Abstract:
In a plasma enhanced physical vapor deposition of a material onto workpiece, a metal target faces the workpiece across a target-to-workpiece gap less than a diameter of the workpiece. A carrier gas is introduced into the chamber and gas pressure in the chamber is maintained above a threshold pressure at which mean free path is less than 5% of the gap. RF plasma source power from a VHF generator is applied to the target to generate a capacitively coupled plasma at the target, the VHF generator having a frequency exceeding 30 MHz. The plasma is extended across the gap to the workpiece by providing through the workpiece a first VHF ground return path at the frequency of the VHF generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 12/077,067, filed Mar. 14, 2008, which is herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Plasma enhanced physical vapor deposition (PECVD) processes are used to deposit metal films such as copper onto semiconductor wafers to form electrical interconnections. A high level of D.C. power is applied to a copper target overlying the wafer in the presence of a carrier gas, such as argon. Plasma source power may be applied via a coil antenna surrounding the chamber. PECVD processes typically rely upon a very narrow angular distribution of ion velocity to deposit metal onto sidewalls and floors of high aspect ratio openings. One problem is how to deposit sufficient material on the sidewalls relative to the amount deposited on the floors. Another problem is avoiding pinch-off of the opening due to faster deposition near the top edge of the opening. As miniaturization of feature sizes has progressed, the aspect ratio (depth/width) of a typical opening has increased, with microelectronic feature sizes having now been reduced to about 22 nanometers. With greater miniaturization, it has become more difficult to achieve minimum deposition thickness on the sidewall for a given deposition thickness on the floor or bottom of each opening. The increased aspect ratio of the typical opening has been addressed by further narrowing of the ion velocity angular distribution, through ever-increasing wafer-to-sputter target distance and ever lower chamber pressures, e.g., less than 1 mT (to avoid velocity profile widening by collisions). This has given rise to a problem observed in thin film features near the edge of the wafer: At extremely small feature sizes, a portion of each high aspect ratio opening sidewall is shadowed from a major portion of the target because of the greater wafer-to-target gap required to meet the decreasing feature size. This shadowing effect, most pronounced near the wafer edge, makes it difficult if not impossible to reach a minimum deposition thickness on the shadowed portion of the side wall. With further miniaturization, it has seemed a further decrease and chamber pressure (e.g., below 1 mT) and a further increase in wafer-sputter target gap would be required, which would exacerbate the foregoing problems. 
         [0003]    One technique employed to supplement the side wall deposition thickness is to deposit an excess amount of the metal (e.g., Cu) on the floor of each opening and then re-sputter a portion of this excess on the opening side wall. This technique has not completely solved the shadowing problem and moreover represents an extra step in the process and a limitation on productivity. 
         [0004]    A related problem is that the sputter target (e.g., copper) must be driven at a high level of D.C. power (e.g., in the range of kW) to ensure an adequate flow of ions to the wafer. Such a high level of D.C. power rapidly consumes the target (driving up costs) and produces an extremely high deposition rate so that the entire process is completed in less than five seconds. This time is about 40% of the time required for the RF source power impedance match to equilibrate following plasma ignition, so that about 40% of the process is performed prior to stabilization of the impedance match and delivered power. 
       SUMMARY 
       [0005]    A method is provided for performing physical vapor deposition on a workpiece in a reactor chamber. The method includes providing a target comprising a metallic element and having a surface facing the workpiece, and establishing a target-to-workpiece gap less than a diameter of the workpiece. A carrier gas is introduced into the chamber and gas pressure in the chamber is maintained above a threshold pressure at which mean free path is less than 5% of the gap. RF plasma source power from a VHF generator is applied to the target to generate a capacitively coupled plasma at the target, the VHF generator having a frequency exceeding 30 MHz. The method further includes extending the plasma across the gap to the workpiece by providing through the workpiece a first VHF ground return path at the frequency of the VHF generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    So that the manner in which the exemplary embodiments of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
           [0007]      FIG. 1  is a diagram of a plasma reactor in accordance with a first aspect. 
           [0008]      FIG. 2  is a graph of a random or near-isotropic velocity distribution of ions at the wafer surface attained in the reactor of  FIG. 1 . 
           [0009]      FIG. 3  is a graph depicting angular velocities in the distribution of  FIG. 2 . 
           [0010]      FIG. 4  is block diagram depicting method in accordance with one embodiment. 
           [0011]      FIG. 5  is a diagram of a plasma reactor in accordance with a second aspect. 
           [0012]      FIGS. 6A and 6B  depict alternative embodiments of variable ground return impedance elements in the reactor of  FIG. 1  or  FIG. 5 . 
       
