US3359418A

US3359418A - Electromagnetic actuating means for a shutter mechanism in an electron microscope

Info

Publication number: US3359418A
Application number: US417845A
Authority: US
Inventors: Gunter F Bahr; Otmar H Sackerlotzky; Elmar H Zeitler
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-12-11
Filing date: 1964-12-11
Publication date: 1967-12-19
Anticipated expiration: 1984-12-19

Description

Dec. 19, 1967v G. F. BAHR ET AL 3,359,418

ELECTROMAGNETIC ACTUATING MEANS FOR A SHUTTER MECHANISM IN AN ELECTRON MICROSCOPE Filed Dec. 11, 1964 -Flc;.5

EE F. gm n OTMAR h. SA 4ERLaTZKy EL/WAR 25/7152 ATTORNEY United States Patent ELECTROMAGNETIC ACTUATING MEANS FOR A SHUTTER MECHANISM IN AN ELECTRON MICROSCOPE Gunter F. Bahr, Chevy Chase, Md'., Otmar H. Sackerlotzky, Kendall Park, N.J., and Elmar H. Zeitler, Washington, D.C., assignors to the United States of America as represented by the Secretary of the Army Filed Dec. 11, 1964, Ser. No. 417,845 6 Claims. (Cl. 250--49.5)

ABSTRACT OF THE DISCLOSURE This invention relates to electron microscopy and more particularly to a novel shutter mechanism for an electron microscope. The automatically operated shutter mechanism normally obstructs passage of an electron beam when electrical current is flowing and when no current is flowing, the shutter does not obstruct the passage of an electron beam.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

For quantitative evaluation of electron micrographs, it is essential that reproducible exposure times be obtained. However, until now the various mechanisms used have proved unsatisfactory for several reasons. The simplest mechanism used for controlling exposure time involves manually moving the plate with respect to the electron beam. This method is employed with the Siemens electron microscope, Elmiskop I. Manual movement of the plate is manifestly unsatisfactory when short exposure times are involved and even with longer eX- posure time uneven exposure over the surface of the plate occurs.

More sophisticated automatically operated mechanisms have been employed, some located at the focal plane of the microscope and others located at various points along the electron beam. While automatic operation increases the speed of the mechanical movement involved in the shutter, the large cross section of the electron beam at the focal plane still causes exposure time variations across the image. The use of an automatic shutter mechanism is also limited since the magnetic and electrical fields produced by the automatic devices effect the electron beam and distort the images produced.

It is accordingly an object of this invention to provide an automatic, quick acting shutter mechanism for an electron microscope.

It is another object of this invention to provide an automatic shutter mechanism that does not produce undesirable electric and magnetic fields during the exposure period.

It is another object of this invention to provide a shutter mechanism in which a small shutter element moves only a short distance between the beam obstructing and beam passing positions.

These and other objects will be readily apparent from the following description with reference to the accompanying drawings wherein:

FIG. 1 is a schematic representation, partly in section, of an electron microscope showing the general arrangement of the elements;

3,359,418 Patented Dec. 19, 1967 FIG. 2 is a plan view of the shutter mechanism shown in FIG. 1;

FIG. 3 is a cross sectional view of the automatic plunger mechanism;

FIG. 4 is a perspective view of the sensor for determining intensity of the electron beam; and

FIG. 5 is an oscillogram tracing showing the electron beam intensity change on operation of the shutter.

The invention has general utility but for purposes of illustration it will be described with respect to the Siemens Elmiskop I the electron microscope in which the shutter mechanism has been used. The electron micro scope is shown generally at 1, having a source of electrons 2, a lens section 3, a sample section 4, an objective lens 6, an intermediate lens 7 and. a screen 8. In the Siemens microscope, the objective and intermediate lenses are contained within one casing or pole piece 9 having a wall 10 defining an annular central passage through which the electron beam passes. Access to the enclosed volume, Within the pole piece, between the objective and intermediate lens is bad by means of threaded holes in the casing, one of which is shown at 11, having the shutter actuating means 12 in threaded engagement therewith. Flexible sealing means can be provided in the hole to prevent leakage through the hole. A hole 13 is provided in the wall 10 to permit passage of the plunger 14 of shutter actuating means 12 as will be explained more fully below.

A hollow cylindrical sleeve 15 is slidably mounted within the annular passage defined by wall 10. The cylindrical sleeve 15 carries the movable shutter blade 16 and has a hole 50 aligned with the hole 13 in wall 10 to permit passage of plunger 14. As can be seen in FIG. 2 shutter blade 16 is pivotally mounted on the cylindrical sleeve 15 by means of pin 17 which passes through a hole inthe shutter blade 16 and is mounted in the bottom face of a notch 18 in the sleeve 115. The shutter blade is biased by resilient means such as spring 19 to normally bein a position that does not obstruct the passage of the electron beam, the locus of which is shown by the dotted circle E in the center of cylindrical sleeve 15. Spring 19 extends around pivot pin 17 having one leg affixed to blade 16 and the other leg abutting the edge of groove 18. For convenience, the sleeve may be made of upper and lower sections which are joined together after the assembly of the spring 19, pin 17 and blade 16.

