Holding means

For especially simple and reliable handling of thin and/or bent semiconductor wafers it is proposed in a corresponding holding means ( 100 ) that there is a gas flowing through at least one first means ( 5 ) for producing the forces which pull a semiconductor wafer toward the means ( 100 ) based on the Bernoulli principle and that there is at least one second means ( 2, 3 ) for holding the semiconductor wafer on the means ( 100 ) as a result of the forces produced by at least one electromagnetic field.

In the embodiment of a gripper 100 as claimed in the invention shown in FIG. 1 in a section and in FIG. 4 in a top view, there are an annular electrode 2 and a round disk-shaped electrode 3 in a base body 1 in the form of a circular wafer in depressions on the top 25 , therefore the side 25 of the base body 1 facing the wafer to be held. The electrodes 2 and 3 are embedded in the dielectric 4 which also covers the surfaces of the electrodes 2 and 3 which point up. In the base body 1 there is a circular nozzle 5 which discharges at an acute angle to the surface 25 of the base body 1 facing the wafer. The nozzle 5 is exposed to pressurized gas, for example air or preferably nitrogen, via a channel 6 which is provided in the base body. The electrodes 2 and 3 are dimensioned such that the area ratio between the electrode 2 and 3 is roughly one. The electrodes 2 and 3 are triggered bipolarly opposite so that an electrostatic field forms which attracts a wafer which has been placed on the gripper 100 (not shown) so that it rests on the surface 25 of the gripper which lies at the top in FIG. 1 . In order to move the wafer very close to the gripper 100 , specifically to the surface 25 which lies at the top in FIG. 1 , at least at the start of placement of a wafer on the gripper 100 the nozzle 5 is supplied with a pressurized gas via the channel 6 so that forces which pull the wafer toward the gripper 100 are produced as a result of the Bernoulli principle by the gas discharging between the base plate 1 and the wafer and emerging from the nozzle 5 . These forces which are produced as a result of the Bernoulli principle by the discharging gas are relatively large in order to pull the wafer reliably against the upper surface 25 of the gripper 100 when the wafer is bent, i.e. bent wafers are planarized or aligned flat at the same time. As soon as the wafer has been pulled against the gripper 100 , the feed of pressurized gas to the nozzle 5 can be interrupted, and the wafer is held securely against the surface 25 of the gripper 100 facing it solely as a result of the electrostatic forces which are produced by the electrodes 2 and 3 . In the embodiment shown in FIG. 2 , the side 25 of the base body 1 facing the wafer is made stepped by there being a circular elevation 8 in the middle of the base body 1 . The nozzle 5 which is supplied with pressurized gas via the channel 6 discharges in the area of the step 7 which has been formed in this way. As FIG. 2 shows, the outflow direction of the pressurized gas from the nozzle 5 in the embodiment shown in FIG. 2 is essentially parallel to the surface extension of the base body 1 . In a depression in the elevation 8 of the base body 1 —this part can be formed by a separate component—there are two electrodes 2 and 3 which are made hemispherical. The electrodes 2 and 3 are covered to the top by a dielectric in the embodiment of a gripper 100 as claimed in the invention which is shown in FIG. 2 and are insulated from one another by the dielectric 4 (see FIG. 5 ). The embodiment of a gripper 100 as claimed in the invention which is shown in FIGS. 2 and 5 can be altered as shown in FIG. 6 and as is indicated by the broken line in FIG. 2 . In this altered embodiment there is a single electrode 3 in the depression in the projecting elevation 8 of the base body 1 . Outside of the circular electrode 3 and outside the step 7 there are electrodes 2 with the shape of a segment of a circular ring. These electrodes 2 are likewise covered to the top by a dielectric 4 . FIGS. 3 and 7 show an embodiment which has been modified compared to the embodiment from FIG. 6 , in which in the middle of the gripper 100 there is an opening 10 which can be exposed to negative pressure in order to produce an additional holding force for a wafer fixed on the gripper 100 . One such opening 10 which is exposed to negative pressure can also be implemented for all other embodiments shown. It should be pointed out that the shape and arrangement of the electrodes 2 and 3 of the gripper as claimed in the invention can be modified in any way. The alignment and type as well as the size of the nozzle 5 by which pressurized gas for producing the forces which pull the wafer toward the gripper 100 as a result of the Bernoulli principle are not important &lsqb;sic&rsqb;. Thus, in addition to the circular or circular arc-shaped nozzles, there can also be nozzles in the form of holes which are located for example along one circle in the base body 1 of the gripper 100 . In all the illustrated embodiments of the gripper 100 as claimed in the invention, instead of an annular nozzle 5 for producing a force which pulls a wafer toward the gripper 100 as a result of the Bernoulli principle, there can also be a plurality of nozzles 5 . These nozzles 5 can be circular arc-shaped nozzles 5 or nozzles 5 which are made as holes. When the nozzles 5 which are provided with pressurized gas for producing a holding force using the Bernoulli principle are made as