A method and system for monitoring defocus in a scanning electron microscope

Abstract

The application discloses a method and system for monitoring defocus of a scanning electron microscope, and the method comprises the following steps: using the scanning electron microscope to measure the graphic critical dimension of the same position on a control sheet to obtain a measurement value; performing difference calculation on the current measurement value obtained by each measurement and the adjacent measurement value obtained by previous measurement to obtain a measurement difference value; comparing the current measurement difference value obtained by each difference calculation with a control limit, and when the current measurement difference value exceeds the control limit, it is judged that the scanning electron microscope is defocused, so that effective monitoring of the measurement state of the scanning electron microscope is realized, the scanning electron microscope can always maintain a good state for measurement, and the accuracy of the output measurement value is ensured.

Description

Technical Field

[0001] This invention relates to the field of semiconductor testing technology, and in particular to a method and system for monitoring defocusing in a scanning electron microscope. Background Technology

[0002] With the increasing variety of semiconductor products, the photolithography, development, and etching processes at each layer of silicon wafers require the measurement of critical dimensions. Current methods typically involve using a scanning electron microscope (SEM) to measure these critical dimensions. However, SEMs are inherently prone to defocusing. Defocusing is usually caused by the electron beam not being focused at the appropriate height, improper magnification leading to blurred images, or suboptimal calibration causing electron beam scattering. When measuring critical dimensions of patterns such as lines and holes, defocusing can cause ghosting and blurred edges at the edges of these patterns, resulting in significant errors in the measured critical dimensions (linewidth, aperture), failing to reflect the true critical dimension values, and thus rendering them meaningless for monitoring and reference.

[0003] The aforementioned defocusing problem is difficult to detect in a timely manner during normal automated measurement using a scanning electron microscope. It can only be identified afterward by examining and comparing the retrieved measurement images based on experience. However, this method is clearly slow and unstable.

[0004] Therefore, in order to ensure the accuracy of product measurement results, it is particularly important to detect defocusing problems in the scanning electron microscope in a timely manner. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned defects in the prior art and to provide a method and system for monitoring defocusing in a scanning electron microscope.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a method for monitoring defocusing in a scanning electron microscope, comprising:

[0008] Using a scanning electron microscope, the key dimensions of the graphic at the same location on the control sheet are measured to obtain the measured values;

[0009] The difference between the current measurement value obtained in each measurement and the adjacent measurement value obtained in the previous measurement is calculated to obtain the measurement difference;

[0010] The current measurement difference obtained from each difference calculation is compared with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope has defocused.

[0011] Furthermore, a measurement value control chart is established to collect the obtained measurement values. Simultaneously, based on the measurement value control chart, a measurement difference control sub-chart is established to calculate the difference between the measurement values ​​in the measurement value control chart, collect the obtained measurement differences, and set the control limit in the measurement difference control sub-chart so that an alarm is triggered when the current measurement difference exceeds the control limit.

[0012] Furthermore, the control limit is determined by the following method:

[0013] The system determines whether the measured values ​​in the measured value control chart are stabilizing. When the measured values ​​are stabilizing, the system calculates the difference between the measured values ​​after stabilization begins in the measured value control chart using the measured difference control sub-chart. Based on the collected measured difference, the system generates and sets the control limit.

[0014] Furthermore, the control limit is generated based on 6 times the standard deviation calculated from the measurement difference.

[0015] Furthermore, a threshold is set in the measurement value control chart as a standard for judging whether the measurement value is stabilizing; wherein, by calculating the tilt angle between the straight line defined by the last n measurement values ​​in the measurement value control chart and the horizontal axis, when the tilt angle reaches the threshold, it is judged that the measurement value is stabilizing, and the difference is calculated on the last n measurement values ​​and the measurement values ​​thereafter through the measurement difference control sub-chart, and the calculated measurement difference is collected.

[0016] Furthermore, n is 3 to 5.

[0017] Furthermore, the graphic includes line graphics.

[0018] Furthermore, the threshold is 170 degrees, and when the tilt angle is greater than or equal to 170 degrees, the measured value is determined to be stable.

[0019] The present invention also provides a system for monitoring defocusing in a scanning electron microscope, comprising:

[0020] The control module is used to control the scanning electron microscope to measure the key dimensions of the image at the same location on the control slide in order to obtain the measurement value;

[0021] The data processing module is used to calculate the difference between the current measurement value obtained from each measurement and the adjacent measurement value obtained from the previous measurement, and obtain the measurement difference.

