Scanning electron microscope

Abstract: The device of the present invention enables high resolution observation even when a sample is tilted. A deflecting electrode device 17 for generating an electric field having a component in the direction perpendicular to the center axis (optical axis) of an objective lens 6 is provided between the objective lens 6 and the sample 8. A voltage applied to the deflecting electrode device 17 is controlled in accordance with the tilting of a sample stage 10. A lateral electric field component generated on the optical axis when the sample stage 10 is tilted is corrected by a deflected electric field generated by the deflecting electrode device 17. This is effective to suppress generation of astigmatism, and to allow effective arrival of a secondary electron at a secondary electron detector disposed at a position closer to the electron source 1 than the objective lens 6.

Description: Background of the Invention The present invention relates to a device suitable for obtaining a scanning image of a sample on the basis of an information signal, peculiar to the sample, obtained by emission of an electron beam to the sample. A method for observing a sample at high resolution using a scanning electron microscope or the like has been described in EP-A-592 899 and in Japanese Patent Laid-open No. Sho 57-172643, in which a lens magnetic field is generated at a lower portion of an objective lens; a secondary electron generated from the sample is accelerated by a positive voltage applied to electrodes disposed on an objective lens portion; and the secondary electron is detected at a portion over the objective lens. On the other hand, a so-called retarding method is intended to decelerate a primary electron directly before a sample by applying a negative voltage to a brake electrode above the sample (US-4 330 707) or directly to the sample. In this retarding method, when secondary electrons generated from a sample are accelerated by a voltage applied to the sample and advance over the objective lens, they must be detected at a portion over the objective lens. In such method, an electric field is generated between the sample and the objective lens. In the above-described prior art methods, when a conductive sample is tilted, an axisymmetric property of an electric field between a sample and an objective lens is put into disorder, and consequently a lateral electric field component is generated on the optical axis of the electron beam. This is inconvenient in increasing astigmatism, thereby making poor resolution. The disorder of the axisymmetric property of an electric field also disturbs the orbit of a secondary electron generated form the sample, and obstructs effective arrival of the secondary electron at a detector disposed over the objective lens. This results in lowering of the detection efficiency of the secondary electron. An example of a scanning electron microscope having a sample stage which allows tilting of a sample is disclosed in US-4,803,358 from which the precharacterising first portion of claim 1 starts out. This microscope has a secondary electron detector positioned between the sample stage and an objective lens. A scanning electron microscope with tiltable sample stage is also disclosed in EP-A-335 398 which discloses to dispose two secondary electron detectors nearly opposite each other with respect to the objective lens but both slightly more towards that side of the objective lens to which the sample is facing when tilted, so as to capture more efficiently secondary electrons emitted from the tilted sample. Summary of the Invention It is an object of the invention to provide a scanning electron microscope or analogous device having a high resolution and a high detection efficiency for secondary electrons. This object is solved by the microscope or device as set forth in claim 1. The subclaims are directed to preferred embodiments of the invention. There are provided means for correcting a nonaxisymmetric property, due to the tilting of the sample, of an electric field applied to the electron beam to be emitted on the sample, wherein the nonaxisymmetric property correcting means includes a deflecting electrode device to which a variable voltage is applied for generating an electric field having a component in the direction perpendicular to the optical axis of the electron beam. Accordingly, when a sample is tilted, generation of astigmatism can be suppressed by changing a voltage applied to the deflecting electrode device for generating an electric field component being large sufficient to compensate for an irregular electric component generated in the direction perpendicular to the optical axis of the electron beam, to thereby prevent lowering of resolution due to the astigmatism. The generation of an effective electric field component by the deflecting electrode device exhibits another effect. Specifically, when a secondary electron generated from a tilted sample is detected after being extracted through an objective lens, an irregular electric field component generated in the direction perpendicular to the optical axis of the electron beam disturbs the orbit of the secondary electron, thereby lowering the detection efficiency of the secondary electron. The lowering of the detection efficiency of the secondary electron, however, can be prevented by changing a voltage applied to the deflecting electrode device for generating an electric field component being large sufficient to compensate for the irregular electric field component. In addition, the device of the present invention is excellent in operability because a voltage applied to the deflecting electrode means can be changed in accordance with the tilting of a sample. Brief Description of the Drawings Fig. 1 is a schematic view showing a configuration of one embodiment of a scanning