Abstract:
A plasma reactor embodying the invention includes a wafer support and a chamber enclosure member having an interior surface generally facing the wafer support. At least one miniature gas distribution plate for introducing a process gas into the reactor is supported on the chamber enclosure member and has an outlet surface which is a fraction of the area of the interior surface of said wafer support. A coolant system maintains the chamber enclosure member at a low temperature, and the miniature gas distribution plate is at least partially thermally insulated from the chamber enclosure member so that it is maintained at a higher temperature by plasma heating.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention is related to a plasma reactor for processing a semiconductor wafer using polymer precursor gases such as fluorocarbon gases, and in particular to a gas distribution plate and ceiling thereof. 
     2. Background Art 
     In plasma processing employed in the fabrication of semiconductor microelectronic integrated circuits, a semiconductor wafer is immersed in a plasma inside the chamber of a plasma reactor. The reactor may be thus employed to carry out any one of various processes on the wafer, such as chemical vapor deposition or reactive ion etching. In certain plasma etch processes carried out in such reactors, the upper most layer (the layer to be etched) may have a lower etch rate than the underlying layer (which must not be etched in most cases). This presents an especially challenging problem because there would be no etch selectivity (or an inverse selectivity) of the upper layer to the lower layer. This situation is characteristic of a plasma process for etching a dielectric layer overlying another layer such as a semiconductor layer (e.g., a silicon dioxide layer overlying a polysilicon layer) using a process gas containing an etchant such as fluorine or fluoride compounds. The problem has been solved by using a fluoride compound such as a fluorocarbon gas or a fluoro-hydrocarbon gas which, upon ionization, tends to break up into fluorine-containing etchant species and polymer precursor species. The polymer precursor species provide the requisite etch selectivity because it tends to accumulate as a hard polymer film on non-oxygen containing materials (such as the underlying polysilicon layer) but does not accumulate on oxygen-containing materials (such as the overlying silicon dioxide layer). Thus, the underlying layer is protected from the etchant by the polymer layer while the overlying layer is left exposed to the etchant, so that the process has a net etch selectivity of the overlying layer. 
     The problem is that the polymer accumulates on the interior reactor surfaces, including the ceiling of the chamber. Typically, the ceiling consists of a gas distribution plate with gas distribution inlets or orifices through which the process gas must be sprayed into the reactor chamber for uniform distribution. The plate must be formed of materials such as quartz which are suitable for carrying the etchant-containing process gases. Such materials do not readily lend themselves to temperature control, and therefore the center of the gas distribution plate tends to be very hot due to plasma heating while the perimeter tends to be colder. The polymer accumulates as a solid film in the colder perimeter region and cannot accumulate in the hot center region. Between these two regions is a transition region where the polymer tends to accumulate as a fine powder, which tends to flake onto the wafer and create contamination. This requires that the gas distribution plate be replaced periodically. The gas distribution plate is on the order of the diameter of the wafer (e.g., 9 inches or 14 inches) and its replacement is expensive due to the cost of the item as well as the non-productive time during which the reactor is disassembled for removal and replacement of the gas distribution plate. However, periodic removal and replacement of the gas distribution plate is not a solution to the problem, as flaking of any accumulated polymer from the gas distribution plate can occur any time up to the replacement of the plate. 
     One solution to this problem might be to cool the entire gas distribution plate so that the polymer deposited thereon is entirely of a hard consistency and will not flake. However, this would eventually block the gas inlets, stopping the inflow of the process gas. Another solution might be to heat the entire gas distribution plate sufficiently to prevent any polymer from accumulating thereon. However, this would expose the entire gas distribution plate to bombardment from the plasma and much faster wear. 
     Therefore, there is a need for a gas distribution plate which is not susceptible to accumulation of polymer or the flaking of accumulated polymer onto the wafer. 
     SUMMARY OF THE INVENTION 
     A plasma reactor embodying the invention includes a wafer support and a chamber enclosure member having an interior surface generally facing the wafer support. At least one miniature gas distribution plate for introducing a process gas into the reactor is supported on the chamber enclosure member and has an outlet surface which is a fraction of the area of the interior surface of said wafer support. A coolant system maintains the chamber enclosure member at a low temperature, and the miniature gas distribution plate is at least partially thermally insulated from the chamber enclosure member so that it is maintained at a higher temperature by plasma heating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional plasma reactor including a gas distribution plate of the prior art. 
     FIG. 2 is a cross-sectional cut-away view of a plasma reactor including a water-cooled ceiling and an array of thermally isolated mini-gas distribution plates embodying the invention. 
     FIG. 3 is a plan view of the ceiling interior surface corresponding to FIG.  2 . 
     FIG. 4 is a plan view of an individual mini-gas distribution plate of the invention having angled gas inlets providing a preferred vortex pattern of gas spray. 
     FIG. 5 is a cross-sectional cut-away view corresponding to FIG.  4 . 
     FIG. 6 illustrates an alternative spray pattern corresponding to FIG.  4 . 
     FIG. 7 is an enlarged cut-away cross-sectional view corresponding to FIG.  2 . 
