Abstract:
Embodiments disclosed herein generally include methods of ensuring uniform deposition on a substrate. The smallest gap between a portion of the substrate and the substrate support upon which the substrate rests may lead to uneven deposition of material or ‘thin spots’ on the substrate. Large area substrates, due to their size, are susceptible to numerous gaps at random locations. By inducing an electrostatic charge on the substrate prior to placing the substrate onto the substrate support, the substrate may be placed generally flush against the substrate support. The electrostatic charge on the substrate creates an attraction between the substrate and substrate support to pull substantially the entire surface of the substrate into contact with the substrate support. Material may then be substantially uniformly deposited on the substrate while reducing ‘thin spots’.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/122,290 (APPM/14129L), filed Dec. 12, 2008, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments disclosed herein generally relate to a method for ensuring uniform deposition on a substrate. 
         [0004]    2. Description of the Related Art 
         [0005]    As the demand for larger flat panel displays (FPDs) continues to grow, so does the size of the substrate that is used to make the FPDs. The size of the substrates now routinely exceeds 1 square meter in area. When compared to the size of semiconductor substrates, which typically are about 300 centimeters in diameter, it can be easily understood that a chamber sized to process a semiconductor wafer may not be sufficiently large to process a substrate of 1 square meter or larger. Thus, larger area processing chambers need to be developed. 
         [0006]    These large area processing chambers may, in some cases, be identical to the semiconductor counterpart chambers where simply scaling up in size achieves acceptable results. In other cases, scaling up the size of the processing chamber is not effective as unforeseen difficulties occur when scaling up the processing chambers. Therefore, care needs to be taken to design a chamber that can process large area substrates. 
         [0007]    Additionally, the process conditions for processes that are performed in the large area processing chambers may need to be adjusted. Determining proper gas flows, timing sequences, power to apply, temperature conditions, and other process variables may require a significant amount of research and experimentation that is beyond routine. 
         [0008]    Therefore, there is a need for new and non-obvious methods for processing large area substrates. 
       SUMMARY OF THE INVENTION 
       [0009]    Embodiments disclosed herein generally include methods of ensuring uniform deposition on a substrate. The smallest gap between a portion of the substrate and the substrate support upon which the substrate rests may lead to uneven deposition of material or ‘thin spots’ on the substrate. Large area substrates, due to their size, are susceptible to numerous gaps at random locations. By inducing an electrostatic charge on the substrate prior to placing the substrate onto the substrate support, the substrate may be placed generally flush against the substrate support. The electrostatic charge on the substrate creates an attraction between the substrate and substrate support to pull substantially the entire surface of the substrate into contact with the substrate support. Material may then be substantially uniformly deposited on the substrate while reducing ‘thin spots’. 
         [0010]    In one embodiment, a method includes inserting a substrate into a chamber on an end effector and lowering the end effector to place the substrate onto one or more lift pins. The method may also include retracting the end effector from the chamber, introducing a gas into the chamber, igniting the gas into a plasma, and extinguishing the plasma. The method may also include exhausting the gas from the chamber and raising a substrate support from a first position to a second position such that the substrate rests on the substrate support. 
         [0011]    In another embodiment, a method includes introducing a gas into a chamber, igniting the gas into a plasma while a substrate is spaced from a substrate support, and bringing the substrate into contact with the substrate support. 
         [0012]    In another embodiment, a method includes inducing an electrostatic charge onto a substrate while the substrate is spaced from a substrate support and bringing the substrate into contact with the substrate support. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical 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. 
           [0014]      FIG. 1  is a schematic cross sectional view of a processing chamber according to one embodiment. 
           [0015]      FIG. 2A  is a schematic top view of a substrate having a layer deposited thereon that has thin spots. 
           [0016]      FIG. 2B  is a schematic cross sectional view of  FIG. 2A  taken along line A-A. 
           [0017]      FIG. 3  is a schematic cross sectional view of a large area substrate disposed on a substrate support. 
           [0018]      FIGS. 4A-4D  are schematic cross sectional views showing a sequence of placing a substrate into a chamber according to one embodiment. 
           [0019]      FIGS. 5A-5C  are graphs showing a comparison of film thickness variations according to several embodiments. 
