Patent Publication Number: US-6709323-B2

Title: Holder for flat workpieces, particularly semiconductor wafers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a holder for flat workpieces, particularly semiconductor wafers. 
     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 printing 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 requires to be planarized by means of suitable devices. 
     A process serving the purpose is the chemico-mechanical polishing method (briefly called CMP). In this process which uses a polishing agent which is both corrosive and abrasive, the wafer is polished on a polishing cloth 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 cloth 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 in that the wafer border is heavily polished, 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 in a range of abt. 20 mm from the wafer border. 
     It is the object of the invention to provide a holder for flat workpieces, particularly semiconductor wafers, in which the non-uniformity of the remaining film thickness is reduced. 
     BRIEF SUMMARY OF THE INVENTION 
     The inventive holder provides a ring-shaped loading member of limited width which is supported to be movable parallel to the axis in the support plate near the border and is displaceable by a loading device towards a workpiece retained by the support plate and away from the device to exert a predetermined pressure on the workpiece. 
     The ring-shaped loading member which is brought very close to the border of the support plate, e.g. to a distance of about 3 mm, and which only extends over a limited width, e.g. from 5 to 10 mm, helps in generating a separate extra pressure if a pressure is exerted on the workpiece, particularly the wafer, by means of the support plate. Such a measure allows to equalize the removal of material across the overall area of the workpiece, particularly the wafer, to a larger extent. 
     According to an aspect of the invention, restoring means are provided which displace the loading member in a direction away from the contact surface of the support plate if the loading mechanism is turned off. This ensures that if the workpiece is received by means of a vacuum in order to make the workpiece bear on the support the loading member does not interfere therewith. 
     Various possible ways are imaginable to form a loading member and to actuate it. For an actuation, it is preferred that a pneumatic pressure be employed all the more so as it is known and advantageous to produce a contact pressure with the support plate via a fluid pressure. It is particularly advantageous to use a ring-shaped hose which is accommodated in a ring-shaped recess of the support plate. The hose, which is preferably elongate in cross-section, may be expanded by means of a gaseous medium and, thus, can exert a pressure on the workpiece. If the material of the hose yields resiliently the hose may be restored automatically if it is relieved from pressure. 
     The fluid pressure in the hose-shaped loading member is preferably controlled via a proportional-pressure control valve. This allows to apply a finely proportioned pressure in a purposeful way. 
     The invention has the advantage that it may be installed in conventional holders. Thus, for example, it is unnecessary to continue employing the retaining ring, which is normally used and which bears against the polishing cloth, in the form which is known. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     An embodiment of the invention will now be explained in more detail with reference to the drawings. 
     FIG. 1 shows a section through a holder according to the invention. 
     FIG. 2 shows a graph of the remaining thickness of a wafer layer which has been polished by means of a holder of FIG.  1 . 
     FIG. 3 shows a detail of FIG. 1 at a larger scale. 
     FIG. 4 shows an enlarged and simplified view of FIG.  1 . 
    
    
     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. 
     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 inversely pot-shaped flange  18  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 lower 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 adjustment to the bottom 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 . 
     A support plate  50  is bolted to the bell-shaped portion  40  at the border as is shown at  52 , for example. The support plate  50  is provided with a plurality of radial bores  54  which are upwardly connected to axially parallel bores  56  with junctions  58 ,  60  which are joined to two junctions  62 ,  64  via flexible lines. The junctions  62 ,  64  are mounted on a sleeve  66  which is accommodated in a bore in the collar  16  and has a central channel  68  which is connected to respective bores in the spindle. A vacuum, a gas pressure or even water may be optionally passed through these channels. The cross-bores  54  are joined to nozzle-like bores  62   a  in the support plate  50  which lead to the lower planar area of the support plate  50 . The bores  62   a  are disposed according to a predetermined pattern and serve for retaining a wafer on the plate  50  by means of a vacuum. A polishing cloth  64   a  which has holes according to the same pattern as that of the support plate  50  is fixed below the support plate  50  by means of a backing film. 
