Electron Beam Device

Abstract

An electron beam deflecting apparatus incorporating two deflecting stages, each of said deflecting stages being provided with two pairs of deflecting coils for generating two directional magnetic fields, the said fields being at right angles to each other and perpendicular to the optical axis. The electron beam deflecting apparatus also incorporates a circuit for supplying current for deflecting the electron beam, the said current supply circuit being sufficient to supply the total current required for independently actuating the individual coils constituting the two deflecting stages.

Description

In order to observe a dark field image and an electron diffraction pattern by means of an electron microscope, it is necessary to irradiate the specimen with the electron beam at various angles of inclination or azimuth. To this end, electron beam deflecting devices incorporating two deflecting stages, each stage being provided with two pairs of deflecting coils have been used. Further, in this arrangement, the respective magnetic fields produced by each pair of coils are perpendicular to the optical axis and at right angles to each other. Utilizing this electron beam deflecting apparatus, the electron beam generated by the electron beam gun is deflected through an angle .alpha. by the first electron deflecting stage and through an angle .beta., which is proportional and opposite to .alpha., by the second electron deflecting stage. The specimen is thus irradiated at an inclination equal to the difference between angles .alpha. and .beta.. It should be noted that tan .alpha. is proportional to tan .beta.. However, in a conventional electron microscope, since both angles .alpha. and .beta. are very small, the angles themselves are substantially proportional.

A drawback of this arrangement, however, is the difficulty in producing coils whose magnetic fields are exactly at right angles to each other and deflecting stages which are accurately parallel with respect to each other. As a result, it is extremely difficult during specimen observation to prevent the irradiation spot from shifting. One attempt to overcome the above defect is described, for example, in German Pat. specification (Auslegeschrift) 1,299,088 where additional deflecting coils have been incorporated as compensators in the deflecting stages. Operation of this apparatus is facilitated by controlling the deflecting current of the compensation coils in accordance with the deflecting current of the regular coils. However, in this arrangement, as a result of resorting to the provision of additional coils in order to surmount the original defect, secondary defects have ensued. These are mainly in connection with the increased size of the deflecting stages, plFs the added difficulty of manufacturing this more complicated deflecting means with sufficiently precision orientation. In short, the above embodiment is feasible in theory but not in practice.

It is an advantage of electron beam deflection devices according to this invention that they overcome the shortcomings inherent in conventional apparatus and at the same time are easy to manufacture. Further, they are easy to operate, whereby the deflection current is automatically controlled, in accordance with the variable incident angle, so as to fix the position irradiated by the electron beam. Still further, the present invention provides a deflecting device which facilitates comparison of the dark field image and bright field image in the same area of an elcectron miscroscope specimen.

The full advantage and novelty of the present invention will be more readily understood by reading the following detailed description in connection with the appended drawings wherein:

FIG. 1a and FIG. 1b are diagrammatic views of the deflecting stages in accordance with the conventional apparatus;

FIGS. 2,3,4 and 5 are block schematics showing the deflecting current supply according to the present invention;

FIG. 6 is an explanatory diagram of the irradiating electron beam path in the deflecting apparatus according to the present invention;

FIG. 7 is a diagram ilustrating alignment of the irradiating electron beam by the deflecting apparatus; and,

FIG. 8 is a schematic diagram showing a preferred circuit incorporating the elements shown in FIGS. 2 and 6 for controlling the beam deflection of an electron microscope.

Referring to FIGS. 1a and 1b (showing different but analogous embodiments of this invention), four coils 2a, 2b, 3a and 3b are wound onto a core 1 whose center axis Z aligns with the optical axis. Coils 2a and 2b generate a magnetic field in the direction of the x-axis and coils 3a and 3b generate a magnetic field in the direction of the y-axis.

FIG. 2 is a circuit diagram of the deflecting current supply according to this invention. Coil L through which current I flows represents any one of the coils 2a, 2b, 3a and 3b shown in FIG. 1. Current I is equal to the sum of the current i.sub.s flowing through resistor Rs and the current i.sub.o flowing through resistor R.sub.L. However, since i.sub.o is normally quite negligible, current I is substantially equal to i.sub.s.

