Patent Document

BACKGROUND OF THE INVENTION 
     The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices. 
     Low density plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent to the target impact the surface of the target to dislodge, i.e., “sputter” material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer. 
     Sputtered material has a tendency to travel in straight line paths from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices having trenches or holes with a high depth to width aspect ratio, can bridge over, which is undesirable; to prevent this, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate to position vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered in a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term “dense plasma” is intended to refer to one that has a high electron and/or ion density. 
     There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil generates a plasma in a region surrounded by the coil, the plasma consisting of electromagnetic currents. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance-matching network, such that the coil acts as the first winding of a transformer. The plasma acts as a single turn second winding of the transformer. 
     A high density plasma typically requires the chamber to be operated at a relatively high pressure. As a result, the frequency of collisions between the plasma ions and the deposition material atoms is increased and the scattering of the deposition atoms is likewise increased. This scattering of the deposition atoms typically causes the deposition layer on the substrate to be thicker on that portion of the substrate aligned with the center of the target and thinner in the outlying regions. Such nonuniformity of deposition is often undesirable in the fabrication of semiconductor devices. 
     It is recognized that the uniformity with which an etching or deposition operation across the surface of a substrate is adversely affected by variations in the density of the ionized plasma across the surface of the substrate. As a consequence, the uniformity of the plasma density across the substrate surface can be improved, it is believed that the uniformity of the resulting etching or deposition operation can likewise be improved. 
     A number of designs and arrangements for plasma generating coils have been proposed which are intended to improve the uniformity of the generated plasma. Typically, one or more coils are disposed at locations above the wafer surface and according to many of these proposals, the coils are located outside of the chamber in which the plasma itself is confined. Investigations have indicated that these prior coil arrangements tend to produce plasma fields which have substantial plasma density nonuniformity across the wafer surface. 
     BRIEF SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to substantially improve the uniformity of plasma potential or density across the surface of a wafer which is being subjected to various processing operations. 
     The above and other objects are achieved, according to one embodiment of the present invention, by an apparatus for processing a substrate in the presence of a plasma, in which an RF coil is disposed at an elevation proximate to a plane containing a substrate support. It has been found that such an arrangement can cause the plasma maintained by the coil to have a uniform potential and density parallel to the substrate support. In the illustrated embodiment, the apparatus comprises a chamber enclosing a region maintained at a low pressure; a substrate support disposed in the region and having a substantially horizontal substrate support surface for supporting a substrate; and an RF coil disposed in the chamber for producing a radio frequency field within the chamber to maintain a plasma above the substrate support surface. The coil surrounds a vertical region which contains the substrate support surface and is disposed near the plane of the substrate support surface to cause the plasma to have a more uniform density and potential across substantially the entire surface of the substrate. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is an elevational, cross-sectional view showing one embodiment of an apparatus according to the present invention. 
     FIG. 2 is a view similar to that of FIG. 1 showing a second embodiment of apparatus according to the invention. 
     FIG. 3 is a view similar to that of FIG. 1 showing a third embodiment of apparatus according to the invention. 
     FIG. 4 is a view similar to that of FIG. 1 showing a fourth embodiment of apparatus according to the invention. 
     FIG. 5 is an elevational, cross-sectional view of an apparatus corresponding essentially to the fourth embodiment of the invention, including a computer simulation of the pattern of equipotential lines believed to be achieved in the fourth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As will become more readily apparent from the detailed description to be presented below, the present invention results from applicants&#39; discovery that the uniformity of the plasma density above the surface of a semicondutor wafer or other substrate can be substantially improved if the plasma generating coil in accordance with one embodiment of the invention, is located in a region which circumscribes the wafer and if at least the lower most turn of the coil is located close to, or below, the wafer. With this coil arrangement, it has been found that the major part of the flux generated by the coil will be located above the horizontal median plane of the coil and lines of equal plasma density will extend substantially parallel to the wafer surface across the entirety of the wafer surface, and this can be true across substantially the entire vertical extent of the plasma. 