electron microscope according to the present invention; Fig. 2 is a view showing a configuration of a deflecting electrode shown in Fig. 1; Fig. 3 is a view showing a positional relationship between an objective lens, a sample stage and the deflecting electrode shown in Fig. 1; Fig. 4 is a view, similar to Fig. 2, showing another configuration of the deflecting electrode; Fig. 5 is a view, similar to Fig. 2, showing a further configuration of the deflecting electrode; and Fig. 6 is a view, similar to Fig. 3, showing another positional relationship between the objective lens, sample stage and deflecting electrode. Description of Preferred Embodiments Fig. 1 is a view showing a configuration of one embodiment of the present invention. When a voltage V1 is applied between a cathode 1 of an electron gun and an extraction electrode 3, a primary electron beam 2 is discharged from the cathode 1. The primary electron beam 2 is accelerated by a voltage Vacc applied between the cathode 1 and an accelerating electrode 4, and is then emitted to be converged to a sample 8 through a convergent lens 5 controlled by a lens controller 14 and an objective lens 6. Deflectors 7a, 7b deflect the primary electron beam 2 two-dimensionally so that the primary electron 2 scans the sample 8 two-dimentionally. A scanning signal supplied to the deflectors 7a, 7b is controlled by a deflection control circuit 12 on the basis of an observation magnification. A secondary electron beam 15 generated from the sample 8 by emission of the primary electron beam is accelerated by a voltage Vse applied to a lifting electrode 16, and is detected by a secondary electron detector 9 by way of the objective lens 6. An enlarged image of the sample is thus displayed on an image display 13. A deflecting electrode device 17 is composed of two electrodes 19, 20 symmetrically disposed with respect the optical axis of the electron beam, as shown in Fig. 2. Different voltages, which are respectively applied to the electrodes 19, 20, are controlled by a control unit 18 in accordance with the tilting of a sample stage 10 mounting the sample 8. The suitable selection of these voltages enables generation of an electric field component being large sufficient to compensate for an irregular electric field component generated in the direction perpendicular to the axis of the electron beam due to the tilting of the sample. This is effective to correct the nonaxisymmetric property, due to the tilting of the sample, of the electric field applied to the electron beam, and hence to suppress generation of astigmatism as a cause of lowering of resolution. The generation of an effective electric field component by the deflecting electrode device 17 exhibits another effect. Specifically, when a secondary electron generated from a tilted sample is detected after being extracted through an objective lens, an irregular electric field component generated in the direction perpendicular to the optical axis of the electron beam disturbs the orbit of the secondary electron, thereby lowering the detection efficiency of the secondary electron. The lowering of the detection efficiency of the secondary electron, however, can be prevented by changing a voltage applied to the deflecting electrode device 17 for generating an electric field component being large sufficient to compensate for the irregular electric field component. A voltage applied to the deflecting electrode device 17 can be manually changed; however, the voltage is desirable to be automatically changed in practice. In this case, experiments have been made for obtaining a relationship for correcting an irregular electric field, that is, a nonaxisymmetric property of an electric field applied to an electron beam. In other words, a relationship of a voltage applied between the electrodes 19, 20 to a tilting angle of a sample has been previously obtained by experiments. Thus, the control unit 18 automatically changes, on the basis of the experimental results, a voltage applied between the electrodes 19, 20 in accordance with a variable tilting angle of the sample. Fig. 3 shows an arrangement example of the deflecting electrode device 17, in which the device 17 is arranged between the lower surface of the objective lens 6 and a sample in such a manner as to be positioned around the optical axis of an electron beam. As shown in Figs. 2, 4 or 5, the deflecting electrode device 17 may be composed of two, four or eight electrodes, to which different voltages are applied. A voltage applied to each electrode of the deflecting electrode device 17 is automatically controlled in accordance with the tilting of the sample stage 10, that is, the sample 8 by the control unit 18. In this case, it is apparent that lowering of resolution can be prevented and also effective detection of a secondary electron can be performed for the same reason as described above. Fig. 6 shows another arrangement example of the deflecting electrode device 17, in which the device 17 is disposed opposite to the sample 8, that is, the sample stage 10 tilted with respect to the optical axis of an electron beam. A voltage applied to the device 17 is controlled in accordance with the tilting of the sample stage 10 by the control unit 18. Even in this case, when a voltage is applied to the deflecting electrode device 17, an electric field having a component in the direction perpendicular to the optical axis of the electron beam is generated. The electric field component thus generated corrects a lateral electric field component generated on the optical axis of the electron beam when the sample stage 10 is tilted, to suppress generation of astigmatism. As a result, it becomes possible to correct astigmatism, and to realize effective detection of a secondary electron 15 generated from the sample 8. According to the present invention, generation of astigmatism