    
    
       [0013]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       DETAILED DESCRIPTION 
       [0014]    In one embodiment, a PEPVD process provides complete uniform sidewall coverage at feature sizes of 25 nm and below (e.g., 18 nm) free of the non-uniformities caused by shadowing. The PEPVD process of this embodiment is carried out with VHF plasma source power on the sputter target to create a capacitively coupled RF plasma at the target. The process further employs a very low (or no) D.C. power on the sputter target. With the low D.C. power level on the target, the process may be performed over a period of time that is longer than the settling time of the impedance match elements, and sufficiently long for good process control and chamber matching. These advantages are realized by using high chamber pressure to at least reduce or totally eradicate the neutral directionality from the metal target to the wafer. If, in fact the remaining deposition were to be accomplished from neutrals emanating from the vacuum gap of the mean free path above the wafer, the features would totally be pinched off. However, the RF power maintains a significant density of metal ions in the plasma, making them available for attraction to the wafer by electric field. The electric field can be derived by either a residual RF field from the VHF plasma source at the ceiling (target) or by a small RF bias power applied to the wafer directly. This then creates a predominantly vertical ion velocity distribution with some perpendicular (horizontal) velocity component for side wall coverage. The result is that the side wall and floor of each high aspect ratio opening is conformally covered with the sputtered material from the target. In summary, the source provides a nearly isotropic distribution of neutral velocity and a distribution of ion velocity including a large vertical component and a relatively smaller non-vertical or horizontal component. The isotropic neutral velocity distribution and somewhat broadened but predominantly vertical ion velocity distribution at the wafer surface is realized by maintaining the chamber at an extremely high pressure (e.g., 100 mT) to ensure an ion collision mean free path that is 1/20 th  of the wafer-to-sputter target gap. A high flux of sputtered ions at the wafer is realized by: (1) minimizing the wafer-to-sputter target gap to a fraction of the wafer diameter, (2) generating a VHF capacitively coupled plasma at the sputter target (as mentioned above) and (3) extending the capacitively coupled plasma down to the wafer. The plasma is extended down to the wafer by providing an attractive VHF ground return path through the wafer. The process window is expanded by reducing (or possibly eliminating) the D.C. power applied to the sputter target, so that the target consumption rate is reduced and the process is less abrupt. This reduction in D.C. target power without loss of requisite sputtering is made possible by the high density plasma generated at the target by application of VHF power to the target and by the reduced wafer-to-sputter target gap. 
         [0015]    Undesirable ion bombardment by ions of the carrier gas (e.g., argon ions) is suppressed by selectively favoring the desired sputter target ions (e.g., copper ions) at the plasma sheath overlying the wafer surface. This selection is made by maintaining the wafer bias voltage below an upper threshold voltage (e.g., 300 volts) above which carrier gas (e.g., argon) ions interact with or damage thin film structures on the wafer. However, the wafer bias voltage is maintained above a lower threshold voltage (e.g., 10-50 volts) above which the sputter target material (e.g., copper) ions deposit on the wafer surface. Such a low wafer bias voltage is achieved by differential control of VHF ground return path impedances for the VHF source power through: (a) the wafer and (b) the chamber side wall, respectively. Decreasing the VHF ground return path impedance through the sidewall tends to decrease the wafer bias voltage. This control is provided by independent variable impedance elements governing ground return path impedances through the side wall and through the wafer, respectively. 
         [0016]    The wafer bias voltage is also minimized as follows: The VHF source power applied to the target creates a modest positive bias voltage on the wafer, in the absence of any other applied RF power. This positive wafer bias may be offset by applying a small amount of optional low frequency (LF) RF bias power to the wafer. The LF bias power tends to contribute a negative bias voltage on the wafer, so that its negative contribution may be adjusted or balanced against the positive contribution of the VHF source power to produce a net wafer bias voltage that is close to zero, if desired, or as small as desired. Specifically, the wafer bias voltage is reduced well below the carrier gas ion bombardment bias threshold referred to above. 
