Method and system for monitoring defocusing of scanning electron microscope

Abstract

The invention discloses a method and system for monitoring defocusing of a scanning electron microscope, and the method comprises the steps: employing the scanning electron microscope, and carrying out the measurement of the key size of a pattern at a same position on a control wafer, so as to obtain a measurement value; carrying out difference value calculation on the current measurement value obtained by each measurement and the adjacent measurement value obtained by the previous measurement to obtain a measurement difference value; and comparing the current measurement difference value obtained by each time of difference value calculation with a control limit, and if the current measurement difference value exceeds the control limit, judging that the scanning electron microscope is defocused, thereby realizing effective monitoring of the measurement state of the scanning electron microscope, enabling the scanning electron microscope to be always kept in a good state for measurement, and improving the measurement accuracy. And the accuracy of the output measurement value is ensured.

Description

Technical Field

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

[0002] There are more and more types of semiconductor products. For each layer of photolithography, development and etching on silicon wafer products, the critical dimensions of the graphics need to be measured. The existing method generally uses a scanning electron microscope to measure the critical dimensions, but scanning electron microscopes objectively have defocusing problems. The reason for defocusing is usually related to the electron beam not being focused at the appropriate height, improper magnification resulting in blurred imaging, or suboptimal calibration resulting in electron beam scattering. When a scanning electron microscope measures the critical dimensions of graphics such as lines and holes, if defocusing occurs, it will be affected and cause smearing, virtual edges and other phenomena on the edges of these graphics, resulting in large errors in the measurement values ​​of the critical dimensions (line width, aperture) of the graphics, and cannot reflect the true critical dimension values, thus losing monitoring and reference significance.

[0003] The above defocusing problem is difficult to be discovered in time during the normal automatic measurement process of the scanning electron microscope. The only way to find the abnormality is to check and compare the retrieved measurement images based on experience afterwards. However, this method obviously has hysteresis and instability.

[0004] Therefore, in order to ensure the accuracy of product measurement results, it is particularly important to promptly detect whether there is a defocus problem in the scanning electron microscope. Summary of the invention

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

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

[0007] The present invention provides a method for monitoring defocus of a scanning electron microscope, comprising:

[0008] Use a scanning electron microscope to measure the key dimensions of the pattern at the same position on the control wafer to obtain a measurement value;

[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 by performing the difference calculation each time is compared with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope is defocused.

[0011] Furthermore, a measurement value control chart is established to collect the obtained measurement values, and at the same time, a measurement difference control sub-chart is established based on the measurement value control chart to perform the difference calculation on the measurement values ​​in the measurement value control chart, collect the obtained measurement differences, and set the control limits in the measurement difference control sub-chart to trigger an alarm when the current measurement difference exceeds the control limits.

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

[0013] It is judged whether the measured value in the measured value control chart is stabilizing, and when it is judged that the measured value is stabilizing, the difference calculation is performed on the measured value after the stabilization starts in the measured value control chart through the measured difference control sub-chart, and the control limit is generated for setting based on the collected measured difference.

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

[0015] Furthermore, a threshold is set in the measurement value control chart as a criterion for judging whether the measurement value is stabilizing; wherein, by calculating the inclination angle between the straight line defined by the last n measurement values ​​in the measurement value control chart and the horizontal axis, when the inclination angle reaches the threshold, the measurement value is judged to be stabilizing, and the difference calculation is performed on the last n measurement values ​​used for calculation and the measurement values ​​thereafter through the measurement difference control sub-chart, and the calculated measurement differences are collected.

[0016] Furthermore, the n is 3-5.

[0017] Furthermore, the graphics include line graphics.

[0018] Furthermore, the threshold is 170 degrees, and when the inclination angle is greater than or equal to 170 degrees, it is determined that the measurement value is stabilizing.

