US3124680A

US3124680A - Agent

Info

Publication number: US3124680A
Authority: US
Priority date: 1958-09-13
Publication date: 1964-03-10
Anticipated expiration: 1981-03-10
Also published as: FR1235119A; GB927845A; NL108507C; NL231361A; DE1152766B; CH379656A

Description

M h 0, 6 A. c. VAN DORSTEN ETAL 3,124,680

- ELECTRON MICROSCOPE SPECIMEN HOLDER POSITIONED BETWEEN A PAIR O TEMPERATURE CONTROLLED APERTURED PLATES Filed. Sept. 14, 1959 s c Q ADRIANUS c W$9z- ADRIANUS .131. FRANKEN Y O a [I a 3 s Q 8 3'38 N N v 0 F 9 g- 21 R United States Patent 3,124,680 ELECTRON MICROSCOPE SPECIMEN HOLDER POSITIDNED BETWEEN A PAIR OF TEMPERA- TURE CONTRGLLED APERTURED PLATES Adrianus Cornelis Van Dorsten, and Adrianus Jacobus Jozef Franken, Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N .Y., a corporation of Delaware Filed Sept. 14, 1959, Ser. No. 839,750 Claims priority, application Netherlands Sept. 13, 1958 Claims. (Cl. 2.5049.5)

This invention relates to electron microscopes comprising devices for examining objects, in which a space containing the object is cooled artificially. In a known device of this kind, the object is at the same low temperature as the space. This device permits of examining objects the properties of which vary due to variations in temperature.

In another case use is made of object cooling for preventing contamination of the surface of the object. During examination, it is found that particles deposit onto the object, which particles cause the microscopic image to be blurred. The formation of particles is attributed to organic vapors which are a constituent of the residual gases still present in the space of the microscope after this space has been evacuated to the desired low pressure. It is necessary to make allowance for the fact that a decrease in the temperature of the object may result in condensation of all of the substances present in the residual gas and which have a comparatively low vapor pressure. The occurrence of a thin layer of condensate on the surface of the object is likewise harmful to the quality of the image produced of the object by electrons.

An object of the invention is to obviate this disadvantage. According to the invention, the space containing the object serves as a cooling chamber. In this chamber, which is bounded by at least two walls to be maintained at a low temperature, the object is arranged and secured to a carrier which extends inwards from the periphery of the housing of the microscope other than that which carries on the object chamber. The arrangement is such that the carrier and the object are not in contact with the walls of this space. The cooling chamber is particularly operative if the walls largely surround the object.

For the dissipation of heat use may be made of a cooling liquid which is supplied and discharged through a channel from outside the housing of the microscope. However, it is possible that vibrations, for example the periodic motions of a pump aggregate pushing the liquid through the channel, are propagated by the liquid and transferred to the cooling chamber.

This disadvantage is not encountered when using a cold gas as the cooling medium, for which use may be made of air or nitrogen obtained by evaporation of liquid gas in a vacuous vessel.

It is preferable to arrange not only the object, but also the object diaphragm in the cooling chamber. However, in this case, it is desirable for a partition wall, which is likewise cooled, to extend between the object and the diaphragm, both the object and the diaphragm then being situated between cold walls.

It is advantageous to prevent the cooling device from extracting heat from the housing of the microscope. For this purpose, a device according to the invention may comprise a tubular carrier, one extremity of which is closed by a metallic block to which the walls of the cooling chamber are secured, and the other extremity of which has an inlet aperture for a cooling medium which is led through a channel in the carrier along the metallic block and discharged through an aperture provided in the wall, said carrier being provided with means of supplying heat 3,124,580 Patented Mar. 10, 1964 to a portion which constitutes the connection with the housing of the microscope. For the supply of heat to the said portion it is possible to employ an electric heater element.

