Charged-particle beam apparatus

Abstract: The charged-particle beam apparatus according to this invention is of the type in which a specimen is inserted between the upper and lower magnetic poles of an objective lens (1), and is constructed such that stages (9a, 9b, 11, 12), which are used to move a specimen in a desired direction, are mounted one on top another from the bottom wall of the objective lens (1), thus preventing the specimen from being affected by external vibration. Devices (9c, 30, 31, 32, 33, 100), which do not generate a magnetic field, are used to drive the stages. This enables the drive devices to be directly connected to the stages, so that the stages are moved accurately.

Description: This invention relates to a specimen stage mounted inside an objective lens for use in a charged particle beam apparatus, according to the preamble of claim 1. In an electron microscope, for example, if a large working distance (W.D.) from the center of the objective lens to the specimen is set, then the aberration of the objective lens becomes large, so that a high resolving power cannot be obtained unless a high accelerating voltage of the objective lens is used. Therefore, as disclosed in JP-A-58-161235, in recent years, a method has been practiced in which a specimen is introduced between the upper and lower magnetic poles of the objective lens and it is observed with less aberration of the objective lens. With an electron microscope of a construction such as this, the interpole gap between the upper and lower poles is so narrow that a conventional large specimen stage cannot be used in its present size, and therefore, it is necessary to make some contrivance for the specimen stage. In JP-A-61-64055, for example, there have been proposed a method in which the specimen stage is made in a cantilever type support structure and a specimen is observed while it is moved from outside, and another meth in which the ends of the upper and lower poles are pointed and the specimen is inclined at an elevated angle. In US-A-4 866 280, according to which the preamble of claim 1 is worded, there has been proposed a method in which the specimen stage is constructed with the bottom wall of the lower magnetic pole as the base. With the specimen stage of cantilever type support structure revealed in the above-mentioned JP-A-61-64055, it is difficult to move a specimen slightly in the X-Y or horizontal direction, and since the end portion of the specimen stage is supported in the form of a cantilever in the magnetic path of the objective lens, the specimen observing section is susceptible to effects of external vibration, and it is difficult to observe the specimen with a high resolving power. As for the inclining method of a specimen, only the upper and lower magnetic poles in shapes to allow inclining to an elevated angle have been proposed, but no concrete specimen inclining method has been proposed. On the other hand, in US-A-4 866 280 mentioned above, though it has been proposed to construct a specimen stage, which moves in the X and Y directions, by basing it on the inside bottom face of the lower magnetic pole. However, no concrete form of construction for cutting off the effects of external vibration has been suggested. In addition, no construction for inclining the specimen has been disclosed. US-A-3,778,621 discloses a specimen stage with inclination means for adjusting the angle of inclination of the specimen. The inclination means as a whole is arranged between the upper and lower poles of the objective lens. SUMMARY OF THE INVENTION The object of the present invention is to provide a specimen stage mounted inside an objective lens which is capable of inclining the specimen relative to the axis of the beam without disturbing the magnetic field between the poles of the lens. This object is met by the features set out in claim 1, according to which the specimen stage comprises an inclination means including a first supporting block supported by the bottom wall of the lens and a first inclining table supported by the first supporting block, said supporting block and said inclining table having a space in which the magnetic pole is inserted for adjusting an angle of inclination of a specimen, the specimen holder being supported on said inclination means. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of an electron microscope according to an embodiment of this invention; Fig. 2 is an enlarged sectional view of the specimen stage and its vicinity shown in Fig. 1; Figs. 3A and 3B are partially transparent perspective views of the specimen stage; Fig. 4 is a diagram for explaining the operation of the actuator; and Fig. 5 is a partially transparent perspective view of the specimen stage according to another embodiment of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention will be described in detail with reference to the accompanying drawings. In Fig. 1, a field emission tip 6 is connected to a high-voltage power source 34, and the field emission tip 6 emits electrons according a potential difference from a first anode 5a. After accelerated by a second anode 5b, the electrons are converted by a condenser lens 4 to a narrow beam, are deflected by a deflection coil 3, and strike a specimen 8 placed between an upper magnetic pole 1a and a lower magnetic pole 1b of an objective lens 1. According to this embodiment, the interpole gap of the objective lens 1 is about 10 mm, by which a focal length of several mm can be obtained, and a high-performance lens with a chromatic aberration coefficient and a spherical aberration coefficient each of three to four mm can be provided. Said first anode 5a is connected to a first anode lead-out power source 19, the condenser lens 4 is connected to a condenser lens power source 20, a deflecting coil 3 is connected through a magnification control device 16 to a deflection power source 21, and a coil 18 of the objective lens 1 is connected to an objective lens power source 25. Secondary electrons emitted from the specimen 8 by irradiation of the electron beam are detected by a secondary electron detector 7 installed above the upper magnetic pole 1a, and a detection signal, after amplified by a preamplifier 17 and a main amplifier 24, is input as a brightness modulation signal into a cathode ray rube 23. A cathode ray deflecting coil 22 of the cathode ray tube 23 is connected to a deflection power source 21, and the cathode rays of the cathode ray tube 23 are deflected according to the deflection of the electron beam by the above-mentioned deflecting coil 3. A specimen 8 is placed on a specimen stage 26 which is chiefly composed of an X-direction movement table 9. A Y-direction movement table 10, a Y-direction inclining stage 11, and an X-direction inclining stage 12. The specimen is introduced and changed by utilizing a specimen exchange chamber 14 provided at a side face of the magnetic path of the objective lens 1. A gate valve 15 is provided between the objective lens and the specimen exchange chamber 14. The interior of the objective lens 1 and the interior of the specimen exchange chamber 14 are evacuated by a vacuum pump, not shown. Referring to Figs. 2 and 3A, the composition of the above-mentioned specimen stage 26 will be described in detail. As shown in Fig. 3A, the specimen stage 26 according to this embodiment comprises an X-Y stage 50 and an inclining stage 60. In Fig. 3A, the X-Y stage 50 and the inclining stage 60 are separated. In practical use, a Y-direction movement support table 10a of the X-Y stage 50 and an inclining table 11a of the inclining stage 50 are fixed, which will be described later. Detailed description is made first of the composition of the X-Y stage 50. The X-Y stage 50 chiefly comprises a wafer table 9a, an X-direction movement support table 9a for supporting the movement of the wafer table 9a in the X-direction, and a Y-direction movement support table 10a for supporting the movement of the X-direction movement support table 9a in the Y-direction. The wafer table 9a mentioned above is supported with both sides thereof fitted in the V-grooves of guides 9h, 9i provided at both end portions of the X-direction movement support table 9b, and the wafer table can slide in the X-direction along the V-grooves. These guides 9h, 9i cannot be formed of a material with magnetic properties, but are made of ceramics, for example. A guide rod 9b and a linear actuator 9c constitute a linear driving device, the guide rod 9d being installed along the guide 9i with both end portions thereof fixed with fixing blocks 9f, 9e, the linear actuator having the guide rod 9d passed therethrough. The linear actuator 9c is connected to the above-mentioned wafer table 9a with the intermediary of a connection fitting 9g. Driven by the linear actuator 9c, the wafer table 9a moves in the X direction on the X-direction movement support table 9b. Thus, the movement of the wafer table 9a is controlled accurately. In other words, if an ordinary motor incorporating a coil is used, in order to preclude the effects of a magnetic field that the coil generates, the motor needs to be installed outside the objective lens 1. If this is done, owing to mechanical errors of a device connecting the wafer table and the motor, e.g. a coupling, it is impossible to accurately control the movement of the wafer table. On the other hand, the Y-direction movement support table 10a has two grooves formed in the Y direction on the surface thereof. At the one side of the grooves, a guide rod 10b is fixed with fixing blocks 10c, 10d in the same manner as with the other guide rod 9d in the X direction. The guide rod 10b has passed thereon a linear actuator (not shown) fixed with the X-direction movement support table 9b in the same manner as with the other guide rod 9d. Driven by the linear actuator, the X-direction movement support table 9b is moved in the Y direction on the Y-direction movement support table 10a. In this embodiment, in order to allow the specimen to move in any direction while keeping a narrow interpole gap of the objective lens 1, a relief space into which the lower magnetic pole can enter is provided in the center areas of the wafer table 9a, the X-direction movement support table 9b, and the Y-direction movement support table 10a. By this means, those moving parts are prevented from contacting the lower magnetic pole 1b of the objective lens 1. The construction of the above-mentioned linear actuator 9c will now be described briefly. In this embodiment, an inchworm-shaped jogging linear actuator utilizing a piezoelectric phenomenon by a piezoelectric element is used for the above-mentioned linear actuator. An example of a linear actuator constructed as described is inchworm motors of Burleigh Instrument, Inc. Fig. 4 is a diagram for explaining the inchworm actuator. As shown in Fig. 4, the inchworm actuator comprises three piezoelectric elements (hereafter referred to as PZT) 31, 32, 33. When a voltage is applied to PZTs 31, 33, they function to grasp the guide rod 30, while PZT32 at the center, when it has a voltage applied, functions to expand along the guide rod 30. The operation progresses as: (1) No voltage is applied to each PZT. PZTs 31, 33 are not holding the guide rod 30, and PZT32 keeps its normal length. (2) Voltage is applied only to PZT31 to let it hold the guide rod 30. (3) While voltage is applied to PZT31, voltage is also applied to PZT32, which thereby expands along the guide rod 30. (4) While voltage is applied to PZTs 31, 32, voltage is also applied to PZT33 to let it grasp the guide rod 30. (5) Only the voltage supply to PZT31 is stopped to let it release the guide rod 30. (6) Furthermore, the voltage supply to PZT32 is also stopped, and PZT32 returns to its normal length. (7) Voltage is applied again to PZT31 to let it grasp the guide rod 30. (8) The voltage supply to PZT33 is stopped to let it release the guide rod 30. By repeating this operation, the inchworm makes a step motion continuously. As is well known, since the piezoelectric phenomenon by a piezoelectric element does not generate a magnetic field, if an inchworm actuator utilizing a piezoelectric phenomenon is used, a specimen (a field of view) can be moved without having any effects on the image observed, and moving means of a specimen can be provided which is smaller in size and more accurately and finely movable than the cantilever type support structure of the prior art. Fig. 3B shows an example in which an ultrasonic motor 100, which does not generate a magnetic field, is used as a drive source to move the X-Y stage. In Fig. 3B, those parts which are identical with those of Fig. 3A are designated by the same reference numerals. In Fig. 3B, reference numeral 109d indicates a screw rod connected to a rotating shaft of an ultrasonic motor 100, and is supported by radial bearings 109e, 109f. The screw rod 109d has a nut 109c screwed thereon. Under this arrangement, the nut 109c moves in the X direction as the screw rod 109d rotates, so that the X-direction stage 9a is moved in the same direction. In Fig. 3B, a screw rod 110b for supporting the movement of the X-direction stage 9b in the Y direction is supported by radial bearings 110c, 110d. Description will then be made of the construction of the inclining stage 60. In Fig. 3A, the inclining stage 60 comprises a Y-direction inclining stage 11 and an x-direction inclining stage 12. The inclining directions are not limited to the X and Y directions, but may be directions 45°, for example, shifted from the X and Y directions shown in Fig. 3A. The Y-direction inclining stage 11 comprises an inclining table 11a, an inclining support block 11b, a circular gear (worm wheel) 11c, and a worm gear 11d. By rotating the worm gear 11d, the inclining table 11a can be inclined. The inclining motion of the inclining table 11a by the circular gear 11c is done with the intersection of the center of the objective lens and the surface of the specimen as the center of the inclining motion. In other words, the inclining table 11a is inclined with the observed spot of the specimen as the center, which is the intersection of the specimen and the beam of electrons. On the other hand, the x-direction inclining stage 12 comprises an inclining table 12a, an inclining support block 12b, a circular gear 12c, and a worm gear 12d. By rotating the worm gear 12d by a motor 40 arranged outside the objective lens 1, the inclining table 12a can be inclined. If an ultrasonic motor is used for the motor 40. The inclining