Scanning electron microscope

Abstract: An electron beam 2 is focused on a specimen 6 by a focusing lens 5. A light beam 26 is incident on the position of irradiation of the specimen 6 with the electron beam 2 and the reflected light beam 28 is detected by a linear light detector 30. The output of the detector 30 is used to measure height of the specimen at the position of irradiation of the electron beam 2. The specimen 6 is moved in a plane perpendicular to an optical axis 36 of the focusing lens 5. A specimen height measuring device (31 to 34) carries out the height measurement of the specimen at a position to be observed on the specimen and at positions thereon which are in the vicinity of the position to be observed, when those positions are located at the position of irradiation of the electron beam. The specimen height measuring device 31 to 34 averages the measured values so as to produce a focusing correction signal on the basis thereof and controls the focusing lens 5 on the basis of the focusing correction signal.

Description: Background of The Invention The present invention relates to a scanning electron microscope, and more particularly to a scanning electron microscope suitable for observing semiconductor wafers with a high resolution in inspecting the same during their production. Disclosed in Japanese Patent Laid-Open No. 63-254649(1988) is a scanning electron microscope in which a specimen is irradiated with a light beam emitted from a light source such as a laser and the reflected light beam from the specimen is detected by a position detector such as a linear sensor. The output of the sensor is used to produce an electric signal representative of the moving distance of the specimen in the direction of an electron beam or the height of the specimen and the produced signal is used to control a focusing electron lens so as to focus the electron beam on the specimen. However, in a specimen such as a semiconductor wafer which has optical disturbances on its surface including a difference in reflectance, the measured result has an error due to the optical disturbances. This is because the intensity or intensity distribution of the light beam detected by the sensor is changed where there is a difference in reflectance depending on a position due to a circuit pattern formed on the wafer. In this connection, in Japanese Patent Application Laid-Open No.57-139607 (1982), measuring accuracy in the height of the specimen is improved by reducing the light spot diameter on the specimen and feedback-controlling the light source so that the amount of light incident on the detector always becomes equal to a pre-set reference value in order to reduce change in light intensity distribution. When the light beam is incident on the specimen such as a semiconductor wafer having optical disturbances, especially, fine pits and projections such as circuit patterns on its surface, the optical disturbances scatter the reflected light to disturb the detected result by the sensor. As a result, the focusing cannot be carried out with a high accuracy. Further, even if the light spot diameter on the specimen is reduced and the light source is feedback-controlled so that the amount of light incident on the detector always becomes equal to a pre-set reference value in order to reduce change in light intensity distribution, it is impossible to remove the error contained in the measured result caused by the scattering of the reflected light due to the pits and projections on the surface of the specimen. EP-A-279 172 relates to an electron beam lithography apparatus and deals also with the problem of focusing the electron beam on the specimen. The technique disclosed in this document resembles that of the above-mentioned publication JP-A-63-254649 in that the height of the specimen is measured by detecting a reflected light beam with a linear sensor. Considering that the light beam reflected from one position on the specimen impinges with an approximately bell-shaped intensity distribution always on a plurality of light sensitive diodes of the linear sensor at the same time, EP-A-279 172 proposes to integrate the intensity distribution of the beam incident on the linear sensor, to divide the integral by 2 to obtain the center of the beam from the division result and to determine the height of the specimen from the position of the beam center along the linear sensor. This technique cannot measure the height of the specimen precisely when the intensity distribution of the reflected light beam is distorted due to a pattern on the specimen surface such as a circuit pattern on a semiconductor wafer. Summary of the Invention It is an object of the invention to provide a scanning electron microscope in which the electron beam can be focused with high accuracy on a specimen even if the specimen has a surface pattern. This object is solved by the electron microscope of claim 1. The subclaims are directed to preferred embodiments of the invention. According to an embodiment of the present invention, a scanning electron