US 3715582 A Abstract A method of and apparatus for attaining the focusing following a variation in the magnification in an electron microscope having an image producing lens system consisting of three or more lenses or generally n lenses including at least an objective lens, an intermediate lens and a projective lens. Means are provided in the apparatus so that, when the value of excitation current for the magnification varying lens is varied to vary the magnification, the excitation current supplied to the focus adjusting lens can be set at a value corresponding to the variation in the magnification, whereby the focusing following the variation in the magnification by the magnification varying lens as well as the adjustment of the intensity of illumination can be achieved by a single regulating operation.
Claims available in Description (OCR text may contain errors) United States Patent 91 Akahori et al. Feb. 6, 1973 54 METHOD OF AND APPARATUS FOR 3,535,514 10/1970 Cardile ..250/49.5 A ATTAINING FOCUSING FOLLOWING 3 32%: gflgg 3 VARIATION IN MAGNIFICATION IN 315091275 4/1970 Deeley......................::::::250/49:5 A ELECTRON MICROSCOPE Inventors: Hiroshi Akahori; Morioki Kubozoe; Toshihiro Furuya; Sadayasu Ueno, all of Katsuta, Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Aug. 21, 1970 Appl. No.: 65,996 Foreign Application Priority Data. - Feb. 13,1970 Japan ..-....45/40945 May 15, 1970 Japan ..4 5/l2377 US. Cl. 250/49.5 A, 250/495 D Int. Cl ..H0lj 37/10 Field of Search ..250/49.5 A, 49.5 C, 49.5 D v References Cited UNITED STATES PATENTS POWER SUPPLY 4/1970 Deeley ..250/49.5 A, PrimaryExaminep-Archie R. Borchelt Assistant Examiner-C. E. Church Attorney-Craig, Antonelli, Stewart & Hill 57 ABSTRACT 1 tion, whereby the focusing following the variation in the magnification by the magnification varying lens as well as the adjustment of the intensity of illumination can be achieved by a single regulating operation. 8 Claims, 7 Drawing Figures PATENTEDFEB 61975 3.715.582 SHEET 30F 3 FIG. 5 IN VEN TORS H\ROS\-\\ HKHHOR\ MORKNM kuzozoe, ADRYHSM ueuo er 'rosmmko Funuom BY A L WM *HUZQ, ATTORNEYS METIIOI) OF AND APPARATUS FOR ATTAINING FOCUSING FOLLOWING VARIATION IN MAGNIFICATION IN ELECTRON MICROSCOPE BACKGROUND OF THE INVENTION tion is varied by varying the value of excitation current supplied to the magnification varying lens in an electron microscope. 2. Description of the Prior Art In an electron microscope, the magnification must be suitably varied for the observation of various kinds of specimens or for the observation of. those specimens under various states. The manipulation for the variation in the magnification in an electron. microscope includes varying the value of excitation current supplied to the magnification varying lens for varying the magnification and then varying the value of excitation current supplied to the focus adjusting lens for attaining the focusing thereby to produce an enlarged image of a specimen on a final image screen. In the electron microscope, the intensity of illumination of the final image must be kept constant since the intensity of illumination of the image varies in inverse proportion to the square of the magnification with an increase or decrease in the magnification. By way of example, in an electron microscope having a three-stage image producing lens system consisting of an objective lens, an intermediate lens and a projective lens, the intermediate or projective lens acts as the magnification varying lens, while the objective lens acts as the focus adjusting lens. l-Ieretofore, a microscopist hasmanually adjusted the value of excitation current supplied to the focus adjusting lens and condenser lens while observing the final image produced on the fluorescent screen. , However, such a focusing system has been defective in that an elongated period of time is required for the focus adjusting manipulation and, since the specimen is exposed to the electron beam for such a long period of time, the surface of the specimen is damaged to such an extent that it is no longer possible to observe the specimen under ordinary conditions. This defect has been especially marked in the case of a specimen such as a high molecular compound which is easily affected by heat. Further, it has been very difficult with the prior art focusing system to attain the focusing following a variation in the magnification in the case of observation of a specimen such as a piece of tissue with a so-called low magnification of the order of several thousand times or less, and the manipulation for the proper gfocusing has required an elongated period of time. Thus, this defect has aggravated the above-described defect. SUMMARY OF THE INVENTION 7 method. Another object of the present invention is to provide an apparatus for attaining the focusing following a variation in the magnification in an electron microscope in which means are provided so that, when the value of excitation current supplied to the magnification varying lens is varied to vary the magnification, the excitation current supplied to the focus adjusting lens can be set at a value corresponding to the above variation in the magnification to attain the focusing in simultaneous relation with the manipulation for the variation in the magnification, and at the same time, the excitation current supplied to the condenser lens can also be set at the value corresponding to the above variation in the magnification to adjust the intensity of illumination, whereby the