Abstract:
Apparatus for supporting a workpiece and method of making same. The apparatus comprises a flex circuit laminated to a contoured support pedestal. The flex circuit includes a reinforced layer to improve puncture resistance of the flex circuit. The top surface of the chuck has a contoured topography that is achieved by machining the upper surface of the pedestal prior to lamination of the flex circuit to the pedestal. The contoured topography improves the flow of backside cooling gas resulting in a more uniform wafer temperature profile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/744,039 now U.S. Pat. No. 5,885,469, filed Nov. 5, 1996, this application is also a continuation-in-part of U.S. patent application Ser. No. 09/015,802 now U.S. Pat. No. 5,986,878, filed Jan. 29, 1998, which is a continuation of U.S. patent application Ser. No. 08/755,716 now U.S. Pat. No. 5,729,423 filed Nov. 25, 1996, now U.S. Pat. No. 5,729,423, which is a continuation-in-part of U.S. patent application Ser. No. 08/381,786, filed on Jan. 31, 1995 now U.S. Pat. No. 5,745,331, which is a continuation-in-part of U.S. patent application Ser. No. 08/189,562, filed Jan. 31, 1994, now abandoned, all of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to electrostatic chucks for holding a workpiece and, more specifically, to an electrostatic chuck having a flex circuit with improved puncture resistance and a surface topography that promotes heat transfer from a semiconductor wafer to the chuck. 
     2. Description of the Background Art 
     Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. Electrostatic chucks secure a workpiece by creating an electrostatic attractive force between the workpiece and the chuck. A voltage is applied to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The opposite charges pull the workpiece against the chuck, thereby retaining the workpiece. In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a support during processing. The support may form both an electrode (in electrostatic chuck applications) and a heat sink. These chucks find use in etching, chemical vapor deposition (CVD), ion implantation, and physical vapor deposition (PVD) applications. 
     The materials and processes used to process a wafer are extremely temperature sensitive. Should these materials be exposed to excessive temperature fluctuations due to poor heat transfer from the wafer during processing, performance of the wafer processing system may be compromised resulting in wafer damage. To optimally transfer heat between the wafer and a chuck, a very large electrostatic force is used in an attempt to cause the greatest amount of wafer surface to physically contact a support surface of the chuck. However, due to surface roughness of both the wafer and the chuck, small interstitial spaces remain between the chuck and wafer that interfere with optimal heat transfer. 
     To achieve further cooling of the wafer during processing, an inert gas such as Helium is pumped into the interstitial spaces formed between the wafer and the support surface. This gas acts as a thermal transfer medium from the wafer to the chuck that has better heat transfer characteristics than the vacuum it replaces. The chuck generally has a contoured surface to maximize heat transfer and to prevent the heat transfer gas from escaping into the surrounding low pressure atmosphere (i.e., the reaction chamber). To further enhance the cooling process, the chuck is typically water-cooled via conduits within the support. This cooling technique is known as backside gas cooling. 
     An electrostatic chuck often has a flex circuit comprised of a conductive material, such as copper, encased in a flexible dielectric material such as polyimide. The flex circuit has a sculptured surface that enhances backside gas cooling by providing specific channels or grooves for conveyance of the heat transfer gas across the entire bottom surface of the wafer and into the interstitial spaces. The flex circuit may be shaped in various configurations such as a flat plate across most of the support surface, as a series of concentric rings or radial arms to disperse the heat transfer gas across the entire bottom surface of the wafer. The sculptured surface of the flex circuit includes contours such as ribs, grooves or channels provided therein to further improve heat transfer gas flow uniformity. 
     A disadvantage of prior art chucks that utilize a laminated flex circuit with a sculptured surface is that dielectric materials, such as polyimide, typically used in existing flex circuits are likely to puncture after repeated clamping of wafers in a wafer processing system. A puncture in the upper dielectric of a flex circuit of an electrostatic chuck can lead to a short circuit between the chuck electrode and the wafer. When the chuck electrode is energized, such a short circuit produces a large current flowing through the wafer that damages the wafer. Such a short circuit can only be repaired by replacement of the chuck causing stoppage of production and a decline in productivity. 
     Therefore, there is a need in the art for a semiconductor wafer processing chuck having a contoured flex circuit that is resistant to punctures and a concomitant method of manufacturing same. 
