Abstract:
The invention is a chemical-mechanical polishing wafer carrier that is able to apply a plurality of different pressures, with minimal discontinuities at the interfaces between different pressures, through a diaphragm to a back surface of a wafer. A plurality of concentric balloons, that may be individually pressurized, is used to support and press on the back surface of the diaphragm. The walls of the balloons are preferably thin and elastic and preferably do not attach to the diaphragm. This helps to minimize any pressure discontinuities on the diaphragm along the interfaces between the balloons. A wafer may be placed against the front surface of the diaphragm allowing the front surface of the diaphragm to retain and press against the back surface of the wafer during a planarization process.

Description:
TECHNICAL FIELD 
     The present invention relates generally to semiconductor manufacturing, and more specifically to a carrier for retaining and pressing a semiconductor wafer against a polishing pad in a chemical-mechanical polishing tool to remove material and planarize the front surface of the wafer. 
     BACKGROUND OF THE INVENTION 
     A flat disk or “wafer” of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness. 
     Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modern complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Non-planar surfaces create poor optical resolution of subsequent photolithography processing steps. Poor optical resolution prohibits the printing of high-density lines. Another problem with non-planar surface topography is the step coverage of subsequent metalization layers. If a step height is too large there is a serious danger that open circuits will be created. Planar interconnect surface layers are required in the fabrication of modem high-density integrated circuits. To this end, CMP tools have been developed to provide controlled planarization of both structured and unstructured wafers. 
     Carriers may generally be grouped into back-reference and front-reference carriers. Back-reference carriers typically have a rigid pressure plate for supporting the back surface of the wafer while the wafer is pressed against the polishing pad. Imperfections on the back surface of the wafer are pressed on by the rigid pressure plate creating areas of non-uniform pressure on the front surface of the wafer. A compliant thin film may be used to cover the rigid pressure plate reducing, but not eliminating, the non-uniform pressure areas. 
     Front-reference carriers typically have a diaphragm for supporting the back surface of the wafer. Imperfections on the back surface of the wafer are better absorbed by the diaphragm than with the thin film allowing for a more uniform pressure to be placed on the front surface of the wafer. However, even with a uniform pressure on the front surface of the wafer, other problems, such as non-uniform slurry distribution or different motions for different points on the front surface of the wafer cause non-uniform planarization results. The non-uniform planarization results are typically manifested as concentric bands on the front surface of the wafer that need an increased or decreased material removal rate. It may therefore be desirable to have different pressures on different concentric bands while maintaining a uniform pressure over each band. 
     Carriers providing different uniform pressures on different concentric bands generally accomplish this by having two or more plenums that may be individually pressurized over a diaphragm separated by barriers. However, these carriers generally have a discontinuity of pressure at the interface between the bands near the barrier. This is generally caused by the barrier experiencing a shear force due to the different pressures within the plenums. The shear force causes the barrier to change position, for example by slightly lifting and puckering the diaphragm, creating a narrow band of discontinuity of pressure on the diaphragm along the barrier. 
     What is needed is a carrier having a plurality of concentric plenums that may be individually pressurized for planarizing the front surface of a wafer that reduces the discontinuities at the barrier between the plenums. 
     SUMMARY OF THE INVENTION 
     The invention is a method and apparatus that may be used in a CMP tool to press the front surface of a wafer against a polishing pad during a planarization process. A puck and a diaphragm may be used, possibly in combination with other features such as a cushion ring, to form a plenum within which concentric balloon may be positioned. Individually controllable fluid communication paths may be used to communicate a pressure to the plenum and/or concentric balloons. The plurality of concentric balloons may be used to apply different pressing forces through the diaphragm to the back surface of a wafer. Each pressing force is preferably uniform within a concentric band. Pressing force discontinuities between concentric bands are minimized by using thin balloons that are not connected to the thicker diaphragm. 
     As an improvement to the invention, the puck may have a plurality of concentric grooves. Double-sided tape may be placed inside each groove and the balloons may be sealed, for example by bonding, to metal rings. The metal rings may be inserted into the grooves and connected to the puck by the double-sided tape. The balloons may then be in position to expand within the plenum and support the diaphragm. The balloons are preferably very thin, highly elastic and sufficiently inflated during a planarization process to substantially fill the plenum to prevent pressure discontinuities at the interface between balloons. The diaphragm is preferably thicker and preferably less elastic than the balloons to average and further reduces any small discontinuities in pressure that may still exist on the diaphragm. 
     As another improvement, a cushion ring may be positioned between the periphery of the bottom surface of the puck and the periphery of the top surface of the diaphragm. In this embodiment, the cushion ring forms part of the plenum and provides space between the puck and diaphragm. The space is preferably substantially completely filled by the balloons when the balloons are expanded during a planarization process. A retaining ring may be connected to the periphery of the bottom surface of the diaphragm below the cushion ring. The cushion ring is preferably elastic, thereby allowing the retaining ring some freedom of movement in relation to the puck. 
