Abstract:
An apparatus for the chemical-mechanical polishing of surfaces of circular flat workpieces, in particular semi-conductor wafers, comprising a loading and unloading station for the workpieces which includes a carrier which is supported for rotation about a vertical axis and is driven by a rotary driving means into a predetermined rotary position, at least two horizontal loading surfaces on the carrier means facing upwardly. With a transfer means the workpieces can be placed on the loading surfaces or removed therefrom. At least two polishing tables in corresponding polishing stations are provided which are located at the circumference of the carrier means and at least two chucks for the workpieces, the chucks being adapted to be moved along a vertical and a horizontal axis by moving means to align the chuck with a loading surface, to hold and discharge a workpiece and for the transfer of the workpiece as well to the associated polishing station and away therefrom and for the cooperation with the polishing table of the associated polishing station and a control means for the rotary driving means, the actuation means and the moving means.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     After each coating of a semi-conductor wafer, e.g. with an oxide layer, a tungsten layer or other metal layers, a processing has to take place in order to achieve planar surfaces. Otherwise, problems may occur with lithographic processes in form of focus failures of the UV stepper or in form of damages of the conductor paths. A common method in the semi-conductor industry for the planarization uses the so-called CMP process. This is a chemical-mechanical treatment by means of the fluid (slurry), whereby the chemically reactive part of the slurry has the objective to convert the material into a polishable condition. The slurry includes an abrasive in the form of colloidal abrasive small particles. 
     From the DE 197 19 503 A1 an apparatus for the chemical-mechanical polishing of surfaces has become known. It includes two polishing stations with vertically movable vacuum chucks for a semi-conductor wafer. The polishing stations have polishing tables which can be rotated about a vertical axis. The vacuum chucks are guided along two parallel horizontally extending guides. By this, two wafers can be polished by a polishing table contemporarily. At least one transfer means for the wafers is provided. Furthermore, on opposing sides of the guides loading and unloading means for the wafers are provided which can be aligned with the vacuum chucks. The transfer means normally are formed by a robot. 
     During the transportation and the processing the wafers are held by a vacuum chuck or a carrier. This has the task to transfer a homogenous pressure field or different pressure profiles onto the back side of the workpiece. The so-called sharp surface, i.e. the surface which is provided with circuits is facing the polishing table. Usually, the chuck is retained and moved by a corresponding actuating means which rotates the carrier about a vertical axis and moves it along linearly in vertical and horizontal direction. 
     The throughput through a CMP apparatus is mainly dependent upon the number of polishing stations. On the other side, the processing times for the planarization are relatively short (typically 90 seconds). Due to the short processing times bottlenecks may occur between the individual sections and limit the throughput. 
     It is an object of the invention to provide an apparatus for the chemical-mechanical polishing of workpieces, in particular of semi-conductor wafers, whereby the complete time of the workpieces within the apparatus can be reduced. 
     BRIEF SUMMARY OF THE INVENTION 
     In the invention the loading and unloading station includes a carrier which is supported for rotation about a vertical axis and which is rotated by a rotary driving means. The rotatable carrier has at least two horizontal loading surfaces exposed upwardly. In the apparatus according to the invention further at least two polishing stations are associated with a circumference of the rotatable carrier. Two polishing stations preferably are located on diametrically opposed sides of the carrier. A third polishing station can be provided which has an offset with respect to the first polishing stations about an angle of 90°. Two transfer means are diametrically opposed to the last-mentioned polishing station. The transfer means is to load and unload the workpieces to and from the loading surfaces. 
     The CMP processes can be carried out by two or more steps, whereby the workpieces are planarized in different polishing stations. By using different chemical substances and polishing cloths in the different polishing stations, different materials, as for example tungsten, copper or titanium nitrite can be worked under optimized conditions. It is important to minimize the transportation times of the workpieces between the polishing stations as the chemical components of the first step may quickly etch the workpiece. In the apparatus according to the invention, a fast transportation from one polishing station to another can take place. By a quick exchange of the workpieces between the polishing stations, the throughput can be increased and the secondary times can be reduced. By the described configuration of the loading and unloading station according to the invention two or more polishing stations can be interconnected so that a fast exchange between the stations can be achieved. Also with a one step process the throughput time can be reduced since the workpieces can be treated during their transport on the loading surface, e.g. a chemical pretreatment can take place and/or a rinsing or cleaning after the polishing step. 
