Abstract:
A direct-deposition system capable of forming a high-resolution pattern of material on a substrate is disclosed. Vaporized atoms from an evaporation source pass through a pattern of through-holes in a shadow mask to deposit on the substrate in the desired pattern. The shadow mask is held in a mask chuck that enables the shadow mask and substrate to be separated by a distance that can be less than ten microns. As a result, the vaporized atoms that pass through the shadow mask exhibit little or no lateral spread (i.e., feathering) after passing through its apertures and the material deposits on the substrate in a pattern that has very high fidelity with the aperture pattern of the shadow mask.

Description:
STATEMENT OF RELATED CASES 
       [0001]    This case claims priority to U.S. Provisional Patent Application Ser. No. 62/340,793 filed on May 24, 2016 (Attorney Docket: 6494-208PR1), which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to thin-film deposition in general, and, more particularly, thin-film deposition equipment. 
       BACKGROUND OF THE INVENTION 
       [0003]    Shadow-mask-based deposition is a process by which a layer of material is deposited onto the surface of a substrate such that the desired pattern of the layer is defined during the deposition process itself. This is deposition technique is sometimes referred to as “direct patterning.” 
         [0004]    In a typical shadow-mask deposition process, the desired material is vaporized at a source that is located at a distance from the substrate, with a shadow mask positioned between them. As the vaporized atoms of the material travel toward the substrate, they pass through a set of through-holes in the shadow mask, which is positioned in contact with or just in front of the substrate surface. The through-holes (i.e., apertures) are arranged in the desired pattern for the material on the substrate. As a result, the shadow mask blocks passage of all vaporized atoms except those that pass through the through-holes, which deposit on the substrate surface in the desired pattern. Shadow-mask-based deposition is analogous to silk-screening techniques used to form patterns (e.g., uniform numbers, etc.) on articles of clothing or stenciling used to develop artwork. 
         [0005]    Shadow-mask-based deposition has been used for many years in the integrated-circuit (IC) industry to deposit patterns of material on substrates, due, in part, to the fact that it avoids the need for patterning a material layer after it has been deposited. As a result, its use eliminates the need to expose the deposited material to harsh chemicals (e.g., acid-based etchants, caustic photolithography development chemicals, etc.) to pattern it. In addition, shadow-mask-based deposition requires less handling and processing of the substrate, thereby reducing the risk of substrate breakage and increasing fabrication yield. Furthermore, many materials, such as organic materials, cannot be subjected to photolithographic chemicals without damaging them, which makes depositing such materials by shadow mask a necessity. 
         [0006]    Unfortunately, the feature resolution that can be obtained by conventional shadow-mask deposition is diminished due to the fact that the deposited material tends to spread laterally after passing through the shadow mask—referred to as “feathering.” Feathering increases with the magnitude of the separation between the substrate and the shadow mask. As a result, this separation is typically kept as small as possible without compromising the integrity of the chucks that hold the substrate and shadow mask. Still further, any non-uniformity in this separation across the deposition area will give rise to variations on the amount of feathering. Such non-uniformity can arise from, for example, a lack of parallelism between the substrate and shadow mask, bowing or sagging of one or both of the substrate and shadow mask, and the like. 
         [0007]    Unfortunately, it can be difficult to hold the shadow mask and substrate close enough to avoid giving rise to significant amounts of feathering. Furthermore, a shadow mask must be supported only at its perimeter to avoid blocking the passage of the vaporized atoms to the through-hole pattern. As a result, the center of the shadow mask can sag due to gravity, which further exacerbates feathering issues. 
         [0008]    In practice, therefore, critical features formed by prior-art shadow-mask-based deposition techniques are typically separated by relatively large areas of open space to accommodate feathering, which limits the device density that can be obtained. For example, each pixel of an active-matrix organic light-emitting-diode (AMOLED) display normally includes several regions of organic light-emitting material, each of which emits a different color of light. Due to feathering issues, prior-art AMOLED displays have typically been restricted to approximately 600 pixels-per-inch (ppi) or less, which is insufficient for many applications, such as near-to-eye augmented reality and virtual-reality applications. In addition, the need for large gaps within and between the pixels gives rise to a reduced pixel fill factor, which reduces display brightness. As a result, the current density through the organic layers must be increased to provide the desired brightness, which can negatively impact display lifetime. 