     FIG. 8 is an view corresponding to FIG. 7 illustrating a method of fastening the mini-gas distribution plate on the ceiling. 
     FIG. 9 illustrates an alternative embodiment of the invention having a mini-gas distribution plate mounted on the side wall of the reactor chamber. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A reactor embodying the present invention has a cooled ceiling formed preferably of a good thermal conductor such as metal and an array of mini-gas distribution plates embedded therein, the gas distribution plates being thermally isolated from the cooled ceiling. The ceiling is sufficiently cooled so that polymer accumulates thereon as a solid film with little or no tendency to flake off, while the mini-gas distribution plates reach a sufficiently high temperature from plasma-heating so that no polymer accumulates thereon. Thus, neither the ceiling nor the array of mini-gas distribution plates harbors polymer having a tendency to flake. As a result, the necessity for replacement of internal chamber parts (such as the ceiling or the mini-gas distribution plates) is greatly reduced if not eliminated. 
     Each mini-gas distribution plate has plural gas injection holes connected to a common manifold within the plate. The area of each of the mini-gas distribution plates facing the plasma is limited so that: (1) the area is contained within a region in which the turbulence from the injected gas in the vicinity of the inlets prevents or impedes polymer accumulation, and (2) the size or thermal mass of the mini-gas distribution plate is sufficiently low to allow rapid plasma-heating of the plate. In order to enhance the gas turbulence across the area of the plate, the gas injection holes in each mini-gas distribution plate are angled relative to the surface of the plate facing the chamber interior. Preferably, the gas injection holes are angled so that the gas injection streams from adjacent holes cross one another or together form a vortex pattern. In a preferred embodiment, the mini-gas distribution plates extend slightly out from the surface of the ceiling, to enhance plasma-heating thereof and to enhance gas injection turbulence. Preferably, the mini-gas distribution plates are each a relatively small fraction of the area of the entire ceiling. 
     FIG. 1 illustrates a conventional plasma reactor chamber  100  having a cylindrical side wall  105  supporting a ceiling  110  consisting of a large gas distribution plate  115 . The gas distribution plate  115  covers a major portion of the ceiling  110 . The gas distribution plate has a gas manifold  120  from which plural gas inlets  125  extend downwardly to the reactor chamber interior. The gas distribution plate  115  overlies a wafer support  130  on which a semiconductor wafer  135  is mounted. The gas distribution plate  115  has a diameter generally corresponding to that of the wafer  135 , and may be on the order of 9 inches or 14 inches in diameter. Process gas is supplied to the gas distribution plate manifold by a process gas source  140  through a pump  145 . The pressure within the chamber is maintained at a desired vacuum level by a vacuum pump  150 . For capacitive coupling of RF power to the plasma within the chamber  100 , RF power is applied to the wafer support  130  by an RF generator  160  through an impedance match circuit  165 . The ceiling  110  or the gas distribution plate  115  may include a conductive material which is grounded to provide a an RF return. 
     For a reactive ion etch process to be carried out on a dielectric layer, the gas source can provide a fluoro-hydrocarbon gas, in which case a polymer layer forms on a major portion of the gas distribution plate  115 . Heating from the plasma generally keeps the center portion of the gas distribution plate too hot to accumulate any polymer, while the peripheral edge portion of the gas distribution plate is sufficiently cool to permit a hard film of polymer to accumulate thereon. An intermediate annular portion  170  of the gas distribution plate  115  is typically at an intermediate temperature at which the polymer can accumulate on the surface but cannot form a hard film. Instead, in the intermediate region  170  the polymer tends to be powdery and flakes easily, leading to contamination of the wafer  135 . Therefore, the gas distribution plate  115  must be replaced frequently. 
     The foregoing problems are overcome in the present invention. Referring to FIGS. 2 and 3, a plasma reactor embodying the present invention has a water-cooled ceiling  210  in which there are embedded an array of showerhead plugs or mini-gas distribution plates  220 . Each mini-gas distribution plate  220  is formed of a semi-metal such as silicon or a dielectric such as silicon dioxide (quartz) or sapphire, and has plural gas inlets  225  through which process gas is sprayed into the reactor chamber interior. Preferably, the mini-gas distribution plates  220  are thermally insulated from the water-cooled ceiling  210 , so that they are readily heated by the plasma within the chamber. Each gas distribution plate  220  is sufficiently small relative to the ceiling—has a sufficiently small thermal mass—so as to be rapidly heated by the plasma upon plasma ignition. (For example, the ceiling  210  may have a diameter in a range of 9 inches to 14 inches, while the gas distribution plate has an exposed diameter on the order of about 0.25-0.5 inch. As a result, the plasma heats each mini-gas distribution plate  220  to a sufficiently high temperature to prevent any accumulation of polymer thereon. The advantage is that the gas inlets  225  of each mini-gas distribution plate  220  can be kept clear of polymer. 