holes, they are preferably located on a circle which is concentric to the center of the gripper 100 and are preferably aligned such that they are aligned obliquely to the surfaces 25 of the gripper 100 which face the wafer. Here an alignment of the nozzles 5 is preferred in which the axes of the nozzles 5 point obliquely to the outside, as is shown for the annular nozzle 5 in the cross section from FIG. 1 . When there are several nozzles 5 made as holes, they can, as mentioned, be located along a circle which is arranged as is shown in FIGS. 4 to 7 for a circular nozzle 5 . In this embodiment the holes are distributed over the periphery of the circle on which the circular ring-shaped nozzle 5 discharges. One embodiment of a gripper 100 as claimed in the invention is also possible in which the nozzles 5 supplied with pressurized gas are located within the electrodes 2 and/or 3 . For example, oblique holes made as nozzles 5 for producing a holding force as a result of the Bernoulli principle can also be routed obliquely also through the middle electrode 3 , in which case there is insulation in the form of a dielectric 4 in these channels. When the gripper 100 as claimed in the invention is made with a means for holding a wafer using negative pressure, the negative pressure can be applied through a middle opening 10 , as is described using FIGS. 3 and 7 . Here it is possible to make the vacuum head which is exposed to negative pressure in order to hold a wafer adjustable and optionally tiltable in the direction perpendicular to the plane of the gripper 100 . Embodiments as can be made in practice—the opening 10 exposed to negative pressure can be movable—are shown in FIGS. 8 to 11 . In the embodiment shown in FIGS. 8 and 9 , there is a vacuum head 12 , specifically a vacuum pin, which has an opening 10 which is exposed to negative pressure in the center. The vacuum head 12 sits with an edge flange 14 from the top on a flexible foil 13 which covers the top of the base body 1 of the gripper 100 . If the vacuum head 12 is exposed to negative pressure, it moves into the position shown in FIG. 9 in which its end surface 15 which points to the outside lies in plane 16 which is defined by the surface 25 of the gripper 100 facing the wafer. In the embodiment shown in FIGS. 10 and 11 , the vacuum head 12 with its edge flange 14 is attached from underneath resting against the flexible foil 13 . In addition, in the embodiment shown in FIGS. 10 and 11 , there is a compression spring 17 which presses the vacuum head 12 into the position which projects over the gripper 100 and which is shown in FIG. 10 . In the embodiment shown in FIGS. 10 and 11 the vacuum head 12 moves perpendicularly to the plane of the gripper 100 when it is exposed to negative pressure and ultimately assumes the position shown in FIG. 11 , in which its end surface 15 is flush with the surface 16 of the gripper 100 . When the gripper 100 as claimed in the invention is additionally equipped with openings which can be exposed to negative pressure, as for example the vacuum heads 12 of FIGS. 8 to 11 , feed of pressurized gas and/or exposure to negative pressure—then only electrostatic forces work—can be ended as soon as the wafer has been placed against the gripper 100 . Then the vacuum heads 12 remain in the lowered position shown for example in FIGS. 9 and 11 , when there is the embodiment of a gripper 100 which as shown in FIG. 1 has a continuous flat surface on the side 25 of the gripper 100 facing the wafer. In a stepped embodiment ( FIGS. 2 and 3 ) of a gripper the vacuum heads 12 , when they have been exposed to negative pressure, are lowered into the gripper 100 only so far that their upper end surface 15 lies in the plane which is defined by the upper end surface of the projection 8 . In the embodiment with vacuum heads 12 , there can be several such vacuum heads 12 with largely any arrangement on the gripper. The vacuum heads 12 are for example mounted on a flexible foil 13 and in the base body 1 of the gripper 100 there is one vacuum chamber 20 underneath each vacuum head 12 . When the vacuum heads 12 are not exposed to negative pressure, the upper end surfaces 15 of the vacuum heads 12 project out of the gripper surface 25 , a projection of a few tenths of a millimeter being possible. In the embodiments shown in FIGS. 8 and 9 , the vacuum head 12 is inserted into an opening of the flexible foil 13 . In the embodiment shown in FIGS. 10 and 11 the vacuum head 12 with its upper end surface adjoins the flexible foil 13 from underneath, in the area of the opening 10 of the vacuum head 12 exposed to negative pressure there being an opening in the flexible foil 13 . Exposing the vacuum heads 12 to negative pressure is implemented for example by a system of channels in the base body 1 of the gripper 100 . The embodiments shown in the drawings are only preferable embodiments of the invention. It is important that there are both means, for example exit openings or nozzles for pressurized gas for producing forces using the Bernoulli principle, and at the same time means, for example electrodes which are triggered bipolarly and oppositely, in order to hold a wafer on the gripper by Coulomb forces of attraction which are produced by electrostatic fields. The segmented electrodes of FIGS. 6 and 7 are preferably interconnected, i.e. charged with rectified potential, this potential however being opposite the potential with which the circular electrode 3 which is located in the middle is charged.