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

[0020] A control module is used to control the scanning electron microscope to measure the key dimensions of the pattern at the same position on the control wafer to obtain a measurement value;

[0021] A data processing module is used to calculate the difference between the current measurement value obtained in each measurement and the adjacent measurement value obtained in the previous measurement to obtain a measurement difference;

[0022] The analysis and judgment module is used to compare the current measurement difference obtained by performing the difference calculation each time with the control limit. If the control limit is exceeded, it is judged that the scanning electron microscope is defocused.

[0023] Furthermore, the control module measures the control film 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 at the same time, based on the measurement value control chart, establishes a measurement difference control sub-chart to calculate the difference of the measurement values ​​in the measurement value control chart and collect the obtained measurement differences; the analysis and judgment module triggers an alarm when the current measurement difference exceeds the control limit by setting the control limit in the measurement difference control sub-chart.

[0024] Furthermore, the analysis and judgment module is also used to judge whether the measurement value in the measurement value control chart is stabilizing, and when judging that the measurement value is stabilizing, the difference calculation is performed on the measurement value after the stabilization starts in the measurement value control chart through the measurement difference control sub-chart, and the control limit is automatically generated for setting based on the collected measurement difference through the measurement 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 criterion for judging whether the measurement value is stabilizing; wherein the data processing module calculates the inclination angle between the straight line defined by the last n measurement values ​​in the measurement value control chart and the horizontal axis. When the inclination 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 ​​used for calculation and the measurement values ​​thereafter through the measurement difference control sub-chart, and collects the calculated measurement differences.

[0026] It can be seen from the above technical scheme that the present invention measures the key dimensions of the graphics at the same position on the control sheet, and calculates the difference between every two adjacent measurement values ​​to obtain the measurement difference. By utilizing the law that the measurement difference will undergo a numerical mutation when defocus occurs, by comparing 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 state of the scanning electron microscope, enabling the scanning electron microscope to always maintain a good state for measurement, and ensuring the accuracy of the output measurement value. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0028] Figure 2 The figure is a schematic diagram of the principle of using a measurement difference control sub-chart to monitor the defocus of a scanning electron microscope according to a preferred embodiment of the present invention.

[0029] Figure 3 The figure is a schematic diagram showing the principle of determining a threshold value by using a measurement value control chart according to a preferred embodiment of the present invention.

[0030] Figure 4 The schematic diagram of a system for monitoring defocus of a scanning electron microscope according to a preferred embodiment of the present invention is shown in FIG. DETAILED DESCRIPTION

[0031] In order to make the purpose, 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 part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be understood by people with general skills in the field to which the present invention belongs. "Including" and similar words used in this article mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0032] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings.

[0033] refer to Figure 1 The method for monitoring the defocus of a scanning electron microscope of the present invention comprises the following steps:

[0034] Step S1: using a scanning electron microscope to measure the key dimensions of the pattern at the same position on the control wafer to obtain a measurement value;

[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 by performing the difference calculation each time with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope is defocused.

[0037] In some embodiments, in the above step S1, the key dimensions of the graphics at the same position on the same control wafer are measured at a certain measurement cycle (for example, once a day or when the measurement batch is changed, or when necessary) (that is, the key dimensions of the graphics at the same position on the control wafer are repeatedly measured one after another according to the measurement cycle) to perform monitoring of the scanning electron microscope.

[0038] In some embodiments, in the above step S1, when measuring the control piece, the measurement formula (measurement recipe) corresponding to the pattern to be measured on the scanning electron microscope is called to measure the control piece. And, based on the measurement values ​​obtained by the scanning electron microscope, a measurement value control chart is established in the control system to collect the obtained measurement values. At the same time, the upper control limit and the lower control limit that are automatically generated in the measurement value control chart are canceled to avoid the problem of measurement termination caused by triggering an alarm (because when measuring the same control piece, there will always be a charging process between the scanning electron microscope and the control piece, so that each measurement result will show a certain trend with the scanning time and number of scanning of the electron beam on the scanning electron microscope, so that after a certain number of measurements, the alarm will be triggered due to the setting of the upper control limit and the lower control limit. For example, for the measurement of line graphics, the charging phenomenon will cause the measured line width size to show an overall trend of gradually decreasing with the number of measurements (refer to Figure 3 ), which is reflected in the measurement value control chart as a decreasing trend in the line width value. Therefore, after a certain number of times, the upper and lower control limits will be touched, which will continue to trigger the alarm).