Further particulars of the electron microscope according to the invention and of the device comprising the object space which serves as the cooling chamber will now be explained in detail, by way of example, with reference to the accompanying drawing, showing part of an electron microscope according to the invention comprising the object and object space and in which:

FIG. 1 shows a cross-section of the arrangement of the object and the object space in the housing of the micro scope, and

FIG. 2 shows longitudinal section thereof.

The housing of the microscope comprises a cylindrical wall 1 which contains poles 2 and 3 of a magnetic elec tron lens having an energizing Winding 4 which surrounds the pole 2. The winding 4 has a coil container 5. The poles 2 and 3 are provided with detachable pole-pieces 6 and 7, which are connected to the poles by means of flange nuts 8 and 9. The poles 2 and 3 are provided with bores 119 and 11 which merge into small apertures 12 and 13 in the pole-pieces 6 and 7 to allow passage of the electron beam.

A disc-like plate 14 is arranged in the space between the poles 2 and 3. Provided between the plate 14 and the poles 2, 3 are rubber sealing rings 15 and 16 which separate the space containing the pole-pieces from the other part of the housing of the microscope and which constitute an air-tight closure so that the volume which is evacuated is small with respect to the Volume of the housing.

The disc-like plate 14 serves to support an object carrier 17 which comprises a thin rod, one extremity of which is provided with a knob 18 and the other extremity of which is a thin pin 19. A sealing ring 26 provides for air-tight sealing of the carrier 17 in aperture 21 of the plate 14.

The object is arranged at the end of pin 19 and lies in the optical axis of the electron lens in the space between two walls 22 and 2-3 which have apertures 24 and 25 to allow passage of electrons which, after the space of the microscope has been evacuated, traverse the apertures in the pole-pieces from above to below when the microscope is put into operation (FIG. 2). The evacuation of the space in the microscope decreases the density of the remaining gas residues to an extent sufiicient to prevent residual gas particles from detrimentally affecting the path of the electron beam. These gas residues would not otherwise be harmful if they did not contain substances from which molecules are dissociated when bombarded by electrons. A portion of the dissociated products deposits on walls bounding the vacuous space, especially at areas struck by electrons, such as along the edges of apertures bounding the beam. Since the object positioned in the electron beam is bombarded by all of the electrons and the gas molecules which have a velocity directed towards the object lose energy upon reaching the surface, the possibility of the gas molecules being dissociated is greater in the vicinity of the object than elsewhere in the microscope. Deposition of the dissociated particles onto the object is noticeable fairly soon from variations in the quality of the image on the viewing screen. It has been found that the use of the invention provides a considerable improvement since the rate of deposition on the object evidently decreases considerably. A possible explanation of this phenomenon is that molecules of the harmful organic vapors, which substantially consist of hydrocarbons, are delayed in the cold space and hence a smaller number of them can reach the object which has a higher temperature. In addition, these gas particles which have a comparatively low vapor pressure condense on the cold walls of the chamber so as to avoid at the same time the formation of condensate on the object.

The walls 22 and 23 between which the object is arranged are connected to a metallic block 26 which closes one extremity of a metallic tube 27. Secured to the other extremity of tube 27 is a plug 28 of heat-insulating material which is provided with a spout 29 around which a rubber tube 30 may be clamped. The plug 28 carries a thinner tube 31 which extends into the cavity of tube 27. These tubes constitute in common a channel through which a cooling medium supplied through an aperture 32 in the plug 29 to the surface of the block 26. The cooling medium is discharged through a bore 33 provided in the wall of tube 27 in which a spout 34 is also secured for providing a rubber tube 35.

The tube 27 is arranged in a holder 36 which is rigidly connected to the housing 1 of the microscope by means of a part 37 which is of a larger size so that it can accommodate the attachment means which are not shown. The part 37 has a cylindrical tube 38. The space for these parts is recessed in the disc 14-, which for this purpose has a wide aperture 39 which communicates through a narrow aperture 40 with the object space of the microscope. A rubber intermediate piece 41 is clamped in position in this aperture by means of a resilient ring 42 and encircles the cylindrical tube 38 so as to close the aperture 40 hermetically. Between the holder 36 and the tube 27 there is provided a rubber ring 43, which likewise serves to prevent air from penetrating the space between the pole-pieces.