         [0009]    The need for a process that enables high-resolution direct patterning remains unmet in the prior art. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention enables high-resolution direct deposition of a patterned layer of material on a substrate without some of the costs and disadvantages of the prior art. The present invention enables high-precision alignment of a shadow mask and substrate that are separated by only a few microns. The present invention also provides for mitigation of gravity-induced sag of a shadow mask that is supported only at its perimeter. Embodiments of the present invention are particularly well suited for applications requiring high-density patterns of material on a substrate, such as dense-pixel displays (DPD), high-definition displays, and the like. 
         [0011]    An illustrative embodiment of the present invention is a direct-patterning deposition system comprising a first chuck having a first mounting surface for holding a substrate and a second chuck having a second mounting surface for holding a shadow mask that comprises a pattern of through-holes. The second chuck includes a frame that surrounds a central opening that exposes the pattern of though-holes in the shadow mask. As a result, during deposition, vaporized atoms of the material can pass through the second chuck and the through-holes to deposit in a desired pattern on a deposition region of the front surface of the substrate. 
         [0012]    The first chuck generates a first electrostatic force that is selectively applied to the back surface of the substrate. The first chuck is also dimensioned and arranged such that it does not project above the front surface of the substrate. In similar fashion, the second chuck generates a second electrostatic force that is selectively applied to the back surface of the shadow mask. The second chuck is also dimensioned and arranged so that it does not project above the front surface of the shadow mask. When the shadow mask and substrate are in alignment for a deposition, no part of either the first and second chuck encroaches into the three-dimensional space between the substrate and the shadow mask. As a result, the substrate and shadow mask can be positioned brought very close during deposition, thereby mitigating feathering. 
         [0013]    In some embodiments, at least one of the first and second attractive forces is a force other than electrostatic, such as a vacuum-generated force, a magnetic force, etc. 
         [0014]    In some embodiments, the second mounting surface is dimensioned and arranged to create a tensile stress in the front surface of the shadow mask that mitigates gravity-induced sag of its central region. In some such embodiments, the frame of the second chuck is shaped such that its mounting surface slopes away from top edge of the inner perimeter of the frame. As a result, when the shadow mask is mounted in the second chuck, the shadow mask becomes slightly bowed, which induces a tensile stress in the front surface of the shadow mask. In some of these embodiments, the mounting surface is curved downward from the top edge of the inner perimeter of the frame. 
         [0015]    An embodiment of the present invention is a system for depositing a pattern of material on a first region of a substrate through an arrangement of through-holes in a shadow mask, wherein the substrate includes a first major surface and a second major surface that comprises the first region, and wherein the shadow mask includes a third major surface and a fourth major surface that comprises the through-holes, the system comprising: a first chuck for holding the substrate, the first chuck being dimensioned and arranged to selectively impart a first attractive force on the first major surface; a second chuck for holding the shadow mask, the second chuck comprising a frame that surrounds a first opening that enables the material to pass through the second chuck to the through-holes, the second chuck being dimensioned and arranged to selectively impart a second attractive force on the third major surface; and an alignment system for controlling the relative position of the first chuck and the second chuck to align the shadow mask and the substrate. 
         [0016]    Another embodiment of the present invention is a system for depositing a pattern of material on a first region of a substrate through an arrangement of through-holes in a shadow mask, wherein the substrate includes a first major surface and a second major surface having a first lateral extent, the second major surface comprising the first region, and wherein the shadow mask includes a third major surface and a fourth major surface that comprises the through-holes, the system comprising: a first chuck for holding the substrate; and a second chuck for holding the shadow mask, the second chuck comprising a frame that surrounds a first opening that enables the material to pass through the second chuck to the through-holes; wherein, when the shadow mask and substrate are aligned, the shadow mask and substrate collectively define a second region, the second region (1) having a second lateral extent that is equal to or larger than the first lateral extent, (2) having a thickness that is equal to a separation between the substrate and the shadow mask, and (3) excluding the first chuck and the second chuck; and wherein the first chuck and second chuck are dimensioned and arranged to enable the thickness to be greater than 0 microns and equal to or less than 10 microns. 