[0039] refer to Figure 2 At the same time, in the above step S2, according to the measurement value control chart, a measurement difference control subchart is established at the next level. The measurement difference control subchart is used to calculate the difference between each current measurement value in the measurement value control chart and the adjacent measurement value obtained by the previous measurement (i.e., to calculate the moving difference between every two adjacent measurement values ​​before and after), and to collect the obtained measurement differences, which are reflected in the measurement difference control subchart.

[0040] Take the line graph as an example. Figure 2 The 1st to 9th measurement differences collected sequentially in the measurement difference control subchart are shown in the figure, and the magnitude of the values ​​is shown on the vertical axis (unit: nm). In this example, among the 9 measurement differences, the largest difference is the 6th measurement difference, whose value is -0.41812, and the smallest difference is the 8th measurement difference, whose value is -0.00094.

[0041] The inventors have discovered that when a scanning electron microscope is defocused, there will be an obvious difference between the current measured result and the previous measured value, which is reflected in the above-mentioned measurement difference. That is, the measurement difference between the current measured value measured after defocusing and the previous adjacent measurement value measured before defocusing is much larger than the other previous adjacent measurement difference.

[0042] Therefore, in the above step S3, when there is a large value mutation between the current measurement difference and the previous adjacent measurement difference, for example Figure 2 In the example, the 10th measured difference is 3.84704, which has an obvious mutation in magnitude from the 9th measured difference of 0.2184, that is, when defocus occurs, the measured difference will immediately increase. Therefore, the above rule can be used to determine that the scanning electron microscope has defocused.

[0043] In some embodiments, a reasonable control limit is set in the measurement difference control sub-chart to quantitatively characterize the above-mentioned sudden change in value. In this way, when the current measurement difference reaches the control limit, it can be determined that the scanning electron microscope has defocused, and an alarm is triggered to stop further measurement and wait for confirmation and processing.

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

[0045] First, each time a scanning electron microscope is used to measure the key dimensions of the graphic at the same position 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 becoming stable. When the measurement value is determined to be stable, the measurement difference control sub-chart is used to calculate the difference of the measurement value after the stabilization begins in the measurement value control chart, and 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 measurement value control chart are stabilizing is to avoid the aforementioned charging process, which causes each measurement result to show a certain trend with the scanning time and number of scanning times of the electron beam on the scanning electron microscope, resulting in the calculated control limit being too wide, thereby failing to accurately judge the occurrence of defocusing.

[0047] In some embodiments, the standard deviation is automatically calculated based on the measurement difference in the measurement difference control sub-chart through the calculation function of the measurement difference control sub-chart, and 6 times the standard deviation is set as the control limit ( Figure 2 The control limits are for reference 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 judging whether the measurement value is stabilizing. The inclination angle between the straight line defined by the last n measurement values ​​collected in the measurement value control chart and the horizontal axis is calculated. When the inclination angle reaches the threshold, the measurement value is judged to be stabilizing. The difference calculation is performed on the last n measurement values ​​used to calculate the inclination angle and the measurement values ​​collected thereafter through the measurement difference control subchart, and the calculated measurement difference is collected.

[0049] In some embodiments, the above n is 3 to 5. That is, the inclination angle between the straight line defined by the last 3 to 5 measured values ​​collected in the measured value control chart and the abscissa 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, it is determined that the measured value is stable. Preferably, the threshold is set to 173.5 degrees.

[0051] Figure 3 The example shows the 1st to 20th measurement values ​​collected sequentially in the measurement value control chart, and the magnitude of the values ​​is reflected on the vertical axis (unit: nm). In this example, the 20 measurement values, according to the measurement time, show that the line width trend decreases due to the charging process and gradually stabilizes (charging saturation).

