Abstract:
A burn-in process is performed in a high density plasma sputtering chamber to remove contaminants from a coil and a sputtering target installed in the chamber. The process includes applying respective power signals to the coil and to the sputtering target while maintaining a pressure level in the chamber that is lower than the conventional pressure level of 40 mT. Preferably the pressure level is maintained at substantially 10 mT.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to semiconductor device manufacturing, and more particularly to a burn-in process employed in an high density plasma physical vapor deposition (HDPPVD) chamber.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 1 is a side diagrammatic illustration, in section, of the pertinent portions of a conventional high density plasma (HDP) sputtering or physical vapor deposition (PVD) chamber  100 . The sputtering chamber  100  contains a coil  102  which is operatively coupled to a first RF power supply  104  via one or more feedthroughs  105 . The coil  102  may comprise a plurality of coils, a single turn coil, a single turn material strip, or any other similar configuration. The coil  102  is positioned along the inner surface of the sputtering chamber  100 , between a sputtering target  106  and a substrate pedestal  108 . Both the coil  102  and the target  106  are formed from the to-be-deposited material (e.g., copper, aluminum, titanium, tantalum, etc.).  
           [0003]    The substrate pedestal  108  is positioned in the lower portion of the sputtering chamber  100  and typically comprises a pedestal heater (not shown) for elevating the temperature of a semiconductor wafer or other substrate supported by the substrate pedestal  108  during processing within the sputtering chamber  100 . The sputtering target  106  is mounted to a water cooled adapter  110  in the upper portion of the sputtering chamber  100  so as to face the substrate receiving surface of the substrate pedestal  108 . A cooling system  112  is coupled to the adapter  110  and delivers cooling fluid (e.g., water) thereto.  
           [0004]    The sputtering chamber  100  generally includes a vacuum chamber enclosure wall  114  having at least one gas inlet  116  coupled to a gas source  118  and having an exhaust outlet  120  coupled to an exhaust pump  122  (e.g., a cryopump or a cryoturbo pump). The gas source  118  typically comprises a plurality of processing gas sources  118   a,    118   b  such as a source of argon, helium and/or nitrogen. Other processing gases may be employed if desired.  
           [0005]    A removable shield  124  that circumferentially surrounds the coil  102 , the target  106  and the substrate pedestal  108  is provided within the sputtering chamber  100 . The shield  124  may be removed for cleaning during chamber maintenance, and the adapter  110  is coupled to the shield  124  (as shown). The shield  124  also supports the coil  102  via a plurality of cups  126   a - b  attached to, but electrically isolated from the shield  124 , and via a plurality of pins  128   a - b  coupled to both the cups  126   a - b  and the coil  102 . The coil  102  is supported by resting the coil  102  on the pins  128   a - b  which are coupled to the cups  126   a - b.  The cups  126   a - b  and the pins  128   a - b  comprise the same material as the coil  102  and the target  106  (e.g., copper) and are electrically insulated from the shield  124  via a plurality of insulating regions  129   a - b  (e.g., a plurality of ceramic regions). The sputtering chamber  100  also includes a plurality of bake-out lamps  130  located between the shield  124  and the chamber enclosure wall  114 , for baking-out the sputtering chamber  100 .  
           [0006]    The sputtering target  106  and the substrate pedestal  108  are electrically isolated from the shield  124 . The shield  124  may be grounded so that a negative voltage (with respect to grounded shield  124 ) may be applied to the sputtering target  106  via a first DC power supply  132  coupled between the target  106  and ground, or may be floated or biased via a second DC power supply  133  coupled to the shield  124 . Additionally, a negative bias may be applied to the substrate pedestal  108  via a second RF power supply  134  coupled between the pedestal  108  and ground. A controller  136  is operatively coupled to the first RF power supply  104 , the first DC power supply  132 , the second DC power supply  133 , the second RF power supply  134 , the gas source  118  and the exhaust pump  122 . The controller  136  includes computer program code adapted to control various operating parameters of the chamber  100  including the power levels provided by the power supplies  104 ,  132 ,  133 ,  134  and the pressure level in the chamber  100 .  
