Abstract:
A holder for semiconductor wafers in an apparatus for chemical-mechanical polishing of semiconductor wafers, having a disk-shaped head, a holding plate and a ring-shaped membrane attached to the carrier section and the holding plate which defines a pressure chamber between these components, the bores in the holding plate being connected with the pressure chamber, a contact membrane of elastomeric gas-impermeable material having a peripheral edge which is fixedly connected to a peripheral portion of the holding plate in a gas-tight manner and engages the lower side of the holding plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     BACKGROUND OF INVENTION 
     The invention relates to a holder for flat workpieces, particularly semiconductor wafers, particularly in an apparatus for chemico-mechanically polishing the semiconductor. 
     The miniaturization of semiconductor components which has steadily intensified over the recent years causes more stringent and new demands to the manufacturing process of the electronic components. Thus, the surface of the semiconductor material to be exposed during the lithographic pitting process has to be very flat (the difference in profile being less than 0.4 μm) if the structure sizes are less then 0.5 μm in order to lie within the focussing plane. To this effect, the material needs to be planarized by means of suitable devices. 
     A process serving the purpose is the Chemical Mechanical Polishing (CM). In this process which uses a polishing agent which is both corrosive and abrasive, the wafer is polished on a polishing clot in plastic at a defined contact force under a rotatory motion of the polishing cloth and the wafer. While the polishing process is under way the polishing agent (a slurry) will flow onto the polishing cloth and form a film between the clot and the wafer. The slurry which is used consists of a chemically offensive solution to which particles such as silica are added in a colloidal suspension. 
     From DE 195 44 328 or the company document “CMP Plaster Tool System Planarization Chemical Mechanical Polishing” published by the Wolters GmbH company in March, 1996, it has been known to provide appropriate stations and devices for such polishing processes. The wafers are retained by holders in processing units and are pressed by them against the polishing working surface. The holders or holding heads are connected to a spindle of a driving machine which is supported to be adjustable in height in order to press the wafer against the working surface. To obtain sufficient planarity, the lower support plate which holds the wafer via vacuum channels or bores is hinged by a universal joint to a carrier portion which, in turn, is connected to the spindle of the driving mechanism. The contact pressure is applied to the support plate via the universal joint. 
     From DE 197 55 975 A1, it further has become known to guide a support plate for the known holder in a carrier so as to be movable in height and to dispose an annularly closed membrane between the carrier portion and the support plate. The enclosed inner space of the membrane is optionally connected to the atmosphere or a vacuum or a fluid source under pressure. The pressure and vacuum help in displacing the support plate relative to the carrier. In this way, the contact pressure is applied to the support plate on a large surface, which causes an improved result to be obtained in planarization. 
     Apart from influencing other parameters such as the speed of the wafer, the speed of the polishing disk, the oscillating motions of the polishing head, the supply of polishing agent, and the condition and wear of the polishing cloths, the accuracy and uniformity which can be achieved will have an effect on the result of polishing in the CMP process. Planarized films of 300 mm wafers which are processed by CMP machines frequently present a rotationally symmetric, differentiated surface geometry which is characterized by a heavily polished wafer border, the removal of material is least at a small distance from the wafer border, i.e. 3 mm, and the largest removal of material is achieved about 20 mm from the wafer border. 
     From EP 0 922 531 A1, it has become known to use membranes of elastomeric material, which are disposed at the underside of the support plate and are pressed against the wafers by means of compressed air, for the described holders for wafers (chucks or chuck plates). This way helps obtain a better compensation of non-uniform areas. The membranes are thin-walled moulded-rubber parts to which compressed air can be applied via bores in the support plate. A holder of this type is constructed as a unit and the contact pressure acting on the wafer during the polishing process is exclusively exerted via the membrane. Apart from performing its function of transmitting the polishing pressure and the torque to the wafer, the holder also has to be capable of lifting the wafer from the polishing disk, thus overcoming adhesion between the wafer and the polishing disk. It is known to realize this operation by producing a negative pressure at the back of the wafer. 
