Abstract:
A sputtering system having a processing chamber with an inlet port and an outlet port, and a sputtering target positioned on a wall of the processing chamber. A movable magnet arrangement is positioned behind the sputtering target and reciprocally slides behinds the target. A conveyor continuously transports substrates at a constant speed past the sputtering target, such that at any given time, several substrates face the target between the leading edge and the trailing edge. The movable magnet arrangement slides at a speed that is at least several times faster than the constant speed of the conveyor. A rotating zone is defined behind the leading edge and trailing edge of the target, wherein the magnet arrangement decelerates when it enters the rotating zone and accelerates as it reverses direction of sliding within the rotating zone.

Description:
RELATED APPLICATION 
       [0001]    This Application claims priority benefit from U.S. Provisional Application Ser. No. 61/556,154, filed on Nov. 4, 2011, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    This application related sputtering systems, such as sputtering system used to deposit thin films on substrates during the fabrication of integrated circuits, solar cells, flat panel displays, etc. 
         [0004]    2. Related Arts 
         [0005]    Sputtering systems are well known in the art. An example of a sputtering system having a linear scan magnetron is disclosed in U.S. Pat. No. 5,873,989. One of the problems to be resolved in such a system is the uniformity of the firm that is formed on the substrate. Another problem to be resolved in such a system is target utilization. Specifically, since the magnets of linear magnetrons scans back and forth, excessive sputtering occurs at both edges of the target, generating two deep grooves along, i.e., parallel to, the scan direction. Consequently, the target has to be replaced, even though the majority of the surface of the target is still usable. Various methods for combating this phenomenon are disclosed in the above cited &#39;989 patent. 
         [0006]    However, another target utilization issue that has not been previously addressed is the erosion caused at the edges of the scan cycle. That is, when the magnets reach an end of the target, the scan direction is reversed. In order to achieve film uniformity, the &#39;989 patent suggests to slow the scan speed towards either end of the target. However, this leads to increased sputtering of the target, leading to excessive erosion at both ends of the target in a direction perpendicular to the scan direction. 
         [0007]    Accordingly, there is a need in the art for a sputtering system that enable uniform film deposition and increased target utilization. 
       SUMMARY 
       [0008]    The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
         [0009]    Disclosed herein is a sputtering system and method that enhance uniformity of the film formed on the substrate, and also enables high throughput. One embodiment provides a system wherein substrates continually move in front of the sputtering target. The magnetron is linearly scanned back and forth at speed that is at least several times higher than the speed on the substrates&#39; motion. The magnetron is scanned in the direction of substrate travel and then in the reverse direction, repeatedly. During most of its travel, the magnetron is moved at a constant speed. However, as it approaches the end of its travel, is decelerates. Then, when is starts its travel in the opposite direction, it accelerates until it reaches the constant speed. The deceleration/acceleration in one embodiment is 0.5 g and in another it is 1 g. This enhances utilization of the target. According to another embodiment, the turning point of the magnetron is changed at successive scans, so as to define a zone of turnaround. This also helps in enhancing target utilization. 
         [0010]    A sputtering system having a processing chamber with an inlet port and an outlet port, and a sputtering target positioned on a wall of the processing chamber. A movable magnet arrangement is positioned behind the sputtering target and reciprocally slides behinds the target. A conveyor continuously transports substrates at a constant speed past the sputtering target, such that at any given time, several substrates face the target between the leading edge and the trailing edge. The movable magnet arrangement slides at a speed that is at least several times faster than the constant speed of the conveyor. A rotating zone is defined behind the leading edge and trailing edge of the target, wherein the magnet arrangement decelerates when it enters the rotating zone and accelerates as it reverses direction of sliding within the rotating zone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
           [0012]      FIG. 1  illustrates part of a system for processing substrate using sputtering magnetron according to one embodiment. 
           [0013]      FIG. 2  illustrates a cross section along lines A-A in  FIG. 1 . 
           [0014]      FIG. 3  illustrates a cross section along lines B-B in  FIG. 1 . 
           [0015]      FIG. 4  illustrates another embodiment, wherein substrates are supported on a conveyor that moves continuously at constant speed. 
           [0016]      FIG. 5  illustrates an example of a system architecture using a sputtering chamber such as that shown in  FIG. 4 . 
           [0017]      FIG. 6  illustrates an embodiment of a movable magnetron, which may be used in any of the disclosed embodiments. 
           [0018]      FIGS. 7A-7D  are plots of deposition uniformity using constant wafer transport speed and different magnets scan speed. 
