Abstract:
A holding device for a vacuum unit has a lifting table transferable between a lower end position and an upper end position for moving a support plate toward the underside of a disc-shaped workpiece. The workpiece is supported by a support ring carrying centring pins spaced to form an equilateral triangle, in the center of which the axis of the support plate is located. A clamping ring clamps the workpiece with the support ring. The centring pins engage the clamping ring to center the support ring and clamping ring relative to the support plate axis so they are not affected by thermal expansion.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a holding device as used in vacuum units. It is used, for example in an intermediate chamber after transfer of a disc-shaped workpiece from a transport mechanism in a transfer position in which a lifting table thereof is, for example, in a lower end position, for lifting said workpiece into a processing position by virtue of the fact that the lifting table performs a lifting movement to an upper end position. In the processing position, the top of the workpiece is present in a process chamber and can be subjected to a processing step. After processing is complete, the lifting table lowers the workpiece again to the transfer position, where it can once again be picked up by the transport mechanism. A corresponding vacuum unit is described in EP-A-0 343 530. 
     In the case of holding devices of this type, it is desired that the support ring on which the workpiece is placed in the transport position is centred as accurately as possible relative to the support plate on which said workpiece rests in the processing position and by means of which it is heated or cooled. Consequently, the recesses at the edge of the support plate, which are necessary for supporting the support ring, can be kept small, and the processing of the workpiece, such as coating, heating or cooling or any other action of the support plate on the workpiece, takes place in a uniform manner. 
     A known holding device of the generic type is described in EP-A-0 452 779. There, the support ring is in the form of a ring directly surrounding the support plate. Since a specific radial distance must be provided between the two because of any different thermal expansions, exact centring of the support ring with respect to the axis of the support plate is not provided for in this design. Since the clamping ring is centred with respect to the support ring, it too is inevitably inaccurately centred with respect to the support plate. 
     SUMMARY OF THE INVENTION 
     The invention is intended to provide a remedy here. The invention, as characterized in the Claims, provides a holding device in which support plate, support ring and clamping ring are centred with respect to a common axis through a common centring device which engages all of said parts and whose function is not impaired by different thermal expansions of said parts, and which does not hinder necessary mutual axial displacements thereof. 
     A fundamental principle of operation of the proposed corresponding centring insensitive to thermal expansion is described for holding a lens in RU-C-2 047 197, where radial lamellae anchored to a housing engage corresponding radial grooves in the lens mount. In the case of this holder, however, no mutual axial displacement of the parts is envisaged. 
     The advantages achieved by the invention are in particular that concentric positioning of the disc-shaped workpiece on the support plate can be ensured and, owing to the accurate centring of the clamping ring with respect to the support plate and hence of the workpiece, the edge waste is very small. The holding device is particularly suitable for workpieces such as storage discs, for example hard discs, magnetooptical storage discs, CDs and especially semiconductor wafers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail below with reference to Figures which show only an exemplary embodiment. 
     FIG. 1 shows a perspective exploded diagram of a cut-out sector of the holding device according to the invention and 
     FIG. 2 shows, on the right, a first radial section through a holding device according to the invention, in a lower end position in which the workpiece can be transferred and, on the left, a corresponding second radial section through the same holding device according to the invention, in the upper end position in which the workpiece can be processed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The holding device comprises (cf. especially FIG. 2, right) a lifting table  1  which is formed to be essentially rotationally symmetrical about an axis  2  and has a double support tube  3  which can be raised and lowered parallel to the axis  2  by a corresponding device not shown and whose inner tube  3   a  carries a baseplate  4  on which a support plate  5  is fastened, while the outer tube  3   b  carries a ring  6  having a driver collar  7  on its outer edge, which ring surrounds the baseplate  4  concentrically a distance away. The underside of the ring  6  is connected gas-tight, by means of a metal bellows  8  surrounding the double support tube  3 , to the edge of an opening in a fixed wall  9  through which said double support tube is passed. Depending on the vacuum chamber for which the device is intended, the support plate  5  is heatable or coolable or both. Its shape depends on that of the workpiece to be processed. Usually, it will be round as shown, but other shapes are also possible; thus, for example, it may be rectangular. A pipe  10  through which gas can be passed in opens in its centre. 