     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 guided 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 to all directions. 
     FIG. 4 is an enlarged and simplified view of FIG.  1 . FIG. 4 also shows the ring-shaped recess  102  (discussed below in connection with FIG.  3 ). FIG. 4 also shows a polishing disk  104  which is used to polish wafer  94  held to the underside of support plate  50  by the vacuum source. 
     The components described and their functions have generally become known already from DE 197 55 975 A1 which was repeatedly mentioned. A particular feature ensues from FIG.  3 . 
     It is apparent from FIG. 3 that the circumference of the support plate  50  has mounted, in a recess thereof, a ring-shaped component  80  which is fixedly connected to the support plate  50  by means of bolts such as the bolt  52 . The ring-shaped component  80  has a ring-shaped recess  102  which faces downwards and which receives a ring-shaped circumferential membrane  82  or a ring-shaped circumferential hose of an elongate cross-section with the largest extension being parallel to the axis of the holder  10 . The ends of the membrane are located in the recess by means of a ring  84  which is pinched between the ring-shaped component  80  and the respective part of the support plate  50 . The inner space of the membrane  82  is in communication, via a line  86 , with a proportional control valve  92 . An appropriate pressure in the membrane  82  causes the membrane to expand downwardly, thus exerting a pressure on the polishing cloth  64   a  and, hence, on a wafer which is shown at  94  in FIG.  3 . Since the material of the membrane  82  is resilient it will automatically be restored once the space in the membrane  82  is relieved from pressure. The membrane is designed so as not to project beyond the underside of the support plate  50  when in a state relieved from pressure. 
     FIG. 3 also illustrates the pressure distribution which can be applied to the wafer  94  by means of the support plate  50 . It can be seen that the pressure is evenly distributed outside the area of the membrane  82 . However, there is a pressure intensification at  96  in the area of the membrane  82 . This compensates the smaller removal of material which is encountered close to the border of the wafer or the support plate  50 . 
     Such a pressure distribution ensues from the graph of FIG.  2 . As is apparent the wafer border undergoes more intense polishing while the least removal of material is achieved at a distance from the wafer border, e.g. 3 mm, and the largest removal of material is attained at a distance of 20 mm. Therefore, a differing removal of material is still achieved by means of the tool in FIG.  2 . Nevertheless, a significant improvement is attained over the previous operations using conventional tools. 
     It is understood that the junction  88  needs to be joined to a respective connection on the carrier component  14  via an appropriate line in the space between the bell-shaped portion  40  and the support plate  50  in order that an appropriate pressure may be set up in the membrane  82  as was described. 
     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 retaining plate  34  or the polishing cloth  64   a  to get into engagement with the wafer surface facing it. Prior to it, the support plate  50  was shifted to the position raised to a maximum with respect to the carrier component  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 bores  62   a  in the way described. This holds the wafer on the support plate  50  and the wafer may now be moved to a working surface, e.g. a polishing disk. Above the polishing disk, the holder  10  is lowered up 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, pressure is applied to the chamber  44 , which action causes the support plate  50  to move downwards and to bring the wafer into engagement with the polishing disk. The force of engagement (the polishing force) is determined by the pressure in the chamber  44 . Subsequently, the head or holder  10  are caused to rotate and the polishing operation starts. The vacuum is maintained at the bores  62   a  during the polishing process. Moreover, a predetermined pressure is set up in the membrane  82  via the proportional control valve  92 , which membrane provides for an additional contact force in the area of the membrane  82  as can be seen with reference to FIG.  3 . This equalizes the removal of material over the entire area of the wafer. 
     Once the polishing operation is completed a vacuum is applied to the chamber  44  again and the membrane  82  is 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 retaining plate  50  if the vacuum is removed from the bores  62   a  and a short shock or the like is applied. It is also possible to convey water to the underside of the retaining plate through the bores  62   a  in order to effect cleaning. 
     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 holder  10 . It is not needed for the operation of 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 the 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.