       
    
    
       [0020]    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 disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
       DETAILED DESCRIPTION 
       [0021]    Embodiments discussed herein relate to methods of ensuring a substantially uniformly thick layer is deposited onto a substrate. In the description that follows, reference will be made to a plasma enhanced chemical vapor deposition (PECVD) chamber, but it is to be understood that the embodiments herein may be practiced in other chambers as well, including physical vapor deposition (PVD) chambers, etching chambers, semiconductor processing chambers, solar cell processing chambers, and organic light emitting display (OLED) processing chambers to name only a few. Suitable chambers that may be used are available from AKT America, Inc., a subsidiary of Applied Materials, Inc., Santa Clara, Calif. It is to be understood that the embodiments discussed herein may be practiced in chambers available from other manufacturers as well. 
         [0022]      FIG. 1  is a schematic cross sectional view of a processing chamber  100  according to one embodiment. The chamber  100  includes a chamber body  102  having a lid  104  coupled thereto. In one embodiment, the chamber body  102  and the lid  104  may comprise aluminum. Within the chamber body  102 , a substrate support  106  may be present to support a substrate  108  during processing. In one embodiment, the substrate  108  may comprise glass. One or more lift pins  110 A,  110 B may extend through the substrate support  106  to support the substrate  108  when the substrate  108  is received from an end effector and when the substrate is ready to be received by an end effector. In one embodiment, the lift pins  110 A,  110 B may comprise a ceramic material. The lift pins  110 A,  110 B may rest on the bottom of the chamber body  102  and support the substrate  108  when the substrate support  106  is lowered by an actuator  138 . In one embodiment, the substrate support  106  may comprise a conductive material. In another embodiment, the substrate support  106  may comprise aluminum. In another embodiment, the substrate support  106  may have a coating of anodized aluminum thereover. 
         [0023]    A showerhead  114  may be present within the chamber  100  and disposed opposite the substrate support  106 . The showerhead  106  may be electrically coupled to a backing plate  116  by a bracket  132 . In one embodiment, the showerhead  114 , backing plate  116 , and bracket  132  may each comprise a conductive material. In another embodiment, the showerhead  114 , backing plate  116 , and bracket  132  may comprise aluminum. 
         [0024]    Gas may be delivered to the chamber  100  from a gas source  122  through a tube  124  that passes through the lid  124  and couples to the backing plate  116 . The gas then travels through the backing plate  116  and disperses within a plenum  126  between the backing plate  116  and the showerhead  114 . The gas substantially evenly distributes within the plenum  126  and then travels through gas passages  128  formed through the showerhead  114 . The gas, when depositing material, is ignited into a plasma within the processing area  130  between the substrate  108  and the showerhead  114 . The chamber  100  may be evacuated by a vacuum pump  118  that is coupled to the chamber body  102 . A valve  120  may be present to regulate the vacuum level. 
         [0025]    The plasma may be ignited in the chamber  100  by supplying power from a power source  136 . In one embodiment, the power source  136  may comprise an RF power source. In one embodiment, the power source  136  may generate RF currents having a frequency of between about 10 MHz and about 60 MHz. The power source  136  is coupled to the tube  124  that feeds the gas into the chamber  100 . The RF current travels along the outside surface of the tube  124  and does not penetrate into the inside of the tube  124  due to the skin effect of RF current. The gas traveling within the tube  124  therefore does not ignite into a plasma within the tube  124 . 
         [0026]    The RF current travels along the back surface of the backing plate  116 , down along the bracket  132  and along the front surface of the showerhead  114  facing the substrate  108 . The RF current returns along the walls of the chamber body  102  as well as the lid  104  until it returns to the power source  136 . 
         [0027]    To perform a process in the chamber  100 , a substrate  108  is first inserted into the chamber  100  through the slit valve opening  112  on an end effector. At this time, the substrate support  106  is in a lowered position such that the lift pins  110 A,  110 B extend above the substrate receiving surface  140  of the substrate support  106 . The end effector then lowers, as does the substrate  108  supported thereon, such that the substrate rests on the lift pins  110 A,  110 B. 