     SUMMARY OF THE INVENTION 
     The disadvantages heretofore associated with the prior art are overcome by a puncture resistant electrostatic chuck having a contiguous workpiece support surface with at least one protruded region to support a backside surface of a wafer. 
     The apparatus comprises a pedestal having a surface with at least one protruded region and a puncture resistant flex circuit disposed above the pedestal surface. The puncture resistant flex circuit comprises a first dielectric layer beneath a conductive layer and a puncture resistant layer above the conductive layer. Preferably, the puncture resistant layer comprises a woven material of aramid fibers. A second dielectric layer above the puncture resistant layer defines the contiguous workpiece support surface. 
     Additionally, the electrostatic chuck has at least one heat transfer gas port extending through it to the workpiece support surface. The protruded region(s) are arranged so that when chucked, the workpiece does not block the flow of heat transfer gas to the backside of the wafer. As such, the gas uniformly extends to all non-protruded regions of the chuck surface and fills the space between the wafer and the chuck surface. 
     An inventive method of fabricating an electrostatic chuck that utilizes a puncture resistant flex circuit includes: forming a laminate of a dielectric layer, a puncture resistant layer and a conductive layer, etching an electrode pattern on the conductive layer, then adhering another dielectric layer to the electrode pattern to form a flex circuit, machining a topography into a pedestal surface of the electrostatic chuck and laminating the aforementioned flex circuit to the sculpted (machined) pedestal surface. The topography created by the machining contains protruded regions and non-protruded regions. The flex circuit is adhered to the surface of the pedestal and conforms to the topography thereof. As such, the support surface of the electrostatic chuck contains protruded regions and non-protruded regions, where the total surface of the protruded regions is less than the total surface area of the non-protruded regions. 
     This invention fulfills the long felt need for puncture resistant apparatus that can efficiently clamp a wafer to an electrostatic chuck and provide improved heat transfer characteristics. Specifically, the puncture resistant layer of the flex circuit protects the electrode from short circuiting to the wafer. This, in turn leads to longer useful chuck life, fewer damaged wafers, less chamber downtime and increased productivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a top view of an electrostatic chuck with an inventive flex circuit; 
     FIG. 2 depicts a cross-sectional view of the electrostatic chuck taken along lines  2 — 2  of FIG. 1; and 
     FIGS. 3A-3E are sectional schematic side views showing successive steps in fabrication of an embodiment of the electrostatic chuck of the present invention. 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 depict a top view and a cross-sectional view respectively of an electrostatic chuck  100  adapted to support and electrostatically retain a workpiece to be processed, such as a semiconductor wafer, on a pedestal  102  within a high density plasma reaction chamber (not shown). Specifically, FIG. 2 depicts a cross-sectional view of the electrostatic chuck of FIG. 1 taken along line  2 — 2  with a semiconductor wafer  111  shown in phantom. To best understand the invention, the reader should simultaneously refer to both FIGS. 1 and 2. 
     For a detailed understanding of the plasma reaction chamber and its operation in processing a wafer, the reader should refer to the drawings and the detailed description contained in U.S. Pat. No. 4,842,683, issued Jun. 27, 1989 and incorporated herein by reference. That patent discloses a illustrative biased, high density plasma etch reaction chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif. 
     The electrostatic chuck  100  contains an inventive flex circuit  120  upon which the wafer  111  rests. Specifically, the chuck  100  contains a pedestal  102  with a contoured surface  101  upon which the flex circuit  120  is adhered. The flex circuit contains a first dielectric layer  114 , a conductive layer  112 , a puncture resistant layer  116  and a second dielectric layer  118 . 
     A laminate containing the second dielectric layer  118 , the puncture resistant layer  116  and the conductive layer  112  is assembled as discussed below. The conductive layer  112  is a conductive material such as copper having a uniform thickness of approximately 5 microns. This layer  112  is used to form an electrode for the electrostatic chuck, i.e., the conductive layer is masked and etched to form an electrode pattern. An adhesive layer  115  bonds the conductive layer side of the laminate to the first dielectric layer  114  and the layer  114  is bonded to the pedestal surface  101  by another adhesive layer  113 . 
     The puncture resistant layer  116  protects against puncture by sharp edged fragments that form when a substrate such as the wafer  111 , chips during processing. The puncture resistant layer  116  preferably comprises a polymer having a high tensile strength, tenacity, stiffness and fracture toughness. The high stiffness and tenacity results in reduced penetration or puncture of the puncture resistant layer  116  by sharp edged fragments. 