         [0017]    Yet another embodiment of the present invention is a method for depositing a pattern of material on a first region of a substrate through an arrangement of through-holes in a shadow mask, wherein the substrate includes a first major surface and a second major surface having a first lateral extent, the second major surface comprising the first region, and wherein the shadow mask includes a third major surface and a fourth major surface that comprises the through-holes, the method comprising: holding the substrate in a first chuck that imparts a first attractive force selectively on the first major surface; holding the shadow mask in a second chuck that imparts a second attractive force selectively on the third major surface, wherein the second chuck enables the passage of particles comprising the material through the second chuck to the through-holes; aligning the substrate and the shadow mask such that the second major surface and the fourth major surface are separated by a distance that is greater than 0 microns and less than or equal to 10 microns; and passing a flow of particles comprising the material through the second chuck and the shadow mask. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  depicts a schematic drawing of the salient features of a high-precision, direct-patterning deposition system in accordance with an illustrative embodiment of the present invention. 
           [0019]      FIG. 2  depicts methods of an operation for forming a patterned layer of material on a substrate in accordance with the illustrative embodiment of the present invention. 
           [0020]      FIG. 3A  depicts a schematic drawing of a cross-sectional view of a mask chuck in accordance with the illustrative embodiment. 
           [0021]      FIG. 3B  depicts a schematic drawing of a cross-sectional view of substrate chuck  102  while holding substrate  114 . 
           [0022]      FIGS. 4A-B  depicts a schematic drawings of top and cross-section views, respectively, of a mask chuck in accordance with the illustrative embodiment. 
           [0023]      FIG. 5  depicts a cross-sectional view of shadow mask  122  mounted in mask chuck  104 . 
           [0024]      FIG. 6  depicts a schematic drawing of a cross-sectional view of a portion of system  100  with substrate  114  and shadow mask  122  in alignment for deposition of material  118 . 
           [0025]      FIG. 7A  depicts a schematic drawing of a cross-sectional view of a portion of a mask chuck in accordance with a first alternative embodiment of the present invention. 
           [0026]      FIG. 7B  depicts a schematic drawing of a cross-sectional view of a portion of a mask chuck in accordance with a second alternative embodiment of the present invention. 
           [0027]      FIGS. 8A-B  depict schematic drawings of top and cross-section views, respectively, of a mask chuck in accordance with a third alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 1  depicts a schematic drawing of the salient features of a high-precision, direct-patterning deposition system in accordance with an illustrative embodiment of the present invention. System  100  includes substrate chuck  102 , mask chuck  104 , source  106 , alignment system  108 , vacuum chamber  110 , and controller  112 . System  100  is operative for evaporating a desired pattern of material onto a substrate surface without the need for subsequent subtractive patterning operations, such as photolithography and etching. 
         [0029]    System  100  is described herein with respect to the deposition of a pattern of light-emitting organic material on a glass substrate as part of the fabrication of an AMOLED display. However, it will be clear to one skilled in the art, after reading this Specification, that the present invention can be directed toward the formation of directly patterned layers of virtually any thin- and thick-film material (organic or inorganic) on any of a wide range of substrates, such as semiconductor substrates (e.g., silicon, silicon carbide, germanium, etc.), ceramic substrates, metal substrates, plastic substrates, and the like. Further, although the illustrative embodiment is a thermal evaporation system, one skilled in the art will recognize, after reading this Specification, that the present invention can be directed toward virtually any material-deposition process, such as e-beam evaporation, sputtering, and the like. Still further, although the depicted example is a deposition system suitable for use in single-substrate planar processing, the present invention is also suitable for use in other fabrication approaches, such as cluster-tool processing, track processing, roll-to-roll processing, reel-to-reel processing, etc. As a result, the present invention is suitable for use in myriad applications including, without limitation, packaging applications, integrated-circuit fabrication, MEMS fabrication, nanotechnology device fabrication, ball-grid array (BGA) fabrication, and the like. 
