzle therethrough and a secondary distributor positioned beside and spaced from the primary distributor, each distributor comprising a plenum member attached to a shell, each plenum member being formed of an elongated piece of rigid material formed to provide two elongated and substantially parallel attachment portions separated by a raised portion, wherein the raised portion of the plenum member and the shell define an enclosed space when the plenum member is attached to the shell. The plenum member may be perforated by a plurality of holes formed in the raised portion. The assembly may further comprise a baffle having one edge attached to the raised portion of the plenum member and the other edge spaced from the plenum member, the baffle being positioned to cover said holes without sealing them. 
     In another aspect, the invention provides a method of making a gas blanket distributor for use in a reaction chamber, wherein the distributor includes a primary distributor having a window for passage of an injection nozzle therethrough. A preferred method comprises attaching a plenum member to a primary shell, attaching a primary screen to the primary shell, then inserting a gas supply tube through the primary screen into the plenum, and attaching the gas supply tube to the primary shell. 
     In yet another aspect, a preferred embodiment of the present invention provides a gas blanket distributor for use in a semiconductor processing apparatus, wherein the distributor has a lateral sealing assembly for inhibiting the flow of gas between a lateral side of the distributor and an enclosure in which the distributor is operatively positioned. The lateral sealing assembly may comprise first and second sealing strips each having a longitudinal spine portion adjacent one edge and a sealing portion adjacent the other edge. The sealing portion may be divided into segments by slots formed across the sealing portion and terminating prior to traversing the spine portion, and the spine portion of the first sealing strip may be attached along its length to the lateral side of the distributor. The spine portion of the second sealing strip may then be overlaid upon the spine portion of the first sealing strip such that the segments of the second sealing strip cover the slots of the first sealing strip. 
     In another aspect, preferred embodiments of the present invention provide a gas blanket distributor for use in a semiconductor processing apparatus, comprising primary distributor means for distributing a uniform flow of gas in the vicinity of a process gas injector, and secondary distributor means for distributing a uniform flow of gas therefrom, said secondary distributor means being spaced from said primary distributor means by an exhaust channel, wherein said secondary distributor means comprises a shell and a plenum member which cooperate to form an enclosed gas distribution chamber. The primary distributor means may comprise a shell and a plenum member which cooperate to form an enclosed gas distribution chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the herein described advantages and features of the present invention, as well as others which will become apparent, are attained and can be understood in detail, more particular description of the invention summarized above may be had by reference to the embodiments of the invention which are illustrated in the appended drawings, which form a part of this specification. 
     FIG. 1 is an elevation view of a CVD nozzle and nitrogen blanket distribution assembly as is known in the prior art. This figure is adapted from FIG. 3 of U.S. Pat. No. 5,304,398. 
     FIG. 2A is a partially cut-away perspective illustration of a nitrogen distributor assembly as is known in the prior art, over which the present invention provides several improvements. 
     FIG. 2B is a perspective view of the prior art nitrogen distributor assembly of FIG. 2A, shown with the corresponding process gas injector head and nozzle, as well as a wafer being processed carried past the assembly on a conveyor belt. 
     FIG. 3 is an exploded perspective view of a primary distributor according to the present invention. 
     FIGS. 3A and 3B are detailed views of the plenum member configuration of this invention. 
     FIG. 4 is an exploded perspective view of a secondary distributor according to the present invention. 
     FIG. 5 is a partially cut-away perspective view of a nitrogen shield distributor assembly according to the present invention. 
     FIG. 6 is an enlarged illustration of the cut-away portion of the embodiment shown in FIG.  5 . 
     FIG. 7 is a perspective cross-sectional view of a nitrogen shield distributor assembly according to the present invention in combination with the corresponding injection head and nozzle and the enclosure in which the assembly is located. 
    
    
     It is noted, however, that the appended drawings illustrate only exemplary embodiments of the invention and are, therefore, not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring briefly to FIG. 5, the nitrogen distributor assembly consists of primary nitrogen distributor  340 , accompanied on each side by a secondary nitrogen distributor  410 . FIG. 3 is an exploded view of a preferred embodiment of primary nitrogen distributor  340 . Primary distributor shell  342  comprises a generally “C” shaped metal form. It may be fabricated as two separate pieces and joined along a longitudinal axis down the center of the distributor. A window  344  may be formed in primary distributor  342  in order for process gas injector nozzle  216  to have access through primary distributor  340  to workpieces (wafers) that are conveyed past it for processing (see FIG.  7 ). In the illustrated embodiment, window reinforcements  346  are affixed to the primary distributor at each end of window  344 . Primary shell ends  348  are also provided as illustrated. 