     Positioning of the plasma generating coil at or below the level of the wafer surface so that the major portion of the plasma located above the coil are features believed to be contrary to the concepts disclosed in relevant published literature and the arrangement provided in commercially available systems of which applicants are aware. 
     A first specific embodiment of apparatus according to the invention is shown in FIG.  1 . This apparatus includes a plasma chamber  1  which is partially enclosed by a shield  2  made of a conductive material and disposed in a cylindrical vacuum chamber  4 . A pedestal  6  extending through an opening in the bottom of vacuum chamber  4  is provided at its upper end with a substrate support portion  7 . The upper surface of the substrate support portion  7  defines a substrate support surface  7   a  which supports a substrate  8  that is to be processed. The lower end of pedestal  6  is mounted on a support surface (not shown) and may contain an elevator mechanism permitting the substrate support surface to be moved vertically within shield  2 . The interior of vacuum chamber  4  is sealed by a bellows  12  connected between the bottom of vacuum chamber  4  and the bottom of the substrate support. Alternatively, the pedestal and associated elevator mechanism may be entirely enclosed within the vacuum chamber  4 , obviating the need for the bellows  12 . 
     The embodiments illustrated in the drawings of the present application are intended for deposition of a layer on the upper surface of substrate  8  by sputtering. For this purpose, a target  14  is mounted within the enclosure formed by shield  2 , a source adaptor  15  mounted on the top of the vacuum chamber  4  supports the target  14 . The target  14  is coupled to a suitable source of biasing voltage. Typically, a negative dc voltage would be applied to target  14  which is insulated from the source adaptor by an insulator ring  17 . 
     Embodiments for deposition of a layer by sputtering have been selected only by way of nonlimiting example. The invention can be applied to any other apparatus in which a plasma is to be created and in which a uniform plasma density across the surface of the substrate is advantageous, including etching apparatus, for example. 
     Furthermore, while target  14  is illustrated as being substantially disk-shaped, it is to be understood that any suitable target configuration adapted to the particular deposition requirements may be employed. 
     Also by way of example, the apparatus illustrated herein is one in which erosion of material from target  14  is produced by a rotating magnet assembly, or a magnetron,  16 . Here again, any suitable known arrangement can be used to control the generation of impacting ions onto the target  14 . 
     Substrate  8  is held in place on the substrate support surface by a conventional clamp ring  18 . A bias may also be applied to the substrate  8  through the clamp ring  18  by suitable bias sources including AC and RF sources. 
     In this embodiment, a plasma is generated and maintained in the region enclosed by shield  2  by means of a coil  20  which is electrically connected to a source  22  of radio frequency (RF) current. Source  22  is connected to opposite ends of coil  20  via conductors  23  which pass through insulators  24 ,  25  mounted in openings in the vacuum chamber  4  and the shield  2 . 
     In the embodiment illustrated in FIG. 1, coil  20  is composed of three turns which are wound in a spiral in a horizontal plane about a vertical axis coaxial with the center axis of pedestal  6 , the center axis of substrate  8  and, preferably, the center axis of target  14 . Coil  20  surrounds the substrate support  6  and, in the illustrated embodiment, is located below the upper surface  7   a  of the substrate support  6  and below the upper surface of substrate  8 . 
     Coil  20  includes vertical conductors  28  which provide RF current to the coil  20 . The conductors  28  are supported by a mechanism which allows coil  20  to be displaced vertically in order to be brought to an elevation which produces an optimal plasma field pattern. In the illustrated embodiment, this mechanism may be composed of two or more hydraulic jacks, or piston-cylinder assemblies  30  which includes a conductive cover member  32  and an insulative base  34 . The cover member  32  is preferably spaced from the shield  2  and the base  34  so as to form labyrinthine passageways to prevent the formation of conductive paths of deposition material which could short the coil  20  to the shield  2 . 
     The coil conductors  28  are coupled to the conductors  23  via the conductive member  32  and a conductive core (not shown) inside the insulative base  34 . 
     Preferably, coil  20  is movable over a range extending between a lower limit which is somewhat below the upper surface of substrate  8  to an upper limit which is somewhat above the upper surface of substrate  8 . 
     It is believed that when coil  20  is at the elevation described above in which the plane of the coil is below the plane of the substrate, the plasma that the coil produces will be characterized by lines of equal electron density (electrons/cc) which extend, with a high degree of uniformity, parallel to the upper surface of substrate  8 , and this uniformity will exist throughout virtually the entire vertical dimension of the plasma field. Even the lines of equal electron density at the center of the plasma field will extend parallel to the upper surface of the substrate across a region coextensive with substantially the entire substrate surface. In contrast, in the case of many prior coil arrangements, these lines of equal electron density at the center of the plasma field are believed to be coextensive with only a portion of the associated substrate surface. 
     Furthermore, in contrast to the conditions believed to exist with prior art coil arrangements, in the embodiment shown in FIG. 1, virtually the entire plasma field will be situated above coil  20 . 