microscope is provided which comprises means for generating an electron beam; means for irradiating a specimen with the electron beam; means for focusing the electron beam on the specimen; means for scanning the specimen with the electron beam so as to generate from the specimen a signal characteristic thereof; means for detecting the generated signal so as to display an image of the specimen on the basis of the detected signal; means for moving the specimen in a plane transverse to the electron beam; means for measuring physical quantities related to height of the specimen at a position to be observed on the specimen and at least one position on the specimen which is in the vicinity of the position to be observed thereon; and means for producing a statistical focusing correction signal on the basis of the measured physical quantities and controlling one of the focusing means and the moving means on the basis of the produced statistical focusing correction signal so as to focus the electron beam on the specimen when the position to be observed on the specimen is subject to irradiation of the electron beam. In the above scanning electron microscope, preferably, the statistical focusing correction signal is representative of an average of the measured physical quantities. In the above scanning electron microscope, preferably, the measuring means measures the physical quantities at a measurement position when the position to be observed on the specimen and the at least one position thereon are positioned at the measurement position by means of the moving means. According to another embodiment of the present invention, a scanning electron microscope is provided which comprises means for generating an electron beam; means for irradiating a specimen with the electron beam; means for focusing the electron beam on the specimen, means for scanning the specimen with the electron beam so as to generate from the specimen a signal characteristic thereof; means for detecting the generated signal so as to display an image of the specimen on the basis of the detected signal; means for moving the specimen in a plane transverse to the electron beam; means for measuring at a measurement position physical quantities related to height of the specimen at a position to be observed on the specimen and at least one position on the specimen which is in the vicinity of the position to be observed thereon; means for producing a statistical focusing correction signal on the basis of the measured physical quantities and controlling one of the focusing means and the moving means on the basis of the produced statistical focusing correction signal so as to focus the electron beam on the specimen when the specimen is so moved that the position to be observed on the specimen is subject to irradiation of the electron beam. According to a further embodiment of the present invention, a scanning electron microscope is provided which comprises means for generating an electron beam; means for irradiating a specimen with the electron beam; means for focusing the electron beam on the specimen, means for scanning the specimen with the electron beam so as to generate from the specimen a signal characteristic thereof; means for detecting the generated signal so as to display an image of the specimen on the basis of the detected signal; means for moving the specimen in a plane transverse to the electron beam; means for measuring at a measurement position physical quantities related to height of the specimen at a plurality of positions of irradiation of the electron beam when the specimen is moved; means for producing a statistical focusing correction signal on the basis of the measured physical quantities and controlling the focusing means on the basis of the produced statistical focusing correction signal so as to focus the electron beam on the specimen when the specimen is so moved that a position to be observed on the specimen is subject to irradiation of the electron beam, the position to be observed on the specimen being one of the plurality of positions and being also a position of irradiation of the specimen with the electron beam when movement of the specimen is stopped. Other objects and features of the present invention will be become apparent from the descriptions of preferred embodiments of the present invention taken in connection with the accompanying drawings. Brief Description of The Drawings FIG.1 is a block diagram of a scanning electron microscope showing an embodiment in accordance with the present invention, FIG.2 is a view for explaining the principle of measurement of the height of the specimen in accordance with the present invention, FIG.3 is a graph showing a position detecting result when the specimen is moved in accordance with conventional art. FIG.4 is a chart showing a calculation flow for obtaining a corrected position measurement result in accordance with the present invention, and FIG.5 is a graph showing a position measurement result in accordance with the present invention. Description of The Preferred Embodiments Referring to FIG.1, an electron beam 2 emitted from an electron gun 1 is focused on a specimen 6 such as a semiconductor wafer by first and second condenser electron lenses 3 and 4 and an objective electron lens 5 which serves as a focusing electron lens. An electron beam deflection controlling circuit 7 generates X- and Y-axis electron beam deflection signals, which are introduced into an electron beam deflector 8 arranged between the second condenser electron lens 4 and the objective electron lens 5 around an electro-optical axis 36, so that the specimen 6 is scanned two-dimensionally with the electron beam 2 because the electron beam 2 is deflected two-dimensionally. The specimen 6 produces secondary electrons, reflected electrons, light, etc. characteristic of the specimen 6, the secondary electrons of which are accelerated by an acceleration electrode 9 and the accelerated secondary electrons are detected by a secondary electron detector 11 through a filter 10 generating a magnetic field and an electric field perpendicular to each other so as to be converted into a video electric signal. The video electric signal is conducted to a cathode ray tube 12 through a video signal amplifier 15 as a brightness modulation signal. The X- and Y-axis electron beam deflection signals generated by the electron beam deflection controlling circuit 7 are also introduced into an electron beam deflector 13 of the cathode ray tube 12. Therefore, an image of the specimen 6 due to the secondary electrons is displayed on the cathode ray tube 12. The specimen 6 is held by a specimen driving mechanism system 14 thereon, which is designed to move the specimen 6 not only in a plane transverse to the electron beam 2 but also in an upward and downward direction, i. e., a direction of height of the specimen 6. In order to observe an arbitrary position on the surface of the specimen 6, the specimen 6 is moved in a plàne transverse to the electron beam 2, specifically that perpendicular to the electro-optical axis 36. When the position of the surface of the specimen 6 in the direction of the electro-optical axis 36 is changed after moving due to some cause, for example, warpage in the surface of the specimen 6 itself or displacement from a certain plane perpendicular to the electro-optical axis 36 of the specimen driving mechanism system 14, the surface of the specimen 6 does not coincide with a point of focus of the electron beam 2. The difference between them is assumed to be focus displacement ΔZ. Where it is necessary to automatically control the focusing electron lens 5, for example, in order to automatically focusing during observation after moving the specimen 6, it is required to know the focus displacement ΔZ. The focus displacement ΔZ may be obtained by using a specimen height measuring device. According to the device, a laser beam emitted from a laser beam source 25 is incident on the surface of the specimen 6 along an optical axis 26 so as to be focused on the surface of the specimen 6 by a lens 27 and is reflected from the specimen 6 into a light detector, i. e., a linear sensor 30 along an optical axis 28 through a lens 29 so as to be converted into an electric signal by the linear sensor 30. Hereinafter, the laser beam is referred to as incident light. Referring to FIG. 2, the surface of the specimen 6 is assumed to be placed at a height level of Z a. The incident light emitted from the laser beam source 5 is reflected at a point P a on the surface of the specimen 1 when it hits the surface. If the height level of the surface of the specimen 6 is displaced by a height of ΔZ from the position P a to a position Z b , the position of the surface becomes as shown by a dashed line in the figure. Since the reflected position of the incident light 26 is shifted to a point P b , the reflected light is directed toward the linear sensor 10 along not the optical axis 28-a but the optical path 28-b keeping the same reflected angle  as before. The light path difference Δl in the direction perpendicular to the optical axes 8-a and 8-b becomes the displacement of focus position of the reflected light on the linear sensor 10. The relationship between the light path difference Δl and the focus displacement ΔZ can be expressed by equation (1) and is linear. ΔZ=Δl×sin Therefore, the focus displacement ΔZ can be obtained or known by detecting the light path difference Δl using the linear sensor 10 and by converting it using Equation (1). The output electric signal of the linear sensor 10 indicating the light path difference Δl is converted into an electric signal indicating the focus displacement ΔZ according to Equation (1) by a sensor output/specimen height converter 31. However, when there are optical disturbances, such as projections and depressions or difference in surface reflectance, on the surface of the specimen 6, the light intensity of the reflected light is changed even when the intensity of the incident light is constant. In addition to this, the intensity distribution of light on the focus point of the reflected