variation in the magnification can be always carried out in the state in which the focusing is properly attained and the intensity of illumination is completely adjusted. A further object of the present invention is to provide an electron microscope having an image producing lens system consisting of three or more lenses or generally ri lenses including at least an objective lens, an intermediate lens and a projective lens, in which means are provided to set the excitation current supplied to the focus adjusting lens at a value corresponding to a variation in the value of excitation current supplied to the magnification varying lens so that the focusing following the variation in the magnification can be carried out by a single regulating operation thereby to eliminate the troublesome manipulation involved in the handling of the electron microscope. A yet further object of the present invention is to provide, in-an electron microscope having an image producing lens system consisting of three or more lenses or generally n lenses including at least an objective lens, an intermediate lens'and a projective lens, a method of varying the magnification of the electron lens comprising fixing the focal length of the intermediate lens and varying the value of excitation current supplied to the lens or lenses disposed in the stage or stages lower than the intermediate lensfor carrying out the variation in the magnification. A still further object of the present invention is to provide, in an electron microscope having an imageproducing lens system consisting of three or more lenses or generally n lenses including at least an objective lens, an intermediate lens and a projective lens, a method and apparatus for varying the magnification of the electron microscope in which the value of excitation current supplied to the intermediate lens is varied stepwise while the value of excitation current supplied to the lens or lenses disposed in the stage or stages lower than the intermediate lens is continuously varied at a portion corresponding to the stepped portion of the intermediate lens excitation current so as to carry out the variation in the magnification. A common object of the present invention is to provide a novel method of varying the magnification of an electron microscope and an apparatus capable of attaining the focusing following the variation in the magnification by a single regulating operation in simultaneous relation with the above variation in the magnification. Other objects, features and advantages of the present invention will be readily apparent from the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view showing the relative position of electron lenses and the positional relation between image planes in an electron microscope having a four-stage image producing lens system consisting of an objective lens, an intermediate lens, a first projective lens and a second projective lens. FIG. 2 is a block diagram of an embodiment of the present invention having interlocking control means for the simultaneous control of the values of excitation current supplied to electron lenses in an electron microscope having an image producing lens system consisting of four or more lenses or generally n lenses including at least an objective lens, an intermediate lens, a first projective lens and a second projective lens. FIG. 3 is a graph showing the relation between objecand the principal plane 2 of the objective lens and the distance between the principal plane 2 and the image forming plane 3 of the objective lens O respectively. Suppose further that f,, a,, b,; f,.,, a,.,, b and 5 fpz, a bpz are those of the intermediate lens I,,,, first tive lens excitation current and-intermediate lens.ex 1 citation current when, in an electron microscope having an image producing lens system consisting of three or more lenses or generally n lenses including at least an objective lens, an intermediate lens and a projective lens, the value of excitation current supplied to the electron lens or lenses disposed in the stage or stages lower than and including the projective lens is kept constant. FIG. 4 is a graph showing themann'er of variation in the values of excitation current supplied to electron lenses for illustrating an embodiment of the method of varying the magnification according to the present invention when it is applied to an electron microscope having a four-stage image producing lens system consisting of an objective lens, an intermediate lens, a first projective lens and a second projective lens. FIG. 5 shows a modification of the embodiment shown inFlG. 2. I FIGS. 6 and 7 are modifications of parts of the embodiment shown in FIG. 5. ' DESCRIPTION OF THE PREFERRED EMBODIMENTS The relative position of electron lensesand the positional relation between image planes in an electron microscope having a four-stage image producing lens system consisting of an objective lens, an intermediate lens, a first projective lens and a second projective lens will be described with reference to FIG. 1. In FIG. 1, a specimen to be observed is designated by the reference numeral 1. The objective lens 0,, has aprincipal plane 2 and an image forming plane 3. Theintermediate lens 1,, has a principal plane 4 and an image forming plane 5. The image forming plane 5 of the intermediate lens Int corresponds at the same time to the object plane for the first projective lens P, disposed in the next stage. The first projective lens P hasa principal plane 6 and an