US2928943A

US2928943A - Electronic microscope for top illumination of surfaces

Info

Publication number: US2928943A
Application number: US759162A
Authority: US
Inventors: Bartz Gunter Erwin; Bill Walter
Original assignee: Ernst Leitz Wetzlar GmbH
Current assignee: Ernst Leitz Wetzlar GmbH
Priority date: 1957-09-11
Filing date: 1958-09-05
Publication date: 1960-03-15
Anticipated expiration: 1977-03-15
Also published as: DE1133841B; CH362772A; FR1227224A

Description

March 15, 1960 G. E. BARTZ EI' 7 2,928,943

ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION 0F. SURFACES Filed Sept. 5. 1958 4 Sheets-Sheet 1 INVENTORS GU/VTER BQW/N BARTZ WAUER BILL BMW ATTORNEYS March 15, 1960 G. E. BARTZ EI'AL 2,928,943

ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION 0F SURFACES Filed Sept. 5. 1958 4 Sheets-Sheet 2 INVENTORS GUNTER ERMA! BARTZ WALTER 8/LL wmm mz,

ATTORNEYS G. E. BARTZ ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION 0F SURFACES 4 Sheets-Sheet 3 March 15, 1960 Y Filed Sept. 5, 1958 v Fig. 'lb

Fig. la

\fa M GUNIERERW/NBARTZ WALTERB/LL v 151 mm Arrows IN VENTORS March 15, 1960 G. E. BARTZ EIAL 2,928,943

ELECTRONIC MICROSCOPE FOR TOP ILLUMINATION OF SURFACES Filed Sept. 5. 1958 4 Sheets-Sheet 4 4a 5a 6a 5/ 7a INVENTORS GUNTER ERM/V BARTZ A WALTER ,B/LL

BYW

ATTORNEYS I ELECTRONIC ,MICROSCOPE FOR TOP ILLUMI- NATION F SURFACES Giinter Erwin Bartz, Dutenhofen, and Walter-Bill, Naunhelm, Germany, assignors to Ernst Leitz, G.m.b.H., WetzlanGermany Y Application September5i1958, Serial No. 759,162

priority, application Germany September 11, 1957 8 Claims. (Cl. 250 -495) This invention relates to an electronic microscope suitable for making surfacesof objects visible by means of top illumination. a

It is already known in the art to make an object surthe surface in the form of material layers which lead to a filling in of depressions in that surface equal to a snowing-in effect, thus progressively hiding details of the original surface in the resulting image.

It is the object of our invention to avoid these and other drawbacks of the presently known apparatus, and to provide an electronic microscope for top illumination of surfaces, which is simpler both in construction and operation than the existing devices while being free in particular, from the snowing-in effect of ion beam microscopes, and subjecting the surface of the object only to a moderate load.

This object is achieved by the electronic microscope according to the invention which is adapted for forming direct images of. the surface of an object by means of secondary electrons, and provided with a conventional electronic beam-operating system comprising an accelerating anode, which system is so disposed as to cause an electron beam to fall under an oblique angle on to the surface area of the object to be depicted, and which microscope is further provided with a shielding cage or can adapted for excluding electrostatic fields in its interior and having a plurality ofopenings and being rotationsymmetrical in shape and enclosing the object, While being at the same time centered about the optical axis of the secondary emission path serving to produce the image, and being at the same electrical potential as the object itself.

According to another feature, the objective system of the electronic microscope according to the invention is formed by the aforesaid shielding can and the object together with a ground-connected electrode disposed behind the can in the electronic path from the object to the image observation means, which latter is preferably a conventional fluorescent screen,

In other embodiments of the electronic microscope according to the invention, the aforesaid electrode forming part of the objective system is replaced by other lens means in the imaging path from the object to the, image-producing screen means, which other means con sist either of an electrostatic unipotential lens, or an electrostatic immersion lens, or a magnetic lens or lens system forming part of the objective system.