     A primary plenum member  350  is formed of stainless steel and attached, for example by welding, to each side of primary distributor  342  as illustrated. Primary plenum member  350 , when attached to primary shell  342 , provides an enclosed space or plenum into which nitrogen may be injected in order to be dispersed by primary nitrogen distributor  340 . Each primary plenum member  350  is perforated by a plurality of holes drilled therein  352 . In some embodiments, each longitudinal edge of plenum member  350  may be bent away from shell  342  in order to provide additional mechanical strength or stiffness to the completed assembly. The holes  352  in primary plenum member  350  are overlaid by primary baffle  354 , which intercepts the directed streams of nitrogen exiting the plenum through holes  352 . Primary baffle  354  is attached, as by welding, to primary plenum member  350  along the peak  353  of plenum member  350 , and the free edge  355  is spaced from plenum member  350  so that nitrogen can flow through the space thus created. (FIG. 6 shows a detail of the plenum and baffle assembly.) In use, nitrogen is supplied to the plenum by tubes  362 , and it exits the plenum through holes  352 . The gas, expanding into the space is part defined by baffle  354  and the outer wall of plenum member  350 , loses much of its local velocity and exits the distributor as a diffuse and uniform flow. 
     Primary screen  356  is configured to cover and enclose primary plenum member  350 , primary baffles  354  and to form an enclosed space between itself and primary shell  342 . Primary screen  356  is provided with an elongated window  358  which aligns with window  344  in primary shell  342 . These windows  358 ,  344 , are provided to permit process gas injector nozzles to be directed therethrough during semiconductor wafer processing. Primary screen  356  is provided with stress relief contours  360  as shown in FIG.  3 . These stress relief contours serve to permit primary screen  356  to flex like a bellows when stressed by thermal cycling of the system. Note that in preferred embodiments the stress relief contours  360  extend across the top surface of primary screen  356  as well as around the curved sides of primary screen  356 , occupying substantially the entire width of primary screen  356 . 
     Note that an enclosed space is formed by primary shell  342 , primary screen  356  and primary shell ends  348 , as well as sealing features around windows  358  and  344 . Nitrogen that exits plenum  350  through holes  352  makes its way around primary baffle  354  and into that enclosed space. From there, it exits primary nitrogen distributor  340  through the plurality of small holes formed in primary screen  356 . 
     In preferred assembly methods, after the above-described components are assembled, then primary tubes  362  are inserted through holes formed in primary screen  356  and into primary plenum  350  and into position as shown in FIG.  6 . Primary tubes  362  may be secured into place by welding to tube attachment tabs  363  formed on primary shell  342 . Primary tubes  362  may then be used to introduce nitrogen into the space formed between primary plenum  350  and primary shell  342 . It is advantageous to be able to assemble the distributor without the tubes  362  attached, and then to install the tubes as a final step in the assembly process. 
     Referring to FIGS. 3A and 3B, plenum member  350  is a folded and elongated piece of sheet metal that is attached to shell  342 . Alternatively, plenum member  350  could be extruded in the form shown. In the illustrated preferred embodiment, plenum member  350  has two attachment portions  370  that are substantially coplanar and that are used to attach the plenum to shell  342 , such as by welding. Plenum member  350  also has a raised portion  372  located between attachment portions  370 . Raised portion  372  cooperates with shell  342  to form an enclosed space  374  into which nitrogen is introduced for distribution. Raised portion  372  is perforated by holes  352  to allow nitrogen to exit enclosed space  374 . As shown in FIG. 3B, one edge of baffle  355  is attached to raised portion  372  of plenum member  350 , and the free edge  355  of baffle  354  is spaced from the surface of plenum member  350 . It will be appreciated that gas that is introduced into enclosed space  374  formed by plenum member  350  will exit through holes  352 , into the space beneath baffle  354 , and flow uniformly outward through the space between free end  355  of baffle and plenum member  350 . Plenum member  350  may also have edge portions  376  that are bent upward from attachment portions  370 , which serve to further stiffen plenum member  350  and the assembly into which it is incorporated. In presently preferred embodiments, this plenum and baffle arrangement is used in the primary distributor and in the secondary distributors, which are described below. 
