Abstract:
An electrostatic chuck is disclosed, which is especially suitable for fabrication of substrates at high throughput. The disclosed chuck may be used for fabricating large substrates or several smaller substrates simultaneously. For example, disclosed embodiments can be used for fabrication of multiple solar cells simultaneously, providing high throughput. An electrostatic chuck body is constructed using aluminum body having sufficient thermal mass to control temperature rise of the chuck, and anodizing the top surface of the body. A ceramic frame is provided around the chuck&#39;s body to protect it from plasma corrosion. If needed, conductive contacts are provided to apply voltage bias to the wafer. The contacts are exposed through the anodization.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority benefit from U.S. Provisional Application Ser. No. 61/554,457, filed on Nov. 1, 2011, the content of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    This disclosure relates to processing of solar cells and, in particular, to electrostatic chucks supporting wafers inside solar cells processing chambers. 
         [0004]    2. Related Art 
         [0005]    Processing chambers, such as plasma chambers, used to fabricate solar cells have the same basic elements of processing chambers used for fabricating integrated circuits (IC), but have different engineering and economic requirements. For example, while chambers used to fabricate integrated circuits have throughput on the order of a few tens of wafers per hour, chambers used for fabricating solar are required to have throughput on the order of a few thousands of wafers per hour. On the other hand, the cost of purchasing and operating a solar cell processing system must be very low. 
         [0006]    Processing systems used for both IC and solar cell fabrication utilize electrostatic chucks to support the wafers during processing. However, the electrostatic chuck for solar cell system must cost a fraction of that for an IC manufacturing, yet it must endure much higher utilization rate due to a much higher throughput of the solar cell fabrication system. Moreover, while in IC systems the electrostatic chuck is stationary, in some solar cell fabrication systems the chuck is movable. Consequently, no connections for cooling fluid can be made, such that active thermal control of the chuck is not possible. 
         [0007]    Various steps involved in the fabrication of solar cells require exposure of the wafer to plasma. During certain processing steps, the plasma is formed using corrosive gases, which attack any exposed part of the chuck supporting the wafers. Therefore, another requirement on the chuck is to be able to withstand such corrosive attacks of the plasma. 
         [0008]    Accordingly, what is needed in the art is an electrostatic chuck that is inexpensive to manufacture, can endure high utilization rates without active cooling, and can withstand corrosive effects of plasma. 
       SUMMARY 
       [0009]    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. 
         [0010]    An electrostatic chuck is disclosed, which is especially suitable for fabrication of substrates at high throughput. The disclosed chuck may be used for fabricating one substrate at a time or simultaneously fabricating several substrates positioned on several chucks. For example, disclosed embodiments can be used for fabrication of multiple solar cells simultaneously, providing high throughput. 
         [0011]    Various embodiments provide an electrostatic chuck which is designed to endure high throughput processing, such as that used in solar fabrication systems, and can withstand corrosive plasmas. Disclosed embodiments take advantage of static mass and processing cycles to thermally control the chuck, and dispense with active fluid cooling. 
         [0012]    According to disclosed embodiments, an electrostatic chuck body is constructed using aluminum having sufficient thermal mass to control temperature rise of the chuck. The top surface of the aluminum body is anodized to provide endurance to high utilization rates. A ceramic frame is provided around the chuck&#39;s body to protect it from plasma corrosion. If needed, conductive contacts are provided to apply voltage bias to the wafer. The contacts are exposed through the anodization. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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. 
           [0014]      FIG. 1A  is a schematic illustrating the major parts of an electrostatic chuck according to one embodiment, while  FIG. 1B  illustrates a partial cross-section along line A-A of  FIG. 1A . 
           [0015]      FIG. 1C  is a flow chart illustrating a process flow for fabricating the chuck illustrated in  FIGS. 1A and 1B . 
           [0016]      FIG. 2  illustrates an example of a plasma chamber for processing substrates, utilizing a chuck according to an embodiment of the invention. 
           [0017]      FIG. 3A  is a schematic illustrating the major parts of an electrostatic chuck according to another embodiment, while  FIG. 3B  illustrates a partial cross-section along line A-A of  FIG. 3A . 
           [0018]      FIG. 4A  is a schematic illustrating the major parts of an electrostatic chuck according to yet another embodiment, while  FIG. 4B  illustrates a partial cross-section along line A-A of  FIG. 4A . 
           [0019]      FIGS. 5A  is a schematic illustrating the major parts of an electrostatic chuck according to yet another embodiment, while  FIG. 5B  illustrates a partial cross-section along line A-A of  FIG. 5A . 
           [0020]      FIG. 6  is a schematic illustrating the major parts of an electrostatic chuck and carrier according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Various features of the electrostatic chuck according to embodiments of the invention will now be described with reference to the drawings. The description will include examples of electrostatic chuck, processing systems incorporating the electrostatic chuck, and methods for making the electrostatic chuck for fabrication of, e.g., solar cells. 
