US3474245A

US3474245A - Scanning electron microscope

Info

Publication number: US3474245A
Application number: US559296A
Authority: US
Inventors: Hirokazu Kimura; Hifumi Tamura; Michiyoshi Maki; Hisayuki Higuchi
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1965-06-23
Filing date: 1966-06-21
Publication date: 1969-10-21
Anticipated expiration: 1986-10-21
Also published as: GB1128107A

Description

Oct. 21, 1969 HIROKAZU KIMURA ETAL.

SGANNING ELECTRON MIcRoscoPE Filed June 21, 1966 3 Sheets-Sheet l Oct 21. 1969 HiRoKAzu KIMURA ETAL. 3,474,245

SCANNING ELECTHON MICROSCOPE Filed June 21, 1966 3 Sheets-Sheet b w) 3A m 0,7 mmv Km W K m -WH w X /0 4 C 0 7. 6. 5 4. 5 2 0 0 0 0. 0 0 0. 0 mbu bmmQ INVENTOR n n ORNEY United States Patent O 3,474,245 SCANNING ELECTRON MICROSCOPE Hirokazu Kimura, Koganei-shi, Hifumi Tamura, Hachiojishi, Mi'chiyoshi Maki, Kodaira-shi, andHisayuki Higuch, Kokubunji-shi, Japan, assignors to Hitachi, Ltd., Tokyo, Japan, a corporation of Japan -Filed June 21, 1966, Ser. No. 559,296 Claims priority, application Japan, June 23, 1965,

Int. c1. Holi 37/26 Us. c1. o-49,5 9 claims ABSTRACT 0F THE DISCLOSURE This invention relates to improvements in the structure of scanning electron microscopes and more particularly to a scanning electron microscope' provided with novel means for detecting secondary electrons.

As is commonly known, a scanning electron microscope operates in such a way that an electron beam having a fine'spot, by being finely focused, is used to scan the surface of a specimen in its longitudinal and lateral directions and reflected electrons or secondary electrons emitted from points of the specimen bombarded by the primary electron beam are detected in the form of signals, which signals are then `supplied to a cathode-ray tube, which is scanned in synchronized relation with the above scanning by the primary electron beam, so as to modulatethe brightness vof the cathode-ray tube and to thereby observe the surface image of the specimen on the fluorescent screen. In -the scanning electron microscope, its resolution is dependent upon the diameter of focused electron beam. It is therefore desirable to minimize the diameter of electron beam spot in order to obtain a high resolution image. To this end, it is necessary that the focusing electron lens has a short focal length and this means that the distance between the focusing electron lens and a specimen for microscopic examination must be as short as possible. In ordinary scanning electron microscopes, however, a detector for refiected electrons or secondary electrons has been located between Ithe focusing electron lens and an observed specimen, with the result that it has generally been difficult to use an electron lens of short focal length and such difficulty has been especially marked when an electron lens of shor-t focal length is intended for use with a detector for secondary electrons.

With such prior difficulty in mind, it is the primary object of the present invention to provide a unique arrangement by which secondary electrons and reflected electrons emitted from a specimen by being bombarded by the primary electron beam can be induced to any desired position outside of the path of the primary electron beam.

Another object of the present invention is to provide the capability` of using a focusing electron lens of short focal length having little spherical aberration and astigmatism, whose capability is derivable from lthe attainment of the first-described object.

i A further object of the present invention is to derive 3,474,245 Patented Oct. 21, 1969 ICC a high resolution image by focusing the primary electron beam to a diameter of the order of several tens to several hundreds of angstroms by the use of the above-described focusing electron lens with short focal length.

In order to attain the objects as described above, the scanning electron microscope according to the present invention is composed of means for emitting a primary electron beam, focusing electron lens means for focusing Said primary electron beam or thereby illuminating a specimen, means for directing said primary electron beam to the specimen for the scanning thereof, means for establishiug a first uniform magnetic field which is disposed in close proximity to the magnetic field established by said focusing lens means and extends linearly along the path of said primary electron beam, means for establishing a second uniform magnetic field in such a manner that it forms a substantially continuous extension of said first magnetic field and extends outwardly away from the path of said primary electron beam, means for detecting those secondary electrons and/or reflected electrons which are derived outwardly after passing through said first and second magnetic fields, means operative in response to a detected electronic signal to modulate the brightness of the scanning electron beam in a cathoderay tube, and means for deflecting said scanning electron beam in synchronous relation with the scanning by said primary electron beam.