     A disadvantage in all of the known designs is the fact that the membranes, in turn, are sucked into ring-shaped or sucker-shaped indentations to produce the vacuum required for suction in order to generate chambers having a negative pressure in this way. This causes the membrane to expand to a relatively large extent and to rapidly undergo wear, as a consequence. Moreover, the membrane has to be designed with very thin walls, the disadvantage being that a torque can be transmitted to the wafer only to a low efficiency. Membranes which are known are about 0.5 mm in thickness. 
     It is the object of the invention to improve a holder provided with a membrane to the effect that it is given a higher stability in standing and may also be employed in a more differentiated manner in order to achieve a geometry of material removal which is as uniform as possible. 
     BRIEF SUMMARY OF THE INVENTION 
     The inventive holder relies on a construction in which the support plate is suspended from the carrier portion via a ring-shaped membrane and compressed air, an atmosphere or a negative pressure can be optionally applied to the pressure chamber defined between these components. In this way, the contact pressure of the support plate on the workpiece, particularly on the wafer, can be produced by compressed air and the suspension of the support plate via a universal joint permits the support plate to soundly rest on the workpiece with no danger of chocking. As was stated above a holder of this type is known as such. According to the invention, it is provided with a contact membrane which is appropriately mounted at the border of the support plate in an air-tight way. Thus, the gap between the membrane and the support plate may be pressurized so that the axial force to press the head against the workpiece is produced, on one hand, and a pressure cushion is built up between the support plate and the membrane and provides for an appropriate yielding resilience to exist between the membrane and the workpiece, which reduces non-uniformities in the removal of material. Changing the pressure allows the contact pressure to vary, which achieves a further advantage, in that, if an atmosphere is applied to the pressure chamber the sole force of the weight exerted by the support plate can be used as a polishing force, which leads to a geometry of material removal which differs from the one if compressed air is applied to the membrane. 
     According to the invention, it is contemplated that the back of the membrane has socket-shaped lugs which are integrally formed to the membrane and extend into bores of the membrane. The socket-shaped lugs are provided with appropriate junctions for connection to a feed line which is located within the pressure chamber and is adapted to be connected, in turn, to a vacuum source. In this way, the negative pressure necessary for holding and, in particular, moving the workpiece is produced directly at the underside of the membrane with no need to deform the membrane for this purpose. Thus, it is unnecessary to expand or upset the membrane to achieve the vacuum required to move the wafer. Therefore, the membrane may be designed with relatively thick walls, e.g. having a thickness of at least 1.5 mm. 
     The invention helps in adjusting the polishing pressure differently. The force by which the head presses a wafer against the polishing cloth is composed of the weighting force of the support plate and the pressure which is produced in the pressure chamber between the support plate and the carrier portion and which also prevails between the underside of the support plate and the membrane. The differing way of applying forces causes a differing geometry of material removal. This one, for example, is not uniform at all for rigid support plates, but it has rather been shown that an intense removal of material is produced at the border of the wafer and is largely reduced at a slight distance from the border and will increase again towards the centre of the wafer. Naturally, efforts are made to obtain a geometry of material removal which is as uniform as possible. The inventive holder comes closer to this aim. 
     In an aspect of the invention, the support plate has a circular recess of a small depth which extends nearly up to the border of the support plate. Now, if the diameter of the membrane is chosen so as to coincide with the diameter of the workpiece, e.g. a wafer, the geometries of material removal which ensue therefrom are quite different, depending on whether working is done only by the weighting force of the support plate or by an extra contact pressure because of a positive pressure between the support plate and the carrier section. 
     According to an aspect of the invention, the socket-shaped lugs of the membrane are provided with junctions which, in turn, are in communication with a manifold line such as a closed-loop line. The manifold line as well as the junctions support themselves on the membrane via the socket-shaped lugs of the membrane. Thus, the manifold line “floats” in the pressure chamber. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAIWNGS 
     The invention will now be explained in more detail with reference to an embodiment shown in the drawings. 
     FIG. 1 shows a section through a holder according to the invention. 
     FIG. 2 shows a graph for material removal in polishing via the effective diameter of the holder of FIG.  1 . 
     FIG. 3 shows a detail of FIG. 1 at a larger scale. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention maybe embodied in many different forms, there are described in detail herein a specific preferred embodiment of the invention. The description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiment illustrated. 
