Abstract:
An article holder has protrusions that contact the article. The friction between the protrusions and the article impedes the article movement relative to the holder yet allows the article to slide when the article is pushed against some object. The article is pushed against the object in order to position the article more precisely.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to handling of semiconductor wafers and other articles. 
     In a typical integrated circuit manufacturing process, semiconductor wafers are handled by a computer controlled robot. The robot is required to position the wafers with much precision to avoid wafer damage or misalignment between different processing stages. A wafer can be damaged, for example, if it is unintentionally rubbed against a wall of a storage container into which the wafer is being lowered by the robot. 
     Absolute precision is impossible to achieve. Usually there are some errors in the wafer&#39;s position relative to the robot and the robot&#39;s position relative to a target station into which the wafer is placed by the robot. 
     SUMMARY 
     Some embodiments of the present invention allow one to reduce or eliminate the impact of wafer positioning errors. In some embodiments, the robot&#39;s wafer holder (“end effector”) is constructed to allow the wafer to slide relative to the holder without the holder losing control of the wafer. For example, the holder may have protrusions on its surface. The robot generates a force that draws the wafer to the holder so that the wafer is pressed against the protrusions. The friction force between the wafer and the protrusions prevents the wafer from moving relative to the holder when the robot is transporting the wafer between stations, but allows the wafer to slide on the protrusions when the robot pushes the wafer against a pin or pins precisely positioned relative to a target station. The pin or pins steer the wafer into a precise position relative to the target station even though the holder position relative to the target station may be imprecise. The robot then places the wafer into the target station. 
     The invention is not limited to the embodiments described above, but is defined by the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an article handling system according to one embodiment of the present invention. 
     FIGS. 2-7 are top views of article handling systems according to some embodiments of the present invention. 
     FIG. 8 is a perspective view of an article handling system according to one embodiment of the present invention. 
     FIG. 9 is a bottom view of an end effector according to one embodiment of the present invention. 
     FIG. 10 is a side view of an end effector holding an article according to one embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1, an article  120  is being placed onto a target station  122  by a robot  124 . For the purpose of illustration, we will assume that article  120  is a semiconductor wafer, though this assumption is neither necessary nor limiting. The article is being held by an article holder (end effector)  130  mounted on a robot arm  134 . Arm  134  is attached to a robot body  124 B which itself may include a number of moving arms. One example of a suitable robot is model GBY7S available from Genmark Automation of Sunnyvale, Calif. The robot is controlled by its computer  140 , which in turn may receive commands from, and send information to, a programmable logic controller (PLC)  150 . Computer  140  and PLC  150  are software programmable. A device  160  controlled by computer  140  and/or PLC  150  generates a force that draws the wafer  120  to end effector  130 . In some embodiments, the wafer is held by gas vortices emitted from the end effector, and device  160  is a gas pump which provides a gas flow for the gas vortices. The invention is not limited to any particular end effector or robot structures or control mechanisms. Other robots and end effectors, known or to be invented, and non-electronically-controlled article handlers, can also be used. 
     Station  122  can be any station involved in wafer storage or processing. In one embodiment, station  122  is part of a transport device that delivers semiconductor wafers to a plasma processing chamber. See U.S. Pat. No. 6,139,678 issued on Oct. 31, 2000 to O. Siniaguine, entitled “Plasma Processing Methods and Apparatus”, incorporated herein by reference. See also U.S. patent application Ser. No. 09/632,236 filed Aug. 4, 2000 by S. Casarotti et al., entitled “Detection and Handling of Semiconductor Wafers and Wafer-Like Objects”, incorporated herein by reference. The invention is not limited to a particular structure or function of the target station. 
     Wafer  120  is round in plan view. Target station  122  has pins  170 . 1 ,  170 . 2 ,  170 . 3 ,  170 . 4  positioned thereon along a circle approximating the contour of wafer  120 . When the end effector  130  approaches the target station, the position of the end effector relative to the target station may deviate from an ideal position as illustrated in FIG.  2 . In the embodiment being described, the end effector should ideally be positioned symmetrically with respect to pins  170 , and wafer  120  should also be in a position symmetric with respect to pins  170  and to the end effector. In FIG. 2, both the wafer and the end effector are shifted sidewise (up in the top view of FIG. 2) from their ideal position relative to the pins, and the wafer is shifted down (in the top view) relative to the end effector. FIG. 2 does not illustrate all of the possible errors that may occur in the wafer and end effector positioning. 