In another form of the present invention, the scanning electron microscope is composed of means for emitting a primary electron beam, focusing lens means for focusing said primary electron beam for thereby illuminating a Specimen, means for directing said primary electron beam to the specimen for the scanning thereof, means for establishing a rst uniform magnetic field which is disposed in close proximity to the magnetic field established by said focusing lens means and extends linearly along the path of said primary electron beam, means for establishing a static field so that the secondary electrons and/or reflected electrons having passed through said first magnetic field can thereby be deflected and derived outwardly of the path of said primary electron beam, means for detecting the derived secondary electrons and/or reliected electrons, means operative in response to a detected electric signal to modulate the brightness of the scanning electron beam in a cathode-ray tube, and means for defiecting said scanning electron beam in synchronous relation with the scanning by said primary electron beam.

The above and other objects, advantages and features of the present invention will become obvious from the following description with reference to the accompanying drawings, in which:

FIG. l is a diagrammatic view showing` the structure of a prior form of scanning electron microscope;

FIG. 2 is a schematic perspective View showing the structure of a prior system used for the detection of secondary electrons;

FIG. 3 is a schematic sectional view showing the structure of part of an embodiment according to the invention;

FIG. 4 is an enlarged view of part of the embodiment shown in FIG. 3 for the sake of explanation of the operating principle of the microscope according to the invention;

FIG. 5 is a schematic sectional view of part of another embodiment according to the invention;

FIG. 6 is a graphic illustration of test results obtained with the embodiment shown in FIG. 5; and

FIG. 7 is a schematic sectional View showing the structure of a further embodiment according to the invention.

yBefore giving any detailed description of the present invention, structure of a prior form of scanning electron microscope will first be described with reference to FIG. 1 so that the improvements effected by the present invention can more clearly be understood.

A scanning electron microscope of prior form is disclosed for example in the Journal of Scientific Instruments, 1960, vol. 37, pages 246248 and in the Journal of Scientific Instruments, 1965, vol. 42, pages 81-85 and generally has a structure as shown in FIG. 1. This scanning electron microscope includes electron beam emission means consisting of a cathode 1 in the form of a hair-pin shaped tungsten filament, a grid 2 generally called the Wehnelt cylinder, and an anode 3. A voltage of 20 to 30 kilovolts is applied -across the anode 3 and the cathode 1, and an electron beam 4 emitted from the heated tungsten filament is focused on a specimen 7 to be observed by means of two

condenser lenses

5 and 6. The specimen 7 is placed on a specimen stage 8 and the specimen position is adjustable in both horizontal and axial directions. The electron beam 4 is deflected by two pairs of defiecting plates 9 so as to scan the specimen surface longitudinally and laterally thereof. When the specimen surface is bombarded by the primary electron beam 4, a secondary electron beam 10 is emitted from the bombarded point and the amount of the secondary electrons is variable depending on the material of that particular bombarded point and the incident angle of the primary electron beam 4. This secondary electron beam 10 generally has an energy of less than 50 electronvolts. A mesh 11 having a central aperture for the passage therethrough of the incident beam is disposed opposite the specimen 7 and is kept at a negative potential of from several to several tens of volts so that the secondary electron beam 10 is deflected in a manner as shown and only those secondary electrons having a certain constant energy are passed through a slit 12. These secondary electrons are then accelerated to hit against a scintillator 13, and a signal whose amplitude is proportional to the number of electrons having passed through the slit 12 is detected by a photomultiplier 14. This signal is then supplied through an amplifier 15 to a grid 17 of a cathode-ray tube 16 for effecting modulation of the brightness of the tube 16. Meanwhile, the output of the scanning power source 18 is simultaneously supplied to the defiecting plates 9 of the scanning electron microscope column, and to defiecting plates 19 of the cathoderay tube 16. By this simultaneous supply of scanning power, a secondary electron image of the specimen 7 can be observed on the fluorescent screen of the cathoderay tube 16.

In addition to the above-described secondary electron detection system in which the energy of the emitter secondary electrons is analyzed and those secondary electrons having a specific energy are solely detected, a simpler form of secondary electron detection system has previously been used and generally has a structure as shown in FIG. 2. This secondary electron detection system includes a mesh 23 which has a central aperture 24 for passage therethrough of an incident electron beam 22 and is electrically insulated from the ground by insulators 25. A secondary electron beam 26 emitted from an observed specimen 21 is trapped by the mesh 23 to be derived in the form of a signal current 27. In this case, reflected electrons having high energies pass through this mesh 23 and do not appear as a signal.

It will be seen that, in both of prior manners of secondary electron detection, their secondary electron detection system occupies a considerable space with the result that the second focusing lens 6 must have a large focal length of the order of several ten millimeters. This large focal length is undesirable in view of the fact that the influence of spherical aberration and astigmatism of the electron lens becomes quite great and that the minimum diameter of the electron beam will only be of the order of 0.1 micron.