[0052] In some embodiments, by Figure 3 The inclination angle α (reflecting the slope) between the straight line A defined by the last 5 measured values ​​in the continuously updated measurement value control chart and the horizontal axis (horizontal axis) is calculated. When the inclination angle α reaches the threshold, the measured value is judged to be stable, and the value is determined by Figure 2 The measurement difference control sub-chart performs difference calculation on the last five measurement values ​​used to calculate the inclination angle and the measurement values ​​collected thereafter, and collects the calculated measurement difference values. On the contrary, if the calculated inclination angle α does not reach the threshold, it is necessary to continue measuring the measurement values, and continue to move the inclination angle α of the straight line defined by the last five measurement values ​​until the latest calculated inclination angle α reaches the threshold.

[0053] For example, Figure 3 In the example, when calculating the 11th to 15th measurement values, the calculated inclination angle α is greater than 170 degrees, indicating that the data has been in a stable state since the 11th measurement value. At this time, through the measurement difference control sub-chart, the difference calculation starts from the 11th measurement value (the first measurement difference is the difference between the 12th measurement value and the 11th measurement value), and is collected to obtain the data reflected in Figure 2 The 1st to 9th measurement differences in .

[0054] The measurement value control chart can calculate the standard deviation based on the collected measurement difference data, and set the control limit at 6 times the standard deviation. Through the continuous accumulation of data, the calculated control limit has more long-term applicability.

[0055] It should be noted that when using a control film to measure and monitor whether a scanning electron microscope is defocused, it is generally necessary to wait until the measured value is judged to be stable before starting the formal monitoring process.

[0056] It should also be noted that the above method of setting a threshold in the measurement value control chart as a criterion for judging whether the measurement value is stabilizing, and calculating the inclination angle α to compare it with the threshold to judge whether the measurement value is stabilizing, belongs to a quantitative implementation method provided by the present invention. Figure 3 By observing the shape of the mid-curve, we can roughly judge whether the measured value is stable, and adjust it according to the actual effect.

[0057] In order to verify the practicality of the method of the present invention, an artificial deterioration experiment can be used to defocus the scanning electron microscope, and then the method of the present invention is used to perform measurements, and the measurement values ​​measured in the defocused state are obtained for data comparison with the measurement values ​​in the normal state.

[0058] For example, after a certain degree of defocusing of the scanning electron microscope through a deterioration experiment, the control film is measured and the 21st measurement value is reflected in Figure 3 The difference between the 10th measurement and the 20th measurement (the measurement under normal conditions) is shown in Figure 2 In. It can be seen that Figure 3 between the 21st and 20th measurements in , and Figure 2 The data immediately increased between the 10th and 9th measurement differences, and Figure 2 The 10th measured difference in φ obviously exceeds the control limit. Therefore, it is confirmed that the method of the present invention can effectively monitor the defocus phenomenon.

[0059] The above-mentioned deterioration test method can adopt at least one of focusing the electron beam emitted during measurement at a height position outside the measurement surface, increasing the magnification to blur the image, and scattering the electron beam.

[0060] In some embodiments, line patterns at the same position in multiple (eg, 5) lithography units may be measured on the control wafer, and an average value of the multiple measured line width dimensions may be used as the measurement value to ensure the stability of the measurement result.

[0061] Taking the conventional measurement of the pitch between line graphics as an example, since the degree of influence of defocus on the smear and virtual edges on both sides of each line graphic is relatively consistent, they will be offset each other when measuring the pitch and will not cause obvious deviation in the pitch measurement result. 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 line width of line graphics, the error in the line width measurement value caused by defocusing will have a superposition effect with the normal fluctuation of the actual line width value, thus masking the existence of the defocusing problem. Therefore, when the line width measurement value is abnormal, the problem is often directed to the investigation of the cause of the process, delaying the processing time.