     The above-described apparatus is preferably used for pressing against a back surface of a wafer during a planarization process. An exemplary method starts by pressurizing the plenum behind the diaphragm to provide a substantially uniform pressing force against a back surface of a test wafer. The test wafer is planarized and then its front surface uniformity is measured. The test wafer is used to assist in determining the optimum pressure to apply to each balloon. Multiple iterations of planarizing, measuring and adjusting the balloons may be done to optimize the planarization process until the planarization process reaches a level suitable for production wafers. Even after production wafers are used in the planarization process, further iterations of measuring the wafer&#39;s front surface and adjusting the pressure within the balloons, and therefore the pressure against the diaphragm, may be performed to further improve the planarization process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the appended drawing figures, wherein like numerals denote like elements, and: 
     FIG. 1 a  is a cross section view of a wafer carrier according to an embodiment of the invention with the supporting balloons in a contracted state; 
     FIG. 1 b  is a cross section view of the wafer carrier illustrated in FIG. 1 a,  but with the supporting balloons in a partially expanded state; 
     FIG. 2 is a cross section view of the wafer carrier illustrated in FIG. 1 a,  but with the supporting balloons in an expanded state; 
     FIG. 3 is an exploded bottom perspective view of the wafer carrier illustrated in FIG. 1 a;  and 
     FIG. 4 is a flow chart illustrating an exemplary method of using the invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     An improved polishing apparatus and method utilized in the polishing of semiconductor substrates and thin films formed thereon will now be described. The invention may also be used to planarize a wide range of workpieces, but is particularly well suited for planarizing raw and STI wafers and wafers covered by a thin metal or dielectric layer. In the following description, numerous specific details are set forth illustrating Applicant&#39;s best mode for practicing the present invention and enabling one of ordinary skill in the art to make and use the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known machines and process steps have not been described in particular detail in order to avoid unnecessarily obscuring the present invention. 
     The invention is preferably attached to, or acts as, a wafer carrier within a CMP tool. An apparatus for practicing the present invention will now be disclosed with reference to FIGS. 1 a  and  3 . A puck  104  acts as an upper housing support for the remaining components of the invention. The puck  104  may also have holes or other features, such as flanges, that allow the puck  104  to be easily connected to the rest of the CMP tool. The puck  104  may be connected to the rest of the CMP tool via a membrane, springs or other manner that allow the puck  104  to have some freedom in movement, but is preferably rigidly connected. 
     The puck  104  may be made from multiple pieces, but is preferably a single solid piece of material that does not adversely react, e.g. corrode, within its operating environment. The puck  104  will typically be exposed to corrosive chemicals contained in slurry as part of a CMP planarization process. Pucks  104  comprising stainless steal or aluminum have been found to perform well in a CMP environment. 
     The puck  104  preferably has a plurality of concentric grooves  112  surrounding a recessed central area  111 . The greater the number of grooves  112  the better the process flexibility, but the greater the complexity and expense of the invention. A central area  111  and two concentric grooves  112  are preferred for planarizing a 200 mm wafer and a central area  111  and four concentric grooves  112  are preferred for planarizing a 300 mm wafer. 
     Double-sided tape  105  may be cut into the shape of the grooves  112  and central recessed area  107  and pressed into place. The double-sided tape  105  may be purchased from 3M as  9469  or  4920  double-sided tape. The tape  105  is preferably slightly compressible and elastic to allow metal rings  106  (described below) and balloons  107 - 109  (described below) to float in relation to the puck  104  during the planarization process. 
     A plurality of balloons  107 - 109  are sealed, preferably by bonding, to a corresponding plurality of metal rings  105 . The balloons  107 - 109  and metal rings  105  should correspond in shape and size to fit into the grooves  112  and central recessed area  111 . The metal rings  105  may then be pressed against the double-sided tape  105  with about 15 psi to connect the metal rings  105  and balloons  107 - 109  to the puck  104 . 
     The balloons  107 - 109  may advantageously be made very thin, about 0.3 mm or less, and preferably about 0.25 mm. The balloons  107 - 109  preferably comprise a super elastic material such as latex. The balloons  107 - 109  may be made very thin since they experience minimal shear force during the planarization process due to the protection of the diaphragm  103 . The thinness and elasticity of the balloons  107 - 109  allows the balloons  107 - 109  to substantially fill the plenum  113  behind the diaphragm  103  when inflated as illustrated in FIG.  2 . This minimizes discontinuities in pressure on the diaphragm  103  as substantially the entire top surface of the diaphragm  103  is supported by a balloon  107 - 109 . The inflated balloons  107 - 109  contact each other and the diaphragm  103 , but are not connected to the diaphragm  103 . This also minimizes discontinuities in pressure on the diaphragm  103  as the pressure within each balloon  107 - 109  may be changed, and thus the contact position of the balloons  107 - 109  moved, without lifting or puckering the diaphragm  103 . 