         [0028]    The lift pins  110 A,  110 B have different heights. The lift pins  110 A that are disposed near the edge of the substrate support  106  extend to a greater height than the lift pins  110 B that are closer to the center. Thus, the substrate  108 , when placed onto the lift pins  110 A,  110 B, sags down as shown in  FIG. 1 . The substrate  108  sags due to its size. The inner lift pins  110 B, extending to a shorter height than the outer lift pins  110 B, permit the center of the substrate  108  to sag closer to the substrate support  106  than the outer lift pins  110 A. Thus, the substrate  108 , when resting on the lift pins  110 A,  110 B and spaced from the substrate support  106 , has a convex surface facing the substrate support  106 . 
         [0029]    After depositing the substrate  108  onto the lift pins  110 A,  110 B, the end effector retracts from the chamber  100 . The substrate support  106  then raises while the lift pins  110 A,  110 B remain stationary. The substrate support  106  rises until it is in the processing position. While on the way to the processing position, the substrate support  106  comes into contact with the substrate  108  supported by the lift pins  110 A,  110 B. The substrate  108  begins to contact the substrate support  106  in a center to edge manner due to the sagging of the substrate  108 . The lift pins  110 A,  110 B remain stationary as the substrate support  106  raises until the substrate support  106  has raised to a position such that the substrate  108  supported by the lift pins  110 A,  110 B is supported by the substrate support  106 . Thus substrate support  106  then continues to raise and thus lifts not only the substrate  108 , but also the lift pins  110 A,  110 B. Because the lift pins  110 A,  110 B have different lengths, the inner lift pins  110 B are raised by the substrate support  106  prior to the outer lift pins  110 A. Nonetheless, both sets of lift pins  110 A,  110 B are raised by the substrate support  106  along with the substrate  108 . 
         [0030]    Once the substrate support  106  is in the processing position and supports the substrate  108  and lift pins  110 A,  110 B, the substrate  108  may be processed. In a PECVD process, a processing gas is introduced through the showerhead  114  and ignited into a plasma that causes material to be deposited onto the substrate  108 . The plasma may cause an electrostatic charge to build up on the substrate  108 . During processing, the substrate support  106  may function as part of the RF return path, or as others refer, ground relative to the hot (or RF biased) showerhead  114 . 
         [0031]    Following processing, the substrate support  106  is lowered. As the substrate support is lowered  106 , the lift pins  110 A,  110 B will eventually come into contact with the bottom of the chamber body  102 . The outer lift pins  110 A, due to their length, will contact the bottom of the chamber body  102  before the inner lift pins  110 B. Thus, the substrate  108  will begin to separate from the substrate support  106  in an edge to center progression until the substrate  108  is entirely supported by the lift pins  110 A,  110 B and spaced from the substrate support  106 . An end effector may then enter into the chamber below the substrate  108 , raise up to lift the substrate  108  off of the lift pins  110 A,  110 B, and retract the substrate  108  from the chamber  100 . 
         [0032]    However, problems may occur in separating the substrate  108  from the substrate support  106 . The substrate  108  may be more tightly adhered to the substrate support  106  due to the electrostatic charge that has built up on the substrate  108  and/or the substrate support  106 . The electrostatic force may cause the substrate  108  to adhere to the substrate support  106  sufficiently such that overcoming the electrostatic force may damage the substrate  108 . 
         [0033]    Therefore, to overcome the electrostatic charge that has built up on the substrate  108  and substrate support  106 , the substrate  108  may be power lifted from the substrate support  106 . To power lift the substrate  108  from the substrate support  106 , a gas may be introduced into the chamber  100 . The gas may be a gas that does not chemically react with the processed substrate  108 . If a gas that chemically reacts with the substrate  108  were used, then undesirable processing of the substrate  108  may occur. Therefore, the gas should be chemically inert relative to the processed substrate  108 . In one embodiment, the gas may be selected from hydrogen, nitrogen, argon, and ammonia. 
         [0034]    The gas that has been introduced is ignited into a plasma. In one embodiment, the RF power used to ignite the plasma is lower than the RF power applied to generate the plasma used to deposited material onto the substrate  108 . The processed substrate  108  is exposed to the plasma for a predetermined time period. In one embodiment, the time period is between about 5 seconds and about 15 seconds. Not wishing to be bound by theory, it is believed that the plasma of non-reactive gas removes, reduces or redistributes the electrostatic charge built up on the substrate  108  and substrate support  106  such that the substrate  108  may be removed from contact with the substrate support  106  without damaging the substrate  108 . The removal, reduction or redistribution of the electrostatic charge reduces the stiction between the substrate  108  and the substrate support  106  and thus allows the substrate  108  to be more easily separated from the substrate support  106 . By using a power lower than used for the depositing of material, the charge applied to the substrate  108  and the substrate support  106  during the power lifting is limited. 
