A scanning electron microscope

Abstract: Provided between an optical axis of a primary electron beam 4 directed to a specimen 7 and a secondary electron detector 50 for detecting secondary electrons 9 emitted from the specimen 7 is a shielding electrode 12 which has a transparency for the secondary electrons 9 and has a shielding effect for an attracting electric field 54 produced by the secondary electron detector 50. An opposed electrode 11 is provided at a position facing the shielding electrode 12 to put the optical path of the electron beam 4 therebetween. Applied across the shielding electrode 12 and the opposed electrode 11 is a voltage which produces a deflecting electric field E in the electron beam path. The deflecting electric field E deflects the secondary electrons 9 to the shielding electrode 12 side to pass the secondary electrons therethrough before they are captured by the secondary electron detector 50. Optical axis correction coils 19 and 20 are provided for generating a magnetic field B to correct an orbit of the primary electron beam bent by the deflecting electric field E.

Description: The present invention relates to a scanning electron microscope that obtains a scan image on the basis of secondary electrons emitted by irradiating an electron beam to a specimen. More particularly, it concerns a scanning electron microscope that is capable of efficiently capturing secondary electrons to obtain an image of high resolution. A most popular method for improving resolution of a scanning electron microscope is to shorten a focal length of an object lens with a specimen placed in a magnetic field of the objective lens. Methods of placing the specimen in the magnetic field of the objective lens include a so-called in-lens method that places the specimen between magnetic poles of the objective lens, as disclosed in, for example, the Japanese Patent Application Laid-Open No. 63-110543, and a method that an objective lens (snorkel lens) is shaped to allow an objective lens magnetic field to leak out to a specimen side so that the specimen is substantially placed in the objective lens magnetic field even if the specimen is put below the objective lens, as disclosed by Mulvey in "Scanning Electron Microscopy," 1974, p44. In the both disclosures, with the specimen placed in the object lens magnetic field, the secondary electrons emitted from the specimen are captured by the magnetic field of the object lens to travel to a top of the objective lens. For this reason, the secondary electrons emitted from the specimen must be detected at the top of the objective lens. Even if the ordinary objective lens out of the magnetic field of which the specimen is placed is used to shorten the focal distance to minimize the lens aberration, the specimen is preferably placed as close to a bottom of the objective lens. In such a method, also, the secondary electrons have to be detected at a top of the objective lens. In order that the secondary electron detector captures the secondary electrons, the secondary electron detector has a high voltage of around 10 kV applied thereto to produce an attracting electric field. The attracting electric field undesirably bends an orbit of the primary electron beam. As a result, the bent orbit changes an incoming position of the primary electron beam on the object lens, causing an axial deviation. Further, the attracting electric field produced by the secondary electron detector causes astigmatism and other disturbances in itself. These difficulties are too high to realize high resolution. To prevent such adverse effects of the attracting electric field produced by the secondary electron detector, many proposals have been made. As an example, the Japanese Patent Application Laid-Open No. 52-91361 has a shielding casing for covering an electron beam path in front of a secondary electron detector. However, the shielding casing cannot fully prevent the effect of the electric field of the secondary electron detector as the secondary electrons cannot be detected unless the electric field leaks onto the optical axis. As another example, the Japanese Patent Application Laid-Open No. 63-110543 has the attracting electric field produced by the secondary electron detector formed axially symmetrical. However, this involves a problem that the detection efficiency of the secondary electrons around the center axis is lowered because of the axial symmetry of the attracting electric field of the secondary electrons. As still another example, the Japanese Utility model Application Laid-Open No. 61-99960 shows a method that an orbit of the primary electron beam bent by an attracting electric field of a secondary electron detector is corrected by a two-stage deflector provided at a top of the secondary electron detector. However, also, this method cannot prevent the attracting electric field produced by the secondary electron detector from leaking into an electron beam path, causing adverse effects, such as astigmatism. As still another example, the Japanese Patent Application Laid Open No. 60-212953 discloses a method that additional electrodes and coils are provided to make the electric field and magnetic field orthogonal to correct a bent orbit of the primary