     In the present CMP process technology it is usual to clean the workpiece after the first polishing step in order to minimize the described disadvantageous effects or to eliminate these effects. In the already discussed publication DE 197 19 503 or U.S. Pat. No. 6,050,885 it has become known to provide a stationary cleaning means. According to the invention, a cleaning means can be associated with the carrier so that during the transport of the workpiece on the carrier a cleaning can take place. As a consequence, undesired etchings on the workpiece can be effectively prevented by cleaning the workpieces during transportation. Additionally, the so-called cross contamination between the polishing stations in a two step process can be eliminated. 
     The positioning of the workpieces on the loading surfaces by means of the transfer means normally is such that the workpieces are centered prior to being picked up by a chuck. Therefore, the loading surfaces of the apparatus according to the invention are associated with center means which cooperate with the circumference of the workpiece on the loading surface in order to align the workpiece to a predetermined vertical axis. The vertical axis of the chuck can be also aligned with this axis so that a lowering of the carrier onto the workpiece on the loading surface the chuck can pick up the workpiece in a centered manner. 
     The chuck for the transport of the workpieces and the cooperation with the polishing tables in the polishing station can be formed in a usual way. Preferably, the workpieces are held by vacuum. For the removing of the workpieces from the chuck an air pressure pulse can be generated after switching off of the vacuum. The movement of the chucks along vertical and horizontal axes has already become known and can be carried out as disclosed by U.S. Pat. No. 6,050,885. 
     From the mentioned publication, it is also known to provide a linear guide for the chucks, with two chucks being provided for each polishing table. The chucks can be moved along the guide independent from each other. For this case, it is of advantage if the carrier has four loading surfaces, with each two loading surfaces having an axis which is in a plane parallel to the guides if the carrier has a corresponding rotary position. By this, per chuck one loading surface can be provided whereby the throughput of the workpieces upon polishing can be considerably increased, in particular in connection with a two or multiple step planarization process. The positioning of the four loading surfaces preferably takes place in steps of 90° or multitude of 90°. 
     A cleaning means is associated with the carrier. For this, the carrier can include a central elevation which per loading surface positions a nozzle which is connected to a fluid source. By the nozzle cleaning liquid can be sprayed onto the processed surface of the workpiece. The nozzle can also serve to wet the surface of the workpiece by a suitable liquid. In such an elevation also a number of detectors can be mounted which detect whether a workpiece is on a loading surface. 
     It is necessary to center the workpieces on the loading surfaces so that they can be picked up by a chuck in a centralized manner. As to this, different known structures can be used. According to an embodiment of the invention, a plurality of centering cams are provided which are located on a circle and which have support surfaces which accommodate a marginal portion of the workpiece. The centering cams further include radially adjustable stop surfaces which may engage the circumference of the workpiece in order to align the workpiece with respect to a predetermined vertical axis. To this purpose, the stop surfaces are synchronously actuated. 
     The loading surfaces can have a concave shape so that the space between an accommodated workpiece and the loading surfaces can serve as cleaning chamber. It is further possible to drain liquid from this cleaning chamber to one or more bores in the loading surface. Furthermore, a nozzle can be arranged in the loading surface for the supply of cleaning fluid to the described chamber between workpiece and loading surface. Finally, by means of such measures the contact surface of the chuck can be cleaned if it is lowered onto the loading surface. 