     Because the plasma generated by coil  20  in accordance with one aspect of the present invention contains lines of equal electron density which extend parallel to the substrate surface over substantially the entirety of the substrate surface, it is believed that any uniformity of the deposition material in transit from the target  14  to the workpiece  8  will tend not to be disrupted by the plasma. Consequently, deposition material from a uniform source can be ionized and deposited with a reduced amount of nonuniformity caused by the ionizing plasma. 
     Furthermore, because of the location of the coil in accordance with the present invention, establishing an optimum vertical spacing between substrate  8  and target  14  is facilitated. 
     In the embodiment illustrated in FIG. 1, the vertical position of coil  20  is such that clamp ring  18  also acts as a heat shield to protect substrate  8  against nonuniform heating by heat radiated from coil  20 . In contrast, in the case of prior art coils which are disposed well above the upper surface of the substrate being processed, uneven heating by radiation from the coil is often a problem. 
     While FIG. 1 illustrates a coil having three turns, it will be appreciated that, depending on the requirements of the particular apparatus, a different number of coil turns can be employed. In some applications, a single turn coil may be used. 
     Adjustment of the height of coil  20 , by operation of jacks  30 , is advantageous because, for processes such as reactive metal ion deposition, using for example a Ti target, different plasma gasses, such as Ar or N 2 , there may be variations in the plasma density distribution due to differences in rate of recombination, etc. It is for this reason that it is desirable to be able to adjust the height of coil  20  in order to optimize the configuration of the plasma field. 
     FIG.  2 . Illustrates a second embodiment of the invention, in which identical components are illustrated with the same reference numerals, and these components are not again described. For the sake of simplicity, FIG. 2 does not illustrate certain components such as, for example, vacuum chamber  4 , target  14 , current source  22 , insulators  24 , jacks  30 . Some or all of these components may be present in the embodiment of FIG. 2 in the same arrangement as shown in FIG.  1 . However, it is contemplated that certain components may be eliminated in some applications. For example, the elevation of the coil may be fixed at an optional height for a particular process. As such, the jacks  30  may be eliminated in those applications. 
     The embodiment shown in FIG. 2 differs from that shown in FIG. 1 in that coil  20  of FIG. 1 is replaced by a coil  40  which is wound in a helical form so that the coil follows a frustoconical path with each turn having a larger diameter than the turn immediately therebelow. This arrangement may be preferred when shield  2  has a relatively small diameter. It may be desirable, in this case, to modify clamp ring  18  in a manner illustrated in FIG. 3, to be described below, to assure adequate heat shielding of substrate  8  from thermal radiation produced by coil  40 . 
     FIG. 3 illustrates a modified version of the embodiment of FIG. 1 in which coil  20  is placed slightly above the plane of the upper surface of substrate  8 . As mentioned earlier herein, such a positioning of coil  8  may be found to produce an optimum plasma field configuration, depending on the nature of the material to be deposited on the substrate surface and the composition of the ionized gas within shield  2 . When coil  20  is to be disposed at the position illustrated in FIG. 3, use is preferably made of a modified clamp ring  48  having an upwardly projecting portion  50  which constitutes a heat shield between coil  20  and substrate  8 . 
     FIG. 4 illustrates another modified version of the embodiment of FIG. 1 in which coil  20  is replaced by a cylindrical coil  60  having two turns with substantially equal diameters. The turns are thus located one above the other, with the lower turn being disposed at approximately the level of substrate  8  and the upper turn being located a small distance above the level of substrate  8 . In order to optimize the distribution of the density of the plasma produced by coil  60 , the lower turn may be slightly below or slightly above the level of substrate  8 . Because the upper turn of coil  60  is located somewhat above substrate  8 , this embodiment includes the same modified clamping ring  48  as that employed in the embodiment of FIG.  3 . 
     FIG. 5 illustrates apparatus essentially according to the fourth embodiment and further illustrates a computer simulation of the equipotential lines of the plasma which is believed to be created within the interior space enclosed by shield  2  when a coil  60  according to the fourth embodiment is utilized to produce the plasma. It is believed that the pattern of equipotential lines in a plasma field corresponds generally to the pattern of equal plasma density lines within the field. It should be readily apparent from FIG. 5 that equipotential lines extend essentially parallel to the upper surface of substrate  8  over substantially the entire width, or diameter of the substrate. 
     Superimposed on the portion of shield  2  shown in FIG. 5 is a coordinate system representing distances from an origin whose abscissa value is the central axis of substrate  8  and whose ordinate value is an arbitrarily selected point below the upper surface of substrate  8 . The abscissa scale of this coordinate system is designated R and the ordinate thereof is designated Z. It will be seen that the upper surface of substrate  8  has an ordinate value of Z≈3 cm. Exemplary values for the plasma potential voltage calculated at selected points in the RF field illustrated in FIG. 5 are shown in the following table. 
     
