JP2010092632A - Adjustment method of charged particle beam and charged particle beam device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a focus adjustment method of charged particle beams, and a charged particle beam device, capable of realizing proper focus adjustment, even in carrying out restraint of shrink due to beam irradiation, and measurement of a three-dimensional structure. <P>SOLUTION: A focus adjustment method of arranging a beam scanning position for obtaining signals for focus adjustment differently from a beam scanning position for obtaining signals for pattern measurement in the same visual field region, as well as the charged particle beam device are proposed. As a more concrete mode, a focus adjustment method of carrying out focus adjustment as well as pattern measurement by alternately arranging (each other, or each several) beam scanning position(s) for obtaining signals for focus adjustment and for obtaining signals for pattern measurement in the same FOV, and the charged particle beam device are proposed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a charged particle beam apparatus, and more particularly to an adjustment method for adjusting a charged particle beam and a charged particle beam apparatus provided with an adjusting means.

  In recent years, semiconductor microfabrication technology has progressed, and a resist (ArF resist) that is sensitive to argon fluoride (ArF) excimer laser light is being used as a photosensitive material for photolithography. However, it has been known that when a pattern formed on the resist is irradiated with an electron beam, the pattern may shrink (shrink).

  Therefore, efforts are made to minimize shrinkage by optimizing the measurement conditions (acceleration voltage, probe current, etc.) on the scanning electron microscope side for measuring the dimension of the pattern. However, since the signal amount is reduced by lowering the acceleration voltage and the probe current, high length measurement accuracy cannot be maintained. In other words, we are facing the problem that it is very difficult to achieve both shrink suppression and high-precision measurement. As a technique for solving these problems, Patent Document 1 discloses a technique for suppressing shrinkage by expanding the electron beam irradiation region in the Y direction with respect to the X direction to lower the electron beam irradiation density. .

  Further, in Patent Document 2, autofocus is executed at a location different from the measurement target pattern, and the focus condition is applied as a focus condition at the time of scanning of the measurement target pattern. A technique for preventing beam scanning is disclosed.

JP 2007-3535 A JP 2005-327578 A

  In recent years, with the high integration and miniaturization of semiconductor elements, a wide variety of patterns are formed on a sample (for example, a semiconductor wafer or a photomask), and the demand for high-precision measurement for these patterns is increasing. As an example, there are devices such as the ArF resist described above and a three-dimensional transistor (for example, a Fin-Field Effect Transistor: Fin-FET). Since the Fin-FET has a three-dimensional structure, the focus adjustment when measuring the three-dimensional structure requires a very high accuracy as compared with the case of a transistor having a planar structure.

  Patent Document 1 does not mention anything about a focus adjustment method when measuring a pattern such as a Fin-FET. Even in the case of a pattern such as an ArF resist, the aspect ratio of the magnification changes, so that it cannot be applied when it is desired to match the aspect ratio.

  Further, in the focus adjustment method at a location (off-site) different from the measurement target pattern disclosed in Patent Document 2, for example, the focus condition may greatly change between the measurement target pattern having a three-dimensional structure and off-site. For this reason, it may not be possible to perform proper focus adjustment on the measurement target.

  Further, when the pattern formed on the sample is an insulator, the trajectory of the electron beam may be bent due to the adhesion of the charge by electron beam irradiation. For example, in order to perform auto-focusing off-site, when the field of view (FOV) is moved to the off-site by image shift, the FOV to the off-site or measurement target pattern is caused by charging adhesion by beam irradiation. In some cases, it is difficult to perform proper alignment.

  In the following, it is possible to perform appropriate beam irradiation condition adjustment while realizing suppression of shrinkage by beam irradiation, or to perform appropriate beam irradiation condition adjustment even when measuring a three-dimensional structure. A target charged particle beam adjustment method and a charged particle beam apparatus will be described.

  In order to achieve the above object, an adjustment method in which a beam scanning position for obtaining an image for beam evaluation and a beam scanning position for obtaining a pattern measurement signal are arranged differently in the same visual field region, and A charged particle beam system is proposed.