     With the invention a multi function apparatus is created by which through a rotary movement the individual polishing stations and the transfer means could be interconnected in order to decrease the transportation times as short as possible. Furthermore, by means of the multi function apparatus the throughput can be increased, in particular in a two step or multiple step process, wherein different materials as for example tungsten, copper or titanium nitrite is to be processed with different chemical substances and polishing cloths in different polishing stations. By the integration of suitable rinsing and cleaning means, it is possible to avoid etching and chemical reactions which can occur by remainders on the workpieces. Furthermore, the multi function apparatus according to the invention prevents the so-called cross contamination, i.e. the transportation of different materials and chemical components between the polishing stations. Furthermore, the rinsing and cleaning means can be used for a chemical pretreatment of the workpieces in order to prepare the workpieces for the second and third polishing step. Since the cleaning, the pretreatment and the like takes place during the transportation, the throughput speed is not affected. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the following, embodiment examples of the invention are described in more detail along accompanying drawings, wherein 
     FIG. 1 shows diagrammatically the processing of a semi-conductor wafer with a polishing table. 
     FIG. 2 shows the view on a diagrammatically depicted apparatus according to the invention. 
     FIG. 3 shows a cross section through the carrier and the loading and unloading station of FIG.  2 . 
     FIG. 4 shows the view onto the loading and unloading station of FIG.  2 . 
     FIGS. 5 a  to  5   o  show diagrammatically the procedure of a two step polishing process according to the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention may be embodied in many different forms, there are described in detail herein a specific preferred embodiment of the invention. This description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiment illustrated. 
     FIG. 1 shows diagrammatically the known structure of a polishing station, e.g. for a semi-conductor wafer. A polishing unit  12  is supported for linear movement along a horizontal linear guide  10  and is moved along the guide by a not shown driving means. This is indicated by double arrow S 1 . The upper portion  14  which is guided by guide  10  supports a spindle  16  which can be rotatably driven by a not shown motor. The spindle is also vertically movable. A chuck  18  is mounted to the lower end of the spindle for the holding and transportation of a semi-conductor wafer not shown. The chuck can be rotatably driven by spindle  16 , i.e. with speed n1. A rotatable driven polishing table  20  is arranged below the chuck  18  as is usually used for the planarization of wafers. The polishing disk or table is rotated with the revolution n 2 . On the polishing cloth of the polishing table  20  a slurry is supplied by a device  22 , e.g. with the amounts of α1 and α2. By means of a not shown mechanism for the elevation of spindle  16  a pressure b1 can be exerted in order to press the wafer with a predetermined pressure against the polishing table  10 . 
     A not shown dressing mechanism  24  includes a dressing disk  26  which is rotatably supported by an arm  28  and is driven by the revolution n 3 . The force by which the dressing disk is pressed is designated with F 2 . 
     In the illustration of FIG. 2, two polishing stations  30 ,  31  are provided which resemble that of FIG. 1, two polishing units  12  being associated with each polishing station which are guided by linear guides  10   a ,  10   b . The linear guides  10   a ,  10   b  are lying on an axis. The structure of the polishing units of FIG. 2 corresponds to that of FIG.  1 . The arrangement of the polishing units on the guides  10   a ,  10   b  corresponds to that described in U.S. Pat. No. 6,050,885. 
     A circular carrier  34  is located between the polishing stations  30 ,  31  and supported for rotation about a central vertical axis. The rotary driving means are not shown. The guides  10   a ,  10   b  are extended right and left and extend over the carrier  34  approximately to the center thereof. The centers of the polishing tables  20   a ,  20   b  and of the carrier  34  are on a common axis which is parallel to the guides  10   a ,  10   b.    
     Two loading and unloading stations  36  are arranged on the carrier on opposite sides of the axis which will be subsequently described in more detail. Their centers are positioned on a circle concentric to the rotary axis of carrier  34 . Each of the four loading and unloading stations  36  is in a position to accommodate a wafer in a centered manner. The loading and unloading of these stations  36  take place by a diagrammatically illustrated robot  38 . 
     In the rotary position shown in FIG. 2, the polishing units  12  can be aligned with two unloading and loading stations  36  in order to accommodate a wafer or to have a wafer removed. It is understood that a third polishing station can be provided. It is then located at the circumference of carrier  34  on the opposite side of robot  38 . 
