Abstract:
A carrier for the masks used in Electron Projection Lithography, or other workpieces used in nanotechnology fields, comprises a rectangular frame having a set of four electrostatic chucks in the top surface for holding the mask above a central aperture that has an electron absorber on the bottom for suppressing backscattering; the frame being supported by a bottom carrier that grips the frame with a set of flexures flexible in the z-direction, stiff in an azimuthal direction and flexible in a radial direction.

Description:
BACKGROUND OF INVENTION 
   The field of the invention is that of electron projection lithography, or other nanotechnology fields, in particular holders for masks or other workpieces used in connection therewith. 
   Electron Projection Lithography (EPL) is one of the leading candidates for Next Generation Lithography, generally considered to be linewidths of 65 nm and below. 
   Other candidate technologies are electron pencil beams in a direct write mode and X-ray lithography. In the former the very thin electron beam is focused to the required size beamspot and is focused directly on the wafer. This technology is used currently and has been used for some time in writing masks (more formally termed reticles) for optical lithography. In the latter, the X-ray beam is spread out to the dimension of the chip being exposed and a method similar to contact printing is used. This type of printing is used because X-Rays cannot be focused. Since the mask and final image are in a 1:1 ratio, accuracy requirements in making the mask are extreme. 
   Electron Projection Lithography has the benefit that the mask is larger than the final image, so that errors in the mask (reticle) are demagnified by the demagnification ratio when they print on the final wafer image. Conventionally, a demagnification ratio of 4:1 is used in electron projection lithography. 
   In currently preferred technology, the EPL mask is formed in a very thin membrane that is selected with a thickness of less than one micron to reduce the heat load of energy deposited by electrons that are not part of the image. In contrast to a stencil mask that absorbs the unwanted beam, such as is used in photon (optical) lithography, the electron mask scatters the unwanted electrons only slightly, and subsequent optical elements remove the unwanted electrons from the beam. 
   EPL masks may be formed of any convenient material. Silicon is preferred because it is durable, its properties are well known and semiconductor techniques can be used to form the desired pattern in the material. 
   An alternative technology, referred to as a stencil mask, employs a mask thickness of one to three microns that absorbs the unwanted electrons. 
   Both of these competing technologies employ masks that are fragile and are susceptible to distortions even when they do not break. This extreme susceptibility arises in part from dimensional changes or distortions that will be reflected in changes in the final image. 
   It is essential, therefore that the masks be held in fixtures that do not distort them, both when being handled and also when in use. 
   Further, it is essential that the masks be clamped in an identical manner during mask formation and in use. 
   At the present state of development, nominal requirements for a mask holder are:
         maintain&lt;2 micron clamp pad coplanarity;   provide&gt;1PSI clamping pressure;   provide&lt;5 micron mask to mask positioning repeatability; and accommodate 200 mm diameter masks.       