motion of the inclining table 12a by the circular gear 12c is done with the intersection of the center of the objective lens and the surface of the specimen as the center of the inclining motion in the same manner as in the inclination of the inclining table 11a. As shown in Fig. 2, the inclining stage 60 constructed as described is installed on the bottom wall of the lower magnetic pole, and a relief space 17 is provided substantially in the center areas of the inclining tables 11a, 12a and the inclining support blocks 11b, 12b so that the lower magnetic pole 1b does not contact those inclining tables and inclining support blocks. The X-Y stage 50 and the inclining stage 60 are placed in a strong magnetic field. Therefore, the parts of those stages need to be made of non-magnetic material. According to this embodiment, the inclining stage 60 and the X-Y stage 50 are installed on the bottom wall of the lower magnetic pole and around the lower magnetic pole 1b. This prevents the specimen from being affected by external vibration and makes observation with high resolution possible. The center point of the inclining motion by the inclining means is the observed point of the specimen, and therefore, it never occurs that a field of view comes out of the observation range when the specimen is inclined. In addition, if the specimen is moved by operating the X-Y stage with the specimen inclined, the specimen is kept at a constant height at the observed position of the specimen, so that the observed spot of the specimen can be moved with the specimen inclined. In the foregoing embodiment, description has been made based on the arrangements that the specimen stage comprises an X-Y stage 50, which uses piezoelectric elements, and an inclining stage 60, and that the specimen stage is capable of both moving in the X and Y directions and inclining in the X and Y directions. However, this invention is not limited to these functions, but can be applied to applications wherein only an X-Y stage using piezoelectric elements is used or the X-Y stage is used in combination with an inclining table capable of inclining either in the X or Y direction only. In the foregoing embodiment, this invention has been described by taking as an example an electron microscope for which a specimen is introduced in the gap between the upper and lower poles of the objective lens, but this invention may be applied to an electron microscope in which a specimen is introduced below the objective lens. In this case, a relief space for the lower magnetic pole need not be provided in the center area of the X-Y stage or the inclining table. In the foregoing embodiment, description has been made of an arrangement such that the inclining stage is made of a combination of an X-direction inclining stage and a Y-direction inclining stage in order to incline the X-Y stage 50 in any desired direction. However, in place of the X-direction inclining stage, as shown in Fig. 5, it is possible to use a Y-direction inclining stage which surrounds the lower pole and is based on the inside bottom face of the lower pole, and have the Y-direction inclining stage held by a rotary support block, which is provided in such a way as to be rotatable about the optical axis of the electron beam block. In Fig. 5, those parts identical with those of Fig. 3 are designated by the same reference numerals. In Fig. 5, reference numeral 112b denotes a cylindrical block fixed to the inside bottom face of the objective lens, and a rotary block 112a is mounted on the block 112b in such a manner that the rotary block 112a can rotate in the circumferential direction. this rotary block 112a has a worm wheel 112c with which a worm gear 112d engages. As is clear from the foregoing description, the present invention offers the following advantages: (1) Piezoelectric elements are used for the linear actuators, and by utilizing a piezoelectric phenomenon, the specimen stage is moved. This makes it possible to form a small specimen stage free from effects of magnetism and move the stage accurately and finely. (2) A relief space is provided for the lower magnetic pole in the specimen stage, and the inclining stage is so formed as to surround the lower magnetic pole and is based on the lower pole. The specimen stage is made less susceptible to adverse effects of external vibration and permits observation of the specimen with high resolution. (3) The center of the inclining motion by the inclining means is the observed spot of the specimen. Therefore, a field of view never comes out of the range observation when the specimen is inclined. (4) Even if the specimen is moved by operating the X-Y stage with the specimen inclined, the specimen is kept at a constant height at the observed position of the specimen, so that the observed spot can be moved with the specimen inclined.