         [0035]    Unfortunately, material does not always deposit uniformly onto a substrate.  FIG. 2A  is a schematic top view of a substrate having a layer  202  deposited thereon that has thin spots  204 ,  206 ,  208 .  FIG. 2B  is a schematic cross sectional view of  FIG. 2A  taken along line A-A. As shown in  FIGS. 2A and 2B , the layer  202  is deposited over the substrate  200 , but the film does not have a uniform thickness across the layer. The thin spots  204 ,  206 ,  208 , are locations where the deposited material is not as thick. Due to the thin spots  204 ,  206 ,  208 , the layer  202  is not uniform across the substrate  200 . The thin spots may be randomly distributed across the layer  202 . 
         [0036]    Thin spots may be caused by the substrate not being perfectly flush with the substrate support during processing.  FIG. 3  is a schematic cross sectional view of a large area substrate  302  disposed on a substrate support  300 . As can be seen from  FIG. 3 , one or more gaps  304  may be present between the substrate support  300  and the substrate  302 . Because of the gaps  304 , portions of the substrate  302  are higher than others such that bumps  306  are present. Even though the substrate  302  may contact the substrate support  300  in a center to edge progression as discussed above, air may still get trapped between the substrate  302  and the substrate support  300 . Not wishing to be bound by theory, it is believed that the gaps  304 , which cause bumps  306  in the substrate  302 , may lead to the thin spots in material deposited over the substrate  302 . 
         [0037]    Not wishing to be bound by theory, it is believed that the thin spots may form on the substrate  302  having the bumps  306  because the deposited material may tend to deposit in the lower areas and build up. The material would continue to deposit until the desired thickness has been reached. Once the desired thickness has been reached, the top surface of the film is expected to be substantially planar. However, if the gaps  304  between the substrate support  300  and the substrate  302  are removed, the bumps  306  are gone. The material deposited on the substrate  302  would no longer be planar due to the absence of the bumps  306 . While no material has disappeared, the layer, because the bumps  306  are gone, is no longer planar. Where the bumps  306  once were, thin spots are present in the deposited layer. 
         [0038]    Another reason that the thin spots may form is due to the plasma density. In the chamber shown in  FIG. 1 , the showerhead  114  is ‘hot’ because it is connected to the RF power source  136 . The substrate support  106  is part of the RF return path and is considered to be ‘RF grounded’. The gaps  304  between the substrate  302  and the substrate support  300  may lead to an uneven power density distribution within the chamber at the gaps  304 . If the substrate  302  is flush against the substrate support  302 , it is believed that the plasma density will be substantially symmetrical. 
         [0039]    To ensure symmetrical plasma density, it would be beneficial to have the substrate flush against the substrate support.  FIGS. 4A-4D  are schematic cross sectional views showing a sequence of placing a substrate into a chamber according to one embodiment such that the substrate is flush against the substrate support. The sequence may be referred to as a pre-plasma loading sequence. 
         [0040]    As shown in the figures, a substrate  404  is supported by an end effector  402  as it is brought into a processing chamber. The end effector  402  is then lowered to place the substrate  404  on the lift pins  410 ,  412  that extend from the bottom  408  of the chamber through the substrate support  406 . Once the substrate  404  is resting on the lift pins  410 ,  412 , the end effector is retracted from the chamber. 
         [0041]    While the substrate  404  is resting on the lift pins  410 ,  412  and before the substrate  404  rests on the substrate support  406 , a gas may be introduced into the chamber. The gas may comprise a gas that does not chemically react with the substrate  404  or cause any deposition onto the substrate  404 . Examples of gases that may be used include hydrogen, nitrogen, ammonia, argon, and combinations thereof. The gas is then ignited into a plasma. 
         [0042]    Similar to the situation that occurs during plasma deposition discussed above, an electrostatic charge develops on the substrate  404  and/or the substrate support  406 . The power applied to ignite the plasma may be discontinued and the chamber may then be pumped down to the base pressure for processing. The substrate support  406  may then be raised and the substrate  404  may contact the substrate support  406  in a center to edge manner at a slow speed. The substrate support  406  is raised without any gas or plasma until the substrate  404  is supported by the substrate support  406 . It is only after the plasma is extinguished that the substrate support  406  is raised. 