           [0007]    To perform deposition within the sputtering chamber  100 , a substrate  138  (e.g., a semiconductor wafer, a flat panel display, etc.) is loaded into the sputtering chamber  100 , is placed on the substrate pedestal  108  and is securely held thereto via a clamp ring  140 . An inert gas such as argon then is flowed from the gas source  118  into the high density plasma sputtering chamber  100  and the first DC power supply  132  biases the sputtering target  106  negatively with respect to the substrate pedestal  108  and the shield  124 . In response to the negative bias, argon gas atoms ionize and form a plasma within the high density plasma sputtering chamber  100 . An RF bias preferably is applied to the coil  102  via the first RF power supply  104  to increase the density of ionized argon gas atoms within the plasma and to ionize target atoms sputtered from the target  106  (as described below).  
           [0008]    Because argon ions have a positive charge, argon ions within the plasma are attracted to the negatively biased sputtering target  106  and strike the sputtering target  106  with sufficient energy to sputter target atoms from the target  106 . The RF power applied to the coil  102  increases the ionization of the argon atoms, and, in combination with the coupling of the coil power to the region of argon and sputtered target atoms, results in ionization of at least a substantial portion of the sputtered target atoms. The ionized, sputtered target atoms travel to and deposit on the substrate  138  so as to form over time a continuous target material film  142  thereon. Because the sputtered target atoms are ionized by the coil  102 , the target atoms strike the substrate  138  with increased directionality under the influence of the electric field applied between the target  106  and the substrate pedestal  108  (e.g., by the first DC power supply  132 ). The second RF power supply  134  may be employed to apply a negative bias to the substrate pedestal  108  relative to both the sputtering target  106  and to shield  124  to further attract sputtered target atoms to the substrate  138  during deposition.  
           [0009]    In addition to target atoms, coil atoms are sputtered from the coil  102  during deposition and deposit on the substrate  138 . Because of the coil&#39;s proximity to the wafer&#39;s edge the sputtered coil atoms predominantly coat the substrate  138  near its edges and, where the flat target atoms tend to deposit a center thick layer, result in overall uniformity of the thickness of the film  142  deposited on the substrate  138 . Following deposition, the flow of gas to the high density plasma sputtering chamber  100  is halted, all biases (e.g., target, pedestal and coil) are terminated, and the substrate  138  is removed from the high density plasma sputtering chamber  100 .  
           [0010]    Occasionally the sputtering chamber  100  must be vented to the atmosphere to permit cleaning or routine maintenance and/or replacement of the sputtering target  106  and the coil  102 . After the chamber  100  has been exposed to atmosphere, and before proceeding with deposition, decontamination processes must be performed to place the chamber in a suitable condition for deposition processing. One decontamination procedure is known as “bake-out”. During bake-out, the chamber  100  is maintained at an elevated temperature (e.g., via the bake-out lamps  130 ) for an extended period of time to de-sorb contaminants such as moisture or other gases from the chamber walls and other chamber components.  
           [0011]    Another decontamination process is referred to as “burn-in” and is applied to the sputtering target  106  and to the coil  102 . During burn-in the respective power signals are applied to the target  106  and to the coil  102 , and contaminants such as surface oxides, or trace metals introduced into the target or coil during manufacturing, are removed by sputtering atoms or molecules from the target and coil. Typically, burn-in is carried out intermittently, with a sequence of substrates present in the chamber  100  for monitoring purposes. That is, a substrate is loaded onto the pedestal  108 , a burn-in cycle is performed, and the substrate is removed and replaced with another substrate, whereupon another burn-in cycle is performed.  
           [0012]    One application that has been proposed for HDP sputtering chambers is deposition of copper as a seed layer for electroplating. To obtain suitable Cu seed layer quality, it has been found to be desirable to provide active cooling of the substrate during the seed layer deposition. In order to provide active cooling, the conventional clamp ring (e.g., clamp ring  140 ) has been replaced with a low temperature biasable electrostatic chuck (LTBESC). However, the LTBESC has proven to be susceptible to malfunction or permanent failure resulting from contamination from copper evaporation from the coil that occurs during burn-in. To prevent chuck malfunction or permanent failure, the conventional burn-in process may be modified to reduce copper evaporation from the coil by reducing the duty cycle of the RF power signal applied to the coil. Also, to avoid substrate breakage, the dwell time in the chamber for each monitor substrate employed during burn-in may be reduced so that the total number of substrates used for monitoring was increased. However, these changes may result in a substantial increase in the total elapsed time required for burn-in, and a corresponding increase in the down-time for the sputtering chamber when maintenance is performed.  