         [0030]    Substrate chuck  102  is a platen for holding substrate  114  via an attractive force applied only to its back surface. In the depicted example, substrate chuck  102  generates electrostatic force to hold a substrate; however, in some embodiments, substrate chuck  102  holds a substrate via a different attractive force, such as a vacuum-generated force, a magnetic force, etc. For the purposes of this Specification, including the appended claims, the term “magnetic force” includes any force arising from the use of permanent magnets and/or electromagnets. Substrate chuck  102  is described in more detail below and with respect to  FIGS. 3A-B . 
         [0031]    In the depicted example, substrate  114  is a glass substrate suitable for use in active-matrix organic-light-emitting-diode (AMOLED) displays. Substrate  114  includes two major surfaces—back surface  116  and front surface  118 , on which the display elements are defined. Front surface  118  defines plane  120 . 
         [0032]    Mask chuck  104  is a fixture for holding shadow mask  122  via an attractive force imparted on only its back surface. In the depicted example, mask chuck  104  holds shadow mask  122  using electrostatic force. In some embodiments, mask chuck  104  holds a shadow mask via a different attractive force, such as a vacuum-generated force, a magnetic force, etc. Mask chuck  104  is described in more detail below and with respect to  FIGS. 4A-B . 
         [0033]    In the depicted example, shadow mask  122  is a high-precision shadow mask comprising handle substrate  124  and membrane  126 , which is suspended over a central opening formed in the handle substrate. Membrane  126  includes through-hole pattern  128 . Shadow mask  122  includes two major surfaces—front surface  130  and back surface  132 . Front surface  130  is the top surface of membrane  126  (i.e., the membrane surface distal to handle substrate  124 ), which defines plane  134 . Back surface  132  is the surface of handle substrate  124  (i.e., the substrate surface distal to membrane  126 ). It should be noted that, while shadow mask  122  is a high-precision, membrane-based shadow mask, mask chucks in accordance with the present invention can be used to hold virtually any type of shadow mask. 
         [0034]    Source  106  is a crucible for vaporizing material  118 , which is an organic material that emits light at a desired wavelength. In the depicted example, source  106  is a single-chamber crucible that is centered with respect to substrate  114 ; however, in some embodiments, source  106  includes a plurality of chambers that are arranged in one- and/or two-dimensional arrangements. When material  118  is melted or sublimed within the low-pressure atmosphere of chamber  110 , vaporized atoms of material  118  are ejected from the source and propagate toward substrate  114  in substantially ballistic fashion. 
         [0035]    Alignment system  108  is a high-precision alignment system for controlling the relative position between substrate  114  and shadow mask  122 . In the depicted example, alignment system  108  is capable of independently controlling the position of each of substrate chuck  102  and mask chuck  104  in six dimensions. It is also capable of controlling the position of source  106  so that the source can be moved relative to the substrate/shadow mask combination to improve deposition uniformity across the substrate, if desired. 
         [0036]    Vacuum chamber  110  is a conventional pressure vessel for containing a low-pressure environment required for the evaporation of material  118 . In the depicted example, vacuum chamber  110  is a standalone unit; however, it can also be realized as a part of a cluster deposition system or track-deposition system without departing from the scope of the present invention. In some embodiments, vacuum chamber  110  includes several evaporation sources/shadow mask combinations that enable formation of different patterns of different materials on substrate  114 , such as, for example, multiple light-emitting subpixels that emit light at different colors (e.g., red, green, and blue). 
         [0037]    Controller  112  is a conventional instrument controller that, among other things, provides control signals  136  and  138  to substrate chuck  102  and mask chuck  104 , respectively. 
         [0038]      FIG. 2  depicts methods of an operation for forming a patterned layer of material on a substrate in accordance with the illustrative embodiment of the present invention. Method  200  begins with operation  201 , wherein substrate  114  is mounted in substrate chuck  102 . 
