Abstract:
An apparatus for performing a global planarization of a surface of a deformable layer of a wafer on a production scale. The apparatus includes a chamber having a pressing surface and containing a rigid plate and a flexible pressing member or “puck” disposed between the rigid plate and the pressing surface. A wafer having a deformable outermost layer is placed on the flexible pressing member so the deformable layer of the wafer is directly opposite and substantially parallel to the pressing surface. Force is applied to the rigid plate which propagates through the flexible pressing member to press the deformable layer of the wafer against the pressing surface. Preferably, a bellows arrangement is used to ensure a uniformly applied force to the rigid plate. The flexible puck serves to provide a self adjusting mode of uniformly distributing the applied force to the wafer, ensuring the formation of a high quality planar surface. The surface of the wafer assumes the shape of the pressing surface and is hardened in a suitable manner while under pressure to produce a globally planarized surface on the wafer. After the force is removed from the rigid plate, lift pins are slidably inserted through the rigid plate and the flexible pressing member to lift the wafer off of the surface of the flexible pressing member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/539,094, filed Mar. 30, 2000, pending, which is a continuation of application Ser. No. 09/287,502, filed Apr. 7, 1999, now U.S. Pat. No. 6,062,133, issued May 16, 2000, which is a continuation of application Ser. No. 08/761,630, filed Dec. 6, 1996, now U.S. Pat. No. 5,967,030, issued Oct. 19, 1999, which is a divisional of application Ser. No. 08/560,552, filed Nov. 17, 1995, abandoned.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to creating planar surfaces on a substrate. More particularly, the present invention relates to global planarization methods and apparatuses designed to produce a microscopically smooth surface on a semiconductor wafer.  
           [0004]    2. Background of Related Art  
           [0005]    Integrated circuits are typically constructed by depositing layers of predetermined materials to form the circuit components on a wafer shaped semiconductor substrate. The formation of the circuit components in each layer produces a rough, or planar topography on the surface of the wafer. The resulting nonplanar surface must be made smooth and planar to provide a proper surface for the formation of subsequent layers of the integrated circuitry. Planarization of the outermost surface of the wafer is performed locally over small regions of the wafers and globally over the entire surface. Typically, a layer of oxide is deposited over the exposed circuit layer to provide an insulating layer for the circuit and to locally planarize regions. A thicker layer is then deposited on top of the insulating layer to provide a surface that can be globally planarized without damaging the deposited circuitry. The thick outer layer is generally composed of an oxide or a polymer material. Spin coating is a commonly used technique to form the thick polymer layers on a wafer. Thick oxide layers can be deposited using conventional deposition techniques. While those techniques are useful in producing uniform thickness layers, neither technique is particularly effective at producing a planar surface when applied to a nonplanar surface. As such, additional surface preparation is generally required prior to forming additional circuit layers on the wafer.  
           [0006]    Conventional methods for globally planarizing the outermost surface of the wafer include chemical etching and chemical mechanical polishing (CMP) of the surface. In chemical etching, a thick layer is produced over the circuit layer as described above and the thick layer is chemically etched back to planarize the surface. Global planarization by this technique is iterative in that following the etching step, if the surface was not sufficiently smooth, a new layer of polymer or oxide must be formed and subsequently etched back. This process is time consuming, lacks predictability due to the iterative procedure for obtaining a planarized surface and consumes significant amounts of oxides and/or polymers in the process.  
           [0007]    In the CMP technique, a reactive chemical slurry is used in conjunction with a polishing pad to planarize the surface of the wafer. Two problems associated with the CMP techniques are that the chemicals may become unevenly distributed in the pad, and particulates removed from the substrate during the polishing process may become lodged in the pad, both of which result in nonuniformity in the substrate surface. As a result, CMP techniques are generally less desirable since the process is often time consuming, exposes the wafers to aggressive chemicals and may not yield the desired results in terms of final surface quality.  