     Referring to FIG. 1, a holder in the form of a retaining head  10  is mounted on a spindle  12  which is only shown in phantom lines. It is mounted by a bolted joint which is not referred to in detail. Mounting is done on a carrier portion  14  of the retaining head  10 , which will be described in more detail below. The spindle  12  forms part of a driving mechanism, which is not further shown, of a device for chemico-mechanically polishing the surface of a semiconductor wafer. The spindle  12  not only is rotated, but can also be adjusted in height as is described, for example, in DE 197 55 975 A1 to which explicit reference is made here. 
     The carrier portion  14  has an axial collar  16  which is joined by an inversely pot-shaped flange  18 . A ring-shaped retaining component  20  is fixed to the border of the flange  18  by means of bolts  22 . Along with the flange  18 , it pinches one end of a ring-shaped rolling membrane  24 . The retaining component  20  further has mounted thereon, in a radially more outward position in a ring-shaped recess, a hose  26  which is adapted to be connected to a pressure source, which is not shown, via a flexible line  28  and respective bores  30  in the collar  16  and the spindle  12  to optionally cause the hose  26  to expand or contract. Finally, a retaining ring  34  is suspended from the ring-shaped component  20 , i.e. via the bias of a spring  36 , by means of pins  32  which are disposed at circumferential spacings. A radially inward portion of the retaining ring  34  bears against the hose  26 . The hose  26  may help in axially moving the retaining ring  34  up and down. A ring-shaped sliding portion  38  made of a low-friction non-abrasive material is mounted at the underside of the retaining ring  34 . 
     A bell-shaped portion  40  is coaxially arranged within the flange  24  at an axial distance therefrom. A ring  42  is fixed by a bolted joint to the upper surface of the bell-shaped portion  40 . The other end of the rolling membrane  24  is pinched between the ring  42  and the bell-shaped portion  40 . As a result, an enclosed chamber  44  is formed between the carrier portion  14  and the bell-shaped portion  40 . This chamber can be optionally connected to a fluid source under pressure or a vacuum source, which is not shown herein. Thus, the fluid may serve for adjusting the bell-shaped portion  40  relative to the carrier portion  14  with downward adjustment being restricted by a pin  46  which is bolted into the flange  18  and has a head which limits the downward motion of the bell-shaped portion  40 . 
     The bell-shaped portion  40  has connected to its border a support plate  50 , i.e. via a locking ring  52  which is disposed between the radial flange of the bell-shaped portion  40  and a ring-shaped recess at the border of the support plate  50 . The locking ring pinches the upwardly and backwardly turned border of a contact membrane  53  on the support plate  50 . This is more evident from FIG.  3 . The turned-up border of the membrane  53  with the bulge on the border is indicated by  55  in FIG.  3 . The contact membrane  53  which is made of an elastomeric material and, for example, has a thickness of 1.5 mm or greater is provided with single socket-shaped lugs  57 , which are integrally formed to the membrane body, at the side facing the support plate  50 . The lugs  57 , for instance, are located on a divided circle at an appropriate distance from each other. The lugs  57  extend through bores  59  of the support plate with the diameter of the bores  59  being clearly larger than the outer diameter of the lugs  57 . A circular recess  60  which is of a relatively small depth is formed at the underside of the support plate  50 . However, the recess does not extend up to the border of the support plate  50 , but terminates at a certain distance therefrom. 
     As was mentioned already a pressure can be built up in the chamber  44  and acts on the support plate  50 , trying to displace it relative to the carrier portion  14 . This pressure also gets to the underside of the support plate  50  through the bore  59  and into the gap between the recess or the bottom of the recess  60  and the side of the contact membrane  53  that faces it. Thus, pressure is exerted on a workpiece such as a wafer via a membrane  53  which, in turn, is supported on an air cushion with the capability of the pressure cushion to transmit a pressure being dependent on the pressure prevailing in the chamber  44 . 