     As shown in FIG. 3, the end effector moves to the left, in the direction of arrow  310 , to some predefined position proximate to pins  170 . The final position of the end effector deviates from the ideal position symmetric with respect to the pins. As the end effector is moving, the wafer comes in contact with pins  170 , and the pins steer the wafer into a circular area defined by the pins&#39; position and the wafer radius. The wafer position is now precise relative to the pins, even though the wafer position relative to the end effector may be erroneous. 
     The end effector does not come in contact with pins  170  in some embodiments. 
     When the pins  170  steer the wafer, the wafer slides relative to the end effector without the end effector losing hold of the wafer. Suitable end effectors are described in detail below. 
     As shown in FIG. 4, the robot now moves the end effector to the right, away from the pins, in the direction of arrow  410 . The end effector moves by some predetermined distance D defined by control circuitry (e.g. computer  140  and/or PLC  150 ). Consequently, the wafer moves by distance D to the right. The wafer remains precisely positioned relative to pins  170  even though the end effector may be positioned imprecisely. 
     Then the end effector lowers the wafer onto the target station and releases the wafer. 
     In some embodiments, the step of FIG. 4 is omitted. The wafer is lowered onto the target station immediately after the step of FIG.  3 . 
     Precise wafer positioning is especially important for thin wafers (e.g. 100 μm or thinner) because they are fragile and can be easily damaged. Such wafers can be easily mis-positioned when being picked up by the end effector because they sag under their own weight and because they can be warped by internal stresses induced by the simultaneous presence in the wafer of different materials (semiconductor materials, metals, dielectrics) that constitute the wafer circuitry. The invention is not limited to thin or deformed wafers however. 
     Some embodiments handle semiconductor wafers which have a flat side portion (a “flat”) used for rotational alignment. The distance between the adjacent pins  170  is chosen to be larger than the length of the flat. 
     The invention is not limited to any particular shape or size of end effector  130  or wafer  120 . In some embodiments, the wafer protrudes from the end effector by at most a few millimeters. The end effector may be round in top view or have some other shape. 
     FIG. 5 illustrates a case in which the wafer positioning technique of FIG. 3 may fail. The end effector is positioned as in FIG. 3, but the wafer does not touch the pins  170  because the wafer is shifted too far right relative to the end effector. This may happen due to an error which occurred when the wafer was picked up. Small errors of this type may be tolerable depending on the particular system, while larger errors may be not. Larger errors can be avoided in a number of ways. In some embodiments, when the end effector picks up the wafer, the end effector is operated so that any possible positioning errors would result in the wafer being shifted too far towards pins  170  at the stage of FIG. 3, not away from the pins. 
     In another embodiment, station  122  has pins  610  (FIG. 6) provided thereon to shift the wafer left. Before the stage of FIG. 3, the end effector is positioned so that the pins  610  are between the end effector and the robot body  124 B. The robot moves the end effector towards pins  610 , in the direction of arrow  620 . If wafer  120  projects beyond the end effector in the direction of the robot&#39;s body by more than a predetermined distance, the wafer  120  will come in contact with pins  610  and will slide left relative to the end effector. Then the steps of FIG.  3  and possibly FIG. 4 are performed. 
     Some embodiments have more or fewer than four pins  170 , and may have any number of pins  610 . Two pins  170  are sufficient for precise positioning of a round article having a predefined radius. If the article is a round wafer having a flat or a notch on its periphery, three pins  170  are sufficient if the distance between the adjacent pins  170  is larger than the length or the flat or the notch. Two pins  170  may be insufficient because one of the pins may come against the flat or the notch during the operation of FIG.  3 . Four pins  170  may be desirable depending on the maximum positioning error at the stage of FIG.  2 . For example, suppose in FIGS. 2,  3 , pin  170 . 4  is omitted. Suppose that at the stage of FIG. 2, the center of wafer  120  is above the pin  170 . 3  (in top view). Then at the stage of FIG. 3 the pin  170 . 3  will push the wafer up, and the wafer will not contact the pins  170 . 1 ,  170 . 2 . More than four pins  170  can be provided. Similar considerations apply to pins  610 . 
     In some embodiments, only one pin  170  and/or only one pin  610  is provided. The article positioning may be imprecise, but the positioning errors are tolerable. 
     The invention is not limited to a particular position of pins  170  or  610  relative to station  122  or each other. Pins  170  can be positioned on some other side of station  122  and not necessarily on the side opposite to the robot&#39;s body as in FIG.  1 . Pins  610  may also be positioned differently than in FIG.  6 . Pins  170 ,  610  do not have to be part of station  122 , they can be at a distance from the station. 
