Patent Number: 042726824
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an ion milling machine and more particularly to an ion milling machine specimen elevator and airlock mechanism which facilitates specimen viewing and specimen exchange. 2. Description of the Prior Art Ion milling machines are used in the preparation of electron microscope specimens. An article entitled "An Improved Ion Thinning Apparatus" by A. H. Heuer, et al. from The Review of Scientific Instruments, Vol. 42, No. 8, Aug. 1971, pp. 1177-1184, which is herein incorporated by reference, provides good background information on ion milling. Ion milling machines are employed for thinning specimens to a thickness of the order of 0.5 micrometers for examination by transmission electron microscopy. Desired thinning is accomplished by placing the specimen in the path of one or more beams of energetic ions and neutral atoms which sputter atoms from the specimen surface. Although material removal is very slow during ion milling, this procedure does much less damage to the underlying specimen than other material removal methods such as cutting or grinding. Ion milling has become widely used as a method for electron microscope specimen preparation, especially for those materials that because of their chemical nature cannot be thinned by electro-polishing or chemical polishing methods. Ion thinning is accomplished in an evacuated chamber; usually the specimen is rotated during exposure to the ion beams to improve the uniformity of thinning across the specimen surface. In order to study the progress of the ion milling operation, the specimen must be periodically examined using a lower power microscope. This inspection is difficult in prior art ion thinning machines because material sputtered from the specimen and specimen holders coats the observation windows and sources of illumination, thereby obscuring the view of the specimen. Further, because the specimen is positioned near the center of the work chamber and is surrounded by the ion guns and other related mechanisms, the specimen must be viewed at an uncomfortably long distance. A second deficiency of presently available ion thinning units is that the entire work chamber must be raised to atmospheric pressure before specimens can be exchanged. Raising the work chamber to atmospheric pressure has at least three disadvantages: (1) the time for specimen exchange is lengthy because a large volume of air has to be evacuated before the ion guns can be turned on; (2) the ion guns are initially less stable after exposure to atmospheric pressure and must be pumped for a long period of time before they restabilize; and (3) a costly valving mechanism is needed to isolate the work chamber from the pumping system. In presently available ion thinning units, it is awkward to load and unload specimens. This is disadvantageous because electron microscope specimens are extremely fragile and are easily damaged by the small mechanical shocks they receive during the loading and unloading operation. SUMMARY OF THE INVENTION The present invention provides a specimen elevator and airlock apparatus which is quick, gentle and convenient in operation and solves many of the problems associated with prior art ion milling apparatus. The specimen elevator for the disclosed ion milling machine comprises a vertically elongated piston which supports a specimen holder at its top. The elongated piston is movable between a raised and lowered position. When the elongated piston is in the lowered position, it is rotated about its vertical axis. The elongated piston extends through an O-ring seal into an evacuated work chamber where the ion milling is accomplished. The piston can be moved either up or down by altering the pressure in a pneumatic cylinder mounted at the bottom of the work chamber around the piston. A sealing plate is disposed around the piston and provides for sealing between the piston and inner diameter of the cylinder walls and allows the piston to be rotated around its vertical axis. A motor is mounted at the bottom of the cylinder for rotating the piston when it is in its lowered position. During ion milling the piston is at its lowered position and is held there by pressurizing the pneumatic cylinder. With the piston in its lowered operating position, the specimen lies inside the evacuated work chamber. If it is desired to remove the specimen from the work chamber without disturbing the work chamber vacuum, the pressure in the pneumatic cylinder is released and the piston automatically moves to its raised position under the force exerted by atmospheric pressure. As the piston is forced upwardly by atmospheric pressure, it passes through a second O-ring pressure seal into a small upper chamber. As the piston moves the specimen holder into the small upper chamber, a seal is provided between the upper chamber and the work chamber. Thus any air admitted to the small upper chamber will not pass to the work chamber. Means are provided for admitting atmospheric pressure to the small upper chamber. The small upper chamber can be lifted from the top of the large work chamber when the internal pressure in the small chamber rises to atmospheric. This exposes the specimen holder and thus permits removal of the specimen or close examination of the specimen to study the progress of the ion milling operation. The specimen is reintroduced into the work chamber by first replacing and evacuating the small chamber, and then repressurizing the bottom control cylinder. When the control cylinder is pressurized, the piston is forced to move downwardly to its lowered operating position. As the piston moves downwardly, the seal between the upper chamber and the working chamber is eliminated and the two chambers are in open communication. The small upper chamber is held to the working chamber by atmospheric pressure. A driven bevel gear is provided at the bottom of the control cylinder. A mating bevel gear is attached to the piston. When the piston is in its lowered position, its attached bevel gear engages the driven bevel gear causing the piston to slowly rotate. The specimen holder is mounted on the top of the piston and, therefore, specimens can be held in place simply by the force of gravity. This arrangement makes unnecessary the use of mechanical clamps and the consequent risk of physical damage to the fragile specimen. It is an object of the present invention to provide a mechanism which enables a specimen undergoing ion thinning to be raised quickly from the center of the work chamber to a more convenient viewing position and also to quickly lower the specimen back to its normal working position after inspection. It is another object of the invention to provide an ion milling machine having a specimen elevator device which raises the specimen to a viewing or exchange position and at the same time maintaining the working chamber in a sealed condition. It is yet a further object of this invention to provide a specimen handling apparatus for an ion milling machine which simplifies specimen handling and is quick and gentle in operation.