Furthermore, the electron microscope according to the invention may comprise an additional electron beami' d States P t ,Q i

:01 can permits a straight-lined travel of the illuminating a Wehnelt cylinder andan accelerating anode.

7 2,928,943 Patented Mar. 1 5,

ing source, which ion source is almost, but not quite at the same potential as the object, and is so disposed in the'device according to the invention that the direction of the ion beam approximately coincides with the direction of the primary electron beam directed towards the object. The emitter system for generating the primary electron beam comprises, for example, an electron source, Such a system is, for instance, described in Manfred von Ardenne: Tabellen der Elektronenphysik, VEB Deutscher "Verlag der Wissenschaften, Berlin (1956), vol. I, p. 131.

It is advisable to have the electron emitter system directed at an oblique angle'toward the object.

The body of the shield can or cage enclosing the object and protecting the same against undesirable electrostatic fields is substantially symmetrical with regard to an axis of rotation which, preferably, is identical with the optical axis of the image-transmiting' system formed by the paths of the secondary electronic beams, and is provided with a plurality of. openings. Primary electrons are permitted to enter the shielding can by a first, lateral opening, while secondary electrons leave the can by way of another opening provided close to the object in the A third opening or bore in the cage Wall serves for introducing and withdrawing the object from the can and for unimpeded displacement of the object carrier or holder,

In a similar manner, several independent illuminating systems for directing primary electron beams toward dilferent portions of the object surface may be provided in the device according to the invention and, in this case, a correspondingly larger number of openings will have to be provided in the can wall, so that the object may be top-illuminated severally either at the same time, or in successive order. I

The field-free space in the interior of the shield cage beam or beams. Furthermore, the can provides a reliable safeguard againstdischarges, for the high field intensity at the object which latter usually bears corners and/ or edges, is only utilized at the spot or area under observation,- and the danger of an electrical arc-over is substantially eliminated or at least greatly reduced. It is correspondingly possible to operate the microscope with extremely high field intensities, for instance in the order of 50 kilovolts/cm., which, of course, signifies a considerable increase in the resolving power of the emission microscope, since the latter increases in linear function with the field intensity at the place under observation.

The ejection of image-forming secondary electrons from the object by primary electrons instead of the conventionally used ions olfers several advantages. First of all, the object rating is considerably less when irradiating the object with electrons instead of ions. Secondly, as has been mentioned above, the snowing-in effect is avoided.

Other advantages reside in the fact that a single high voltage source will sufiice for providing voltage to the system, and that the angle of the incident primary beam relative to the object can easily be altered by changing the object voltage. Other advantages are of a vacuum-technical nature, it being possible to obtain an extremely pure vacuum in the interior of the electronic tube which could not be attained if an ion beam is directed into the interior of the apparatus.

On the other hand, when objects of electrically'nonconductive or semiconductive materials are to be made visible, it may be desirable to prevent the object from .tron beam-producing system. This ion-beam producing source should be provided with means for adjusting the intensity and direction of the beam, the latter to co1nc1de approximately with that of the primary electron beam,

and should be held approximately at object potential, as stated hereinbefore.

spasms The invention will be still further explained with the aid of the accompanying drawings in which Figure 1 is a schematical sectional view of an embodiment of the top-illumination electronic microscope according to our invention;

Figure 1a is a longitudinal sectional view of the micro- I :metric means for adjusting the diaphragm 27 suitable for adjusting the object-holder 9;

Figure 1b is a cross sectional view of the micrometric adjusting means taken along the lines IIII in Figure la;

Figure 2 is a partial sectional view of a somewhat different embodiment of certain electrode means used in the electronic microscope shown in Figure 1;

Figure 3 shows in partial sectional view another embodiment of the electrode means in the electronic microscope shown in Figure 1; and

Figure 4 illustrates a third embodiment of the electrode means in a partial sectional view of the electronic microscope according to the invention.

