US3188465A - Two stage electron beam magnification device comprising plural adjustable magnetic lens system - Google Patents

Description

June 8, 1965 KAZUO ITO ETAL. 3,188,465

TWO STAGE ELECTRON BEAM MAGNIFICATION DEVICE COMPRISING PLURAL ADJUSTABLE MAGNETIC LENS SYSTEM Filed Dec. 20, 1960 2 Sheets-Sheet 1 as/2x 65x 460x J30 0x c h a g hr Emfmr s June 8, 1965 KAZUO ITO ETAL 3,188,465

TWO STAGE ELECTRON BEAM MAGNIFICATION DEVICE COMPRISING PLURAL ADJUSTABLE MAGNETIC LENS SYSTEM Filed Dec. 20, 1960 2 Sheets-Sheet 2 IQ. Z

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61+! F300 2m 1 d f=df F 5 2*h2 157F517 furs aze/o ffo aah/ Yanma United States Patent Office 3,188,465 Patented June 8, 1965 1 Claim. rel. 250-495 This invention relates to an electron lens system and more particularly to an improved electron lens system which is more applicable to an electron microscope.

Generally in an electron microscope and the like, it is required to obtain as large a ratio as possible between maximum and minimum magnifications obtained by the device.

In the case of a single electron lens the minimum focal length produced by the single lens is subject to some restrictions due to electron-optical or constructional conditions, hence it has been usual in an electron microscope having a higher magnification that three electron lenses are generally combined.

Various images from high to low magnifications can be obtained by changing excitation conditions of lenses in this electron lens system, but it has disadvantages that as the magnification goes down under several thousands an electron microscopic image begins to distort generally and that when the magnification is further lowered to several hundreds the observation field of the electron microscopic image produced on a projection fluorescent screen comes to be so reduced as to be unpracticable.

The reason is as follows:

It has been well known that distortion aberrations of an electron lens increase in proportion to the cube of the distance from a specimen to be focused to an optical axis of the electron lens and that the proportioanl constant is determined in accordance with the shape, dimension and degree of excitation of the magnetic pole piece of the lens. It the dimension of the magnetic pole piece is fixed, the above mentioned proportional constant has a tendency to become minimum at a constant value of large excitation of the electron lens and to increase with the reduction of the excitation.

Accordingly in order to keep the distortion aberrations of an electron lens small, it is required to increase the excitation degree of the lens to be maintained at the above constant value orits vicinity. Therefore, in the case where the dimension of the electron lens is fixed and the magnification is lowered by only decreasing the excitation current it is inevitable that an image to be observed is much distorted owing to the increases of the distortion coeflicient and of the observation field of a specimen to be magnified. Namely, if the dimension of an electron lens is predetermined, the magnification range which can be changed Without distortion in a lowe magnification will be very narrow.

For the above reason, in the case of using an electron microscope in a lower magnification range, it is required to successively exchange the aperture or gap of an electron lens magnetic pole piece for those of larger dimension by means of a turret system or the like. In a microscope in which a broader magnification range from high to low magnifications is obtained by such a turret system, a plurality of projector lenses must be provided.

It is inevitable in an optical microscope in which focal length of respective lenses are unvariable, but a plurality of projector lenses are not always necessary in an electron microscope in which focal length can be variable by the excitation of the electron lenses.

In order to remove the above mentioned defects to be observed when a magnification is lowered by changing excitation of a single lens only, it has been considered to use two electron lenses in such a manner that magnifications of both lenses are reduced.

That is, in case of the single lens, only its distortion aberrations will offer a problem, but if the two electron lenses are used with their magnifications lowered at the same time, the last image of an electron microscope to be produced on the projection fluorescent screen will have the resultant distortion of the two lenses.

The resultant distortion aberration has been reported by those skilled in the art.

It has also been well known that when two electron lenses are associated in a certain excitation relationship, respective distortion aberrations are cancelled with each other to result a distortionless electron microscopic image.

However, the above said excitation relationship is complicated in general, and further a range in which magnification can be changed without distortion is as narrow as several times, hence such a lens system has not been used for a lower magnification electron microscope.

Therefore, an object of this invention is to provide an electron lens system in which no distortion aberration appears even in a low magnification by simple operations of an electrical circuit without exchanging electron lenses and having a wide magnification range.

Another object of this invention is to provide a novel electron lens system in which a large ratio between a minimum and a maximum magnification is obtained without distortion aberration in a combination of two electron lenses.

Other objects, features and advantages of this invention will become more apparent from the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating an electron lens system according to this invention which is employed as intermediate and projector lenses of a three stage electron microscope; an electrical. circuit being connected to the lens system.

FIG. 2 is a partially enlarged view of the intermediate and projector lenses shown in FIG. 1, showing the principle of the electron lens system of this invention.

FIG. 3 shows curves for illustrating the relationship between focal length of two electron lenses to be arranged for compensating distortion or distortion aberrations in a two stage electron lens system.