     In some embodiments, pins  610  are omitted, and their function is performed by one or more of pins  170 . For example, the end effector positions the wafer to the left of pins  170 , and moves the wafer right. Pins  170 . 2 ,  170 . 3  steer the wafer left. Then the stage of FIG. 3 is performed. 
     The invention is not limited to articles being positioned on top of station  122 . Station  122  may be a wafer storage cassette such as described in the aforementioned U.S. patent application Ser. No. 09/632,236. Wafers are inserted into the cassette horizontally as the end effector moved horizontally. Pins  170  and/or  610  can be mounted on top of the cassette or at some other place. Other stations and wafer placement techniques, known or to be invented, can be used. 
     As noted above, the invention is not limited to any particular shape of article  120 . In FIG. 7, the article is rectangular. It can be a flat panel display for example. Pins  170 . 1 ,  170 . 2  are positioned along one side of the article, and pins  170 . 3 ,  170 . 4  are positioned along an adjacent side of the article. To bring the article in contact with the pins, the robot moves the article diagonally, in the direction of arrow  710 . Alternatively, the robot may move the article to the left to push the article against pins  170 . 3 ,  170 . 4 , then right, then down (in the view of FIG. 7) to push the article against the pins  170 . 1 ,  170 . 2 , then up. Then the article is placed onto or into station  122 . 
     Objects other than pins can also be used. For example, one or more of pins  170 ,  610  can be replaced with a vertical wall having a circular surface against which a circular article can be pushed at the stage of FIG.  3 . Multiple walls can be used. The pins or other pin positioning objects do not have to be vertical, and may have any shape and orientation. 
     In FIG. 8, target station  122  is a horizontal wafer shipment container (a “pod”). The pod stores a stack  810  of semiconductor wafers separated by paper or plastic inserts. Foam is provided at the top and bottom of the stack. See U.S. patent application Ser. No. 10/071,689 filed by F. E. Kretz et al. on Jul. 13, 2001, entitled “Article Holders with Sensors Detecting a Type of Article Held by the Holder”, incorporated herein by reference. The pod has walls  122 . 1 ,  122 . 2 ,  122 . 3 ,  122 . 4  around the wafer/paper/foam stack  810 . The end effector pushes the article against the walls  122 . 2 ,  122 . 3  as in FIG. 3, then optionally moves the article to the left as in FIG. 4, then lowers the article into the pod. The pod is tilted by a pneumatic device  820  inserted under the pod near the walls  122 . 2 ,  122 . 3 . The tilt allows the article to travel above the walls  122 . 1 ,  122 . 4  when the article is being pushed horizontally against the walls  122 . 2 ,  122 . 3 . Device  820  can be replaced by a non-pneumatic device, or block or a wedge, or some other tilting means, known or to be invented. 
     In some embodiments, the pod is not tilted. In some embodiments, the robot tilts the end effector so that the end effector moves at an angle relative to the pod when the end effector pushes the wafer against the walls  122 . 2 ,  122 . 3 . Also, walls other than  122 . 2 ,  122 . 3  can be used for wafer positioning. Some of the walls can perform the function of pins  610  (FIG.  6 ). 
     In some embodiments, the positioning steps of FIGS. 3,  4 ,  6  are omitted for the embodiment of FIG. 8 when the end effector places paper or foam in the pod because the paper and foam are less likely to be damaged by imprecise positioning relative to the pod. 
     FIG. 9 is a bottom view of an end effector that allows the wafer to slide while holding the wafer. The end effector is a non-contact type, described in detail in the aforementioned U.S. patent application Ser. No. 10/071,689. It holds articles with gas vortices emitted from openings  920  in its flat bottom surface. Only a few of the openings are labeled in the drawings. Gas vortex end effectors are described in U.S. Pat. No. 6,095,582 issued Aug. 1, 2000 to Siniaguine et al. and incorporated herein by reference. See also U.S. patent application Ser. No. 09/632,236 filed Aug. 4, 2000 by S. Casarotti et al.; U.S. patent application Ser. No. 09/633,086 filed Aug. 4, 2000 by S. Kao (now U.S. Pat. No. 6,427,991 B1); U.S. patent application Ser. No. 09/877,366 entitled “Article Holders That Use Gas Vortices To Hold An Article In A Desired Position”, filed Jun. 8, 2001 by S. Kao. Numeral  120  denotes a contour of an article (wafer, paper or foam) held in the end effector. In an exemplary embodiment, the end effector has a body  130 B made of a top plate and a bottom plate. A number of vortex chucks are positioned in a hollow region (not shown) between the two plates. Each opening  920  is an opening of one such chuck. A tangential passage  924  goes through a vertical cylindrical wall of each chuck. Gas is supplied under pressure, by pump  160  (FIG. 1) into the hollow region between the plates of the end effector. The high pressure forces the gas to enter the vortex chucks through passages  924  and form gas vortices. The gas vortices emerge from openings  920  and create an attraction force that holds the wafer, paper, or foam article near the body of the end effector. The gas also creates a cushion that prevents the article from touching the bottom surface of the end effector except at brake pads  930 . Brake pads  930  protrude from the bottom surface of the end effector. See the side view in FIG.  10 . The article is pressed against the brake pads by the attraction force of the gas vortices. 