electron beam before leading secondary electrons to a secondary electron detector positioned above the electrodes and coils. However, this method does not take into account any of leaking of a non-uniform attracting electric field produced by the secondary electron detector onto the optical axis. A similar scanning electron microscope, which uses an electric field to deflect the secondary electrons to a detector and a magnetic field to compensate the effect of the electric field on the primary electron beam is disclosed in DE-A-35 32 781. As described above, each of the prior arts has its merits and demerits. It has been demanded to have a scanning electron microscope capable of detecting secondary electrons further efficiently. In view of solving the foregoing problems of the prior arts, it is an object of the present invention to provide a scanning electron microscope capable of efficiently detecting secondary electrons without astigmatism and axial deviation even with detection of the secondary electrons above an objective lens. This object is achieved by the scanning electron microscope set forth in claim 1. The subclaims are directed to preferred embodiments of the invention. The shielding electrode can shield the attracting electric field produced by the secondary electron detector to capture the secondary electrons, thereby preventing the attracting electric field from leaking into the electron beam path. Besides, the magnetic field produced by the coils can correct the orbit of the primary electron beam bend by action of the deflecting electric field produced by the shielding electrode, thereby preventing the primary electron beam from deviating axially. As for the secondary electrons, these can be easily detected as the deflection direction of the magnetic field is made the same as that of the deflecting electric field to make deflection of the secondary electrons large. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a cross-sectional view illustrating principles of the present invention, Fig. 2 is a schematic diagram illustrating a first embodiment of the present invention, Fig. 3 shows behaviors of a primary electron beam and secondary electrons deflected by a deflecting electric field E, Fig. 4 shows behaviors of a primary electron beam and secondary electrons deflected by a magnetic field B, Fig. 5 shows behaviors of a primary electron beam and secondary electrons deflected by interaction of a deflecting electric field E and a magnetic field, Fig. 6 is a schematic diagram illustrating a portion of a second embodiment of the present invention, and Fig. 7 is a schematic diagram illustrating a portion of a third embodiment of the present invention. The following describes in detail a basic concept according to the present invention by reference to Fig. 1. Fig. 1 depicts a cross-sectional view illustrating a secondary electron detector 50 and its peripheries of a scanning electron microscope according to the present invention. An electron source emits a primary electron beam 4. The primary electron beam is irradiated depthwise from outside to inside in the drawing. A secondary electron detector 50 comprises an attracting electrode 51, a scintillator 52, and light guide 53. Secondary electrons are attracted by an attracting electric field 54 generated by the attracting electrode 51 and are accelerated to the scintillator 52 by a positive potential applied thereto to thereby radiate light from the scintillator 52. The radiated light is guided by the light guide 53 to a photomultiplier (not shown) to be converted into an electric signal, which is fed to an intensity modulation input of a CRT. A shielding electrode 12 is provided between the secondary electron detector 50 and the electron beam path to cover parts of an electron beam path. An opposed electrode 11 is also provided at a position facing the shielding electrode 12 to put the electron beam path there between. The shielding electrode 12 can be a porous plate or mesh-like member having a transparency for the secondary electron and having a shielding effect for the attracting electric field 54. In such a construction as described above, a potential difference V is applied between the shielding electrode 12 and the opposed electrode 11 to produce a deflection electric field E. The primary electron beam 4 is accelerated by an acceleration voltage Vacc and, as shown in Fig. 3, is deflected an angle e = Ke × V/Vacc in proportion to V/Vacc to the shielding electrode 12 side by the deflecting electric field E where Ke is a constant determined in terms of shapes and positions of the shielding electrode 12 and the opposed electrode 11. On the other hand, two sets of optical axis correction coils 19 and 20 are provided around the electron beam path and are supplied with excitation currents so as to produce a composite or combined excitation current I(A) to generate a corresponding composite or combined magnetic field B in the electron beam path in a direction orthogonal to the deflecting electric field E and the electron beam optical path. As shown in Fig. 4, the primary electron beam 4 is deflected b = Kb × I/(Vacc) 1/2 in proportion to I/(Vacc) 1/2 by a Lorenz's force in a direction reverse to that of the deflection