           [0019]      FIG. 8A  is a plot illustrating that the uniformity drops as the magnet scan speed increases. 
           [0020]      FIG. 8B  is another pot illustrating a strange behavior of film deposition uniformity versus magnet scan speed at higher speed than the scan speed. 
           [0021]      FIG. 8C  is an enlargement of the portion circled in  FIG. 8B . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Embodiments of the inventive sputtering system will now be described with reference to the drawings. Different embodiments may be used for processing different substrates of to achieve different benefits, such as throughput, firm uniformity, target utilization, etc. Depending on the outcome sought to be achieved, different features disclosed herein may be utilized partially or to their fullest, alone or in combination, balancing advantages with requirements and constraints. Therefore, certain benefits will be highlighted with reference to different embodiments, but are not limited to the disclosed embodiments. 
         [0023]      FIG. 1  illustrates part of a system for processing substrates using sputtering magnetron, according to one embodiment. In  FIG. 1 , three chambers,  100 ,  105  and  110 , are shown, but the three dots on each side indicates that any number of chambers may be used. Also, while here three specific chambers are shown, it is not necessary that the chamber arrangement shown here would be employed. Rather, other chamber arrangements may be used and other type of chambers may be interposed between the chambers as shown. For example, the first chamber,  100 , may be a loadlock, the second,  105 , a sputtering chamber, and the third,  110  another loadlock. 
         [0024]    For illustration purposes, in the example of  FIG. 1 , the three chambers  100 ,  105  and  110  are sputtering chambers; each evacuated by its own vacuum pump  102 ,  104 ,  106 . Each of the processing chambers has a transfer section,  122 ,  124  and  126 , and a processing section  132 ,  134  and  136 . Substrate  150  is mounted onto a substrate carrier  120 . In this embodiment, the substrate  150  is held by its periphery, i.e., without touching any of its surfaces, as both surfaces are fabricated by sputtering target material on both sides of the substrate. The carrier  120  has a set of wheels  121  that ride on tracks (not shown in  FIG. 1 ). In one embodiment, the wheels are magnetized so as to provide better traction and stability. The carrier  120  rides on rails provided in the transfer sections so as to position the substrate in the processing section. In one embodiment, motive force is provided externally to the carrier  120  using linear motor arrangement (not shown in  FIG. 1 ). When the three chambers  100 ,  105 , and  110 , are sputtering chambers, it is assumed that the carrier  120  enters and exits the system via a loadlock arrangement. 
         [0025]      FIG. 2  illustrates a cross section along lines A-A in  FIG. 1 . For simplicity, in  FIG. 2  substrate  250  is illustrated without its carrier, but it should be appreciated that the substrate  250  remains on the substrate carrier  120  throughout the processing performed in the system of  FIG. 1 , and is continuously transported from chamber to chamber by the substrate carrier, as illustrated by the arrow in  FIG. 2 . In this illustrative embodiment, in each chamber,  200 ,  205  and  210 , the substrate  250  is processed on both sides. Also shown in  FIG. 2  are isolation valves  202 ,  206 , that isolate each chamber during fabrication; however, since in one embodiment the substrates continuously move, the isolation valves can be replaced with simple gates or eliminated. 
         [0026]    Each chamber includes a movable magnetron  242 ,  244 ,  246 , mounted onto a linear track  242 ′,  244 ′,  246 ′, such that it scans the plasma over the surface of the target  262 , as shown by the double-headed arrows. The magnets are scan back and forth continuously as the substrates are transported in the chambers on the carriers. As illustrated with respect to magnets  242 , as the magnets reach the leading edge  243  of the target  262 , it reverses direction and travels towards the trailing edge  247  of target  262 . When it reaches the trailing edge  247 , it again reverses direction and is scanned towards the leading edge  243 . This scanning process is repeated continuously. 
         [0027]      FIG. 3  illustrates a cross section along lines B-B in  FIG. 1 . Substrate  350  is shown mounted onto carrier  320 . Carrier  320  has wheels  321 , which ride on tracks  324 . The wheels  321  may be magnetic, in which case the tracks  324  may be made of paramagnetic material. In this embodiment the carrier is moved by linear motor  326 , although other motive forces and/or arrangements may be used. The chamber is evacuated and precursor gas, e.g., argon, is supplied into the chamber to maintain plasma. Plasma is ignited and maintained by applying RF bias energy to the movable magnetron  344 , situated behind target  364 . 