[0022] The analysis and judgment module is used to compare the current measurement difference obtained from each difference calculation with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope has defocused.

[0023] Furthermore, the control module measures the control slide by calling the measurement program of the scanning electron microscope; the data processing module collects the obtained measurement values ​​by establishing a measurement value control chart, and simultaneously calculates the difference between the measurement values ​​in the measurement value control chart and collects the obtained measurement difference by establishing a measurement difference control sub-chart based on the measurement value control chart; the analysis and judgment module triggers an alarm when the current measurement difference exceeds the control limit set in the measurement difference control sub-chart.

[0024] Furthermore, the analysis and judgment module is also used to judge whether the measured value in the measured value control chart is stabilizing, and when the measured value is judged to be stabilizing, the module calculates the difference between the measured values ​​after the stabilization begins in the measured value control chart through the measured difference control sub-chart, and automatically generates the control limit to set based on the collected measured difference through the measured difference control sub-chart.

[0025] Furthermore, the analysis and judgment module is also used to set a threshold in the measurement value control chart as a standard for judging whether the measurement value is stabilizing; wherein, the data processing module calculates the tilt angle between the straight line defined by the last n measurement values ​​in the measurement value control chart and the horizontal axis. When the tilt angle reaches the threshold, the analysis and judgment module judges that the measurement value is stabilizing, and performs the difference calculation on the last n measurement values ​​and the measurement values ​​thereafter through the measurement difference control sub-chart, and collects the calculated measurement difference.

[0026] As can be seen from the above technical solution, the present invention measures the key dimensions of the graphic at the same position on the control plate and calculates the difference between every two adjacent measurements. Utilizing the law that the measurement difference will undergo a sudden change in value when defocusing occurs, the present invention compares the measurement difference with the set control limit. When the measurement difference exceeds the control limit, it can be determined that the scanning electron microscope has defocused, thereby achieving effective monitoring of the measurement status of the scanning electron microscope. This ensures that the scanning electron microscope can always maintain a good state for measurement and guarantees the accuracy of the output measurement values. Attached Figure Description

[0027] Figure 1 This is a flowchart of a method for monitoring defocusing of a scanning electron microscope according to a preferred embodiment of the present invention.

[0028] Figure 2 This is a schematic diagram illustrating the principle of monitoring defocusing of a scanning electron microscope using a measurement difference control sub-chart, according to a preferred embodiment of the present invention.

[0029] Figure 3 This is a schematic diagram illustrating the principle of determining a threshold using a measurement value control chart, according to a preferred embodiment of the present invention.

[0030] Figure 4 This is a schematic diagram of a system for monitoring defocusing of a scanning electron microscope, according to a preferred embodiment of the present invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art. The terms "comprising" and similar expressions used herein mean that the element or object preceding the word covers the element or object listed after the word and its equivalents, but does not exclude other elements or objects.

[0032] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0033] refer to Figure 1 The method for monitoring defocusing in a scanning electron microscope according to the present invention includes the following steps:

[0034] Step S1: Use a scanning electron microscope to measure the key dimensions of the graphic at the same location on the control sheet to obtain the measured values;

[0035] Step S2: Calculate the difference between the current measurement value obtained in each measurement and the adjacent measurement value obtained in the previous measurement to obtain the measurement difference;

[0036] Step S3: Compare the current measurement difference obtained from each difference calculation with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope has defocused.

[0037] In some embodiments, in step S1 above, the key dimensions of the graphic at the same position on the same control sheet are measured at a certain measurement cycle (e.g., once a day or when the measurement batch is changed, or when needed) to perform monitoring of the scanning electron microscope.

[0038] In some embodiments, in step S1 above, when measuring the control sheet, the measurement program (measurement recipe) corresponding to the pattern to be measured on the scanning electron microscope is called to measure the control sheet. Furthermore, based on the measurement values ​​obtained from the scanning electron microscope, a measurement value control chart is established in the control system for collecting the obtained measurement values. Simultaneously, the automatically generated upper and lower control limits in the measurement value control chart are removed to avoid triggering alarms and causing measurement interruption (because when measuring the same control sheet, there is a continuous charging process between the scanning electron microscope and the control sheet, causing each measurement result to show a certain trend with the scanning time and number of scans of the electron beam on the scanning electron microscope. Therefore, after a certain number of measurements, due to the setting of the upper and lower control limits, alarms will be continuously triggered. For example, for the measurement of line graphics, the charging phenomenon will cause the measured line width to show a general decreasing trend with the number of measurements (see reference). Figure 3 In the measurement control chart, this is reflected as a decreasing trend in line width. Therefore, after a certain number of times, it will reach the upper and lower control limits, thus continuously triggering the alarm.

[0039] refer to Figure 2 Simultaneously, in step S2 above, a measurement difference control sub-chart is established at the next level based on the measurement value control chart. The measurement difference control sub-chart is used to calculate the difference between each current measurement value in the measurement value control chart and the adjacent measurement value obtained from the previous measurement (i.e., to calculate the moving difference for every two adjacent measurement values), and the obtained measurement differences are collected and reflected in the measurement difference control sub-chart.

[0040] Taking line drawings as an example, Figure 2 The example shows the 1st to 9th measurement differences collected sequentially in the measurement difference control subchart, with their values ​​represented on the vertical axis (unit: nm). In this example, the largest difference among the 9 measurement differences is the 6th measurement difference, with a value of -0.41812, and the smallest difference is the 8th measurement difference, with a value of -0.00094.

[0041] The inventors discovered that when a scanning electron microscope defocuses, it causes a significant difference between the current measurement and the previous measurement. This difference is reflected in the measurement difference, which shows that the measurement difference between the current measurement after defocusing and the previous adjacent measurement before defocusing is much greater than the previous adjacent measurement difference.

[0042] Therefore, in step S3 above, when a large numerical abrupt change occurs between the current measurement difference and the previous adjacent measurement difference, for example... Figure 2 In the example, the 10th measurement difference is 3.84704, which shows a significant abrupt change in magnitude compared to the 9th measurement difference of 0.2184. This indicates that when defocusing occurs, the measurement difference immediately increases. Therefore, this pattern can be used to determine if defocusing has occurred in the scanning electron microscope.

[0043] In some embodiments, a reasonable control limit is set in the measurement difference control sub-chart to quantify the aforementioned numerical abrupt changes. Thus, when the current measurement difference reaches the control limit, it can be determined that the scanning electron microscope has defocused, triggering an alarm to stop further measurements and await confirmation and processing.

[0044] In some embodiments, the control limits are determined by the following methods:

[0045] First, each time a scanning electron microscope is used to measure the key dimensions of the graphic at the same location on the control sheet, and the current measurement value is entered into the measurement value control chart, a process is performed to determine whether the measurement value in the measurement value control chart is stabilizing. When the measurement value is determined to be stabilizing, the difference between the measurement values ​​after the stabilization begins is calculated through the measurement difference control sub-chart. Based on the collected measurement difference, control limits are generated and set in the measurement difference control sub-chart.

[0046] The purpose of judging whether the measured values ​​in the control chart are stable is to avoid the aforementioned charging process causing each measurement result to show a certain trend with the scanning time and number of scans of the electron beam on the scanning electron microscope, resulting in an overly wide calculated control limit, which would fail to accurately determine the occurrence of defocus.

[0047] In some embodiments, the standard deviation is automatically calculated based on the measurement difference in the measurement difference control sub-chart using its calculation function, and a control limit is set at 6 times the standard deviation. Figure 2 The control limits shown are for illustrative purposes only and are not actual calculated values.

[0048] refer to Figure 3 In some embodiments, a threshold is set in the measurement value control chart as a criterion for determining whether the measurement values ​​have stabilized. Specifically, the tilt angle between the line defined by the last n measurement values ​​collected in the measurement value control chart and the horizontal axis is calculated. When the tilt angle reaches the threshold, the measurement values ​​are considered to have stabilized. A measurement difference control sub-chart is then used to calculate the difference between the last n measurement values ​​used to calculate the tilt angle and the measurement values ​​collected thereafter, and the calculated measurement difference is collected.

[0049] In some embodiments, n is 3 to 5. That is, the tilt angle between the straight line defined by the last 3 to 5 measurements in the measurement control chart and the horizontal axis is calculated.

[0050] In some embodiments, the threshold is set to 170 degrees. When the calculated tilt angle is greater than or equal to 170 degrees, the measured value is considered to be stable. Preferably, the threshold is set to 173.5 degrees.

[0051] Figure 3 The example shows the 1st to 20th measurements collected sequentially in the measurement control chart, with their values ​​represented on the vertical axis (unit: nm). In this example, the 20 measurements, measured in chronological order, exhibit a decreasing linewidth trend due to the charging process, gradually stabilizing (charging saturation).

[0052] In some embodiments, by means of Figure 3 The tilt angle α (reflecting the slope) between the straight line A defined by the last 5 continuously updated measurement values ​​in the measurement control chart and the horizontal axis is calculated. When the tilt angle α reaches a threshold, it is determined that the measurement value has stabilized, and then... Figure 2 The measurement difference control subchart calculates the difference between the last five measurements used to calculate the tilt angle and subsequent measurements, and collects the calculated measurement differences. Conversely, if the calculated tilt angle α does not reach the threshold, measurements need to be taken again, and the tilt angle α of the newly formed line defined by the last five measurements needs to be recalculated until the latest calculated tilt angle α reaches the threshold.

[0053] For example in Figure 3 In the example, when the 11th to 15th measurements were calculated, the calculated tilt angle α was greater than 170 degrees, indicating that the data had stabilized starting from the 11th measurement. At this point, the difference was calculated starting from the 11th measurement using the measurement difference control subchart (the first measurement difference was the difference between the 12th and 11th measurements), and the results were collected to obtain the data reflected in... Figure 2 The 1st to 9th measurement differences.

[0054] The measured value control chart calculates the standard deviation based on the collected measurement difference data, and sets the control limits at six times the standard deviation. Through continuous data accumulation, the calculated control limits become more applicable over the long term.

[0055] It should be noted that when using control films to measure and monitor whether defocusing occurs in a scanning electron microscope, it is generally necessary to wait until the measured values ​​stabilize before starting the formal monitoring process.

[0056] It should also be noted that the above method, which sets a threshold in the measured value control chart as a standard for judging whether the measured value is stabilizing, and compares the calculated tilt angle α with the threshold to determine whether the measured value is stabilizing, is a quantitative implementation method provided by this invention. In actual monitoring, considering efficiency, a different approach can also be adopted. Figure 3 By observing the shape of the curve, we can roughly determine whether the measured value is stabilizing and make adjustments based on the actual results.

[0057] To verify the practicality of the method of the present invention, an artificial deterioration experiment can be conducted to cause the scanning electron microscope to defocus, and then the method of the present invention can be used to measure the data. The measured values ​​under the defocused state are compared with the measured values ​​under the normal state.

[0058] For example, after defocusing the scanning electron microscope to a certain extent through a degradation experiment, the control slide was measured, and the 21st measurement value was reflected in... Figure 3 In this context, the difference between the 10th measurement and the 20th measurement (the measurement under normal conditions) is reflected in... Figure 2 In the middle. It can be seen that, Figure 3 Between the 21st and 20th measurements, and Figure 2 Between the 10th and 9th measurement differences, a sudden and significant increase in data occurred. Figure 2 The 10th measurement difference significantly exceeded the control limit. This confirms that the method of the present invention can effectively monitor defocusing.

[0059] The aforementioned degradation test method may employ at least one of the following: focusing the electron beam emitted during measurement at a height position outside the measurement surface, increasing the magnification to the point of blurring the image, and causing the electron beam to scatter.

[0060] In some embodiments, line patterns at the same position in multiple (e.g., 5) photolithography units on the control chip can be measured, and the average of the multiple measured line width dimensions can be used as the measurement value to ensure the stability of the measurement results.

[0061] Taking the conventional measurement of pitch between line graphics as an example, since the effect of defocus on the trailing shadows and blurry edges produced on both sides of each line graphic is relatively consistent, they will cancel each other out when measuring pitch, and will not cause significant deviation in the pitch measurement results. Therefore, it is impossible to monitor the defocus problem in a timely manner by setting the deviation rate of the pitch measurement value.

[0062] When measuring the critical dimension of line width in graphic lines, the error in the measured line width due to defocus can have an additive effect with the normal fluctuations in the actual line width value, thus masking the existence of the defocus problem. Therefore, when abnormal line width measurements occur, the problem is often directed at troubleshooting the process, delaying the processing time.

[0063] However, if a fixed control plate is used, and the defocusing phenomenon is monitored by measuring the line width of the lines at the same position on the control plate each time, the measurement results will show a certain downward trend as the scanning electron microscope and the control plate are constantly charging. As a result, after a certain number of measurements, the control limit settings in the control chart will continuously trigger alarms. Therefore, it is necessary to frequently replace different measurement points and new control plates, which not only wastes control plate resources, but also makes it difficult to effectively monitor defocusing.

[0064] This invention measures the line width of lines at the same position on the control plate and calculates the difference between every two adjacent measurements. Utilizing the principle that the measurement difference changes abruptly when defocusing occurs, the measurement difference is compared with a set control limit. When the measurement difference exceeds the control limit, defocusing of the scanning electron microscope is detected, thus enabling effective monitoring of the scanning electron microscope's measurement status. This effectively solves the problem in existing technologies where it is difficult to detect defocusing during normal automatic measurement of a scanning electron microscope, requiring post-operative inspection and comparative analysis of retrieved measurement images, which introduces lag and instability. This invention ensures the accuracy of the product's measurement results.

[0065] The following detailed description, in conjunction with specific embodiments and accompanying drawings, provides a further detailed explanation of a system for monitoring defocusing in a scanning electron microscope according to the present invention.

[0066] refer to Figure 4 In conjunction with references Figures 2-3 This invention provides a system for monitoring defocusing in a scanning electron microscope, which can be used to implement the aforementioned method for monitoring defocusing in a scanning electron microscope. The system for monitoring defocusing in a scanning electron microscope includes: a control module, a data processing module, and an analysis and judgment module.

[0067] The control module is used to control the scanning electron microscope to measure the key dimensions of the image at the same location on the control slide in order to obtain the measurement value.

[0068] The data processing module is used to calculate the difference between the current measurement value obtained from each measurement and the adjacent measurement value obtained from the previous measurement, and obtain the measurement difference.

[0069] The analysis and judgment module is used to compare the current measurement difference obtained from each difference calculation with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope has defocused.

[0070] In some embodiments, the control module calls the measurement program of the scanning electron microscope to measure the control slide. The data processing module collects the measured values ​​by establishing a measurement value control chart. Simultaneously, based on the measurement value control chart, it establishes a measurement difference control sub-chart to calculate the difference between the measured values ​​in the measurement value control chart and collect the resulting measurement differences. The analysis and judgment module sets control limits in the measurement difference control sub-chart to determine that the scanning electron microscope has defocused and trigger an alarm when the current measurement difference exceeds the control limit.

[0071] In some embodiments, the analysis and judgment module is further configured to determine whether the measured values ​​in the measured value control chart are stabilizing. When it is determined that the measured values ​​are stabilizing, the difference between the measured values ​​after the start of stabilization in the measured value control chart is calculated through the measured difference control sub-chart, and control limits are automatically generated based on the collected measured differences and set in the measured difference control sub-chart.

[0072] In some embodiments, the analysis and judgment module is further configured to set a threshold in the measurement value control chart as a standard for determining whether the measurement values ​​have stabilized. Specifically, the data processing module calculates the tilt angle α between the straight line defined by the last n (e.g., 3-5) measurement values ​​and the horizontal axis in the measurement value control chart. When the tilt angle α reaches the threshold, the analysis and judgment module determines that the measurement values ​​have stabilized and calculates the difference between the last n measurement values ​​used for calculation and subsequent measurement values ​​through the measurement difference control sub-chart, collecting the calculated measurement differences. If the tilt angle α does not reach the threshold, indicating that the measurement values ​​have not stabilized, the calculation of the tilt angle α continues.

[0073] In conjunction with the manufacturing execution system's (MES) inhibitor system, abnormal conditions of the scanning electron microscope can be monitored in real time, problems can be correctly analyzed, and such defocusing problems can be resolved through machine adjustment and calibration. Ultimately, the scanning electron microscope can always maintain a good measurement state and accurately output actual critical dimension values.

[0074] In summary, this invention measures the key dimensions of the graphic at the same position on the control plate and calculates the difference between every two adjacent measurements. Utilizing the principle that the measurement difference changes abruptly when defocusing occurs, the measurement difference is compared with a set control limit. When the measurement difference exceeds the control limit, it can be determined that the scanning electron microscope has defocused. This effectively monitors the measurement status of the scanning electron microscope, ensuring it maintains a good measurement state and guarantees the accuracy of the output measurements.

[0075] While embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations fall within the scope and spirit of the invention as set forth in the claims. Furthermore, the invention described herein may have other embodiments and can be implemented or carried out in various ways.

Claims

1. A method for monitoring defocusing in a scanning electron microscope, characterized in that, include: Using a scanning electron microscope, the key dimensions of the graphic at the same location on the control sheet are measured to obtain the measured values; The difference between the current measurement value obtained in each measurement and the adjacent measurement value obtained in the previous measurement is calculated to obtain the measurement difference; The current measurement difference obtained from each difference calculation is compared with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope has defocused. A measurement value control chart is established to collect the obtained measurement values. At the same time, based on the measurement value control chart, a measurement difference control sub-chart is established to calculate the difference between the measurement values ​​in the measurement value control chart and collect the obtained measurement difference. The control limits are determined by the following method: The system determines whether the measured values ​​in the measured value control chart are stabilizing. When the measured values ​​are stabilizing, the system calculates the difference between the measured values ​​after stabilization begins in the measured value control chart using the measured difference control sub-chart. Based on the collected measured difference, the system generates and sets the control limit.

2. The method for monitoring defocusing in a scanning electron microscope according to claim 1, characterized in that, The control limit is set in the measurement difference control sub-chart to trigger an alarm when the current measurement difference exceeds the control limit.

3. The method for monitoring defocusing in a scanning electron microscope according to claim 1, characterized in that, The control limits are generated based on six times the standard deviation calculated from the measurement difference.

4. The method for monitoring defocusing in a scanning electron microscope according to claim 1, characterized in that, A threshold is set in the measurement value control chart as a standard for judging whether the measurement value is stabilizing. The tilt angle between the straight line defined by the last n measurement values ​​and the horizontal axis in the measurement value control chart is calculated. When the tilt angle reaches the threshold, the measurement value is judged to be stabilizing. The difference is calculated on the last n measurement values ​​and the measurement values ​​thereafter through the measurement difference control sub-chart, and the calculated measurement difference is collected.

5. The method for monitoring defocusing in a scanning electron microscope according to claim 4, characterized in that, The value of n is 3 to 5.

6. The method for monitoring defocusing in a scanning electron microscope according to claim 4, characterized in that, The graphics include line graphics.

7. The method for monitoring defocusing in a scanning electron microscope according to claim 6, characterized in that, The threshold is 170 degrees. When the tilt angle is greater than or equal to 170 degrees, the measured value is determined to be stable.

8. A system for monitoring defocusing in a scanning electron microscope, characterized in that, include: The control module is used to control the scanning electron microscope to measure the key dimensions of the image at the same location on the control slide in order to obtain the measurement value; The data processing module is used to calculate the difference between the current measurement value obtained from each measurement and the adjacent measurement value obtained from the previous measurement, and obtain the measurement difference. The analysis and judgment module is used to compare the current measurement difference obtained from each difference calculation with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope has defocused. The control module measures the control slide by calling the measurement program of the scanning electron microscope; the data processing module collects the obtained measurement values ​​by establishing a measurement value control chart, and simultaneously calculates the difference between the measurement values ​​in the measurement value control chart and collects the obtained measurement difference by establishing a measurement difference control sub-chart based on the measurement value control chart. The analysis and judgment module is also used to judge whether the measured value in the measured value control chart is stabilizing, and when the measured value is judged to be stabilizing, the module calculates the difference between the measured values ​​after the stabilization begins in the measured value control chart through the measured difference control sub-chart, and automatically generates the control limit to set based on the collected measured difference through the measured difference control sub-chart.

9. The system for monitoring defocusing of a scanning electron microscope according to claim 8, characterized in that, The analysis and judgment module triggers an alarm when the current measurement difference exceeds the control limit set in the measurement difference control sub-chart.

10. The system for monitoring defocusing of a scanning electron microscope according to claim 8, characterized in that, The analysis and judgment module is further configured to set a threshold in the measurement value control chart as a standard for judging whether the measurement value is stabilizing; wherein, the data processing module calculates the tilt angle between the straight line defined by the last n measurement values ​​in the measurement value control chart and the horizontal axis. When the tilt angle reaches the threshold, the analysis and judgment module judges that the measurement value is stabilizing, and performs the difference calculation on the last n measurement values ​​and the measurement values ​​thereafter through the measurement difference control sub-chart, and collects the calculated measurement difference.

Images