US3737659A - Field of view adjusting device - Google Patents

Abstract

Field of view adjusting device for use in electron microscopes or the like having a mechanical sample adjusting means and electron deflecting means disposed in or at the rear of an objective lens, coarse adjustment being carried out by the mechanical moving means and fine adjustment by the electron deflecting means.

Description

Uliitfl St3tS Patent 91 Yanaka et al.

FIELD OF VIEW ADJUSTING DEVICE Inventors: Takashi Yanaka, Hino-shi, Tokyo; Kohei Shirota, Akishima-shi, Tokyo, both of Japan Assignee:

Nihoa Denshi Kabushiki, Tokyo,

Japan Filed:

Nov. 17, 1971 Appl. No.: 199,702

Related 0.8. Application Data Continuation of Ser. No. 199,702, Nov. 17, 1971,

abandoned.

Foreign Application Priority Data April 8, 1969 Japan ..44/27401 US. Cl ..250/49.5 A, 250/495 B, 250/495 D Int. Cl

............. ..H0lj 37/26, G0ln 23/00 Field of Search ..250/49.5 A, 49.5 B,

7 5 0. c, CONTROL) saunas 1 June 5, 1973 Primary ExaminerWilliam F. Lindquist Attorney-Webb, Burden, Robinson & Webb [57] ABSTRACT Field of view adjusting device for use in electron microscopes or the like having a mechanical sample adjusting means and electron deflecting means disposed in or at the rear of an objective lens, coarse adjustment being carried out by the mechanical moving means and fine adjustment by the electron deflecting means.

6 Claims, 9 Drawing Figures PATENTELJJUH 51975 Isotropic Astig mdtiSml'fl) Off-axial Chromdtic Aberr 1ti0 mg) Resolving Power (Ii) SHEET 2 UP 5 m Radius of Movementw) A INVENTOR. TAV-ASm YANAM 4;

BY 1 03 E\ swab-m PATENTEDJUH 5 I973 SHEET 3 OF 5 FL 4/0255 CENT Mme l/VG V255 R un w 3 w v m mms W (PE 3 M K. A T

Y M B M m w 6 1 c 6 a I. o m 47 N w A-C. SOURCE PATENTEUJL'N 5 I573 SHEET U 0F 5 INVEINTOR. TAKAiJ? YANAKA i AEl sauzan FIELD OF VIEW ADJUSTING DEVICE This is a continuation, of application Ser. No. 199,702 filed Nov. 17, 1971, now abandoned.

This invention relates to an electron beam apparatus, and more particularly to an improved specimen field of view adjusting device for use in electron microscopes or the like.

In electron microscopy, it is necessary to adjust or shift the specimen field of view to be observed accurately, finely, smoothly, and without causing aberrations or affecting image resolution. It is also necessary to minimize specimen transverse drift and-vibration. At magnifications in the order of 100,000 times or over, these factors are especially important. In a conventional mechanical specimen adjusting device, as described, for example, in U.S. Pat. No. 2,423,158 and U.S. Pat. No. 2,858,444, the specimen is adjusted with very limited accuracy.

Generally, a specimen stage is supported on ball bearings, in which case, it is difficult to maintain the machined accuracy of the respective contact surfaces. These surfaces tend to become scored by the bearings, a factor which hampers smooth specimen adjustment.

Another adverse factor arises from the unbalanced thermal expansion of the stage, the adjusting linkage and the upper part of the objective lens on which the stage mechanism is supported. This causes the specimen to drift or shift from the electron beam axis in a plane perpendicular to the axis.

Yet another adverse factor arises from the tangential adjusting linkage for the stage. When the specimen stage is moved mechanically, it continues to move with decreasing speed for some ten seconds or more after manipulation has been terminated in the direction moved. This factor makes accurate adjustment of the specimen field of view difficult and destroys the image quality of a photograph taken immediately after completing the specimen shift adjustment. As long as the specimen stage is adjusted by mechanical means only, the aforementioned drawbacks will persist.

It is an advantage of adjusting devices according to this invention that the field of view may be adjusted extremely, precisely, finely and smoothly at magnifications in the order of 100,000 times or more. It is a further advantage of adjusting devices according to this invention that specimen drift or shift due to thermal expansion or contraction and inertial mechanical move ment are almost entirely eliminated. The velocity of specimen drift and the magnification of the image can be measured with these adjusting devices.

