Method for monitoring focal length of photoetching machine

Abstract

The invention discloses a method for monitoring the focal length of a photoetching machine, which comprises the following steps of: 1, performing exposure to form a first photoresist pattern group comprising a plurality of first photoresist patterns on a reference wafer, the design values and exposure energy of the critical dimensions of the first photoresist patterns are respectively the same, and the exposure focal lengths are sequentially changed; step 2, performing electron beam scanning with a first angle by adopting a CDSEM to image each first photoresist pattern and measure the side width, and then establishing a first relational expression between the side width of the first photoresist pattern and the exposure focal length; and step 3, exposing an online second wafer by using the photoetching machine and forming a second photoresist pattern. And 4, performing electron beam scanning with a first angle by adopting a CDSEM to image at least one second photoresist pattern and measure the side width. And obtaining the actual exposure focal length of the photoetching machine according to the side width of the second photoresist pattern and the first relational expression. According to the invention, the focal length of the photoetching machine can be monitored in time, and the measurement accuracy, efficiency and product yield are improved.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a semiconductor integrated circuit, and in particular to a method for monitoring the focus of a photolithography machine (scanner). Background Art

[0002] With the advancement of technology nodes, device nodes will shrink, the lithography depth of focus (DoF) will become smaller and smaller, the focal process window of the lithography machine of the lithography process will become much smaller, and the focus error will become more and more important to product performance. Scanner focus monitoring is also becoming more and more important. Currently, the scanner focus is monitored by offline monitoring of wafers to monitor the focus, which cannot provide timely feedback on the machine focus. This offline method of obtaining scanner focus has a lag, and the focus of some product pieces is offset and cannot be discovered immediately, which ultimately affects the product yield. In addition, at present, the focal condition of the lithography machine is most commonly monitored through the Focus Energy Matrix (FEM). The information obtained by checking the focal length using FEM is inaccurate and the measurement time is long.

[0003] In the photolithography process, if the focal length of the photolithography machine is offset and cannot be discovered in time, it will cause huge losses to production. Some existing methods mainly use methods of measuring key dimensions or measuring overlay accuracy for monitoring, but they all require special exposure of a wafer to monitor the status of the machine, and at the same time, measuring key dimensions has high requirements for the measurement position. When the measurement position changes, it will cause great interference to the final result. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for monitoring the focal length of a lithography machine, which can timely monitor the focal length of the lithography machine and improve the accuracy of focal length monitoring, measurement efficiency and product yield.

[0005] In order to solve the above technical problems, the method for monitoring the focal length of a lithography machine provided by the present invention comprises the following steps:

[0006] Step 1: providing a reference wafer, and using a photolithography machine to perform exposure to form at least one first photoresist pattern group on the reference wafer, wherein the first photoresist pattern group includes a plurality of first photoresist patterns.

[0007] The critical dimensions of the first photoresist patterns have the same design value and the same corresponding exposure energy.

[0008] The exposure focal lengths of the first photoresist patterns change sequentially, and one first photoresist pattern corresponds to one exposure focal length.

[0009] Step 2: Use a critical dimension scanning electron microscope (CDSEM) to perform electron beam scanning with a first angle to image each of the first photoresist patterns and measure the side width of each of the first photoresist patterns, and then establish a first relationship between the side width of the first photoresist pattern and the exposure focal length.

[0010] Step three: using the photolithography machine to expose the second wafer online and form at least one second photoresist pattern.

[0011] Step 4: Use CDSEM to perform electron beam scanning at the first angle to image at least one of the second photoresist patterns and measure the side width of the second photoresist pattern.

[0012] The actual exposure focal length of the photolithography machine during online exposure is obtained according to the side width of the second photoresist pattern and the first relationship.

[0013] A further improvement is that the reference wafer directly adopts the FEM wafer, and the first photoresist pattern group adopts a group of the first photoresist patterns corresponding to a selected exposure energy in the FEM wafer.