           [0013]    It would be desirable to provide a burn-in process that can be performed more rapidly, but without compromising the functioning of the LTBESC.  
         SUMMARY OF THE INVENTION  
         [0014]    An aspect of the invention provides a method of performing a burn-in process wherein contaminants are removed from a coil and a sputtering target installed in a high density plasma PVD chamber. The inventive method includes applying respective power signals to the coil and the sputtering target while maintaining a pressure level in the chamber of less than 40 mT. In one embodiment, the pressure level in the chamber is maintained at less than 25 mT, and in another embodiment at substantially 10 mT.  
           [0015]    The present inventors have discovered that a burn-in process performed at a lower pressure is more efficient than a conventional burn-in process (e.g., typically performed at a pressure of at least 40 mT or higher), thereby permitting the total elapsed time for burn-in to be decreased and reducing overall down-time required for chamber maintenance.  
           [0016]    Other objects, features and advantages of the invention will become more fully apparent from the following detailed description of the exemplary embodiments, the appended claims and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a side diagrammatic illustration, in section, of components of a conventional high density plasma sputtering chamber;  
         [0018]    [0018]FIG. 2 is a graph of coil voltage data obtained from coils that were burned-in using processes that varied in terms of chamber pressure, power level applied to target, and power level applied to coil;  
         [0019]    [0019]FIG. 3 is a graph of copper resistivity data gathered from wafers processed according to conventional and inventive burn-in processes; and  
         [0020]    [0020]FIG. 4 is a graph of data indicative of full-width-half-max (FWHM) readings for the Cu ( 111 ) peak obtained from monitor wafers processed in accordance with conventional and inventive burn-in processes. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    The present inventors carried out a series of experiments to determine the effects of process pressure, level of DC power applied to the target, and level of RF power applied to the coil on the efficiency of burn-in processes. These experiments were performed using an HDP sputtering chamber like that illustrated in FIG. 1, except that, as noted before, a LTBESC (not shown) was installed in place of the clamp ring  140 . The coil voltage was taken as an indicator of the efficiency of the burn-in. In the experimental burn-in processes, the chamber pressure was varied in a range of 60 mT to 10 mT; the DC power supplied to the target was set at 1 kW, 1.5 kW and 2 kW; and the RF power applied to the coil was varied in a range of 2-5 kW.  
         [0022]    [0022]FIG. 2 presents data indicating the coil voltages obtained from the respective experimental burn-in processes. The left-hand third of the graph of FIG. 2 indicates results obtained from burn-in processes carried out with a DC power level applied to the target of 1 kW. The middle third indicates results obtained with a DC power level of 1.5 kW. The right-hand third of the graph indicates results obtained with the DC power level at 2 kW.  
         [0023]    The results presented in FIG. 2 indicate that lowering the process pressure of the sputtering chamber, raising the DC power level applied to the target and raising the RF power level applied to the coil all generally have a positive effect on coil voltage. Of these three factors, lowering the process pressure is the most significant in increasing coil voltage (e.g., increase coil voltage and thus evidence improved burn-in efficiency). Moreover, there are constraints upon increasing the DC and RF power levels applied to the target and coil, respectively. As to increasing the RF power level applied to the coil, at increased levels the coil temperature is increased, leading to coil evaporation. As noted before, coil evaporation may interfere with the functioning of the LTBESC. On the other hand, increased DC power applied to the target may lead to net deposition from the target on the coil. Deposition from the target to the coil may generate particles, and may trap contaminants on the coil. However, lowering the pressure within the sputtering chamber does not suffer from these adverse effects, and therefore was considered to be the best way of improving the efficiency of the burn-in process.  
         [0024]    With these factors in mind, the present inventors determined that an optimal recipe for the burn-in process called for 1 kW of DC power applied to the target, 3 kW of RF power applied to the coil, and a process pressure of 10 mT. This is in contrast to a conventional burn-in recipe of 1 kW-DC/3 kW-RF/40 mT. The duty cycle in both cases was 1:1 on:off. The improved efficiency of the lower-pressure recipe as compared to the conventional recipe is indicated by comparing data points  201  and  202  in FIG. 2. Data point  201  indicates that a coil voltage of 245 V was produced by the 1 kW-DC/3 kW-RF/10 mT recipe (the “new recipe”), whereas data point  202  indicates that a coil voltage of 170 V was produced by the 1 kW-DC/3 kW-RF/40 mT recipe (the “old recipe”). As stated previously, a higher coil voltage is indicative of a more effective burn-in process.  