     Centring pins  12 , preferably comprising nonconducting, in particular ceramic material, are supported on the wall  9 , i.e. in a fixed location, in the lower end position by means of three supports  11  whose positions form an equilateral triangle, in the centre of which is the axis  2 . The support  11  is in each case passed through a hole in the ring  6  and projects into a hole in the baseplate  4  which hole is located above and is provided with a guide bush (not shown), the internal diameter of the hole being substantially greater than the diameter of the support  11  at the corresponding height. On the other hand, the centring pin  12  supported on the support  11  has a greater diameter corresponding approximately to the internal diameter of the last-mentioned hole or, more precisely, of the guide bush, so that it is guided axially displaceably therein but otherwise essentially without play. Fastened at slightly more than half height on the centring pin  12  is a stop ring  13  which surrounds said centring pin and whose underside forms a stop limiting the axial displaceability of the centring pin  12  relative to the baseplate  4 . 
     A support ring  14  rests on the tops of the stop rings  13  and is provided there in each case with a groove which is radial, i.e. directed towards the centre of the ring, and which partly holds the stop ring  13  whose thickness is slightly greater than the depth of the groove while the width of the latter is greater than its diameter. However, it continues upwards in a radially oriented slot  15  through which the centring pin  12  is passed azimuthally essentially without play, so that the support ring  14  is fixed by the three centring pins  12  essentially nonrotatably and, transversely to the axis  2 , nondisplaceably. On the other hand, limited radial movement is possible in each case between the individual centring pin  12  and support ring  14 . 
     The support ring  14 , which surrounds the support plate  5  at a slight distance, has, in the azimuthal positions of the centring pins  12 , on its inner edge, three supports  16  which point upward and are provided with recesses in the insides for securely holding a workpiece  17 , which may be, for example, a wafer. The supports  16  lie in corresponding grooves  18  (FIG. 1) which are let into the outer edge of the support plate  5 . However, the supports  16  are also a slight radial distance away therefrom. The surface of the support plate  5  is only very slightly reduced in size by the grooves  18 . It is also possible to provide more or differently arranged supports or a single support which surrounds the support plate continuously or in segments, for example in the form of a horseshoe. 
     Vertical pins  19  on which an intermediate ring  20  is supported are fastened to the wall  9  outside the centring pins  12  but in the same azimuthal positions. Said intermediate ring has, in the underside, grooves  21  which receive the tips of the vertical pins  19 . The intermediate ring  20  has an approximately L-shaped cross-section with an inward-pointing flange which, in its rest position, supports a clamping ring  22  which projects inwards substantially beyond the intermediate ring  20  and likewise has there, in the underside, radial grooves  23  which are suitable for receiving the upper end regions of the centring pins  12 , and likewise do so in such a way that in each case radial movement is possible but transversely thereto, azimuthally, the end regions engage the grooves  23  essentially without play. On the inside, the clamping ring  22  has an inner flange  24  which is offset slightly upwards and, with its edge bent slightly downwards and following the shape of the workpiece, for example of a circular shape as shown, overlaps the supports  16 . Instead of a closed edge, however, the inner flange may also have individual fingers. On its outer ring surrounding the flange, the intermediate ring  20  has a protective profile  25  whose inward-pointing flange covers the top of the clamping ring  22 , with the exception of the inner flange  24 . 
     In the lower end position of the holding device, shown on the right in FIG. 2, the workpiece  17  can be placed centred relative to the support plate  5  on the supports  16  of the support ring  14  by means of a robot loader  26 . The accuracy of positioning is, for example, ±0.13 mm. The workpiece  17  is then centred with corresponding accuracy relative to the support plate  5 . After retraction of the robot loader  26 , the lifting movement of the lifting table  1  can begin. First, the support plate  5  touches the underside of the workpiece  17  and lifts it from the supports  16  of the support ring  14 . Thereafter, the baseplate  4  reaches the stop formed by the undersides of the support rings  13 , and the centring pins  12  are carried along during the further lifting movement. Since, in its rest position, the support ring  14  is supported on the stop rings  13 , it is also lifted from the beginning. The stop between the baseplate and the stop rings can also be adjusted so that the support ring is already carried along when the support plate has reached the workpiece or has approached the latter up to an envisaged minimum distance. 
