Abstract:
A hollow cathode magnetron comprises an open top target within a hollow cathode. The open top target can be biased to a negative potential so as to form an electric field within the cathode to generate a plasma. The magnetron uses at least one electromagnetic coil to shape and maintain a density of the plasma within the cathode. The magnetron also has an anode located beneath the cathode. The open top target can have one of several different geometries including flat annular, conical and cylindrical, etc.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to Physical Vapor Deposition (“PVD”) systems, and more particularly to an apparatus and method for improving PVD processes using an open top Hollow Cathode Magnetron (“HCM”) source. 
     2. Description of the Background Art 
     Physical vapor deposition (“PVD”) is a well known technique for depositing metal layers onto semiconductor wafers (also referred to herein as “substrates”). These thin metal layers can be used as diffusion barriers, adhesion or seed layers, primary conductors, antireflection coatings, and etch stops, etc. 
     In a conventional Hollow Cathode Magnetron (“HCM”), magnetic fields are used to generate a high density plasma of Argon (“Ar”) or other suitable inert gas and target material within a cathode of the HCM. The magnetic fields are also used to confine the plasma within the HCM. The cathode has a target, which can be made of metals such as Tantalum (“Ta”), Aluminum (“Al”), Titanium (“Ti”), Copper (“Cu”), or other suitable metal. A power supply supplies a negative potential to the target such that the magnetic fields in combination with the negative potential cause plasma ions to hit the target with high energy, which in turn cause target atoms to dislodge from the surface of the target by direct momentum transfer and also create secondary electrons. These dislodged atoms and ions (created by the secondary electrons) are then deposited on the semiconductor wafer. 
     However, the atoms are typically dislodged from the sidewall of the target at a higher rate than at the top of the target. This can lead to more deposition than erosion at the top of the target. Redeposition has a higher potential to form particles or delamination than an eroded area of the target. 
     SUMMARY 
     The present invention provides a system for performing PVD using an open top HCM source, increasing the efficiency of using target material and decreasing redeposition. In one embodiment, the apparatus comprises a HCM with an open top cylindrical sputtering target having a negative bias, thereby forming an electric filed within the HCM to generate a plasma. To seal the HCM for PVD processes, a shield is placed on the top of HCM in place of target material, thereby allowing the HCM to be evacuated. The HCM also comprises electromagnetic coils that shape and increase density of a plasma of inert gas within the HCM. Plasma ions strike the target, thereby dislodging target atoms, which then deposit on the wafer. 
     In another embodiment of the invention, the HCM uses a flat annular target arranged in a horizontal position in place of the cylindrical target. In a third embodiment of the invention, the HCM uses a conical open top target arranged at a 45-degree angle. It will be recognized by one skilled in the art that in alternative embodiments the open top target can be shaped and aligned at any angle. 
     As the target in each of the above-mentioned embodiments does not have a top/central portion, the HCM is more efficient to operate since the target comprises less metal but does not lower deposition rates. Further, using the conical open top target leads to more target material flowing out of a mouth of the target than in a conventional HCM because the target mouth has many times the area of the open top of the target, thereby yielding higher deposition rates. In addition, redeposition becomes less of a problem since the target does not contain a top portion on which redeposited particles can form. 
     The present invention further provides a method for PVD using a topless target. The method uses the apparatus described above and comprises the steps of maintaining a plasma within a cathode of the HCM; sputtering target material from the open top target; and depositing the sputtered target material onto the wafer. 
     The system and method may advantageously perform PVD onto a wafer using an open top target while improving on the deposition rate of a conventional HCM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a cylindrical open top HCM according to an embodiment of the invention; 
     FIG. 2 shows a magnetostatic simulation plot for the open top HCM of FIG. 1; 
     FIG. 3 shows a magnetostatic simulation plot for a double separatrix configuration of the open top HCM of FIG. 1; 
     FIG. 4 shows a magnetostatic simulation plot for a pinched top configuration of the open top HCM of FIG. 1; 
     FIG. 5 shows a schematic diagram of a flat annular HCM according to an embodiment of the invention; 
     FIG. 6 shows a magnetostatic simulation plot for the flat annular HCM of FIG. 5; 
     FIG. 7 shows a schematic diagram of a conical open HCM according to an embodiment of the invention; and 
     FIG. 8 shows a magnetostatic simulation plot of the conical open top HCM of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is provided to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles, features and teachings disclosed herein. 