[0014] A further improvement is that the size of the first angle is set to ensure that the side width of the first photoresist pattern meets the measurement accuracy requirement in the CDSEM imaging image, and the larger the first angle is, the higher the measurement accuracy is.

[0015] A further improvement is that the minimum side width of the first photoresist pattern reaches 2nm to 3nm.

[0016] A further improvement is that the first angle includes 5 degrees.

[0017] A further improvement is that the second wafer is a product wafer.

[0018] A further improvement is that the first photoresist pattern group includes a corresponding exposure focal length equal to the reference exposure focal length of the photolithography machine; the exposure focal length of each first photoresist pattern is represented by an offset value from the reference exposure focal length.

[0019] A further improvement is that, in step 1, the change in the exposure focal length between two adjacent first photoresist patterns is within 0.2 μm.

[0020] The present invention uses CDSEM to scan the side widths of each first photoresist pattern in the first photoresist pattern group with exposure focal lengths changing in sequence by using an electron beam with a first angle, so as to obtain a first relational expression between the side widths of the first photoresist pattern and the exposure focal length. Since the electron beam scanning with the first angle can magnify a side dimension of the first photoresist pattern in the CDSEM imaged pattern by a certain proportion, the measurement accuracy of the side width of the first photoresist pattern can be improved, so that the accuracy of the first relational expression can be improved. After exposing the second wafer, the actual exposure focal length during lithography exposure can be obtained by measuring the side width of the second photoresist pattern and inversely calculating it according to the first relational expression. Therefore, the present invention can timely monitor the focal length of the lithography machine by monitoring the online product through CDSEM; the accuracy of focal length monitoring can be improved by CDSEM measurement of the side width of the photoresist pattern; compared with the existing method of using FEM to monitor focal length, the method of the present invention is simpler and can improve monitoring efficiency; finally, the product yield can also be improved. Furthermore, the present invention uses CDSEM to monitor the actual focal length of the online product during the photolithography process. Therefore, the present invention can use CDSEM to simultaneously monitor the line width of the photoresist pattern and monitor the focal length of the photolithography machine. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0022] Figure 1A This is a SEM photo of the existing photoresist FEM;

[0023] Figure 1B yes Figure 1A Magnified photos of photoresist patterns at different focal lengths;

[0024] Figure 2A It is a front view of the photoresist pattern in the existing CDSEM imaging;

[0025] Figure 2B It is a top view of the photoresist pattern in the existing CDSEM imaging;

[0026] Figure 2C It is a photograph corresponding to the front view of the photoresist pattern obtained by existing CDSEM imaging;

[0027] Figure 2D It is a photograph corresponding to the top view of the photoresist pattern obtained by existing CDSEM imaging;

[0028] Figure 3 It is a curve of each photoresist pattern of the photoresist FEM obtained by existing CDSEM imaging;

[0029] Figure 4 is a flow chart of a method for monitoring the focal length of a lithography machine according to an embodiment of the present invention;

[0030] Figure 5A is a front view of a photoresist pattern during CDSEM imaging in a method for monitoring the focal length of a photolithography machine according to an embodiment of the present invention;

[0031] Figure 5B is a top view of a photoresist pattern during CDSEM imaging in a method for monitoring the focal length of a photolithography machine according to an embodiment of the present invention;

[0032] Figure 5C It is a photograph corresponding to a top view of a photoresist pattern during CDSEM imaging in a method for monitoring the focal length of a photolithography machine according to an embodiment of the present invention;

[0033] Figure 6 is a curve of the critical dimensions of each photoresist pattern of the photoresist FEM obtained by CDSEM imaging in the method for monitoring the focal length of a photolithography machine in an embodiment of the present invention;

[0034] Figure 7 It is a curve between the side width of the first photoresist pattern and the exposure focal length in the method for monitoring the focal length of a lithography machine according to an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The method of the embodiment of the present invention is obtained on the basis of a detailed analysis of the existing method. Before the method of the embodiment of the present invention is introduced in detail, the existing method is further described as follows:

[0036] An existing method is to use FEM to monitor the focal length of the lithography machine, such as Figure 1A As shown, it is a SEM photo of the existing photoresist FEM; the photoresist pattern on the wafer reflects the FEM, that is, the matrix of the photoresist pattern corresponds to the FEM. As can be seen from FIG1, the FEM wafer includes a plurality of exposure areas 101, and the exposure area 101 has a plurality of photoresist patterns 102. The exposure areas 101 are arranged in a matrix structure. The focal lengths corresponding to the exposure areas 101 in the same row are the same, and the exposure energy, that is, the exposure dose, will gradually change; the exposure energy corresponding to the exposure areas 101 in the same column is the same, and the focal length will gradually change.

[0037] The right side of FIG. 1 shows a plurality of exposure focal lengths, where F=0.0 μm represents a base exposure focal length, and specific values ​​of other exposure focal lengths are also given, with a deviation between two adjacent exposure focal lengths being 0.2 μm.

[0038] like Figure 3 As shown, it is the curve of each photoresist pattern of the photoresist FEM obtained by conventional CDSEM imaging; Figure 3 In the figure, the horizontal axis is the focal length, the vertical axis is the critical size, and it includes multiple curves and each curve corresponds to an exposure energy. Figure 6 The bottom of the figure shows a total of 9 exposure energies in mJ. Figure 3 The corresponding curve is used to monitor the focal length, but the monitoring result has the disadvantages of being inaccurate and taking a long time.

[0039] The applicant has found through analysis that the morphology of the photoresist pattern 102 formed by exposure at different focal lengths in different FIG. 1 is different. For a clearer understanding, please refer to Figure 1B As shown, Figure 1B yes Figure 1A Enlarged photos of photoresist patterns at different focal lengths. Figure 1B In the figure, the photoresist pattern 102a is a pattern when negative defocus, the photoresist pattern 102b is a pattern when the best focus is achieved, and the photoresist pattern 102c is a pattern when positive defocus is achieved. Negative defocus corresponds to the focal length F in FIG. 1 taking a negative value, such as one of -0.2 μm to -0.8 μm, and positive defocus corresponds to the focal length F in FIG. 1 taking a positive value, such as one of 0.2 μm to 0.8 μm. Best focus corresponds to F=0 μm.

[0040] It can be seen that the photoresist pattern 102a has a cross-sectional structure of an inverted trapezoid with a large top width, the cross-sectional structure of the photoresist pattern 102b is close to a rectangle, and the photoresist pattern 102b has a cross-sectional structure of a regular trapezoid with a small top width. Therefore, the applicant believes that the photoresist pattern 102 will reflect the characteristics of the exposure focal length. If the side width (SW) of the photoresist pattern 102 can be measured, the relationship between the focal length and the side width can be obtained. However, the side width of the photoresist pattern 102 is small, such as only 2nm to 3nm, and the CDSEM cannot be used to accurately measure the side width of the photoresist pattern 102.

[0041] like Figure 2A , which is a front view of a photoresist pattern in existing CDSEM imaging; Figure 2A In the embodiment, the photoresist pattern 201 has a regular trapezoidal cross-sectional structure with a narrow top and symmetrical side surfaces 201a and 201b. During CDSEM imaging, the electron beam 202 scans the top surface of the wafer, namely the photoresist pattern 201, vertically.

[0042] like Figure 2B As shown, it is a top view of the photoresist pattern in the existing CDSEM imaging; when the electron beam 202 is irradiated vertically, Figure 2B The width of the side surface 201a observed in Figure 2A The width of the projection of the side surface 201a on the bottom surface of the photoresist pattern 201 may reach 2nm to 3nm, which is not easy to measure accurately.

