Low pressure CVD apparatus comprising gas distribution collimator
This is a continuation of application Ser. No. 07/881,643, filed May 12, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a low-pressure CVD apparatus (hereinafter referred to as &#34;LP-CVD apparatus&#34;) and, more particularly, to a LP-CVD apparatus capable of forming a tungsten blanket layer in improve step coverage. 
     2. Description of the Prior Art 
     A nonselective tungsten blanket CVD process (hereinafter referred to as &#34;W-CVD process&#34;), which is capable of filling up minute contact holes and through holes of 0.4 μm square or below in a contact resistance smaller than that of polyplugs formed by filling up minute holes with polycrystalline silicon, has become an attractive technique in the field of the future-generation VLSI industry. 
     As shown schematically in FIG. 7, a conventional LP-CVD apparatus for forming a tungsten blanket layer has a CVD reaction chamber 6 provided with a source gas inlet 5 through which a source gas 4 is supplied into the CVD reaction chamber 6, a susceptor 1, i.e., a support table, for supporting a wafer or workpiece 2, disposed within the CVD reaction chamber 6, and a shower head 3, i.e., a source gas distributing means, for distributing the source gas 4 over the surface of the wafer 2. 
     The shower head 3 of this known LP-CVD apparatus of a single-wafer processing system is a ceramic plate having a thickness on the order of about 1 to 2 mm provided with holes of about 2.0 mm in diameter in a random arrangement. The shower head 3 is disposed so as to distribute the source gas 4 uniformly over the wafer 2. 
     Since the LP-CVD apparatus shown in FIG. 7 does not control the direction of travel of molecules of the source gas 4, molecules of the source gas fall in a substantially isotropic mode into a contact hole (or a trench) 13 as shown in FIG. 8. During this W-CVD process, a seam 14 having a void is formed in the contact hole 13 as shown in FIG. 8, because the tungsten blanket layer 12 grows at substantially the same deposition rate in a vertical direction and in a horizontal direction over a barrier metal layer 11 so that A and B are approximately equal to each other and the aspect ratio of the contact hole 13 increases with the progress of deposition. Thus, it has been impossible to fill up the contact hole 13 perfectly without forming any void therein by the W-CVD process. Accordingly, in etching back the surface of the tungsten blanket layer to form a tungsten plug, the upper portion of the seam is enlarged as shown in FIG. 9 and the enlarged seam forms a void 14a under a wiring layer, not shown, formed over an interlayer insulating film 9, adversely affecting the reliability of the wiring layer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a LP-CVD apparatus capable of forming a film in an improved step coverage and of filling up contact holes and the like without forming any seams and voids. 
     In one aspect of the present invention, a LP-CVD apparatus comprises: a CVD reaction chamber provided with a source gas inlet; a susceptor for supporting a wafer or workpiece; and a gas distributing means interposed between the source gas inlet and the susceptor; wherein the gas distributing means is provided with a plurality of minute pores serving as passages for a source gas and extending substantially perpendicularly to the surface of a wafer supported on the susceptor. 
