Charged particle beam apparatus

Abstract: In a scanning electron microscope, a pair of fiducial patterns (10, 11) are so laid out on an X-Y stage so that they are separated with a distance equivalent to the distance between the optical axis of an optical microscope (5) and that of an electron optical system (3). The image signals of these patterns (10, 11) are captured by an image processing circuit (12). A position shift of the optical axis of the optical microscope (5) and a position shift of the optical axis of the electron optical system (3) with respect to the fiducial patterns (10, 11) are obtained by numerical calculation, and this position shift is used as an offset value in the actual measurement by the scanning electron microscope, which leads to an accurate positioning.

Description: Background of the Invention The present invention relates to a charged particle beam apparatus, especially to a charged particle beam apparatus good for a semi-conductor production equipment for producing semi-conductors, and for a measurement and evaluation apparatus used in the measurement and evaluation process for the semi-conductor production. In recent years, what is used in producing semi-conductor devices and their characteristics measurement and evaluation process is an apparatus using electron beams or ion beams as charged particles. For example, there is a scanning electron microscope in which an image is formed by detecting the secondary electrons which are generated when the electron beams are irradiated onto the specimen to be measured with synchronizing the scanning operation of the irradiation electron beams, and a fine pattern is observed, and furthermore, the dimension of a designated pattern in the formed image is measured. There is also an electron beam exposure apparatus for forming a fine pattern on the wafer by processing the photoresist with electron beams. There is also an ion beam apparatus for observing and processing the semi-conductor devices with ion beams. Those systems are composed of a charged particle beam optical system including an generation source of the charged particle beams such as electron beam or ion beam, and a means for directing the charged particle beam to the specimen; an X-Y stage for fixing the specimen to a designated position and a control system for controlling those. In the scanning electron microscope having a function for measuring the dimension of fine patterns, an optical microscope is installed in addition to the electronic optical system, and the patterns formed on the wafer can be observed and measured with low magnification, as disclosed, for example, in Japanese Patent Publication No.3-52178. The following effects can be obtained by using this function. When fine patterns are observed and measured with high magnification in the electronic optical system, as the magnification is so high for the direct observation in the electronic optical system, it is not possible to fix the pattern in a designated position. In order to solve this problem, at first, a designated pattern and its neighboring pattern are observed and their positions are fixed by using an optical microscope, and next, the designated pattern with its position being fixed by the optical microscope is fixed at a observation position for the electronic optical system so that the fine pattern may be observed and measured with high magnification by the electronic optical system. In the scanning electron microscope, it is necessary to obtain precisely the position of the wafer on the X-Y stage in order to measure the pattern at a pre-determined position on the wafer. In order to solve this problem, before the measurement of the pattern on the wafer, the position of the wafer is measured by using a specific alignment pattern on the wafer, and the wafer alignment is adjusted. This measurement can be done by the optical microscope. It is required to keep precisely the distance between the optical axis of the optical microscope and the optical axis of the electronic optical system. The distance between these optical axis is about 50 mm, and is calibrated on the basis of the positioning detector of the stage when assembling and adjusting the apparatus. However, there is an upper-bound for the accuracy in the positioning of the stage, and the error from ±2 µ m to ±5 µ m can not be avoided. For this reason, it is practically difficult to calibrate precisely the distance between a couple of optical axes. In addition, many trial-and-error operations are required for the calibration work and it takes a lot of time. The distance between the optical axis of the optical microscope and the optical axis of the electric optical system may be deviated when operating the apparatus. In case that the distance between these optical axes changes by ±5 µ m, when the designated pattern is observed and measured by the electronic optical system with high magnification after the designated pattern is observed and measured by the optical microscope and the position of the wafer is fixed, there is the disadvantage that the pattern positioned with the X-Y stage cannot be fixed within the observation range of the electronic optical system. Thus, if there is a deviation in the distance between the optical axes, the distance must be calibrated again, and as the calibration operation is done in a try-and-error manner similar to that in the assembling and adjusting the apparatus using the pattern formed on the wafer which is used normally for measurement, the adjusting operation takes a lot of time. US-A-4,785,187 discloses a charged particle beam apparatus with the features included in the first part of claim 1. In this apparatus, alignment is done by means of a pair of reference marks which are provided on a reference plate provided independently of the respective specimen to be observed. Therefore, environmental influences, such as changes in temperature, will have different effects on the sample and reference marks, thereby impairing the precision of the alignment. US-A-4,447,731 relates to an exterior view examination apparatus in a scanning-type electron microscope in which the same cross hair is used for prealignment and alignment, so that the problem of alignment between different optical axes does not exist. Similarly, GB-A-2,105,858 discloses an inscribing apparatus in which an alignment mark is formed on the back surface of a specimen stage and detected by the use of a second beam. Again, a calibration of the distance between two optical axes does not take place. In the apparatus disclosed in US-A-5,117,110 and JP-A-64-61718, two reference marks are provided for adjusting the distance between two optical axes. The specific problem underlying the invention is neither discussed nor solved. GB-A-2,263,335 discloses an electron beam lithography apparatus in which two marks formed on the wafer being treated are detected. The marks, however, are not used for calibrating the distance between two optical axes. Summary of the Invention It is an object of the present invention to provide a charged particle beam apparatus which enables precise and quick calibration of the distance between the optical axes of the particle beam optical system and that of a second optical system. This object can be attained by providing the charged particle beam apparatus defined in claim 1. According to the invention, as the calibration of the distance between the optical axes of the two optical systems is enabled by means of first and the second fiducial patterns which are formed in the specimen itself or on the stage mounting the specimen and are independently observed with their positions being measured individually by the two optical systems, the calibration operation can be done precisely. Other objects and features of the invention will become clear from the following description of the preferred embodiments referring to the accompanied drawings. Brief Description of the Drawings FIG. 1 is a view showing the construction of a scanning electron microscope having an optical microscope as an embodiment of a charged particle beam apparatus in accordance with the present invention. FIG. 2 is a perspective view of another embodiment of a specimen stage portion. FIG. 3 is a front view showing the concept of a control system. FIG. 4 is a view showing the construction of an electron beam drawing apparatus as an embodiment of a charged particle beam apparatus in accordance with the present invention. Detailed Description of the Preferred Embodiments FIG.1 shows the structure of the scanning electron microscope having an optical microscope in one embodiment of the charged particle beam apparatus of the present invention. In the specimen chamber 1, a wafer (not shown in FIG.1) to be supplied as a specimen is mounted on the specimen holder 6, and the specimen holder is mounted on the X-Y stage 2 used for moving the wafer mounted on the specimen holder in X and Y directions. An electronic optical system is incorporated in the optical column assembly 3 above the specimen chamber 1, which includes electron lenses 13 and 14 for converging the generated electron beam onto the wafer and a bias coil 9 and so on. The lenses 13 and 14 and the bias coil 9 are controlled by the control system 15. The electron gun 4 is connected to the high-voltage power supply unit 16 which is also controlled by the control system 15. The secondary electron detector 18 for detecting the secondary electron generated from the wafer or the fiducial pattern to be explained later by electron beam exposure is mounted in the specimen chamber 1, and the optical microscope 5 is also mounted at the side part of the column 3. The fiducial patterns 10 and 11 are placed on the specimen holder 8 mounted on the stage 2. Both fiducial patterns are placed in the position other than the place in which the wafer as a specimen is mounted, that is, the position around the specimen holder 8. The fiducial pattern 11 is one specifically used for the electronic optical system, and the fiducial pattern 10 is one specifically used for the optical microscope 5. The distance between the fiducial patter 10 and the fiducial pattern 11 is so defined as to be equal to the distance between the optical axis of the electronic optical system and the optical axis of the optical microscope 5. For example, the distance is set to be 50 mm. When the fiducial pattern 10 is positioned at the center of the optical axis of the optical microscope 5, the fiducial pattern 11 is positioned at the center of the optical axis of the electronic optical system. In this time, the image signal of the fiducial pattern 10 obtained by the optical microscope 5 is supplied to the image processing circuit 12. In addition, The image signal of the fiducial pattern 11 defined by the secondary electrons detected by the secondary electron detector 18 when the electron beam is biased in X and Y directions by using the bias coil 9 is supplied to the image signal processing circuit 12. The image processing signal from the image processing circuit 12 is supplied to the control circuit 15, where the images of the fiducial patterns 10 and 11 are displayed on its display part. The inside gas of the specimen chamber 1 is evacuated so that a designated vacuum level may be established in the specimen chamber 1. This evacuation operation is performed by the turbo molecular pump 6 and the rotary pump 7 used for coarse evacuation. Next, what is described is the method for calibrating the distance between the optical axes of the electronic optical system and the optical microscope by using the fiducial patterns 10 and 11. Before starting the observation and size measurement of the pattern by using the scanning electron microscope, the relative position between the electronic optical system and the optical microscope is calibrated. At first, the fiducial patterns 10 and 11 are positioned by moving the X-Y stage 2, using a drive mechanism 17, so as to be located in their corresponding visible scopes for the optical microscope and the electronic optical system, respectively. Next, the images of the fiducial patterns captured by the electronic optical system and the optical microscope are supplied to the image processing circuit 12, where the relative position shift A1 and A2 between the individual fiducial pattern and its corresponding optical axis are obtained by numerical calculation. The distance between a couple of fiducial patterns is defined as A which is made beforehand to be equal to the distance L between the optical axis of the electronic optical system and the optical axis of the optical microscope. For example, if the design value of L is 50 mm, A becomes 50 mm. Therefore, the positional relations A1 and A2 contain the errors in positioning the X-Y stage and the deviation of the distance L between the optical axis of the electronic optical system and the optical axis of the optical microscope. In principle, if L is exactly equal to A, then A1 must be equal to A2. However, in case that L changes, L is not equal to A and hence, A1 is not equal to A2, and thus, the deviation component is given as A1 - A2. This relation can be expressed in the following formula. L = A + (A1 - A2) From the above formula, the deviation value accompanied with the detection of the fiducial patterns is proved to be (A1 - A2). In the measurement with the scanning electron microscope, the positional relationship between the optical microscope and the electronic optical system can be precisely evaluated by using the deviation value (A1 - A2) of the distance L between both optical systems as an offset value. At first, the fine pattern to be observed and measured by the electronic optical system is positioned by using the optical microscope, and next, the pattern can be positioned on the optical axis of the electronic optical system by using the X-Y stage with consideration of the offset value obtained at first. In case that the offset value is defined on the basis of the optical axis of the electronic optical system, the offset is added to the position of the optical axis of the optical microscope. On the other hand, in case that the offset value is defined on the basis of the optical axis of the optical microscope, the optical axis of the electronic optical system is biased by adding the offset value onto the current flowing in the bias coil, which establishes the offset of the center of the optical axis of the electronic optical system. When positional relationships between the fiducial pattern and each of the centers of the optical axes are expressed as (A1 X , A1 Y ), (A2 X , A2 Y ), the deviation value can be obtained as follows. The distance A X , A Y between the two fiducial patterns is preset to the distance L X , L Y between the optical axis of the electronic optical system and the optical axis of the optical microscope. Therefore, the positional relationships (A1 X , A1 Y ),(A2 X , A2 Y ) include the positioning error of the X-Y stage and the deviation value of the distance L X , L Y between the optical axis of the electronic optical system and the optical axis of the optical microscope. However, since the positioning error of the X-Y stage is commonly included in the both fiducial patterns, the positional relationships (A1 X , A1 Y ), (A2 X , A2 Y ) directly become the deviation value of the distance L X , L Y between the optical axis of the electronic optical system and the optical axis of the optical microscope. Therefore, the deviation value of the distance L X , L Y between the optical axes can be expressed as the following equations. L X =A X +(A1 X -A2 X ) L Y =A Y +(A1 Y -A2 Y ) Accordingly, the deviation values can be known as follows by detecting the fiducial pattern. ΔX=(A1 X -A2 X ) ΔY=(A1 Y -A2 Y ) Then, when measurement is performed in the scanning electron microscope, the positional relationship between the optical microscope and the electronic optical system can be accurately obtained by offsetting the deviation values ΔX, ΔY in the distance L X , L Y between the optical axes. In the above described configuration and method, in the electronic optical system, it is possible to establish a high accuracy in the positioning operation for a designated fine pattern without any positioning error even in high magnification. On the other hand, as shown in FIG.2, by forming a couple of fiducial patterns 10 and 11 on a single member 22 mounted on the specimen holder 8 on the stage 2, it is enabled to establish more precisely the distance between the fiducial patterns 10 and 11. The component 24 is a wafer, and the component 20 is a position detector for detecting the position of the X-Y stage 2. When observing the fiducial patterns by the electronic optical system and the optical microscope, in case that the number of monitors installed in the display part is only one, it is possible to observe them by altering the display objects on the single monitor, but this method requires complicated operations for the operator. In order to solve this problem, different arts in the display panel 31 in the display part of the control system as shown in FIG.3, that is, both of the display panels 32 and 33, are displayed at the same time. Though a single display panel is displayed in a normal mode, by displaying a couple of display panels at the same time, a first image obtained by the electronic optical system can be observed on the monitor display panel 32, and a second image obtained by the optical microscope can be observed on the monitor display panel 33 at the same time. In addition, the status when measuring the position relation can be monitored at the same time. In FIG.3, the components 36 and 37 are icons. In the scanning electron microscope having the above described optical microscope, in order to evaluate the position relation of the wafer set on the X-Y stage beforehand before measuring a designated pattern formed on the wafer, the position relation with respect to the coordinates of the X-Y stage is measured on the basis of a typical wafer pattern by using the optical microscope. Next, now that the position relation is evaluated explicitly, the coordinate system of the wafer with respect to the optical microscope is transformed so as to be equivalent to the coordinate system of the X-Y stage. As the relative distance between the electronic optical system and the optical microscope is calibrated strictly by using the fiducial pattern based on the present invention, the coordinate system of the wafer with respect to the optical axis of the electronic optical system is similarly transformed accurately. Therefore, in the observation and measurement of a designated pattern formed on the wafer by using the electronic optical system so calibrated as described above, the positioning of the pattern can be enabled in a short time even with high magnification. Further, the present invention discloses the following construction as another embodiment. Among the fiducial patterns, a plurality of patterns are provided in the side being detected by the charged particle beam and have different dimensions. The dimensions of the plurality of patterns are determined based on the measuring magnifications by the charged particle beam. With such a construction, positioning can be performed after the charged particle mean apparatus is set to a desired magnification. When a magnification is changed after positioning, there is possibility to change the irradiation position of the beam. Therefore, it is preferable that positioning is performed after changing the magnification. The aforementioned construction is for this purpose. In the present invention, it is necessary that the patterns detected by the optical microscope are positioned corresponding to the different pattern positions. That is, a plurality of patterns for the optical microscope are respectively formed on positions in the same distance from the patterns observed or detected by the charged particle beam. It is also possible to form only a single pattern for the optical microscope depending on positioning of patterns for the charged particle beam apparatus. This can be attained by forming the patterns for the charged particle beam apparatus in gathering to a single place. In detail, one idea is that a smaller mark is successively arranged on a larger mark. When the center of the larger mark agrees with the center of the smaller mark, it is sufficient to form only a single pattern for the optical microscope. As for another embodiment, even in an ion beam apparatus for the observation, analysis and work of the fine patterns by using ion beams, in case of using an optical microscope in addition to the ion beam optical system, the present invention similarly enables the precise calibration and positioning of the distance between the optical axis of the ion beam optical system and the optical axis of the optical microscope. As for another embodiment different from above described embodiments, a scanning electron microscope and an electron-beam exposure apparatus having 2 or more electronic optical systems exist. Those are apparatuses for the observation, or measurement and work of the specimen by 2 or more electronic optical systems. In FIG.4, one embodiment of the electron-beam exposure apparatus having two electronic optical systems is shown. In this apparatus, it is required to keep the position relation between a couple of electronic optical systems 41 and 42 in higher precision. In order to keep precisely the distance S between the electronic