[0043] like Figure 2C As shown, it is a photograph corresponding to the front view of the photoresist pattern obtained by conventional CDSEM imaging, and the photograph of the photoresist pattern 201 is separately indicated by the mark 201 ′.

[0044] like Figure 2D As shown, it is a photograph corresponding to the top view of the photoresist pattern obtained by conventional CDSEM imaging; the photograph of the side surface 201a is separately indicated by the mark 201a'.

[0045] like Figure 4 , which is a flow chart of a method for monitoring the focal length of a lithography machine according to an embodiment of the present invention; the method for monitoring the focal length of a lithography machine according to an embodiment of the present invention comprises the following steps:

[0046] Step 1: Provide a reference wafer, and use a photolithography machine to perform exposure to form at least one first photoresist pattern group on the reference wafer, wherein the first photoresist pattern group includes a plurality of first photoresist patterns 301. The first photoresist pattern 301 is shown in FIG. Figure 5A shown.

[0047] The critical dimensions of the first photoresist patterns 301 have the same design value and the same corresponding exposure energy.

[0048] The exposure focal lengths of the first photoresist patterns 301 change sequentially, and one first photoresist pattern 301 corresponds to one exposure focal length.

[0049] In the embodiment of the present invention, the reference wafer directly adopts the FEM wafer, and the first photoresist pattern group adopts a group of the first photoresist patterns 301 corresponding to a selected exposure energy in the FEM wafer. The FEM wafer represents a wafer that has been exposed to photoresist according to the distribution of the FEM. The photoresist on the wafer reflects the FEM. Please refer to the SEM photo of the photoresist FEM as shown in FIG1, which includes a plurality of exposure areas 101, and the exposure area 101 has a plurality of photoresist patterns 102. Each exposure area 101 is arranged in a matrix structure. The focal lengths corresponding to the exposure areas 101 in the same row are the same, and the exposure energy, that is, the exposure dose, will gradually change; the exposure energy corresponding to the exposure areas 101 in the same column is the same, and the focal length will gradually change. When the FEM wafer is used, the first photoresist pattern group can be formed by using a plurality of the first photoresist patterns 301 arranged continuously on the exposure areas 101 in a column.

[0050] In other embodiments, a FEM wafer may not be used, that is, a matrix structure in which rows and columns are gradually changed is not required, but only a gradual change in focal length is required.

[0051] In the embodiment of the present invention, the first photoresist pattern group includes a corresponding exposure focal length equal to the base exposure focal length of the lithography machine; the exposure focal length of each first photoresist pattern 301 is represented by an offset value from the base exposure focal length. Preferably, the change in the exposure focal length between two adjacent first photoresist patterns 301 includes 0.2 μm. For further understanding, please refer to FIG. 1 at the same time. The right side of FIG. 1 shows the change in the exposure focal length, where F=0.0 μm represents the base exposure focal length, and the specific values ​​of the other exposure focal lengths are also given. Please refer to FIG. 1 for specific values.

[0052] Step 2: Using CDSEM to perform electron beam scanning with a first angle θ to image each of the first photoresist patterns 301 and measure the side width of each of the first photoresist patterns 301, and then establish a first relationship between the side width of the first photoresist pattern 301 and the exposure focal length. Performing electron beam scanning with a first angle θ utilizes the angle (tilt) function of CDSEM.

[0053] like Figure 5A As shown, it is a front view of a photoresist pattern, i.e., a photoresist pattern, during CDSEM imaging in a method for monitoring the focal length of a photolithography machine according to an embodiment of the present invention; the angle between the electron beam 302 and the normal line of the top surface of the first photoresist pattern 301 is the first angle θ; Figure 5A As shown in FIG. 3 , the first photoresist pattern 301 is rotated by the first angle θ.