     FIG. 1 shows a schematic diagram of a cylindrical open top HCM  100  according to an embodiment of the invention. The HCM  100  is symmetrical in shape around axis  102  as indicated by arrow  104 . HCM  100  has a cathode  132 , wherein the plasma is generated, and an electrically floating anode  130  located beneath the mouth of target  106 . Target  106  is composed of tantalum (“Ta”), which may be biased to about −300 to −400 volts using voltage from a power supply (not shown). In an alternative embodiment of the present invention, the target  106  is composed of titanium, copper, or other suitable material. The target  106  is aligned vertically in the HCM  100  and has an open top. In order to complete a seal for the HCM  100  for forming a vacuum before generating a plasma, a top shield  125  is located on the top of HCM  100  where the top of target  106  would be in a conventional HCM. As shown in FIG. 1, HCM  100  further includes a wafer pedestal  120 . Pedestal  120  is also shown in FIGS. 2 and 3. 
     Argon, nitrogen or other suitable gases and mixtures known in the art of physical vapor deposition that do not appreciably react with the target  106  is injected into HCM  100  by an injector (not shown). Pressure within the HCM  100  is generally maintained at about 0.5 mTorr but may be up to several hundred mTorrs. The negatively biased voltage between target  106  and HCM  100  generates an electric field that generates plasma from the inert gas. 
     HCM  100  also comprises electromagnetic (“EM”) coils  108 ,  110 ,  112 , and  114 , which, in concert, generate a magnetic field that enhances and shapes the plasma formed from electric fields within the HCM source  100 . The plasma is generally a high-density plasma on the order of 10 13  ions/cm 3 . Due to the negative potential of the target  106 , ions from the plasma impact the tantalum target  106  causing Ta atoms to sputter off of the surface of the target  106  due to direct momentum transfer. The Ta atoms then enter the plasma where they may be ionized and transported to the wafer as an ion. The electromagnetic coils  116  and  118  are used to shape the plasma and achieve uniform ion transport to the wafer. 
     HCM  100  also comprises a water jacket  122 , which cools the target  106 . As target  106  is an open top geometry, HCM  100  does not require a rotating permanent magnet above cathode  132  to cause the plasma ions to sputter target material from the top of target  106 . Further, additional or fewer electromagnetic coils may be added or subtracted from the HCM  100 , as long as a plasma can be shaped and maintained at a sufficient density within cathode  132 . 
     In an alternative embodiment of the invention, HCM  100  comprises an additional wafer pedestal (not shown) above cathode  132 . As target  106  comprises an open top geometry, the target  106  has, in effect, two target mouths from which target material exits. Accordingly, a second wafer pedestal for PVD can be placed above the open top of target  106 , thereby doubling the rate at which wafers can be processed in HCM  100 . 
     FIG. 2 shows a magnetostatic simulation plot for the open top HCM  100  (FIG.  1 ). The HCM  100  is symmetrical about axis  200  as indicated by arrow  202 . Coils  108 ,  110 ,  112 , and  118 , which have strengths of 0, 1500, 1200, −2400, amp-turns respectively, are used to generate magnetic field lines  210 ,  215 ,  220 ,  225 ,  230 ,  235 ,  240 ,  245 , and separatrix  250 . Generally speaking, a separatrix is a three-dimensional surface that divides the magnetic flux into two pieces: that which returns to the opposite magnetic pole by flowing inside the target and that which flows outside the target. Commonly owned U.S. Pat. No. 6,179,973 also discusses the concept of separatrix in HCMs. The just-mentioned patent is incorporated herein by reference in its entirety. The strength of the field lines  210 ,  215 ,  220 ,  225 ,  230 ,  235 ,  240 , and  245  can be varied by modifying the current flow in the coils  108 ,  110 ,  112 , and  118 . Alternatively, coils  108 ,  110 ,  112 , and  118  may be permanent magnets. 