Current i.sub.o is equal to the sum of the currents i.sub.1 , i.sub.2 , and i.sub.3 , flowing through resistors r.sub.1 , r.sub.2 , and r.sub.3 , respectively. By means of the operational or differential amplifiers 6 and 7 (power supplies not shown for simplicity) the following relationships were established:

I .apprxeq. i.sub.s = E's/R.sub.s (1)

E's = - R.sub.L .sup.. i.sub.o (2)

Hence,

I = -(R.sub.L /R.sub.s) .sup.. i.sub.o

= - (R.sub.L /R.sub.s) .sup.. i.sub.1 - R.sub.L /R.sub. s .sup.. i.sub.2 - R.sub.L /R.sub.s .sup.. i .sub.3

= - (R.sub.L .sup.. Ei.sub.1 /R.sub.s) .sup.. (1 /r.sub.2 ) - (R.sub.L .sup.. Ei.sub.2 /R.sub.s) .sup.. (1/r.sub.2) - (R.sub.L .sup.. Ei.sub.3 /R.sub.s) .sup.. (1/r.sub.3)

(3)

It is apparent from equation (3) that the deflecting current I is independently determined by the variable resistors R.sub.1 , R.sub.2 , and R.sub.3 .

FIG. 3 shows another embodiment of the deflecting current supply incorporating two current control transistors 11 and 12. Here, the input voltage Ei of the operational or differential amplifier 9 is substantially equal to voltage E's. Accordingly, the following relationships were established:

I = i.sub.s = E' s/R.sub.s = Es/R.sub.s (4)

Es = - R.sub.L.sup.. i.sub.o (5)

Now, since current i.sub.o , is equal to the sum of the currents i.sub.1 , i.sub.2 , and i.sub.3 , flowing through resistors r.sub.1 , r.sub.2 , and r.sub.3 , respectively, the following relationship is established:

I = -(R.sub.L /R.sub.s) .sup.. i.sub.o

= - (R.sub.L /R.sub.s) i.sub.1 .sup.. (R.sub.L /Rs) i.sub. 2 - (R.sub.L /Rs) i.sub.3

= - (R.sub.L .sup.. Ei.sub.1 /R.sub.S) .sup.. (1/r.sub.1) - (R.sub.L .sup.. Ei.sub.2 /R.sub. s ) .sup.. (1/r.sub.2) - (R.sub.L.sup.. Ei.sub.3 /R.sub.s) .sup.. (1/ r.sub.3) ta (6)

It is thus apparent from equation (6) that the circuit described in FIG. 3 can substitute for that described in FIG. 2.

FIG. 4 shows yet another embodiment of the deflecting current supply this time incorporating three current control resistors 13, 14 and 15. Here, currents I.sub.1 , I.sub.2 , and I.sub.3 flowing through transistors 13, 14 and 15 respectively, produce current I flowing through coil L. Expressed algebraically:

I = I.sub.1 - I.sub.2 -I.sub.3 (7 )

Thus, by controlling input voltages Es.sub.1 , Es.sub.2 and Es.sub.3 of the operational or differential amplifiers 16, 17 and 18, the deflecting current I can be controlled in the same way as in FIG. 2.

FIG. 5 shows a further embodiment of the deflecting current supply incorporating operational or differential amplifiers 19 and 20. Here current I.sub.1 and I.sub.2 , flowing through resistors R.sub.1 and R.sub.2 , respectively, produce current I flowing through coil L, expressed algebraically, equation (7) can be expressed as:

I = Ei .sup.. ?1/Rs) + (1/R.sub.1) + (1/R.sub.2)! - (e.sub.1 /R.sub.1) - (e.sub.2 /R.sub.2)

(8)

thus, by controlling variable resistors r.sub.1 , r.sub.2 and r.sub.3 , voltages Ei, e.sub.1 and e.sub.2 are controlled thereby determining the deflecting current I the same way as in FIG. 2.

In the case of the deflecting current supplies described with reference to FIGS. 2 to 5, there are three independent currents. This number, however, can be easily increased or decreased as required.

The method of deflecting an irradiating electron beam by means of the deflecting apparatus as described in the aforegoing will be apparent in the following.

Referring to FIGS. 6 and 8, the deflecting current I.sub.1 x for generating the magnetic field in the x-axis direction in the first deflecting stage is expressed as follows:

I.sub.1 x = i.sub.1 x.sub.1 + i.sub.1 x.sub.2 + i.sub.1 x.sub.3 (9)

Similarly, the deflecting current I.sub.1 y for generating the magnetic field in the y-axis direction in the first deflecting stage, the deflecting current I.sub.2 x for generating the magnetic field in the x-axis direction in the second deflecting stage, and the deflecting current I.sub.2 y for generating the magnetic field in the y-axis direction in the second deflecting stage are expressed as follows:

I.sub.1 y = i.sub.1 y.sub.1 + i.sub.1 y.sub.2 + i.sub.1 y.sub.3 (10)

I.sub.2 x = i.sub.2 x.sub.1 + i.sub.2 x.sub.2 (11)

I.sub.2 Y = i.sub.2 Y, + i.sub.2 Y.sub.2 (12)

in the conventional type deflecting apparatus as used with an electron microscope, the irradiating electron beam is deflected in the amount .theta.ox1 by the first deflecting stage and is the amount .theta.ox2 by the second deflecting stage, so as to irradiate a point 28 where the optical axis intersects the plane of specimen 27. By so doing, the irradiation angle .theta. is controlled. However, since the magnetic field generated by the first deflecting stage in the x-axis direction and the magnetic field generated by the second deflecting stage in the x-axis direction are not absolutely parallel, a component of the magnetic field in the y-axis direction is inadvertently generated, resulting in the irradiating electron beam being deflected in the amount .theta..sub.1 x in the said direction. Such being the case, it is extremely difficult to prevent the irradiation spot from shifting without the aid of either one or both y-axis magnetic fields to act as shift compensators.

Since .theta.ox.sub.1, .theta.ox.sub.2, .theta..sub.1 x and .theta. are proportional to each other, it is possible to determine the proportional coefficient existing between .theta.ox.sub.1, .theta.ox.sub.2 and .theta..sub.1 x and that existing between i.sub.1 x.sub.1, i.sub.2 x.sub.1 and i.sub.1 y.sub.2. The deflecting current supply circuit is designed to satisfy the relation between i.sub.1 x.sub.1, i.sub.2 x.sub.1 and i.sub.1 y.sub.2 by, for example, interlocking variable resistors r.sub.1, r.sub.2 and r.sub.3 in FIG. 2.

By designing the deflecting current supply circuit as described above, the irradiating electron beam in the x-axis direction is fully controlled. Similarly, by controlling i.sub.1, y.sub.1, i.sub.2 y.sub.1 and i.sub.1 x.sub.2, the irradiating electron beam in the y-axis direction is also fully controlled. As a result, the irradiating electron beam in any azimuth is fully controlled.

Referring to FIG. 8, in order to control the deflecting current component i.sub.1 x.sub.1, it is necessary to control variable resistor r.sub.1 x.sub.1. Similarly, to control i.sub.2 x.sub.1, it is necessary to control r.sub.2 x.sub.1, and to control i.sub.1 y.sub.2, r.sub.1 y.sub.2 must be controlled.

The deflecting angle .theta.ox.sub.1 as shown in FIG. 6 is controlled by i.sub.1 x.sub.1 in turn controlled by r.sub.1 x.sub.1. Similarlyy, .theta.ox.sub.2 and .theta..sub.1 x are controlled by i.sub.2 x.sub.1 and i.sub.1 y.sub.2 respectively, in turn controlled by r.sub.2 x.sub.1 and r.sub.1 y.sub.2 respectively.

A plurality of individually adjustable potentiometers provides current to the individual deflection coils because r.sub.1 x.sub.1, r.sub.2 x.sub.1 and r.sub.1 y.sub.2 (interlocked to form control Dx) must be controlled proportionally, in order to satisfy the proportionally of .theta.ox.sub.1, .theta.ox.sub.2, .theta..sub.1 x and .theta. whose said proportionality is necessary to prevent the irradiation spot from shifting.

Similarly, r.sub.1 y.sub.1, r.sub.2 y.sub.1, and r.sub.1 x.sub.2 (interlocked to form control Dy) are used to deflect the irradiation electron beam in the y-axis direction.

Variable resistors r.sub.1 x.sub.3 , r.sub.1 y.sub.3 , r.sub.2 x.sub.2 and r.sub.2 y.sub.2 are utilized to control the alignment deflecting current. Normally, these four resistors are controlled individually.

Thus far, it has been assumed that the irradiating electron beam aligns with the optical axis. If not, the beam must be either aligned mechanically by shifting the position of the electron beam generator or deflected electromagnetically. In the case of the latter, two deflecting stages are necessary.

Referring to FIG. 7, the irradiating electron beam is deflected by the first deflecting stages 31a so as to intersect the cross point between the optical axis and the second deflecting stages 31b. It is then further deflected by 31b so as to align with the optical axis.

In order to effect the above alignment, the conventional deflecting apparatus requires additional deflecting coils. This is not so in the deflecting apparatus according to this invention, the deflecting currents i.sub.1 x.sub.3, i.sub.1 y.sub.3, i.sub.2 x.sub.2 and i.sub.2 y.sub.2 refer to equations (9), (10), (11) and (12) being used in lieu.