image'forming plane7. The image forming plane 7 of the first projective lens P corresponds at the same time to the object plane for the second projective lens P, disposed in the next stage. The second projective lens P, has a principal plane 8and an image forming plane 9. Generally, a fluorescent screen for the image observation'or the emulsion surface of a photographic plate or film is disposed at the image forming plane 9 of the second projective lens P Suppose that f ',a,, and b, are the focal length of the objective lens O,,,,'the distance between the specimen 1 projective lens P, and second projective lens P respectively, and u, v and w are the distances between the principal planes of the four lenses. Then, the following equations hold: ' l/a,,+ 1Ib,,=l/f 1 /fI l/a 'l' I/bp =1/fp l pz P2 /fP2 a, b u Pl 1 v P2 l! w Suppose further that M M,, M and Mp2 are the magnification of the objective lens O,,,, that of the intermediate lens I,,,, that of the first projective lens P and that of the second projective lens P respectively. Then, the total magnification M is given by MT: a/ o X l/ l X Pll Pl X P2 P2 o l MP1 P2 In the electron microscope, the variation in the magnification is generally carried out by varying the value of excitation current supplied to the intermediate lens I, thereby varying the focal length f, of the intermediate lens I,,,, that is, by shifting the position of the object plane 3 for the intermediate lens l,,,. More precisely, when the distance a, is varied to a, 01,, the magnification of the intermediate lens I is now given by M, b,/(a,'+ a and the magnification of the objective lens O is given by M (b a,)/a,,. Thus, the magnification at the portion including the objective lens O and the intermediate lens I, is varied from MD'MI t0 M 1 (b a,)/a,, b1/(Ll1 a However, due to the fact that the image forming plane 3 of the 1 objective lens O is shifted by --a,, the image would be out of focus unless the focal length f, of the objective lens O is varied correspondingly provided that the distance a, is fixed. Thus, with the system described above, the variation in the magnification by the intermediate lens results in blurring of the image, and therefore, the value of excitation current supplied to the objective lens must be adjusted for attaining the proper focusing each time the .sure' to the electron beam for a long period of time. With a view to overcome the above drawbacks, the present invention contemplates the provision of an electron microscope which is provided with means for setting the excitation current supplied to the focus adjusting lens at a value corresponding to a variation in the magnification in simultaneous relation with the variation in the magnification by the magnification varying lens. The present invention contemplates also the provision of a novel method of carrying out the variation in the magnification. The method of varying the magnification according to the present invention will be described in detail hereunder. l. At first, the variation in the focal length f, of the objective lens 0,, when the magnification is varied by varying the value of excitation current supplied to the intermediate lens I, in the relative position shown in FIG. 1 will be discussed. In this case, the focal length f, of the intermediate lens 1,, is solely subject to variation. Thus, the focal lengths f,., and f are fixed and the distances a a,, a bp a and b are kept constant. The total magnification M in this case is given by i= o MI MP1 MP2 K1 e where 1 l/ Pll Pl) ps P2)- Substituting a, and b, in the equation (8) by those obtained from the equations (2) and M is expressed MT: K1 ["UF'fl) lfr/ llfl The focal length f, is sought from the equation (9) as 0 u heft? The equation (12) is differentiated with M to find the variation Af, of f], when f, is varied to vary M The result is given by fa)fI 1/ 'flf' r' r (13) The above equation represents the variation Af of the focal length f of the objective lens 0,, when the total magnification is varied by AM by varying the value of excitation current supplied to the intermediate lens 1, 2. The next discussion is directed to the case in which the magnification is varied by varying the value of excitation current supplied to another lens, for example, the first projective lens P, instead of the intermediate lens l,,,. In this case, the focal length fp is solely subject to variation. Thus, the focal lengths f, and f are fixed and the distances a bp a and b are kept constant. The total magnification M in this case is given by 1 o' l' m' rz u/ 0 1)'( Pi/ m)'( Pz/ pz) i( o l)'( l/ m) 1( Pr/ m) where [61 (hm/ 0 bra/ P2 The equation (17) is differentiated with M to find the variation Af of f, when M is varied by varying fp The result is given by flv){ 1/ 2)' fo F T The above equation represents the variation Af of the focal length f, of the objective lens O when the magnification is varied by varying the value of excitation current supplied to the first projective lens P 3. The ratio between the equations (13) and (18) is sought to seek the ratio between the variations Af of the focal length f, of the objective lens 0,, when the magnification is varied to the same value. For the sake of simplicity, it is assumed that u z v most cases. when u V,fpz is constant and (h (b h Since the focal length f, of the intermediate lens I is used in the range in which it is smaller than u and the relation f, u holds at a high magnification, the following equation can be obtained: ft!) fp1 The above