FIG. 4 shows a curve for illustrating a relationship between a dimension ratio of the same two electron lenses and the focal length of the first lens tobe given at that time, and

FIG. 5 shows curves for illustrating an example of measured values showing the relationship between excitation currents of the two lenses and the magnification of this lens system to be obtained at that time in the arrangement shown in FIG. 1. V

For better understanding the principle of this invention, we will hereinafter explain one example in which an electron lens system' of this invention is employed.

FIG. 1 illustrates diagrammatically an electro-optical system and lens exciting power source system in which an electron lens system of this invention is applied to the intermediate and projector lenses of an enlarged three stage electron microscope.

FIGS. 1 and 2 show enlarged parts of the intermediate and projector lens in which this invention is embodied.

In FIGS. 1 and 2, 1 is a cathode having a tungsten filament for emitting electron beams and an electron gun is made by including the cathode, a Wehnelt cylinder 2 and an anode 3.

Passages of electron beams emitted from the cathode 1 are shown by the numerals l and 16. The electron beams 15 and 16 are focused into a parallel and fine beam by the first and second focusing lenses 4 and 5 and strike a specimen 14, are thereafter magnified by an objective lens 6.

V The electron beams further pass through intermediate and projector lenses 7 and it) which form an electron lens system according to this invention and are focused into an image on the projection fluorescent screen 13.

In the explanation of the electron lens system of this invention it is convenient to designate the lenses the first lens and second lens according to the advancing direction of the electron beams, hence we will hereinbelow refer to the intermediate lens I to as the first lens 7 and the projector lens to as the second lens 10.

Namely,'the electron beams passed through the objective lens 6 are affected by the first lens 7 and which consists of a coil 8 and magnetic pole piece 9 and which is kept excited at a constant value so as not to result in distortion aberrations in spite of changing excitation of the second lens and by the second lens which is made up of a coil 11 and magnetic pole piece 12 and which is excited to various values by a variable resistor 19 of the electrical circuit 18. The electron beams are finally projected on the fluorescent screen 13 to form an electron microscopic image without distortion aberrations.

Referring to FIG. 2 we will explain a process in which the distortion aberrations are canceled by the first and second lenses to produce an image without aberrations. The full line shows an actual path of an incident electron beam and the dotted line illustrates a route of an incident electron beam in the case where it is regarded as a paraxial ray whose distortion aberrations to the first and second lenses can be negligible.

When the incident electron beam 15 passes through the first lens it refracts more sharply than the assumed route of the paraxial ray and enters into the second lens, where it retracts less sharply than the assumed route.

In the end, since the incident electron beam coincides with the assumed route of the paraxial ray by passing through the first and second lenses, an electron microscopic image without distortion aberrations can be obtained on the fluorescent screen.

Now, we will show such conditions to be satisfied of the electron lens system in FIG. 3, in which the focal distance of the first lens his represented by the abscissa and the focal distance of the second lens f by the ordinate.

As has been previously described, in order that the I two lenses may compensate their distortion aberrations with each other the relationship between the focal length 7; and f of the first and second lenses comes to be a complicated function of the dimensions of magnetic pole pieces of the respective electron lenses, of the distance between the two lenses and of the distance between the second lens and the fluorescent screen. However, we have found that the above mentioned relationship may be made ,very simple in the case of selecting properly the dimen sions of the lens system. Specifically, we found it out by illustrating the relationship established between the focal length f and f as shown in FIG. 3 when the magnification is changed and distortion aberrations are corrested simultaneously with this invention. (1 desginaites the distance between the first and second lenses and D the distance between the second lens and the fluorescent screen. The arrow indicates the direction of magnification increase. In FIG. 3, when the parameter is made smaller than 1 a linear part which will approach the horizontal axis of almost in parallel appears. Considering the linear part S, in this invention the dimensions of the two electron lenses 7 and 1d are so determined that l+ l 2 V S2+ z becomes enough larger than 1 and the coil of the first lens '7 is so excited by the electrical circuit 17 that the focal length f is maintained at a constant value h (s) and only the focal distance f of the second lens It! is changed by a variable resistance 19 of the electrical circuit 18, thereby obtaining an electron lens system which has no distortion aberration in its low magnification but a broad magnification variable range.

Now, a conventional method of compensating the distortion aberrations is corresponding to use a part where is near 1 such, for example, as a curve C, but a curve C which is similar to the curve C is used in order to simplify the lens magnetization circuit. The magnification of an image increases in the direction of the arrow, hence it is difiicult to vary the magnification broadly along such curve C and the curve C deviate from the curve C, so that the distortion aberrations increase. For this reason, a range in which the magnification can be controlled without distortion aberration has been very narrow as previously described. Further, it is very diflicult to change the excitation in such a manner that focal distances of the two electron lenses run along the curve C. Accordingly it is not practicable for a low magnification electron microscope that both the magnetic excitations of the two stage lenses are changed, as the arrangement becomes com plicated as has already been explained.

