Patent Abstract:
The invention is embodied in a plasma reactor for processing a semiconductor wafer, the reactor having a gas distribution plate including a front plate in the chamber and a back plate on an external side of the front plate, the gas distribution plate comprising a gas manifold adjacent the back plate, the back and front plates bonded together and forming an assembly. The assembly includes an array of holes through the front plate and communicating with the chamber, at least one gas flow-controlling orifice through the back plate and communicating between the manifold and at least one of the holes, the orifice having a diameter that determines gas flow rate to the at least one hole. In addition, an array of pucks is at least generally congruent with the array of holes and disposed within respective ones of the holes to define annular gas passages for gas flow through the front plate into the chamber, each of the annular gas passages being non-aligned with the orifice.

Full Description:
BACKGROUND OF THE INVENTION 
     Various types of plasma reactors employed in the manufacture of semiconductor microelectronic circuits require a large RF electrode at the reactor chamber ceiling that overlies the semiconductor workpiece. Typically, the workpiece is a semiconductor wafer supported on a conductive pedestal. RF power is applied to the support pedestal, and the ceiling or overhead electrode is a counter electrode. In some reactors, the RF power applied to the support pedestal is the plasma source power (determining plasma ion density) and is also the plasma bias power (determining ion energy at the wafer surface). In other reactors, an RF power applicator other than the wafer pedestal furnishes the plasma source power, while the RF power applied to the wafer pedestal serves only as plasma RF bias power. For example, the plasma source power may be applied by an inductive antenna or may be applied by the ceiling electrode. Thus, the ceiling electrode may either be a grounded counter electrode for the RF power applied to the wafer support pedestal or it may be connected to an independent RF power generator and function as an independent RF power applicator. In either case, the most uniform distribution of process gas is obtained by introducing the process gas through the ceiling. This requires that the ceiling electrode be a gas distribution plate. 
     There is a continuing need to improve the uniformity of process gas distribution across the wafer surface in a plasma reactor, particularly in a plasma reactor used for semiconductor etch processes as well as other semiconductor processes. This need arises from the ever-decreasing device geometries of microelectronic circuits and minimum feature sizes, some approaching 0.15 microns. Such small device geometries are dictated in most cases by the desire for higher microprocessor clock speeds, and require corresponding improvements in etch rates, etch uniformity across the wafer surface and damage-free etching. Previously, with devices having relatively large feature sizes, a single gas inlet in the plasma reactor overhead ceiling electrode/gas distribution plate provided adequate process gas distribution uniformity. A single inlet would necessarily be of a large size in order to meet the requisite gas flow requirements. 
     One problem with such a large inlet is that it is more susceptible to plasma entering the inlet and causing arcing or plasma light-up within the inlet. Such arcing damages the plate and/or enlarges the inlet and consumes power. Sputtering of the plate material around the inlet can also contaminate the plasma with by-products of the sputtering. With a large hole, the maximum electric field occurs near the center of the hole, and this is the likliest location for plasma light-up or arcing to begin. One solution proposed for reactors having a single gas inlet was to juxtapose a disk or puck in the center of the hole to keep gases away from the intense electric field at the hole center (U.S. Pat. No. 6,885,358 by Dan Maydan). However, with current device geometries incorporating very small feature sizes, much better process gas distribution uniformity across. the wafer surface is required. As a result, a single gas distribution inlet or orifice in the ceiling gas distribution plate is inadequate to provide the requisite gas distribution uniformity. Thus, an overhead gas distribution plate is currently made by drilling thousands of fine holes or orifices through the plate. The spatial distribution of such a large number of orifices improves gas distribution uniformity across the wafer surface. The smaller size makes each hole less susceptible to plasma entering the hole. 
     Unfortunately, it has not been practical to place or hold an individual puck in the center of each one of the thousands of holes to ward the gas away from the high intensity electric fields near the hole centers. Thus, in order to reduce plasma arcing, the gas inlet holes must be of minimal diameter and within a small dimensional hole-to-hole tolerance to ensure uniform gas distribution. Drilling such a large number of holes is costly. This is because the holes must have such a high aspect ratio, must be drilled through very hard material (such as silicon carbide) and sharp hole edges must be avoided. Moreover, the very need for such accurately sized holes means that performance is easily degraded as hole sizes are enlarged by plasma sputtering of the hole edges. Depending upon plasma ion density distribution across the ceiling surface, some holes will be widened at a greater rate than other holes, so that a gas distribution plate initially having highly uniform gas distribution across the wafer surface eventually fails to provide the requisite uniformity. 