Figure 5 is a partial, schematical sectional view of amodification of the primary electron beam generating system in the electronic microscope of the present invention;

Figure 6 is a partial, schematical sectional view of yet another modification of the primary electron beam generating system in the electronic microscope of the present invention;

Figure 7 is a schematical, sectional view of another embodiment of the invention, similar to Figure 1, but

f showing another preferred embodiment of the top-illuminated electronic microscope of the present invention having a plurality of electron beam generating systems and being equipped with an ion beam generating-system.

Figure 7a is a schematical, cross sectional view, taken along II of Figure 1.

More particularly, the embodiment of the electron microscope according to the invention as illustrated in Figure 1 comprises a microscope hull or casing 1 having a main tube 2 which is cocentrically arranged about the optical axis of the object-image path, and a lateral angular I tube 3 which houses the primary electron beam generating system consisting of an electron emitting cathode 4, a modulating (Wehnelt cylinder) electrode 5 and an accelerating anode 6. In the path of the primary electron stance in the form of an annular body of high tension insulating material, in which there is mounted a shield .can-12.

, Can 12 is symmetrical to its axis of rotation which coincides with optical axis 10, and is preferably made of stainless steel, and free from corners or edges and of a highly polished, completely smooth surface, whereby the formation of corona discharges is substantially suppressed.

beam toward the approximate center of the casing 1, 3

Can 12 is electrically connected, on the one hand, by means of a number of sliding contacts 13 (only one of which is shown in Figure 1) with object 8, and, on the other hand with a source of high voltage potential at 14.

Openings

15, 16 and 17 are provided in the wall of can 12, of which opening 15 is provided in a neck portion 18 of can 12, the front wall of which is somewhat flattened at 19 in a plane which is vertical to the optical axis 20 of the illuminating system (see Figure 7).

Opening 16 is provided in an inwardly curved or conical wall' portion 21 of can 12 whereby the opening is located close to the surface of object 8 where the primary electron beam along axis 20 impinges on the latter.

This opening 16 is centered with extreme accuracy on the common optical axis through the subsequently described elements of the imaging system.

The further, larger opening 17 in the wall of can 12 opposite. opening 16 serves for the introduction, withdrawal and unimpeded displacement of object holder 9 in any lateral direction, and for the evacuation of the interior of can 12, which is efiected together with the interior of easing 1 by means of a vacuum pump (not shown) connected to outlet pipe 22.

At its free end main tube 2 bears a fluorescent screen 23 or similar means for making the electron-beam produced image of the top-illuminated object surface area visible.

An electrode 24 which, is connected to ground via line 25, is located in the path of the secondary electron beams justing means which permit the distance between the opening 16 of can 12 and the electrode to be altered. A micrometrically adjustable or iris diaphragm 27 is disposed at the focal point of electrode 24 behind the latter as seen from the object 8. Between the diaphragm 27 and fluorescent screen 23, there is disposed an electronic lens 28 of known type which serves as a projective.

Such lenses are described, for instance, in Von Ardenne, op. cit., vol. I, p. 407, 412, 416.

The adjustment of diaphragm 27 can be effected with cross slide macrometric adjusting means known per se and shown, for example, in Figures 1a and 1b. The diaphragm 27 can be displace in the directions of arrow 63 by means of the adjusting screw acting against a spring 61 and it can be moved in the directions of arrow 62 by means of the screw 64 engaging the ratchet portion 65 provided in the slidable sleeve 66 within the 50 cylinder 2.

The analogous micrometric adjusting means can be used for displacing the object-holder 9.

The electronic microscope shown in Figure 1 operates as follows:

Primary electrons are emitted from the system of cathode 4, modulated by electrode 5 and accelerated by anode 6, and then focussed by condenser 7 in the direction toward object 8 along optical axis 20.

A voltage, the so-called object voltage is applied at both the object 8 and shield can 12, which is positive with regard to the potential of cathode 4 and can be taken from potentiometer 29 and varied, for instance,

between 0 and 5000 volts.

