Abstract:
A mask fixture for etching an item includes: a top fixture disposed over the item, including a reservoir centered within the top fixture for containing an etchant; a bottom fixture underneath the item to be etched including a recessed surface area centered within the bottom fixture; and an etch-resistant window for holding the item to be etched, the etch-resistant window disposed entirely within the recessed surface area. In addttion, a small via centered within and intersecting both the top and bottom fixtures acts as a path for a high intensity light beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of, and claims priority from U.S. patent application Ser. No. 12/172,876, filed on Jul. 14, 2008, which application is incorporated by reference in its entirety herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT 
     None. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     None. 
     FIELD OF THE INVENTION 
     The invention disclosed broadly relates to the field of transmission electron microscopy (TEM) and more particularly relates to the field of TEM sample etching fixtures. 
     BACKGROUND OF THE INVENTION 
     During the preparation of transmission electron microscopy (TEM) samples, a mask or template is placed over a sample or specimen to allow a selective area to be eroded through acid etching. One common factor when preparing samples that require the use of highly corrosive etchants has been to use wax to mask the area not to be etched. One such process is the jet etch process where the sample is masked and mounted with wax. 
     Another procedure is to manually prepare samples which are entirely covered with wax except for the portion to be etched. Because manually prepared samples need little hardware set-up, the costs of the operation are mostly personnel related costs, therefore process steps that can be removed result in money saved. The drawback is the difficulty of monitoring the etch erosion. 
     In either case, once the etching is complete, the wax or stop-off must be removed with an adequate solvent. For the manual preparation of a common 100 micron thick sample, this process can be rather labor intensive and create chemical waste from both the etchant as well as the solvent used for the mask removal. In the jet etch process, the wax removal cannot be automated because the sample is also mounted using the wax. This necessitates the manual handling of the sample specifically for post etch cleaning. 
     SUMMARY OF THE INVENTION 
     Briefly, according to an embodiment of the invention, a mask fixture for etching an item includes: a top fixture disposed over the item, including a reservoir centered within the top fixture for containing an etchant; a bottom fixture underneath the item to be etched including a recessed surface area centered within the bottom fixture; and an etch-resistant window for holding the item to be etched, the etch-resistant window disposed entirely within the recessed surface area. In addition, a small via centered within and intersecting both the top and bottom fixtures acts as a path for a high intensity light beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To describe the foregoing and other exemplary purposes, aspects, and advantages, we use the following detailed description of an exemplary embodiment of the invention with reference to the drawings, in which: 
         FIG. 1   a  is an illustration of the mask fixture, showing an oblique view of the three main components of the fixture, according to an embodiment of the present invention; 
         FIG. 1   b  is an illustration showing a cross-section view of the top and bottom portions of the fixture, and the sapphire window, according to an embodiment of the present invention; 
         FIG. 2   a  is a close-up cross-section view of the sample on the sapphire backing disk, according to an embodiment of the present invention; 
         FIG. 2   b  is a cross-section view showing the different portions of the fixture, according to an embodiment of the present invention; and 
         FIG. 3  is an illustration of a person peering through a microscope at the progress of the etching process, according to an embodiment of the present invention; 
     
    
    
     While the invention as claimed can be modified into alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the present invention. 
     DETAILED DESCRIPTION 
     We discuss a sample holder for the micro analysis of a sample item. The holder is shaped like a hockey puck, with a top section and a bottom section sandwiching the sample item with a mask, all contained within the hockey puck-shaped holder. This sample holder is a non-consumable mask fixture used for TEM sample preparation. It is clamped about the specimen and is self-sealing, eliminating the need for masking and mounting waxes as well as the solvents needed to remove them. Integral to the fixture is an etch-resistant window to allow in situ observation of the etch progress. When etching certain materials such as silicon, light will begin to be transmitted through the sample as the eroded portion approaches a usable thickness. By providing a path for a high intensity light in the fixture, the transmission of light through the sample can be monitored either manually or automatically. 
     Referring now in specific detail to the drawings, and particularly  FIG. 1 , there is illustrated a mask fixture  100  for TEM sample preparation. The specific geometry of the mask fixture  100  is tailored to the thickness of the specimen and the area to be etched. The mask fixture  100  is essentially a three piece fixture: a puck-shaped bottom  180 , a puck-shaped top  120  and a sapphire window  170 . Note that we discuss a hockey puck, or disc, shape, but those with knowledge in the art will appreciate that other shapes can be advantageously used within the spirit and scope of the invention. 
     In a preferred embodiment, the puck-shaped base  180  is constructed of PVDF (polyvinylidene fluoride) and is fabricated with a recess  182  which holds an etch-resistant wafer  170  made from sapphire. In other embodiments, the base  180  may be constructed of polytetrafluoroethylene (PTFE), polyethylene, or any other suitable hydrofluoric acid resistant material. 