Briefly, this invention pertains to an electron microscope, comprising, for example, an electron beam source, a sample holder, a magnetic objective lens, and an image producing device, such as a fluorescent screen arranged along an optical axis. Usually an intermediate magnetic lens and a magnetic projector lens are located between the objective lens and the fluorescent screen. According to this invention, the specimen field of view is coarsely adjusted with mechanical devices and finely adjusted by optical devices, that is, by at least one electron deflecting device positioned adjacent the image side of the objective lens. It is preferable if the deflecting means is positioned at or about the back focal plane of the objective lens. It is also preferable that the mechanical adjustment devices be disengagable from the sample holder immediately after the coarse adjustment and during the optical adjustments.

A preferred embodiment of this invention, the power source for the deflecting means, can be modulated to cause a double image on the imaging device, for example, fluorescent screen.

This invention will be more fully understood by reading the following detailed description made with reference to the accompanying drawings, in which:

FIGS. 1 and 2 schematically illustrate the adjusting devices according to this invention;

FIG. 3 is a graph showing variation in the isotropic astigmatism, the off-axial chromatic aberration and the image resolution as a function of the radius of movement when the specimen is adjusted by the deflecting means illustrated in FIG. 2;

FIG. 4 is a sectional view showing one embodiment of this invention;

FIG. 5 is a sectional view showing another embodiment of this invention;

FIG. 6 shows one observing method using this inventlon;

FIGS. 7, 8 and 9 are schematic diagrams showing other embodiments of this invention.

Referring now to FIG. 1, electrons scattered by a specimen 1 pass through an objective lens 2, (shown in the drawing by an optical analog) forming an image of the specimen 1 on a plane 3. The image is magnified by an intermediate lens 4 (shown as an optical analog) and a projector lens (not shown) arranged at the rear of the objective lens 2, and a final image is formed on a fluorescent screen (not shown). One set of electron deflecting means 5 is arranged on the back focal plane or in the vicinity of the objective lens 2. The electron deflecting means may, for example, comprise two pairs of mutually parallel charged plates, the two pairs of plates being at right angles to each other. The electron deflecting means might also, for example, be two pairs of magnetic field forming coils arranged perpendicular with each other. A D.C. source 6 provides the said electron deflecting means 5 with a current or voltage for deflecting the electrons from the specimen 1. A control means 7, arranged between the deflecting means 5 and the D.C. source 6, serves to control the deflecting current or voltage provided by the D.C. source 6.

Now if the deflecting means 5 is not energized, that is to say, when the control means 7 is so adjusted that no current or voltage flows from the D.C. source 6 to the deflecting means 5, only the electrons from the center portion (a) of the specimen pass through the intermediate lens 4 and the projector lens so as to be projected onto the screen. In this case, the electrons emanating from the edge portion (b) form an image at point (b) as shown by the broken line. In other words, the electrons are unable to pass through the intermediate lens 4 and the projector lens. Consequently, the image of the part of the specimen designated by (b) is not projected on the screen.

When it is necessary to vary the specimen field of view, a suitable deflecting current or voltage is supplied to the deflecting means 5 by adjusting the control means 7 accordingly. For example, in order to project an image of portion (b), the control means 7 must be adjusted so as to produce a deflecting angle of a. By so doing, the electrons normally forming at (b) are directed along the electron beam axis, so as to be projected onto the fluorescent screen. In this way, the image of any part of the specimen can be optionally observed.

Referring now to FIG. 2, this shows the arrangement when the deflecting means cannot be arranged in the back focal plane of the objective lens 2. In this case, an extra set of deflecting means are required in order to obtain the same results as in FIG. 1.

It is known to those skilled in the art that the variation of the field of view by the deflecting means causes various forms of aberration to occur in the image and further adversely affects image resolution. However, it has been found that these aberrations are only very slightly increased for small adjustments, e.g., less than about 3 microns. FIG. 3 is a graph showing the effect of adjustments by deflection on certain aberrations for a particular electron microscope. The accelerating voltage of the electrons was lOOKV, the magnetomotive force of the objective lens was 7,000 amp. turn, a half width of the magnetic field in the said lens was 1.5 m.m., and the image magnification was 250,000 times. The abscissa of FIG. 3 represents the radius of movement when the specimen is adjusted by the deflecting means 5a and Sb, and the three ordinates represent the isotropic astigmatism, the off-axial chromatic aberration and the resolving power respectively. Curves A, B and C correspond to the isotropic astigmatism, the offaxial chromatic aberration and the resolving power of the final image on the photographic plate respectively.

It will be readily understood from the graph that the above aberrations, etc. become quite large as the radius of specimen movement increases. It is also quite apparent, however, that by limiting the radius of movement to 3 microns or less, the above aberrations, etc. remain quite small indeed. This is especially true in the case of image resolving power which remains virtually constant. This is due to the fact that the image dimness is less than the resolution of the photographic plate.