can be prevented by changing a voltage applied to the deflecting electrode device for generating an electric field component being large sufficient to compensate for an irregular electric component generated in the direction perpendicular to the optical axis of the electron beam when a sample is tilted, to thereby prevent lowering of resolution due to the astigmatism. The generation of an effective electric field component by the deflecting electrode device 17 exhibits another effect. Specifically, when a secondary electron generated from a tilted sample is detected after being extracted through an objective lens, an irregular electric field component generated in the direction perpendicular to the optical axis of the electron beam disturbs the orbit of the secondary electron, thereby lowering the detection efficiency of the secondary electron. The lowering of the detection efficiency of the secondary electron, however, can be prevented by changing a voltage applied to the deflecting electrode means for generating an electric field component being large sufficient to compensate for the irregular electric field component. In addition, the device of the present invention is excellent in operability because a voltage applied to the deflecting electrode device can be changed in accordance with the tilting of a sample. The change in a voltage applied to the deflecting electrode device 17 will be fully described using the deflecting electrodes shown in Fig. 3. The orbit of a secondary electron generated from a sample by emission of an electron beam is required to be changed in such a manner that the secondary electron is introduced to a secondary electron detector by way of the objective lens. To satisfy such requirement, in the case where the sample stage 10 mounting the sample is tilted as shown in Fig. 3, a voltage applied to the electrode which is separated apart from the sample stage is set on the negative side as compared with a voltage applied to the electrode which moves near the sample stage. Specifically, although a secondary electron generated from the sample on the sample stage 10 directs in the perpendicular direction of the sample stage 10 due to the tilting of the sample stage 10, it is pushed back to the center axis of the objective lens by applying a voltage on the negative side to the electrode apart from the sample stage as compared with the electrode near the sample stage, thereby improving the detection efficiency of the secondary electron. As long as the above relationship is kept, that is, a voltage on the negative side is applied to an electrode apart from the sample stage as compared with an electrode near the sample stage, the same effect as described above can be usually obtained even when a voltage applied to one electrode is zero or when positive voltages are applied to both the electrodes. Additionally, in the case where both the electrodes are applied with positive voltages, they function as the deflecting electrodes serving as extraction electrodes for a secondary electron.

Claims: A scanning electron microscope, comprising: an electron gun (1) for generating an electron beam (2), an objective lens (6) for converging the electron beam onto a sample (8), so as to allow emission of the electron beam (2) on the sample; a deflector (7a, 7b) for deflecting the electron beam (2) so as to allow the electron beam to scan the sample (8); a sample stage (10) for mounting the sample and capable of tilting it with respect to the electron beam, means (16) for generating an electric field extracting secondary electrons from the sample in the direction of the electron gun, and a detector (9) placed between said electron gun (1) and said means (16) for generating the electric field, for detecting said secondary electrons extracted, characterised by a deflecting electrode device (17) arranged between the sample stage (10) and the detector (9) and by means (18) for applying a variable voltage to said deflecting electrode device and forming an electric field suppressing a non-axisymmetric electric field formed by tilting of said sample (8), and having a component in the direction perpendicular to the optical axis of the electron beam (2). A microscope according to claim 1, wherein said deflecting electrode device (17) comprises a plurality of electrodes (17, 19, 20) arranged around the optical axis between said sample stage (10) and said objective lens (6). A microscope according to claim 2, wherein voltages applied to a first group of said plurality of electrodes (17, 19, 20) being more apart from the sample stage (10) when the latter is tilted are negative with respect to voltages applied to the remaining group of electrodes (17, 19, 20) closer to the sample stage when the latter is tilted. A microscope according to claim 3, wherein as the tilting angle of the sample stage (10) is increased, the voltage difference between the first group of electrodes and the remaining group of electrodes (17, 19, 20) is increased. A microscope according to any of claims 1 to 4, further comprising means (18) for detecting a tilting angle of the sample (8) and controlling the voltage applied to the deflecting electrode device (17) in accordance with the tilting angle detected. A microscope according to any of claims 1 to 5, wherein said deflecting electrode device (17, 19, 20) includes a deflecting electrode (17) disposed opposite the tilted sample (8) with respect to the optical axis of the electron beam (2). A microscope according to any of claims 1 to 6, wherein said means (16) for generating an electric field are arranged to extract said secondary electrons through said objective lens (6).

Inventors: IWABUCHI YUKO, OSE YOICHI, SATO MITSUGU, IWABUCHI, YUKO, OSE, YOICHI, SATO, MITSUGU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1996
Main classification: H01J37
IPC codes: H01J37