Claims: A specimen stage mounted inside an objective lens (1) having a magnetic pole (1b) projecting from a bottom wall of the lens, for use in a charged particle beam apparatus, said specimen stage (26), supported by said bottom wall of the lens, having a space (17) in which the magnetic pole (1b) is inserted and comprising a specimen holder (9a), characterised by said specimen stage (26) comprises an inclination means (60) including a first supporting block (12b) supported by said bottom wall and a first inclining table (12a) supported by said first supporting block (12b), said supporting block and said inclining table having a space (17) in which said magnetic pole (1b) is inserted for adjusting an angle of inclination of a specimen (8), said specimen holder (9a) being supported on said inclination means (60). The specimen stage of claim 1, further comprising exciting means (18) disposed on said bottom wall of the objective lens (1) for exciting said magnetic pole (1b). The specimen stage of claim 1, wherein said first inclining table (12a) inclines substantially around a portion of the specimen (8) on which the beam impinges as a center of the inclination thereof. The specimen stage of claim 1, wherein said first supporting block (12b) supports said first inclining table (12a) with a surface of arc, and the center of the arc being substantially in agreement with the center of inclination of said first inclining table (12a). The specimen stage of claim 1, further comprising: moving means (12c, 12d, 40) for tilting said first inclining table (12a) and maintaining the table at a predetermined slant, said moving means also being disposed inside the objective lens (1). The specimen stage of claim 4, further comprising: a worm wheel (12c) mounted on said first inclining table (12a) and having a shape corresponding to said surface of arc, a worm gear (12d) engaging with said worm wheel and a motor (40) for rotating said worm gear, wherein said motor is also disposed inside the objective lens (1). The specimen stage of claim 1, wherein said inclination means further comprises: a second supporting block (11b) supported by said first inclining table (12a), and a second inclining table (11a) supported by said second supporting block (11b), wherein said second inclining table inclines in a different direction from the first inclining table, said specimen holder (9a) being supported by said second inclining table. The specimen stage of claim 3, wherein said second inclining table (11a) inclines substantially around a portion of the specimen (8) on which the beam impinges as a center of the inclination thereof. The specimen stage of claim 8, wherein said second supporting block (11b) supports said second inclining table (11a) with a surface of arc, and the center of the arc is substantially in agreement with the center of inclination of said second inclining table. The specimen stage of claim 7, further comprising: moving means (11c, 11d) for tilting said second inclining table (11a) and maintaining the second inclining table at a predetermined slant, said moving means also being disposed inside the objective lens (1). The specimen stage of claim 9, further comprising: a worm wheel (11c) mounted on said second inclining table (11a) and having a shape corresponding to said surface of arc, a worm gear (11d) engaging with said worm wheel and a motor for rotating the worm gear, wherein said motor is also disposed inside the objective lens. The specimen stage of any of the preceding claims, comprising: a first magnetic pole (1b) on a bottom wall of the lens and a second magnetic pole (1a) on an upper wall of the lens, wherein the relief space is formed between the inclination means (60) and the second magnetic pole (1a) to enable the inclination means to move without interference with the second magnetic pole when the inclination means inclines. The specimen stage of claim 12, wherein the second magnetic pole (1a) has a conical shape and a center axis disposed in agreement with the trajectory of the charged particle beam, and the relief space is formed between a slope of the second magnetic pole and an upper surface of the inclination means (60). A speciment stage according to claim 12, wherein the first magnetic pole (1b) projects from the bottom wall of the lens, said inclination means (60) being supported by the bottom wall and having said space (17) into which the first magnetic pole is inserted. The specimen stage of any of the preceding claims, further comprising: exciting means (18) for generating a magnetic field in the lens, said exciting means being disposed on the opposite side of said holding means (9a) from