         [0025]    To confirm the improved effectiveness of the new recipe relative to the old recipe, further experiments were undertaken. In one set of experiments the resistivities of Cu films deposited on monitoring wafers were compared for burn-in processes using the old and new recipes. Results of this experiment are presented in FIG. 3. In FIG. 3, curve  203  plots resistivity data for Cu layers deposited during the new recipe burn-in process. Curve  204  plots resistivity data for Cu layers deposited during an old recipe burn-in process. In the case of both processes, resistivity decreases with increased burn-in duration, but after an initial period lower resistivity levels are achieved with the new recipe process. Taking a 2.5 ohm-cm deposited copper film as a benchmark indicative of a satisfactory burn-in, it will be noted that this level of resistivity is achieved with the new recipe burn-in process after applying a total of 3 kW-hr of DC power to the target. With a 1:1 on:off duty cycle, this amount of total applied power results in about a six-hour elapsed time for burn-in for a 1 kW level of DC power. On the other hand, with the old recipe process, the benchmark is not achieved until a total of 5 kW-hr has been applied to the target which requires about 10 hours.  
         [0026]    In another confirming experiment, the texture of the Cu thin films deposited on monitor wafers was compared for the new recipe and old recipe processes. The full width half max (FWHM) of the Cu ( 111 ) peak was detected for each deposited copper film, and a benchmark of 3.8 or below was considered to indicate a satisfactory burn-in. FIG. 4 presents the results of this experiment. In FIG. 4 the diamond-shaped data points represent FWHM-Cu ( 111 ) peak figures for monitor wafers for the new recipe burn-in process. The square data points represent the results for monitor wafers for the old recipe burn-in process. Once more it will be observed that the desired benchmark was achieved with a burn-in corresponding to 3 kW-hr of DC power applied to the target with the new recipe, as compared to 5 kW-hr being required to achieve this benchmark using the old recipe process.  
         [0027]    The monitor wafers were also examined using secondary ion mass spectroscopy (SIMS) and it was determined that a 3 kW-hr target burn-in using the new recipe performed satisfactorily in terms of eliminating trace metal contaminants.  
         [0028]    Based on these results, it was determined that a burnin process using the new recipe could be satisfactorily terminated upon 3 kW-hr of DC power having been applied to the target. This is in contrast to the conventional process using the old recipe, in which a total of 5 kW-hr of DC power was applied to the target. The total elapsed time for the burn-in process using the new recipe was about 6 hours, as compared to 10 hours for the burn-in process using the old recipe. This represents a substantial reduction in time required for burn-in, and a corresponding reduction in the down-time required for maintenance of the HDP sputtering chamber.  
         [0029]    The foregoing description discloses only exemplary embodiments of the invention; modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For example, although the invention has been described in connection with burn-in of a copper target and copper coil, it is also applicable to burn-in of targets and coils for depositing other metals, such as Ti, W, and Ta. Moreover, the HDP chamber to which the invention is applicable need not be equipped exactly as described herein. Although the invention is particularly advantageous when used in a HDP sputtering chamber which uses an LTBESC, an LTBESC need not necessarily be employed.  
         [0030]    Also, although it is preferred to perform the burn-in process at a chamber pressure of substantially 10 mT, it is within the scope of the invention to employ any pressure level that is less than the conventional level of 40 mT. Further, the inventive burn-in processes described herein may be performed within an HDP sputtering chamber during and/or after a bake out process is performed within the chamber (e.g., any conventional bake-out procedure used to bake out the walls, shield, target, pedestal or any other chamber surface). In accordance with at least one embodiment of the invention, an inventive HDP sputtering chamber may be provided based on the conventional HDP sputtering chamber  100  of FIG. 1 (or based on any other HDP sputtering chamber such as one that employs a LTBESC) by providing the controller  136  with computer program code adapted to perform a burn-in process by applying respective power signals to the coil and the target while maintaining the pressure level in the chamber at less than 40 mT.  
         [0031]    Accordingly, while the present invention has been disclosed in connection with a preferred embodiment thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.