           [0008]    An alternative to the above techniques is the use of a press planarization technique to globally planarize the surface of the wafer. In global press planarization, a deformable layer is deposited on the surface of the wafer containing the circuit components by conventional processes known in the art, such as by spin coating. The surface of the deformable layer, which is usually an uncured polymer, is pressed against a surface having surface characteristics which are desired for the surface of the wafer. The deformable layer is typically then cured while under pressure to harden the deformable layer to produce a planarized outermost surface of the desired surface quality.  
           [0009]    Apparatuses used to perform the global press planarization are known in the art, such as those disclosed in U.S. Pat. No. 5,434,107 to Paranjpe. A problem with those global planarization apparatuses is encountered due to the need to apply a uniform force to the deformable layers while providing an apparatus to be used in production scale operations. For instance, the pressing surfaces of such apparatuses contain holes to allow loading fingers to pass through the surface and lift the wafer; these holes will invariably lead to nonuniform pressure distributions across the surface of the wafer and in the surface of the deformable layer. Additionally, the force used to planarize is applied directly to the surface of the wafer; therefore, any nonuniformities in the application of the force will be directly propagated to the surface layer resulting in less than optimal surface characteristics. The Paranjpe patent suggests a possible solution to the potential direct application of a nonuniform force through the use of direct fluid contact with the wafer and the application of the planarizing force to the wafer by pressurizing the fluid. However, the use of pressurized fluid contact results in substantial complications involved with handling pressurized fluid, as well as exposing the wafer to the fluid and the necessary addition of drying steps to the process. The aforementioned difficulties result in increased throughput time, require precise production controls and a higher potential for damage to the wafers during processing.  
           [0010]    It is therefore an object of the present invention to provide a method and an apparatus for global process planarization of the surface layer of a semiconductor wafer that is conducive to automated handling and provides for a uniform distribution of force to planarize the surface.  
         SUMMARY OF THE INVENTION  
         [0011]    The above objects and others are accomplished by a global planarization method and apparatus in accordance with the present invention. The apparatus includes a chamber having a pressing surface and containing a rigid plate and a flexible pressing member or “puck” disposed between the rigid plate and the pressing surface. A semiconductor wafer having a deformable outermost layer is placed on the flexible pressing member so the surface of the deformable layer of the wafer is directly opposite and parallel to the pressing surface. Force is applied to the rigid plate which propagates through the flexible pressing member to press the surface of the wafer against the pressing surface. Preferably, a bellows arrangement is used to further ensure a uniformly applied force to the rigid plate. The flexible puck serves to provide a self adjusting mode of uniformly distributing the applied force to the wafer ensuring the formation of a high quality planar surface. The surface of the wafer assumes the shape of the pressing surface and is cured in a suitable manner while under pressure so that the surface of the wafer maintains the shape of the pressing surface after processing to produce a globally planarized surface on the wafer. After the force is removed from the rigid plate, lift pins are slidably inserted through the rigid plate and the flexible pressing member to lift the wafer off the surface of the flexible pressing member.  
           [0012]    Accordingly, the present invention provides an effective solution to problems associated with planarizing the surfaces of semiconductor wafers on a production scale. These advantages and others will become apparent from the following detailed description of the present invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The preferred embodiment of the present invention will be described in greater detail with reference to the accompanying drawings, wherein like members bear like reference numerals and wherein:  
         [0014]    [0014]FIG. 1 is a side view of a preferred embodiment of the present invention in a first position; and  
         [0015]    [0015]FIG. 2 is a side view of a preferred embodiment of the present invention in a second position.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The operation of the global planarization apparatus  10  will be described generally with reference to the drawings for the purpose of illustrating presently preferred embodiments of the invention only and not for purposes of limiting the same. The global planarization apparatus  10  of the present invention serves to press the surface of a semiconductor wafer  20  having multiple layers including a deformable outermost layer  22  against a fixed pressing surface  32 . The surface of deformable layer  22  will assume the shape and surface characteristics of the pressing surface  32  under the application of a force to the wafer  20 . The deformable layer  22  can then be cured in a suitable manner while pressed against the pressing surface  32  so that the surface of the wafer maintains the surface characteristics corresponding to the pressing surface  32 . FIGS. 1 and 2 show one embodiment of the global planarization apparatus  10  in the rest and pressing modes, respectively. While preferred embodiments of the invention will be discussed with respect to producing a globally planarized highly smooth surface, one skilled in the art will appreciate that the invention can be suitably modified to produce a curved or a textured surface on the wafer  20 .  