equation shows the fact that, when the mag nification is varied by varying the focal length f, of the intermediate lens l,,,, the variation Af of the focal length f of the objective lens O is greater by the magnification of the first projective lens l than when the magnification is varied by varying the focal length f of the first projective lens P In other words, the above equation implies that the variation f, in the focal length f, of the objective lens for the variation in the magnification is smaller when the magnification is varied by varying the focal length f of the first projection lens than when the magnification is varied by varying the focal length f, of the intermediate lens. The ratio between the two cases is about l/M Ordinarily the value of M is 10 to 20. Consequently, when the magnification is varied by varying the focal length f of the first projection lens, the image falls within the depth of focus of the objective lens 0,, to become a focused or distinct image even without varying the focal length f, of the objective lens. This applies also to an electron microscope having a three-stage image producing lens system consisting of an objective lens, an intermediate lens and a projective lens. In this case too, the variation in the magnification can be carried out in a substantially sharply focused state by varying the magnification by the projective lens while fixing the value of excitation current supplied to the intermediate lens. However, the variation in the magnification solely by the first projective lens is not practical because its magnification varying range is narrow. In such a case, as will be described later with reference to FIG. 4, the entire magnification range of the electron microscope is divided into N sections, at the boundaries of which the excitation current of the intermediate lens is varied discontinuously, but within each of which it is kept constant. Within each section the excitation current of the lens of a subsequent stage to the intermediate lens, that is, of the first or second projection lens is finely varied so that the magnification is varied without a substantial variation in the focal length f, of the objective lens, that is, in the substantially focused state. In order to keep the focal length of the objective lens substantially constant irrespective of the variation in the magnificatiomthe variation in the excitation current of the objective lens corresponding to the variation in the excitation current of the intermediate lens, which has been calculated or measured in advance, is imparted to the excitation current of the objective lens in coordination with the switching of the excitation current of the intermediate lens. In this manner the magnification can be varied over its entire range in a substantially focused state. The above description has referred to the method of varying the magnification of an electron microscope having a three-stage image producing lens system consisting of an objective lens, an intermediate lens and a projective lens. FIG. 2 is-a block diagram of means for controlling the excitation current supplied to electron lenses in an electron microscope having an image producing lens system consisting of four lenses or generally n lenses including at least an objective lens, an intermediate lens, a first projective lens and a second projective lens As for the means for controlling the excitation current suppliedto electron lenses in an electron microscope having a three-stage image producing lens system, the case in which the excitation current supplied to the lenses disposed in the stages lower than the third stage is fixed at a predetermined value in FIG. 2 may be con sidered. Referring to FIG. 2, the reference numerals 20, 21, 22 and 23 designate an objective lens excitation current control system, an intermediate lens excitation current control system, a first projective lens excitation current control system, and a second projective lens excitation current control system, respectively. The reference numeral 24 designates generally an nth projective lens excitation current control system. The electron lens control systems 20, 2I 22, 23 and 24 are supplied with current from a common stabilized power supply 25. The objective lens excitation current control system 20 includes an objective lens coil 0,,, a detecting resistor R, for detecting the excitation current flowing through the objective lens coil 0,,, a voltage-dividing resistor R, for the detecting resistor R,, an error amplifier D, such as a balanced DC amplifier for detecting and amplifying the difference between a voltage detected by the voltage-dividing resistor R, and a reference voltage V,,,, and an output control means C, which is controlled by the output from the amplifier D, for controlling the excitation current flowing through the coil 0,,. The lens excitation current flowing through the objective lens coil 0,, is converted into a voltage by the combination of the current detecting resistor R, and the voltage-dividing resistor R,, and the difference between this voltage and the reference voltage V,,, is detected and amplified by the error amplifier D,. The output from the error amplifier D, is supplied to the output control means C, for stabilizing the current flowing through the excitation coil 0,,. That is, the lens excitation current is maintained at a predetermined value depending on the value of the detecting resistor R,. The setting of the lens excitation .current flowing through the