JP2014143031A - Charged particle beam device and sample observation method - Google Patents

Abstract

In order to observe the pattern shape of the bottom and lower layers such as holes and trenches, there is a technique of increasing the acceleration voltage. However, when a multilayer film sample is observed under a high acceleration condition, the pattern appears to penetrate to the lower layer, so the focus position misdetects the pattern of another layer, and the focus position can be adjusted to the target layer. There may not be. Therefore, there is a need for a method for stably focusing on a layer to be imaged in observation of a high acceleration voltage.
A second acceleration voltage condition higher than the first acceleration voltage condition is obtained by obtaining a focus position on the sample surface under the first acceleration voltage condition and adding a predetermined offset amount to the focus position. Find the in-focus position.
[Selection] Figure 5

Description

  The present invention relates to a charged particle beam apparatus that inspects, observes, and measures a sample from an image acquired using a charged particle beam. In particular, it relates to focus adjustment of a charged particle beam apparatus.

  In recent years, the pattern line width of semiconductor devices has been miniaturized from 2X nm to beyond, but pattern miniaturization, which has been a driving force for higher speed and higher integration, is approaching its limit. As one measure, development of a three-dimensional structure device by three-dimensionalization has been advanced. Since this three-dimensional structure device has a laminated structure, the layer becomes thicker than before, and the aspect ratio tends to be further increased in processes such as holes and trenches. Therefore, in the process development of such a three-dimensional structure, an inspection / measurement means capable of judging whether the quality of a wafer is good or more becomes more important.

  In a semiconductor device manufacturing factory, a scanning electron microscope is used to manage the quality of wafers for process processing. So far, scanning electron microscopes have improved resolution in order to cope with miniaturization, and conversely, the depth of focus has been in the direction of decreasing. Since the aspect of a three-dimensional structure device is even larger, it is becoming difficult to observe the bottom of holes and trenches with conventional methods.

  To compensate for this, using high acceleration voltage conditions, it is possible to observe holes, trenches, and lower layer patterns by irradiating electrons deep into the sample and efficiently detecting secondary electrons and reflected electrons. Inspection / measurement becomes possible.

  Patent Document 1 discloses focusing on a height position different from the in-focus position in a scanning electron microscope.

JP 7-176285 A (US Pat. No. 5,512,747)

  As described above, in order to observe the bottom and lower layer pattern shapes such as holes and trenches, there is a technique of increasing the acceleration voltage and observing. However, when a multilayer film sample is observed with a high acceleration voltage, the focal position may not be adjusted to the target layer. This is because when the sample is a multilayer film, the pattern appears to pass through to the lower layer depending on the imaging position, and the focal position erroneously detects the pattern of another layer. Therefore, there is a need for a method for stably focusing on a layer to be imaged in observation of a high acceleration voltage.

  In Patent Document 1, focus shift when changing the acceleration voltage is not taken into consideration, and when the acceleration voltage is changed, the target layer may not be stably focused.

  An object of the present invention is to provide an apparatus and a method capable of stably focusing on a target layer even when the acceleration voltage is changed, in particular, in order to obtain a sample image deeper than the surface layer. .

  In order to solve the above-described problem, in the present invention, the in-focus position on the sample surface is obtained under the first acceleration voltage condition, and the offset amount obtained in advance is added to the in-focus position from the first acceleration voltage condition. The in-focus position when the second acceleration voltage condition is high is obtained.

  According to the present invention, it is possible to provide an apparatus and a method capable of performing stable focusing on a target layer even when the acceleration voltage is changed, particularly in order to obtain a sample image deeper than the surface layer. .

  Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

The block diagram of the scanning electron microscope of a present Example. The figure explaining the spread of the incident electron by an acceleration voltage. A figure showing a captured image of low acceleration voltage The figure which shows the picked-up image focused on the B layer or the C layer by the change of the acceleration voltage. The figure which shows the relationship between the focus height and focus evaluation value by acceleration voltage conditions. The whole flowchart for observing using the focusing function of a present Example. The figure explaining the correction process of the focus position at the time of observation condition creation and product wafer observation. The flowchart which shows the calibration procedure of the focus position between acceleration voltage conditions. The flowchart which shows the observation condition preparation procedure containing the correction value between acceleration voltages. The flowchart which shows the procedure which observes a product wafer.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  Hereinafter, an example using a scanning electron microscope as an example of a charged particle beam apparatus will be described. However, this is merely an example of the present invention, and the present invention is not limited to the embodiments described below. In the present invention, the charged particle beam apparatus widely includes apparatuses that take an image of a sample using a charged particle beam. As an example of the charged particle beam apparatus, an inspection apparatus using a scanning electron microscope, a review apparatus, and a pattern measurement apparatus can be given. The present invention can also be applied to a general-purpose scanning electron microscope, a sample processing apparatus equipped with a scanning electron microscope, and a sample analysis apparatus. Hereinafter, the charged particle beam apparatus includes a system in which the above charged particle beam apparatuses are connected via a network and a composite apparatus of the above charged particle beam apparatuses.

  In this specification, the “defect” is not limited to a pattern defect, but includes a wide range of observation objects such as foreign matter, pattern dimension abnormality, and structural defect.

  In this specification, the “sample” is described by taking a semiconductor wafer having a lower layer pattern as an example, but is not limited thereto. For example, the present invention is effective for a bottom portion of a hole or an observation target having a fine three-dimensional structure.

  In this specification, the “defect image” is an image (inspected image) to be subjected to defect inspection, and includes not only a true defect image but also a defect candidate image and a pseudo defect image. The “reference image” is an image used for comparison with a defect image for defect extraction, and is an image of a normal region, that is, a region estimated to have no defect.

  In this specification, “high acceleration” is, for example, 10 kV or more, and “low acceleration” means 1 kV or less, but the acceleration voltage is not limited thereto.

  FIG. 1 is a view showing a charged particle beam apparatus using a scanning electron microscope (SEM) of this embodiment. This scanning electron microscope incorporates an automatic focus control function described later.

  First, a set of samples inspected by an appearance inspection apparatus is conveyed. One set of samples is, for example, one lot of wafers. The scanning electron microscope is provided with a sample storage means (not shown) for storing one set of transported samples. Samples to be observed are sequentially selected from the one set of samples, and a sample preparation chamber ( The sample is moved to the sample chamber through (not shown) and placed on the sample stage 101.

  The scanning electron microscope of this example has an imaging device 102. The imaging device 102 constitutes an SEM.

  The electron optical system that irradiates the electron beam EB includes a condenser lens 104, a deflection scanning coil 105, an objective lens 106, and detectors 109 and 125. The electron beam EB emitted from the electron source 103 is converged by the condenser lens 104 and deflected by the deflection scanning coil 105 so as to scan the sample 107 to be imaged on the sample stage. Furthermore, the electron beam EB is converged on the sample 107 mounted on the sample stage 101 by the objective lens 106 and irradiated and scanned. Here, in the electron source 103, the voltage applied between the cathode and the anode of the electron gun can be changed, and accelerated electrons are emitted. This voltage is called an acceleration voltage. In this embodiment, electron beam irradiation can be performed under a plurality of acceleration voltage conditions.

  By this irradiation, secondary charged particles having sample information such as secondary electrons and reflected electrons are emitted from the sample 107, and this is detected by the detectors 109 and 125. Outputs from the detectors 109 and 125 are digitized by the A / D conversion unit 110, and an image is generated by the image calculation unit 111 in which the scanning position of the electron beam is associated with the pixel. Furthermore, image processing such as defect extraction is performed by obtaining a difference image between the image to be inspected and a reference image that is a normal image corresponding to the defect position as necessary. The output from the image calculation unit 111 is sent to the monitor 113 via the control unit 112 that controls the entire apparatus, and an SEM image of the sample 107 is displayed.

  The configuration of the system including the image calculation unit 111 and the control unit 112 is not limited to this, and a part or all of the devices constituting the system may be a common device. The processing described below can be realized by either hardware or software. When configured by hardware, it can be realized by integrating a plurality of arithmetic units for executing processing on a wiring board or in a semiconductor chip or package. When configured by software, a program for executing desired arithmetic processing by a central processing unit (CPU) mounted on a device constituting the system or a general-purpose CPU mounted on a general-purpose computer connected to the system. It can be realized by executing. It is also possible to upgrade an existing apparatus with a recording medium in which this program is recorded.

  The operation of the scanning electron microscope until SEM image capturing is instructed from the control unit 112 to each unit based on a recipe that specifies observation conditions such as electron beam irradiation conditions and image acquisition conditions.

  The user inputs input items such as a defect observation condition and an operation result monitoring condition via the input unit 114 such as a mouse, a controller, or a console. The input parameter is sent to the control unit 112. The control unit 112 sends a control signal to the electron optical system control unit 115 that controls the lenses, coils, and the like constituting the electron optical system and the high voltage stabilized power supply 116, and sets the imaging conditions of the SEM. Each part of the electron optical system operates based on this imaging condition.

  The electron optical system control unit 115 also includes a focus control unit, which controls each optical component so as to focus on the observation target position. Specifically, the focus control can be performed by adjusting the excitation current of the objective lens 106 by the focus control signal from the focus control unit. The automatic focus control is a control for automatically setting the focus condition of the lens to an optimum value. In this method, two frames obtained from the detectors 1009 and 125 are obtained by scanning a plurality of frames while changing the lens condition. Focus evaluation values are obtained from detection signals of secondary electrons and backscattered electrons, are compared and evaluated, and a focus condition when an optimum focus evaluation value is obtained is set as a control condition for the electron lens. The focusing process described below is performed by the focus control unit unless otherwise specified.

  The calibration sample 108 is a sample that calibrates the focal position according to the conditions of the electron optical system. The calibration sample 108 is provided on the stage. Further, as will be described later, the calibration sample 108 is for grasping in advance the appearance when the acceleration condition is changed, and needs to have a multilayer structure, that is, a structure composed of at least two layers.

  Defect coordinate data from the inspection apparatus is sent to the control unit 112 via a network (not shown), and an observation position can be determined based on the defect coordinate data. As another method, the determination of the observation position may be a method in which the user manually designates the observation position through the input unit 114. In addition, relative position coordinates indicating the position of the observation position from the origin in the die may be set in advance in a recipe so that the corresponding locations of a plurality of dies can be automatically observed by executing the recipe. This method is sometimes called fixed point observation. Note that a recipe is a data file in which alignment positions corresponding to device and process information, image acquisition conditions during observation, image processing conditions such as comparative inspection and length measurement, and observation flow are stored in advance. Other parameters such as the focal position can be appropriately stored in the recipe as will be described later.

  The control unit 112 controls the stage control unit 117 based on the determined observation position. The sample stage 101 moves in the X direction and the Y direction under the control of the stage control unit 117.

  Further, the control unit 112 has a recipe creation unit 119. The recipe creation unit 119 creates a recipe based on the imaging conditions designated by the observer through the monitor 113 and the input unit 114. The imaging condition specifically refers to setting items for acquiring an image including focal position information, the center position of the imaging field, the field size, the imaging magnification, the resolution, and the like. The focal position information is a value indicating the height (range) for determining the focal position, the pitch, the offset value from the focal position, and the like. The created recipe is stored in the storage device 118 and can be read out at the next observation.

  Recipe creation may be performed by a recipe creation apparatus connected via a network (not shown). In this case, the charged particle beam apparatus is provided with a network interface for receiving a recipe from the recipe creation apparatus.

  FIG. 2 is a diagram for explaining the spread of the incident electron beam in the sample at the low acceleration voltage and the high acceleration voltage. FIG. 2 shows a sample having a hole pattern and a lower layer pattern in the film. The hole pattern on the left side of FIGS. 2A and 2B is a hole pattern obtained by etching the surface layers A to C. In addition, a film is present on the right side of the hole, and a B layer pattern 13 and a lower layer C layer pattern 14 are present in the film.

  FIG. 2A is a diagram for explaining a state at a low acceleration voltage. The electron beam 11 accelerated to a low acceleration voltage by the electron source 103 passes through the electron optical system and converges on the sample 107 to be irradiated. Since the electron beam 11 has a small incident electron energy inside the sample, electrons reach only a shallow portion from the hole pattern and the surface layer A, and the information in the depth direction obtained is very small.

  On the other hand, FIG. 2B is a diagram for explaining a state at a high acceleration voltage. Similarly to (a), the electron beam accelerated to the high acceleration voltage condition by the electron source 103 passes through the electron optical system and is converged and irradiated on the sample 107. FIG. As the acceleration voltage increases, the diffusion region 12 of electrons irradiated on the sample expands. Therefore, information on the surface layer can be obtained under the low acceleration voltage condition, but more internal information can be obtained under the high acceleration voltage condition. That is, information generated from the region where electrons irradiated to the sample are diffused at the bottom of the hole of the sample or the B layer and C layer inside the film is obtained. As a result, it is possible to observe a pattern including depth information under a high acceleration voltage condition. The focusing method described in this embodiment is for always stably focusing on the observation target when the lower layer pattern or the bottom of the hole is set as the observation target under the high acceleration voltage condition. .

  FIG. 3 is a diagram illustrating a captured image under a low acceleration voltage condition.

  FIG. 3 is a captured image of the sample in FIG. 2 as viewed from above, and is a diagram when a hole and a lower wiring portion are imaged under a low acceleration voltage condition. As described above, in the case of the low acceleration voltage condition, information in the film is not obtained, and information on the shallow portion from the surface layer is obtained, so that only an information close to the surface layer of the hole can be observed. For this reason, the pattern of B layer and C layer in a film | membrane cannot be observed on low acceleration voltage conditions. As described above, when the automatic focus positioning is performed using the image acquired under the low acceleration voltage condition, the focus position is aligned with the surface layer of the film.

  FIG. 4 is a diagram illustrating a captured image under a high acceleration voltage condition.

  FIG. 3 is a captured image of the sample of FIG. 2 as viewed from above, and is an image when a hole and a lower wiring portion are imaged under a high acceleration voltage condition.

  By observing under high acceleration voltage conditions, the hole bottom of the hole pattern and the B layer and C layer patterns in the film can be confirmed. When the focal position is automatically detected under a high acceleration voltage condition, electrons enter the sample deeply, so that information on the surface layer is reduced and a pattern of a plurality of layers in the film can be seen. However, when automatic focus detection is performed when there are B layer and C layer patterns in the same field of view, whether the pattern of B layer or C layer is in focus depends on the layout and material of the pattern. End up. That is, depending on the layout and material of the B layer and C layer, the place where the contrast becomes large may change and the automatic detection of the focus signal may fail. If it fails, the focus position matches the unintended layer, and the focus position cannot be adjusted to the target layer.

  FIG. 4A is a diagram when the pattern of the B layer is focused under a high acceleration voltage condition. Since the focal position matches the pattern of the B layer, the pattern of the B layer can be clearly confirmed. However, since the hole bottom of the hole is located at the same position as the C layer from FIG. 2, when the B layer is in focus, the hole bottom and the focal position are shifted, so that the hole bottom cannot be clearly observed. . Further, the pattern of the C layer in the film is also out of focus, so the pattern is not clear and looks blurred.

  FIG. 4B is a diagram when the pattern of the C layer is focused under a high acceleration voltage condition. Since the focal position matches the pattern of the C layer, the pattern of the C layer can be clearly confirmed. Further, since the focus is on the bottom of the hole, the bottom of the hole can be clearly seen, and the pattern of the C layer in the film can be clearly observed.

  In this way, the appearance changes greatly depending on which layer is focused on, so in order to stably obtain the focal position of the observation target at any coordinate on the wafer surface regardless of the layout and material, the observation target It is necessary to focus by specifying the position of the layer where the pattern is present.

  FIG. 5 is a diagram showing the relationship between the focus height and the focus evaluation value according to the acceleration voltage condition. FIG. 5 shows the relationship between the focus height and the focus evaluation value corresponding to the sample structure of FIG. By using this, it becomes possible to adjust to the focal position of the observation target by the method described below.

  The curve on the left side of FIG. 5 is a curve 40 when automatic focus detection is performed under a low acceleration voltage condition. Since it is a low acceleration voltage condition, the lower layer pattern cannot be observed, and the surface layer pattern A layer is focused. In order to detect the height position in the vicinity of the surface layer, an image is acquired while shifting the focus at a certain height step. A point represented by a black circle in the graph corresponds to one image. A focus evaluation value is obtained from the acquired image. The optimum focus condition for the A layer can be obtained by taking the place where the focus evaluation value takes the maximum value as the maximum detection value. In this case, the point “a” is the point most focused on the pattern in the A layer.

  Further, a curve 41 and a curve 42 on the right side of FIG. 5 are curves representing the focal position of the high acceleration voltage condition. A curve 41 is a curve in the case of the image obtained in FIG. It is in focus at point b on the graph. If the focus is adjusted to the height corresponding to the point b in the graph, the focal position is in the B layer pattern. A curve 42 is a curve of the image obtained in FIG. 4B, and a point c is a condition in which the focal position is matched to the pattern of the C layer.

  As will be described in detail below, by performing automatic focusing on the surface of the sample under low acceleration voltage conditions to obtain a focus position, and then adding an appropriate offset amount obtained in advance for each layer to be observed The focus position when observing the lower layer pattern under high acceleration voltage conditions can be set. When it is desired to take an image of the C layer pattern, after detecting the point a of the surface layer A, an offset value of the difference between the points a and c is obtained, and this value is registered in the recipe. This ensures that the target layer is always focused at any coordinate of the wafer.

  FIG. 6 is an overall flowchart for observation using the focusing function of the present embodiment.

  First, calibration between acceleration voltage conditions is performed (S101). That is, the focal position between the reference low (reference) acceleration voltage condition and the high acceleration condition for observation is obtained using the calibration sample 108. The focus changes even if the acceleration voltage condition is changed. By obtaining this fluctuation amount using a calibration sample, it is possible to obtain a change value over time due to a change in acceleration voltage. For example, a field emission electron gun or a Schottky electron gun are known as the electron source 103. In particular, when a field emission electron gun is used, gas molecules are formed on the cathode surface, which is an electron emission source, with the passage of time. By adhering, it is affected by fluctuations in the amount of electron current emitted from the electron gun, and fluctuations such as brightness that occur in the image occur. In order to release the adhering gas, it is necessary to periodically perform a flushing process and an emission output value. However, by performing these processes, there is an effect due to fluctuations in the current value. As a result, optical axis misalignment or focal position changes due to thermal fluctuations occur, maintaining a stable state. Difficult to do. These cause changes in the amount of focus variation with time when the acceleration voltage condition is changed. For this reason, it is possible to always perform the focal position in a stable state by performing calibration for each acceleration voltage condition to be used and obtaining a difference in focal position between the acceleration voltage conditions. Details of the calibration processing performed in S101 will be described later with reference to FIG.

  Next, observation conditions are created (S102). Here, the focus height in the observation sample is manually measured using the reference acceleration condition (low acceleration) and the observation acceleration condition (high acceleration). Thereby, the focus correction value from the sample surface to the layer where the observation target pattern exists can be obtained. This focus correction value is stored, and a recipe is created using this value. Details of the calibration processing performed in S102 will be described later with reference to FIG.

  Next, observation of the product wafer is started (S103). Here, based on the description of the recipe created as an observation condition in S102, the product wafer is observed.

  Next, when observing the product wafer, it is determined whether calibration between acceleration voltage conditions has been performed (S104). The criterion here can be set so that the time and current value are monitored and the calibration is automatically performed when the set value is exceeded. In this case, in order to confirm whether the calibration process is performed, for example, in the case of time management, it is determined whether the calibration is performed within the set reference time before the measurement is performed. As another example, calibration may be performed in wafer units or lot units.

  If it is determined in S104 that the set value is exceeded, that is, it is determined that calibration is necessary, calibration between acceleration voltage conditions is performed (S105).

  If the set value is not exceeded in S104, the product wafer processing is observed as it is because calibration is unnecessary (S106).

  In S106, according to the recipe created in S102 for the observation condition for the product wafer, after detecting the automatic focus position under the reference acceleration condition, the observation acceleration condition can be changed, and the image can be moved to the target observation height and imaged. . Thereby, it is possible to focus on the lower layer pattern and the hole hole bottom even when observing at a high acceleration voltage, and it is possible to image under stable conditions when observing the product wafer.

  FIG. 7 is a diagram showing a focus position correction process when creating an observation condition and observing a product wafer.

  Here, when the observation conditions are created and when observing the product wafer, even if the same acceleration voltage is set, the focal position may be deviated due to changes in the beam current or optical axis deviation. Perform the process. This process will be described according to the flow described in FIG.

  FIG. 7 shows the focal position when the focal position A under the reference acceleration condition is used as a reference value and the focal position under the observation acceleration condition is acquired. The vertical direction in the figure represents the depth of the focal position, and the horizontal direction in the figure represents the processing procedure.

  First, before performing the observation condition creation (S102), the calibration 1 automatically obtains the relationship between the reference acceleration condition and the focal height of the observation acceleration condition using the calibration sample. Calibration 1 is executed by the time the recipe is created. The focal position under the reference acceleration condition is d0, and the focal position under the observation acceleration condition is d1. Let this difference be ΔD1. ΔD1 is a change amount of the focal position due to the change of the acceleration voltage.

  Next, in the observation condition creation step (S102), the amount of change in the focal height between the reference acceleration condition and the observation acceleration condition is obtained using the sample to be observed. If the focus position under the observation acceleration condition is automatically obtained, there is a possibility that the wrong position will be focused as described above. Therefore, in this step, the user manually adjusts to the focus position of the pattern to be observed. The focal position under the reference acceleration condition is e0, and the focal position under the observation acceleration condition is e1. Let this difference be ΔE. ΔE represents the difference between the focal position of the reference acceleration condition and the in-focus position when the pattern of the observation object is observed under the observation acceleration condition when the observation object sample is used. This value ΔE becomes an offset value from the focal position under the reference acceleration condition to the focal position under the observation acceleration condition, and is registered in the recipe.

  From the difference between ΔE obtained from the observation sample at the time of creating the observation condition and ΔD1 obtained from the calibration sample, it can be seen that the difference in focus position between the sample to be observed and the calibration sample under the observation acceleration condition is g1. The state of the apparatus is recorded as the sum of ΔD1 and g1 (ΔD1 + g1).

  However, when the observation conditions are created and when the observation of the product wafer is performed, an error occurs due to the temporal change of the focal position when the acceleration voltage condition is changed. That is, ΔD1 and ΔD2 do not necessarily match.

  In order to correct this, calibration 2 is performed in the same manner as calibration 1 after the recipe is created and before the product wafer is observed. The calibration 2 is executed in units of lots or wafers, for example. Here, the relationship between the focal position of the reference acceleration condition and the observation acceleration condition is automatically obtained using the calibration sample used in calibration 1. Here, the focal position under the reference acceleration condition is d0, and the focal position under the observation acceleration condition is d2. Let this difference be ΔD2. ΔD2 represents the amount of change in the focal position due to the change in the acceleration voltage during actual observation. ΔD2 obtained here is different from ΔD1 obtained in calibration 1 because of the error due to the above-mentioned change with time.

  To correct the offset amount ΔE registered in the recipe from the focus position (e10) automatically focused under the reference acceleration condition during the observation of the product wafer, the apparatus side only observes the calibration value (ΔD1 + g1) acquired when the observation condition is created. Sometimes it is necessary to add an offset to the focus height. However, since the amount corresponding to ΔD1 at the time of creating the observation condition is changed to ΔD2 in the calibration 2 before observing the product wafer, ΔE obtained at the observation condition creation step or the calibration value (ΔD1 + g1) obtained at the time of creating the observation condition is simply In addition, the focal point is deviated from the focal position (e11) of the actual pattern to be observed only by adding to e10. Therefore, the difference g2 is obtained by (ΔD1 + g1) −ΔD2, and the value of ΔD2 + g2 is offset from e10 when observing the product wafer. That is, the offset amount is determined through the amount of change in the focal position due to the change in the acceleration voltage during actual observation. As a result, an offset amount that takes into account the temporal change (ΔD2−ΔD1) of the focus change amount due only to the change of the acceleration voltage condition can be obtained, and therefore the same offset as ΔE at the time of creating the observation condition is applied to the product wafer. It can be performed stably.

  A detailed flow will be described with reference to FIGS.

  FIG. 8 is a flowchart showing details of the calibration procedure in the focal position calibration step between the acceleration voltage conditions in S101 of FIG.

  Since focus positioning is performed by combining the reference acceleration condition and the observation acceleration condition, it is necessary to grasp the state of the acceleration voltage condition to be used in the automatic focus operation. “The state of the acceleration voltage condition” means, for example, the relationship between the reference acceleration condition before the preparation of the observation condition (calibration 1 in FIG. 7) and the offset value under the observation acceleration condition.

  In FIG. 8, when calibration is started, the stage position is first moved so that the calibration sample is irradiated with an electron beam (S201). Here, it is assumed that the calibration sample 108 is provided on the stage.

  Next, the reference acceleration condition is switched (S202). It is automatically switched by predetermining the optical conditions used as a reference. Here, the reference acceleration condition is specifically a so-called low acceleration condition suitable for observing the sample surface. Since only the surface layer of the sample is imaged under the reference acceleration condition, it is possible to automatically focus on the sample surface.

  Next, automatic optical axis adjustment under reference acceleration conditions is performed (S203). In order to maximize the performance of the apparatus, it is desirable to perform electromagnetic alignment (beam alignment, aperture alignment, stigma alignment, etc.) before image observation. In particular, in this embodiment, by performing optical axis adjustment before focusing, focus adjustment can be performed based on an image under optimum optical conditions.

  Next, focusing is performed and the focal position is read (S204). An image is picked up while changing the focal position by the objective lens 106, and the place where the focus evaluation value is maximized is detected. The place where the focus evaluation value is maximized means the point a in FIG. 5 and is in a focused state. After detecting the maximum focus evaluation value, the value of the focus position is read. The value at this time is d0.

  Next, it switches to observation acceleration conditions (S205). Here, the observation acceleration condition is specifically an optical condition for observing the target pattern in the lower layer, and is a so-called high acceleration condition. Under the high acceleration condition, the lower layer pattern of the sample is also imaged.

  Next, automatic optical axis adjustment under observation acceleration conditions is performed (S206). In order to maximize the performance of the apparatus, it is desirable to perform electromagnetic alignment for this optical axis adjustment before image observation. In particular, since changing the acceleration voltage condition generally requires a new optical axis adjustment, in this embodiment, the optical axis adjustment is performed again before focusing in the observation acceleration condition.

  Next, focusing is performed and the focal position is read (S207). The focus position is changed by the objective lens 106, and the point where the focus evaluation value is maximized is detected. The place where the focus evaluation value is maximized means the point b or c in FIG. 5 and is in a focused state. After detecting the maximum focus evaluation value, the value of the focus position is read. The value at this time is d1.

  Next, a difference between values read under the reference acceleration condition and the observation acceleration condition is obtained (S208). That is, ΔD1 = d1−d0 is calculated. Data of the correction value ΔD1 is managed as device status information.

  Next, it is determined whether the calibration process is performed under other observation acceleration conditions (S209). If other acceleration voltage conditions are also used, processing is performed in the same flow from S205. In that case, a plurality of acceleration voltage conditions to be used may be registered as calibration target conditions.

  If it is not necessary to calibrate under other optical conditions, the calibration process flow ends.

  FIG. 9 is a flowchart showing a processing procedure in the observation condition creation step of the correction value between the acceleration voltages in S102 of FIG. In FIG. 9, the procedure for adjusting the focal position under the observation acceleration condition and registering it in the recipe will be described.

  First, the observation target position of the sample is aligned with the center of the visual field (S301). The defect coordinate data from the inspection apparatus may be used to move to the observation position, or the observer may manually move the stage to the target position.

  Next, the reference acceleration condition is set (S302). Here, since the automatic focusing is performed, the low acceleration condition is set. The acceleration voltage condition may be input numerically or may be selected from a plurality of modes prepared in advance.

  Next, automatic focusing is performed, and the obtained in-focus position is read (S303). The focus height value (e0) obtained here is automatically stored in the reading device.

  Next, the observation acceleration condition is set (S304). Here, an acceleration voltage condition is set so that a target pattern to be actually observed can be confirmed. The observation acceleration condition is a higher acceleration condition than the reference acceleration condition. Determine the observation magnification.

  Next, the observer adjusts the focal position to the target pattern to be observed by manual operation (S305). In addition, when there is information that indicates the height of the surface layer pattern and the target pattern, it can be dealt with by inputting the numerical value.

  Next, the adjusted focal position is read (S306). The value read in S306 is defined as (e1).

  Next, a correction value (ΔE) is obtained from the difference between the in-focus position (e0) under the reference acceleration condition and the position (e1) where the focus position is adjusted under the observation acceleration condition. The obtained value is registered as a correction value (S307). Finally, information such as the above-described focal position, imaging magnification, and imaging position is also registered in the recipe.

  As described above, the correction value is registered, and the observation condition creation step ends.

  FIG. 10 is a flowchart showing a procedure for executing observation of a product wafer. It is a flow corresponding to S103-S106 of FIG. This corresponds to the calibration 2 and the product wafer observation step in FIG.

  Even when the product wafer is observed under the focus condition included in the recipe registered in the observation condition creation step S102 (described in FIG. 9), a stable focus position must be maintained continuously.

  First, if the observation condition is registered as a recipe, a registered recipe is selected based on the wafer device and process information from the sample information. The correction value ΔE registered at the time of creating the observation condition is read from the recipe information (S401). ΔE is stored as ΔD1 + g1.

  Next, it moves to the calibration sample, finds the in-focus position of the calibration sample, that is, d0 and d2 under the reference acceleration condition and the observation acceleration condition, and obtains a calibration value ΔD2 that is the difference between them (S402). ΔD2 is an amount representing a focal position shift due to a change in acceleration voltage condition immediately before the product wafer observation. In FIG. 10, S402 is described as a flow performed for each wafer, but may be performed in lot units. Note that S402 is substantially the same as the processing described in S204 to S208 in FIG.

  Next, the difference between the read correction value ΔE (= ΔD1 + g1) and the calibration value ΔD2 between the acceleration voltage conditions performed before observing the product wafer is obtained. The difference here is set as a fluctuation correction value g2 (S403).

  Next, in order to observe the observation coordinates registered in the recipe or the defect position from the inspection apparatus, the stage is moved to the target coordinates of the sample according to the defect coordinate data obtained from the inspection apparatus ( S404).

  Next, the reference acceleration condition indicated by the recipe information is changed and automatic focusing is performed. The obtained focal position is read. The focus height value (e10) obtained here is automatically read and stored in the apparatus (S405).

  Next, the observation acceleration condition is changed according to the recipe information (S406). The observation acceleration condition is a higher acceleration condition than the reference acceleration condition.