     Another problem is that the need for greater etch rate has dictated a smaller wafer-to-ceiling gap in order to obtain denser plasma. The small gas orifices produce very high velocity gas streams. The high velocity gas streams thus produced can be so narrowly collimated within the narrow wafer-to-ceiling gap that the hole-to-hole spacing in the gas distribution plate produces corresponding peaks and valleys in gas density at the wafer surface and corresponding non-uniformities in etch rate across the wafer surface. 
     As a result, there is a need for an overhead gas distribution plate that functions as an electrode or counter electrode, and that is not susceptible to plasma arcing in the gas injection passages, that does not have high gas injection velocities and in which the gas distribution uniformity and velocity are not affected by enlargement of the gas injection passages. 
     SUMMARY OF THE DISCLOSURE 
     The invention is embodied in a plasma reactor for processing a semiconductor wafer, the reactor having a gas distribution plate including a front plate in the chamber and a back plate on an external side of the front plate, the gas distribution plate comprising a gas manifold adjacent the back plate, the back and front plates bonded together and forming an assembly. The assembly includes an array of holes through the front plate and communicating with the chamber, at least one gas flow-controlling orifice through the back plate and communicating between the manifold and at least one of the holes, the orifice having a diameter that determines gas flow rate to the at least one hole. In addition, an array of pucks is at least generally congruent with the array of holes and disposed within respective ones of the holes to define annular gas passages for gas flow through the front plate into the chamber, each of the annular gas passages being non-aligned with the orifice. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified cut-away cross-sectional side view of a plasma reactor embodying the present invention. 
     FIG. 2A is a partially exploded cross-sectional side view of a gas distribution plate of the plasma reactor of FIG. 1 in accordance with a first embodiment. 
     FIG. 2B is a side view of an assembled gas distribution plate of the plasma reactor corresponding to FIG.  2 A. 
     FIG. 3A is a plan view of one implementation of the front plate of the gas distribution plate of FIG.  2 B. 
     FIG. 3B is a plan view of the front plate of FIG. 3A bonded to the back plate in accordance with this implementation. 
     FIG. 4 is a cross-sectional side view of the assembly of FIG. 3B corresponding to lines  4 — 4  of FIG.  3 B. 
     FIG. 5 is a cross-sectional side view of a gas distribution plate of the plasma reactor of FIG. 1 in accordance with a second embodiment. 
     FIG. 6 is a cut-away partially exploded perspective view of a gas distribution plate of the plasma reactor of FIG. 1 in accordance with a third embodiment. 
     FIG. 7 is a cross-sectional view corresponding to lines  7 — 7  of FIG.  6 . 
     FIGS. 8A,  8 B,  8 C and  8 D are sequential cut-away partial side views of one portion of a gas distribution plate of FIG. 6, illustrating a first process for fabricating the gas distribution plate of FIG.  6 . 
     FIGS. 9A,  9 B,  9 C and  9 D are sequential cut-away partial side views of one portion of a gas distribution plate of FIG. 6, illustrating a second process for fabricating the gas distribution plate of FIG.  6 . 
     FIG. 10 is a cross-sectional side view of a gas distribution plate of the plasma reactor of FIG. 1 in accordance with a third embodiment. 
     FIG. 11 is a cross-sectional side view of an alternate gas distribution plate as shown in FIG.  10 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a plasma reactor includes a vacuum chamber  100  bounded by a reactor chamber cylindrical side wall  105 , a ceiling  110  and floor  115 . A vacuum pump  120  maintains a vacuum within the chamber at a desired chamber pressure. A wafer support pedestal  125  for supporting a semiconductor wafer or workpiece  130  is disposed at the bottom of the chamber  100  so that the wafer  130  faces the ceiling  110 . The wafer support pedestal  125  has conductive elements so that the pedestal  125  can serve as an electrode or RF power applicator. For this purpose, an RF generator  135  is connected to the pedestal  125  through an RF impedance match circuit  140 . The ceiling  110  is conductive in the illustrated embodiment and is connected to the RF return terminal of the RF generator  135  so that the ceiling  110  serves as a counter electrode for the wafer pedestal  125 . In some types of reactors, another RF generator  145  may be connected to the ceiling  110  through an RF impedance match circuit  150 , so that the ceiling  110  also serves as another RF power applicator. In this case, the frequencies of the two RF generators  135 ,  145  are very different so that the two RF generators  135 ,  145  function independently. 