[0054] like Figure 5B , which is a top view of a photoresist pattern during CDSEM imaging in a method for monitoring the focal length of a photolithography machine according to an embodiment of the present invention; when the electron beam 302 has a first angle θ, the widths observed from the top views of the side surfaces 301a and 301b of the first photoresist pattern 301 are not symmetrical, wherein the width of the side surface 301a increases, thereby facilitating accurate measurement of the side width of the first photoresist pattern 301, Figure 5C It is a photograph corresponding to a top view of a photoresist pattern during CDSEM imaging in a method for monitoring the focal length of a photolithography machine according to an embodiment of the present invention; Figure 5C In the side view 301a' Figure 5B An imaging photograph of the middle side 301a.

[0055] This can ultimately increase the measurement accuracy of the side width and of the first photoresist pattern 301 and the fitting accuracy of the first relational expression. Figure 7 As shown, a curve 401 is shown between the side width of the first photoresist pattern 301 and the exposure focal length in the method for monitoring the focal length of a lithography machine according to an embodiment of the present invention. The curve 401 corresponds to the first relational expression.

[0056] like Figure 6 , which is a curve of the critical dimensions of each photoresist pattern of the photoresist FEM obtained by CDSEM imaging in the method for monitoring the focal length of a photolithography machine according to an embodiment of the present invention. Figure 6 In the figure, the horizontal axis is the focal length, the vertical axis is the critical size, and it includes multiple curves and each curve corresponds to an exposure energy. Figure 6 The bottom of the figure shows a total of 9 exposure energies, in mJ. In the embodiment of the present invention, the subsequent step 3 does not use Figure 6 The curve is used to obtain the actual exposure focal length, and it is necessary to use Figure 7 The actual exposure focal length can be obtained by using curve 401 in or directly using the first relationship.

[0057] In the embodiment of the present invention, the size of the first angle θ is set to ensure that the side width of the first photoresist pattern 301 meets the measurement accuracy requirement in the CDSEM imaging image. The larger the first angle θ is, the higher the measurement accuracy is.

[0058] In some embodiments, the minimum side width of the first photoresist pattern 301 is 2 nm to 3 nm.

[0059] The first angle θ comprises 5 degrees. Figure 6 and Figure 7 The first angle in the corresponding CDSEM imaging is 5 degrees.

[0060] Step three: using the photolithography machine to expose the second wafer online and form at least one second photoresist pattern.

[0061] In the embodiment of the present invention, the second wafer is a product wafer. The second wafer directly uses the product wafer, so that the actual exposure focal length of the lithography machine can be monitored in real time.

[0062] In other embodiments, the second wafer may be a test wafer.

[0063] Step 4: Using CDSEM to perform electron beam scanning with the first angle θ to image at least one of the second photoresist patterns and measure the side width of the second photoresist pattern.

[0064] The actual exposure focal length of the photolithography machine during online exposure is obtained according to the side width of the second photoresist pattern and the first relationship.

[0065] In the embodiment of the present invention, the side width of each first photoresist pattern 301 whose exposure focal length changes sequentially in the first photoresist pattern group is scanned by an electron beam with a first angle θ by CDSEM, and a first relational expression between the side width of the first photoresist pattern 301 and the exposure focal length can be obtained. Since the electron beam scanning with the first angle θ can magnify a side dimension of the first photoresist pattern 301 in the CDSEM imaged pattern by a certain proportion, the measurement accuracy of the side width of the first photoresist pattern 301 can be improved, and thus the accuracy of the first relational expression can be improved. After the second wafer is exposed, the actual exposure focal length during lithography exposure can be obtained by measuring the side width of the second photoresist pattern and inferring it according to the first relational expression. Therefore, the embodiment of the present invention can timely monitor the focal length of the lithography machine by monitoring the online product by CDSEM, and the accuracy of focal length monitoring can be improved by CDSEM measurement of the side width of the photoresist pattern. Compared with the existing method of monitoring focal length by FEM, the method of the embodiment of the present invention is simpler and can improve monitoring efficiency. Finally, the product yield can be improved. Furthermore, the embodiment of the present invention uses CDSEM to monitor the actual focal length of the online product during the lithography process. Therefore, the embodiment of the present invention can use CDSEM to simultaneously monitor the line width of the photoresist pattern and monitor the focal length of the lithography machine.