Improvements effected by the present invention over the prior art will now be described in detail with reference to FIGS. 3 to 7.V Referring first to FIG. 3, a second focusing lens includes an upper magnetic pole 31 and a lower magnetic pole 32 disposed opposite to each other with a spacer 33 of non-magnetic material interposed therebetween. A coil 34 establishes a magnetic field of several thousand gauss across this lens gap. Further, a solenoid coil 35 is disposed adjacent the above magnetic field and consists of a straight portion 35 (FIG. 4) of suitable length and a curved portion 35" which is gradually bent towards a detector 36 disposed outside of the path of a primary electron beam 38 so that secondary electrons (or reflected electrons) emitted from a specimen 39 can be guided into the detector 36. Other parts of the embodiment shown in FIG. 3 have a structure generally similar to that shown in FIG. 1.

An aperture 37 is provided at a suitable portion of the solenoid coil 35 to permit passage therethrough of the incident primary electron beam 38. The primary electron beam 38 is focused onto the specimen 39 by the magnetic field established by the focusing lens and its point being bombarded on the specimen surface is successively longitudinally and laterally moved by defiecting plates 44 for the scanning of the specimen 39. Secondary electrons or refiected electrons 40 emitted from the point bombarded by the primary electron beam are worked upon by the magnetic field established by the focusing lens and then by the succeeding magnetic field established by the solenoid coil 35, with the result that these electrons 40 are confined substantially in the vicinity of the central axis of the solenoid coil 35 to move along a spiral path, `as shown by a dotted line, into the detector 36. A movable aperture 41 is disposed in the vicinity of the central axis of the focusing electron lens and is adapted to be controlled from outside of vacuum. An accelerating electrode 42 for the acceleration of secondary electrons or refiected electrons is disposed in close proximity to the specimen surface and is led outside of the vacuum to have a positive potential of from several to several tens of volts. An electrical insulator 43 insulates the accelerating electrode 42 from the focusing electron lens. Another accelerating electrode 45 is disposed at the outlet of the solenoid coil 35 to accelerate the secondary electrons or refiected electrons leaving the solenoid coil 35. The

r specimen 39 is placed on a specimen stage 46 and its position is vertically and horizontally adjustable from the exterior.

The principle of operation of the embodiment shown in FIG. 3 will be described with reference to FIG. 4. As Will be apparent from FIG. 4, secondary electrons or refiected electrons 40 having an initial velocity v are emitted from the specimen 39 when the primary electron beam 38 hits against the specimen 39. A magnetic field 47 is established in the vicinity of the specimen 39 by the straight portion 35 of the solenoid coil 35. For ease of explanation, the velocity v of the secondary electron or reected electron 40 is divided into two velocity components, that is, a velocity component vy in the same direction as that of the magnetic field having a field strength H1 and a velocity component vx in a direction at right angles with respect to the velocity component vy. It is then known that the secondary electron or reflected electron 40 makes a gyrating motion since the velocity component vX is perpendicular to the direction of the magnetic field while the velocity component vy is entirely free from the effect of the magnetic field. The radius r1 of gyration of the secondary electron or refiected electron 40 is given by the following formula:

mUx GHI along a spiral path 48 while being confined within a cylindrical area substantially coaxial with the ,central axislof the solenoid coil 35 and having a diameter of from `severalto several tens of microns.

Suppose then that the curved portion 35" of the solenoid coil 35 is disposed at an angle 0 with respect to the straight portion 35' of the solenoid coil 35. Then, a velocity u of the secondary electron or reflected electron 40 coming into the solenoid coil portion 35'.' at an angle a with respect to the central axis thereof can .likewise be divided into two velocity components, that is, a velocity component uy in the axial direction of the coil portion 35" and avelocity component ux in a direction at right angles with respect to the component uy. Suppose the solenoid coil portion 35" establishes a magnetic field having a field strength H2, then the radius r2 of gyration of the secondary electron or reliected electron is likewise given by a formula max SH2 In this manner, the secondary electrons or reflected electronsadvance in the axial direction of the solenoid coil 35 while depicting a spiral trajectory, and it is therefore possible to guide the secondary electrons or reflected electrons to vany desired place outside the path of the primaryelectron beam. In the above embodiment, the primary electron beam 38 for bombarding the specimen 39 maybe admitted through the gap between the

coil portions

35 and 35'. instead of through the aperture 37 shown in FIG. 3.