     Magnetic field lines  210 ,  215 ,  220 ,  225 ,  230 ,  235 ,  240 , and  245  represent the strength of the magnetic field in the plasma, which is used to increase plasma density in the target  106  and downstream and to shape the plasma to get better uniformity at the substrate or wafer. Plasma ions impacting target  106  cause target atoms to dislodge from the target  106  due to direct momentum transfer. The impact also forms secondary electrons, which ionize a fraction of the dislodged target atoms. In contrast to a conventional HCM, uniform erosion of the target  106  is not a problem since target  106  has an open top geometry. Further, redeposition is less of a concern due to the open top geometry of target  106 . If the shield  125  becomes contaminated with redeposited particles, the shield  125  can be easily replaced at low cost. In comparison, if target  106  did not have an open top geometry, the plasma in cathode  132  would not uniformly erode the top of the target, leading to a waste of target material. 
     FIG. 3 shows a magnetostatic simulation plot for a double separatix configuration of the open top HCM  100  (FIG.  1 ). In the magnetostatic simulation plot of FIG. 3, as in the plot of FIG. 2, the HCM  100  is symmetrical about axis  300  as indicated by arrow  302 . The plot of FIG. 3 varies from the plot of FIG. 2 in that the FIG. 3 plot has two separatrixes  310  and  320 . Separatrix  310  and  320  are generated by the magnetic coils  108 ,  112 ,  114 , and  118 , which have strengths of −5,000 amp-turn; 1,000 amp-turn; −4,500 amp-turn; and −3,000 amp-turn, respectively. The advantage of using a double separatrix configuration is that the configuration increases magnetic flux lines parallel to the target  106 , along which electrons travel, thereby creating a high density plasma adjacent to the target  106  leading to erosion of the target  106 . 
     FIG. 4 shows a magnetostatic simulation plot for a pinched top configuration of the open top HCM  100  (FIG.  1 ). The HCM  100  is symmetrical about axis  400  as indicated by arrow  402 . The FIG. 4 embodiment of HCM  100  comprises an additional coil  410  located above the top shield  125 . In this embodiment, coil  410  has a strength of 2,500 amp-turn, and coils  108 ,  110 ,  112 , and  118  have strengths of 3,000 amp-turn, 1,500 amp-turn, 1,200 amp-turn; and −3,000 amp-turn, respectively. Coil  410  creates a magnetic mirror that repels plasma from shield  125 , thereby preventing redeposition onto the shield  125 . 
     FIG. 5 shows a schematic diagram of a flat annular HCM  500  according to an embodiment of the invention. The HCM  500  is symmetrical about axis  502  as indicated by arrow  504 . HCM  500  comprises a flat annular target  506  of Ta, Ti, Cu or other suitable material charged to a negative bias so as to form an electric field within the HCM  500  that generates a plasma. HCM  500  further comprises a top shield  525  in place of a target  506  top. The HCM  500  also has a water jacket  522 , which cools target  506  during PVD processes. HCM  500  also comprises electromagnetic coils  508 ,  510 ,  512 , and  514  for shaping and increasing density of a plasma within a cathode  532  of the HCM  500 . Additionally, HCM  500  comprises anode coils  516  and  518 . While HCM  500  comprises six electromagnetic coils in total, any number of coils may be used as long as the number of coils is sufficient to shape and maintain a sufficient density of a plasma within cathode  532 . Further, in another embodiment of HCM  500 , electromagnetic coils  508 ,  510 ,  512 ,  514 ,  516 , and  518  may be replaced with permanent magnets or HCM  500  may comprise a combination of electromagnetic coils and permanent magnets. 
     An advantage of the flat annular geometry of HCM  500  is that the target  506  is directly facing the wafer pedestal  520 . Accordingly, sputtered target material will usually have a velocity in the direction of the wafer pedestal  520 , thereby increasing the deposition rate over a conventional HCM, wherein the target sidewalls are at right angles to the wafer pedestal and therefore sputtered target material must undergo a collision with plasma in order to be redirected towards the wafer pedestal. 