objective lens coil 0,, can be varied by varying the resistance of the detecting resistor R, or the voltage division ratio of the voltagedividing resistor R,'. In the present embodiment the value of the excitation current flowing through the objective lens coil 0,, is varied by varying the voltage division ratio of the voltage-dividing resistor R, connected in parallel with the detecting resistor R,. For this purpose, the voltage-dividing resistor R, may have a variable resistance so that it may be varied continuously or the voltage-dividing resistor R, may be divided stepwise as shown in FIG. 2 so that its resistance may be changed over by a changeover switch 30,. The operation of the remaining electron lens excitation current control systems 21, 22, 23 and 24 is similar to the operation of the objective lens excitation current control system 20, and suffixes I, P,, P, and P, are attached to the corresponding components of the intermediate lens excitation current control system 21, first projective lens excitation ,current control system 22, second projective lens excitation current control system 23, and nth projective lens excitation current control system 24, respectively. The variation in the values of excitation current in the electron lens excitation current control systems is carried out by an interlocking change-over means 40 such as a rotary switch which changes over the voltage division ratios R of the detecting resistors which are preset to supply required current values in a fixed relationship with each other. As an example, when the excitation currents for the electron lenses disposed in the stages lower than and including the first projective lens P, are kept at respective fixed values and the focal length f, of the intermediate lens I, is varied to vary the magnification of the electron micriscope shown in FIG. 2, a corresponding variation Af, of the focal length f, of the objective lens 0,, will be as shown in the equation (13). It is known that the focal length of a magnetic lens generally varies in inverse proportion to the square of the excitation current. Suppose, in this case, that I, and I, are the excitation current for the objective lens 0,, and the excitation current for the intermediate lens I respectively. Then, the following equation is derived from the equation (12): o 1+[ 2 l 3)] (21) where C,, C and C are constants. This relation is illustrated in FIG. 3. Therefore, the voltage division ratio of the voltagedividing resistor R, for the detecting resistor R, in the intermediate lens excitation current control system 21 and the corresponding voltage division ratio of the voltage-dividing resistor R, for the detecting resistor R, in the objective lens excitation current control system 20 may be determined on the basis of the relation between the intermediate lens excitation current I, and the objective lens excitation current I, shown in FIG. 3, and the change-over switches 30, and 30. for the changeover of these voltage division ratios may be mechanically coupled to each other by the interlocking changeover means 40 as to attain the focusing following the variation in the magnification by a single regulating operation. The interlocking change-over means 40 may be disposed between the objective lens excitation current control system 20 and the intermediate lens excitation current control system 21 or it may be arranged to simultaneously control the values of all the electron lens excitation currents as shown in FIG. 2. In this latter case, the change-over of the values of excitation current for the lenses disposed in the stages lower than the intermediate lens I may be such that the excitation currents are kept at the same values in spite of changeover of the steps for varying the voltage division ratio of the voltage-dividing resistors R. For example, the change-over steps may be connected to the same voltage-dividing point so as to show the same resistance. A condenser lens excitation current control system 26 is so arranged that the excitation current for a condenser lens coil C is set at a value corresponding to the variation in the value of excitation current for the intermediate lens coil O,,, in the intermediate lens excitation current control system 21 in order to adjust the intensity of illumination of the image following the variation in the magnification. Since the intensity of illumination of the final image can be kept always constant in spite of the variation in the magnification, photographing, especially continuous photographing can be done without any fear of underexposure or overexposure. The above description has referred to the simultaneous control means for the electron lens excitation currents on the basis of the discussion given in paragraph I. The following description is directed to a method and apparatus for varying the magnification on the basis of the discussion given in paragraphs 2 and 3. FIG. 4 is a graphic illustration of one form of the method of varying the magnification in an electron microscope having a four-stage image producing lens system consisting of an objective lens an intermediate lens I,,,, a first projective lens P, and a second projective lens P In FIG. 4, the horizontal and vertical axes represent the magnification and the lens excitation current, respectively. As will be apparent from FIG. 4, the value of excitation current for the intermediate lens 1,, is varied stepwise and the value of excitation current for the first projective lens P or second projective