CARBON FILMS AND SPECIMEN STABILITY

In recent years a subject which has received little attention is the question of motion of the specimen relative to the objective lens of the electron microscope. In ordinary electron microscopy, where resolutions better than 30 A are not required, one can tolerate drift of the object (during a 5 second exposure of the plate) of perhaps 1 to 3 A per second, while for high resolution microscopy this figure may drop as low as 0.2 to 0.6 A per second. When we consider that the distance between adjacent atoms of many substances is on the order of 1 A, it is clear that our stability requirements are rather stringent. For purposes of analysis we can find two general sources of specimen drift: motion of the specimen holder arising in the stage mechanism and motion of the specimen relative to the grid. We will consider these in order. Inherent in the design of any useable stage mechanism is its ability to move the specimen over distances hundreds of thousands of times greater than the size of objects one desires to resolve. Any part of this mechanism subject to mechanical relaxation or to sensitivity to vibration may cause undesired motion of the specimen. Proper adjustment of the stage mechanism will minimize the mechanical problems mentioned above, but may not be permanent. There remains, in addition, the long mechanical pathway between the specimen and the objective which is subject to thermal effects during warmup, drafts around the microscope, etc. Although the stage may be perfectly stable in one position, this is no guarantee that it is stable in another position. For this reason it is d~Scult to be certain that the mechanism is operating properly. Operating practice leading to consistently good results from the microscope requires certainty on this point. In addition, if other sources of image drift, such as drift in the power supplies, are present as well, one is in doubt as to where to look for the trouble. These points, in my opinion, are strong arguments for eliminating insofar as possible all sources of stage drift. The device to be described provides an arrangement whereby the chances for motion of the specimen holder are very substantially reduced. While this device has been primarily designed for the RCA types EMU-1 and -2 microscopes, it probably can be adapted to certain other instruments as well. Essentially, the specimen holder is arranged so that it rests on top of the objective polepiece and is in contact with the stage only when it is being moved. Text-fig. 1 shows how this is achieved. 31