         [0017]    In a preferred embodiment, the global planarization apparatus  10  includes a fully enclosed apparatus having a hollow cylindrical chamber body  12  formed from a rigid material, such as aluminum, other metals or hard composites, and having open top and bottom ends,  13  and  14 , respectively, an interior surface  16  and an evacuation port  11 . A base plate  18  having an inner surface  17  is attached to the bottom end  14  of chamber body  12 , by conventional means, such as bolts  94  shown in FIGS. 1 and 2. A press plate  30  is removably mounted to the top end  13  of chamber body  12  with pressing surface  32  facing base plate  18 . The interior surface  16  of chamber body  12 , the pressing surface  32  of press plate  30  and the inner surface  17  of base plate  18  define a sealable chamber. It will be appreciated that evacuation port  11  can be positioned through any surface defining the sealed chamber but not used to engage wafer  20 , such as through base plate  18 , and not solely through chamber body  12 .  
         [0018]    The press plate  30  has a pressing surface  32  with dimensions greater than that of the wafers  20  and is of a sufficient thickness to withstand applied pressures. Press plate  30  is formed from non-adhering material capable of being highly polished, preferably with surface variations less than 500 Angstroms, so that pressing surface  32  will impart the desired smooth and flat surface quality to the surface of the deformable layer  22  on wafer  20 . In a preferred embodiment, the press plate  30  is a disc shaped quartz optical flat. However, material selection for the press plate  30  can be specifically tailored to meet process requirements by considering factors such as the range of applied pressures and the method of hardening the deformable layer, such as heat or radiation (UV, IR, etc.), as well as whether the surface of deformable layer  22  of the wafer  20  will be planar, curved or textured.  
         [0019]    A rigid plate  50  having top and bottom surfaces  52  and  54 , respectively, and lift pin penetrations  56  therethrough is disposed within chamber body  12  with the top surface  52  substantially parallel to and facing the pressing surface  32 . In the case where the surface of wafer  20  is to be curved, the term parallel is understood to mean that all points of the top surface  52  of rigid plate  50  are equidistant from the corresponding points on pressing surface  32 . The rigid plate  50  is constructed from a material of sufficient rigidity, such as aluminum, to transfer a load under an applied force with minimal deformation.  
         [0020]    In a preferred embodiment, a uniform force is applied to the bottom surface  54  of rigid plate  50  through the use of a bellows arrangement  40  and relatively pressurized gas to drive rigid plate  50  toward pressing surface  32 . Such relative pressure can be achieved by supplying gas under pressure or, if the chamber body  12  is under vacuum, allowing atmospheric pressure gas into bellows  40 . The bellows  40  is attached at one end to the bottom surface  54  of rigid plate  50  and to the inner surface  17  of base plate  18  with a bolted mounting plate  15  to form a pressure containment that is relatively pressurized through port  19  in base plate  18 . As shown in FIG. 2, when the bellows  40  is relatively pressurized, a force will be applied on the bottom surface  54  driving the rigid plate  50  toward the pressing surface  32 . As shown in FIG. 1, one or more stand off brackets  42  are mounted to the inner surface  17  of the base plate  18  to limit the motion toward base plate  18  of the rigid plate  50 , when the bellows  40  is not relatively pressurized. The application of force through the use of a relatively pressurized gas ensures the uniform application of force to the bottom surface  54  of rigid plate  50 . The use of rigid plate  50  will serve to propagate the uniform pressure field with minimal distortion. Alternatively, the bellows  40  can be replaced with any other suitable means for consistently delivering a uniform force such as hydraulic and pneumatic linear drives or mechanical or electrical linear displacement mechanisms.  