lens P, is varied continuously or stepwise at portions corresponding to the stepped portions of the intermediate lens excitation current'for varying the magnification. Points bearing the marks on the characteristic curves for the first and second projective lenses P and I represent the current values varied stepwise. In the magnification range A of from 600 to 3,000 times, the values of excitation current for the intermediate lens I, and first projective lens P are fixed at about 2.17A and 0.83A, respectively, while the value of excitation current for the second projective lens P is varied continuously or discontinuously (stepwise) from about 2.67A to 4.17A. P P P represent the stepwise varying values of excitation current for the second projective lens P and P P P represent the settings of excitation current for the objective lens 0 corresponding to the stepwise varying values of excitation current for the second projective lens P Therefore, the voltage division ratios of the voltagedividing resistors R for the detecting resistors R in the respective electron lens excitation current control systems may be set to have a relation as described above and these voltage-dividing resistors may be mechanically coupled to each other by the interlocking change-over means 40 shown in FIG. 2. The above arrangement applies also to the magnification ranges described below. In the magnification range B of from 3,000 to 30,000 times, the values of excitation current for the intermediate lens I and second projective lens P are kept constant, while the value of excitation current for the first projective lens P is varied continuously or stepwise. In this range, the value of excitation current for the objective lens 0,, is varied in a manner as shown. In the magnification range C of from 30,000 to 300,000 times, the values of excitation current for the intermediate lens I, and second projective lens P are kept constant, while the value of excitation current for the first projective lens P is varied continuously or stepwise. In this range, the value of excitation current for the objective lens 0,, is varied in a manner as shown. It will be apparent from FIG. 4 that the value of excitation current for the objective lens O is varied stepwise in a relation similar to the stepped variation of the value of excitation current for the intermediate lens 1, but the variation in the value of excitation current for the objective lens 0,, at the portions corresponding to the stepped portions of the excitation current for the intermediate lens I, is very little compared with the variation in the value of excitation current for other electron lenses. It will be understood that the variation in the magnification according to the socalled Zoom lens system can be carried out by an arrangement in which the values of excitation current for the respective electron lenses are varied in a relation as shown in FIG. 4 and an interlocking change-over means 40 as shown in FIG. 2 is used for the simultaneous control of the variation of these current values. While the above description has referred to the case of varying the values of excitation current by varying the resistances of the detecting resistors or the voltage division ratios of the voltage-dividing resistors in the respective excitation current control systems, such may be carried out by varying the voltage value of the reference voltage source V Another embodiment of the present invention in which the value of reference voltage is varied to vary the value of excitation current will be described with reference to FIG. 5. Although a three-stage image producing lens system consisting of an objective lens 0, an intermediate lens I, and a projective lens P is shown in FIG. by way of example, it will be apparent for those skilled inthe art that the present embodiment is also applicable to an n-stage lens system. The suffixes O, I and P are attached to components belonging to excitation current control systems 200, 210 and 220 for the objective lens 0 intermediate lens I,,, and projective lens P, respectively. The reference numerals 50 and 60 designate a source of reference voltage and a voltage-dividing resistor group, respectively, which are common to the electron lens excitation current control systems 200, 210 and 220. A change-over means S for the objective lens excitation current control system 200 includes a change-over switch 70,, and a plurality of stationary contacts 0,, 0,, 0,. A change-over means S, for the intermediate lens excitation current control system 210 includes a change-over switch 70, and a plurality of stationary contacts 1,, I 1,. Similarly, a change-over means S for the projective lens excitation current control system 220 includes a change-over switch 70 and a plurality of stationary contacts P,, P P,. The voltage-dividing resistor group 60 has a plurality of voltage-dividing terminals T T T,, T,,. The precision of the setting of the value of excitation current is different in each electron lens, but a very high degree of precision is required for every electron lens. For example, in order to eliminate any fluctuation inthe intensity of illumination of the image as well as in Y the focal point when the magnification is varied in a lens system consisting of a plurality of stages, the values of excitation current must not deviate beyond 0.001 percent in the case of the objective lens and 1 percent in the case of the condenser and projective lenses. Therefore, the voltage-dividing resistor group 60 