by the deflecting electric field E where Kb is a constant determined in terms of shapes and positions of the optical axis correction coils 19 and 20. If the deflecting electric field E and the magnetic field B being orthogonal to each other exist at the same time, a composite or combined force of the deflecting electric field E and the magnetic field B determines an orbit of the primary electron beam 4. If the potential difference V and the combined excitation current I can be selected to satisfy e = b, as shown in Fig. 5, the primary electron beam 4 can be made to go straightly forward without any action. That is, if K = Ke/Kb, the combined excitation current I for the optical axis correction coils 19 and 20 can be controlled to always make the primary electron beam 4 go straightly forward with any acceleration voltage Vacc so that the combined excitation current I meets I = K × V/(Vacc) 1/2 . As the deflecting electric field E and the magnetic field B are virtually uniform around the optic axis, there will not occur an astigmatism due to non-uniform electromagnetic distribution. On the other hand, as shown in Fig. 3, the deflecting electric field E can deflect the secondary electrons 9 emitted from a specimen 7 in the same direction as the primary electron beam 4. The magnetic field B, as shown in Fig. 4, can deflect the secondary electrons 9 in the direction reverse to the primary electron beam 4 since the secondary electrons 9 go opposite to the primary electron beam 4. That is, as both the deflecting electric field E and the magnetic field B make action to the secondary electrons 9 to lead the same to the secondary electron detector 50, they sharply bend the orbit of the secondary electrons 9 having little energy to transmit through the shielding electrode 12 so that the secondary electrons 9 are captured by the secondary electron detector 50. As the non-uniform attracting electric field 54 produced from the secondary electron detector 50 is shielded by the shielding electrode 12, the attracting electric field 54 cannot affect the primary electron beam 4, causing no astigmatism. Means making the deflecting electric field E and the magnetic field B orthogonal to each other has been known as a kind of energy filter called the Wien filter. The electromagnetic field strength needed for the energy filter causes the electron beam to disperse the energy, resulting in a large chromatic aberration. However, the electromagnetic field strength for bending the orbit of the secondary electron having a few to some ten electron volts causes so little energy dispersion in the primary electron beam having tens of times the energy that the effect to the chromatic aberration can be virtually ignored. In turn, the following describes an embodiment of the present invention in detail. Fig. 2 depicts a schematic diagram illustrating a scanning electron microscope of the embodiment of the present invention. Arrangements and parts in the Figure identical with those mentioned above are indicated by the same numbers as above. The primary electron beam 4 is radiated from a cathode 1 by an extracting voltage V1 applied between the cathode 1 and a first anode 2. The primary electron beam 4 then is accelerated by the accelerating voltage Vacc applied between the cathode 1 and a second anode 3 to travel to a succeeding lens system. The accelerated primary electron beam 4 is focused into a minute spot on the specimen 7 by the lens action of a focusing lens 5 and an objective lens 6 controlled by a lens control circuit 17. The focused primary electron beam 4 is scanned on the specimen 7 by a scanning coil 8 two-dimensionally. In the embodiment, the scanning coil 8 is formed of two stages so that the scanned primary electron beam 4 can be made always to pass through a center of the objective lens 6. A scanning signal fed to the scanning coil unit 8 is controlled by a magnification control circuit 15 depending on the observation magnifying power. An aperture angle of the primary electron beam 4 at the objective lens 6 is determined by an aperture of the aperture plate 18 positioned below the focusing lens 5. There is arranged the secondary electron detector 50 outside the electron beam path between the scanning coil unit 8 and objective lens 6. There also are arranged around the electron beam path near the secondary electron detector 50 the above mentioned shielding electrode 12, the opposed electrode 11, the two sets of optical axis correction coils 19 and 20 for generating the combined magnetic field B orthogonal to the deflecting electric field E produced by the electrodes 11 and 12 (Fig. 1). Note that Fig. 2 has the optical axis correction coils 19 and 20 omitted, but only the magnetic field B produced thereby for ease of seeing the drawing. A secondary electron accelerating electrode 13 is arranged between the electrodes 11 and 12 and the object lens 6 and has a positive voltage Va applied there across. A control electrode 14 is arranged just below the secondary electron accelerating electrode 13 and has a positive voltage Vb thereacross ( 0 ≥ Vb > Va). As a result, as shown in Fig. 6, the secondary electron accelerating electrode 13 and the control electrode 14 produce an equipotential surface 60. The