         [0021]    In a preferred embodiment, a flexible pressing member or “puck”  60  is provided having upper and lower surfaces  62  and  64 , respectively, which are substantially parallel to the top surface  52  of rigid plate  50  and pressing surface  32 . Lift pin penetrations  66  are provided through the puck  60 . The flexible puck  60  is positioned with its lower surface  64  in contact with the top surface  52  of rigid plate  50  and lift pin penetrations  66  aligned with lift pin penetrations  56  in rigid plate  50 . The upper surface  62  of the flexible puck  60  is directly opposite and substantially parallel to the pressing surface  32  of press plate  30 . The flexible puck  60  is formed from a material, such as  30  durometer silicone or other materials of similar low viscosity, that will deform under an applied force to close lift pin penetrations  66  and uniformly distribute the applied force to the wafer, even when the top surface  52 , the upper surface  62  and/or the lower surface  64  is not completely parallel to the pressing surface  32  or when thickness variations exist in the wafer  20 , rigid plate  50  or puck  60 , as well as other sources that result in nonuniformities in the applied force. It is also preferred that puck  60  is formed from a material that is thermally resistant in the temperature ranges of interest.  
         [0022]    In a preferred embodiment, lift pins  70  are slidably disposable through lift pin penetrations,  56  and  66 , respectively, in the form of apertures, to contact the bottom surface  26  of wafer  20  for lifting the wafer  20  off of the top surface  62  of flexible puck  60 . Movement of the lift pins  70  is controlled by a lift pin drive assembly  72 , which is mounted on the inner surface  17  of the base plate  18 . The lift pin drive assembly  72  can provide for either manual or automatic control of the lift pins  70  through the use of pneumatic, hydraulic or other conventional drive means as is known in the art. Lift pins  70  and lift pin drive assembly  72  are preferably positioned outside of the pressure boundary defined by the bellows  40  to minimize the number of pressure boundary penetrations. However, the lift pin  70  and lift pin drive assembly  72  can alternatively be located within the pressure boundary when used in conjunction with vacuum seals in the lift pin penetrations  56 , as is known in the art to maintain the pressure boundary.  
         [0023]    In a preferred embodiment, a multi-piece assembly consisting of lower lid  80 , middle lid  82 , top lid  84 , gasket  86  and top clamp ring  88  is used to secure the press plate  30  to the top end  13  of chamber body  12 . The ring-shaped lower lid  80  is mounted to the top end  13  of chamber body  12  and has a portion with an inner ring dimension smaller than press plate  30 , so that press plate  30  can be seated on lower lid  80  as shown in FIGS. 1 and 2. Middle lid  82  and top lid  84  are ring-shaped members of an inner ring dimension greater than press plate  30  and are disposed around press plate  30 . Middle lid  82  is affixed between lower lid  80  and top lid  84 . A gasket  86  and top clamp ring  88  are ring-shaped members with an inner ring dimension less than press plate  30  and are seated on the surface of press plate  30  external to the chamber. Conventional means, such as bolts  94  shown in FIGS. 1 and 2, are used to secure the press plate  30  to the chamber body  12 . While a multi-piece assembly is used to secure press plate  30 , one skilled in the art will appreciate that other suitable attachment designs are possible, including providing access to the interior chamber through any surface defining the chamber not used to engage wafer  20 .  
         [0024]    In a preferred embodiment, heating elements  90  and thermocouples  92  are provided to control the temperature of the flexible puck  60 . However, it can be appreciated that additional heating elements  90  and thermocouples  92  can be added to the press plate  30  and/or to rigid plate  50 . In a preferred embodiment, any conventional means, such as a vacuum pump, for evacuating the chamber body  12  prior to pressing the wafer  20  against the pressing surface  32  can be used with the present invention.  