     FIG. 6 shows a magnetostatic simulation plot for the flat annular HCM  500  (FIG.  5 ). The HCM  500  is symmetrical about axis  602  as indicated by arrow  604 . Electromagnetic coils  510 ,  512 ,  514 , and  518  have strengths of −1,200 amp-turn, −1,200 amp-turn, −800 amp-turn, and 1,500 amp-turn, respectively. The plasma is located in cathode  532  above and below the separatrix  620  and electrons are generally confined by the magnetic field, as shown by field lines  625 , causing plasma density near the target  506  to be high. An electric field causes electrons from the plasma to strike target  506  and dislodge target atoms due to direct momentum transfer. As can be seen for the flat annular geometry, only a minimum of magnetic fields lines  625  cross shield  525 , leading to a minimum of redeposition onto shield  525 . If shield  525  becomes contaminated with material, the shield  525  can easily be replaced with another inexpensive shield. In comparison, if target  506  did not have an open top geometry, the plasma in cathode  532  would not uniformly erode the top of the target, leading to a waste of target material. 
     FIG. 7 shows a schematic diagram of a conical open top HCM  700  according to an embodiment of the invention. The HCM  700  is symmetrical about axis  702  as indicated by arrow  704 . HCM  700  comprises a conical open top target  706  of Ta, Ti, Cu or other suitable material charged to a negative bias so as to form an electric field within HCM  700 . HCM  700  further comprises a top shield  725  in place of a target  706  top, and a water jacket  722 , which cools target  706  during PVD processes. HCM  700  also comprises electromagnetic coils  708 ,  710 ,  712 , and  714  for shaping and increasing the density of a plasma within a cathode  732  of the HCM  700 . Additionally, HCM  700  comprises anode coils  716  and  718 . While HCM  700  comprises six electromagnetic coils in total, any number of coils may be used as long as the number of coils is sufficient to shape and increase the density of a plasma within cathode  732 . Further, in another embodiment of HCM  700 , electromagnetic coils  708 , 710 , 712 ,  714 , 716 , and  718  may be replaced with permanent magnets or HCM  700  may comprise a combination of electromagnetic coils and permanent magnets. As shown in FIG. 7, HCM  700  further includes a wafer pedestal  720 . 
     An advantage of the conical open top geometry of HCM  700  is that the geometry provides for about nine times the open area at the target mouth  740  than at the open top  745 . Accordingly, a higher proportion of target material will flow out of target mouth  740  than out of open top  745 , leading to faster deposition rates. In comparison, in HCM  100 , using the magnetic field configuration of FIG. 2, an equal amount of target material may flow out of the target  106  (FIG. 1) mouth and the target  106  open top, leading to conventional deposition rates. 
     FIG. 8 shows a magnetostatic simulation plot for the conical open top HCM  700  (FIG.  7 ). The HCM  700  is symmetrical about axis  802  as indicated by arrow  804 . Electromagnetic coils  708 ,  710 , 712 ,  714 , and  716  have strengths of −300 amp-turn, −700 amp-turn, −700 amp-turn, −400 amp-turn, and 900 amp-turn, respectively. The plasma is located in cathode  732  above and below the separatrix  820  and electrons are generally confined by the magnetic field, as shown by field lines  825 , causing plasma density near the target  706  to be high. An electric field causes electrons from the plasma to strike target  706  and dislodge target atoms due to direct momentum transfer. As can be seen for the conical open top geometry, only a minimum of magnetic fields lines  825  cross shield  725 , leading to a minimum of redeposition onto shield  725 . If shield  725  becomes contaminated with material, the shield  725  can easily be replaced with another inexpensive shield. In comparison, if target  706  did not have an open top geometry, the plasma in cathode  732  would not uniformly erode the top of the target, leading to a waste of target material. 
     The foregoing description of the preferred embodiment of the present invention is by way of example only, and other variations and modifications of the above-described embodiments and methods are possible in light of the foregoing teaching. For example, target  106  may be comprised of titanium instead of tantalum. Further, the number, type and shape of components or magnetic materials shown can be varied to achieve the same effect as that disclosed herein. The present invention is limited only by the following claims.