     Preferably, the length of the plurality of minute pores of the gas distributing means is not smaller than the mean free path of the source gas, because minute pores of such a length are able to bias the gas velocity distribution curve so that the probability of a velocity perpendicular to the surface of the wafer is increased. Since the plurality of minute pores extending substantially perpendicularly to the surface of the wafer supported on the susceptor increase the probability of a velocity of molecules of the source gas perpendicular to the surface of the wafer, step coverage is improved in filling up contact holes and the like by deposition and contact holes can be filled up with, for example, tungsten without forming any seam in the contact holes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic view of a LP-CVD apparatus in a preferred embodiment according to the present invention; 
     FIGS. 2(a) and 2(b) are schematic views of a shower head with a collimator employed in the LP-CVD apparatus of FIG. 1; 
     FIG. 3 is a sectional view of a work provided with a tungsten blanket layer formed by the LP-CVD apparatus of FIG. 1; 
     FIG. 4 is a sectional view of assistance in explaining a process of forming a tungsten layer by the LP-CVD apparatus of FIG. 1; 
     ,FIG. 5 is a diagrammatic view showing the result of simulation using a deposition/etching model on an assumption that molecules are deposited in an anisotropic mode (present invention); 
     FIG. 6 is a diagrammatic view showing the result of simulation using a deposition/etching model on an assumption that molecules are deposited in an isotropic mode; 
     FIG. 7 is a schematic view of a conventional LP-CVD apparatus; 
     FIG. 8 is a sectional view of a work provided with a tungsten blanket layer formed by the LP-CVD apparatus of FIG. 7; and 
     FIG. 9 is a sectional view of the work after etching back the tungsten blanket layer of FIG. 8. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a LP-CVD apparatus embodying the present invention is provided with a shower head 10 with a collimator. As shown in FIGS. 2(a) and 2(b), the shower head 10 with a collimator is a disk formed of, for example, aluminum and provided with a plurality of minute pores 16 serving as passages 10a for a source gas and having a thickness L1 of about 2 cm, which is greater than the mean free path λ (cm) of molecules of a source gas determined by an expression (1). 
     
         λ=1/√2πα.sup.2 n                    (1) 
    
     where α is the size of molecules and n is the number of molecules of an ideal gas per cubic meter. 
     The mean free path λ is about 1.1 cm when the source gas is WF 6  for depositing tungsten by CVD, the size of molecules of WF 6  is about 4 Å, the temperature is 470° C. and the pressure is 1 mtorr. A stable laminar flow of the source gas is produced when the thickness L1 of the shower head 10 is greater than the mean free pass λ and greater than the distance L2 between the lower surface of the shower head 10 with a collimator and the upper surface of a wafer or workpiece 2 supported on a susceptor 1. 
     Tungsten was deposited over the entire surface of a work having contact holes in a tungsten layer 12 by the LP-CVD apparatus. As shown in FIG. 3 the tungsten layer 12 has a short seam 17 separated from the surface of the work and no seam is formed in portions of the tungsten layer 12 filling up a contact hole 13. The shower head 10 with a collimator increases the probability of a velocity of molecules perpendicular to the surface of the wafer 2 in the gas velocity distribution curve, so that the deposition rate on horizontal surfaces is higher than that on vertical surfaces. Consequently, tungsten layers 12a, 12b and 12c meet an inequality A &gt;B and the thickness A&#39; of the tungsten layers 12a, 12b and 12c on the bottom surface of the contact hole 13 is approximately equal to the thickness A of the same on the surface of the work (FIG. 4). In FIGS. 3 and 4, parts like or corresponding to those shown in FIG. 8 are denoted by the same reference characters. FIGS. 5 and 6 show the results of simulation of deposition/etching conducted on an assumption that molecules fall on the work in an anisotropic mode and in an isotropic mode, respectively, in forming a film by a LP-CVD process. The simulation was conducted on an assumption that the sticking coefficient is 0.1. The simulation used a mechanism disclosed in J. P. McVittie (Stanford Univ.) et al., &#34;A Profiles Simulator for Etching and Deposition, SPEEDIE&#34; made public in the Dry Process Symposium, 1990. 
     As is obvious from films 20 shown in FIG. 5 and 6 formed by LP-CVD processes, the LP-CVD apparatus of the present invention forms the films 20 in a satisfactory step coverage, and the difference between the thickness of a portion of the film 20 near the upper end of the contact hole 13 and that of a portion of the film 20 on the bottom surface of the contact hole 13 is small. 
     The LP-CVD apparatus of the present invention is effective also when a source gas other than the WF 6 , such as a mixed gas of O 3  and TEOS gas for forming a SiO 2  film. 
     Although the invention has been described in its preferred form with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing the spirit and scope thereof.