     The structure of the loading and unloading stations is more clearly seen in FIGS. 3 and 4 which are to be described hereinafter. 
     A stationary frame  40  has an opening wherein the carrier  34  is supported for rotation about a vertical axis. It comprises a plurality of parts. A circular plate  42  is connected to a wheel  44  for rotation therewith, the wheel being driven about a vertical axis through a gear  46  and a driving motor  48 . Plate  42  rotates with wheel  44 . A trunnion-shaped holder  50  is mounted to plate  42 . The holders  50  support cap-shaped elements  52 . This support is axially resilient in axial direction by means of a spring  51 . The upper side of the elements  52  form a loading surface  54  for wafers  56  which can be placed on the loading surfaces. Four centering cams  58  are positioned at the circumference on the loading surface  54  in a circumferentially spaced manner. The centering cams include a support surface not shown in detail for the wafers  56 . Thereby, the wafers  58  are only supported on four spots at a marginal portion thereof (in FIG. 3 only two centering cams  58  can be seen). In FIG. 4 four centering cams  58  can be recognized. The radially movable centering cams have a stop surface which is radially moved by an actuation mechanism  60 . This mechanism includes a motor  61  which effects on four rods  65  through a gear  63  in order to move the cams  58 . These are formed as levers which are pivoted by the rods  65 . The stop surfaces are also not shown. By means of the stop surfaces or the centering cams  58 , respectively, a wafer disk accommodated can be centered with respect to a predetermined axis, e.g. the center axis of element  52 . 
     The top wall of element  52  includes a throughbore  62  which is provided with a connection fitting  64  for a fluid. Through this fitting fluid can be conveyed to the lower side of the wafer accommodated. Furthermore, bores can be provided to remove liquid from the loading surface. 
     Spaced from plate  42  a plate  66  can be fixedly attached to plate  42  which in the area of element  52  has openings  68 . In the center, plate  64  has an elevation  70  which has an inner hollow space, the elevation being aligned with an axial passage  72  from wheel  44  to plate  42 . In the slightly oblique wall of elevation  70  a number of nozzles is arranged in the upper portion which is shown at  74 . Each loading and unloading station  36  is associated with a nozzle  74  which is directed to a loading surface. A conduit connected to a fluid source is connected with nozzle  74  in order to spray a fluid onto the upper side of a wafer accommodated. Also a radiation source  78  is provided for each loading and unloading station  36  which is directed to the loading surfaces  54  and cooperates with a receiver  79  which indicates whether a wafer  56  is accommodated. 
     The carrier  34  is encircled by a sealing ring  80  of frame  40 , a labyrinth sealing  82  being located between ring  80  and plate  66 . A dripping tub (not shown) is below ring  80 . Each cap-shaped element  52  is also provided with a dripping tub  82  in order to accommodate liquid or slurry, respectively, and to drain it to the tub for the complete system. 
     According to FIG. 2, the robot  38  can load wafers on two associated loading and unloading stations or remove wafers therefrom. It is also conceivable to bring the carrier into a rotary position wherein only one station  36  can be served by the robot  38 . In the rotary position according to FIG. 2 the polishing unit then can only pick up one wafer from the loading and unloading means or place one wafer thereon. If the left polishing station is for the first processing while the next processing takes place in the right polishing station, the carrier  34  carries out a rotation about 180° after the placement of wafers on the associated loading and unloading stations so that the associated polishing unit can pick up the wafer and transport it to the associated half of polishing table  20   b . During the rotation of carrier  34  the surface of the wafers can be cleaned, e.g. by means of nozzle  74  in order to remove remainders of a treating substance and to avoid an undesired etching. Thus, the loading and unloading station  36  in conjunction with carrier  34  is not only a means to center accommodated wafers to allow a centered pick up by chuck  18 , rather, also a transportation means between two or more polishing stations and a cleaning station as well for the cleaning process to wafers prior to the further transport to the next polishing station or prior to the removal by robot  38 . 
