A monitoring method for overlay accuracy

Abstract

The present invention provides a method for monitoring overlay accuracy, which collects alignment parameters of multiple wafers in multiple batches on a photolithography machine that have completed a certain layer of photolithography and have stable and normal overlay accuracy during photolithography; sets the mean value of the alignment parameters as a reference value for the overlay accuracy of the photolithography layer; provides a wafer for photolithography of the layer and collects the measured values ​​of the overlay accuracy of multiple points or all points on the wafer in real time; makes a difference between the measured value and the reference value to obtain a subtraction difference; determines the overlay error according to the subtraction difference; compares the overlay error with a preset threshold value to realize monitoring of the overlay accuracy. The present invention establishes a reference in advance, makes a difference between the measured value of the overlay accuracy and the reference value during photolithography of subsequent batches of wafers, and then performs card control on the differential result, which can effectively monitor the abnormal overlay accuracy caused by position change, thereby realizing effective monitoring of the overlay accuracy.

Description

Technical Field

[0001] The invention relates to the technical field of semiconductor manufacturing, and in particular to a method for monitoring overlay accuracy. Background Art

[0002] Overlay is an important parameter in the integrated circuit manufacturing process. If the overlay accuracy is abnormal, it will lead to abnormal alignment between different layers, thus causing circuit failure and wafer scrapping. Therefore, monitoring the overlay accuracy becomes increasingly important.

[0003] At present, the common method for monitoring overlay accuracy is to measure the overlay accuracy and monitor the mean, maximum, minimum and standard deviation of the offset value. However, in actual production, if only the position of the maximum and minimum values ​​changes, the mean, maximum, minimum and standard deviation of the above four parameters cannot monitor the abnormal overlay accuracy, resulting in monitoring failure. This monitoring failure phenomenon is more likely to occur when measuring multiple points or all points. Summary of the invention

[0004] In view of this, the present invention provides a method for monitoring overlay accuracy, so as to solve the problem that the existing monitoring method cannot fully and effectively realize the monitoring of overlay accuracy.

[0005] The present invention provides a method for monitoring overlay accuracy, comprising the following steps:

[0006] Step 1, collecting alignment parameters of multiple wafers in multiple batches on a lithography machine that have completed lithography of a certain layer and have stable and normal overlay accuracy;

[0007] Step 2, setting the mean value of the alignment parameter as the reference value of the overlay accuracy of the photolithography layer;

[0008] Step 3, providing a wafer for photolithography of the layer and collecting the measured values ​​of overlay accuracy of multiple points or all points on the wafer in real time;

[0009] Step 4: subtract the measured value from the reference value to obtain a difference value;

[0010] Step 5, determining the overlay error according to the subtraction difference value;

[0011] Step six: compare the overlay error with a preset threshold to monitor the overlay accuracy.

[0012] Preferably, the multiple batches in step one are at least three batches.

[0013] Preferably, the plurality of wafers in step one is at least 75 wafers.

[0014] Preferably, the alignment parameter in step one and the measured value in step three are at least one of the alignment accuracy X in the horizontal direction and the alignment accuracy Y in the vertical direction.

[0015] Preferably, the overlay error in step five is the sum of the mean of the subtraction difference value and three times the standard deviation of the subtraction difference value.

[0016] Preferably, the preset threshold in step six is ​​a pre-set value for determining the overlay accuracy.

[0017] Preferably, the step of comparing the overlay error with a preset threshold to monitor the overlay accuracy in step six includes: when the overlay error does not exceed the preset threshold, the overlay accuracy is normal; when the overlay error exceeds the preset threshold, the overlay accuracy is abnormal.

[0018] Preferably, the monitoring of the overlay accuracy in step six is ​​performed through a statistical process control system.

[0019] Preferably, the method is used to monitor the overlay accuracy of positional anomalies of multiple points or all points on a wafer.

[0020] The present invention first determines the baseline value of the current level overlay accuracy, and then uses the baseline value as the basis to differentiate the monitored overlay accuracy measurement value, and performs SPC (Statistical Process Control) specification monitoring on the differential result. This can more sensitively detect overlay accuracy anomalies and directly observe the location of the overlay accuracy anomalies, which is beneficial to subsequent improvement and compensation of the overlay accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0022] Figure 1 A schematic diagram showing the overlay accuracy of a layer under stable conditions;

[0023] Figure 2 A schematic diagram showing the overlay accuracy of an abnormal batch;

[0024] Figure 3 Shown is a flow chart of a method for monitoring overlay accuracy according to an embodiment of the present invention;

[0025] Figure 4 Displays the normal batch overlay accuracy and Figure 1 The difference diagram of the benchmark overlay accuracy;

[0026] Figure 5 Displayed as abnormal batch overlay accuracy and Figure 1 Difference plot of the benchmark overlay accuracy. DETAILED DESCRIPTION

[0027] The present invention is described below based on embodiments, but the present invention is not limited to these embodiments. In the detailed description of the present invention below, some specific details are described in detail. It is possible for a person skilled in the art to fully understand the present invention without the description of these details. In order to avoid confusing the essence of the present invention, known methods, processes, flows, components and circuits are not described in detail.