[0066] Measurement is like an eye, and its accuracy is crucial. Only with sufficient accuracy can the problem be correctly reflected and a reasonable basis be provided for subsequent optimization. Therefore, better measurement accuracy is the goal pursued by lithography and even semiconductor practitioners. Currently, CDSEM is mainly used to monitor line width (Width) and does not obtain other information. The embodiment of the present invention uses inline measurement of PRwidth and monitors scanner focus at the same time to monitor inline product focus, which can improve product yield.

[0067] In the embodiment of the present invention, the CDSEM tilt function is used to monitor the photoresist profile, i.e., the side width, to obtain a clearer photoresist signal and save more measurement time. The CDSEM tilt function is used to measure the line width and edge width (EW), i.e., the side width, of the photoresist pattern. The side width SW combined with the critical dimension (CD) of the line width can distinguish different focal lengths. Therefore, the embodiment of the present invention can obtain the instantaneous state of the photoresist focal length according to the tilt measurement result.

[0068] The present invention has been described in detail above through specific embodiments, but these do not constitute limitations of the present invention. Without departing from the principle of the present invention, those skilled in the art may also make many variations and improvements, which should also be considered as the protection scope of the present invention.

Claims

1. A method for monitoring the focal length of a lithography machine, characterized in that: The steps include: Step 1, providing a reference wafer, and using a photolithography machine to perform exposure to form at least one first photoresist pattern group on the reference wafer, wherein the first photoresist pattern group includes a plurality of first photoresist patterns; The critical dimensions of the first photoresist patterns have the same design value and the same corresponding exposure energy; The exposure focal lengths of the first photoresist patterns change sequentially, and one of the first photoresist patterns corresponds to one of the exposure focal lengths; Step 2: using CDSEM to perform electron beam scanning at a first angle to image each of the first photoresist patterns and measure the side width of each of the first photoresist patterns, and then establishing a first relationship between the side width of the first photoresist pattern and the exposure focal length; Step 3: using the photolithography machine to expose the second wafer online and form at least one second photoresist pattern; Step 4: using CDSEM to perform electron beam scanning at the first angle to image at least one of the second photoresist patterns and measure the side width of the second photoresist pattern; The actual exposure focal length of the photolithography machine during online exposure is obtained according to the side width of the second photoresist pattern and the first relationship.

2. The method for monitoring the focal length of a lithography machine according to claim 1, wherein: The reference wafer directly adopts the FEM wafer, and the first photoresist pattern group adopts a group of the first photoresist patterns corresponding to a selected exposure energy in the FEM wafer.

3. The method for monitoring the focal length of a lithography machine according to claim 1, wherein: The size of the first angle is set to ensure that the side width of the first photoresist pattern meets the measurement accuracy requirement in the CDSEM imaging image. The larger the first angle is, the higher the measurement accuracy is.

4. The method for monitoring the focal length of a lithography machine according to claim 3, wherein: The minimum side width of the first photoresist pattern is 2nm-3nm.

5. The method for monitoring the focal length of a lithography machine according to claim 3, wherein: The first angle comprises 5 degrees.

6. The method for monitoring the focal length of a lithography machine according to claim 1, wherein: The second wafer is a product wafer.

7. The method for monitoring the focal length of a lithography machine according to claim 1, wherein: The first photoresist pattern group includes a corresponding exposure focal length equal to a reference exposure focal length of the photolithography machine; the exposure focal length of each of the first photoresist patterns is represented by an offset value from the reference exposure focal length.

8. The method for monitoring the focal length of a lithography machine according to claim 7, wherein: In step 1, the exposure focal length variation between two adjacent first photoresist patterns is within 0.2 μm.

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