     A preferred material that provides a high hardness, stiffness, tenacity and toughness values, excellent chemical compatibility, and strong adhesion with polymeric materials is a polyamide material comprising a high molecular weight polymer having hydrocarbon chains containing amide or —CONH— molecular groups. More preferably, the puncture resistant layer comprises highly aromatic polyamides that exhibit increased puncture resistance because of their higher hardness as compared with conventional polyamide materials. The highly aromatic polyamides can be in the form of a continuous planar layer disposed directly above and covering the electrodes or in the form of a layer of intermeshed fibers covering the electrodes. Most preferably, the highly aromatic polyamides comprise aramid materials. The puncture resistant layer can be in the form of a sheet of aramid, or a layer of aramid fibers. The aramid sheet and/or fibers comprise highly aromatic polyamide chains that are preferably oriented parallel to the plane of the sheet, or to the longitudinal axis of the fiber. The highly aromatic molecular chains of aramid provide a hard polymer that exhibits excellent puncture resistance. Preferably, the aramid sheet, or layer of fibers, comprise KEVLAR® or NOMEX® aramid materials fabricated by DuPont de Nemours Company, Wilmington, Del. For example, Kevlar typically has a tensile strength of 400×10 3  psi, a dry tenacity of 22 g/denier, a percent elongation at break of 2.5%, and a stiffness of 975 g/denier. Nomex has a tensile strength of 90×10 3  psi, a dry tenacity of 4-5.3 g/denier and a toughness of 0.85 g-cm. Aramid materials are typically derived from phenylenediamine and terepthaloyl chloride. 
     In the preferred version, the aromatic polyamide layer comprises an intermeshed layer of fibers such as a paper, felt, cloth, or woven structure. It has been discovered that the intermeshed fiber-containing layer provides increased hardness, fracture toughness, and puncture resistance. The fiber-containing layer can be a paper or felt structure comprising non-woven, randomly oriented, short fibers or whiskers which comprise fibers of polyamide embedded in a polymer matrix. When the aromatic polyamide is present in the form of fibers, a surrounding matrix material is used to hold the fibers together. Preferably, the matrix material provides good adhesion and chemical compatibility between the material comprising the conformal holding surface and the aromatic polyamide fibers. 
     In a preferred configuration, the aramid fibers form a paper or felt structure is easier to fabricate into the chuck because it is easier to cut or laser ablate this randomly oriented structure. Alternatively, the aramid fibers can comprise an oriented woven structure, such as cloth or a weave pattern. A suitable cloth is a “THERMOUNT®” cloth fabricated by DuPont de Nemours Company, Wilmington, Del., having a yarn of 40-60, a denier of 40-60, a fill of 50-60, and a warp of 60-80. While the oriented woven structure is difficult to cut, the woven structure provides more predictable properties in the plane parallel to the fibers, due to the predetermined non-random orientation of the fibers. 
     In a preferred embodiment of the invention, the puncture resistant layer  116  is Thermount®. Thermount® is a registered trademark of the DuPont Corporation of Wilmington, Del. and is preferred because of its dimensional stability and low weight. Thermount® has a tensile strength of about 80×10 3  psi, a dry tenacity of about 10 g/denier, and a stiffness of about 10-500 g/denier and a fracture toughness of about 0.79 g-cm. Alternatively, other puncture resistant materials having similar physical characteristics and dielectric properties such as Kevlar may be used for the puncture resistant layer  116 . 
     To form the laminate, the second dielectric layer  118  is placed on top of the puncture resistant layer  116  which is placed on top of the conductive layer  112 . The layers are heated and pressed together to form the laminate. Thermount® is an epoxy based material having a Kevlar cloth incorporated within the epoxy resin. Thus, the Thermount® forms an adhesive bond to the conductive layer  112  and the dielectric layer  118  when the layers are heated to between 150 and 175° C. and pressed together using a pressure of 170 to 220 psi. The second dielectric layer  118  has an upper surface that defines a workpiece support surface  107 . 