used for the setting of the current value requires such a number of resistors which will correspond to the very finely divided units of the current value and the in dividual resistors must also be of high precision. Taking the above point into consideration, the voltage-dividing resistor group 60 is composed of a series connection of n unit resistors having the same rated resistance R, and the stationary contacts 0,, O 0,; 1,, I 1,; and P,, P P, of the respective change-over means 8,, S, and S, are connected to the voltage-dividing terminals corresponding to the desired current settings in a manner as, for example, shown in FIG. 5. When, for example, the change-over switch 70, of the change-over means Sp for the projective lens excitation current control system 220 is brought into contact with the stationary contact P,, the input terminals of an error amplifier D, is connected through the change-over switch 70, and stationary contact P, to the voltage-dividing terminal T, of the voltage-dividing resistor group 60 connected to the reference voltage source 50. Thus, a reference voltage which is S/n of the voltage V,, of the reference voltage source 50 is applied to the error amplifier D Similarly, a voltage which is any desired division of the reference voltage V,, can be obtained when each of the change-over switches 70,, and 70, is brought into contact with a suitable stationary contact. Therefore, the variation in the magnification can be always carried out in a sharply focused state when these stationary contacts are previously connected to give respective fixed voltages as described in the first embodiment and the change-over switches 70,, 70, and 70,. are simultaneously changed over by an interlocking change-over means 40. Two or three of the stationary contacts in the change-over means 5,, S, and S, are connected to the same voltage-dividing terminal of the voltage-dividing resistor group 60 as shown in FIG. 5. In order to deal with possible fluctuation in the reference voltage value applied to the error amplifier D,,, D, or D, when these stationary contacts are connected to the same terminal, the input resistors for the error amplifiers must have a sufficiently large resistance. When it is required that the divided voltage of the reference voltage V,, applied to the error amplifier has a more precise value lying intermediate between the divided voltage values appearing at the adjacent terminals T, and T of the voltage-dividing resistor group 60, a series circuit including ten resistors each having a resistance of 10R may be connected in parallel with the resistor R lying between the voltage-dividing terminals T, and T as shown in FIG. 6 or a series circuit including ten resistors each having a resistance of R/l0 may be connected in series across the terminals T, and T,,, as shown in FIG. 7. By this arrangement, the excitation current can be set at a value which is one place higher in precision than that obtained with the arrangement shown in FIG. 5. We claim: 1. In an electron microscope comprising a threestage image forming electron lens system consisting of an objective lens, an intermediate lens and a projection lens arranged in this order in the direction of an electron beam axis, and excitation current sources each connected to each of said electron lenses for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation current sources including means for adjusting said excitation currents; a focusing apparatus comprising means for mechanically coupling said excitation current adjusting means to simultaneously control said excitation cur rents, so that when the magnification of said electron microscope is varied by varying the excitation current of said intermediate lens while keeping the excitation current, of said projection lens constant, the excitation current of said objective lens is'set at the value compensating forthe variation of said excitation current of said intermediate lens to provide proper focusing. 2. In an electron microscope having a three-stage image producing electron lens system consisting of an objective lens, an intermediate lens and a projective lens which are disposed in the above order in the direction of the electron beam axis, an apparatus for attaining the focusing following a variation in the magnification comprising an excitation power supply connected to each of said electron lenses for supplying excitation current to said electron lenses and stabilizing the excitation current, each said excitation power supply including an excitation current control means for controlling the excitation current flowing through the lens coil, a detecting resistor for detecting the excitation current flowing through said lens coil, means for varying the resistance of said resistor, a source of reference voltage for supplying a reference voltage for comparison between'it and the voltage detected by said detecting resistor, and an-amplifier for detecting and amplifying the difference between said detected voltage and the voltage of said reference voltage source, said amplifier having its first input terminal connected to said detecting resistor, its second input terminal connected to said reference voltage source and its output terminal connected to said excitation current control means, and means mechanically coupling said resistance varying means for said detecting resistors in said excitation power supplies for the simultaneous control of the resistances of said detecting resistors so as to set the excitation current for said objective lens at a value corresponding to a variation of the value of excitation current for said intermediate lens when the value .of excitation current for said intermediate lens is varied while maintaining the excitation current for said projective lens at a constant value. 