As a consequence of the above dimness, the maximum moving distance of the specimen in this invention is 3 microns, a region which is 225 times that of the area of the image observed on the screen when the diameter of the screen is 120 mm. and the image magnification is 300,000 times. It has been found that regardless of magnification changes, the radius of movement is hardly affected.

FIG. 4 shows one embodiment of this invention. An objective lens 2 consisting of a lens yoke 8, pole pieces 9 and an energizing coil 10, is mounted on an intermediate lens 4, consisting of a lens yoke 11, a nonmagnetic spacer 12 and an energizing coil 13. A specimen stage 14 is arranged on the objective lens 2 via balls 15, the said stage supporting a holder 16 for holding the specimen. Two moving rods 17 are mounted at suitable positions in the specimen chamber wall 18, one end of each of which are in contact with the stage 14. A spring 19 is suitably arranged between the said wall 18 and the stage 14. By means of this arrangement, any manipulation of the moving rods 17 causes the stage 14 to move in a plane perpendicular to the electron beam axis, thereby varying the specimen field of view.

Further, a non-magnetic cylinder 20 is arranged in the objective lens yoke 8 coaxially with the beam axis. A first deflecting means 5a and a second deflecting means 5b are attached to the upper portion and the lower portion of the cylinder 20 respectively. In this embodiment, the mechanical specimen moving device consisting of the moving rods 17 and the spring 19 is used for coarse adjustment of the specimen and the deflecting means 5a and 5b is used for fine adjustment. As

a result, the specimen field of view, is adjusted accurately and smoothly with extreme ease.

FIG. 5 is a variation of the arrangement described in FIG. 4. In this case, the stage 14 includes two members 21 and 22, member 22 acting as a support for the specimen holder 16. Members 21 and 22 are held firmly in contact by springs 23 which are suspended between the specimen chamber wall 18 and the member 21. The member 22 is slidably disposed on supporting legs 24 mounted on the upper surface of the objective lens 2. Springs 25, suspended between the objective lens 2 and the member 22, serve to hold the member 22 in slidable contact with the supporting legs 24. The moving rods 17 are mounted so that one end of each rod is in contact with the member 21. Thus, by manipulating the said rods, member 21 is moved which in turn causes member 22 to move, thereby changing the specimen field of view.

In addition, a plurality of levers 26 are pivotally mounted in the specimen chamber 18 above which rods 27 are arranged in contact. By depressing the said rods, the member 21 is made to separate from the member 22 by the depressing action of the levers 26. The member 22, in the meantime, remains stationary on the supporting legs 24.

Thus, by isolating the member 21, together with its manipulating means 17, from the member 22 which acts as a support for the specimen holder 16, specimen drift due to thermal expansion or contraction in the member 21 and moving rods 17, and specimen movement occuring after mechanical manipulation has been terminated is completely eliminated.

As soon as coarse adjustment has been completed in the above fashion, a deflecting current or voltage is applied to the deflecting means 50 and 5b, and the specimen is finely adjusted within a radius of 3 microns.

FIG. 6 illustrates a method in accordance with this invention by which high resolution and broad field image photographs can be obtained. In this method the deflecting means operates so that the specimen image is formed on the screen in the order A B C D E F G II I. Each of the said image portions is photographed and after being developed, each film is pieced together in accordance with the above order.

FIGS. 7, 8 and 9 show other embodiments of this invention.

In FIG. 7, the external end of the moving rod 17 contacts a lever 30 which is activated by a screw bar 31. The other end of the said screw bar is fitted with a bevel gear 32 which is in turn meshed with a second bevel gear 33 attached to a shaft 34 which passes through a gear box 35. The shaft 34 also carries a spur gear 36 which is shown meshed with a second spur gear 39 carried by a slidable shaft 37 to the external end of which knob 38 is fitted. A further shaft 40 likewise carrying a spur gear 41 runs from the gear box 35 through to the control means 7. A potentiometer forming part of the said control means is adjusted by rotating the said shaft 40, thereby varying the deflecting current or voltage.

When spur gears 36 and 39 are meshed, stage 14 is moved by rotating knob 38 via the train of linkages and gears, etc. In other words, lever 30 pushes rod 17 according to the degree in which knob 38 is rotated, thereby controllably moving the specimen stage 14.

On the other hand, when knob 38 is pushed, spur gears 39 and 36 are unmeshed forcing the spur gear 39 to mesh with the spur gear 41. When this occurs, the

potentiometer forming part of the control means 7 is controllably adjusted by knob 38 with the result that the deflecting current or voltage supplied to the deflecting means 5 is similarly adjusted, and the specimen field of view is varied by electron deflection.