[0063] However, if a fixed control piece is used and the line width of the line graphic at the same position of the control piece is measured each time, the defocus phenomenon is monitored by the deviation rate of the measured value. However, there is always a charging process between the scanning electron microscope and the control piece, so that each measurement result will show a certain downward trend with the scanning time and number of times. After a certain number of measurements, the alarm will be triggered due to the setting of the control limit in the control chart. Therefore, different measuring points and new control pieces need to be replaced frequently and regularly, which not only causes a waste of control piece resources, but also makes it difficult to effectively monitor defocus.

[0064] The present invention measures the line width of the line graphics at the same position on the control sheet, and performs difference calculation on every two adjacent measurement values ​​to obtain the measurement difference. By utilizing the law that the measurement difference will undergo a sudden change in value when defocus occurs, the measurement difference is compared with a set control limit. In this way, when the measurement difference exceeds the control limit, it can be determined that the scanning electron microscope is defocused, thereby achieving effective monitoring of the measurement state of the scanning electron microscope, thereby effectively solving the problem in the prior art that it is difficult to timely discover the defocus problem during the normal automatic measurement process of the scanning electron microscope, and the abnormality can only be discovered by checking and comparing the retrieved measurement images based on experience afterwards, which brings about the shortcomings of hysteresis and instability, thereby ensuring the accuracy of the product measurement results.

[0065] The following is a further detailed description of a system for monitoring defocus of a scanning electron microscope of the present invention through specific implementation modes and in conjunction with the accompanying drawings.

[0066] refer to Figure 4 Combined with reference Figure 2-Figure 3 The system for monitoring the defocus of a scanning electron microscope of the present invention can be used to implement the method for monitoring the defocus of a scanning electron microscope of the present invention. The system for monitoring the defocus of a scanning electron microscope of the present invention comprises: 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 graphics at the same position on the control wafer to obtain the measurement value.

[0068] The data processing module is used to 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.

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

[0070] In some embodiments, the control module measures the control film 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 based on the obtained measurement values. At the same time, according to the measurement value control chart, a measurement difference control sub-chart is established to calculate the difference of the measurement values ​​in the measurement value control chart and collect the obtained measurement differences. The analysis and judgment module sets the control limit in the measurement difference control sub-chart so that when the current measurement difference exceeds the control limit, it is determined that the scanning electron microscope is defocused and an alarm is triggered.

[0071] In some embodiments, the analysis and judgment module is also used to judge whether the measured value in the measured value control chart is stabilizing. When it is judged that the measured value is stabilizing, the difference of the measured value after the stabilization in the measured value control chart is calculated through the measured difference control sub-chart, and the control limit is automatically generated through the measured difference control sub-chart according to the collected measured difference to be set in the measured difference control sub-chart.

[0072] In some embodiments, 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. Among them, the data processing module calculates the inclination angle α between the straight line defined by the last n (for example, 3 to 5) measurement values ​​in the measurement value control chart and the horizontal coordinate. When the inclination angle α reaches the threshold, the analysis and judgment module determines that the measurement value is stabilizing, and calculates the difference between the last n measurement values ​​used for calculation and the measurement values ​​thereafter through the measurement difference control sub-chart, and collects the calculated measurement difference. When the inclination angle α does not reach the threshold and it is judged that the measurement value is not stabilizing, the calculation of the inclination angle α continues.

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

[0074] In summary, the present invention measures the key dimensions of the graphics at the same position on the control sheet, and calculates the difference between every two adjacent measurement values ​​to obtain a measurement difference. By utilizing the law that the measurement difference will undergo a sudden change in value when defocus occurs, the measurement difference is compared with the set control limit. When the measurement difference exceeds the control limit, it can be determined that the scanning electron microscope is defocused, thereby achieving effective monitoring of the measurement state of the scanning electron microscope, enabling the scanning electron microscope to always maintain a good state for measurement, and ensuring the accuracy of the output measurement value.

[0075] Although the embodiments of the present invention are described in detail above, it is obvious 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 are within the scope and spirit of the present invention as described in the claims. Moreover, the present invention described herein may have other embodiments and may be implemented or realized in a variety of ways.