Next, we will show measured data of an embodiment of the electron lens system according to this invention.

In accordance with this invention the first lens is kept at a constant focal length f (s), which is given as follows. Namely, a graph such as shown in FIG. 5 can be obtained in advance by experiments and calculations, the abscissa being determined in accordance with the dimensions of the two lenses. The abscissa represents and the ordinate shows r /r which is related tothe magnification of the two lens system. Both r and r are shown in FIG. 2 and the former r designates the focal distance between the incident electron beam '15 in parallel with the optical axis and an intersected optical axis with a perpendicular plane with respect to the axis passing through the center of the first lens 9, and the latter r shows the focal length between the electron beam 15 after passing through the first lens and the intersecting point of the optical axis and a perpendicular plane to the axis at the center of the second lens. It will be easily understood that the relationship shown by is) d is given between the above r /r and the focal distance h (s) of the first lens whose excitation is to be kept at a constant value. Accordingly, if the dimensions of the electron lens system of this invention are as follows:

S =S mm. 12 mm.

S =l.4 mm.

k 1.0 mm.

d: 100 mm.

D=300 mm.

l is obtained as a corresponding value of that of the second lens i Numerals attached in the graph show magnifications of an electron microscopic image according to this invention at their respective points.

That is, the first lens current i is fixed at the value of 13 ma. and the second lens current i is made variable from 29 ma. to 70 ma. whereby the magnification comes to 'be successively variable from 10 times of point a to 65 times of point b without the distortion aberration. In the same figure, the tWo electron lenses are not excited in such a manner that parts of (b)-(c) and (d)(e) compensate the distortion aberrations of each other strictly. Namely, as for the part of (b)(c), when the first lens current i is decreased to make f large, the incident electron beam to the second lens approaches to the optical axis in parallel and refracts greatly by the second lens, hence the magnification increases up to a point 0 of 460 times. As for the part of (d)-(e), the focal distance h of the first lens is made large and the incident electron beam to the second lens is so made as to enter into the second lens at a large angle after passing across the optical axis, accordingly the (d)(e) part shows an increase of the magnification from the point d of 460 times to point e of 3300 times.

Anyway, in the parts of (b)(c) and (d)(e) the magnification is large, accordingly image formation is made by using paraxial ray only, hence the distortion aberrations do not matter in this invention.

In the embodiment of this invention, the magnification can be successively adjusted from 10 times of the lowest to 3300 of the highest without distortion aberration and without diminishing the observation field. If an objective lens 6 having the magnification of times is provided in the electron lens system of this invention to constitute a three stage electron microscope, a highly efficient electron microscope can be obtained the magnification of which is 600 times at the minimum to 200,000 times at the maximum.

It will be understood from the above explanation of the embodiment that this invention has not such disadvantage as a complicated non-successive operation for adjusting the magnification even in a low magnification. The method of the magnification adjustment of this invention is diiferent from that of an ordinary electron microscope which is taken by exchanging magnetic pole pieces of the electron lens'by means of a turret.

That is, when the electron lens system of this invention is employed a highly eificient electron lens system can be obtained in which the magnification can be made variable successively over a considerably broad range from a low magnification to a high one without distortion aberration by means of electrical operations only. Therefore, this invention has an advantage that a highly sensitive electron beam magnification device can be produced more easily and cheaper.

The above explanations of this invention have been made in connection with a magnetic field type electron lens using an electromagnet. It will, however, be very easy to apply this invention to a magnetic field type electron lens employing a permanent magnet or to an electric field type electron lens. Moreover, it will also be apparent to those skilled in the art that the electron lens system of this invention can be employed in an electron beam magnification device other than an electron microscope.

It Will be apparent that many modifications and variations can be effected without departing from the scope of the novel concepts of the present invention.

What is claimed is:

A two stage electron beam magnification device comprising a first electron lens, a second electron lens spaced from said first electron lens in the direction of travel of the electron beam, means tor exciting said first lens, and separate means for exciting said second lens, the lens dimensions being such that the Value is sufiiciently greater than unity so that the excitation of one of said lenses may be varied while holding the excitation of the other lens constant to vary the focal length of said one lens without introducing distortion aberration, S and b being the gap width of the magnetic pole pieces and the diameter of the first lens, respectively, and S and b being the gap width of the magnetic pole pieces and the diameter of the second lens, respectively.

References ited by the Examiner UNITED STATES PATENTS 2,219,405 10/40 Sukurnlyn 25049.5 2,233,264 2/41 Marton 250-49.5 2,313,018 3/43 Krause 25049.5 2,418,349 4/47 Hillier et al. 25049.5 2,802,111 8/57 Reisner 250-495 RALPH G. NILSON, Primary Examiner.

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