equipotential surface 60 can focus the secondary electrons 9 so that the secondary electrons 9 emitted in a wide area can be efficiently led to the secondary electron detector 50. On the other hand, the secondary electrons 9 emitted from the specimen 7 are attracted into the objective lens 6 before being screwed up above a top (the electron source side) of the objective lens 6 by effect of the magnetic field of the objective lens 6. The secondary electrons 9 will expand quickly when the lens magnetic field disappears. In the embodiment, however, the secondary electron accelerating electrode 13 is provided in front of expansion of the secondary electrons 9 to accelerate the secondary electrons 9 to tens to hundreds of electron volts. The secondary electrons 9, therefore, will not divert much even when the magnetic field of the object lens 6 disappears. Most of the secondary electrons 9 emitted from the specimen 7 will not divert much after the magnetic field disappears, but can come into the area where the deflecting electric field E and magnetic field B exist. When passing the secondary electron accelerating electrode 13, the secondary electrons 9 return to the original low energy again before being forced in the same direction (to the secondary electron detector 50 side) by the deflecting electric field E and magnetic field B. For this reason, most of the secondary electrons 9 transmit through the shielding electrode 12 before being captured by the secondary electron detector 50. An image display apparatus 16 displays a magnified image of the specimen 7. The primary electron beam 4 is forced by the deflecting electric field E produced by the shielding electrode 12 and the opposed electrode 11. The force, as described above, can be canceled by the magnetic field B produced by the optical axis correction coils 19 and 20. The primary electron beam 4, therefore, can be made to come into the objective lens 6 finally without any effect. The non-uniform attracting electric field 54 produced by the secondary electron detector 50 will not affect the primary electron beam 4 at all as being shielded by the shielding electrode 12. The deflecting electric field E is produced only by a potential difference of the voltage V applied between the shielding electrode 12 and the opposed electrode 11. Therefore, one of the electrodes, for example, the opposed electrode 11 can be made a ground potential and only the shielding electrode 12 can have the positive voltage V. Otherwise, the opposed electrode 11 and the shielding electrode 12 can be at the potentials V = Vc ± V/2 where vc is a center potential. The control electrode 14 can be cooled by, for example, a liquid nitrogen 24 as shown in Fig. 7. The cooled control electrode 14 can increase a degree of vacuum around the specimen 7 and at the same time, can prevent the specimen 7 from being contaminated by the electron beam irradiation. Variable excitation currents I 1 and I 2 are supplied from a function generator 23 to the two sets of optical axis correction coils 19 and 20 so as to produce the combined current I corresponding to the combined magnetic field B. The coils 19 and 20 can produce an additional magnetic field in superposition on the magnetic field B for canceling the deflection of the primary electron beam 4 due to the deflecting electric field E to correct an axis deviation of the primary electron beam 4 due to other causes. Alignment signal generators 21 and 22 generate signals I a1 and I a2 , respectively, which are added to the variable excitation currents I 1 and I 2 , respectively, so as to produce the additional magnetic field described above. In this way, The optical axis correction coils 19 and 20 cannot only correct the action of the deflecting electric field E for deflecting the secondary electrons 9, but also correct the axial deviation due to other causes. As described so far, the present invention provides the following effects: (1) The attracting electric field produced for capturing by the secondary electron detector is shielded by the shielding electrode to prevent the attracting electric field from leaking out to the electron beam path. This dissolves the astigmatism so that the high resolution observation can be made. (2) The magnetic field produced by the optical axis correction coil means can correct the orbit of the primary electron beam bent by action of the deflecting electric field E produced by the shielding electrode. This can prevent the primary electron beam from being deviated. (3) The secondary electron accelerating electrode is provided above the objective lens to accelerate the secondary electrons emitted from the specimen to a degree that the secondary electrons cannot be dispersed and can come into the area of the deflecting electric field E. Thus, the secondary electrons can be efficiently detected. (4) The control electrode having an opening for passing the electron beam is provided on the optic axis below the secondary electron accelerating electrode. Thus, the secondary electrons emitted from the wide area of the specimen can be efficiently detected. (5) The control electrode can be cooled so that contamination of the specimen due to the irradiation of the electron beam can be minimized.