     Preferably, the diameter of each mini-gas distribution plate  220  is sufficiently small so that the entire bottom surface  220   a  of the gas distribution plate  220  is enveloped within a region of gas flow turbulence of the process gas spray from the inlets  225 . Thus, for example, each mini-gas distribution plate  220  has an exposed diameter on the order of about 0.25-0.5 inch. This region has sufficient gas turbulence to retard or prevent the accumulation of polymer on the surface  220   a.    
     Referring to FIGS. 4 and 5, the gas turbulence around the bottom surface  220   a  is enhanced by introducing a crossing pattern of gas spray paths from the plural gas inlets  225  of the mini-gas distribution plate  220 . The embodiment of FIGS. 4 and 5 provides a vortex pattern (indicated by the arrows of FIG.  4 ). This is accomplished by drilling each of the gas inlets  225  at an angle A relative to the outlet surface  220   a  of the mini-gas distribution plate  220 . Preferably, the angle A is in the range of about 20 degrees to 30 degrees. In an alternative embodiment illustrated in FIG. 6, the gas spray paths of the plural gas inlets  225  are directed at other inlets in order to enhance the gas turbulence. 
     As a further aid in inhibiting the accumulation of polymer on the mini-gas distribution plates  220 , the outlet surface  220   a  of the plate  220  extends slightly below the surface of the ceiling  210  by a distance d, as shown in FIG.  7 . The distance d is preferably about 0.02 inch to 0.03 inch or a fraction of the thickness of the gas distribution plate  220 . The enlarged cross-sectional view of FIG. 7 illustrates one preferred implementation in which the gas inlets  225  are angled holes passing entirely through the mini-gas distribution plate  220 . Process gas is supplied to the gas inlets  225  by a common manifold  230  formed in the ceiling  210 . A water jacket  240  of the water-cooled ceiling  210  is also shown in the drawing of FIG.  7 . Preferably, a thermal insulation layer  250 , which may be aluminum nitride for example, is trapped between the mini-gas distribution plate  220  and the ceiling  210 . 
     The water-cooled ceiling  210  is maintained at a sufficiently low temperature so that polymer accumulates on the entire ceiling as a very hard film which is virtually immune from flaking or contributing contamination to the chamber interior. The thermally isolated mini-gas distribution plates  220  are heated by the plasma to a sufficiently high temperature to inhibit accumulation of polymer thereon. Thus, the gas inlets  225  are kept clear of any polymer. The small size of the mini-gas distribution plates  220  not only enables the plasma to heat them to the requisite temperature. It also permits the concentration of gas inlets  225  over the small surface  220   a  to provide sufficient gas turbulence to further inhibit the accumulation of polymer on the surface  220   a  or inlets  225 . The gas turbulence is enhanced by providing a crossed or vortex pattern of gas spray paths from each of the gas inlets  225  of the mini-gas distribution plate  220 , and having the outlet surface  220   a  below the ceiling  210 . The combination of all of the foregoing features prevents any observable accumulation of polymer on any portion the mini-gas distribution plate  220 . 
     In a preferred embodiment, there are four mini-gas distribution plates  220  mounted on the ceiling  210  at four symmetrically spaced locations overlying the periphery of the wafer  135 . of course, additional mini-gas distribution plates may be provided in other embodiments, or their placement modified from the arrangement illustrated in FIG.  5 . 
     The advantage is that the ceiling and the gas distribution plate need not be periodically replaced, at least not as frequently as in the prior art, a significant advantage. Moreover, the system is more immune from contamination from polymer flaking regardless of the frequency with which the ceiling and gas distribution plates are replaced. 
     FIG. 8 illustrates one mode for mechanically holding the mini-gas distribution plate  220  in place on the ceiling  210 . The mini-plate  220  has an annular ear  280  extending radially from its periphery. The ceiling  210  has a hole  290  in which the mini-plate  220  is nested, the ceiling  210  having upper and lower sections  210 - 1 ,  210 - 2  joined together by a threaded fastener  295 . Each section  210 - 1 ,  210 - 2  has an annular shelf  210 - 1   a ,  210 - 2   a  which together form an annular pocket  297  for receiving and holding the annular ear  280 . 
     In a preferred embodiment, polymer flaking from the ceiling  210  is inhibited not only by cooling the ceiling but, in addition, by providing a “waffled” surface on the ceiling. The waffled surface, partially illustrated in FIG. 3, consists of an array of 1 mm half-spherical “bumps”  300  spaced apart by about 4 mm. The bumps  300  are arrayed in this manner across the entire interior surface of the ceiling  210 . They tend to force the solid polymer film accumulated thereon to form local crystalline regions which are less susceptible to cracking than a large crystalline region. 
     While the invention has been described with reference to a preferred embodiment in which the mini-gas distribution plates are mounted in the reactor chamber ceiling, in an alternative embodiment mini-gas distribution plates may be mounted at other locations within the chamber, such as the chamber side wall, as illustrated in FIG.  9 . In this alternative embodiment, the side wall  105  preferably is water-cooled for the same reasons that the ceiling  210  is water cooled as explained above. The mini-gas distribution plates  220  on the side wall  105  may be provided in addition to or in lieu of the gas distribution plates  220  on the ceiling  210 . 
     While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications may be made without departing from the true spirit and scope of the invention.