     The dielectric layers  114  and  118  are preferably an electrically insulative polymeric material, such as polyimide, polyketon, polyetherketone, polysulfone, polycarbonate, polystyrene, nylon, polyvinylchloride, polypropylene, polyetherketones, polyethersulfone, polyethylene terephthalate, fluoroethylene propylene copolymers, cellulose, triacetates, silicone, and rubber. A preferred material to form the conformal holding surface comprises a layer of polyimide. Polyimide is resilient enough to deform slightly under the electrostatic clamping pressure to provide enhanced heat transfer when a heat transfer fluid is introduced in the space  109  between the substrate  111  and the resilient polyimide layer  118 . Polyimide also provides superior temperature resistance and excellent dielectric properties. For example, polyimide has a high dielectric breakdown strength, ranging from 3,000 to 3,500 volts per mil, which allows use of a thin layer that maximizes the electrostatic attractive force. Polyimides are also resistant to temperatures in excess of 100° C., and more preferably in excess of 200° C., allowing use of the chuck for high temperature processes. Preferably, the polymeric material also has a high thermal conductivity so that heat generated in the substrate during processing can dissipate through the chuck, a suitable thermal conductivity being at least about 0.10 Watts/m/° K. Furthermore, the dielectric layers  114  and  118  should be stable at temperatures in excess of 50° C. Preferably, the dielectric material used for the first and second dielectric layers  114  and  118  is a sheet of polyimide sold under the brand name UPILEX®. UPILEX® is a registered trademark of UBE Industries, Ltd. of Yamaguchi, Japan. The preferred thickness of the dielectric layers  114  and  118  is 0.002 inches. 
     Before attaching the flex circuit  120  to the pedestal  102 , the topographical structure of the chuck  100  is enhanced by machining the pedestal surface  101  to create contours, such as one or more ribs  106  and  110 . Such an electrostatic chuck is disclosed in U.S. patent application Ser. No. 08/744,039, filed Nov. 5, 1996. The pedestal surface  101  is machined to a specific topographical pattern which allows the flex circuit  120  above it to take the shape of the pattern. Specifically, a rim of approximate height 1 mil and width between 1.5 to 2 mm is shown as a preferred pattern in FIG.  2 . The height may be altered depending on the thickness of the flex circuit  120 . The width may be altered depending on the desired amount of area of the wafer to be exposed to the backside gas to provide adequate cooling. 
     As an example of the topography, the pedestal  102  is provided with a raised rim  104  at its outer diameter. The diameter of the rim  104  is approximately equal to a diameter of the wafer to be processed. As such, a circumferential edge  122  of the flex circuit  120  follows a contour of the rim  104  such that the wafer  111  is supported in part by the rim via contact with the circumferential edge  122  of the flex circuit  120 . The chuck  100  also contains at least one port  108  for conducting heat transfer gas through the chuck  100  to a bottom surface of the wafer. In a preferred embodiment of the invention, thirty-six (36) ports are equidistantly spaced about the circumferential edge  122 , six (6) ports are equidistantly spaced radially inward of the circumferential edge  122  and one (1) port is at the center of the flex circuit  120 . When the wafer  111  is placed on the chuck  100 , it is supported solely by the rim  104 . As such, a volume of space  109  is created below the wafer  111  and above the wafer support surface  107 . When power is applied to the electrode (i.e., conductive layer  112 ), it creates an electrostatic force that draws the wafer toward the wafer support surface  107 . This electrostatic force is also known as a clamping force. Since the dielectric material of the flex circuit  120  is semi-pliant, a relatively tight seal is created at the rim to wafer contact area  103 . As such, the volume of space  109  between the wafer and the flex circuit covered wafer support surface  107  is sealed from the chamber vacuum when the wafer  111  is clamped to the chuck  100 . 
     To create a topography for the wafer support surface  107  upon which the wafer rests when clamped, the surface  101  of the pedestal  102  is preferably machined into a series of protruded and non-protruded regions such as one or more ribs  106  and  110  rising above a common level. The ribs  106  and  110  may be continuous or broken into radial segments about the pedestal  102 . In this manner, the protruded regions of support surface that contact the back of the wafer have an area that is smaller than the area of the non-protruded regions. As such, the backside of the wafer is mostly contacted by the heat transfer medium, e.g., argon or helium gas. 
