1. Field of the Invention
The present invention relates generally to a pattern dimension measuring system and a pattern dimension measuring method. More specifically, the invention relates to a system and method for measuring the dimensions of a pattern formed on the surface of a sample, while moving a stage, on which the sample is mounted.
2. Description of the Prior Art
In recent years, pattern dimension measuring systems are widely utilized for measuring the dimensions of a pattern formed on the surface of a semiconductor device, such as a very large scale integration (VLSI).
Referring to the accompanying drawings, a conventional pattern dimension measuring system will be described below. Furthermore, in the following drawings, the same reference numbers are assigned to the same portions, and the descriptions thereof are suitably omitted.
FIG. 1 is a schematic block diagram showing an example of a conventional pattern dimension measuring system. In this figure, a pattern dimension measuring system 110 comprises an electron beam lens-column 111, a vacuum sample chamber 2 and a host computer 104.
The electron beam lens-column 111 includes an electron gun part 11 and an electron lens system, and has a resolving power of about 5 nm corresponding to the scale down of semiconductor devices. The electron gun part 11 is designed to irradiate a sample 5 with electron beam 96. The electron lens system has a condenser lens 21, a deflecting lens 102 and an objective lens 103. The electron lens system is designed to control the trajectory and focal length of the electron beam 96 so that the electron beam 96 focuses on the sample 5.
The vacuum sample chamber 2 houses an X-Y stage 3, a sample conveyance system 12 and a secondary electron detector 31 in vacuum atmosphere. The sample conveyance system 12 is designed to convey a sample 5, such as a semiconductor wafer, to the X-Y stage 3. The X-Y stage 3 is designed to support the conveyed wafer 5 (sample) on the upper surface thereof, and to move in an optional direction on the X-Y plane with a high stopping accuracy of about 1 xcexcm on the basis of a control signal supplied from a stage control part 113. The secondary electron detector 31 is designed to detect secondary electrons, reflected electrons and back scattered electrons (which will be hereinafter referred to as xe2x80x9csecondary electrons and so forthxe2x80x9d), which are emitted from the surface of the sample 5 irradiated with the electron beam 96, to supply the detected results to an image data processing part 132. The image data processing part 132 is designed to receive the detected results of the secondary electron detector 31 to supply image data, which form a SEM image by a predetermined data processing, to the host computer 104. The host computer 104 has a pattern dimension calculating part 16 for calculating the dimensions of a target pattern on the basis of the image data, which are fed from the image data processing part 132, to suitably store the calculated results in a memory 14.
Referring to FIG. 2, an example of a sequence for measuring the dimensions of a pattern, which is formed on the surface of the wafer 5 using the pattern dimension measuring system 110 shown in FIG. 1, will be described below. FIG. 2 is a schematic diagram showing the moving direction of the X-Y stage 3. In this example, the stage 3 is designed to move from a measurement start position Ps to a measurement end position Pe while drawing a locus shown by the dotted line in FIG. 2.
First, by the sample conveyance system 12, the wafer 5 is conveyed into the vacuum sample chamber 2 to be mounted on the upper surface of the X-Y stage 3.
Then, global alignment marks {circle around (1)} and {circle around (2)}, which are formed on the surface of the wafer 5 at substantially center and peripheral portion thereof, respectively, are used to carry out the global alignment to calculate a correlation between a pattern layout coordinate system and a stage coordinate system on the wafer 5.
Then, the stage 3 is moved so that the position of a target pattern to be measured, e.g., the vicinity of pattern {circle around (3)} shown in FIG. 2, is a position irradiated with the electron beam 96, and stopped at this position. Then, the exciting current of the objective lens 103 is controlled so that the edges of the target pattern are within a beam focal depth by the automatic focus. Then, while the stage 3 is moved again in the direction of the dotted line arrow in FIG. 2, the electron beam 96 is scanned on the pattern {circle around (3)} to detect secondary electrons and so forth, which are emitted from the surface of the wafer 5, by unit of the secondary electron detector 31. The detected signal is data-processed by the image data processing part 132 to be inputted to the host computer 104 as an image signal constituting a SEM image. The host computer 104 detects the target pattern {circle around (3)} existing in the SEM image by the pattern recognition processing. The pattern dimension calculating part 16 in the host computer 104 detects the bottom edges of the detected target pattern {circle around (3)} on the basis of the optimum measuring algorithm to measure the dimensions of the pattern. Moreover, if the next target pattern ({circle around (4)}-{circle around (7)}) exists, the X-Y stage 3 is moved again toward the next target pattern to be stopped again in the vicinity thereof, and then, the above described operations are repeated. Such a series of operations are controlled by the host computer 104 in accordance with a sequence which is set by a recipe file stored in the memory 14 of the pattern dimension measuring system or the like.
However, in the above described sequence, the measurement of the dimensions is carried out by repeating the movement and stopping of the X-Y stage 3 any number of times, so that it takes a lot of processing time in the case of a multipoint measurement for measuring the dimensions of patterns at a large number of measuring places.
FIGS. 3A and 3B are graphs for explaining the throughput, of the pattern dimension measuring system shown in FIG. 1, and show the variation in stage traveling speed. It can be also understood from FIGS. 3A and 3B that the stage 3 is stopped in front of the global alignment mark and the pattern to be measured for focusing (AF) and pattern recognition (PM).
Particularly in recent years, the need for multipoint measurement is enhanced (a) at the initial stage of the development of process devices, (b) in the evaluation of the lens aberration of an aligner and in the evaluation of a wafer for making exposure conditions, and (c) due to the increase of the number of measured points as the increase of the diameter of the wafer. However hand, the throughput in the above described sequence is 30 wafers/hour to 40 wafers/hour, so that there is a problem in that it takes several hours to carry out a multipoint measurement even with a full automatic measurement in the present circumstances.
It is a first object of the present invention to provide a pattern dimension measuring system with a high throughput.
It is a second object of the present invention to provide a pattern dimension measuring method with a high throughput.
According to the first aspect of the present invention, there is provided; a pattern dimension measuring system comprising: a movable stage for mounting a sample on the upper surface thereof, the sample having a pattern to be measured formed thereon; a first control unit for moving the stage; an electron beam irradiation unit for irradiating the sample with an electron beam; an electron beam deflecting/scanning unit for deflecting and scanning the electron beam in a region on the sample, the region including a first portion normally scanned with the electron beam along and around an outgoing beam axis, and a second portion outside the first portion, the second portion being scanned with the electron beam deflected apart from and in parallel to the outgoing beam axis and; a stage coordinate detecting unit for detecting X-Y coordinates of the stage; a secondary electron detecting unit for detecting a secondary electron and a deflected electron which are emitted from the sample by an irradiation with the electron beam and for outputting an image signal which forms an electron image, the electron image representing a shape of the surface of the sample; a pattern dimension calculating unit for obtaining a dimension of the pattern to be measured by recognizing side edges thereof using the image signal and by calculating the dimension thereof; and a second control unit for controlling the first control unit so that the stage continuously moves without stopping and for controlling the electron beam deflecting/scanning unit using the detected result of the stage coordinate detecting unit so that the electron beam is scanned while a scanning start position thereof moves in synchronism with movement of the stage.
According to the present invention, due to a continuous movement of the stage in measurement, it is possible to measure the dimensions of a pattern on a sample with a high throughput.
In a preferred embodiment of the pattern dimension measuring system according to the invention, the pattern dimension measuring system further comprises a focal length measuring unit for detecting a focal length of the electron beam deflecting/scanning unit, and, the second control unit receives the detected result of the focal length measuring unit and optimizes the focal length of the electron beam deflecting/scanning unit on the basis thereof while the stage moves continuously at a constant velocity.
It is preferable that the electron beam deflecting/scanning unit scans each frame to which the pattern to be measured is divided, the frame being defined by the maximum deflection width thereof, and that any one of a continuous scanning mode, in which the plurality of frames are continuously scanned, and a frame accumulating mode, in which the same frame is scanned a plurality of time to output the optimum pattern dimensions, is able to be selected.
The region on the sample preferably further includes a third portion where the image signal is to be acquired and a fourth portion where an irradiation with the electron beam is to be stopped, and the second control unit preferably supplies a control signal to the first control unit so that the stage moves at a first velocity in a third portion of the region and moves at a second velocity which is higher than the first velocity in a fourth portion of the region.
Furthermore, the second control unit sets the second velocity on the basis of a correlation between a distance between the patterns to be measured and on the basis of a processing time required for recognizing a pattern by the pattern dimension calculating unit.
In another preferred embodiment of the invention, the pattern dimension measuring system further comprises image processing unit for processing the image signal so that the electron image is a mirror image with respect to the central axis-in X or Y directions in accordance with a variation in-moving direction of the stage.