Another embodiment according to theinvention is shown .in FIG. 5 in which three

coils

49, 50 and 51 of beam 38 is focused on a specimen 39 by this electron lens and is deflected by the deecting plates 44 disposed outside the solenoid coil 58 so that the specimen 39 can be longitudinally and laterally scanned by the primary electron beam 38 in a manner as described previously. A secondary electron or reflected electron beam 40 derived from each point struck by the primary electron beam 38 flies while depicting a spiral trajectory about the central axis of the solenoid coil 58 in a manner as described previously. The secondary electron or reflected electron beam 40 is then accelerated by an accelerating electrode 59 disposed in the vicinity of the outlet of the solenoid coil 58 and kept at a positive potential of several elec- V tron volts and its electron trajectory is deflected by an same. shape are employed in deriving secondary electrons or reflected electrons from a specimen 39 bombarded by a primary electron beam 38. Each of these

coils

49, 50 and 51 consists of 300 turns of wire wound to an inside diameter of 10 mm., an outside diameter of 18 mm. and a heightof l2 mm. The primary electron beam 38 strikes the specimen 39, which is a piece of silicon, through the ,coils 49 and :50, and a secondary electron or reflected electron beam is` derived from the specimen 39. The secondary electrons or reflected electrons pass 'through the

coils

49, 50 and 51 while gyrating substantially about lthe central axis of these coils for the reason as described previously and finally reach a detectory or a Faradays cup 46. The Faradays cup 46 is grounded' through an ammeter 52 and a power supply 53 at several volts of .positive value.'FIG. 6 shows the relation between the exciting currentsupplied to the above threefcoilsin series andthe detected current when the accelerating voltage and the electron currentv of the primary electron beam are-20 kilovoltsand 4 109 ampere, respectively. From. FIG. 6 it will be seen that the detected current increase monotonically with the increase in the coil current and finally reaches a value of 0.6\ 10-9 ampere at a coil current, of 300 milliamperes in spite of the fact that a dark current of 0.05 10-9 ampere can only be obtained at zero coil current. The result shown in FIG. 6 apparently proves the fact that the secondary electron current-passes through the three successive coils to reach the Faradays cup 46. In this connection, there maybe the fear thatv the primary electron beam is deopposite electrode at a negative potential of from several to several tens of electron volts, with the result that only those electrons having a certain specific energy pass through a slit 61 to reach a detector 62.

The just-described embodiment is advantageous over the embodiment shown in FIG. 3 in that the incident primary electron beam does not pass an aperture provided on the side face of the solenoid' coil but passes through a magnetic field of axial symmetry and hence the electron beam is not subjected to any deflection or distortion by a non-uniform magnetic field.

It is to be understood that the

detectors

36, 46 and 62 appearing in the embodiments of the invention may be detectors of secondary electrons or reflected electrons or detectors of both of these electrons. It is apparent that the present invention is also applicable to scanning electron microscopes of the type employing only one focusing lens, and is applicable to electron beam devices of the type analogous to the device of the present invention in the entirely same manner. For example, marked effects substantially similar to those obtained by the invention can be expected when the present invention is applied to an X-ray microanalyser of the kind disclosed in The Elion Reports, vol. l, No. 3, May 1961, or an electron beam machining apparatus of the kind appearing in FIGS. 5 and 6 on page 295 of the Proceedings of the Third Symposium on Electron Beam Processes, March 23 and 24, 1961.

From the foregoing description it will be appreciated that, according to the invention, the specimen can be positioned in close proximity to the focusing lens in the scanning electron microscope since secondary electrons or reflected electrons derived from the observed specimen can be guided to any desired place outside of the path-of the primary electron beam for the detection thereof. This specific feature permits the use of a focusing lens of short focal length and thus minimizes the spherical aberration and astigmatism inherent in the lens. As a result of such unique feature, the diameter of the electron beam spot can be reduced to such a small value of ilected by the magnetic fields established by the solenoids, but such deflection measured. on the specimen was only about 0.5 mm. and was thus negligible as a matter of practical use. 1 A- j In a further embodiment of the invention shown in FIG. 7, an electron lens is formed by an upper magnetic pole piece 54 and a lower magnetic pole piece 55 With a spacer 56 of non-magnetic material interposed therebetween. A coil 57 on the electron lens excites the same. A solenoid coil 58 is interposed in the magnetic path in a partly overlapped relation with the magnetic liield established across the electron lens gap. A primary electron the order of from several vtens to several hundreds of angstroms and a high resolution image can thereby be obtained.