         [0022]      FIG. 1A  is a schematic illustrating the major parts of an electrostatic chuck according to one embodiment, while  FIG. 1B  illustrates a partial cross-section along line A-A of  FIG. 1A . The chucks body  105  is made of aluminum slab and is configured to have sufficient thermal mass to control heating of the chuck during plasma processing. The top surface of the body  105  is anodized, thereby forming electrically insulating anodized aluminum layer  110 . The sides of the chuck are encased by ceramic layer or frame  115 . Ceramic layer  115  may be a ceramic coating applied to all four sides of the aluminum body, e.g, using standard plasma spray coating or other conventional methods. In the embodiment shown in  FIGS. 1A and 1B , the aluminum body  105  is placed inside a ceramic “tub” such that all four sides and the bottom of the aluminum body  105  are covered by a ceramic frame  115 . The body  105  is bonded to the ceramic frame  115 . The top of the ceramic frame  115  is level with the top of the anodized aluminum layer  110 . Also, the chuck is sized so that the chucked wafer extends beyond the ceramic sides  115 , so as to cover the top of the ceramic sides  115 . This is illustrated by the broken-line outline of wafer  150  in  FIG. 1A . 
         [0023]    The chuck is attached to a base  120 , which may be made of an insulative or conductive material. An aperture is formed through the base  120  and an insulating sleeve  142  is positioned therein. A conductor contact rod  144  is passed through the insulating sleeve  142  so as to form electrical contact to the aluminum body  105 . Conductor rod  144  is used to conduct high voltage potential to form the chucking force to chuck the wafers. 
         [0024]    In some processing chambers it is necessary to bias the processed wafers so as to attract ions from the plasma towards the wafers. For such processing, the chuck is provided with contact points  130  to deliver voltage bias to the wafers. Each contact point  130  is formed by an insulating sleeve  132 , which passes through the base  120  and though the body  105 . A contact rod  134 , which may be spring biased or retractable (not shown), passes through the insulating sleeve  132 . 
         [0025]    The protective ceramic frame  115  may be made of materials such as, e.g., alumina (aluminum oxide), SiC (silicon carbide), silicon nitride (Si 3 N 4 ), etc. The selection of ceramic material depends on the gasses within the plasma and on potential contamination of the processed wafers. 
         [0026]    The arrangement illustrated in  FIGS. 1A and 1B  provides certain advantages over prior art chucks. For example, due to its simple design, it is inexpensive to manufacture. Also, the anodized surface can endure repeated processing, while the ceramic frame protects the anodization and the chuck&#39;s body from plasma corrosion. Since the ceramic frame is designed to be slightly smaller than the chucked wafer, the ceramic frame is sealed by the chucked wafer, thereby preventing plasma attack on the edges of the chuck/ceramic frame. 
         [0027]      FIG. 1C  is a flow chart illustrating a process flow for fabricating the chuck illustrated in  FIGS. 1A and 1B . In step  161  an aluminum block is machined to form the chuck&#39;s body  105 . In step  162  the top surface of the aluminum body is anodized using standard anodization process. In step  163  ceramic frame  115  is fabricated and in step  164  the aluminum body  105  is bonded to the ceramic frame  115 . In step  165  the assembly of the body and frame is bonded to a base  120 . In step  166  the various electrical contacts and insulation sleeves are attached to the chuck. 
         [0028]      FIG. 2  illustrates a schematic cross-section of one example of plasma system utilizing the chuck illustrated in  FIGS. 1A and 1B . Since  FIG. 2  is provided in order to provide an example of the use of the transportable electrostatic chuck, various elements not relating to that function are omitted. The processing chamber  230  shown in  FIG. 2  may be any plasma processing chamber, such as etch, PECVD, PVD, etc. 
         [0029]    The following is an example of a processes sequence using the embodiment of  FIG. 2 . The wafers  258  are delivered to the system on an incoming conveyor  202 . In this example, several wafers  258  are placed abreast in the direction orthogonal to the conveyor&#39;s travel direction. For example, three wafers  258  can be arranged in parallel, as shown in the callout, which is a top view of the substrates on the conveyor, with the arrow showing the direction of travel. 
         [0030]    A wafer transport mechanism  204  is used to transport the wafers  258  from the conveyor  202  onto the processing chucks  215 . In this example, the transport mechanism  204  employs an electrostatic pickup chuck  205 , which is movable along tracks  210  and uses electrostatic force to pick up one or more wafers, e.g., one row of three wafers, and transfer the wafers to the processing chucks  215 . In this example, three processing chucks  215  are used to receive the three substrates held by the pickup chuck  205 . As shown in  FIG. 2 , the loading of wafers onto the processing chucks  215  is done at the loading station C. The processing chucks  215  are attached to carriers  217 , which are transported into the first processing chamber  230  via shutter  208 . 