One application of this invention is to measure the magnification of the image. This is explained by referring to FIG. 8. Here an A.C. source 42 and a switching means 43 are provided in addition to the DC. source 6 and the control means 7. When it is required to measure the image magnification, the switching means 43 is connected to the A.C. source 42 whereby the AC. current or voltage is supplied to the deflecting means 5a and 5b. The electrons from the specimen are then periodically directed along the lines a and a". As a result, a double image of the specimen is formed on the screen, and the magnification is determined by measuring the distance between the two images.

With devices according to this invention, the field of view is coarsely adjusted mechanically, and finely adjusted optically, that is, by electron deflection. By combining mechanical and optical adjustments, the inherent drawbacks of each are almost entirely eliminated. An accurate, fine and smooth adjustment is provided without causing aberrations or affecting image resolution. Further, the devices according to this invention have added capabilities, that is, they can be adapted to measure specimen drift and magnification.

Having thus described my invention in detail with the particularity required by the patent laws, what is desired to have protected by Letters Patent follows:

1. In an electron microscope or the like comprising:

an electron gun for creating an electron beam;

an electron optical system, having an optical axis, for focusing and projecting said beam comprising at least an objective lens;

an imaging device; and,

a means for positioning a specimen about the optical axis whereby the electron beam emanating from the specimen comprises an image beam which in the plane of the imaging device is larger than the device;

the improvement comprising a field of view adjusting 6 device comprising:

mechanical means for driving the positioning means in a plane perpendicular to the optical axis for coarse adjustment of the field of view projected on the imaging device; and,

at least one electron deflection means disposed adjacent the image side of the objective lens for deflecting the image beam for fine adjustment of the field of view projected centrally on the imaging device within limits representative of a three micron radius of the specimen traversely of the optical axis and thereby maintaining an image of the desired portion of the specimen within the three micron radius projected centrally on the imaging device.

2. The improvement in specimen field of view adjusting device set forth in claim 1, wherein an electron defleeting means is disposed in the back focal plane of the objective lens.

3. The improvement in specimen field in view adjusting device set forth in claim 1, wherein two electron deflecting means are disposed between the objective lens and the image producing device.

4. The improvement in specimen field of view adjusting device set forth in claim 1 comprising means for isolating the specimen stage from the mechanical driving means during the time the deflecting means is being operated.

5. The improvement in specimen field of view adjusting device set forth in claim 1 comprising a DC. voltage source for actuating the deflecting means, means for controlling the said deflecting means by adjusting the DC. voltage source, an AC. voltage source, and a switching means for engaging or disengaging the DC. and AC. voltage sources such that when the A.C. source is applied to the deflecting means a double image suitable for determination of magnification is produced on the imaging device.

6. The improvement in specimen field of view adjusting device set forth in claim 1 comprising interlocking means for controlling said deflecting means and said driving means such that only one can be adjusted at one time.

Claims (6)

1. In an electron microscope or the like comprising: an electron gun for creating an electron beam; an electron optical system, having an optical axis, for focusing and projecting said beam comprising at least an objective lens; an imaging device; and, a means for positioning a specimen about the optical axis whereby the electron beam emanating from the specimen comprises an image beam which in the plane of the imaging device is larger than the device; the improvement comprising a field of view adjusting device comprising: mechanical means for driving the positioning means in a plane perpendicular to the optical axis for coarse adjustment of the field of view projected on the imaging device; and, at least one electron deflection means disposed adjacent the image side of the objective lens for deflecting the image beam for fine adjustment of the field of view projected centrally on the imaging device within limits representative of a three micron radius of the specimen traversely of the optical axis and thereby maintaining an image of the desired portion of the specimen within the three micron radius projected centrally on the imaging device.

2. The improvement in specimen field of view adjusting device set forth in claim 1, wherein an electron deflecting means is disposed in the back focal plane of the objective lens.

3. The improvement in specimen field in view adjusting device set forth in claim 1, wherein two electron deflecting means are disposed between the objective lens and the image producing device.

4. The improvement in specimen field of view adjusting device set forth in claim 1 comprising means for isolating the specimen stage from the mechanical driving means during the time the deflecting means is being operated.

5. The improvement in specimen field of view adjusting device set forth in claim 1 comprising a D.C. voltage source for actuating the deflecting means, means for controlling the said deflecting means by adjusting the D.C. voltage source, an A.C. voltage source, and a switching means for engaging or disengaging the D.C. and A.C. voltage sources such that when the A.C. source is applied to the deflecting means a double image suitable for determination of magnification is produced on the imaging device.

6. The improvement in specimen field of view adjusting device set forth in claim 1 comprising interlocking means for controlling said deflecting means and said driving means such that only one can be adjusted at one time.

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