A New Universal Electron Microscope of Hiffh Resolvini^ Power 



3J 



to about 5 microns in dieimeter. The first lens de- 

 magnifies the electron source while the second is the 

 normal condenser lens. Three apertures are provided 

 which can be adjusted by means of a rod control 

 which is easily withdrawn for cleaning purposes. 

 Molybdenum discs are used for the apertures as this 

 metal contaminates at a lower rate than platinum. 

 They are easily cleaned by vacuum heating on a 

 molybdenum boat in a small evaporating plant. 

 Many different sizes of aperture are available rang- 

 ing from 25 microns to I mm in diameter. 



In order to avoid the instability inherent in any 

 form of mechanical beam alignment stage electric 

 aligments are used. The system employed was de- 

 scribed by Haine and Agar at the Reading Conference 

 and consists of a combination of magnetic and 

 electric fields. The advantage of this method is that 

 large angles of tilt can be obtained in one direction 

 when required for reflection microscopy with only 

 two levels of deflection instead of the usual four. A 

 two-level all-magnetic deflector system for normal 

 small angles of tilt has also been designed. 



The adaptability of a microscope depends very 

 much on the design of the specimen stage and cham- 

 ber and a considerable effort has been made in this 

 model to make a design which combines sufficient 

 mechanical stability for high resolution together 

 with airlock insertion of specimens and accessibility 

 for maintenance or special purpose stages. The 

 transmission stage is designed to allow stereo-tih 

 angles of ± 5' and is traversed by push rods operated 

 through bell-cranks with special pivots which are 

 free from backlash. The airlock mechanism is at- 

 tached to the chamber through the medium of 

 flanges and ports so that it can easily be detached. 

 A special stage for reflection working can be inserted 

 into a large port at the front of the chamber. This 

 stage is accurately calibrated for specimen tilt and 

 twist and is capable of accepting a wide range of 

 specimens. 



The design of the objective lens allows for low 

 magnetic flux densities at all parts of the iron circuit 

 resulting in a very low level of stray fields which 

 would adversely affect accurate alignment and cause 

 astigmatism. The construction is simple since the 

 entire body including the lower pole piece is turned 

 from solid steel. The iron circuit is completed with 

 a steel lid which incorporates the upper pole piece. 

 The only magnetic junction is therefore well removed 

 from the electron path so eliminating another pos- 

 sible cause of stray fields. The lens is thus demount- 

 able and the winding, water cooling duct, and 

 aperture mechanism can all be separately removed. 



The focal length is 4 mm which allows the speci- 

 men to be just outside the magnetic field, and at 

 this value the resolving power as limited by spherical 

 abberation and diffraction is better than 5 A. The 

 inherent astigmatism of this type of lens is usually 

 not more than 1 micron. It is therefore necessary to 

 improve this by a factor of at least 5 to attain a 



3 — 568204 Electron Microscopy 



resolving power of 10 A. A four pole rotating turret 

 corrector is provided with which it is possible to 

 attain this degree of correction by visual inspection 

 of the fresnel fringe pattern through the 10 viewing 

 telescope ( I ). 



The objective lens incorporates an adjustable 

 aperture which is quickly and easily centred. The 

 rod carrier contains three molybdenum disc aper- 

 tures and a clear space and can be removed for 

 changing apertures without breaking vacuum. 



There is a preset alignment section between objec- 

 tive and projectors which includes the control for 

 rotation of the astigmatism corrector and two sets 

 of intermediate apertures for dilTraction purposes. 

 One of these provides for variable area selection of 

 the field from which a pattern is required. 



The projector system uses two water cooled lenses, 

 again of integral pole construction similar to the 

 objective. By variation of the power of the inter- 

 mediate lens magnifications ranging from 1500 to 



Fig. 1. Metro- Vick Type I: M6(protoiype) electron microscope. 