       
         
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 COORDINATE VALUES OF POINT 
                 PLASMA POTENTIAL 
               
             
          
           
               
                 Z (cm) 
                 R (cm) 
                 VOLTAGE 
               
               
                   
               
             
          
           
               
                 5 
                 0.5 
                 6.87 
               
               
                 5 
                 5 
                 6.98 
               
               
                 5 
                 10 
                 6.92 
               
               
                 7 
                 0.5 
                 7.46 
               
               
                 7 
                 5 
                 7.53 
               
               
                 7 
                 10 
                 7.48 
               
               
                 9 
                 0.5 
                 7.57 
               
               
                 9 
                 5 
                 7.60 
               
               
                 9 
                 10 
                 7.49 
               
               
                   
               
             
          
         
       
     
     The precise locations of the turns of coil  60  can be determined from the coordinate scales of FIG.  5 . While FIG. 5 shows one-half of shield  2  and substrate  8 , it will be understood that the other half of the plasma is mirror-symmetrical to the half which is illustrated, i.e., the equipotential lines and equal flux density lines of the plasma are symmetrical about the vertical, or Z, axis of the coordinate system, which axis passes through the origin of the coordinate system. 
     In view of the high degree of uniformity which can be imparted to a plasma field in accordance with the present invention, the possibility of successfully processing large diameter substrates, up to 300 mm in diameter, particularly for deposition of metal coatings, is presented. 
     It should be noted that embodiments of the present invention can also include single turn coils and coils whose turns are spaced vertically from one another, i.e. constitute a cylindrical coil. 
     It will, of course, be understood that modifications of the present invention, in its various aspects, will be apparent to those skilled in the art, some being apparent only after study, others being matters of routine mechanical and electronic design. Other embodiments are also possible, their specific designs depending upon the particular application. As such, the scope of the invention should not be limited by the particular embodiments herein described but should be defined only by the appended claims and equivalents thereof.

Technology Category: h