   SUMMARY OF INVENTION 
   The invention relates to a holder for 200 mm diameter masks that holds the masks repeatably with acceptable distortion. 
   A feature of the invention is that the holder fits within a vertical space of illustratively 13 mm. 
   Another feature of the invention is that the material is both non-magnetic and slightly conductive. 
   Another feature of the invention is that the assembly is used both to hold the mask during the writing process and also to hold the mask during transport. 
   Yet another feature of the invention is that the carrier is separable, with a bottom layer that is removed during a projection exposure. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows an overall view of the carrier with a mask. 
       FIG. 2  shows an overall view of the carrier without a mask. 
       FIG. 3  shows a view of the intermediate layer of the carrier. 
       FIG. 4  shows a detail of the flexure mounts. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an overall view of the invention holding a mask. Mask  10  rests on a chuck plate  100  that, in turn, rests on a carrier body  150 . The mask is held during the exposure process by the electrostatic attraction of a set of electrostatic chucks formed as described below. At other times, mechanical clamps hold the mask with respect to the assembly. 
   Mask  10  is illustratively formed in a 200 mm diameter silicon wafer. Other mask materials may also be used with the invention. 
   Mask  10  is positioned with respect to the chuck assembly  100  by a three point system with two reference pins  15  and a flexure assembly  25 . The flexure assembly includes a pin  26  that fits into a notch on the mask. Flexure  25  is rigid for motions to the left and right in the figure (which are tangential with respect to the mask) and flexible for motions that are substantially radial with respect to the mask. Flexure  25  is also rigid with respect to vertical motion, but that aspect is not relevant to the mask position because the flexure does not restrict vertical motion of the mask. 
   For purposes of description, a coordinate system with x and y axes in the plane of the mask and a z axis perpendicular to that plane will be used. 
   Two clips  20  at three o&#39;clock and nine o&#39;clock on the mask (using the convention that the upper right direction in the figure represents 12 o&#39;clock on the mask face) provide an electrical ground connection to reduce or drain away electrostatic charge and also provide a slight vertical retaining force. 
   Clips  20  are held in place by retaining screws. The clips are sufficiently flexible that they do not distort the planarity of the mask to any significant degree, so that the amount of torque on the screws is not relevant. 
     FIG. 2  shows the same chuck assembly with the mask removed. Four electrostatic chucks  120  are visible on the four sides of the central open area (central aperture) of chuck plate  100 , denoted with the numeral  102 . Chucks  120  may be formed by embedding electrodes below the surface of the chuck plate  100  and depositing an insulator such as another thin layer of alumina over the electrodes; or by any other convenient technique. 
   Chuck plate  100  illustratively is made from alumina ceramic, which is rigid, non-magnetic and is not a conductor. As discussed below, the frame may be thinly coated with a conductive material, so that it does not support eddy currents during operation of the electron beam writing process, but also drains away stray electrons that will inevitably find their way to the surface of the frame. 
   One of the requirements on the particular version of the invention illustrated is that the entire assembly, including the wafer, fits in a space 13 mm high. The wafer is 0.7 mm thick, the chuck plate  100  is 6.5 mm thick and the bottom carrier body  150  is 5.8 mm thick. 
   Those skilled in the art will appreciate that shaping a magnetic field is much more difficult than shaping an optical lens, so that it is important to minimize the space that is outside the path for the magnetic flux. The mask assembly has to fit in such a confined space, so that trade-offs in constructing the machine that writes the masks were made that constrain the space available for the mask and its support. 
   In the center of aperture  102 , the surface of the bottom of the chuck plate is visible. Aperture  102  in this embodiment does not extend through chuck plate  100 , but has an aperture depth less than a chuck plate thickness, therefore having a bottom surface. In this case, it is a layer of carbon  130  that absorbs electrons that pass through the mask during the process of mask writing. This absorption is an advantageous feature of the invention, since many other materials will scatter electrons back up toward the mask. When that happens, the image being formed on the mask is distorted by exposure of the resist that is being patterned in the mask writing process. 
   The carbon  130  is nominally 1 mm thick and the aperture is nominally 5.5 mm deep, leaving a bottom layer thickness of 1 mm for the remaining material of the chuck plate. 
   Alternatively, the aperture could extend through chuck plate  100  and the carbon would then be on the top surface of carrier body  150 . This arrangement would require that the carrier body  150  remains in place during the mask writing process. 
   On the bottom of carrier body  150 , there is another electrostatic chuck. It holds the carrier body  150  in place during the mask writing process, with both chuck  100  and carrier body  150  in position. 
   Carrier body  150  and electron absorber  130  are removed during the projection lithography exposure. Chuck plate  100  may be used to hold the mask in a projection lithography tool, in which the beam has to pass unhindered along the next section of the tool. 
     FIG. 3  shows the carrier body  150  with the chuck plate  100  removed. Three flexure mounts  110  are positioned at a nominal zero degrees, 120 degrees and 240 degrees about the circumference of the chuck  120 . Each flexure mount  110  comprises a thin strip  112 , illustratively formed from a strip of BeCu 40–60 mm long, 12 mm wide and 0.25 mm thick. These dimensions give flexibility in the vertical (z) direction together with azimuthal rigidity in the horizontal (x-y) plane. 
   The mounts are flexible in the radial direction (about the center of chuck  100 ) to reduce distortion caused by thermal stress (due to differences in material expansion rates) and mechanical stress induced by handling of and electrostatic clamping of carrier body  150  to the mask exposure tool. 
   Details of flexures  110  are shown in  FIG. 4 , in which two screws mount the strip  112  to the base of carrier body  150 . A block  116 , illustratively of BeCu is mounted at the outer end of strip  112  and is threaded to hold a z-adjusting screw  115  that adjusts the end vertically over a range of 0.25 mm. The vertical screw  115  rests against bearing pad  114 , formed from titanium which is more resistant to particles flaking off it than the low expansion glass ceramic body of carrier body  150 . At the sides of the structure, tangent flexures  117  extend outward from block  115  to hold end blocks  113 , also of BeCu. These end blocks are provided with clearance holes to accept fastening screws  118  which engage with corresponding threaded holes on the chuck plate  100 . 
   When the structure is assembled, a chuck plate  100  is placed within the set of flexures  110  with screws  118  retracted. Screws  118  are tightened to hold chuck plate  100  in place. Also, vertical screws  115  are adjusted to level the top of chuck plate  100 , nominally with a tolerance of less than 2 microns. 
   The screws and other metal components are made from a non-magnetic material such as BeCu or 6AL-4V titanium. The body of chuck plate  100  is formed from alumina and coated with a conductive ceramic material on regions which would be in the line of sight of stray electrons during use. Carrier body  150  is formed from low expansion glass ceramic and coated with material such as TiN or with Au or Ag, in order to provide a conductive path to drain away stray electrons that would otherwise charge up the material and deflect the electron beam in unpredictable ways. 
   The sequence of mounting a wafer is:
         Assemble the chuck plate  100  to the carrier body  150 ;   Tighten the horizontal gripping screws  118 ;   Level the frame of chuck plate  100  in the z-direction;   Place the mask on the top surface of chuck plate  100 ;   Actuate flexure assembly  25  to simultaneously position the mask against the two reference pins  15  and align the mask notch to pin  26 ; and   Tighten the ground clips  20  to make contact.       

   The electrostatic attraction between chuck plate  100  and wafer  10  and between carrier body  150  and the exposure tool is only energized when the chuck assembly is in place in that tool (a mask writing tool or a projection exposure tool). 
   The mask may be stored in the assembly, and the chuck plate  100  with the mask attached may be removed from the carrier when the mask pattern is to be printed in a projection lithography tool. 
   Those skilled in the art will appreciate that the structure may be used with a semiconductor wafer instead of a mask in direct-write electron beam lithography. This direct-write may be used in forming integrated circuits or it may also be used in forming components for nanotechnology. For convenience in the claims, the term “workpiece” will be used to denote both a mask, a semiconductor wafer, and a material for nanotechnology components. 
   While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.