The shutter actuating means 12 comprises a housing 20 having an externally threaded portion 21 on one end adapted to engage the threads of hole 11 in the microscope pole piece housing 9. A central bore extends through the housing 20 having a small diameter section 22 in the externally threaded end of housing 20, a larger diameter intermediate portion 23 and an overboard terminal portion 24 provided with internal threads. Field coil 25 having a central bore rests in the intermediate portion 23 and is held in place on collar 26, which rests on the lower face of the intermediate portion 23, by retaining ring 27 which sits in a groove cut in the wall of intermediate portion, 23. The plunger 14 made of a non-magnetic material such as brass may have a narrow inner end 28, a wide intermediate portion 29 and a narrow outer end 30. The outer end 30 fits slidably within collar 26 and is threadably engaged at its upper end with soft iron armature 31. A hearing 33 is provided in bore 22 within which slides intermediate portion 29 of the plunger 14.

A return spring 32 is connected at one end to armature 31 and at the other end to plate 34 mounted in an annular groove cut in the wall of intermediate portion 23 of the bore adjacent the outer end thereof. An ring 35 is mounted in a groove cut into the face of overbored portion 24 and a brass sealing plate 36 is pressed into contact with O ring 35 by retainer ring 37 which is threadably engaged with the internal threads in overbored portion 24.

Electrical conductors

38 and 39 are connected to coil 25 and extend from the actuator unit 12 for connection to a source of current which may be an automatic timer 60. To maintain a vacuum seal,

conductors

38 and 39 pass through a rubber grommet 40 extending through aligned holes in plate 34 and sealing plate 36. A Plexiglas housing 41 may be mounted on the end of casting 20 to prevent flexing of the conductors at grommet 40'.

In use when current is not flowing through coil 25 the plunger 14 is in its Withdrawn position and shutter blade 16 does not obstruct the electron beam, as shown in solid lines in FIG. 2. To obstruct the beam, current is passed through coil 25, extending the plunger 14 and pushing the blade 16 to the portion shown in dotted line in FIG. 2. When current flow stops spring 32 pulls plunger 14 to its withdrawn position and spring 19 moves blade 16 to its non-obstructing position. As can be seen, no current flows through the solenoid during exposure of the plate to the electron beam and distortion of the image caused by stray magnetic fields created by the operation of the shutter is eliminated. The relative strength of springs 19 and 32 and the electromagnetic solenoid are not critical and can be readily determined by a Worker in the art within the following criteria. Both return springs should be sufliciently powerful with respect to the mass of the shutter and plunger to move them rapidly to their return positions. Likewise the number of turns in the coil and the current flow should be chosen to create a force on the armature that is sufliciently greater than the combined strengths of the return springs to allow the beam to be rapidly broken.

The length of the stroke is dependent on the dimensions of the electron microscope itself and upon the size of the shutter blade. The necessary stroke length is obtained by proper selection of the length of the neck of collar 26 for the power stroke and the length of narrow outer end 30 of the plunger for the return stroke.

To obtain reproducible exposure it is necessary to determine the beam intensity for each exposure. A simple detector is shown in FIG. 4 and comprises a metallic disc 43 soldered or brazed to a metallic rod 44. The rod 44 is mounted in an insulator 45 so that metallic disc 43 is slightly above the microscope screen 8. Electrical conductors 46 and 47 connect the rod to a micro-microammeter 48. Since exposure is a product of beam intensity and time, the intensity readings obtained from the meter 48 can be readily used to calculate the exposure time.

FIG. illustrates the rapidity of shutter mechanism. The oscillogram tracing was obtained by placing a sensor, the size of the screen 8, connected to the oscillogram on the screen 8 and actuating the shutter mechanism for an 0.2 second exposure. As can be seen the beam intensity increased sharply with no edge effects and decreased just as sharply.

While the invention has been described with respect to a specific electron microscope, it is readily apparent that the shutter mechanism can be adapted for use in any electron microscope in accordance with the following principles:

First of all it is essential that the shutter mechanism be located in the lens system at a point where the electron beam has its narrowest cross section. By so locating the shutter blade only a small mechanical movement is required to break the beam and the shutter itself can have a small cross section. As a result the entire beam cross section can be broken almost instantaneously.

Secondly the motion of the shutter should be perpendicular to the axis of the beam to reduce to a minimum the length of travel of the shutter blade.

Thirdly the shutter and actuating mechanisms must be so biased that current only flows through the solenoid when the beam is obstructed thereby preventing distortion of the image by magnetic fields during exposure.