In order to observe a bright field image with the apparatus according to this invention, the deflecting current i.sub.1 x.sub.1 , i.sub.1 x.sub.2 , 1 y.sub.2 , i.sub.1 y.sub.1 , i.sub.2 x.sub.1 and i.sub.2 y.sub.1 are set to zero and the deflecting currents i.sub.1 x.sub.3 , i.sub.1 y.sub.3 , i.sub.2 x.sub.2 and i.sub.2 y.sub.2 are used for alignment purposes. Again, in order to observe a dark field image, the deflecting currents i.sub.1 x.sub.3 , i.sub.1 y.sub.3 , i.sub.2 x.sub.2 and i.sub.2 y.sub.2 are used for alignment purposes and the deflecting currents i.sub.1 x.sub.1 , i.sub.1 x.sub.2 , i.sub.1 y.sub.1 , i.sub.1 y.sub.2 , i.sub.2 x.sub.1 and i.sub.2 y.sub.1 are used for controlling the inclination of the irradiating electron beam. irradiating electron beam.

Accordingly, by incorporating deflecting current changeover switches, comparison of the bright and dark field images with the same areas of the specimen will be appreciably facilitated.

Sometimes, an electron deflecting apparatus incorporating only one deflecting stage is used for alignment. In such cases, it is possible to facilitate alignment by incorporating the deflecting current supply according to this invention since the two directional magnetic fields can be accurately orientated at right angles to each other by supplying compensating curent to one of the two deflecting coils for generating the said two directional magnetic fields by the said deflecting current supply.

Having thus described my invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.

Claims

I claim:

1. In an electron microscope or the like having an electron optical axis, an apparatus for deflecting an electron beam to irradiate a location on a specimen at different easily selectable azimuthal angles of incidence comprising:

A. a deflection stage being equipped with deflecting coils L.sub.1x and L.sub.1y for generating magnetic fields substantially at right angles to each other and perpendicular to the optical axis;

B. a second deflection stage being equipped with deflecting coils L.sub.2x and L.sub.2y for generating magnetic fields substantially at right angles to each other and perpendicular to the optical axis, said magnetic deflecting fields of said second stage being substantially aligned with the magnetic deflecting fields of the first stage;

C. a deflecting current supply for supplying individual coil currents I.sub.1x, I.sub.1y, I.sub.2x and I.sub.2y to each of said deflecting coils L.sub.1x, L.sub.1y, L.sub.2x and L.sub.2y respectively, said individual coil currents being the total of a plurality of individually variable analog currents, such that

I.sub.1x = i.sub.1x1 + i.sub.1x2 + i.sub.1x3

I.sub.1y = i.sub.1y1 + i.sub.1y2 + i.sub.1y3

I.sub.2x = i.sub.2x1 + i.sub.2x2

I.sub.2y = i.sub.2y1 + i.sub.2y2

means for varying simultaneously and proportionally at least some of the analog currents controlling coil current to both first and second stages, such that the means for controlling i.sub.1 , i.sub.2x1 and i.sub.1y2 are interconnected and means for controlling i.sub.1y1, i.sub.2y1 and i.sub.1x2 are interconnected, such that the one deflection stage redirects the electron beam deflected by the other stage to the location on the specimen thus changing the azimuth angle of incidence without moving the location on the specimen on which the beam is incident notwithstanding slight misalignment of the deflection stages and slight deviation from right angles between the fields of the separate stages.

2. An apparatus according to claim 1 in which the i.sub.1x3, i.sub.1y3, i.sub.2x2 and i.sub.2y2 analog currents are used for aligning the irradiating beam.

3. In an electron microscope or the like having an electron optical axis an apparatus for deflecting an electron beam to irradiate a location on the specimen comprising:

A. a deflection stage equipped with deflecting coils L.sub.1x and L.sub.1y for generating magnetic fields, said fields being substantially at right angles to each other and perpendicular to the optical axis;

B. a deflecting current supply for supplying individual coil currents I.sub.1x and I.sub.1y to coils L.sub.1x and L.sub.1y respectively, said individual coil currents being the total of a plurality of individually controlled analog currents, such that I.sub.1x = i.sub.1x1 + i.sub.1x2 and I.sub.1y + i.sub.1y1 + i.sub.1y2 and means for varying the analog currents i.sub.1x1 and i.sub.1y1 to control coil currents and vary the deflection of the electron beam.