     The loading surfaces  54  can be shaped concavely so that a chamber is formed at the back side of the wafer  56  as already described. The loading surface can be provided with bores for the drainage of fluid or for the supply of fluid. In this way, also the back side of the accommodated wafers  56  can be cleaned. Furthermore, the contact surface of the chuck can be cleaned if it is lowered onto the loading surface. 
     It is understood that the described driving means for individual parts of the polishing system and the cooperation of these driving means can be controlled by a suitable not shown control device. Such control devices are generally known. 
     In the following, a two step polishing process is explained along FIGS. 5 a  to  5   o . A rotating carrier is located between two polishing disks POT 1  and POT 2 . The carrier has four loading surfaces WLT 1  to WLT 4 . An arrangement can be used as shown in FIGS. 2 to  4 . The transfer means  38  is not shown and also not the chuck (polishing units  18 ) by which the wafers can be transported and held against the polishing disk POT 1  and POT 2 . In case of FIG. 5, the transfer means or robot is on side A of the shown arrangement. The opposing side is designated with B. For the sake of comprehensiveness in FIGS. 5 a  to  5   o  a radial line is shown. In FIG. 5 a  this line indicates the zero position of the carrier. In the other Figures, the position is indicated with 90° or a multitude of 90°. 
     In FIG. 5 a , the loading surfaces WLT 1  and WLT 2  are loaded with workpieces W 1  and W 2 . This takes place with the not shown transfer means and the loading can take place contemporarily or step-by-step. Subsequently, the carrier according to FIG. 5 b  is rotated about −90°, whereby the workpieces W 1  and W 2  are facing the first polishing disk POT 1 . In this position, the wafers can be picked up by the chucks and moved above the polishing disk POT 1 . This can be seen in FIG. 5 c . Now, in this first polishing station the processing of the wafers W 1  and W 2  can take place. 
     As soon as wafers W 1  and W 2  are removed from the carrier, two further wafers W 3  and W 4  are placed on the loading surfaces WLT 1  and WLT 4 . Afterwards, the carrier is rotated back about 90° into the zero position as can be seen in FIG. 5 e . In this position, the wafers W 1  and W 2  can be brought back to the loading surfaces WLT 2  and WLT 3  after finishing of the polishing process. This is shown in FIG. 5 f . Thereafter, the carrier is rotated about 180° as can be seen in FIG. 5 g . In this position, the chucks which are associated with the polishing disk POT 2  can transport the wafers W 1  and W 2  to the second polishing disk POT 2  as shown in FIG. 5 h . Contemporarily, the wafers W 3  and W 4  can be moved to polishing disk POT 1  by the associated chucks. 
     During processing of the wafers W 1  to W 4  by the polishing disk POT 1  and POT 2 , the loading surfaces WLT 1  to WLT 4  are empty. So, they can be loaded with further wafers W 5  and W 6  as shown in FIG. 5 j . According to FIG. 5 k , the carrier is rotated in clockwise direction so that wafers W 5  and W 6  are aligned to polishing disk POT 1  while the empty loading surfaces WLT 2  and WLT 3  as associated with polishing disk POT 2 . In this position, the finished wafers W 1  and W 2  can be placed on the associated loading surfaces as shown in FIG. 5 l . Thereafter, the carrier is rotated about further 90° so that the wafers W 1  and W 2  can be removed (as shown in FIGS. 5 m  and  n ). Thereafter the carrier is again rotated about 90° so that the wafers W 5  and W 6  are aligned with polishing disk POT 2 . Thus, the wafers W 3  and W 4  processed in the first station can be placed on the carrier. Thereafter, the further processing takes place as described in connection with FIG. 5 f  and the following. 
     During the presence of the wafers W 1  to W 6  on the loading surfaces they can be pretreated, rinsed and cleaned as already described above. By these process steps the complete throughput time in a two step polishing process for the wafers is not extended.