optical systems 41 and 42, two fiducial patterns 44 and 45 based on the present invention which has a distance Sp equivalent to the distance between electronic optical systems are formed. By using the fiducial pattern, the mutual distance between two electronic optical systems can be maintained in high precision. In the scanning electron microscope having two electronic optical systems, in case of observing and measuring the patterns, the detection of identical patterns can be performed in high precision and high speed. In FIG.4, the component 46 is a specimen holder, the components 47 and 48 are secondary electron detectors, the components 49 and 50 are bias coils, the components 55 to 58 are electron lenses, the component 52 is a high voltage power unit used for the electron gun, the component 51 is an X-Y stage drive mechanism, the component 53 is a control system, and the component 54 is an image processing circuit, where like parts shown in FIG.1 have substantially the same functions. As seen from the above description, in the scanning electron microscope having an optical microscope, the position relation between the optical microscope and the electron microscope can be maintained in high precision, and the pattern observed and measured by the optical microscope can be positioned to its corresponding visible scopes with high magnification for the electronic optical system in high precision and high speed. As a result, in observing a designated pattern and measuring its size, though much time has been spent for exploring an optimal pattern by shifting up the magnifications in many steps in a conventional method, the present invention enables to make the optimal positioning search time for a single pattern at most 10 seconds in contrast to the conventional search time from 15 to 20 seconds, and therefore, the positioning operation can be performed very quickly. The similar effect can be also brought to the electron-beam exposure apparatus and the ion beam apparatus, each having an optical microscope. On the other hand, the relative position between a couple of patterns, each formed at the same time by using an electron beam exposure apparatus having two electronic optical systems, can be determined in high precision. The similar effect can be also brought to the ion beam exposure apparatus.

Claims: A charged particle beam apparatus comprising a charged particle beam optical system (3; 41) including means (4) for generating a charged particle beam and means (9; 49) for deflecting the beam onto a specimen, a second optical system (5; 42) for irradiating said specimen, first and second fiducial patterns (10, 11; 44, 45), and means (5, 18; 47, 48) for separately detecting the positions of said first and second fiducial patterns (10, 11; 44, 45) by using said charged particle beam optical system (3; 41) and said second optical system (5; 42), respectively, and means for calibrating, on the basis of the positions detected, the distance (L; S) between the optical axes of said charged particle beam optical system (3; 41) and said second optical system (5; 42), characterised in that said fiducial patterns (10, 11; 44, 45) are provided on said specimen or on a stage (2) mounting the specimen. The apparatus of claim 1, wherein said second optical system is another charged particle beam optical system (42) including a charged particle source for generating a second charged particle beam and means (50) for directing the beam onto said specimen. The apparatus of claim 1 or 2, wherein said fiducial patterns (10, 11; 44, 45) are formed on the same member (8) of said specimen stage (2). The apparatus of any preceding claim, wherein said position detecting means measures said fiducial patterns (10, 11; 44, 45) at a same time. The apparatus of any preceding claim, wherein said calibrating means adjusts the position of the optical axis of said charged particle beam optical system (3; 41) by shifting said position by an offset value obtained on the basis of the position of the optical axis of said second optical system (5; 42). The apparatus of any preceding claim, wherein said calibrating means adjusts the position of the optical axis of said second optical system (5; 42) by shifting said position by an offset value obtained on the basis of the position of the optical axis of said charged particle beam optical system (3; 41). The apparatus of any preceding claim, comprising display means (31) having a first display panel (32) for displaying an image of said first fiducial pattern (11; 44) obtained by using said second optical system (5; 42) and a second display panel (33) for displaying an image of said second fiducial pattern (10; 45) obtained by using said charged particle beam optical system (3; 41), both patterns being displayed at a same time. The apparatus of preceding claim, wherein one of said fiducial patterns (10, 11; 44, 45) is formed in a plurality of differently sized patterns, and the other fiducial pattern is spaced from said plurality of differently sized patterns. The apparatus of claim 8, wherein said differently sized patterns are formed so as to have dimensions respectively matching measuring magnifications of said charged particle beam apparatus.

Inventors: KUROSAKI TOSHIEI, KUROSAKI, TOSHIEI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1996
Main classification: H01J37
IPC codes: H01J37, H01L21