  Next, the focal position is moved to a target height (S407). Specifically, the height of the focal point is moved from the in-focus position (e10) obtained under the reference acceleration condition by using the value (ΔD2 + g2) obtained by adding the calibration value ΔD2 obtained previously and the fluctuation correction value g2 as an offset. Thus, it is possible to move by the offset value (ΔE ′) considering the apparatus state.

  Next, an image is picked up under the observation acceleration condition at the focal position (S408). As a result, it is possible to acquire an image in a stable focus state at the target position.

  If there is a next imaging position, the same flow is used.

  Note that, here, it is assumed that after one image pickup position is imaged under high acceleration conditions, the image is moved to the next position and imaged in the same flow, but in the case of imaging a plurality of positions. First, the focusing and position reading process S405 may be performed at each imaging position under the low acceleration condition, and after the completion, the process may return to the first position and the processes after S406 may be performed at each imaging position.

  As described above, according to the present invention, in particular, in order to obtain a sample image deeper than the surface layer, even when a high acceleration condition is used, the target pattern position existing in an arbitrary target layer is focused. It becomes possible to adjust stably. Further, when actually observing the product wafer, it is possible to stably adjust the focal point to the same focus position as when the observation conditions are created.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. In addition, it is possible to add, delete, and replace other configurations for a part of the configuration of the embodiment. Each of the above-described configurations, functions, processing units, processing means, and the like may be realized by hardware by designing a part or all of them with, for example, an integrated circuit. Each of the above-described configurations, functions, and the like may be realized by software by interpreting and executing a program that realizes each function by the processor.

  Information such as programs, tables, and files for realizing each function can be stored in a recording device such as a memory, a hard disk, or an SSD (Solid State Drive), or a recording medium such as an IC card, an SD card, or an optical disk.

  Further, the control lines and information lines indicate what is considered necessary for the explanation, and not all the control lines and information lines on the product are necessarily shown. Actually, it may be considered that almost all the components are connected to each other.

40: Curve 41 when focus detection is performed with autofocus under low acceleration conditions: Curve representing focus position under high acceleration conditions (B layer focus)
42: Curve representing focus position under high acceleration conditions (C layer focus)

Claims (8)

In a charged particle beam apparatus that irradiates a charged particle beam focused on a sample and captures an image of the sample,
Acceleration voltage changing means capable of changing to a plurality of acceleration voltage conditions;
A focus control unit that controls to focus on the observation target position of the sample,
The focus control unit obtains a focus position on the surface of the sample under a first acceleration voltage condition, and a second acceleration voltage condition higher than the first acceleration voltage condition by adding an offset amount to the focus position. A charged particle beam apparatus, characterized in that a focusing position when obtained is determined. The charged particle beam apparatus according to claim 1,
The charged particle beam apparatus according to claim 1, wherein a focusing position in the second acceleration voltage condition is a position corresponding to a lower layer pattern of the sample or a hole bottom of the hole of the sample. The charged particle beam apparatus according to claim 1,
The focus control unit
The difference in focus position in the calibration sample when the first acceleration voltage condition and the second acceleration voltage condition are switched is determined after the creation of the recipe in which at least the observation condition of the sample is registered and before the observation of the sample. The charged particle beam apparatus is characterized in that the offset amount is obtained using the difference. In the charged particle beam device according to claim 3,
The focus control unit
The first difference in the focal position in the sample when switching between the first acceleration voltage condition and the second acceleration voltage condition registered in advance in the recipe;
The second difference in the focal position in the calibration sample obtained when the first acceleration voltage condition and the second acceleration voltage condition were switched after the recipe was created and before the sample was observed, A charged particle beam apparatus characterized in that a difference is added to the second difference to obtain the offset amount. The charged particle beam device according to claim 4,
The sample is a semiconductor wafer,
The charged particle beam apparatus characterized in that the second difference is obtained for each lot of the semiconductor wafer or for each wafer. In the charged particle beam device according to claim 3,
A sample stage on which the sample is placed;
The charged particle beam apparatus, wherein the calibration sample is provided on the sample stage. In the charged particle beam device according to claim 3,
The charged particle beam apparatus characterized in that the calibration sample has a lower layer pattern. In a sample observation method for capturing an image of the sample by irradiating a charged particle beam focused on the sample,
Find the first in-focus position on the sample surface under the first acceleration voltage condition,
Switch to a second acceleration voltage condition higher than the first acceleration voltage condition,
Obtain the second focus position when the second acceleration voltage condition is satisfied by adding an offset amount to the focus position,
A sample observation method, wherein an image of the sample is picked up under the second acceleration voltage condition by controlling a focal position to the second focus position.