light in the linear sensor 30 is shifted from the Gaussian distribution which the incident light has. In such a case, the linear sensor 30 erroneously detects the spot indicating the focus position of the reflected light in the direction perpendicular to the optical axis 28-b of the reflected light. That is, when the intensity distribution of light in the spot is changed, a position of center to be the detecting position of the spot on the linear sensor 30 is changed and consequently the measured value of the focus displacement ΔZ contains a measurement error. Since the specimen 6 is moved in the plane perpendicular to the electro-optical axis 36 by the specimen driving mechanism system 14, the output of the sensor output/specimen height converter 31 is as shown in FIG.3. The measured result of the focus displacement ΔZ is changed along a straight line or a curved line L according to change in the surface height of the specimen. However, at the time t a or t b when the position of the optical disturbance on the specimen surface coincides with the focus position of the incident light P a or P b , the measured result of the focus displacement ΔZ departs from the line L to make a portion A or B. The output electric signal of the sensor output/specimen height converter 31 indicating the measured value of the focus displacement ΔZ is switched every certain period using the time dividing switch 33 to distribute and store to a plurality of holding circuits 32-a to 32-f of a holding circuit unit 32 with time. The initial output of the sensor output/sample height converter 11 is input to the holding circuit 32-a by the time dividing switch 33. The next output is input to the holding circuit 32-b, and the remaining outputs are input to the selected holding units 32-c to 32-f in order. After the last output is input to the holding unit 32-f, the time dividing switch 33 is switched to the holding circuit 32-a having the first output value. Then, the operation is repeated, so that the measured results of the focus displacement ΔZ are always temporarily held retroacting by the number of the holding circuits 32-a to 32-f, i.e., six in the case of this embodiment. Steps of focusing will be explained with reference to a flow chart of FIG.4. Step 45 Initially, the specimen 6 is moved by the specimen driving mechanism system 14 so that a position to be observed on the specimen 6 coincides with a measurement position at which the measurement of height of the specimen 6 is to be carried out. The measurement position is also a position which is subject to irradiation of the electron beam. Step 46 As the specimen driving system 14 starts to be driven, the measurement of height of the specimen is started by the specimen height measuring device to obtain a measured value of the focus displacement ΔZ from the sensor output/specimen height converter 31. The measured value of the focus displacement ΔZ is input to the holding circuit unit 32 in synchronism with the time dividing switch 33. Here, let the m-th measured result be ΔZ m . Step 47 Next, the focus displacement ΔZ m of the output of the sensor output/specimen height converter 31 is temporarily stored in one selected from the holding circuits by the time dividing switch 33. Step 48 When the position to be observed on the sample coincides with the measurement position, the driving of the specimen driving mechanism system 14 is completed and further the measurement of the focus displacement ΔZ and switching operation by the time dividing switch 33 are completed. If not coincident, the processing is returned to the step 46 to perform (m+1)-th measurement next. Step 49 If it is assumed that the last measured result is ΔZ m and the number of the holding circuits is n, in an averaging calculation circuit 34, the focus displacement at the position of the irradiation of the specimen with the electron beam after completion of driving of the specimen driving mechanism system 14 can be obtained according to equation (2). ΔZ={ΔZ m-n +ΔZ m-n+1 +...ΔZ m }/n That is, the final focus displacement ΔZ is defined as the average value of the measured values stored in the n holding circuits. It should be noted that the values stored in the holding circuits 32-a to 32-f are related to the height of the specimen at corresponding positions on the specimen which are in the vicinity of one another and are measured when those positions are located at the measurement position. Step 50 A lens power supply and it's controller 35 generates a focusing correction signal for cancelling out the focus displacement ΔZ and controls the focusing electron lens 5 on the basis of the focusing correction signal so that the focus displacement ΔZ obtained in step 49 becomes zero by changing the position of focus of the electron beam in the direction of the electro-optical axis 36. By doing so, control of the focusing electron lens 5 becomes equivalent to control using the measured result of the focus displacement ΔZ which does not depart from the line L at the time t a and t b when the position of the optical