         [0039]      FIG. 3A  depicts a schematic drawing of a cross-sectional view of a mask chuck in accordance with the illustrative embodiment. Mask chuck  102  includes platen  302  and electrodes  304 - 1  and  304 - 2 . 
         [0040]    Platen  302  is a structurally rigid platform comprising substrate  306  and dielectric layer  308 . Each of substrate  306  and dielectric layer  308  includes an electrically insulating material, such as glass, ceramic, anodized aluminum, composite materials, Bakelite, and the like to electrically isolate electrodes  304 - 1  and  304 - 2  from each other and from substrate  114  when it is mounted in the substrate chuck. 
         [0041]    Electrodes  304 - 1  and  304 - 2  are electrically conductive elements formed on the surface of substrate  306  and overcoated by dielectric layer  308  to embed them within platen  302 . Electrodes  304 - 1  and  304 - 2  are electrically coupled with controller  112 . It should be noted that electrodes  304 - 1  and  304 - 2  are depicted as simple plates; however, in practice, mask chuck  102  can have electrodes that are shaped in any manner, such as interdigitated comb fingers, concentric rings, irregular shapes, etc. 
         [0042]    Dielectric layer  308  is a structurally rigid layer of glass disposed over electrodes  304 - 1  and  304 - 2  to give rise to mounting surface  310 . 
         [0043]      FIG. 3B  depicts a schematic drawing of a cross-sectional view of substrate chuck  102  while holding substrate  114 . 
         [0044]    To hold substrate  112  in substrate chuck  102 , control signal  136  generates a voltage potential between electrodes  304 - 1  and  304 - 2 . When back surface  116  is brought into contact with mounting surface  310  (i.e., the top surface of dielectric layer  308 ), sympathetic charge regions develop within substrate  114  as shown. As a result, an electrostatic force is selectively imparted on back surface  116 , thereby attracting it to mounting surface  310 . 
         [0045]    Although the illustrative embodiment includes a substrate chuck that holds substrate  114  via electrostatic force, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments wherein a substrate is held in a substrate chuck via an attractive force other than an electrostatic force, such as a vacuum-generated force, a magnetic force, and the like. 
         [0046]    At operation  202 , shadow mask  122  is mounted in mask chuck  104 . 
         [0047]      FIGS. 4A-B  depicts a schematic drawings of top and cross-section views, respectively, of a mask chuck in accordance with the illustrative embodiment. The cross-section depicted in  FIG. 4B  is taken through line a-a shown in  FIG. 4A . Mask chuck  104  includes frame  402 , electrodes  404 - 1  and  404 - 2 , and pads  406 . 
         [0048]    Frame  402  is a structurally rigid circular ring of electrically insulating material. Frame  402  surrounds opening  408 , which is sufficiently large to expose the entirety of through-hole pattern  128 . In some embodiments, frame  402  has a shape other than circular, such as square, rectangular, irregular, etc. In some embodiments, frame  402  comprises an electrically conductive material that is coated with an electrical insulator. 
         [0049]    Electrodes  404 - 1  and  404 - 2  are electrically conductive elements formed on the surface of frame  402 . Electrodes  404 - 1  and  404 - 2  are electrically coupled with controller  112 . 
         [0050]    Pads  406  are structurally rigid plates of electrically insulating material disposed on electrodes  404 - 1  and  404 - 2 . Each of pads  406  includes mounting surface  410 , against which shadow mask  122  is held when mounted in the mask chuck. 
         [0051]      FIG. 5  depicts a cross-sectional view of shadow mask  122  mounted in mask chuck  104 . 
         [0052]    Shadow mask  122  is held in mask chuck  104  by an electrostatic force imparted between mounting surface  410  and back surface  132 . The electrostatic force arises in response to a voltage potential between electrodes  404 - 1  and  404 - 2 , which is generated by control signal  138 . When back surface  132  is brought into contact with mounting surface  410 , sympathetic charge regions develop within handle substrate  124  as shown. As a result, the electrostatic force is selectively imparted between back surface  132  and mounting surface  410 . 