     FIGS. 3A-3E depict the steps used to form the flex circuit  120  and adhere the circuit  120  to the pedestal  102 . The process starts with a pre-fabricated laminate  300  of the dielectric layer  110 , the puncture resistant layer  116  and the conductive layer  112 . The laminate is fabricated by stacking the layers  118 ,  116  and  112  and heating them under pressure (e.g., heating to 165° C. under 200 psi) such that the epoxy in the puncture resistant layer  116  bonds to the dielectric layer  118  and the conductive layer  112 . As shown in FIG. 3B, the conductive layer is masked and etched to form the electrode structure. A conventional copper photoresist and wet etching process is used. In FIG. 3C, an adhesive layer  115  is applied between the electrode side of the laminate  300  and the first dielectric layer  114  is applied to the adhesive. Suitable adhesives include, for example, acrylics such as methacrylate, polyesters, polyamide, polyurethanes, epoxies, silicone containing adhesives, and mixtures thereof. The flex circuit  120  is completed by compressing the circuit at a pressure of between 170 and 220 psi and a temperature of between 150 and 175° C. for a period of between 60 and 90 minutes. 
     Suitable pressure forming apparatus include autoclaves, platen presses or isostatic presses. An autoclave is preferred because autoclaves apply a more uniform pressure on the flex circuit assembly. Typical autoclaves comprise pressure resistant steel chambers having diameters ranging from about 1 to 10 feet. A pressurized nonreactive gas such as carbon dioxide or nitrogen is used to pressurize the autoclave. Suitable autoclaves include those fabricated by “BARON AUTOCLAVES,” Santa Fe Springs, Calif.; “AOV INDUSTRIES” in Anaheim, Calif.; and “MELCO STEEL,” in Azusa, Calif. Instead of using an autoclave, a platen press or isostatic press can also be used to complete the flex circuit  120 . When a platen press is used (not shown), a pressure-distribution sheet such as a silicone or stainless steel plate, is placed on the electrically insulator film to uniformly distribute the platen pressure across the flex circuit. Isotatic presses are used by placing the flex circuit in a suitable isostatic molding bag, and applying a sufficient pressure on the bag using the isostatic press. 
     After the flex circuit  120  has been formed, it is laminated the pedestal surface  101 . For example, in FIG. 3D, a second adhesive layer  115  is applied to the pedestal surface  101 , and in FIG. 3E, the flex circuit  120  is pressed against the pedestal surface  101  at a pressure of approximately 2,000 psi and a temperature of approximately 163° C. The second adhesive is generally the same adhesive as the first adhesive. The flex circuit  120  is pressed against the pedestal surface  101  for approximately 60 minutes or until the second adhesive layer  115  sets. Since the puncture resistant layer  116  has already been cured, it will not flow under the heat and pressure of the laminating process. However, the puncture resistant layer is still flexible and, therefore, will conform to the topography machined onto the pedestal surface  101 . The flex circuit can be laminated to the pedestal surface using a press such as an autoclave platen press or isostatic press. 
     The result is a topography across the wafer support surface  107  that lends additional support to the clamped wafer  111 . Specifically, the wafer  111  (of FIG. 2) is supported at the outer diameter rim contact area  103  and at least one other point radially inward therefrom (e.g., ribs  106  and  110 ). The contoured workpiece support surface  107  thus formed is contiguous (i.e., not cut to form channels or grooves). 
     To achieve cooling of the wafer during processing, a heat transfer gas, preferably Helium, is pumped into the volume  109  between the backside of the wafer  111  and the wafer support surface  107  via the heat transfer gas ports  108 . With the addition of the ribs  106  and  110 , the uncontrolled interstitial spaces between the wafer and the wafer support surface  107  are eliminated. Thus, the surface area of the wafer exposed to the heat transfer gas is greatly increased. This contributes to greater and more uniform gas pressure distribution than experienced by the prior art. As a result, heat transfer from the wafer to the pedestal is uniform. The improved topography is capable of reducing wafer temperature nonuniformity to approximately 5° C. across the wafer. 
     The wafer support surface may also be formed as a pattern of islands distributed over the wafer support surface  107 , a series of concentric circles or segments or a combination of both. The pattern lends additional support to the wafer beyond that provided by the outer diameter rim as well as contributing to the uniform distribution and flow of Helium to all areas of the wafer. For example, if a wafer were placed on a conventional support surface, the wafer may block one or more of the Helium ports because of wafer flexing. The ribs of the present invention can be patterned to maintain the wafer above the ports to avoid blockage and improve gas distribution. However, in keeping with the teachings of the invention, the area of the wafer physically contacting the support surface at the chuck is substantially less than the area of the wafer that is exposed to the heat transfer gas. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.