         [0017]    The foregoing process has been discussed above with reference to a PEPVD process for depositing copper. However, the process can be used to deposit a wide range of materials other than copper. For example, the sputter target may be titanium, tungsten, tin (or other suitable metallic materials or alloys) for PEPVD deposition of the metallic target material (e.g., titanium, tungsten or tin). Moreover, a metallic (e.g., titanium) target may be employed with a nitrogen process gas for deposition of titanium nitride or other metallic nitride, where a carrier gas (argon) is used for plasma ignition and is then replaced by nitrogen for the metallic (titanium) nitride deposition. 
         [0018]    Referring to  FIG. 1 , a PEPVD reactor in accordance with a first embodiment includes a vacuum chamber  100  defined by a cylindrical sidewall  102 , a ceiling  104  and floor  106 , the chamber containing a wafer support  108  for holding a wafer  110  in facing relationship with the ceiling  104 . A metal sputtering target  112  is supported on the interior surface of the ceiling  104 . A vacuum pump  114  maintains pressure within the chamber  100  at a desired sub-atmospheric value. A conventional rotating magnet assembly or “magnetron”  116  of the type well-known in the art overlies the ceiling  104  directly above the sputtering target  112 . A process gas supply  118  furnishes a carrier gas such as argon into the chamber  100  through a gas injection apparatus  120 , which may be a gas injection nozzle, an array of nozzles or a gas distribution ring. 
         [0019]    The wafer support  108  may embody an electrostatic chuck including a grounded conductive base  108 - 1 , an overlying dielectric puck  108 - 3  having a wafer support surface  108 - 5 , and an electrode or conductive mesh  108 - 7  inside the puck  108 - 3  and separated from the wafer support surface by a thin layer  108 - 9  of the dielectric puck  108 - 3 . 
         [0020]    A D.C. power supply  122  is connected to the center of the target  112  through a central aperture  116 - 1  in the magnetron  116 . An RF plasma source power generator  124  having a VHF frequency is coupled through a VHF impedance match  126  to the center of the target  112  through the center magnetron aperture  116 - 1 . A D.C. chucking voltage supply  128  is connected to the chuck electrode  108 - 7 . 
         [0021]    In operation, the VHF power generator  124  provides about 4 kW of plasma source power to support a capacitively coupled plasma at the target  112  initially consisting of ions of the carrier gas. This plasma sputters the target  112  to generate free target (e.g., copper) atoms which become ionized in the plasma, the rotation of the magnetic fields of the magnetron  116  helping to distribute the consumption of the target  112  and promote ionization near the target  112 . The reactor includes features that enable the plasma generated at the target  112  to reach the wafer  110 . In accordance with one such feature, the plasma formed at the target  112  by the capacitively coupled VHF power from the generator  124  is made to extend down to the wafer  110  by providing an attractive VHF ground return path through the wafer  110  (i.e., through the wafer support  108 ). For this purpose, a variable impedance element  130  is coupled between the electrode  108 - 7  and ground. Other than this connection, the electrode  108 - 7  is insulated from ground, so that the impedance element  130  provides the only connection between the electrode  108 - 7  and ground. The variable impedance element  130  in one implementation consists of a series reactance  132  such as a capacitor, and variable parallel reactances  134 ,  136 , of which the reactance  134  may be a variable capacitor and the reactance  136  may be a variable inductor. The reactances  132 ,  134 ,  136  are selected to provide an impedance at the frequency of the VHF generator  124  that allows current at that frequency to flow from the electrode  108 - 7  to ground. 
         [0022]    Another feature that enables the plasma to extend to from the target to the wafer is a reduction in the gap between the wafer  110  and the target  112 . The wafer-target gap is reduced to a distance less than or a fraction of the wafer diameter. For example, the gap may be ⅕ th  of the wafer diameter. For a 300 mm wafer diameter, the wafer-target gap may be 60 mm. 
         [0023]    The pump  114  is set to provide a high chamber pressure (e.g., 50-200 mT). The chamber pressure is sufficiently high to set the mean free path to less than 1/20 th  of the length of the wafer-to-target gap. The space between the target  112  and the wafer  110  is empty, i.e., free of other apparatus, to ensure maximum dispersion of angular velocity distribution of the neutrals by the large number of collisions in their transit from the target  112  to the wafer  110 . The resulting angular distribution of velocity of the neutrals in the plasma is very broad (nearly uniform) at the wafer surface within a hemispherical angular range from 0° (normal to the wafer surface) up to nearly 90° (parallel to the wafer surface). The ions have a less uniform angular distribution that peaks at the perpendicular direction, due to the attraction presented by the wafer bias voltage.  FIG. 2  is a simplified diagram depicting the distribution of ion population as a function of direction in a cone of ion trajectories depicted in  FIG. 3 , showing a peak at the perpendicular direction. This peak has been broadened from a high variance (e.g., 0.8) to a variance (e.g., 0.2) by the large number of collisions, mainly with neutrals, that the ions experience within the wafer-target gap. These collisions compete with the electric field to reduce the sharp peak of ion velocity distribution about the vertical and provide a small component of non-vertical (e.g., horizontal) ion velocity. This broadening of the ion angular trajectory distribution is combined with the nearly isotropic angular distribution of neutral velocities. This combination improves the uniformity or conformality of the deposited film. As a result of the broadened angular distribution of ion velocity at the wafer surface, coupled with the relatively small wafer-to-target gap, metal is deposited on interior surfaces of high aspect ratio openings in the surface of the wafer with very high conformality and uniform thickness. 
         [0024]    The capacitively coupled plasma produced at the target  112  contains ions from the carrier gas (e.g., argon ions) and ions from the target (e.g., copper ions). The copper ions require a relatively low plasma bias voltage on the wafer to deposit on the wafer surface, typically around 50 volts or less. The argon ions are more volatile than the copper ions, and at the low sheath voltage tend to elastically collide with the features on the wafer surface and disperse, rather than imparting damage. At slightly greater bias voltage levels (e.g., 300 volts), the argon ions collide inelastically with thin film features on the wafer and damage them. Therefore, ideal results can be achieved by limiting the wafer bias voltage to about 50 volts or less, for example. The problem is how to limit the wafer bias voltage to such a low level. 
         [0025]    A first feature for limiting wafer bias voltage is one that diverts a selected portion of the plasma ions away from the wafer  110  to the chamber sidewall  102  (which is formed of a metal). This feature employs the variable VHF ground return impedance element  130  coupled to the chuck electrode  180 - 7  and, in addition, a second variable VHF ground return impedance element  140  coupled to the side wall  102 . The sidewall variable VHF impedance element  140  has a structure similar to that of the element  130 , and (in one implementation) includes a series capacitor  142 , a variable parallel capacitor  144  and a variable inductor  146 . The sidewall variable impedance element  140  is connected between the sidewall  102  and ground, the sidewall  102  being insulated from ground with the exception of this connection. The impedances of the two elements  130 ,  140  are independently adjustable, and determine the apportionment of the plasma current between the wafer  110  and the sidewall  102 . Adjustment of the impedances of the elements  130 ,  140  is performed to reduce the wafer bias voltage down to a low level and to accurately select that level. For example, a reduction in wafer bias voltage may be obtained by increasing the resistance at the VHF source power generator frequency presented by the chuck electrode impedance element  130  (rendering the VHF ground return path through the wafer  110  less attractive) while reducing the resistance at the VHF source power generator frequency presented by the chamber sidewall impedance element  140  (rendering the VHF ground return path through the sidewall  102  more attractive). The relative impedances of the two impedance elements  130 ,  140  at the VHF source power frequency for a given apportionment of plasma current depends upon the relative areas of the wafer  110  and the conductive side wall  102 . 
         [0026]    In order to reduce the conductance through a selected one of two variable impedance elements  130 ,  140 , the impedance may be chosen to either behave as a very high resistance or open circuit at the frequency of the VHF generator  124 , or to behave as a very low resistance or short circuit at harmonics of the frequency of the VHF generators (e.g., 2 nd , 3 rd , 4 th  harmonics). Such behavior at the harmonic frequencies may be implemented in either or both of the impedance elements  130 ,  140  with the addition of further adjustable tank circuits for each of the harmonics of interest. This may be accomplished by adding more variable reactances to the impedance elements  130 ,  140  to achieve the desired filter or pass band characteristics. For example, referring to  FIG. 6A , the variable impedance element  130  may further include variable resonant circuits  130 - 1 ,  130 - 2 ,  130 - 3  that may be tuned to provide selected impedances at the second, third and fourth harmonics, respectively. Referring to  FIG. 6B , the variable impedance element  140  may further include variable resonant circuits  140 - 1 ,  140 - 2 ,  140 - 3  that may be tuned to provide selected impedances at the second, third and fourth harmonics, respectively. 
         [0027]    A second feature for limiting wafer bias voltage exploits the tendency of the VHF source power applied to the target  112  to produce a modest positive bias voltage on the wafer  110  (in the absence of any other RF power being applied). This second feature involves coupling an optional low frequency RF bias power generator  150  through an impedance match  155  to the chuck electrode  108 - 7 . As RF bias power from the generator  150  is increased from zero, the wafer bias voltage, which is initially positive under the influence of the VHF source power from the VHF generator  124 , is shifted down and at some point crosses zero and becomes negative. By carefully adjusting the output power level of the bias power generator  150  to a small power level (e.g., about 0.1 kW), the wafer bias voltage can be set to a very small value (e.g., to less than 50 volts). 
         [0028]    Since the VHF source power generator  124  provides plasma source power for the generation of plasma ions near the target  112 , the D.C. power source  122  is not the sole source of power for plasma generation. The demands on the D.C. power source  122  are further lessened because the reduced wafer-target gap reduces loss of plasma density between the target  112  and the wafer  110 . Therefore, the D.C. power level of the D.C. supply  122  may be reduced from the conventional level (e.g., 35-40 kW) to as low as 2 kW. This feature reduces the sputtering rate of the target  112  and therefore reduces the consumption of the target, cost of operation and thermal load on the entire system. Moreover, it reduces the deposition rate on the wafer  110 . At the higher D.C. power level (e.g., 38 kW) the deposition rate was extremely high, and the deposition time had to be limited to about 5 seconds for a typical copper film deposition thickness, of which the first 2 seconds were spent by the impedance match  126  reaching equilibrium or stability following plasma ignition. At the new (reduced) D.C. power level (of a few kW or less), the deposition time may be on the order of 30 seconds, so that the impedance match  126  is stable for a very high percentage of the process time. 
         [0029]    The increase in uniformity of the metal coating on interior surfaces of high aspect ratio openings increases the process window over which the reactor may be operated. In the prior art, the metal deposition on the interior surfaces of high aspect ratio openings was highly non-uniform, which allowed for only a very small wafer-to-wafer variation in performance and a very narrow process window within which adequate metal coating could be realized for all internal surfaces of a high aspect ratio opening. Furthermore, the inadequate deposition thickness on sidewalls of high aspect ratio openings required the performance of a second step following deposition, namely a re-sputtering step in which excess material deposited on the floor or bottom of an opening is transferred to the sidewall. The re-sputtering step has typically required an excess thickness to be deposited on the floor of the high aspect ratio opening. With the present embodiment, the improved uniformity of the deposition on the floor and sidewall of the opening eliminates the need for re-sputtering and the need for excess thickness on the bottom of the opening. This increases productivity and reduces the amount of material that must be removed when opening a via through the floor of the high aspect ratio opening. 
         [0030]    The total power applied to the chamber is reduced in the embodiment of  FIG. 1  from a convention PEPVD process, as can be seen in the following table (Table I). 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 conventional 
                 reactor of 
               