disturbance on the sample surface coincides with the measurement position, i. e., the position of focus of the incident light. Instead of the method in which focusing is performed by moving the position of focus in the direction of the axis 36 as described above, it is possible that the focus displacement ΔZ, i. e., difference between the position of focus of the electron beam and the height of the sample surface, is made 0 (zero) by moving the specimen driving mechanism system 14 in the direction of the axis 36. Though the ΔZ is a simple mean value as apparent from the above, it may be a weighted mean value of the output signals of the holding circuits 32-a to 32-f. In this case, weight coefficients may be determined according to the degree of importance of the measured values. In general, the degree of importance of the measured values depends on a distance between a position to be observed on the specimen and a position thereon which is in the vicinity thereof at which the height of the specimen is measured at the measurement position. Namely, it is increased as the distance becomes short. In order to measure the height of the specimen at a plurality of positions thereon which are in the vicinity of one another, a light beam scanner may be so used as to deflect or scan the incident light beam while keeping the specimen stationary in place of the moving of the specimen in the plane perpendicular to the electro-optical axis 36 while keeping the incident light beam stationary. In this case, after the specimen is so moved that one of the plurality of positions, i. e., a position to be observed is subject to irradiation of the electron beam, the incident light beam may be deflected or scanned so as to irradiate the plurality of positions therewith in turn. The light beam scanner itself is omitted in the figures, since it is well known. Since the ΔZ is a simple mean as apparent from the above, the focusing correction signal can be called a so-called statistical focusing correction signal. With the series of operations, the position of focus of electron beam can coincide with the surface of the specimen 6 without being affected by the optical disturbance on the surface of the sample. According to the embodiment of the present invention, the focusing of an electron beam on a specimen can be carried out with a high accuracy even if the specimen has its surface on which fine pits and projections are formed. Further, focusing of a scanning electron microscope can be stably performed with high speed, and accordingly it is possible to improve throughput in a scanning electron microscope used as an inspection apparatus in the field of semiconductor industry. It is obvious that many changes and modifications can be made in the above described details without departing from the scope of the present invention as claimed.

Claims: A scanning electron microscope, comprising: means (1 to 4, 7, 8) for irradiating a specimen (6) with an electron beam (2), means (5) for focusing the electron beam, means (25 to 31) for measuring a physical quantity related to the height of the specimen, and means (32 to 35) for causing the electron beam to be focused on the specimen depending on the measured physical quantity characterised in that said measuring means (25 to 31) are adapted to conduct at least two measurements of the physical quantity at different positions at or in the vicinity of an area to be observed on the specimen by the electron microscope, and said means (32 to 35) for causing the electron beam to be focused on the specimen comprises means (32 to 34) for producing from said at least two measurements a statistical focusing correction signal representative of an average of the measured physical quantity, and for causing said electron beam to be focused in accordance with said statistical focusing correction signal. A scanning electron microscope according to claim 1, comprising means (14) for moving the specimen in a plane transverse to the electron beam. A scanning electron microscope according to claim 2, wherein said means (32 to 34) for producing the statistical focusing correction signal are adapted for controlling the focusing means (5) and/or the moving means (14) so as to focus the electron beam on the specimen when the specimen is so moved that the area to be observed on the specimen is irradiated by the electron beam. A scanning electron microscope according to claim 3, wherein irradiation of the specimen (6) with the electron beam (2) is performed when movement of the specimen is stopped. A scanning electron microscope according to any of claims 2 to 4, wherein the measuring means (25 to 31) measures the physical quantity at a measurement location while said different positions on the specimen (6) are each moved to the measurement location by the moving means (14).

Inventors: KOBARU ATSUSHI, MAEDA TATSUYA, OTAKA TADASHI, SASADA KATSUHIRO, KOBARU, ATSUSHI, MAEDA, TATSUYA, OTAKA, TADASHI, SASADA, KATSUHIRO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1996
Main classification: H01J37
IPC codes: H01J37