         [0028]      FIG. 4  illustrates another embodiment, wherein substrates  450  are supported on a conveyor  440  that moves continuously for “pass-by” processing. This arrangement is particularly beneficial when only one side of the substrates needs to be sputtered, such as when fabricating solar cells. For example, several substrates can be positioned abreast such that several are processed simultaneously. The callout in  FIG. 4  illustrates three substrates abreast, i.e., arrange perpendicularly to the direction of motions, as indicated by the arrow. In such an embodiment, when the target  464  is longer relative to the size of the substrates, then several substrates can be processed simultaneously in columns and rows as the belt continuously move the substrate under the target  464 . For example, when using three rows, i.e., three wafers abreast, the size of the target can be designed so as to enable processing of four substrates in three rows, thus simultaneously processing twelve substrates. As before, the magnetron  444  moves back and forth linearly between the leading and trailing edges of the target, as shown by the double-headed arrow. 
         [0029]      FIG. 5  illustrates an example of a system such as that shown in  FIG. 4 . An atmospheric conveyor  500  continuously brings substrates into the system, and the substrates are then transported on conveyors inside the systems so as to traverse a low vacuum loadlock  505 , a high vacuum loadlock  510 , and, optionally, a transfer chamber  515 . Then the substrates, while continuously moving on conveyor, are processed by one or more successive chambers  520 , here two are shown. The substrates then continue on conveyors to an optional transfer chamber  525 , then to high vacuum loadlock  530 , low vacuum loadlock  535 , and then to atmospheric conveyor  540 , to exit the system. 
         [0030]      FIG. 6  illustrates an embodiment of the movable magnetron, which may be used in any of the above embodiments. In  FIG. 6 , the substrates  650  are moved on the conveyor  640  at constant speed. The target assembly  664  is positioned above the substrates, and movable magnetron  644  oscillates back and forth linearly behind the target assembly, as sown by the double-headed arrow. The plasma  622  follows the magnetron, causing sputtering from different areas of the target. In this embodiment, during normal travel the speed of the magnetron is constant and is at least several times the speed of the substrates. The speed is calculated such that during the time a substrate traverses the sputtering chamber, it is sputtered several times by the moving magnetron. For example, the speed of the magnetron can be five to ten times faster than the speed of the substrate, such that by the time the conveyor moves the substrate past the entire length of the target, the magnets have been scanned back and forth several times behind the target so as to deposit multiple layers on the substrate. 
         [0031]    As shown in  FIG. 6 , in this embodiment each substrate is of length Ls, which is defined in the direction of travel of the conveyor belt. Similarly, the target has a length Lt, which is defined in the direction of travel of the conveyor, which is parallel with the direction of travel of the magnets. In this embodiment, the target&#39;s length, Lt, is several times longer than the substrate length Ls. For example, the target length can be four times longer than the pitch length, which is defined as one substrate length plus the length of separation S between two substrates on the conveyor. That is, the pitch P=(Ls+S). 
         [0032]    The problem with linear motion of magnetron behind a target is that when it reaches the leading or trailing end of the target, it stops and starts motion in the reverse direction. Consequently, the edges of the target get eroded much more than the main surface of the target. When the erosion at the edges of the target exceed specification, the target needs to be replaced, even though the center of the target is still usable. This problem is addressed using various embodiments, as described below. 
         [0033]    According to one embodiment, offsets E and F are designated at the leading and trailing edges of the target, respectively. When the magnetron reaches the offset, it decelerates at a prescribed rate, e.g., 0.5 g, 1 g, etc. At the end of the offset the magnetron changes direction and accelerates at the prescribed rate. This is done at both ends of travel of the magnetron, i.e., at the leading and trailing edges of the target. 
         [0034]    According to another embodiment, a rotation zone is prescribed, e.g., zones E and F are designated at the leading and trailing edges of the target, respectively. When the magnetron reaches either of the rotation zones, it changes travel direction at a point within the rotating zone. However, over time the magnetron changes direction at a different points within the rotating zone. This is exemplified by the callout in  FIG. 6 . As illustrated, at time t 1  the point of reversing direction is designated as F 1 . At time t 2 , the point of reversing direction is designated F 2 , and is further towards the trailing edge of the target as point F 1 , but is still within the zone designated F. At time t 3 , the point of reversing direction F 3  is even further towards the trailing edge of the target, while at time t n , point F n  is back away from the trailing edge of the target. However, all points F i  are within the zone F. A similar process takes place over zone E on the other side, i.e., the leading edge of the target. 