     Process gas is introduced so as to provide maximum gas distribution uniformity across the top surface of the wafer  130  by injecting it through many uniformly spaced gas injection inlets  160  in the ceiling  110 . The ceiling  110  is thus a gas distribution plate. A gas source or supply  165  is coupled to a gas manifold  170  in the ceiling/gas distribution plate  110 , and the gas manifold  170  feeds each of the inlets  160 . As shown in FIGS. 2A and 2B, the inlets  160  of the gas distribution plate  110  are formed by two parallel planar plates, namely a back plate  205  and a front plate  210  which are manufactured separately (FIG. 2A) and then bonded together (FIG.  2 B). The back plate  205  is on top and the front plate  210  is on the bottom and faces the plasma in the interior of the chamber  100 . The back plate  205  consists of an array of relatively large cylindrical openings  215  in its bottom surface while the front plate  210  consists of an array of cylindrical pucks  220  matching the array of openings  215 . As shown in FIG. 2B, the pucks  220  of the front plate  210  fit within the openings  215  of the back plate  205 , the clearance between each opening  215  and matching puck  220  forming an annular gap therebetween, the annular gap being the gas inlet  160 . Gas feed orifices  230  in the back plate  205  are sized to provide the precise gas flow desired extend vertically from the gas manifold  170  overlying the back plate  205  to the annular gas inlets  160 . Since the gas distribution plate  110  consists of an array of hundreds or thousands of annular inlets  160  to achieve spatially uniform gas distribution across the entire wafer surface, the inlets  160  would in most cases allow too much gas flow. Therefore, the finely-sized orifices  230  provide the requisite flow control. 
     Significantly, each orifice  230  faces a horizontal gap  235  between the respective puck  220  and the back plate  205 , so that the gas is forced to make an abrupt turn to enter the gap  235  and another abrupt turn to enter the annular inlet  160 . It is difficult if not impossible for plasma in the chamber travelling upward in the annular inlets  160  to make both of these turns without being extinguished by collisions with the gas distribution plate surfaces within the annular inlet  160  and the horizontal gap  235 . A result is that the precisely sized orifices  230  are protected from plasma sputtering. This leaves only the annular inlets  160  subject to distortion in size from plasma sputtering or attack. However, the area of each annular inlet  160  is so large that plasma sputtering introduces only a small fractional difference in area from inlet to inlet, so that gas distribution uniformity across the wafer surface is virtually immune to such changes. Moreover, in the embodiment of FIGS. 2A and 2B, gas flow uniformity is determined by the uniformity of the orifices  230  only, so that changes in the sizes of the various annular inlets  160  have virtually no affect on gas flow uniformity. Thus, performance of the gas distribution plate  110  is virtually immune to changes induced by plasma sputtering or attack, a significant advantage. 
     In one embodiment, the back plate  205  and front plate  210  are formed of silicon carbide and are bonded together using existing techniques in silicon carbide manufacturing. One advantage of using silicon carbide as the material of the gas distribution plate  110  is that such material is practically impervious to attack by certain process gases and plasma species, such as halogen-containing process gases and plasma species. Also, silicon carbide is relatively compatible with silicon semiconductor wafer processing, so that contamination from plasma sputtering of such material is not as harmful as are other materials such as aluminum. 
     Another advantage of the annular-shaped gas inlets  160  is that each puck  220  keeps the plasma ions and gases away from the center of each opening  215  where electric fields are maximum. This feature helps prevent arcing or plasma light-up. The two-plate structure  205 ,  210  of the gas distribution plate  110  enables cost-effective manufacture of hundreds or thousands of holes  215  and pucks  220  centered in each of the holes. The invention thus provides an economical gas distribution plate with sufficient uniformity of gas distribution to process extremely fine device features (e.g., 0.15 microns) on a very large wafer (10 inch to 20 inch diameter) with minimal plasma arcing while being impervious to long-term wear from plasma sputtering. 