[0028] In addition, persons of ordinary skill in the art will appreciate that the drawings provided herein are for illustration purposes and are not necessarily drawn to scale.

[0029] Unless the context clearly requires otherwise, the words "include", "comprising" and similar words throughout the application should be interpreted as including rather than exclusive or exhaustive; that is, the meaning is "including but not limited to".

[0030] In the description of the present invention, it should be understood that the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. In addition, in the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0031] Overlay accuracy (ovl) refers to the accuracy of the overlap between the current layer pattern and the previous layer pattern in the photolithography manufacturing process. Since the manufacture of integrated circuit chips is made by stacking multiple circuit layers, if the current layer and the previous layer are not aligned, the chip will not work properly. Therefore, it is extremely important to monitor the overlay accuracy.

[0032] like Figure 1 and Figure 2 As shown, Figure 1 A schematic diagram showing the overlay accuracy of a layer under stable conditions; Figure 2 The diagram shows the overlay accuracy of an abnormal batch. The stable situation refers to the situation where the overlay accuracy is normal and stable. Figure 1 The overlay accuracy of this overlay accuracy is used as the overlay accuracy benchmark for this layer. The overlay accuracy can be monitored through the mean, maximum, minimum, and standard deviation of the overlay accuracy of this layer. The sum of the mean and three times the standard deviation (M3S) is usually used to determine whether the overlay accuracy is normal or not. For example, Figure 1 The medium overlay accuracy of M3S is 71.2nm. Figure 2 The M3S of the medium overlay accuracy is 72.3nm. The numerical difference between the two is not large, and the monitoring result of the overlay accuracy should be normal. However, in fact, Figure 2 Set the engraving accuracy for an abnormal batch.

[0033] That is to say, in Figure 2 The existing overlay accuracy monitoring method fails in the monitoring of the overlay accuracy shown. After analysis, if only the positions of the maximum and minimum values ​​in the overlay accuracy measurement change, then the mean, maximum, minimum, and standard deviation of the above four parameters cannot monitor the overlay accuracy abnormality, resulting in monitoring failure. In other words, the overlay mark graphics are not aligned, but the values ​​of the overlay accuracy measurement are similar, but the positions are different, which will cause the overlay accuracy monitoring to fail. This monitoring failure phenomenon is more likely to occur when measuring multiple points or all points.

[0034] In view of the above situation, the present invention proposes a new monitoring method for overlay accuracy to solve the problem in the prior art that the overlay accuracy abnormality caused by position change cannot be detected when monitoring the overlay accuracy of multiple points or all points. The technical solution of the present invention is further explained below in conjunction with the accompanying drawings and through specific implementation methods.

[0035] Figure 3 The flowchart of the method for monitoring the overlay accuracy according to the embodiment of the present invention is shown. Figure 3 As shown, the following steps are included:

[0036] Step 1: Collect alignment parameters of multiple wafers that have completed photolithography of a certain layer in multiple batches on a photolithography machine and have stable and normal overlay accuracy.

[0037] Step 2: Setting the mean value of the alignment parameters as the reference value of the overlay accuracy of the lithography layer.

[0038] Collecting overlay accuracy data with good overlay accuracy performance of multiple batches of multiple wafers can ensure the accuracy of the set overlay accuracy benchmark. Therefore, in the embodiment of the present invention, the multiple batches are at least three batches, and the multiple wafers are at least 75 wafers. The alignment parameter is at least one of the horizontal alignment accuracy X and the vertical alignment accuracy Y. The average value of each collected alignment parameter is used as the benchmark value.

[0039] Step 3: Provide the wafer for photolithography of the layer and collect the measurement values ​​of the overlay accuracy of multiple points or all points on the wafer in real time.

[0040] Complete overlay accuracy is to measure the vector position change between the current layer and the previous layer of each exposure field on the entire wafer. However, in actual production, 5-10 points are usually measured on a wafer to characterize the overlay accuracy of the wafer. In an embodiment of the present invention, the measured values ​​of the overlay accuracy of multiple points or all points on the current lithography wafer are collected. The measured value is at least one of the horizontal alignment accuracy X and the vertical alignment accuracy Y. Of course, the measurement value type is consistent with the alignment parameter in step one.