According to the second aspect of the invention, there is provided; a pattern dimension measuring method using a pattern dimension measuring system comprising: a movable stage for mounting thereon a sample, on which a pattern to be measured is formed; a stage coordinate detecting unit for detecting X-Y coordinates of the stage; a electron beam irradiation unit for irradiating the sample with an electron beam; an electron beam deflecting/scanning unit for deflecting and scanning the electron beam on the sample; a focal length measuring unit for detecting a focal length of the electron beam deflecting/scanning unit; a secondary electron detecting unit for detecting a secondary electron and a deflected electron which are emitted from the sample by the irradiation with the electron beam and for outputting an image signal which forms an electron image; and a pattern dimension calculating unit for calculating a dimension of the pattern by recognizing side edges of the pattern using the image signal and by calculating the dimension of the pattern, the pattern dimension measuring method comprising: a first step of calculating a correlation between a coordinate system of the stage and a pattern layout coordinate system; a second step of detecting the position of a pattern to be measured on the basis of the correlation while the stage moves-continuously without stopping; a third step of detecting the variation in distance between the sample and the electron beam irradiation unit due to movement of the stage, and optimizing the focal length of the electron beam deflecting/scanning unit before the pattern to be measured reaches an area in which the electron beam can be scanned; a fourth step of acquiring an electron image of the pattern to be measured, by deflecting the electron beam in synchronism with the movement of the stage, and by scanning the electron beam in a region including a region, the region including a first portion normally scanned with the electron beam along and around an outgoing beam axis, and a second portion outside the first portion, the second portion being scanned with the electron beam deflected apart from and in parallel to the outgoing beam axis; and a fifth step of recognizing the pattern to be measured, using the electron image, and calculating the dimensions thereof by a predetermined algorithm.
According to a pattern dimension measuring method of the present invention, the detection of the position of a pattern to be measured and the adaptation of the focus of electron beam is processed in real time even during the movement of a stage, so that it is not required to stop the stage before incorporating a SEM image. Therefore, it is possible to measure the dimensions of a pattern while moving the stage at a constant velocity.
In addition, since the scanning range of the electron beam can be enlarged to a range where all of beam trajectories are away from the outgoing beam axis, it is possible to scan electron beam on a pattern to be measured, in synchronism with the constant velocity movement of the stage. Thus, it is possible to further increase the traveling speed of the stage, so that it is possible to improve the throughput in the pattern dimension measurement. On the other hand, when the traveling speed of the stage remains being relatively low, it is possible to scan electron beam on the same pattern to be measured, several times. Thus, it is possible to further enhance the accuracy of the dimension measurement.
In a preferred embodiment of the invention, the pattern includes a reference pattern serving as a reference of the detection of the pattern to be measured, the correlation includes a relationship between a position of the reference pattern and a positions of the pattern to be measured, and the second step includes a step of detecting the position of the reference pattern on the basis of the correlation, and detecting the position of the pattern to be measured on the basis of the detected position of the reference pattern and the relationship between the detected position of the reference pattern and the position of the pattern to be measured.
Furthermore, the fourth step may include a step of dividing the pattern to be measured to a plurality of frames, the frame being defined by the maximum deflection width of the electron beam deflecting/scanning unit, and continuously scanning the plurality of frames (a continuous scanning step) or alternatively, scanning the same frame a plurality of times and outputting an optimum pattern dimension (a frame integrating step).
Thus, when a continuous scanning procedure is selected, it is possible to improve the throughput in the pattern dimension measurement. On the other hand, when a frame accumulating procedure is selected, it is possible to improve S/N.
In the above mentioned pattern dimension measuring method, the region on the sample may include a third portion where the image signal is to be acquired and a fourth portion where no electron beam is to be irradiated, and the stage may be moved at a first constant velocity in the third region on the sample and at a second constant velocity which is higher than the first constant velocity in the fourth portion of the region after an irradiation of the electron beam is stopped and before an irradiation with the electron beam is restarted.
Thus, it is possible to enhance the throughput in the dimension measurement.
The second velocity may be set on the basis of a correlation between a distance between the patterns to be measured and on the basis of a processing time required for recognizing a pattern by the pattern dimension calculating unit.
Thus, in accordance with the specification of a measuring system, to which a pattern dimension measuring method according to the present invention is applied, it is possible to complete a dimension measurement processing until the acquisition of the electron image of the next pattern to be measured is started after the acquisition of the electron image of a certain pattern to be measured.
The fourth step may include a step of processing the image signal so that the electron image is a mirror image with respect to the central axis in X or Y directions in accordance with a variation in moving direction of the stage.
Thus, it is possible to reduce the time required to carry out the above described pattern recognition and dimension calculating processing, and it is possible to further improve the throughput in the dimension measurement.
The second step may be a step of detecting a position of the pattern to be measured, at a first measuring magnification, and the fourth step may be a step of acquiring the electron image at a second measuring magnification which is greater than the first measuring magnification.