     FIG. 4 shows an exploded view of secondary nitrogen distributor  410 . Secondary shell  412  is a formed piece of metal sheet that provides a foundation for secondary distributor  410 . In preferred embodiments, secondary shell  412  has end sections  414  bent at ninety degree angles to the plane of shell  412 , and it has tube attachments tabs  416  formed thereon as shown in FIG.  4 . Secondary plenum member  418  and baffle  420  may be formed and assembled in the same manner as the corresponding components of primary distributor  340  described above. Secondary plenum member  418  is attached by welding to the surface of secondary shell  412  forming an enclosed space there between. This arrangement of a rigid formed plenum member  418  welded to secondary shell  412  provides a relatively stiff secondary nitrogen distributor  410  as compared to the prior art. Secondary plenum member  418  is perforated by a plurality of holes  419  which allow nitrogen to exit the plenum space into to the space formed between secondary shell  412  and secondary screen  422 . Secondary baffle  420  is provided to aid in forming a uniform flow of nitrogen within secondary distributor  410 , identical in function to primary baffle  354  discussed above. Secondary screen  422  is a stainless steel screen material that is installed around secondary plenum member  418  and secondary baffle  420  and attached to secondary shell  412 . The combination of secondary shell  412 , secondary screen  422 , and end portions of  414  provide an enclosed space. After the above referenced assembly is put together, secondary delivery tubes  424  may be inserted through holes in secondary screen  422  provided for that purpose and into the appropriate openings in secondary plenum member  418 . Tubes  424  may then be secured in place by welding to tube attachment tabs  416 . The assembly of the secondary shell, plenum, baffle and delivery tubes may be identical to the assembly of the corresponding primary components, as may be seen in the detailed view of FIG.  6 . 
     In order to ensure that the exhaust gasses from the CVD process exit the vicinity of the processing operation by passing between the primary and secondary distributors, sealing elements are provided for blocking the spaces between the outboard sides of the secondary distributors and the enclosure into which the assembly is inserted. In the prior art embodiments, the seal that is disposed on the outboard side of the secondary distributors is a single sheet of flexible steel  226  (FIG. 2B) welded to the outboard side of the secondary shell  227 . This arrangement has been improved on in the present invention. 
     Referring to FIG. 4, in preferred embodiments of the present invention, secondary seal  425  is composed of two layers of flexible metal provided on the outboard side of secondary shell  412 . Secondary seal  425  comprises inner secondary seal  426  and outer secondary seal  428 . Each of these secondary seals  426 ,  428 , comprises a spine section  430  along one edge, which may be perforated by a number of holes  432  positioned at regular intervals. Connected to one side of spine  430  is a continuous series of integral independent segments  434 , which may be formed by starting with a strip of metal and cutting it part way across from one side at regular intervals. The segmented portion of each seal is outwardly bent or curved away from the back of secondary shell  412  in order to effect a somewhat resilient seal between secondary shell  412  and the enclosure into which the nitrogen distributor assembly is inserted during use. (See detail in FIG. 6.) The segments  434  may be uniformly curved as shown in FIG. 6 to facilitate insertion of the assembly into its enclosure while ensuring an adequate seal. Inner secondary seal  426  is first attached to secondary shell  412  by din spine portion  430  of seal  426  to shell  412 , with the weldments formed between holes  432  in spine  430 . Outer seal  428  is then positioned on top of inner seal  426 , such that the segments and holes of outer seal  428  are offset from those of inner seal  426  with the spines  430  of both seals overlying one another. The spine  430  of outer secondary seal  428  is then welded directly the back surface of secondary shell  412  through the holes  432  formed in inner secondary seal  426 . Persons skilled in the art will recognize that other methods may be used for attaching the secondary seals to the secondary shell. The segmented portions of outer seal  428  and inner seal  426  are arranged similar to roofing shingles, such that each slit between segments of the inner seal is covered by a segment of the outer seal. The secondary seal arrangement  425  described above and illustrated in FIG. 4 is conformal, and it will resist buckling or leaking even if secondary distributor  410  flexes somewhat due to mechanical or thermal stresses, due to the two layer, segmented arrangement provided by this invention. 
     In presently preferred embodiments, the primary and secondary screens  356 ,  422  are made of 0.008 inch  316  stainless steel, distributor shells  342 ,  414  are made of 0.035 inch 304 stainless steel, and tubes  424 ,  362  are made of 304 stainless steel, {fraction (3/16)} inch diameter, with a 0.020 inch wall thickness. The plenums may also be manufactured from 0.035 inch 304 stainless steel. Of course, other materials may be used to implement the invention. 
     FIG. 5 shows a preferred embodiment of completed nitrogen distributor assembly  500  with the outer layers of the foremost comer cut away so that the underlying configuration may be seen. (FIG. 6 shows a detailed view of that comer.) Note that each secondary distributor is attached to the primary distributor by two end brackets  510 , one attached to each end of the apparatus. A gap is preserved between secondary screen  422  and primary screen  356 , this providing an exhaust channel for process gasses and shield nitrogen to exit the vicinity of the workpiece. 