3. An apparatus for effecting focusing dependent upon the variation in magnification of an electron microscope, comprising: an electron source for producing electrons; an irradiation electron lens system including a plurality of condenser lenses for converging said electrons into an electron beam to direct said electron beam to a specimen; a three-stage image forming electron lens system consisting of an objective lens, an intermediate lens and a projection lens; excitation current sources each connected to each of said lenses for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation current sources including means for adjusting each respective excitation current; and means for mechanically coupling said excitation current adjusting means to simultaneously control said excitation currents, so that when the magnification of said electron microscope is varied by varying the excitation current of said intermediate lens while keeping the excitation current of said projection lens constant, the excitation current of said objective lens is set at the value compensating for the variation of said excitation current of said intermediate lens to provide proper focusing necessitated by the variation in the magnification and at the same time the excitation currents of said condenser lenses are set at the values corresponding to the variation of said magnification to adjust the brightness dependent upon said variation in magnification. 4. In an electron microscope comprising a three stage image forming electron lens system including an objective lens, an intermediate lens, and a projection lens arranged sequentially in the direction of the electron beam axis, and excitation sources connected to each respective electron lens for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation sources including means for adjusting said excitation currents, an improved focusing apparatus comprising: control means, coupled to each of said current sources and said lenses, for simultaneously maintaining the excitation current of said projection lens constant, for varying the excitation current of said intermediate lens, and for adjusting the excitation current of said objective lens to a predetermined value to provide proper focusing in response to said variation of the excitation current of said intermediate lens, wherein each of said excitation sources comprises a power supply, a switching transistor connected between sar power supply and a respective electron deflection element associated with each lens and a control circuit responsive to said control means for appropriately energizing said switching transistor to control the flow of current to said deflection element, and wherein said deflection element comprises a lens coil and wherein said control circuit comprises a voltage dividing circuit, connected to said lens coil, and a differential amplifier having one input connected to a respective source of reference potential and another input adjustably connected to said voltage dividing circuit, the output of said differential amplifier being connected to control the operation of said switching transistor. 5. An improved focusing apparatus according to claim 4, wherein said control means further includes a mechanically ganged switching arrangement having respective switches connected between each voltage divider and the corresponding input to each of said differential amplifiers. 6. A method of varying the magnification of an electron mic roscope having a four-stage image forming electron lens system consisting of an objective lens, intermediate lens, and first and second projection lenses arranged in this order in the direction of an electron beam axis, excitation current sources each connected to each of said electron lenses for supplying an excita- 1 tion current thereto and for stabilizing said excitation current, each of said excitation current sources including means for adjusting said excitation current, and means for mechanically'coupling said excitation current adjusting means to simultaneously control said excitation currents in a predetermined relation therebetween, comprising the steps of: a. dividing the entire magnification range into a plurality of sections and varying the excitation current of said intermediate lens discontinuously at the boundaries of said sections while keeping said excitation current of said intermediate lens constant within each section; . varying the excitation current of one of the electron lenses disposed downstream of said intermediate lens within said each section to vary the magnification of said electron microscope; and c. keeping the excitation current of said objective lens, within said each section, substantially constant at avalue compensating for the variation of the excitation current of said intermediate lens. 7. A method of varying the magnification of an electron microscope according to claim 6, in which said one electron lens of the step (b) is said first projection lens. 8. A method of varying the magnification of an electron microscope according to claim 6, in which said one electron lens of the step (b) is said second projection lens. 1 Patent Citations
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