Since the can is connected with potentiometer 29 having a voltage in the order of 2 kilovolts, whereas the anode 6 has a voltage in the order of 50 kilovolts (see Figure 1), the electrons of the primary beam have to travel against a negative voltage and are thus braked upon approaching opening 15 of can 12 down to a speed which corresponds to the positive value of the object voltage or landing voltage of the primary electrons on the object surface spot or area to be topilluminated. In the space free from electrostatic field which exists in the interior of can 12, the primary electron beam travels in a straight line toward, the

. aforesaid object spot b1 area. The electrons which impinge on this spot or area, eject froin the same secondar electrons which travel substantially along opti- 'e'al'axis 10. 7

Together with can 12 being at even potential with object 8, the normally grounded electrode 24 produces an. axially symmetrical accelerating field, which grips "through opening '16 of can 12 and accelerates the secondary electrons leaving the surface of the object from a speed of about 2 kilovolts up to 50 kilovolts. This acceleration is necessary in order to create an image and to obtain the requisite speed in order to create an image on screen 23.

This accelerating field also causes a deflection of the illuminating primary electron beam in the vicinity of opening. 16 in such a manner, that the angle of incidence a of the primary beam with the object surface becomes smaller. p

H A smaller angle of incidence is desirable whenever the surface of the object is comparatively planar, it is undesirable whenever the surface of the object is irregular and has indents and projections. In that case undesirably elongated shadows would be produced which jcjan be avoided by choosing a greater angle of incidence. This can be done by changing the voltage of the voltage sources by means of the potentiometer 29. The. angle of incidence is much smaller by lowering the voltage at the potentiometer '29 and vice versa.

-In case of substantially planar objects an object voltage in the range of 1000-2000 volts is chosen; in case of objects with more irregular surfaces the object voltage is to be inthe order of 4000-5000 volts. In both instances the accelerating voltage is in the order of from 40-50 kilovolts. The angle of incidence a is in the order of 20.

By selectinga suitable landing voltage and correspondinglyadjusting the emission of primary electrons fromthe electron

beam generating system

4, 5, 6, it is, however, possible to provide for optimal conditions of illumination.

Can "12, electrode 24 and the object 8 form together an electrostatic focussing lens which serves as an objective system and produces a real intermediary image. In the focus of the objective behind the latter, seen from the object, the iris diaphragm 27 is preferably provided for reducing aberrations occurring in the image. This "diaphragm may have an inner diameter of about 5 to 20 microns. The real intermediary image produced by the objective is enlarged with the aid of the abovementioned projective lens 28 and appears on the fluores- 5 ing electrode 24-is replaced by an electrostatic unipotential lens 30, in Figure 3 by an electrostatic immersion double aperture lens 31 and in Figure 4 by an iron clad magnetic lens 32. In Figure 2, the electrode 33 of uni- 'potential lens 30 is connected to ground via line 34, while the electrode 35 is connected via line 36 to a potentiorneter 37, whereby the potential at electrode 35* can be varied by means of the potentiometer 37.

In a similar manner, the electrode 38 of the double aperture lens 31 is connected via line 39 to ground, while the electrode 40 is connected via line 41 to potentiom- "titer 42.

The arrangement and operation of unipotential lenses in electronic microscopes is described in detail, for instance, in Von Ardenne, op. cit., vol. II, p. 802, double aperture lenses in H. Johannson, in Annalen der Physik (1933), vol. 18, p. 385 and magnetic lenses in Von Ardenne op. cit., vol. II, p. 803-805.

The electronic microscope of the present invention can also be equipped with a modified primary electron beam generating system as shown, for example, in Figures 5 and '6.

"Ksshown in Figure 5,-the'-anode 6 mayhavea tubular .75

. 6 elongation 62: extending into the opening 150i cage 12. The condenser lens 7 is disposed around the tubular elongated portion 6a substantially in the middle thereof with an interposed insulating layer 7a.

Due to this arrangemennan even narrower pencil of primary electrons is created since exterior field influences are excluded. Consequently, the speed of the primary electrons is more even and hence the speed of the secondary electrons is also more even thereby creating a clearer image.