         [0025]    In the operation of the present invention, the top clamp ring  88 , gasket  86 , upper lid  84  and middle lid  82  are removed from the chamber body  12  and the press plate  30  is lifted from lower lid  80 . At this stage, the bellows  40  is deflated and rigid plate  50  is seated on stand off brackets  42 . The wafer  20  is then placed on the flexible puck  60  with the side of the wafer  20  opposite the deformable layer  22  in contact with flexible puck  60 . Thereafter, the press plate  30  is returned to its position on the lower lid  80 , and the middle lid  82  and upper lid  84  are reinstalled and tightened down using gasket  86  and top clamp ring  88  thereby sealing press plate  30  between top clamp ring  88  and lower lid  80 . If desirable, the temperatures of flexible puck  60 , press plate  30  and rigid plate  50  can be adjusted through the use of heating elements  90  and monitored by thermocouples  92  to vary the deformation characteristics of the outermost deformable layer  22  of wafer  20 . Preferably, chamber body  12  is then evacuated through port  19  to a pressure of approximately 50 millitorr.  
         [0026]    A pressure differential is established between the interior and exterior of the bellows  40 , either by pressurizing or by venting when the chamber body  12  has been evacuated, to drive rigid plate  50 , puck  60  and wafer  20  toward press plate  30  and bring deformable layer  22  of wafer  20  into engagement with pressing surface  32  of press plate  30 . Upon engagement of the wafer  20  with the press plate  30 , the continued application of force will deform the flexible pressing member  60  which serves to close lift pin penetrations  66  and to distribute the force to ensure the wafer  20  experiences a uniform pressure on its deformable layer  22 . After the wafer  20  has been in engagement with pressing surface  32  for a sufficient time to cause its deformable layer  22  to correspond to the pressing surface  32 , the deformable layer  22  may be cured, if necessary, in any conventional manner, such as radiation or heat, so that the deformable layer  22  of the wafer  20  maintains the shape and surface characteristics corresponding to the pressing surface  32 . The air pressure is then released from the bellows  40  thereby retracting wafer  20 , puck  60  and rigid plate  50  from the press plate  30 . The downward movement of rigid plate  50  will be terminated by its engagement with stand off offset brackets  42 .  
         [0027]    Once the rigid plate  50  is fully retracted, the vacuum is released in chamber body  12 . Lift pins  70  are moved through lift pin penetrations  56  in the rigid plate  50  and lift pin penetrations  66  in the flexible puck  60  to lift wafer  20  off of the flexible puck  60 . The top clamp ring  88 , gasket  86 , top lid  84 , middle lid  82  and press plate  30  are removed and the wafer  20  is removed off of lift pins  70  for further processing.  
         [0028]    A specific example is provided to further illustrate the method and operation of the apparatus. A wafer  20  having a nominal 1-5 micron thick deformable layer  22  consisting of a UV curable epoxy resin is placed on the flexible puck  60  within chamber body  12 . Chamber body  12  is evacuated to a pressure of approximately 50 millitorr. A pressure differential is established across the bellows  40  by venting the bellows  40  to atmosphere to drive wafer  20  against press plate  30 . A pressure of 100 psi is then applied to the wafer  20  for 1 minute at a temperature of approximately 50° C. to shape the surface of the epoxy resin to correspond to that of the pressing surface  32 . The deformable layer  22  is then cured while in contact with press plate  30  by the application of ultraviolet radiation for approximately 15-30 seconds through a quartz optical flat used as press plate  30 .  
         [0029]    Those of ordinary skill in the art will appreciate that the present invention provides great advantages over other options for planarizing deformable surface layers. In particular, the subject invention is designed such that the pressure boundary does not have to be breached to operate the apparatus as is true with prior art designs. The subject invention also eliminates the need to use a pressure boundary as the supporting surface for the wafers. Also, the subject invention has the advantage of providing for the automated handling of the wafers, which was not present in the prior art. Thus, the present invention provides a significant reduction in the overall cost associated with the production of semiconductor wafers. While the subject invention provides these and other advantages over other planarization apparatuses, it will be understood, however, that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.