     The invention is not limited to gas vortex end effectors or end effectors made of two plates. 
     Friction between article  120  and brake pads  930  prevents the article from sliding horizontally when the end effector carries the article from one station to another. The friction force is small enough however to allow the article to slide when the article is being pushed against pins  170  or  610 . In some embodiments, the friction force is varied by varying the gas flow through openings  920 . During the steps of FIGS. 3 and 6, when the article is being pushed against the pins, the gas flow is reduced so that the friction force is lowered. For example, in one semiconductor wafer embodiment, the gas flow is normally about 50-240 slpm (standard liters per minute) for wafers weighing about 50 to 250 g. The break pads are 0.25 mm to 1.5 mm high. At the stage of FIG. 3 or  6 , when the end effector is about to move the wafer towards pins  170  or  610 , the gas flow is reduced to about 15-120 slpm. The attraction force acting on the wafer is reduced by a value exceeding 10% of the wafer weight. When the stage of FIG. 3 or  6  is completed, the gas flow may optionally be increased again. 
     FIG. 10 is a side view of end effector  130  holding a wafer  120 . Only three brake pads are shown. In this embodiment, each pad  930  has a rounded bottom surface to avoid wafer damage when the wafer is pushed against the pins  170  or  610 . The brake pads can be made of a smooth material, e.g. plastic. In some embodiments designed to handle semiconductor wafers whose top surface has been passivated with silicon nitride or silicon dioxide, the brake pads are made of plastic, for example, ABS or Kynar. 
     In the embodiment of FIG. 9, the article physically contacts the end effector only at the brake pads. The invention is not limited to such embodiments. The end effector may have limiter pins extending vertically around the wafer to limit the lateral wafer motion. For example, limiter pins may be provided on the side of the robot&#39;s arm  134  (opposite from pins  170 ) in the view of FIG.  3 . 
     In FIG. 9, the brake pads are elongated from left to right. The invention is not limited to any particular shape or number of brake pads  930 . The brake pads can be positioned in the middle of the end effector. They can form continuous lines on the bottom surface of the end effector. 
     In FIG. 10, the bottom surface of the end effector is flat, and so is the top surface of article  120 . The break pads are of a uniform height. These features do not limit the invention. 
     The invention is not limited to gas vortex end effectors. In some embodiments, the end effector holds articles with non-vortex gas flows by using the Bernoulli effect. Vacuum may also be used. For example, openings may be provided in the end effector&#39;s bottom surface to draw air into the end effector&#39;s body and create vacuum between the end effector and the article. The openings may be surrounded by a continuous brake pad rim so that when the article slides relative to the end effector, the article continually contacts the rim along the whole length of the rim so as not to brake the vacuum. Different rims may be provided around different openings. The vacuum may be broken at some of the openings but not at others. An end effector may hold articles using electrostatic or magnetic forces, or using a mechanical clamp. In some embodiments, the electrostatic or magnetic forces are reduced, or the mechanical clamp is loosened, when the article is pushed against the pins  170  or  610 . 
     The invention can be applied to achieve precise positioning of the article relative to the end effector. For example, if the end effector can be positioned precisely relative to pins  170  or  610 , but the article&#39;s position is imprecise, then the steps of FIG.  3  and/or FIG. 6 may position the article precisely relative to the pins and hence relative to the end effector. 
     FIG. 9 illustrates a teaching hole  940  formed through the end effector body  130 B. The teaching hole is used to train the robot to a desired position. For example, if the end effector must place wafers on a platform of a film frame machine which attaches adhesive film frames to wafers, or on a platform of some other station, a mark (e.g. a cross “X”) can be made on the platform, and the end effector can be manipulated to a position in which a human operator can see the mark through the teaching hole (with a naked eye or an optical instrument). Then the robot can be caused to remember that position. The invention is not limited to teaching holes. 
     In some embodiments, the article is positioned above the end effector. The end effector and the article can also occupy a non-horizontal position. Other embodiments and variations are within the scope of the invention, as defined by the appended claims.