         [0035]    The selection of the points of reversing scan direction can be done using various ways. For example, a random selection can be done at each scan, at each two scans, or after x number of scans. Conversely, a program can be implemented wherein at each scan the point is moved a distance Y in one direction until the end of the zone is reached, and then the points start to move a distance Y towards the opposite end. On the other hand, the movement can be designed to generate an interlaced pattern by moving in one direction a Z amount and then in the next step moving in the reverse direction a −w amount, wherein |w|&lt;|Z|. 
         [0036]    In the embodiments described herein, over the processing regime the magnetron is scanned at constant speed, as it has been found that varying the scan speed adversely affects film uniformity on the substrates. Notably, in configurations where the substrates continuously moves in front of the target, slowing down or speeding up the magnet array over the processing area is unadvisable, even for controlling the film thickness uniformity. 
         [0037]    In the disclosed embodiments, moving many substrates on a conveyor can be thought of as a continuous (infinitely long) substrate that is moving at a constant speed. The scan speed must be selected so as to give good uniformity on a substrate moving at a constant speed. In these embodiments, special use is made of the start position, the stop position, acceleration, and deceleration to control target utilization. This has the effect of spreading out the deep grooves that occur at the ends when reversing the motion. 
         [0038]    A pole design is used to reduce the deep grooves at the top and bottom of the plasma track. A thicker target can be used or higher power can be utilized into the targets because the scan is done at a fairly high speed, spreading the power out over the full surface of the substrate. Because each substrate sees multiple target passes of the plasma, the start and stop position can be varied with each pass and the effect of changing the scan length from one pass to the next will not be seen in the film uniformity. That is, while the embodiment of  FIG. 6  was described such that the rotating zone is designed to be outside of the processing area, this is not necessary when having the substrates continuously move, as described herein. Rather, the rotating zone can be within the processing area. 
         [0039]    For example, according to one embodiment the system is used to fabricate solar cells at a rate of 2400 substrates per hour. The conveyor continuously moves the substrates at a rate of about 35 mm/sec. The magnetron is scanned at a speed of at least 250 mm/sec, i.e., more than seven times the speed of the substrate transport. The target and magnetron are designed such that the stroke of the magnetron scan is about 260 mm. This provides film uniformity of over 97%. The acceleration/deceleration can be set at 0.5 g with a distance of about 6.4 mm or 1 g, for about half that distance. 
         [0040]      FIGS. 7A-7D  are plots of deposition uniformity using constant wafer transport speed and different magnets scan speed.  FIG. 7A  is a plot of uniformity for magnets scan speed that is 5% of the wafer transport speed. For example, for a wafer transport speed of 35 mm/s, the magnets were scan at 1.75 mm/s. The resulting film uniformity was 90%, which is not adequate for production of devices such as solar cells. When the magnet scan speed was increased to 7.5% of the wafer speed, the uniformity dropped to 86%, as shown in  FIG. 7B . Moreover, as the speed was increased to 10% the uniformity dropped to 82%, and when the sped was increased to 12.5% the uniformity dropped even further to 78%. Thus, it appeared that increasing the magnet scan speed causes a corresponding reduction of film uniformity, suggesting that the magnet scan speed should be a small fraction of the wafer transport speed. This conclusion was further supported by the plot shown in  FIG. 8A , wherein uniformity drops as the magnet scan speed increases. 
         [0041]    However, the plot of  FIG. 8A  also shows that the maximum achievable uniformity may be about 90% or so. As noted above, such uniformity is not acceptable for many processes. Therefore, further investigation was undertaken, resulting in the plot of  FIG. 8B . The plot of  FIG. 8B  illustrates a strange behavior of film deposition uniformity versus magnet scan speed. Indeed, as magnet scan speed increases, film uniformity drops. However, at a certain point, as the magnet scan speed increases further, uniformity suddenly starts to improve, such that at about magnet scan speed that is three times the wafer transport speed, a uniformity peak of about 98% is achieved. Thereafter a short drop in uniformity is observed, but then uniformity is recovered and remains high when the magnet scan speed that is about 5 times the wafer transport speed and beyond, which is illustrated in the plot of  FIG. 8C . As shown in  FIG. 8C , which is an enlargement of the portion circled in  FIG. 8B , at speeds beyond 5 times the wafer transport speed, the uniformity remains above 97% and, at speeds of about 10 times the transport speed the uniformity remains at over 98%. Higher speeds are not recommended from the mechanical load and machine design perspective, and the uniformity does not seem to improve that much for higher speeds. Thus, the cost in design complexity and potential higher maintenance may not warrant going to scan speeds beyond 10 times the wafer transport speed. 
         [0042]    It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations will be suitable for practicing the present invention. 
         [0043]    Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.