     The pressure for the balloons  107 ,  108  and  109  may be controlled through fluid communication paths P 1 , P 2  and P 3  respectively. The pressure for the plenum  113  may be controlled through fluid communication path P 4 . The fluid communication paths P 14  preferably include a pressure regulator for each fluid communication path and a common pump. A control system (not shown) may be used to set the pressure regulators, before, during or after the planarization process, thereby automating the process of pressurizing the balloons  107 - 109  and plenum  113 . The pressure within the plenum  113  and balloons  107 - 109  may be customized to optimize the planarization process. Typically, lower pressures are beneficial for planarizing softer materials while higher pressures are needed for harder materials. Current materials, such as copper and silicon dioxide are preferably planarized with the balloons  107 - 109  pressurized between about 2 to 6 psi and the plenum  113  pressurized between about 4-5 psi. The optimum pressure for the balloons  107 - 109  and plenum  113  may vary substantially from CMP tool to CMP tool and from type of workpiece to type of workpiece. Therefore, the optimum pressure settings for the balloons  107 - 109  and plenum  113  will generally need to be found empirically for every CMP tool and for every type of workpiece. 
     A cushion ring  110  may be used to create a space, i.e. plenum  113 , between the puck  104  and the diaphragm  103 . The cushion ring  1   10  preferably has an outer diameter equal to the outer diameter of the puck  104 . The cushion ring  110  preferably has a height sufficient to give the balloons  107 - 109  adequate space to inflate within the plenum  113 . The cushion ring  110  may be rigid, but is preferably slightly elastic to allow the retaining ring  101  (described below) some freedom of movement in relation to the puck  104 . 
     The diaphragm  103  is preferably connected to the puck  104  via the cushion ring  110 . The diaphragm  103  is preferably thicker than the balloons  107 - 109 , e.g. about 0.5 to 3 mm, to average the pressures exerted on the back surface of the diaphragm  103  at the interface between balloons  107 - 109  having different pressures. The diaphragm should be elastic and may be made from EPDM or SBR. One or more holes (not shown) may be made in the diaphragm  103  above where the wafer  100  makes contact with the diaphragm  103 . The wafer  100  seals the holes during a planarization process. The holes allow a vacuum to be applied in plenum  113  by fluid communication path P 4  to pick-up wafers  100  or to evacuate the air from the plenum  113  to more fully allow the balloons  107 - 109  to inflate within the plenum  113 . 
     A retaining ring  101 , in combination with the bottom surface of the diaphragm  103 , may be used to create a pocket for retaining the wafer  100  during a planarization process. A fastener  102  may be used to attach the retaining ring to the diaphragm  103 , cushion ring  110  and puck  104 . One specific fastener that may be used is a plurality of screws  102  positioned around the periphery of the puck  104 . Of course, those skilled in the art will appreciate that other fastening methods may easily be used. The retaining ring  101  should be non-corrosive, and when worn, should not give off particles that will scratch the wafer  100 . Examples of suitable materials for comprising the retaining ring  101  are PEEK, SiC, PET or Aluminum. The inside diameter of the retaining ring  101  is preferably rounded to avoid damaging the wafer  100 . 
     A method for planarizing a wafer  100  will now be disclosed with reference to FIG.  4 . The balloons  107 - 109  are initially not inflated so that they do not press on the back surface of the diaphragm  103  (as shown in FIG. 1 a ). The plenum  113  may then be pressurized, for example to 5 psi, through fluid communication path P 4  to provide a uniform pressing surface against the back surface of the wafer  100 . (step  400 ) The wafer  100  may then be planarized (step  401 ) and the material removal rate profile determined. Measurements may be taken before, during and/or after the planarization process to determine where and how much material was removed across the front surface of the wafer  100  during the planarization process (step  402 ). The measurements may be analyzed to determine if concentric bands exist on the front surface of the wafer that needed an increased removal rate to improve the planarization process (step  403 ). Because CMP tools are generally able to repeat a process given the same type of wafer, this information may be used to predict how the next wafer is likely to be planarized. One or more of the balloons  107 - 109  may be pressurized behind the locations adjacent concentric bands that are predicted to need an increased removal rate. The number of balloons  107 - 109  inflated and the pressure within each balloon  107 - 109  may be customized depending on the desired adjustments that are needed for the planarization process (step  404 ). FIG. 1 b  illustrates the balloons  107 - 109  inflated to a point where the balloons  107 - 109  do not totally fill the plenum  113 . While a wafer  100  may be planarized with the balloons only partially filling the plenum  113 , wafers are preferably planarized with the balloons substantially filling the plenum  113 . The next wafer may then be planarize using this customized combination of pressures (step  405 ). Of course, the planarization results of all or some of the future wafers may also be measured to assist in continually adjusting the number and pressure of the balloons  107 - 109  to continually improve the planarization process. If the CMP tool is capable of taking in-situ measurements of the wafer  100 , the number and pressure of the balloons  107 - 109  may even be adjusted during a planarization process to further improve the planarization results for that particular wafer  100 . 
     While the invention has been described with regard to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention.