Patent Number: 047059494
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Identical reference numerals refer to similar structures throughout the several views wherein FIG. 1 illustrates the specimen cell 8 in accordance with one embodiment of the present invention. Although the invention is applicable to all directed beam irradiation devices, the following discussion is with reference to the scanning electron microscope (SEM). A closed specimen module 10 is located within the cell 8 and has a small aperture 12 located therein. The aperture comprises a means for bidirectionally passing electrons into and out of the closed specimen module 10 and for restricting passage of vapor out of the inner portion of the specimen module. Such a means in a preferred embodiment comprises an aperture approximately 200 microns in diameter. However, different aperture sizes could be utilized. The aperture must be small enough to slow the escape of vapor to the point where the pressure within the module is maintained higher than the vapor pressure of the volatile constituent of the sample while at the same time allowing the electron beam to reach the solid surface of the specimen in the module and then allow sufficient number of electrons to escape and reach the detector which is external to the specimen module. There appears to be a range of from 200 angstroms to 500 microns which will operate satisfactorily although a different diameter aperature may be desirable depending upon the thickness and geometric configuration of the aperture. In most instances where a biological specimen is being observed, the pressure within the specimen module should be 17.5 mm Hg at 20.degree. C. Of course, it is desirable that the specimen module be maintained vacuum tight so as to prevent vapor loss except through the aperture 12 when opened for observation. In order to prevent any significant escape of vapor from the aperture of the specimen module 10 while the microscope is being pumped down to its operating pressure, door 14 is slid over and lowered into place sealing aperture 12. The door can pivot about fixed support 16 under the influence of an opening and closing means 18 schematically indicated in FIG. 1. This could comprise any means for moving the door 14 between the illustrated open position and its closed position shown in phantom line. Such a means could be a motor/gear train combination, a solenoid and linkage arrangement, or other pneumatic/hydraulic/electro-mechanical devices. It is only important that the door be controllably movable between its two positions. Additional details of the improved specimen cell 8 can be seen in FIG. 2 which is a section of the apparatus in FIG. 1 taken along section lines 2--2. It can be seen that in a preferred embodiment, the closed specimen module 10 comprises an upper and a lower portion (containing a sealed port 11 for initial induction of specimen) which are mated together by any convenient means such as threaded fasteners. The aperture 12 can be blocked and/or sealed during non-observational periods by a door which is pivotally connected to fixed support 16 by arm 20. In a preferred embodiment, the upper portion of door may be comprised of a metal plate 14 welded to arm 20 with a lower portion comprising an elastomeric sealing material 22. Should it be desirable to heat or maintain the temperature of specimen module 10, a heater element 30 could be inserted into the lower portion 40 of the cell. As shown in FIG. 2, such a heater is comprised of a porcelain insulator 32 (998 alumina) with nichrome resistance wires 34 (28 gauge, 4.1 ohms per foot) connected by means of outlet wires 36 to a suitable power supply. The porcelain insulator is threadably received into the lower portion 40 of the specimen cell and is sealed therewith by means of high temperature glass 42. In order to maintain the temperature of a specimen below ambient temperature, e.g., a frozen specimen, conventional cooling means (not shown) can be used. The details of a preferred embodiment of a closed specimen module can be seen in FIG. 3 where the upper and lower halves are joined by a plurality of machine screws around the periphery of the chamber. In order to obtain the best scanning resolution, it is desirable that the chamber volume be relatively large but the distance from the aperture 12 to the specimen be minimized. This is to reduce, to the extent possible, the vapor thickness through which the electron beam must penetrate before reaching the specimen surface. The closed specimen module 10 can merely be placed in the upper portion 8 of the specimen cell, without any further anchoring means located therein, and, as long as its upper surface is relatively smooth and aperture 12 can be sealingly closed by door 14 in the closed position, the apparatus will work properly. However, it is often desirable to be able to add liquid or vapor to the specimen either immediately prior to or during an observation (as in the studies of hydrating cement) and thus an open specimen module 50 is illustrated in FIG. 4. The reference to an open specimen module system is because there are other openings besides the aperture, leading to the specimen chamber of the module, which serve as attachments for various peripheral embodiments. However, the specimen system is closed to the vacuum chamber of the SEM except for aperture 12. Such a module could comprise a mesh specimen cage 52 surrounded by a liquid/gas induction shroud (to prevent injected liquid/gas from directly impinging upon and disturbing the observed specimen) and the mounting structure for maintaining the cage position. In this