  As a more specific aspect, in the same FOV, the beam scanning position for obtaining the focus adjustment signal and the beam scanning position for obtaining the pattern measurement signal are alternately (every one or plural). A focus adjustment method and a charged particle beam apparatus for performing focus adjustment and pattern measurement are proposed.

  According to the above configuration, it is possible to perform appropriate beam irradiation condition adjustment while realizing suppression of shrinkage by beam irradiation, or to perform appropriate beam irradiation condition adjustment even when measuring a three-dimensional structure. Can be done.

  Hereinafter, an embodiment in which beam scanning for focus adjustment and beam scanning for measurement are performed in the same region will be described. Between the beam scanning for focus adjustment and the beam scanning for measurement, the process of moving the FOV position itself (center position of FOV) such as image shift and stage movement is not performed, and the position of the scanning line Is selectively changed.

  The scanning position of the focus adjustment beam and the scanning position of the measurement beam are set so as not to overlap.

  Hereinafter, an example of a scanning electron microscope that performs focus adjustment beam scanning and measurement beam scanning in the same region will be described with reference to the drawings. In the following description, a scanning electron microscope (SEM), which is a kind of charged particle beam apparatus, will be described as an example. However, the present invention is not limited to this, and a focused ion beam (Focused) that scans an ion beam is described. Application to an Ion Beam (FIB) apparatus is also possible. In the case of the FIB apparatus, it is possible to realize reduction of sample damage due to sputtering of the sample by beam scanning for focus adjustment.

  FIG. 1 is a schematic configuration diagram of a scanning electron microscope. The primary electron beam 4 emitted from the cathode 1 is controlled and accelerated by the anode 2, and a sample on which a pattern to be measured is formed such as a semiconductor wafer held on the sample stage 9 by the condenser lens 3 and the objective lens 6. 8 is converged and irradiated.

  These controls are performed by the control arithmetic unit 17. A deflector 5 is provided in the path of the primary electron beam 4, and a predetermined deflection current is supplied from the deflection coil control power source 12 in accordance with an arbitrary set magnification set from the input device 19, thereby the primary electron beam. 4 is deflected to scan the surface of the sample 8 two-dimensionally. The movement to the measuring point is configured such that the stage control 15 can move in at least two directions (X direction and Y direction) in a plane perpendicular to the primary electron beam 4 by controlling the sample stage 9. It can be moved by controlling the deflection current from the deflection coil control power supply 12.

  Secondary electrons 7 generated by irradiating the sample with an electron beam are detected by the secondary electron detector 16, amplified by the signal amplifier 13, and stored in the image processing unit 21. Then, using the stored image, length measurement is performed in the length measurement unit of the image processing unit 21. The image signal at this time is displayed on the SEM image display device 18.

  This scanning electron microscope has a scanning control unit 20 and an image processing unit 21. The scanning control unit 20 performs control for scanning an electron beam for forming an autofocus image with respect to the visual field including the dimension measuring region between the scanning lines for dimension measuring. The image processing unit 21 is divided into an AF (Auto Focus) unit and a length measuring unit. The AF unit focuses using the secondary electrons 7 obtained when the sample 8 is scanned with the electron beam 23 for autofocus. The adjustment is performed, and the length measurement unit measures the length using the secondary electrons 7 obtained when the sample 8 is scanned with the length measurement electron beam 22. The scanning control unit 20 and the image processing unit 21 may be provided outside the control arithmetic device 17 or may be realized by software of the control arithmetic device 17.

  Autofocus is performed by evaluating the focus evaluation value of an image, for example. More specifically, a focus evaluation value is calculated and evaluated from detection signals of secondary electrons and reflected electrons obtained by scanning a plurality of frames while changing the conditions of the objective lens 6, and the optimum value is determined as an electronic lens. It is set to the conditions.

  Here, the focus evaluation value uses a differential value between pixels, etc. When the sum of the differential values is calculated for each frame taken while changing the conditions of the electronic lens, and the value becomes the largest The condition of the electron lens is the focused condition. In addition, it is possible to adjust the focus by adjusting the negative voltage (retarding voltage) applied to the sample. Based on a plurality of images acquired when the retarding voltage is changed, an appropriate value can be obtained. A focus condition may be selected.

  A beam scanning method for obtaining a focus adjustment signal will be described with reference to FIG. FIG. 2A shows a general scanning method at the time of image acquisition. The electron beam trajectory 23 for autofocus and the electron beam trajectory 22 for length measurement are not distinguished, and the electron beam is scanned continuously in a monotonous manner. .

  FIG. 2B is a diagram illustrating an example in which the beam scanning line position for focus adjustment and the beam scanning line position for measurement are shifted and the positions are alternately arranged. The electron beam trajectory 22 for length measurement and the electron beam trajectory 23 for autofocus are alternately arranged. Although the positions of the scanning lines are alternately arranged, the scanning order is such that the focus adjustment beam is scanned first, the focus adjustment is performed, and then the measurement beam scan is performed.

  FIG. 2C is a diagram for explaining an example in which the ratio of the electron beam trajectory 22 for length measurement to the electron beam trajectory 23 for autofocus is 2: 1 in one FOV. In the example of FIG. 2C, the electron beam trajectory 22 for length measurement and the electron beam trajectory 23 for autofocus are arranged in the order of two, one, two, one ... However, the electron beam trajectories may be arranged at other ratios. However, since autofocus calculates the focus evaluation value based on the sharpness of the image on which the pattern is formed, the autofocus beam has a scanning line density capable of forming an image sufficient for the sharpness evaluation, for example. Need to be scanned.

  In the example of FIG. 2D, the electron beam trajectory 23 for autofocus is arranged in the upper region of the FOV, and the electron beam trajectory for measurement is arranged in the lower region of the FOV. In the examples of FIGS. 2B to 2D described above, FIGS. 2B and 2C are selected in that focusing can be performed in a form that most closely matches the situation of the measurement target region. It is desirable to do. In the case of the example in FIG. 2D, since the focusing area (upper area) and the measurement area (lower area) are different, the pattern formation status of the upper area and the lower area is the same (for example, the upper pattern). And the lower pattern may be selectively used in a case where the pattern is mirror-symmetrical with respect to the boundary between the upper region and the lower region. If the upper pattern state and the lower pattern configuration are the same, the focus condition can be expected to be almost the same, so that the focus condition performed on one side can be applied to the other.

  In the examples of FIGS. 2B and 2C, the region where the measurement beam is scanned and the region where the focus adjustment beam is scanned are substantially the same, and the beam scanning trajectories of the two are different. Therefore, it is possible to detect the focus condition for the measurement pattern with high accuracy while suppressing shrinkage.

  Further, referring to the design data of the measurement location, the focus adjustment beam was scanned by scanning the focus adjustment beam in the region where the beam scan is performed under the focus condition equivalent to the measurement target pattern, although it is not the measurement target. Later, a measurement beam may be scanned at a measurement target location.

  For example, when the measurement target is a Fin-FET, the following focus adjustment method can be considered. First, in the Fin-FET, a gate electrode is formed on a thin strip portion called a fin so as to straddle the fin. Since the fins cut into strips become channels, the channel width that determines the current driving force is determined by the height of the fins and is an important evaluation item in the measuring apparatus. Fin width and gate length are also important measurement items.

  A three-dimensional structural element such as a Fin-FET has a larger step in the depth direction (Z direction) in the FOV than a planar device. Therefore, when focusing on a certain part of the device, focusing on another part at a height different from that part may not be achieved. On the other hand, if only a certain part of the Fin-FET is to be measured, focus adjustment is performed in a region having a height (and / or charging condition) equivalent to that part and different from the part to be measured. It is preferable to selectively perform beam scanning for acquiring a work image and perform focus adjustment.

  More specifically, for example, in a scanning electron microscope for length measurement (Critical Dimension-SEM: CD-SEM), a cursor for selecting an area to be used for measurement called a length measurement box may be selected. For example, based on the position information of the cursor, a part having the same height (and / or charging condition) is automatically selected from the design data and automatically registered in a recipe for setting the operating condition of the CD-SEM. You may do it. Alternatively, an area having height information equivalent to that of the cursor may be displayed on the screen, and the operator may select an arbitrary area as the focus adjustment visual field.

  As described above, the scanning deflector is controlled to scan the sample 8 so that the autofocusing electron beam 23 and the length measuring electron beam 22 do not overlap, thereby suppressing damage to the sample. Therefore, proper focus adjustment can be performed.

  The total number of the electron beam 23 for autofocus and the electron beam 22 for length measurement is not fixed, and can be arbitrarily set to 512 or 1024. In the embodiment shown in FIGS. 2A, 2B, 2C, and 2D, the case where the aspect ratio of the scanning of the electron beam to be two-dimensionally scanned is 1: 1, but 1: 1. There is no necessity and scanning may be performed at different ratios. In order to reduce sample damage due to electron beam irradiation during length measurement, the energy of the electron beam 23 for autofocus and the electron beam 22 for length measurement is changed. Also good.

  According to the above-described embodiment, by shifting the scanning position of the electron beam between autofocus and length measurement while keeping the same scanning area, it is possible to achieve both accurate autofocus and shrink suppression suitable for the length measurement point. Can measure length with high reliability and high accuracy.

  Next, the relationship between the number of scanning lines of the measuring electron beam 22 and repeated length measurement accuracy will be described. In the scanning electron microscope, length measurement is performed from the line profile of the secondary electron signal having the unevenness information of the sample. Therefore, the S / N of the line profile at this time affects the repeated measurement accuracy, and the repeated measurement accuracy decreases as the S / N decreases. Therefore, in order to improve the length measurement accuracy repeatedly, it is necessary to improve the S / N, and the S / N is improved by statistical processing and an increase in the signal amount.

  The former is a spatial process in which a plurality of scanning lines are overlapped by using the fact that the noise positions of the respective line profiles are different, and further smoothing (smoothing) by moving average is performed, and an image of the secondary electrons 7. It is performed by a time process that accumulates.

  The latter is improved by increasing the signal amount by increasing the acceleration voltage and the probe current. FIG. 3 is a diagram for explaining the relationship between the number of scanning lines to be overlapped by the electron beam 22 for length measurement and repeated length measurement accuracy. The length measurement accuracy decreases as the number of superposed scanning lines increases. However, it becomes flat when the number of scanning lines reaches an S / N more than necessary. The number (n) of scanning lines of the electron beam 22 for length measurement and the repeated length measurement accuracy (s) are expressed by the equation (1).

s = (1 / √n) × k (k: coefficient) (1)
The required number of scanning lines (n) of the electron beam 22 for length measurement can be set by repeatedly setting the length measurement accuracy (s) from the equation (1).

  FIG. 4 is a diagram for explaining an algorithm for determining the number of scanning lines of an electron beam based on the length measurement accuracy. First, the necessary repeated measurement accuracy (s) is set from the input device 19. Next, it is sufficient to satisfy the repetition accuracy (s) by obtaining the coefficient (k) of the equation (1) from the measured value of the repeated length measurement accuracy (s1) when scanning with the number of scanning lines (n1). The number of scanning lines (n2) of the electron beam 22 for length measurement is calculated.

  As described above, in this embodiment, an arithmetic function for determining the number of scanning lines (n) required by the scanning control unit 20 by repeatedly inputting the length measurement accuracy (s) from the input device 19 is provided.

  Here, the shrinkage of the ArF resist caused by the interaction with the primary electron beam 4 scanning on the sample 8 will be described with reference to FIG. The figure represents the surface of the ArF resist pattern 24. In the portion irradiated with the primary electron beam 4 of the ArF resist pattern 24, shrinkage occurs in a semicircular shape as shown in (a). When the interval between the scanning lines of the primary electron beam 4 becomes narrow, such as at high magnification, the semicircular shrinks overlap as shown in (b), and the shrinking of the overlapping portions further proceeds. .

  In order to reduce the shrinkage of the ArF resist pattern 24 caused by the irradiation of the primary electron beam 4, a method of reducing the length measurement magnification was adopted in order to reduce the electron dose or increase the scanning interval of the electron beam. There was a problem that the length measurement accuracy repeatedly deteriorated by lowering the magnification. Prior to length measurement, an ArF resist pattern 24 must be irradiated with an electron beam for adjustment such as autofocus, and the primary electron beam 4 in FIG. In other words, the electron beam 22 can be said.