         [0053]    At operation  203 , alignment system  108  aligns substrate  114  and shadow mask  122  by controlling the position of substrate chuck  102 . In some embodiments, alignment system aligns the substrate and shadow mask by controlling the position of mask chuck  104 . In some embodiments, the positions of both chucks is controlled to align the substrate and shadow mask. 
         [0054]    It is an aspect of the present invention that, in some embodiments, neither substrate chuck  102  nor mask chuck  104  includes any structural element that projects past its respective mounting surface. As a result, a substrate and shadow mask can be aligned with little or no separation between them to mitigate feathering during deposition. One skilled in the art will recognize that in conventional direct-deposition systems, the separation between substrate and shadow mask must be at least a few tens, or even hundreds, of microns. 
         [0055]      FIG. 6  depicts a schematic drawing of a cross-sectional view of a portion of system  100  with substrate  114  and shadow mask  122  in alignment for deposition of material  118 . 
         [0056]    When the substrate and shadow mask are aligned, they collectively define region  602  between them. Region  602  has a lateral extent, L 1 , which is equal to that of front surface  118 . Region  602  also has a thickness that is equal to the separation, s 1 , between planes  120  and  134  (i.e., the separation between the substrate and the shadow mask). 
         [0057]    Because no portion of substrate chuck  102  extends past plane  120  into region  602 , there is no obstruction between the substrate and shadow mask. As a result, separation, s 1 , between substrate  114  and shadow mask  122  can be extremely small (10 microns). In fact, if desired, the substrate and shadow mask can be brought into contact with one another. The ability to perform direct patterning with a substrate/shadow mask separation equal to or less than 10 microns affords embodiments of the present invention particular advantage over prior-art direct-patterning deposition systems because it enables feathering to be significantly reduced or even eliminated. 
         [0058]    At operation  204 , source  106  is heated to vaporize material  118  to realize a patterned layer of the material on front surface  118  of substrate  114 . 
         [0059]    In some embodiments, mask chucks in accordance with the present invention include one or more features that mitigate or eliminate gravity-induced sag of a shadow mask when the shadow mask is mounted. As discussed in detail in U.S. patent application Ser. No. 15/597,635 filed on May 17, 2017 (Attorney Docket: 6494-208US1), which is incorporated herein by reference, a shadow mask can sag by several microns in the center due to its own mass and the effect of gravity. This gravity-induced sag leads to several significant issues that exacerbate feathering. First, it increases the separation between the shadow mask and the substrate in the center of the deposition region, which is typically centered on the shadow mask. As discussed above, feathering increases with substrate/shadow mask separation distance. Second, it leads to a non-uniform separation between the substrate and the shadow mask, which gives rise to a variation in the degree of feathering that occurs across the substrate surface. The non-uniformity makes it difficult, if not impossible, to compensate for feathering via creative mask layout. 
         [0060]    It is yet another aspect of the present invention that a mask chuck can include features that mitigate gravity-induced sag of a shadow mask. 
         [0061]      FIG. 7A  depicts a schematic drawing of a cross-sectional view of a portion of a mask chuck in accordance with a first alternative embodiment of the present invention. The cross-section depicted in  FIG. 7A  is taken through line a-a shown in  FIG. 4A . Mask chuck  700  includes frame  402 , electrodes  404 - 1  and  404 - 2 , and pads  702 . 
         [0062]    Pads  702  are analogous to pads  406  described above; however, each pad  702  has a mounting surface that is designed to induce or increase tensile strain in the shadow-mask when it is mounted in the mask chuck. Pad  702  has mounting surface  704 , which is linearly tapered downward from inner edge  706  (i.e., the edge proximal to opening  408 ) to outer edge  708 . In other words, mounting surface  704  tapers in the negative z-direction, as shown, from point  714  to point  716  (i.e., where from it meets inner edge  706  at plane  710  to where it meets outer edge  708  at plane  712 ). In embodiments in which inner edge  706  is perpendicular to plane  710 , therefore, inner edge  706  and mounting surface  704  form interior angle, θ, such that it is an acute angle. 