     Another advantage is that the relatively large annular openings  160  provide a much lower gas injection velocity. Although each finely sized orifice  230  produces a very high velocity gas stream into the respective horizontal gap  235 , passage through the horizontal gap  235  and through the large annular inlet  160  dissipates its velocity. As a result, the gas flow from the bottom of the front plate  210  is much more uniform and free from high velocity narrow gas streams and plasma plumes. Therefore, a small wafer-to-ceiling gap does not lead to spatial non-uniformities in the gas distribution at the wafer surface using the gas distribution plate  110 , a significant advantage. 
     Many of the advantages enumerated above are pertinent to problems encountered in high power plasma reactors capable of high plasma ion densities. One of these problems is that high plasma ion density over the wafer surface is achieved in some reactors by a small wafer-to-ceiling gap to better confine the plasma. As noted above, the gas distribution plate  110  provides uniform gas distribution within such a small gap because of the large size of the annular inlets  160 . Another one of these problems is that high plasma ion density is achieved in some reactors by applying plasma source power to the ceiling or overhead gas distribution plate, which leads to arcing in the gas inlets. As noted above, the gas distribution plate  110  includes the pucks  220  that confine the gas closer to the periphery of each hole  215  where electric fields are minimum so as to suppress or prevent arcing. Thus, the gas distribution plate  110  is inherently suitable for use in high density plasma reactors. 
     FIGS. 3A,  3 B and  4  illustrate one implementation of the embodiment of FIGS. 2A and 2B. FIG. 3A shows that the front plate  210  having the array of pucks  220  consists of a web of longitudinal arms  310  and lateral arms  315  formed with the pucks  220  and holding them in the fixed array. Referring to FIGS. 3B and 4, the back plate  205  has longitudinal channels  320  and lateral channels  325  that receive the longitudinal and lateral arms  310 ,  315  when the plates  205 ,  210  are joined together. The pucks  220  are centered in the respective holes  215  and spaced apart from the back plate  205  by the horizontal gaps  235  and the annular inlets  160  and therefore do not contact the back plate  205 . Contact between the back plate  205  and the front plate  210  is along the longitudinal and lateral arms  310 ,  315  that fit snuggly within the corresponding longitudinal and lateral channels  320 ,  325 . It is along these contacting surfaces that the two plates  205 ,  210  are bonded together. As noted previously above, if the two plates are silicon carbide material, then the bonding is carried out using standard silicon carbide bonding techniques. 
     FIG. 5 illustrates an embodiment in which a single orifice  235   a  feeds a group of neighboring annular gas inlets  160   a ,  160   b ,  160   c . The single orifice  235   a  feed the middle annular gas inlet  160   b  directly via the horizontal gap  235   b , and feeds the adjacent annular inlets  160   a,    160   c  through internal channels  505 ,  510  connecting the adjacent annular inlets  160   a ,  160   c  with the middle annular inlet  160   b . One advantage of this embodiment is that the number of finely sized orifices  235  that must be drilled in the back plate  205  is greatly reduced. 
     FIG. 6 illustrates an embodiment in which a back plate  600  has parallel lateral slots  605  and a front plate  610  has an array of holes  615  and pucks  620 . The circular holes  615  and the cylindrical pucks  620  are concentrically arranged so that they define corresponding annular gas ports  616 . The slots  605  are aligned with respective rows of the holes  615  and pucks  620 . The width of each slot  605  is less than the diameter of each hole  615  (e.g., less than half). The plates  600 ,  610  are joined together so that each slot  610  is centered with a respective row of the array of holes  615 . Referring to the cross-sectional view of FIG. 7, the resulting gas passage aligned with each hole  615  consists of a pair of arcuate slots  630   a ,  630   b  which appear in FIG. 7 in solid line. Process gas is fed into each slot  605  by a single fine orifice  635  through the back plate  600 . The diameter of the orifice  635  is selected to provide the requisite gas flow rate. 
     The embodiment of FIGS. 6 and 7 is simpler to form because there is no horizontal gap (e.g., the horizontal gap  235  of FIG. 2) between the puck  620  and the back plate  600 . Instead, the bond between the plates  600 ,  610  is formed along the entirety of their adjoining surfaces. The pucks  620  are similarly bonded across the entirety of their top surfaces to the bottom surface of the plate  600 . The only areas of the top surfaces of the pucks  620  not thus bonded are the areas facing the narrow slots  605 . 