instance, the shroud 54 partially supports the cage 52 and is in turn supported by a frame surrounding the metallic, plastic or elastic diaphragm 56 which is located between upper portion 38 and central portion 39 of the specimen cell and is affixed to the perimeter of the floor of the cell cavity for receiving the specimen modules. The upper portion of the open specimen module comprises a chamber lid 57 shaped so as to sealingly compress seals 58 in grooves 60 around the periphery of the cavity. The chamber lid is affixed securely by any convenient means such as welding or thread fasteners to the upper surface of the open specimen module system and is also affixed to the upper surface of the controlled environmental chamber of the specimen cell by non-permanent means, such as thread fasteners, in such a manner so as not to disturb the planar profile of the lid surface, thus hindering proper sealing of the door over the aperture. Therefore, any specimen located within cage 52 can have a pressure greater than the high vacuum external to the chamber lid 57. As noted, it may be desirable to maintain the open specimen module at a certain gas pressure or add liquid to the vicinity of the specimen before or during observation. Accordingly, an injection port 62 is provided through the side of the upper portion of the specimen cell and threadably received therein is an injector 64 which is in fluid communication with tubing 66. A plurality of injectors could be located around the periphery of the open specimen module to provide either liquid or gas immediately adjacent the specimen during observation. It may also be desirable to have the capability of agitating or raising and lowering the specimen during observation and to this end, an induction port 70 is provided in an intermediate portion of the specimen cell allowing communication between induction tube 72 and the lower side of elastic diaphragm 56. If a rubber diaphragm is used, it can be inflated and raised to a desired position thus raising the specimen to a level just below the aperture. By modulating the pressure of air or other gas on the bottom side of diaphragm 56, the diaphragm can oscillate the specimen located within cage 52. A typical scanning electron microscopic examination might initially require 15 to 20 minutes in which to pump down the microscope's vacuum chamber into which specimen cell 8 has been inserted. During this entire time, of course, the door 14 would be in place blocking vapor passage through the bidirectional electron and vapor retaining window comprised of aperture 12. Only when the microscope has reached its operating pressure, and is ready to begin observation of the specimen, is door 14 removed and it is only at this point that, due to the higher vapor pressure in the specimen module does vapor begin to flow through aperture 12. Because the electron beam can be focused relatively quickly based upon preliminary settings, scanning of the specimen and detection of the reflected electrons is usually achieved in the space of two or three minutes, there has been a significant, i.e. on the order of a ten-fold decrease in the time of exposure of the specimen to a reduced pressure or vapor loss. Furthermore, the utilization of vapor replenishment through the injection port (for an open specimen module) can maintain the vapor pressure relatively easy for this short observation time without the additional strain on the microscope's vacuum pumping ability which would be present in a constant or continuous supply system such as disclosed in Kalman et al. It will be remembered that the Kalman et al pressure-balancing buffer system would be supplying a pressure greater than the vapor pressure of the specimen to the specimen chamber from the beginning of vacuum chamber pump-down through the completion of the observation period. Induction port 70 can also be used if it is desirable to cool the specimen in either an open or closed specimen module by introducing small amounts of a liquid refrigerant, nitrogen, etc. into the region of the diaphragm. In view of the above disclosure, many modifications and variations of the present invention will be obvious to those of ordinary skill in the art. For example, a pivoting rather than sliding door could be utilized with any number of variations on the door opening mechanism. Different materials could be used for the door with the only critical requirement being that performance of the door is optimized if the sealing relationship of the aperture 12 by the door is maximized when in the closed position. Utilization of the cell with most existing scanning electron microscopes is enhanced by utilizing a sliding door such that the door is away from the path of any electrons travelling from aperture 12 to the detector of the microscope. Although two preferred embodiments of the specimen modules have been disclosed (one closed and one open), different embodiments could be utilized depending upon the particular specimen desired and the specific process to be observed. For example, the diaphragm 56 could be utilized to raise one constituent into physical contact with another constituent in order to observe catalytic reaction, polymerization, etc. The arrangement of injection ports, induction ports, heaters, etc. can be modified depending upon the needs of a particular specimen and scanning electron microscope. Therefore, although the present specification is by way of example and description of preferred embodiments of the present invention, it is by no means limiting thereof of applicants' invention. The invention is limited only by the following claims appended hereto.