Claims: A scanning electron microscope, comprising an electron source (1) for emitting a primary electron beam (4), focusing means (5, 6) for focusing the primary electron beam onto a specimen (7), scanning means (8) for scanning the specimen with the electron beam, a secondary electron detector (50) for detecting secondary electrons (9) emitted from the specimen, electrode means (11, 12) to produce a deflecting electric field (E) acting on the secondary electrons, and magnetic field generation means (19, 20) for producing a magnetic field (B), characterised in that the secondary electron detector (50) comprises means (51) for producing an attracting electric field (54) to capture the secondary electrons (9), that said electrode means comprises a shielding electrode (12) provided between a path of the primary electron beam (4) and the secondary electron detector (50) and having a transparency for the secondary electrons (9) and a shielding effect for the attracting electric field, and the electrode means comprises an opposed electrode (11) opposite the shielding electrode (12) with respect to the electron beam path to produce the deflecting electric field (E) in the region of the electron beam path, and that the magnetic field generating means comprises two sets (19, 20) of coils arranged around the electron beam path, and means (21, 22, 23) to adjust a respective current through each set of coils so as to direct said magnetic field (B), as generated by the set of coils, orthogonal to the deflecting electric field (E) and the electron beam path. A scanning electron microscope according to Claim 1, wherein the shielding electrode (12) is a porous-plate member. A scanning electron microscope according to Claim 1, wherein the shielding electrode (12) is a mesh-like member. A scanning electron microscope according to Claim 1, 2 or 3, wherein the sets of coils (19, 20) are excited to meet an equation I = K × V/(V acc ) 1/2 , I being a combined exciting current of the sets of coils (19, 20), V being a voltage applied across the shielding electrode (12) and the opposed electrode (11), V acc being an accelerating voltage for the primary electron beam (4) and K being a constant. A scanning electron microscope according to Claim 4, which further comprises means for superimposing alignment signals (I a1 , I a2 ) for aligning the optical axis of the primary electron beam (4) on the exciting currents of the sets of coils, respectively. A scanning electron microscope according to Claim 5, which further comprises a secondary electron accelerating electrode (13) having a predetermined positive potential and being arranged between a spatial area in which the deflecting electric field (E) is produced and the specimen (7) to accelerate the secondary electrons (9) emitted from the specimen(7). A scanning electron microscope according to Claim 6, which further comprises a control electrode (14) arranged between the secondary electron accelerating electrode (13) and the specimen(7), the control electrode (14) having an opening for passing the primary electron beam (4) and the secondary electrons (9) from the specimen(7), the control electrode (14) further having a lower positive potential than the positive potential of the secondary electron accelerating electrode(13). A scanning electron microscope according to Claim 7, which further comprises a cooling means (24) for cooling the control electrode (14).

Inventors: OTAKA TADASHI, SATO MITSUGU, TODOKORO HIDEO, OTAKA, TADASHI, SATO, MITSUGU, TODOKORO, HIDEO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: H01J37
IPC codes: H01J37, G01T1