[0041] Step 4: Difference the measured value and the reference value to obtain the difference value.

[0042] In the embodiment of the present invention, the monitoring failure phenomenon is avoided by making a difference between the measured value and the reference value, that is, subtracting the measured value of the overlay accuracy from the reference value of the overlay accuracy.

[0043] Step 5: Determine the overlay error based on the difference value.

[0044] In order to ensure that the circuits designed in the upper and lower layers can be reliably connected, the overlay error between a point in the current layer and the corresponding point in the reference layer must be less than 1 / 3 of the minimum spacing of the pattern. In the embodiment of the present invention, the overlay error is the sum of the mean of the difference value and the standard deviation of three times the difference value. Of course, it can also be other settings according to actual conditions.

[0045] Step 6: Compare the overlay error with a preset threshold to monitor the overlay accuracy.

[0046] In the embodiment of the present invention, the preset threshold is a value pre-set to judge the overlay accuracy, and the monitoring of the overlay accuracy is performed through SPC (Statistical Process Control). When the overlay error does not exceed the preset threshold, the overlay accuracy is normal. When the overlay error exceeds the preset threshold, the overlay accuracy is abnormal. Figure 4 As shown, Figure 4 Displays the normal batch overlay accuracy and Figure 1 The difference diagram of the benchmark overlay accuracy shows that the overlay error is 16.6nm. Figure 5 As shown, Figure 5 Displayed as abnormal batch overlay accuracy and Figure 1 The difference diagram of the benchmark overlay accuracy. Among them, the abnormal batch is Figure 2 The batch shown. The overlay error is 47.5nm. According to conventional thinking, the overlay error between a normal batch and the reference difference is 16.6nm, so the preset threshold should be within a smaller range of about 16.6nm, but Figure 2 The overlay error between the batch shown and the reference batch is 47.5nm, much higher than 16.6nm. Figure 2 The batch shown is an abnormal batch. Figure 2 Compared with the conclusion of a normal batch, the monitoring result of the overlay accuracy in the embodiment of the present invention is correct. In other words, the overlay accuracy monitoring method in the embodiment of the present invention can monitor the overlay accuracy more effectively.

[0047] Moreover, in contrast Figure 2 and Figure 5 It can be seen that, compared with the existing overlay accuracy monitoring method, the monitoring of the overlay accuracy after the difference with the reference value in the embodiment of the present invention can more sensitively detect the overlay accuracy anomaly and can intuitively see the position of the anomaly.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for monitoring overlay accuracy, characterized in that: The following steps are involved: Step 1, collecting alignment parameters of multiple wafers in multiple batches on a lithography machine that have completed lithography of a certain layer and have stable and normal overlay accuracy; Step 2, setting the mean value of the alignment parameter as the reference value of the overlay accuracy of the photolithography layer; Step 3, providing a wafer for photolithography of the layer and collecting the measured values ​​of overlay accuracy of multiple points or all points on the wafer in real time; Step 4: subtract the measured value from the reference value to obtain a difference value; Step 5, determining the overlay error according to the subtraction difference value; Step six: compare the overlay error with a preset threshold to monitor the overlay accuracy.

2. The method for monitoring overlay accuracy according to claim 1, characterized in that: The multiple batches described in step one are at least three batches.

3. The method for monitoring overlay accuracy according to claim 1, characterized in that: The plurality of wafers in step 1 is at least 75 wafers.

4. The method for monitoring overlay accuracy according to claim 1, characterized in that: The alignment parameter in step 1 and the measured value in step 3 are at least one of the horizontal alignment accuracy X and the vertical alignment accuracy Y.

5. The method for monitoring overlay accuracy according to claim 1, characterized in that: The overlay error in step 5 is the sum of the mean of the subtraction difference value and three times the standard deviation of the subtraction difference value.

6. The method for monitoring overlay accuracy according to claim 1, characterized in that: The preset threshold in step six is ​​a preset value for judging the overlay accuracy.

7. The method for monitoring overlay accuracy according to claim 6, characterized in that: The step of comparing the overlay error with the preset threshold to monitor the overlay accuracy in step 6 includes: when the overlay error does not exceed the preset threshold, the overlay accuracy is normal; when the overlay error exceeds the preset threshold, the overlay accuracy is abnormal.

8. The method for monitoring overlay accuracy according to claim 1, characterized in that: The monitoring of the overlay accuracy described in step six is ​​performed through a statistical process control system.

9. The method for monitoring overlay accuracy according to claim 1, characterized in that: The method is used for monitoring the overlay accuracy of positional anomalies of multiple points or all points on a wafer.

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