     Additionally, the sapphire wafer  170  shown here may be substituted with CVD silicon carbide or CVD glassy carbon film deposited onto a transparent substrate. Other materials may be contemplated that are transparent to visible light. The function of the disc  170  is to protect the sample by providing adequate support when being clamped. 
     When the etch-resistant wafer  170  is placed in the recess  182 , it fills the entire recess  182  and does not extend above the top surface of the bottom fixture  180 . This is to facilitate a complete seal when the top and bottom fixtures are secured together. The recess  182  is formed with a small aperture  172 , or via, through which light can pass. The aperture  172  extends through the base  180 , with the light source emanating from underneath the fixture bottom  180 . 
     When the sapphire  170  is situated within the recess, light is transmitted up through the aperture  172  and through the sapphire window  170 . Someone observing from above the fixture  100  with a microscope can tell when the etching of the sample  190  has progressed to the point where the beam of light is visible through the sample  190 . This indicates that the sample is at the desired thickness; therefore, the etching is complete. 
     The top of the fixture  120 , also constructed of PVDF and puck-shaped, contains a reservoir  130  for the etchant. Note that the etchant is preferably a liquid etchant, but a powder etchant, and even a gas etchant can also be used, in keeping with the spirit and scope of the invention. The reservoir  130  is made up of two cones. The top cone  124  has a 90 degree inclusive angle and extends to half-way through the puck  120 . The second cone  126  has a 120 degree inclusive angle and begins at an intersection of the first cone  124  and terminates with a small hole  128  at the bottom. The small hole  128  is positioned directly in line with the aperture  172  running through the fixture bottom  180 , providing a direct line of sight to the light beam emanating up from beneath the fixture bottom  180 . 
     The bottom cone  126  is shallower than the top cone  124  to facilitate the upward release of bubbles as the etching proceeds, when a liquid etchant is used. Below the small aperture  128  in the cone, facing down, the surface of the bottom cone  126  forms a very shallow cone which, when put in assembly, allows the sample  190  to contact the edge of the small aperture  128  first. Upon tightening, this edge is compressed against the sample  190  to a pre-set amount resulting in a fluid-tight seal. 
     The base  180  and top  120  fixtures can be secured with screws  190  as shown, or with a clamp, or other type of interconnect. This being done, the etchant is applied to the reservoir  130  and flows down through the cone-shaped reservoir  130 , through the small aperture  128  and onto the wafer  190 . The etch progress can be monitored through a microscope looking down through the reservoir  130  in situ. The illumination is provided by a high intensity light source passing up through the hole  172  in the base puck  180  and through the sapphire  170  on which the sample  190  rests. 
     Although we limit our examples to visible light being transmitted, other wavelengths could be used for sample specific transmission. The illumination allows an observer to easily determine when the etching process has etched the sample  190  to a desired thickness. The sample  190  may be a silicon wafer, germanium or other similar material. The function of this fixture depends upon the characteristic of silicon and similar materials whereby they begin to become transmissive to light when thinned to tens of microns in thickness. 
     At completion, the remaining etchant is drained from the reservoir  130  and the sample  190  removed and rinsed without any further solvents or steps necessary. The fixture  100  is immediately ready for the next sample  190 . Real savings in labor and equipment are realized in that the only hardware necessary for this process is the etch fixture  100  shown in the illustrations and standard lab equipment such as the microscope and light source. 
     Referring now to  FIG. 2   a  we provide a close-up cross-section view of the sample  190  situated above the sapphire backing disk or window  170 . This close-up view provides a clearer view of the shallow angle where the contact is made with the sample  190  by the shallow cone as previously discussed. 
       FIG. 2   b  is a cross-section view showing the positioning of the different components of the mask fixture, the sample, and the light source.  FIG. 2   b  introduces one optional component of the invention not previously discussed. A mirror  230  is used to re-direct the high intensity light source upwards through the sapphire  170 . 
       FIG. 3  shows a viewer peering through a microscope  320  to determine the etching progress.  FIG. 3  also shows the mirror  230  used to reflect a light beam or other high intensity light source upwards through the sapphire window  170 . 
     Therefore, while there has been described what is presently considered to be the preferred embodiment, it will understood by those skilled in the art that other modifications can be made within the spirit of the invention. The above description of an embodiment is not intended to be exhaustive or limiting in scope. The embodiment, as described, was chosen in order to explain the principles of the invention, show its practical application, and enable those with ordinary skill in the art to understand how to make and use the invention. It should be understood that the invention is not limited to the embodiments described above, but rather should be interpreted within the full meaning and scope of the appended claims.