     Junctions  61  are connected to the socket-shaped lugs  57  by inserting a socket  63  thereof into the lugs  52  under a press fit. The individual junctions  61  are connected to line portions which define a closed-loop line  65 . The closed-loop line  65  is joined, via flexible lines, to two connections  62 ,  64  which are mounted on a sleeve  66  which is seated in a bore in the collar  16 . The sleeve  66  has a central channel  68  which is in communication with respective bores in the spindle  12 . A vacuum, a gas pressure or even water can be passed through these channels. In this way, a vacuum can be produced at the underside of the contact membrane  50  in order to move a workpiece from one processing position to another location. 
     The support plate  50 , via a cardan joint  70  which is not shown in detail, is coupled to a cylindrical component  72  which, in turn, is axially guide in a casing  74  by means of a ball-type guide which cannot be seen. The casing  74  is located in the collar  16  of the carrier portion  14 , which fact is not described in detail. This axially guides the support plate  50  in a precise way if displaced by a gaseous medium and the plate may be easily tilted in all directions. 
     Referring to FIG. 3, two pressure graphs which represent the polishing pressure in the external area of the support plate  50  are shown below the detail on the retaining head of FIG.  1 . The upper graph illustrates the case in which an atmospheric pressure exists in the chamber  44  and, thus, the assembly comprising the support plate  50  and the bell-shaped portion  40  rests on the wafer by the force exerted by its weight. Since the membrane directly bears on the underside of the support plate  50  in the external area of the support plate  50 , which is contrary to the area below the recess  60 , a polishing pressure which is somewhat larger is obtained in this area. This can be useful because there is no uniform removal of material across the diameter of the wafer as ensues from FIG.  2 . Such non-uniformity is due to the fact that the polishing pressure which is applied is not equal at all points although such equality is naturally aimed at. Therefore, if the polishing pressure is increased in the area of the graph of FIG. 2 in which otherwise a minimal removal of material is found to exist an equalization of material removal is obtained during polishing. 
     The lower graph of the contact pressure of FIG. 3 depicts the case where a positive pressure is produced in the chamber  44  and will produce a polishing pressure on the wafer that is higher altogether, but does not make itself felt in the external area to a large extent because the external area of the membrane  53  directly bears on the support plate  50 . Thus, the removal of material is smaller here, which might be desirable in certain cases and phases of polishing. 
     On the whole, the graph of FIG. 2 already shows a certain equalization of material removal by means of the tool shown as compared to the use of conventional tools. 
     The holder  10  which is shown operates as follows. A lowering motion onto a wafer, which is provided by means of the spindle  12  which is adjustable in height, causes the underside of the membrane  53  to get into engagement with the wafer surface facing it. Prior to it, the support plate  50  was shifted to the position raised at a maximum with respect to the carrier portion  14  by applying a vacuum to the chamber  44 . Shortly before or during the contact with the wafer, the vacuum source applies a vacuum to the underside of the membrane  53  in the way described. This holds the wafer on the support plate and the wafer may now be moved to a working surface, e.g. a polishing disk. Above the polishing disk (not shown), the holder  10  is lowered to a predetermined position in which the wafer is at a minimum distance from the polishing cloth of the polishing disk, but does not contact it yet. Subsequently, the chamber  44  is connected to the atmosphere or a fluid source under pressure, which action moves the support plate  50  downwards and brings the wafer into engagement with the polishing disk. As was mentioned already the force of engagement (the polishing force) is determined by the pressure in the chamber  44  or in the gap between the support plate  50  and the membrane  53  or possibly by the weighting force alone. There is no need for a vacuum during the polishing process because the wafer is secured from rotation by means of the retaining ring  34 . 
     Once the polishing operation is completed a vacuum is applied to the chamber  44  again and the hose  26  which was loaded before to press the retaining ring  34  to the polishing cloth is now relieved from load. The support plate  50  is slightly raised. The spindle  12  is moved up at the same time. The driving mechanism is moved to another position to deposit the wafer in another place. To this effect, the spindle is lowered in the new place and the wafer is released from the membrane  53  if the vacuum is removed from the underside of the membrane  53 . It is also possible to apply a pressure shock instead via the lines and the socket-shaped lug  57  which were described. 
     Finally, it is to be noted that a protective hood  100  is mounted at the upper surface of the flange  18  and protects the interior of the retaining head  10 . The hood  100  is not needed to operate the retaining head  10 . 
     The above Examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto.