Measurement device and scan image acquisition method

Abstract

To suppress adverse effect caused by laser beam irradiation while achieving high-accuracy dimension measurement.SOLUTION: A measurement device measures the dimension of a prescribed pattern on a wafer at a plurality of measurement points, and comprises: an imaging part 101 including a charged particle optical system and a detector 108; an image processing part 111 receiving a detection signal from the detector to form a scan image; an irradiation optical system 120 including a laser source 121 and a shutter 122 controlling irradiation of light from the laser source to an area; and a controller 112 controlling the imaging part and the irradiation optical system. The controller performs time adjustment such that amounts of light emitted by the laser source since the shutter is opened and light irradiation from the laser source is started till the imaging part starts to image a scan image are equal to each other at the plurality of measurement points.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a measurement device and a method for acquiring a scanned image. [Background technology]

[0002] In semiconductor device manufacturing lines, measurement equipment installed on the manufacturing line measures the shape of patterns formed on semiconductor wafers and monitors their quality to improve yield. As semiconductor devices become smaller and more three-dimensional, higher measurement accuracy is required, while there is also a strong demand for improved throughput in measurement equipment to accommodate mass production. However, there is a trade-off between measurement accuracy and throughput.

[0003] Patent Document 1 discloses an inspection device that irradiates a wafer with a laser beam to inspect the position and type of defects on the wafer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-151483 Summary of the Invention [Problem to be solved by the invention]

[0005] Sample charging is one of the factors that reduces measurement accuracy. One of the adverse effects of sample charging is the disappearance of pattern edges. A large amount of secondary electrons is emitted from the pattern edge, causing a strong positive charge on the sample. As the signal electrons emitted from the pattern edge are returned to the sample side by the positive charge, only a small number of signal electrons from the pattern edge are detected by the detector, making it impossible to identify the pattern edge from the scanned image.

[0006] Conventionally, the scanning speed of the electron beam has been increased to reduce sample charging, but this does not fundamentally eliminate the charging. In contrast, as disclosed in Patent Document 1, sample charging can be removed by laser irradiation.

[0007] However, when a sample is irradiated with laser light, the pattern shape changes due to damage or contamination of the sample. Because the amount of pattern shape change depends on the amount of laser light irradiation, highly accurate control of pattern dimension variation requires controlling the laser irradiation amount. [Means for solving the problem]

[0008] A measurement apparatus that is one embodiment of the present invention is a measurement apparatus that measures the dimensions of a predetermined pattern on a wafer at multiple measurement points, and includes: a charged particle optical system; a stage; an imaging unit that includes a detector that detects signal electrons emitted when a charged particle beam from the charged particle optical system is irradiated onto a wafer placed on the stage; an image processing unit that creates a scanned image by receiving a detection signal output by the detector as the charged particle optical system scans the charged particle beam over the wafer and the detector detects the signal electrons; an irradiation optical system that includes a laser light source, an optical element for irradiating an area including the field of view of the charged particle optical system with light from the laser light source, and a shutter that controls the irradiation of the area with light from the laser light source; and a controller that controls the imaging unit and the irradiation optical system, and the controller adjusts the time so that the amount of irradiation of light from the laser light source is equal at multiple measurement points from when the shutter is opened to start irradiating light from the laser light source until when the imaging unit starts capturing the scanned image. [Effects of the Invention]

[0009] The present invention enables highly accurate dimension measurement while suppressing side effects caused by laser light irradiation. Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a charged particle beam device. [Figure 2] 1 is an example of a hardware configuration of an information processing device. [Figure 3] FIG. 2 is a diagram schematically showing a wafer as a sample. [Figure 4] 10 is a flowchart for acquiring an SEM image used for pattern measurement. [Figure 5] 1 is a time chart (schematic diagram) for acquiring an SEM image. [Figure 6] 10 is an example of an alarm screen. [Figure 7] 10 is a display example of measurement result data. [Figure 8] 10 is a flowchart for creating a measurement recipe. [Figure 9] 10 is an example of a confirmation screen. [Figure 10] 10 is an example of a data configuration of a processing method database. [Figure 11] 10 is an example of a selection screen. DETAILED DESCRIPTION OF THE INVENTION

[0011] FIG. 1 is a schematic diagram of a charged particle beam device used as a measurement device. Here, the charged particle beam device is exemplified by a scanning electron microscope (SEM) 100 that scans an electron beam over a sample to acquire an electron beam image. The SEM 100 measures the length of a predetermined location (called a measurement point) of a pattern formed on the sample from the acquired electron beam image (SEM image) and monitors the quality of the formed pattern. A measurement recipe for measuring the measurement point on the sample is set in advance in the controller of the SEM 100, and the SEM 100 functions as a measurement device by acquiring a scanned image of the sample and measuring the length of the measurement point according to the set measurement recipe.

[0012] The SEM 100 includes, as its main components, an imaging unit 101, a controller 112, and an irradiation optical system 120 that irradiates the specimen with laser light to control the specimen charging.

[0013] The imaging unit 101 mainly comprises an electron optical system, a stage 110 on which a sample is placed, and a detector 108 that detects signal electrons emitted when the sample is irradiated with an electron beam from the electron optical system. The electron optical system includes an electron source 102, focusing lenses 103 and 105 and an aperture 104 that control the probe diameter and probe current of the electron beam irradiated from the electron source 102 toward a sample 109 placed on the stage 110, an image shift deflector 106a that controls the irradiation position of the electron beam on the sample 109, a scanning deflector 106b that scans the electron beam over the sample 109, and an objective lens 107 that focuses the electron beam on the sample 109. These components are arranged in a housing, and the inside of the housing through which the electron beam passes is kept in a vacuum environment. An image processing unit 111 receives a detection signal output by the detector 108 after detecting the signal electrons and creates an SEM image (scanned image).

[0014] The irradiation optical system 120 controls the charging of the sample 109 by irradiating the sample 109 with light. In this example, the irradiation optical system 120 is placed in an atmospheric environment, and the light from the irradiation optical system 120 is irradiated onto the sample 109 through a window (not shown) provided in the imaging unit 101. The irradiation optical system 120 includes optical elements such as a laser light source 121, a shutter 122 that controls the on / off of the light from the laser light source 121, and a mirror 123 that guides the light to an area on the sample 109 that includes the field of view of the electron optical system. By using the laser light source 121 as the light source for irradiating the sample 109, it is possible to irradiate the sample 109 with light of a wavelength selected according to the material of the sample 109 for which charging control is to be performed, for example, ultraviolet light of a specific wavelength, thereby enabling efficient charging control. Furthermore, a portion of the light from the laser light source 121 is guided by a half mirror 124 to a light intensity monitor 125, which monitors the amount of light output from the laser light source 121. For example, the light intensity monitor 125 receives light from the laser light source 121 and measures its power.

[0015] The shutter 122 is controlled by the controller 112 , and the value of the light amount (power) output by the laser light source 121 measured by the light amount monitor 125 is transmitted to the controller 112 .

[0016] The controller 112 is realized by an information processing device (computer) 200 including, as shown in FIG. 2, a processor (Central Processing Unit: CPU) 201, a memory 202, a storage device 203, an input interface (I / F) 204, an output I / F 205, a communication I / F 206, and a bus 207 as its main components. The processor 201 functions as a functional unit that provides a predetermined function by executing processing in accordance with a program loaded into the memory 202. The storage device 203 stores data and programs used by the functional unit. The input I / F 204 is connected to input devices such as a keyboard, a pointing device, and an operation panel, and the output I / F 205 is connected to a display device. The communication device I / F 206 enables communication with other information processing devices via a network. These are connected to each other via the bus 207 so that they can communicate with each other.

[0017] In the following description, when describing processing by a program, the program, functional units, etc. may be described as the main focus, but the main focus of the hardware in these cases is a processor or an information processing device (computer) configured to include the processor, etc. The information processing device executes processing in accordance with a program read into memory using resources such as memory and communication interfaces as appropriate through the processor. While FIG. 2 shows an example of a CPU as the processor, a GPU (Graphical Processing Unit) or the like may also be used. Furthermore, processing to realize a function is not limited to software program processing, and can also be implemented using a dedicated circuit. The dedicated circuit may be a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or the like. [Example]

[0018] FIG. 3 shows a schematic diagram of a wafer 300 as an example of the sample 109. The wafer 300 includes multiple chip regions 301. A predetermined pattern of semiconductor layers, insulating layers, or conductor layers is stacked on each of the multiple chip regions 301, and finally, the chip regions 301 are separated by dicing to obtain multiple semiconductor chips. Here, the pattern to be measured is a line-and-space (L / S) pattern. For example, an L / S pattern is assumed in which a resist 311 is formed on an underlying conductor layer 312, and the width W of the resist 311 is measured. In this case, the wafer 300 includes an L / S pattern to be measured within the same chip region or another chip region. Therefore, measurement points 302 and 303 are set, including the L / S pattern, to acquire an SEM image. While two measurement points are shown here, in reality, multiple measurement points are set on the wafer. The pattern to be measured is not limited to an L / S pattern; it may also be a pattern or via having a specific shape.

[0019] 4 shows a flow in which the SEM 100 shown in FIG. 1 acquires an SEM image to be used for pattern measurement. Here, an example will be described in which an L / S pattern (see FIG. 3) on a wafer 300 is measured. This flow starts by setting a measurement recipe corresponding to the measurement in the controller 112. The measurement recipe defines the procedure and content of the measurement in advance, and the controller 112 controls the imaging unit 101 and the irradiation optical system 120 in accordance with the measurement recipe.

[0020] First, the wafer 300 is placed on the stage 110 of the SEM 100 (S01), and alignment points within the wafer 300 are used to align the wafer 300 in two steps, using an optical microscope (not shown) and an SEM image (S02). This makes it possible to recognize coordinates defined on the wafer 300 in the SEM 100.

[0021] After moving the stage 110 toward the measurement point (S03), the controller 112 opens the shutter 122 of the irradiation optical system 120 to start irradiating the sample 109 with light (S04). The timing of starting this irradiation is determined according to the definition of the measurement recipe. This timing is just an example, and it is also possible to set a different timing for starting irradiation with light in the measurement recipe.

[0022] Next, the controller 112 moves the field of view of the imaging unit 101 to the measurement point (S05: addressing). For addressing, the controller 112 holds addressing control data that defines in advance a specific pattern on the sample and the positional relationship between the specific pattern and the measurement point. In step S05, pattern matching is performed to search for the specific pattern, and the field of view is moved from the specific pattern to the measurement point. The movement of the field of view is performed by the image shift deflector 106a.

[0023] Next, the controller 112 controls the objective lens 107 so that the electron beam is focused on the wafer surface at the measurement point (S06: autofocus). In step S06, for example, an SEM image is acquired while changing the excitation intensity of the objective lens 107, and it is determined that the electron beam is focused on the wafer surface at the excitation intensity at which the SEM image with the highest definition is obtained, and the excitation intensity of the objective lens 107 is set to the excitation intensity at that time.

[0024] Next, the controller 112 adjusts the gain and offset of the detector 108 in order to adjust the brightness of the SEM image obtained by the image processing unit 111 (S07: brightness adjustment).

[0025] After step S07 is completed and a predetermined waiting time has elapsed (S08), the controller 112 acquires an SEM image (S09). For example, the SEM image is acquired by repeatedly scanning the electron beam at the measurement point using the scanning deflector 106b and accumulating the frame images obtained by one scan. Thereafter, the shutter 122 is closed to terminate the light irradiation of the wafer 300 (S10). The waiting time in step S08 is for adjusting the time so that the amount of light irradiation from the laser light source 121 is equal at multiple measurement points, and how to apply this time will be described later.

[0026] If acquisition of SEM images for all measurement points on wafer 300 has not been completed (NO in S11), stage 110 is moved toward the next measurement point (S12), and the processes from step S04 onward are executed to acquire an SEM image at the next measurement point. On the other hand, if acquisition of SEM images for all measurement points on wafer 300 has been completed (YES in S11), wafer 300 is removed from stage 110 of SEM 100 (S13).

[0027] FIG. 5 shows a time chart (schematic diagram) for acquiring an SEM image at any measurement point. Specifically, it is a time chart for the period from when the shutter 122 is opened to when it is closed, and the controller 112 executes steps S05 to S09. The horizontal axis represents time, and an example of a time chart for three measurement points (measurement points 1 to 3) is shown. For simplicity of explanation, the time required for addressing (S05) and imaging (S09) is the same for all three measurement points, the time required for autofocus (S06) is the same for measurement points 2 and 3, and measurement point 1 takes longer than that, T + The time required for brightness adjustment (S07) is the same at measurement point 1 and measurement point 2, but it takes longer at measurement point 3, T - It was just short.

[0028] At this time, the waiting time at measurement point 2 is set as T wait Then, the controller 112 determines that the waiting time in the time chart of the measurement point 1 is (T wait -T+ ), and the waiting time in the time chart at measurement point 3 is (T wait +T - As a result, the time variance of each process before the image capture (S09) is offset by the waiting time, and the light irradiation time until the image capture starts at all measurement points is set to T pre This makes it possible to control the amount of light irradiation at any of measurement points 1 to 3 so that it is constant at the time when imaging (S09) is started.

[0029] On the other hand, if the waiting time (S08) is not set, the measurement point 1 is measured at a time T + Imaging (S09) is started with the light irradiated for a longer time than measurement point 2, and measurement point 3 is irradiated for a time T - Since imaging (S09) is started with the light irradiation time being as short as possible, the magnitude of the influence of the light irradiation varies for each measurement point. Therefore, the measurement value contains information on the variation in the influence of the light irradiation, which differs for each measurement point, in addition to the process variation information that is the original object of measurement. In this embodiment, by suppressing the variation in the influence of the light irradiation for each measurement point, it becomes possible to detect process variation with higher accuracy.

[0030] The controller 112 may set the waiting time at each measurement point based on the light intensity output from the laser light source 121 measured by the light intensity monitor 125. That is, in the measurement recipe, the light irradiation time T total 5, the length of the waiting time (S08) is set so that the irradiation amount of light from the laser light source 121 monitored by the light amount monitor 125 becomes equal. For example, in the measurement recipe, the light irradiation time T totalA predetermined amount of power is defined as the amount of light irradiation from the laser light source 121 at each inspection point, and the controller 112 starts imaging (S09) when the accumulated power monitored by the light amount monitor 125 reaches the predetermined amount of power. This method makes it possible to suppress the influence of temporal fluctuations in the amount of light output by the laser light source 121 and to more accurately suppress variations in the influence of light irradiation at each inspection point, compared to when the length of the waiting time is defined by time on the assumption that the output of the laser light source 121 is constant.

[0031] Furthermore, in the configuration of FIG. 1, the controller 112 can monitor the amount of light irradiated at the measurement point, so the amount of light irradiated at the measurement point can be saved as additional information on the measurement result and used as information indicating the reliability of the measurement result. For example, if the amount of light irradiated at the measurement point exceeds a predetermined amount, an alarm screen such as that shown in FIG. 6 can be displayed. When the user presses the details button 401 on the alarm screen 400, the measurement result data shown in FIG. 7 is displayed. The measured dimensions for each measurement point are displayed, and the measurement result for measurement point 2 is displayed with an alarm mark 411 indicating that the amount of light irradiated has exceeded the predetermined amount. This allows the user to understand that the measurement value may be affected by light irradiation. [Example]

[0032] In the second embodiment, a description will be given of the creation of a measurement recipe for performing the measurement described in the first embodiment. Here, an example will be described in which the controller 112 creates the measurement recipe, but the measurement recipe may also be created using an information processing device different from the controller 112.

[0033] FIG. 8 shows the flow for creating a measurement recipe. However, this flow is an extracted portion related to the method for setting the timing of light irradiation for acquiring an SEM image. First, measurement point candidates are read (S21). The measurement point candidates are assumed to be created, for example, from CAD data of the wafer 300. Next, it is confirmed whether light irradiation by the irradiation optical system 120 is necessary (S22). FIG. 9 shows an example of a confirmation screen 500. If necessary, the output power of the laser light source 121 to be used is also set. If not, this flow ends.

[0034] Next, a measurement point to be actually measured is selected from the measurement point candidates (S23). In this selection, the spot diameter of the light from the irradiation optical system 120 is large enough to include multiple measurement point candidates, so the selection condition is that the measurement point is not irradiated with light multiple times in an overlapping manner. The presence or absence of overlap can be determined from the coordinates of each measurement point candidate and the size of the light spot diameter.

[0035] Next, for the cases where there are multiple possible processing methods during light irradiation, a processing method is selected (S24). In Example 1, specific processing methods are exemplified for steps S05 to S07 performed during light irradiation, but there are also steps for which processing methods other than those exemplified can be selected. In such cases, the time variation that can occur in a step may differ depending on the processing method. FIG. 10 is an example of a processing method database stored in the controller 112 for appropriately setting the waiting time when a processing method is selected. The processing method database 600 registers the methods that can be used for each process and the waiting time in each case. Here, the waiting time is calculated as the variation in processing time required when that method is adopted for the step.

[0036] Next, the light irradiation period is set (S25). FIG. 11 shows an example of the selection screen 700. Example 1 corresponds to a flow executed when all steps (processes) displayed on the selection screen 700 are selected. On the other hand, if it is desired to shorten the light irradiation period as much as possible, it is possible to minimize the influence of light irradiation while suppressing variations in the influence of light irradiation by excluding specific steps from the light irradiation period. For example, in the example of FIG. 11, brightness adjustment (S07) and imaging (S09) are included in the light irradiation period. Therefore, the time variation of brightness adjustment (S07) is absorbed by the waiting time. However, light is not irradiated during addressing (S05) and autofocus (S06). Therefore, the light irradiation period is shortened. Furthermore, the only time variation that needs to be absorbed is the time variation of brightness adjustment (S07), and the waiting time (S08) itself can also be shortened.

[0037] Next, the length of the waiting time is set (S26). The waiting time is set based on the light irradiation period set in step S25 and the processing method selected in step S24. If the waiting time is set as the amount of light irradiation from the laser light source 121, the product of the total waiting time defined in the processing method database 600 for the steps included in the light irradiation period and the output power of the laser light source 121 entered on the confirmation screen 500 may be set as the target value of the amount of light irradiation.

[0038] The present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail to make the present disclosure easier to understand, and are not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment or modification with the configuration of another embodiment or modification, and it is also possible to add the configuration of another embodiment or modification to the configuration of one embodiment or modification. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment or modification with other configurations. [Explanation of symbols]

[0039] 100: scanning electron microscope, 101: imaging unit, 102: electron source, 103, 105: focusing lens, 104: aperture, 106a: image shift deflector, 106b: scanning deflector, 107: objective lens, 108: detector, 109: sample, 110: stage, 111: image processing unit, 112: controller, 120: irradiation optical system, 121: laser light source, 122: shutter, 123: mirror, 124: half mirror, 125: light intensity monitor, 200 : Information processing device, 201: Processor (CPU), 202: Memory, 203: Storage device, 204: Input I / F, 205: Output I / F, 206: Communication I / F, 207: Bus, 300: Wafer, 301: Chip area, 302, 303: Measurement point, 311: Resist, 312: Conductive layer, 400: Alarm screen, 401: Details button, 411: Alarm mark, 500: Confirmation screen, 600: Processing method database, 700: Selection screen.

Claims

1. A measurement apparatus that measures dimensions of a predetermined pattern on a wafer at a plurality of measurement points, comprising: an imaging unit including a charged particle optical system, a stage, and a detector that detects signal electrons emitted when the wafer placed on the stage is irradiated with a charged particle beam from the charged particle optical system; an image processing unit that generates a scanned image in response to a detection signal output by the detector after the charged particle optical system scans the charged particle beam on the wafer and the detector detects the signal electrons; an irradiation optical system including a laser light source, an optical element for irradiating a region including a field of view of the charged particle optical system with light from the laser light source, and a shutter for controlling irradiation of the region with light from the laser light source; a controller that controls the imaging unit and the irradiation optical system, The controller is a measurement device that adjusts the time so that the amount of light irradiated from the laser light source from the time the shutter is opened to start irradiating light from the laser light source until the time the imaging unit starts capturing the scanned image is equal at the multiple measurement points.

2. In claim 1, The controller is a measuring device that closes the shutter and terminates irradiation of light from the laser light source when capturing of the scanned image is completed.

3. In claim 2, the irradiation optical system includes a light amount monitor that monitors the amount of light from the laser light source, The controller adjusts the time so that the amount of light emitted from the laser light source monitored by the light amount monitor during the period when the shutter is open is equal at the multiple measurement points.

4. In claim 3, The controller is a measuring device that displays an alarm when the amount of light irradiated from the laser light source monitored by the light amount monitor during the period when the shutter is open exceeds a predetermined value.

5. In claim 1, the irradiation optical system includes a light amount monitor that monitors the amount of light from the laser light source, The controller adjusts the time in accordance with a measurement recipe set for measurements at the plurality of measurement points so that the amount of light irradiation from the laser light source monitored by the light intensity monitor during the period in which the shutter is open is equal at the plurality of measurement points.

6. A measurement apparatus that measures dimensions of a predetermined pattern on a wafer at a plurality of measurement points, comprising: an imaging unit including a charged particle optical system, a stage, and a detector that detects signal electrons emitted when the wafer placed on the stage is irradiated with a charged particle beam from the charged particle optical system; an image processing unit that generates a scanned image in response to a detection signal output by the detector after the charged particle optical system scans the charged particle beam on the wafer and the detector detects the signal electrons; an irradiation optical system including a laser light source, an optical element for irradiating a region including a field of view of the charged particle optical system with light from the laser light source, and a shutter for controlling irradiation of the region with light from the laser light source; a controller that creates a measurement recipe that defines a procedure and content of measurements at the plurality of measurement points, The controller defines in the measurement recipe a procedure for adjusting time so that the amount of light irradiation from the laser light source is equal at the multiple measurement points from the time the shutter is opened to start irradiating light from the laser light source until the time the imaging unit starts capturing the scanned image.

7. In claim 6, the irradiation optical system includes a light amount monitor that monitors the amount of light from the laser light source, The controller defines in the measurement recipe a procedure for adjusting time so that the amount of light irradiation from the laser light source monitored by the light intensity monitor during the period when the shutter is open becomes equal at the multiple measurement points.

8. In claim 6, The controller selects from a plurality of measurement point candidates so that light from the laser light source is not irradiated overlappingly, and defines the plurality of measurement points in the measurement recipe.

9. In claim 6, The measurement apparatus wherein the controller is capable of setting different lengths of waiting time for the time adjustment in the measurement recipe depending on the type of processing executed by the imaging unit while the shutter is open.

10. In claim 6, The controller defines in the measurement recipe when to open the shutter and when to start which process executed by the imaging unit.

11. In claim 6, The controller is a measurement apparatus that controls the imaging unit and the irradiation optical system in accordance with the measurement recipe.

12. 1. A scanning image acquisition method for a measurement apparatus that measures dimensions of a predetermined pattern on a wafer at a plurality of measurement points, comprising: The measurement device includes an imaging unit including a charged particle optical system, a stage, and a detector that detects signal electrons emitted when the wafer placed on the stage is irradiated with a charged particle beam from the charged particle optical system; an image processing unit that creates a scanned image when the charged particle optical system scans the charged particle beam over the wafer and the detector detects and outputs a detection signal for the signal electrons; an irradiation optical system that includes a laser light source, an optical element for irradiating a region including a field of view of the charged particle optical system with light from the laser light source, and a shutter that controls irradiation of the region with light from the laser light source; and a controller that controls the imaging unit and the irradiation optical system. The controller opens the shutter to start irradiating light from the laser light source, A scanning image acquisition method in which the controller adjusts the time so that the amount of light irradiation from the laser light source from the start of irradiation of light to the start of imaging of the scanning image in the imaging unit is equal at the multiple measurement points, and then starts imaging of the scanning image.

13. In claim 12, The controller closes the shutter and terminates irradiation of light from the laser light source when capturing of the scanned image is completed.

14. In claim 13, the irradiation optical system includes a light amount monitor that monitors the amount of light from the laser light source, A scanning image acquisition method in which the controller adjusts the time so that the amount of light irradiation from the laser light source monitored by the light intensity monitor during the period when the shutter is open is equal at the multiple measurement points.

15. In claim 14, The controller displays an alarm when the amount of light irradiated from the laser light source monitored by the light amount monitor during the period when the shutter is open exceeds a predetermined value.

Images