     In the foregoing embodiments, the pucks  620  function as flow diversion elements for transforming gas flow between the front and beck plates  610 ,  600  from stream patterns in the back plate  600  to annular flow patterns in the front plate  610 . The stream patterns correspond to a first radius (i.e., the radius of the top orifices  635 ) and the annular patterns correspond to a second radius (i.e., the radius of each annular opening  660 ) which is larger than the first radius. The flow diversion elements  620  induce a rapid change of gas flow (a) from a vertical flow of the stream pattern in each orifice  635  (b) to a horizontal flow from the first radius (of each orifice  635 ) to the second radius (of the corresponding annular opening  660 ) and (c) to a vertical flow in each corresponding annular opening  660 . 
     FIGS. 8A-8D illustrate one method for fabricating the gas distribution plate of FIGS. 6 and 7 as a monolithic silicon carbide piece. In FIG. 8A, the back plate  600  is formed of sintered silicon carbide and the slots  605  are milled in the plate  600 . In FIG. 8B, graphite inserts  805  are placed in the slots  605 . In FIG. 8C, the front plate  610  is formed by chemical vapor deposition of silicon carbide on the bottom surface  600   a  of the back plate  600 . Then, the graphite inserts are all removed by heating the entire assembly until the graphite material burns away, leaving the slots  605  empty, as shown in FIG.  8 D. In FIG. 8D, an array of annular openings  660  are milled completely through the entire thickness of the front plate  610 , corresponding to the holes  615  and pucks  620  illustrated in FIG.  6 . FIG. 8D also depicts the orifice  635 , which may be milled during one of the foregoing steps. 
     FIGS. 9A-9D illustrate another method for fabricating the gas distribution plate of FIGS. 6 and 7 as a monolithic silicon carbide piece. In FIG. 9A, the back plate  600  is formed of sintered silicon carbide and the slots  605  are milled in the plate  600 . In addition, a wide shallow channel  810  is formed in the back plate  600  centered along and parallel to each slot  605 . In FIG. 8B, silicon carbide inserts  815  are placed in the wide shallow slots  810 . In FIG. 8C, the front plate  610  is formed by chemical vapor deposition of silicon carbide on the bottom surface  600   a  of the back plate  600 . In FIG. 8D, an array of annular openings  660  are milled completely through the combined thicknesses of the front plate  610  and the silicon carbide inserts  815 , corresponding to the holes  615  and pucks  620  illustrated in FIG.  6 . 
     FIG. 10 illustrates yet another embodiment in which the back plate  600  and the front plate  610  are both formed of anodized aluminum. The anodization produces an alumina thin film  600 - 1  on the back plate  600  and an alumina thin film  610 - 1  on the front plate  610 . The anodization layer protects the aluminum plates from the plasma. 
     While the invention has been described with reference to embodiments in which the ceiling gas distribution plate must function as an electrode (and therefore comprise conductive material), the gas distribution plate of the invention is also well suited to applications in which the gas distribution plate does not function as an electrode. 
     In those embodiments in which the ceiling gas distribution plate functions as an overhead electrode, it may consist of silicon carbide, as described above. If it is desired that the gas distribution plate have a resistivity less than that of silicon carbide (0.005-1.0 Ohm-cm), then each of the silicon carbide plates  600 ,  610  may be fabricated in such a manner as to have a thin highly conductive graphite layers  910 ,  920  running through the center of the plates and co-planar with the respective plate, as illustrated in FIG.  11 . This is accomplished by forming each plate  600 ,  610  as a graphite plate. Each graphite plate is machined to form the structural features described above with reference to FIGS. 6 and 7. Then, each graphite plate  600 ,  610  is siliconized using conventional techniques. However, the siliconization process is carried out only partially so as to siliconize the graphite plates to a limited depth beyond the external surface of the graphite. This leaves an interior portion of the graphite un-siliconized, corresponding to the graphite layers  910 ,  920  enclosed within the silicon carbide plates  600 ,  610 . The graphite layers  910 ,  920  have a resistivity about one order of magnitude less than that of silicon carbide. Since the graphite layers  910 ,  920  are completely enclosed in silicon carbide, they are protected from the plasma. 
     While the gas distribution plate of FIGS. 2A and 2B has been described as being formed of silicon carbide, it may, instead, be formed of silicin. 
     While the invention has been described in detail with reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.

Technology Classification (CPC): 7