         [0043]    The electrostatic charge that has built up on the substrate  404  and/or the substrate support  406  may pull the substrate  404  into greater contact with the substrate support  406  such that the amount of gaps that may be present between the substrate  404  and the substrate support  406  may be reduced below what would be present in absence of the pre-plasma loading process. 
         [0044]    Once the substrate  404  is supported by the substrate support  406 , processing gases may be introduced into the chamber and ignited into a plasma by RF power. The substrate  404  may thus be processed. The substrate  404  may then be power lifted off of the substrate support  406  as discussed above. 
         [0045]    In addition to building up electrostatic charge on the substrate  404  and/or substrate support  406 , it is believed that the ignited plasma heats the substrate  404  and enables the substrate  404  to be more flexible. The greater the flexibility of the substrate  404 , the less likely gaps may form between the substrate  404  and the substrate support  406  during the center to edge progression. 
         [0046]    The pre-plasma loading discussed above is distinct from what has been termed ‘plasma loading’. Plasma loading is a process for thermophoresis that is used to heat the substrate to a temperature greater than its surroundings. Because the substrate is heated to a temperature greater than its surroundings, any negatively charged particles or other contaminants tend to gravitate towards the coolest surface. When a substrate is introduced into a processing chamber, the substrate may be the coolest surface and thus, risk contamination. By heating the substrate to a temperature greater than the surroundings, the negatively charged particles may gravitate to a surface other than the substrate. Plasma loading, which is different from the pre-plasma loading discussed above, involves rapidly raising the temperature of the substrate. 
         [0047]    A plasma loading sequence involves inserting a substrate into a processing chamber and placing the substrate onto the substrate support. No plasma is ignited prior to placing the substrate onto the substrate support. Then, the pressure of the chamber is increased above the normal processing pressure. An inert gas such as a noble gas or a gas that does not chemically react with the substrate is introduced into the chamber and ignited into a plasma. The plasma heats the substrate up to a temperature that is greater than the other electrode (a showerhead in a PECVD system). Then, the plasma is extinguished, the gas evacuated, and the pressure reduced to normal. The substrate may then be processed. Alternatively, plasma loading may comprise igniting a plasma while the substrate support is moving upwards to make contact with the substrate, which is still different than pre-plasma loading where the plasma is ignited and extinguished before the substrate support ever moves. 
         [0048]    Because the substrate is brought into contact with the substrate support prior to igniting the plasma in a plasma loading, plasma loading and pre-plasma loading are different. Additionally, pre-plasma loading may occur at the normal operating pressures rather than an increased pressure. By inducing an electrostatic charge on the substrate and/or substrate support prior to the substrate resting on the substrate support, the gaps or bumps may be reduced and/or avoided. On the other hand, plasma loading does not induce an electrostatic charge until after the substrate is resting on the substrate support. 
         [0049]      FIGS. 5A-5C  are graphs showing a comparison of film thickness variations according to several embodiments. In each of  FIGS. 5A-5C , it can be seen that when pre-plasma loading occurs, the deposited film has greater uniformity. When no-pre-plasma processing occurs, thin spots are present. The lines labeled “reference BL-TR” and “reference TL-BR” are results for depositions onto substrates that were not pre-plasma loaded. The lines labeled “Pre-plasma” are results for depositions onto substrates that were pre-plasma loaded. 
         [0050]    In the embodiments discussed above, large area substrates may be subject to a pre-plasma process whereby an electrostatic charge may be induced onto a substrate and/or a substrate support prior to coming into contact with each other. By inducing an electrostatic charge, the substrate and substrate support may be brought into intimate contact with each other such that few gaps, if any, are present between the substrate and substrate support. Because few gaps, if any, are present, the plasma density during plasma processing may be substantially symmetrical such that a uniformly thick film is deposited over the substrate. 
         [0051]    While the embodiments discussed above have referred to large area substrates, it is believed that the pre-plasma loading may be beneficial to smaller substrates as well. The benefits of using the pre-plasma loading for smaller substrates include the symmetrical plasma density as discussed above, and also potentially the removal of a clamp ring to press the substrate into intimate contact with the substrate support. 
         [0052]    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.