It is also possible-to dispense with the condenser lens 7 by disposing the primary electron

beam generating system

4, 5, and 6 close to the opening 15 of cage 12 as shown, for example, in Figure 6. In that case the cage 12, the accelerating anode 6 and the Wehnelt cylinder 5 form an ele'ctro-o'ptical system. The adjustment of cathode 4 can be effected with adjusting means of the type shown in Figures 1a and lb.

According to another embodiment the electronic microscope of the present invention is equipped with a plurality of primary electron beam generating systems as, for example, three

systems

50, 51, and 52 as shown in Figure 7. The casing 1 bears three cylinder portions 3,

3a, 3b, housing a

cathode

4, 4a, 4b, and a Wehnelt cylinder '5, 5a, 5b, an

anode

6, 6a, 6b, and a

condenser

7, 7a, 7b, respectively. The optical axis is designated with 20, 20a, 20b, respectively. These axes pass through

openings

15, 15a, and 15b defined by the flattened

front portions

19, 19a, and 19b and the

surfaces

53, 53a and 53b. The object holder 9 is disposed within the cage 12.

According to still another preferred embodiment the electronic microscope of the present invention is provided with an ion beam generating system 55. This system is housed in a further cylinder portion 30 of the casing 1 situated as close as possible to one of the three cylinder portions 50, '51, or 52. This ion source which is, of course, known per se, consists of a gas inlet pipe 56 with a hollow space inside. At the end opposite to the entrance 57 of the inlet pipe. there is provided a small exit opening 58. The gas discharge pipe is embedded in an insulating layer 70 within the electrode 59. The gas inlet pipe is connected with the positive pole of a 'voltage source 72 of approximately 2000 volts, whereas tive pole of a-further voltage source 73 of approximately 100 volts. The negative pole of this second voltage source is also connected with the cage 12.

This ion generating source operates as follows:

The gas leaving the small opening 58 of the gas inlet pipe 56.results in a gas discharge due to the voltage of about 2000 volts between the inlet pipe 53 and the electrode 59 which causes the gas to be ionized. The ions leavingthe opening 75 of the electrode 59 are accelerated towards the object holder by the voltage in the range of 10m volts between the electrode 59 and the cage 12. If a low accelerating voltage is chosen, for example 10 volts, the positive ions will cause a discharging of the object which is negatively charged by the beam of primary electrons, if the object'consists of a non-conductor.

By choosing a high accelerating voltage, for example 100 volts, the ions will have an etching effect on the surface of the object, for example a metallic object. In this case the beam of electrons may be directed to the object only for a few seconds since otherwise too many ions would cover the object thereby producing the undesirable snow effect. The etching performed in such a manner ofiers great advantages over the etching done by means of acids since a much finer etching eifect is obtained.

' It will be understood that this invention is susceptible to modification in order to adapt it to different usages and-conditions, and, .accordingly, -it is desired to comprehend such modifications within this invention as may surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising means for producing primary electrons, anode means for accelerating said primary electrons, means for directing said primary electrons toward the object, and a shielding can enclosing said object and provided with openings for the entry of said primary electrons, for the passage of said secondary electrons from said object toward said fluorescent screen, as well as for the introduction, withdrawal and displacement of the object relative to said can, said can presenting rotation symmetry and being centered upon the aforesaid optical axis, and means for applying the same electrostatic potential to said object and said can.

2. An electronic miscroscope for direct imaging of the surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising means for producing primary electrons, anode means for accelerating said primary electrons, means for directing said primary electrons at an oblique angle toward the object and a shielding can enclosing said object and provided with openings for the entry of said primary electrons, for the passage of said secondary electrons from said object toward said fluorescent screen, as well as for the introduction, withdrawal and displacement of the object relative to said can, said can presenting rotation symmetry and being centered upon the aforesaid optical axis, and means for applying the same electrostatic potential to said object and said can.