said second magnetic pole (1a). The specimen stage of claim 15, wherein said exciting means is a coil (18) which is disposed inside said first supporting block (12b). The specimen stage of claim 16, wherein said coil (18) for exciting the lens (1) is disposed around the first magnetic pole (1b), the inner surface of the coil being substantially in contact with the first magnetic pole. The specimen stage of claim 1 or 2 further comprising: horizontal shifting means (50) for shifting said specimen holder (9a) in a direction parallel to the bottom wall, said horizontal shifting means (50) being disposed between said specimen holder (9a) and said inclination means (60), being supported by the inclination means and having a second space in which the projecting magnetic pole (1b) is inserted. The specimen stage of claim 18, further comprising driving means (100, 109, 110) for shifting the horizontal shifting means (50), said driving means being disposed inside the objective lens (1) and being supported by the inclination means (60). The specimen stage of claim 19, wherein said horizontal shifting means (50) comprises a first shifting table (9b) for shifting said holding means in a first direction and a second shifting table (9a) for shifting said holding means in a second direction. The specimen stage of claim 20, wherein the first shifting table (9b) is supported by an upper surface of the inclination means (60) so as to be movable along said upper surface; the second shifting table (9a) is supported by an upper surface of the first shifting table (9b) so as to be movable along said upper surface of the first shifting table; said driving means comprising first driving means (100, 110b) for driving said first shifting table (9b), said first driving means being disposed between said inclination means (60) and said first shifting table, and second driving means (100, 109c, 109d) for driving said second shifting table (9a), said second driving means is being disposed between said first shifting table and said second shifting table. The specimen stage of claim 21, wherein a surface defining said first space in said inclination means (60) has substantially equal diameter to that of a surface defining said second space in said horizontal shifting means (50). The specimen stage of claim 22, wherein said inclination means (60) comprises a first supporting block (12b) supported by the bottom wall of the lens, a first inclining table (12a) supported by said first supporting block, a second supporting block (11b) supported by said first inclining table and a second inclining table (11a) supported by said second supporting block. The specimen stage of claim 1, further comprising horizontal shifting means (50) for shifting the holding means in a direction parallel to a bottom wall of said lens (1), said horizontal shifting means being supported by said bottom wall of the lens; and driving means (100, 109, 110) for driving said horizontal shifting means and for stopping it at a predetermined position, said driving means being disposed inside the objective lens (1). The specimen stage of claim 24, further comprising inclination means (60) for inclining the holding means, wherein said horizontal shifting means (50) is supported by said bottom wall through said inclination means (60). The specimen stage of claim 24, wherein said horizontal shifting means (50) comprises a first shifting table (9b) for shifting said holding means in a first direction and a second shifting table (9a) for shifting said holding means in a second direction. The specimen stage of claim 26, wherein: said first shifting table (9b) is supported by said bottom wall of the lens so as to be movable along the wall; and said second shifting table (9a) is supported by an upper surface of said first shifting table so as to be movable along said upper surface of said first shifting table. The specimen stage of claim 27, wherein said driving means comprises first driving means (100, 110b) for driving said first shifting table (9b), said first driving means being disposed between said inclination means (60) and said first shifting table, and second driving means (100, 109c, 109d) for driving said second shifting table, said second driving means being disposed between said first shifting table and said second shifting table. The specimen stage of claim 24, wherein said objective lens (1) has a magnetic pole (1b) projecting from the bottom wall of the lens, and said horizontal shifting means (50) has a space in which said magnetic pole (1b) is inserted.

Inventors: OTAKA TADASHI, OTAKA, TADASHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: H01J37
IPC codes: H01J37