               
                   
                 reactor 
                 FIG. 1 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 D.C. power 
                 38 
                 kW 
                 2 
                 kW 
               
               
                   
                 RF source power 
                 1 
                 kW 
                 4 
                 kW 
               
               
                   
                 RF bias power 
                 1 
                 kW 
                 0.1 
                 kW 
               
               
                   
                 TOTAL POWER 
                 40 
                 kW 
                 6.1 
                 kW 
               
               
                   
                 gap 
                 390 
                 mm 
                 60 
                 mm 
               
               
                   
                 pressure 
                 0.5 
                 mT 
                 100 
                 mT 
               
               
                   
                 mean free path 
                 550 
                 mm 
                 2.7 
                 mm 
               
               
                   
                 collisions/transit 
                 0.30 
                   
                 22.22 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    The disclosed process overcomes the problem of shadowing during PECVD processes for metal deposition in high aspect ratio openings. Such openings may be on the order of 22 nanometers in diameter and have a 7:1 aspect ratio (height:width). Prior to the invention, PECVD metal deposition in such openings near the edge of a 300 mm wafer exhibited great non-uniformity due to shadowing effects. The ratio between the metal deposited on radially inner and outer sides of the opening sidewall was as great as 50:1, corresponding to a highly non-uniform sidewall deposition. With the process disclosed above, it is improved to nearly 1:1, a uniform sidewall deposition. 
         [0032]      FIG. 4  depicts a method in accordance with one aspect. A target is provided having a material (such as copper) requiring a low wafer bias voltage (e.g., 10-50 volts) for deposition (block  410  of  FIG. 4 ). A carrier gas such as argon is introduced into the chamber that tends not to produce ion bombardment damage at a low wafer bias voltage (block  412 ). A plasma is generated at the target by capacitively coupling VHF source power using the target as an electrode (block  414 ). This plasma is extended down to the wafer by providing a VHF ground return path through the wafer (block  416 ), and by establishing a narrow wafer-to-target gap (block  418 ). The chamber pressure is maintained at a sufficiently high level (e.g., 100 mT) to ensure an ion mean free path length that is less than 1/20 th  of the wafer-target gap (block  420 ), to establish a random or nearly isotropic ion velocity distribution at the wafer surface (block  422 ). The wafer bias voltage is minimized by apportioning the plasma between a ground return path through the wafer at a first impedance and a ground return path through the chamber sidewall at a second impedance (block  424 ). The wafer bias voltage is further minimized by offsetting a positive bias voltage induced by the VHF source power with a negative bias voltage induced by an LF bias power generator (block  426 ). The steps of blocks  424  and  426  are performed so as to keep the wafer bias below a threshold at which the carrier gas ions can inflict ion bombardment damage but above the necessary threshold to deposit the target material. 
         [0033]      FIG. 5  depicts a modification of the embodiment of  FIG. 1 , in which the sputter target D.C. power supply  122  is eliminated and in which the optional RF bias power generator  150  and match  155  are not present. In the embodiment of  FIG. 5 , the wafer-target gap is sufficiently small and the VHF source power generator  124  has a sufficiently high output power level to provide, with the attraction of a VHF return path through the wafer  110 , the plasma ion density at the target  112  necessary for a desired sputtering rate at the target and deposition rate at the wafer. In the absence of the D.C. power on the target, the target consumption is slower and the thermal load on the system is less and the process window is wider. 
         [0034]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.