     In the course of the further lifting movement, the centring pins  12  enter the grooves  23  in the underside of the clamping ring  22  and do so until their upper end regions have been received therein azimuthally virtually without play but so as to be radially movable. Thus, the clamping ring  22 , too, is centred with respect to the support plate  5 . Shortly before their tips each reach the base of the corresponding groove  23 , the surface of the workpiece  17  comes up against the edge of the inner flange  24  of the clamping ring  22 , so that the latter is raised from its rest position and, resting on the workpiece  17 , fixes the latter in its position relative to the supports  16  and the support plate  5 . Shortly afterwards, the driver collar  7  reaches the underside of the intermediate ring  20 , whereupon this too is carried along during the further lifting movement. Friction processes during introduction of the workpiece  17  occur virtually only underneath said workpiece, so that the danger of soiling by particles is very greatly reduced. 
     In the upper end position (FIG. 2, left) of the holding device, the workpiece  17  has now reached its processing position in which its surface can be covered with a layer of, for example, aluminium or silica by, for example, sputtering or can be etched or otherwise treated. Its underside can be heated or cooled by means of the support plate  5 , gas being introduced through the pipe  10  and producing a gas cushion between the support plate  5  and the underside of the workpiece  17 , which gas cushion ensures uniform heat transmission. 
     The lifting table  1  with the support plate  5 , the support ring  14  with the supports  16 , the clamping ring  22  and the intermediate ring  20  with the support profile  25  are preferably all produced from electrically conducting material, in particular metal. The centring pins  12  on the other hand, by means of which these parts are kept a distance apart from one another consist, like the guide bushes (not shown) in the baseplate  4 , preferably of insulating, in particular ceramic material, so that the above-mentioned parts are all electrically isolated from one another and can be kept at different potentials. This can be used in various processing operations for controlling the electric field in the process chamber. Moreover, ceramic material has a low coefficient of sliding friction, so that its use for guide bushes or centring pins or both moreover has the advantage that they make little contribution to particle formation. Its low coefficient of thermal expansion furthermore ensures substantial dimensional stability also when the device is under thermal load. Owing to the substantially rotationally symmetric design of the holding device and the coaxial conductor structure, the danger of uncontrolled discharge of currents, for example in RF bias applications, and of interfering fields on measuring and control units is greatly reduced. 
     After the processing operation, the holding device is brought back to the lower end position. The workpiece  17  is once again in the transfer position and can be picked up by the robot loader  26  and withdrawn. 
     The holding device has only one drive axis since all movements are effected by raising and lowering the lifting table  1  alone. It is thus very simple mechanically and in terms of control technology and requires little maintenance. Short loading times are achievable, which increases throughput. Many parts are simply placed one on top of the other and not further fixed, so that they can be easily and rapidly changed if required. These properties of the holding device also have an advantageous effect on the availability of the vacuum unit. 
     Since large temperature differences often occur during the processing operations, it is of considerable importance that, in the device according to the invention, the mutual centring of the support plate  5 , of the support ring  14  and of the clamping ring  22  is retained even in the case of different thermal expansions of these parts. This is achieved if the centring pins  12  each have sufficient play radially relative to the support ring  14  and the clamping ring  22  but engage all stated parts azimuthally essentially without play. In the case of a thermal expansion of the baseplate  4 , which, owing to the sufficient lateral play of the supports  11 , is not hindered by said supports, the centring pins  12  are radially offset, which however, owing to the radial extension of the grooves and of the slots  15  in the support ring  14  and of the grooves  23  in the clamping ring  22 , does not lead to difficulties there, similarly to thermal expansions of said rings. The contact points between one of the rings and the centring pins  12  are displaced by different thermal expansions on the ring, while the position of its centre does not change. In a manner which is basically similar to the support ring  14  and the clamping ring  22  with respect to the axis  2 , the intermediate ring  20  with the protective profile  25  is centred with respect to a fixed axis which coincides with the axis  2 . 
     In principle, the support plate, the support ring and the clamping ring could also be centred by only two centring elements in a manner not impaired by different thermal expansions, if the planes determined by said elements and the axis make an angle ≠180° there and said parts are secured to prevent mutual rotation about a vertical axis, for example by forming one of the centring elements as a radially oriented plate, but three centring elements, as described, simultaneously offer a support for the support ring  14 , which support accurately determines the position of said support ring. It is therefore advantageous to provide at least three centring elements which as far as possible are also equal distances from the axis, although the latter is not absolutely necessary. Preferably, their positions form a regular polygon, in particular triangle, in the centre of which the axis is present. It would also be possible, according to the same principle, to centre said concentric parts in pairs, for example the support ring relative to the support plate and the clamping ring relative to the support ring, but this would not lead to such a compact, robust and simple design.