         [0031]    The process chamber is isolated from the loading station and other chambers by shutter  208 . Shutter  208  greatly reduce conductance to adjacent chambers, allowing for individual pressure and gas control within the process chambers without vacuum valves and o-ring seals. In this example only a single processing chamber  230  is used. However, as can be understood, additional chambers can be added serially, such that the substrates will be moving from one chamber directly to the next, via isolation shutters  208  placed between each two chambers (not shown). 
         [0032]    Once chuck  215  is positioned inside the processing chamber  230 , electrical contact is made to the contact rods  134  and  144 , by contacts  252  and  254 , to deliver the required voltage potential. Plasma processing then commences and the substrates are processed. Once processing is completed at the last chamber in the series of chambers, the last shutter  208  is opened and the chuck  215  is transported on carrier  217  to the unloading station H. 
         [0033]    At the unloading station H, a wafer transport mechanism  203  is used to unload wafers from the chuck  215  and transport the wafers onto unload conveyor  201 . Transport mechanism  203  employs an electrostatic wafer pickup head  225 , which rides on tracks  220 , similar to the pickup chuck  205 . The pickup head  225  uses electrostatic forces to transfers wafer from process chucks  215  to outgoing conveyor  201 . Outgoing wafer conveyor  201  receives the wafers from the pickup head  225  and conveys them to further processing downstream. 
         [0034]    The chucks  215  are then lowered by elevator  250  and are transported by chuck return module  240  to elevator  255 , which returns the chucks to station C for receiving another batch of wafers. As can be understood, several processing chucks are used, such that each station is loaded and the processing chamber is always occupied and processing wafers. That is, as one group of chucks leaves the processing chamber into station H, another group from station C is moved into the chamber and a group from elevator  255  is moved into station C. Also, in this embodiment, as the elevators  250  and  255  move chucks between process level and return level, they actively cool the process chuck  215  using, e.g., heat sinks Alternatively, or in addition, cooling station J is used to cool the chucks by contacting the chuck with a heat sink. The process chucks  215  are returned from unload station H to load station C via a return tunnel  240 , which is positioned under the process level. 
         [0035]    Electrical contacts  252  to the chuck are located on each elevator and in each process chamber for electrostatic chucking of wafers. That is, as explained above, since the chucks are movable, no permanent connections can be made to the chucks. Therefore, in this embodiment, stations C and H and each processing chamber  230  include electrical contacts  252  to transfer electrical potential to the chuck, via contact  144 , and enable electrostatic chucking Additionally, DC bias contacts  254  are located in each process chamber  230  for DC bias of wafer if required. That is, for some processing, DC bias is used in addition to plasma RF power, in order to control the ion bombardment from the plasma on the wafer. The DC potential is coupled to the wafers by contacts  134 , which receive the DC bias from contacts  254 . 
         [0036]    Thus, as seen from the above, the system illustrated in  FIG. 2  may utilize several process chucks  215 , which continuously move from load position C, through a series of process chambers  230 , to an unload position H. The process chambers  230  are individually pumped and separated from each other and from the load and unload zones by shutters  208 . The shutters provide vacuum and plasma zone separation for each chamber. This allows for individualized gas species and pressure control in each zone. For simplicity, only one processing chamber  230  is illustrated in  FIG. 2 , but a series of chambers may be connected serially, such that a chuck exiting one chamber directly enters a second chamber. 
         [0037]    The chucks return from the unload station H to the load station C via a vacuum tunnel  240 , located under the process chambers  230 . The chucks recirculate through the system, so they cannot have any fixed connections such as wires, gas lines or cooling lines. Contact for bias and chucking is made at each location the chuck stops in. Chuck cooling is achieved by active cooling on the unload and load elevators  250  and  255 , respectively, and/or cooling station J. In this example, when the chuck is cooled it is mechanically clamped against a cooled heat sink. 
         [0038]    In the example of  FIG. 2 , several chucks  215  are present in each process chamber during processing, so that multiple substrates are being plasma processed simultaneously. In this embodiment, the wafers are processed simultaneously by being supported on several individual chucks, e.g., three chucks, situated abreast. In one specific example, each chamber is fabricated to hold one row of three individual chucks, so as to simultaneously process three wafers. Of course, other arrangement may be used, e.g., a two by three array of chucks, etc. 
         [0039]      FIG. 3A  is a schematic illustrating the major parts of an electrostatic chuck according to another embodiment, while  FIG. 3B  illustrates a partial cross-section along line A-A of  FIG. 3A . Elements in  FIGS. 3A and 3B  that are similar to those of  FIGS. 1A and 1B  are indicated with the same reference numerals, except that they are in a different centennial series. As seen in  FIG. 3A , No contact are made for directly applying bias to the wafer  350 . Instead, capacitive coupling from the plasma to the chuck is relied upon to provide RF path to the chuck and bias to the wafer. 