What is claimed is:

1.' In an electron microscope including electron beam generating means for generating and directing a beam of primary electrons along a beam path toward a point at which a specimen may be supported and at least one focusing means positioned closely adjacent said point between said beam generating means and the specimen for focusing said electron beam to a spot on the specimen, the improvement comprising detector means displaced from said beam path and 1ocated at a distance from said focusing means between said focusing means and said beam generating means for detecting secondary and reflected electrons emitted from the specimen as a result of bombardment thereof by said beam of primary electrons, and field generating means for guiding said secondary and reflected electrons from the specimen to said detector means, at least a part of the path between the specimen and said detector means being concident with said beam path of the primary electrons on the side of said focusing means opposite the specimen.

2. The combination defined in claim 1 wherein said field generating means includes first means for generating a magnetic field along a path between the specimen and said detector means.

3. The combination defined in claim 1 wherein said first means is provided as a continuous coil extending from said focusing means to said detector means and being provided with an aperture therein for passage of said beam of primary electrons to the specimen.

4. The combination defined in claim 3 wherein said field generating means further includes an accelerating electrode positioned at each end of said coil.

5. The combination defined in claim 1 wherein said first means includes a plurality of coils having their axes aligned with the path between the specimen and said detector means.

6. The combination defined in claim 1 wherein said first means is provided as a cylindrical coil coaxial with said beam path of the primary electrons, and further including means for defiecting said secondary and reflected electrons from the end of said coil opposite the end adjacent said focusing means to said Idetector means.

7. The combination defined in claim 1 wherein said first means includes means for establishing a first uniform magnetic field which is disposed in close proximity to the magnetic field generated by said focusing means and extends linearly along said beam path and means for establishing a second uniform magnetic field forming a substantially continuous extension of said first magnetic field and extending outwardly away from said beam path of primary electrons.

8. In a scanning electron microscope:

beam generating means for emitting a primary electron beam,

support means for supporting a specimen,

at least one focusing electron lens of the magnetic type positioned between said beam generating means and said support means for focusing the primary electron beam onto the specimen,

first ldeflecting means positioned between said beam generating means and said focusing means for defiecting the primary electron beam to scan the specimen,

detector means for detecting electrons which are emitted from the portion of the specimen struck by the primary electron beam,

a cathode-ray tube having a scanning electron beam for displaying an image of the specimen, means for modulating the intensity of the scanning electron beam in the cathode-ray tube in response to the output signal of said detecting means, and

second defiecting means for deilecting the scanning electron beam in the cathode-ray tube in synchronous relation to the scanning operation of the primary electron beam,

the improvement comprising:

said support means being disposed in the vicinity of the central axis of the focusing electron lens with the specimen positioned within the housing of the focusing electron lens,

first solenoid coil disposed in close proximity to the specimen and on the primary electron beam incident side of the focusing electron lens for developing a first uniform magnetic field extending linearly along the path of the primary electron beam to guide the electrons emitted by the vspecimen alongspiral paths in the vicinity of the central axis of the first solenoid coil, and

a second solenoid coil disposed in close proximity to the first solenoid coil for developing a second uniform magnetic field contiguous to the first uniform magnetic field to guide the electrons emitted by the specimen to the detector means.

9. In a scanning electron microscope:

beam generating means for emitting a primary electron beam,

support means for supporting a specimen,

at least one focusing electron lens 'of the magnetic type positioned between said beam generating means and said support means for focusing the primary electron beam on the specimen,

first defiecting means positioned between said beam generating means and said focusing means for de- -flecting the primary electron beam to scan the speci-l men,

a cathode-ray tube having a scanning electron beam for displaying an image of the specimen,

means for modulating the intensity of the scanning electron beam in the cathode-ray tube in response to the output signal of said detector means, and

second deflecting means for deffecting the scanning electron beam in the cathode-ray tube in synchronous relation with the scanning operation of the primary electron beam,

the improvement comprising:

the support means being disposed in the vicinity of the central axis of the focusing electron lens with the specimen positioned within the housing of the focusing electron lens,

a solenoid coil disposed in close proximity to the specimen and on the primary electron beam incident side of the focusing electron lens for developing a uniform magnetic field .extending linearly along the path of the primary electron beam to guide the electrons emitted by the specimen along spiral paths in the vicinity of the central axis of the first solenoid coil,

a first electrode disposed in close proximity to the solenoid coil for accelerating the electrons passed through the solenoid coil in a direction opposite to the primary electron beam, and

a second electrode disposed opposite to the first electrode for deflecting the electrons accelerated by the first electrode to direct the accelerated electrons to the ,detector means,

the first and second electrodes having apertures through which the primary electron beam passes.

References Cited UNITED STATES PATENTS 11/1956 Warmoltz. 12/1965 Shapiro et al.

OTHER REFERENCES Journal of Scientific Instruments, 1960, vol. 37, pp. 246-248.