         [0063]    When shadow mask  122  is held in mask chuck  700 , back surface  132  is attracted to mounting surface  704 , thereby inducing a curvature in the shadow mask that increase the laterally directed tension in front surface  130  of the shadow mask. As a result, the membrane is pulled tighter and gravity-induced sag is reduces or eliminated. 
         [0064]      FIG. 7B  depicts a schematic drawing of a cross-sectional view of a portion of a mask chuck in accordance with a second alternative embodiment of the present invention. The cross-section depicted in  FIG. 7B  is taken through line a-a shown in  FIG. 4A . Mask chuck  718  includes frame  402 , electrodes  404 - 1  and  404 - 2 , and pads  720 . 
         [0065]    Pads  720  are analogous to pads  406  described above; however, like pads  702 , each pad  720  has a mounting surface that is designed to induce or increase tensile strain in the shadow-mask when it is mounted in the mask chuck. Pad  720  has mounting surface  722 , which curves downward (i.e., in the negative z-direction, as shown) from inner edge  706  to outer edge  708 . In other words, mounting surface  722  tapers in the negative z-direction, as shown, from point  714  to point  716 . 
         [0066]    When shadow mask  122  is held in mask chuck  718 , back surface  132  is attracted to mounting surface  722 , thereby inducing a curvature in the shadow mask that increase the laterally directed tension in front surface  130  of the shadow mask. As a result, the membrane is pulled tighter and gravity-induced sag is reduces or eliminated. In some embodiments, the amount of additional tension induced in front surface  130  can be controlled by controlling the magnitude of the voltage differential applied to electrodes  404 - 1  and  404 - 2 . 
         [0067]    It will be clear to one skilled in the art, after reading this Specification, that the directions in which mounting surfaces  704  and  722  slope (or curve) would be reversed for a deposition system in which the mask were mounted upside down as compared to its orientation depicted in  FIG. 1 . Further, in such a configuration, it would typically be necessary that substrate chuck  102  be designed to enable substrate  114  to reside within opening  408  to enable a substrate/shadow mask separation of less than or equal to 10 microns. 
         [0068]      FIGS. 8A-B  depict schematic drawings of top and cross-section views, respectively, of a mask chuck in accordance with a third alternative embodiment of the present invention. Mask chuck  800  includes mask chuck  104  and support grid  802 . 
         [0069]    Support grid  802  includes plate  804  and support ribs  806 . 
         [0070]    Plate  804  is a rigid plate from which support ribs  806  extend. In some embodiments, plate  804  and support ribs  806  are machined from a solid body of structural material. Materials suitable for use in plate  804  and support ribs  806  include, without limitation, metals, plastics, ceramics, composite materials, glasses, and the like. Plate  804  is designed to mount to frame  402  to locate support grid  802  within opening  408  such that it mechanically supports membrane  126  when shadow mask  122  is mounted in mask chuck  800 . 
         [0071]    Support ribs  806  are arranged to support shadow mask  122  in regions that lie between the through-holes of through-hole arrangement  128 . Typically, the through-holes of a shadow mask are arranged in clusters that correspond to different die regions on the substrate. Since these die regions are usually separated by “lanes” intended for removal by a dicing saw, support ribs  806  are preferably arranged to match the arrangement of these lanes. It should be noted, however, that any suitable arrangement of support ribs can be used in support grid  802 . 
         [0072]    Support grid  802  is formed such that their top surfaces  808  are coplanar and define plane  810 . Plane  810  lies above mounting surface  410  by a distance equal to the thickness of frame  124 . As a result, when frame  124  is in contact with mounting surface  410 , support ribs  806  are in contact with membrane  126 . 
         [0073]    In some embodiments, shadow mask  122  is held upside down in mask chuck  800  such that membrane  126  is in contact with mounting surface  410 . In such embodiments, support grid  802  is designed to fit within opening  408  such that plane  810  is coplanar with mounting surface  410 . As a result, membrane  126  is supported by support grid  802  such that it is perfectly level all the way across opening  408 . 
         [0074]    It is to be understood that the disclosure teaches just some embodiments in accordance with the present invention and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.