At the end of cooling tube 27 which projects from the housing of the microscope there is arranged an electric heater element 44 having connections 45 and 46 for current supply wires 47 and 48. The object of the heat produced by element 44 is to increase the temperature of the discharged cooling medium up to the ambient temperature, thus preventing the cooling device from extracting heat from the housing of the microscope.

What is claimed is:

1. In an electron microscope, the combination of a housing, a pair of spaced members within the housing defining an object space and having coaxial apertures in the electron optical axis of the microscope, an object holder positioned between said members in the path of the electron beam, means to cool said spaced members to a temperature below the temperature of the object holder extending through a portion of the wall of said housing, a supporting member for said object holder extending through another portion of the wall of said housing without being in contact therewith for maintaining a small temperature difference between the object holder and the cooled spaced members, and means within the housing to hermetically seal the supporting member.

2. In an electron microscope, the combination of a housing, a pair of spaced members within the housing defining an object space and having coaxial apertures in the electron optical axis of the microscope, an object holder positioned between said members in the path of the electron beam, means to supply a cold gas extending through a portion of the Wall of the housing to cool said spaced members to a temperature below the temperature of the object holder, a supporting member for said object holder extending through another portion of the wall of said housing without being in contact therewith for maini taining a small temperature difference between the object holder and the cooled spaced members, and means within the housing to hermetically seal the supporting member.

3. In an electron microscope, the combination of a housing, a pair of spaced members defining an object space and having coaxial apertures in the electron optical axis of the microscope, an object holder positioned between said members in the path of the electron beam, a tubular member extending through one portion of the wall of the housing, a metal member surrounding a portion of said tubular member for supporting said spaced members, and means to introduce a cooling medium into said tubular member for cooling said spaced members to a temperature below the temperature of the object holder, a supporting member for said object holder extending through another portion of the wall of said housing without being in contact therewith for maintaining a small temperature difference between the object holder and the cooled spaced members, and means within the housing to hermetically seal the supporting member.

4. In an electron microscope, the combination of a housing, a pair of spaced members defining an object space and having coaxial apertures in the electron optical axis of the microscope, an object holder positioned between said members in the path of the electron beam, a tubular member extending through a portion of the housing, a metal member surrounding a portion of said tubular member for supporting said spaced members, means to introduce a cooling medium into said tubular member for cooling said spaced members to a temperature below the temperature of the object holder, means in the wall of the tubular member to heat the portion of the tubular member in contact with the wall of the housing, a supporting member for said object holder extending through the wall of said housing without being in contact there with for maintaining a small temperature difference between the object holder and the cooled spaced members, and means within the housing to hermetically seal the supporting member.

5. In an electron microscope, the combination of a housing, a pair of spaced members defining an object space and having coaxial apertures in the electron optical axis of the microscope, an object holder positioned between said members in the path of the electron beam, a tubular member extending through a portion of the housing, a metal member surrounding a portion of said tubular member for supporting said spaced members, means to introduce a cooling medium into said tubular member for cooling said spaced members to a temperature below the temperature of the object holder, electrical heater means in the wall of the tubular member to heat the portion of the tubular member in contact with the wall of the housing, a supporting member for said object holder extending through the wall of said housing without being in contact therewith for maintaining a small temperature difference between the object holder and the cooled spaced members, and means within the housing to hermetically seal the supporting member.

References Cited in the file of this patent UNITED STATES PATENTS 2,826,701 Columbe Mar. 11, 1958 2,858,444 Leisegang Oct. 28, 1958 FOREIGN PATENTS 903,128 France Jan. 8, 1945 236,120 Switzerland May 16, 1945