         [0040]    The structure of the electrostatic chuck will now be described with reference to  FIG. 3B . The chuck of this embodiment is fabricated by machining an aluminum body  305 . All the surfaces of the body  305  are then anodized, to provide a hard insulative surface, shown as top anodization layer  310 , bottom anodization layer  311 , and side anodization layer  312 . The anodized aluminum body is bonded onto a ceramic tub  315  made out of, e.g., alumina, and serving as an insulator and protecting the sides of the anodized aluminum body from plasma corrosion. The ceramic tub is bonded onto an insulating plate  322 , made of, e.g., polyimide, Kapton®, etc. The thickness of the insulating plate  322  is determined depending on the dielectric constant of the plate&#39;s material, so as to provide the required capacitive coupling of RF power to the base plate  320 . Base plate  320  is made of aluminum and is also anodized, and is used to capacitively couple RF from the plasma. The amount of coupling depends, in part, on the properties, such as thickness and dielectric constant, of the insulating plate  322 . Also, alternatively, rather than using insulative plate, the bottom plate of tub  315  can be made thicker to provide the same insulating properties. Also, threaded holes  370  are provided to attach the chuck to a carrier, which is described below. 
         [0041]    As noted above, the aluminum body  305  is anodized on all sides. Therefore, to make the electrical contact with contact rod  344 , the anodization is removed from area of the contact on the bottom of the aluminum body. Additionally, the area where the anodization was removed is plated with a conductive layer such as, e.g., nickel, chromium, etc. When the contact rod  344  is inserted into the insulating sleeve  342 , it contacts the plated conductive layer and good electrical contact is then maintained. 
         [0042]    As can be understood from the above, to make the chucks simple, inexpensive, and transportable, no connections for bias power to the wafer and no cooling are provided. Also, unlike semiconductor chucks, wherein the chucked wafer is round, here the wafer is square to comply with solar cell processing. Consequently, the plasma over the wafer can be very non-uniform, leading to a non-uniform processing of the wafer. The embodiment illustrated in  FIGS. 4A and 4B  is designed to overcome such plasma non-uniformity. 
         [0043]    The structure of the chuck illustrated in  FIGS. 4A and 4B  is similar to that of  FIGS. 3A and 3B , and elements in  FIGS. 4A and 4B  that are similar to those of  FIGS. 3A  and  3 B are indicated with the same reference numerals, except that they are in a different centennial series. However, in order to overcome plasma non-uniformity, in the embodiment of  FIGS. 4A and 4B  the insulating plate  422  has a non-flat bottom surface, and the top surface of the base plate has a matching surface. In the embodiment of  FIGS. 4A and 4B , the bottom surface of the insulating plate  422  is convex, while the top surface of the base plate  420  has a matching concave shape. That is, the insulating plate is thinner at its edges than in its middle. Consequently, less insulation is provided at the edges of the chuck between the body  405  and the base plate  420 , such that better RF coupling is achieved at the edges, leading to better plasma uniformity. 
         [0044]    The plasma non-uniformity can be addressed by other means. For example, the insulating plate may be made to have variable dielectric constant, such that it is higher at the center of the plate than at the edges. For example, the insulating plate may be made of a series of rings, each made of different dielectric constant material. An alternative arrangement is illustrated in  FIGS. 5A and 5B . Elements in  FIGS. 5A and 5B  that are similar to those of  FIGS. 3A and 3B  are indicated with the same reference numerals, except that they are in a different centennial series. As shown in  FIG. 5B , a series of trenches  580  are formed on one surface of the insulating plate  522 . The trenches reduce the dielectric insulation of the insulation plate  522  and can be filled with lower dielectric material or with conductor, depending on the insulation required. For example, the trenches can be filled with the same adhesive, such as Kapton® or conductive adhesive, used to bond the insulating plate  522  to the base plate  520 . 
         [0045]      FIG. 6  illustrates an arrangement for utilizing any of the chucks described above in a plasma processing system, such as that illustrated in  FIG. 2 . Generally, the chuck is connected to a carrier  685 , e.g., by bolting the base  620  to the carrier  685 . The carrier  685  has one set of vertically-oriented wheels  690  and one set of horizontally oriented wheels  695 , which are fitted to ride on rails  692 . In this embodiment, motive force is provided by a linear motor which is partially positioned on the carrier in vacuum and partially positioned outside vacuum beyond the vacuum partition  698 . For example, a series of permanent magnet  694  can be provided on the bottom of the carrier, while a series of coils  696  are positioned in atmospheric environment outside of partition wall  698 . 
         [0046]    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. 
         [0047]    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.