  Accordingly, when the electron beam is irradiated onto the ArF resist pattern 24 without controlling the electron beam 22 for length measurement and the electron beam 23 for autofocus as shown in (c), the primary electron is doubled at the same location. Irradiation of the line 4 will cause shrinkage to proceed. Therefore, as in the example shown in (d), an electron beam for forming an adjustment image is scanned between the scanning lines for dimension measurement with respect to the visual field including the dimension region, and the scanning lines are scanned. Scanning so that the scanning lines for length measurement do not overlap has great significance for suppressing shrinkage.

  The electron beam for forming the image for adjustment is not limited to autofocus, but may be an auto stigma or an alignment such as an axis adjustment.

  Next, an embodiment in which the secondary electron 7 images are integrated will be described with reference to FIG. The scanning control unit 20 of the scanning electron microscope scans the length measurement object 25 on the sample 8 with the primary electron beam 4 according to the image registration number set from the input device 19 so that the respective scanning regions 26 do not overlap. be able to. At this time, it is displayed on the SEM image display device 18 as one superimposed image. According to such a scanning method, since electron beam scanning for acquiring a plurality of images for repeatedly improving length measurement accuracy is not performed at the same location, shrinkage can be suppressed.

The schematic block diagram of a scanning electron microscope. The figure explaining an example of the scanning method of the electron beam for acquiring the visual field for preparation. The figure explaining the relationship between the scanning line number of an electron beam, and length measurement precision. The figure explaining the algorithm which determines the scanning line number of an electron beam based on length measurement precision. The figure explaining the shrinkage | shrink which arises when an electron beam is scanned to an ArF resist. The figure explaining the Example which integrate | accumulates the secondary electron image obtained in a different area | region.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cathode 2 Anode 3 Condenser lens 4 Primary electron beam 5 Deflector 6 Objective lens 7 Secondary electron 8 Sample 9 Sample stage 10 High voltage control power supply 11 Converging lens control power supply 12 Deflection coil control power supply 13 Signal amplifier 14 Objective lens control power supply 15 Stage control 16 Secondary electron detector 17 Control operation device 18 SEM image display device 19 Input device 20 Scan control unit 21 Image processing unit 22 Electron beam trajectory 23 for length measurement Electron beam trajectory 24 for autofocus ArF resist pattern 25 Measurement Long object 26 Scanning area

Claims (5)

A charged particle source;
A lens that converges the charged particle beam emitted from the charged particle source;
A scanning deflector for scanning the charged particle beam on a sample;
A detector for detecting charged particles emitted from the sample by scanning the charged particle beam;
A charged particle beam comprising a control device that evaluates an image obtained based on the charged particle beam scanning based on the charged particle detected by the detector and adjusts the irradiation condition of the charged particle based on the evaluation. In the device
The control device includes a scanning line for obtaining information for evaluating the image and a scanning line for measuring a pattern on the sample in the same visual field region of the charged particle beam scanned by the scanning deflector. A charged particle beam device characterized by being arranged at different positions. In claim 1,
The control device is characterized in that scanning lines for obtaining information for evaluating the image and scanning lines for measuring a pattern on the sample are alternately arranged for each one or a plurality of scanning lines. Charged particle beam device. In claim 1,
The control device arranges a scanning line for obtaining information for evaluating the image at a position different from the measurement location based on the setting of the measurement location of the pattern on the sample. Wire device. In claim 3,
The control device refers to the design data of the sample, selects a region having the same height as the measurement location, and arranges a scanning line for obtaining information for evaluating the image in the selected region. A charged particle beam device. In an adjustment method of a charged particle beam apparatus that evaluates an image obtained by scanning charged particles emitted from a charged particle source and adjusts irradiation conditions of the charged particles based on the evaluation,
A scanning line for obtaining information for evaluating the image and a scanning line for measuring a pattern on the sample are arranged at different positions in the same visual field region of the charged particle beam. Method for adjusting charged particle beam apparatus.

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