     In operation, nitrogen is supplied through secondary tubes  424  and primary tubes  362 . The nitrogen enters primary plenums  350  and secondary plenums  418 , exits the plenums through the holes drilled therein, and emerges beneath primary baffles  354  and secondary baffles  420 , respectively. The nitrogen then travels around the free edges of the baffles and into the spaces formed between primary shell  342  and primary screen  356 , and between secondary shell  412  and secondary screen  422 . The nitrogen then uniformly exits primary distributor  340  and secondary distributors  410  through primary screen  356  and secondary screen  422 , respectively, in order to blanket the work piece outside of the immediate vicinity of the process nozzle and to dilute any unused or spent reactant gasses, before they are removed by the exhaust system. It has been found that the nitrogen distributor assembly as illustrated and described herein reduces powdering due to the uniform flow and unobstructed egress of the nitrogen from the distributor assembly. 
     Referring to FIG. 6, an expanded view of the cutaway comer of the preferred embodiment shown in FIG. 5 is illustrated. The shape of primary plenum member  350  and secondary plenum member  418  are shown clearly in this figure, as is the placement of primary baffle  354  and secondary baffle  420  over the holes in each of the plenums. The upper edge of each baffle  354 ,  420  is free and spaced from the respective plenum member, so the nitrogen may flow through the gap formed between the plenum member and the baffle. The plenum shape is designed to provide structural stiffness to the assembly when a plenum is welded to primary shell  342  or secondary shell  412 . The penetration of secondary tube  424  into secondary plenum  418  is clearly indicated in this figure. Primary tube attachment tab  416  can be seen adjacent primary tube  362 . A welded connection may be formed between these two components when final assembly is performed in order to retain primary tube  362  in place inserted into primary plenum  354 . This arrangement may be used to connect all of the nitrogen supply tubes to the assembly. Note that in this embodiment a supply tube is connected to each end of each plenum member. 
     In an alternative embodiment of this invention, the primary screen  356  as well as secondary screens  422  may be constructed of a woven wire mesh material rather than wire screen material as in the preferred embodiment. As discussed above, the bellows-shaped configuration of the primary screen, which may alternatively be a wrinkled finish, prevents failure due to stresses related to thermal cycling. In one aspect, the invention comprises extending the stress relieving features around the curved sides of primary screen  356 . 
     Referring to FIG. 7, in its operative condition a preferred embodiment of the present invention is slidably inserted into enclosure housing  808  in an atmospheric pressure chemical vapor deposition apparatus used for processing semiconductor wafers. FIG. 7 shows the nitrogen distributor assembly  500  of FIG. 5, illustrated in cross-sectional perspective view, positioned within enclosure  808 . Injector head  214  and associated injector nozzle  216 , which are outside of the scope of this invention, are shown operatively engaged with primary distributor  340 , with nozzle  216  protruding through window  358 . Secondary distributors  410  are shown positioned on both sides of primary distributor  340 , with exhaust spaces maintained between the lateral edges of primary distributor  340  and each secondary distributor  410 . 
     Secondary seals  425  are fastened to the back of each secondary distributor  410 , and when the nitrogen distributor assembly  500  is operatively positioned in enclosure  808  the secondary seals  425  engage the interior surfaces of enclosure  808  to prevent process gasses or blanket gasses from traveling between secondary distributors  410  and enclosure walls  808 . As was described above in connection with FIG. 4, in preferred embodiments secondary seal  425  comprises multiple layers of flexible slotted sealing elements or segments which cooperate to ensure a good conformal seal between secondary distributor  410  and enclosure wall  808 , even if there is some degree of bending in secondary distributor  410  due to stresses applied thereto. In presently preferred embodiments, the secondary seals are formed from 0.005 inch thick 302 stainless steel, which is slotted or segmented similar to roof shingles. 
     In operation, wafer  210  is conveyed past injector nozzle  216  which directs a flow of a selected process gas or combination of process gasses toward the surface of wafer  210 , where the process gasses react with wafer  210  to accomplish a desired process step. At the same time, a flow of inert gas such as nitrogen is injected into primary distributor  340  and secondary distributors  410 , and an exhaust flow is established through the gaps between the lateral edges of primary distributor  340  and each secondary distributor  410  and out of enclosure  808 , as indicated by arrows  810 . This arrangement establishes a protective blanket of inert gas over the surface of the wafer, thereby facilitating precise control of the APCVD process by shielding the processing area from environmental contaminants. The flow of inert gas also serves to dilute and remove any excess or spent process gas or reaction products that may be present in the vicinity of the wafer. 
     This description of the invention has been directed toward describing an exemplary preferred embodiment, which is a nitrogen blanket distributor in an APCVD apparatus. The invention may be beneficially used in other applications as well, for distributing other gasses or for use in other processes. The scope of the invention is not intended to be limited to the specifically described embodiments and applications. 
     Further modifications and alternative embodiments of this invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the invention. It is to be understood that the forms of the invention herein shown and described are to be taken as the presently preferred embodiments. Various changes may be made in the shape, size and arrangement of parts and in material selection and specification. For example, equivalent elements may be substituted for those illustrated and described herein, and certain features of the invention may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the invention.