The invention therefore is not limited to the details of the specific device; rather it covers mechanical modifications necessary to utilize the shutter in other electron miscroscopes and is limited only by the following claims.

We claim:

1. In combination with an electron microscope having at least one coil for the focusing of a beam of electrons, an electrically operated shutter mechanism which obstructs the beam when current is flowing and does not obstruct the beam when current is not flowing in the shutter mechanism, said mechanism comprising:

(a) a shutter blade pivotally mounted within said electron microscope in close proximity to a point where the electron beam cross section is narrowest, said blade being capable of movement in a plane perpendicular to the axis of the electron beam from a position where said beam is obstructed by said blade to a position where said beam is not obstructed by said blade;

(b) biasing means coacting with said shutter blade to maintain said blade in a position where said beam is not obstructed;

(c) actuating means abutting said shuter blade, said actuating means being biased to maintain said blade in a position where said blade does not obstruct said electron beam; and

(d) electromagnetic means for moving said actuating means and shutter blade, against their combined biases from the point where said shutter blade does not obstruct the said beam to a point where said shutter blade obstructs said beam.

2. The combination of claim 1 wherein said electromagnetic rneans comprises an armature and a coil having a central passage in which said armature slides and said actuating means is connected to said armature.

3. The combination of claim 2 further comprising:

(a) a housing having a first and second end and a central bore having an overbored portion adjacent said second end, said overbored portion being provided with internal threads and said bore having an internal groove adjacent said overbored portion;

(b) means for fixedly mounting said coil in said housing with said actuating means extending through said first end of said bore;

(c) a backing plate mounted in said internal groove;

(d) spring means connecting said backing plate to said armature;

(e) sealing means disposed in said overbored portion of said housing; and

(f) externally threaded closure means in threaded engagement with said internal threads on said housing, whereby said second end of said housing may be. sealed.

4. The combination of claim 3 further comprising electrical conductors connecting the coil to a source of electrical current, said conductors passing in vacuum sealed relationship through said backing plate and sealing means.

5. The combination of claim 3 wherein said electron microscope has a casing provided With at least one hole communicating with the interior of said microscope adjacent that portion thereof at which the electron beam is narrowest, said hole being provided with internal threads at the outer end thereof; and said housing is provided with 5 external threads adjacent said first end thereof whereby said housing may be threadably engaged with said microscope casing.

6. The combination of claim 2 wherein said actuating means is constructed from non-magnetic material.

References Cited UNITED STATES PATENTS 3,102,194 8/1963 Van Der Broek et al. 250-495 5 ARCHIE R. BORCHELT, Primary Examiner.

RALPH G. NILSON, WILLIAM F. LINDQUIST,

A. L. BIRCH, Assistant Examiner.

Examiners.

Claims

1. IN COMBINATION WITH AN ELECTRON MICROSCOPE HAVING AT LEAST ONE COIL FOR THE FOCUSING OF A BEAM OF ELECTRONS, AN ELECTRICALLY OPERATED SHUTTER MECHANISM WHICH OBSTRUCTS THE BEAM WHEN CURRENT IS FLOWING AND DOES NOT OBSTRUCT THE BEAM WHEN CURRENT IS NOT FLOWING IN THE SHUTTER MECHANISM, SAID MECHANISM COMPRISING: (A) A SHUTTER BLADE PIVOTALLY MOUNTED WITHIN SAID ELECTRON MICROSCOPE IN CLOSE PROXIMITY TO A POINT WHERE THE ELECTRON BEAM CROSS SECTION IS NARROWEST, SAID BLADE BEING CAPABLE OF MOVEMENT IN A PLANE PERPENDICULAR TO THE AXIS OF THE ELECTRON BEAM FROM A POSITION WHERE SAID BEAM IS OBSTRUCTED BY SAID BLADE TO A POSITION WHERE SAID BEAM IS NOT OBSTRUCTED BY SAID BLADE; (B) BIASING MEANS COACTING WITH SAID SHUTTER BLADE TO MAINTAIN SAID BLADE IN A POSITION WHERE SAID BEAM IS NOT OBSTRUCTED; (C) ACTUATING MEANS ABUTTING SAID SHUTER BLADE, SAID ACTUATING MEANS BEING BIASED TO MAINTAIN SAID BLADE IN A POSITION WHERE SAID BLADE DOES NOT OBSTRUCT SAID ELECTRON BEAM; AND (D) ELECTROMAGNETIC MEANS FOR MOVING SAID ACTUATING MEANS AND SHUTTER BLADE, AGAINST THEIR COMBINED BIASES FROM THE POINT WHERE SAID SHUTTER BLADE DOES NOT OBSTRUCT THE SAID BEAM TO A POINT WHERE SAID SHUTTER BLADE OBSTRUCTS SAID BEAM.