Claims

1. A method for monitoring defocus of a scanning electron microscope, characterized in that: include: Use a scanning electron microscope to measure the key dimensions of the pattern at the same position on the control wafer to obtain a measurement value; 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 by performing the difference calculation each time is compared with the control limit. If the difference exceeds the control limit, it is determined that the scanning electron microscope is defocused.

2. The method for monitoring defocus of a scanning electron microscope according to claim 1, characterized in that: 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 perform the difference calculation on the measurement values ​​in the measurement value control chart, collect the obtained measurement differences, and set the control limits in the measurement difference control sub-chart to trigger an alarm when the current measurement difference exceeds the control limits.

3. The method for monitoring defocus of a scanning electron microscope according to claim 2, characterized in that: The control limits are determined by the following method: It is judged whether the measured value in the measured value control chart is stabilizing, and when it is judged that the measured value is stabilizing, the difference calculation is performed on the measured value after the stabilization starts in the measured value control chart through the measured difference control sub-chart, and the control limit is generated for setting based on the collected measured difference.

4. The method for monitoring defocus of a scanning electron microscope according to claim 3, characterized in that: The control limits are generated as 6 times the standard deviation calculated from the measurement differences.

5. The method for monitoring defocus of a scanning electron microscope according to claim 3, characterized in that: A threshold is set in the measurement value control chart as a criterion for judging whether the measurement value is stabilizing; wherein, the inclination angle between the straight line defined by the last n measurement values ​​in the measurement value control chart and the horizontal axis is calculated, and when the inclination angle reaches the threshold, the measurement value is judged to be stabilizing, and the difference calculation is performed on the last n measurement values ​​used for calculation and the measurement values ​​thereafter through the measurement difference control sub-chart, and the calculated measurement differences are collected.

6. The method for monitoring defocus of a scanning electron microscope according to claim 5, characterized in that: The n is 3-5.

7. The method for monitoring defocus of a scanning electron microscope according to claim 5, characterized in that: The graphics include line graphics.

8. The method for monitoring defocus of a scanning electron microscope according to claim 7, characterized in that: The threshold is 170 degrees. When the inclination angle is greater than or equal to 170 degrees, it is determined that the measurement value is stable.

9. A system for monitoring defocus of a scanning electron microscope, characterized in that: include: A control module is used to control the scanning electron microscope to measure the key dimensions of the pattern at the same position on the control wafer to obtain a measurement value; A data processing module is used to calculate the difference between the current measurement value obtained in each measurement and the adjacent measurement value obtained in the previous measurement to obtain a measurement difference; The analysis and judgment module is used to compare the current measurement difference obtained by performing the difference calculation each time with the control limit. If the control limit is exceeded, it is judged that the scanning electron microscope is defocused.

10. The system for monitoring defocus of a scanning electron microscope according to claim 9, characterized in that: The control module measures the control piece 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 at the same time, according to the measurement value control chart, establishes a measurement difference control sub-chart to calculate the difference of the measurement values ​​in the measurement value control chart, and collects the obtained measurement difference values; The analysis and judgment module uses the control limit set in the measurement difference control sub-chart to trigger an alarm when the current measurement difference exceeds the control limit.

11. The system for monitoring defocus of a scanning electron microscope according to claim 10, characterized in that: The analysis and judgment module is also used to judge whether the measurement value in the measurement value control chart is stabilizing, and when judging that the measurement value is stabilizing, the difference calculation is performed on the measurement value after the stabilization starts in the measurement value control chart through the measurement difference control sub-chart, and the control limit is automatically generated for setting based on the collected measurement difference through the measurement difference control sub-chart.

12. The system for monitoring defocus of a scanning electron microscope according to claim 11, characterized in that: 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 inclination angle between the straight line defined by the last n measurement values ​​in the measurement value control chart and the horizontal axis. When the inclination 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 ​​used for calculation and the measurement values ​​thereafter through the measurement difference control sub-chart, and collects the calculated measurement differences.

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