3. An electronic miscroscope for direct imaging of the surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising means for producing primary electrons, anode means for accelerating said primary electrons, means for directing said primary electrons toward the object, a shielding can enclosing said object and provided with openings for the entry of said primary electrons, for the passage of said secondary electrons from said object toward said fluorescent screen, as well as for the introduction, withdrawal and displacement of the object relative to said can, said can presenting rotation symmetry and being centered upon the aforesaid optical axis, and means for applying the same electrostatic potential to said object and said can; and electrode means intermediate said can and said fluorescent screen, said electrode means being grounded and constituting together with said can the objective system of the microscope.

4. An electronic microscope for direct imaging of the surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising means for producing primary electrons, anode means for accelerating said primary electrons, means for directing said primaryelectrons toward the object, a shielding can enclosing said object and provided with openings for the entry of said primary electrons, for the passage of said secondary electrons from said object toward said fluorescent screen, as well as for the introduction, withdrawal and displacement of the object relative to said can, said can presenting rotation symmetry and being centered upon the aforesaid optical axis, and means for applying the same electrostatic potential to said object and said vcan; and electrostatic unipotential lens means intermediate said can and said fluorescent screen, said electrostatic unipotential lens means being grounded and constituting together with said can the objective system of the microscope.

5. An electronic microscope for direct imaging of the surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising means for producing primary electrons, anode means for accelerating said primary electrons, means for directing said primary electrons toward the object, a shielding can enclosing said object and provided with openings for the entry of said primary electrons, for the passage of said secondary electrons from said object toward said fluorescent screen, as well as for the introduction, withdrawal and displacement of the object relative to said can, said can presenting rotation symmetry and being centered upon the aforesaid optical axis, and means for applying the same electrostatic potential to said object and said can; and electrostatic immersion lens means intermediate said can and said fluorescent screen, said electrostatic immersion lens means being grounded and constituting together with said can the objective system of the microscope.

6. An electronic microscope for direct imaging of the surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising means for producing primary electrons, anode means for accelerating said primary electrons, means for directing said primary electrons toward the object, a shielding can enclosing said object and provided with openings for the entry of said primary electrons, for the passage of said secondary electrons from said object toward said fluorescent screen, as well as for the introduction, withdrawal and displacement of the object relative to said can, said can presenting rotation symmetry and being centered upon the aforesaid optical axis, and means for applying the same electrostatic potential to said object and said can; and magnetic lens means intermediate said can and said fluorescent screen, said magnetic lens means being grounded and constituting together with said can the objective system of the microscope.

7. An electronic microscope for direct imaging of the surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and

wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising a plurality of systems for producing beams of primary electrons, anode means for accelerating said primary electrons, means for directing said primary electrons toward the object, and a shielding can enclosing said object and provided with openings one each for the entry of one of said beams of primary electrons, for the passage of said secondary electrons from said object toward said fluorescent screen, as well as for the introduction, withdrawal and displacement of the object relative to said can, said can presenting rotation symmetry and being centered upon the aforesaid optical axis, and means for applying the same electrostatic potential to said object and said can.

8. An electronic microscope for direct imaging of the surface of an object by means of secondary electrons ejected from said surface toward a fluorescent screen adapted for making the imaged object surface visible, and wherein the object and fluorescent screen are disposed along an optical axis of the microscope, comprising means for producing primary electrons, anode means for acpglerating said primary electrons toward the object,

said object and said 'can, and anqadditional source of slow ions, means for applyingto said ion source an e1ectrostatic potential close to that of said object, said ion source being so arranged in the microscope that the direction of the ions produced thereby substantially coincides with the direction of the aforesaid primary electrons, and comprising means for controllingthe intensity, speed and direction of the ion emission.

References Sited in the file of this patent UNITED STATES PATENTS Dornfeld Feb. '28, 1950 Reisner June 6, 1950 Weissenberg July 16, 1957 FOREIGN PATENTS Germany July 4, 1955