Methods for monitoring the focal length of a lithography machine

Abstract

This invention discloses a method for monitoring the focal length of a lithography machine, comprising: Step 1, forming a first photoresist pattern group comprising multiple first photoresist patterns on a reference wafer through exposure. The design values ​​of the critical dimensions and exposure energy of each first photoresist pattern are identical, and the exposure focal length varies sequentially. Step 2, imaging each first photoresist pattern using a CDSEM with an electron beam scan at a first angle and measuring its side width, then establishing a first relationship between the side width of the first photoresist pattern and the exposure focal length. Step 3, exposing a second wafer in-line using a lithography machine to form a second photoresist pattern. Step 4, imaging at least one second photoresist pattern using a CDSEM with an electron beam scan at the first angle and measuring its side width. The actual exposure focal length of the lithography machine is obtained based on the side width of the second photoresist pattern and the first relationship. This invention enables timely monitoring of the lithography machine's focal length and improves measurement accuracy, efficiency, and product yield.

Description

Technical Field

[0001] This invention relates to a method for manufacturing semiconductor integrated circuits, and more particularly to a method for monitoring the focus of a lithography machine. Background Technology

[0002] As technology nodes advance and device nodes shrink, the depth of focus (DoF) in lithography becomes increasingly smaller. This leads to a significantly smaller focal length process window for lithography machines, making focus error increasingly critical to product performance. Consequently, scanner focus monitoring becomes paramount. Currently, monitoring scanner focus offline via wafer monitoring cannot provide timely feedback on machine focus. This offline method suffers from latency; focus shifts in some wafers may not be immediately detected, ultimately impacting product yield. Furthermore, the most common method for monitoring lithography machine focal length is currently using a focus energy matrix (FEM). However, using FEM to check focus provides inaccurate information and is time-consuming.

[0003] In the photolithography process, if the focal length of the photolithography machine shifts and is not detected in time, it will cause huge losses to production. Some existing methods mainly use the measurement of critical dimensions or the measurement of overlay accuracy for monitoring, but they all require special exposure of a wafer to monitor the machine's condition. At the same time, the measurement of critical 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 monitor the focal length of the lithography machine in a timely manner and improve the accuracy, measurement efficiency and product yield of focal length monitoring.

[0005] To solve the above-mentioned technical problems, the method for monitoring the focal length of a lithography machine provided by the present invention includes the following steps:

[0006] Step 1: Provide a reference wafer and use a photolithography machine to expose and form at least one first photoresist pattern group on the reference wafer. The first photoresist pattern group includes multiple first photoresist patterns.

[0007] The design values ​​of the key dimensions of each of the first photoresist patterns are the same, as are the corresponding exposure energies.

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

[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. Then, establish a first relationship between the side width of the first photoresist pattern and the exposure focal length.

[0010] Step 3: Use the lithography machine to expose the online second wafer and form at least one second photoresist pattern.

[0011] Step 4: Use CDSEM to perform electron beam scanning with 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 lithography machine during in-line exposure is obtained based on the side width of the second photoresist pattern and the first relationship.

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

[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 requirements in the CDSEM image; the larger the first angle, the higher the measurement accuracy.

[0015] A further improvement is that the minimum side width of the first photoresist pattern is 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 lithography machine; the exposure focal length of each of the first photoresist patterns is represented by an offset value from the reference exposure focal length.

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

[0020] This invention employs a CDSEM (Catalyst Digital Separation) sensor to scan the side widths of each first photoresist pattern within a first photoresist pattern group, with the exposure focal length varying sequentially. This allows for the determination of a first relationship between the side width of the first photoresist pattern and the exposure focal length. Since scanning with the first angle magnifies one side dimension of the first photoresist pattern in the CDSEM image, the measurement accuracy of the side width of the first photoresist pattern is improved, thus enhancing the accuracy of the first relationship. After exposing the second wafer, the actual exposure focal length during lithography exposure can be deduced by measuring the side width of the second photoresist pattern and applying the first relationship. Therefore, this invention enables timely monitoring of the lithography machine's focal length through CDSEM monitoring of online products. Furthermore, CDSEM measurement of the side width of the photoresist pattern improves the accuracy of focal length monitoring. Compared to existing methods using FEM (Focus Electron Microscope) for focal length monitoring, this invention is simpler, improves monitoring efficiency, and ultimately increases product yield. Furthermore, this invention uses CDSEM to monitor the actual focal length during the online photolithography process. Therefore, this invention can simultaneously monitor the linewidth of the photoresist pattern and the focal length of the photolithography machine using CDSEM. Attached Figure Description

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

[0022] Figure 1A These are SEM images of existing photoresist FEMs;

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

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

[0025] Figure 2B This is a top view of the photoresist pattern in 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 These are curves of various photoresist patterns obtained from existing CDSEM imaging and photoresist FEM.

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

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

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

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

[0033] Figure 6 These are curves showing the key dimensions of each photoresist pattern in the CDSEM imaging obtained by the photoresist FEM in the method for monitoring the focal length of a lithography machine according to an embodiment of the present invention.

[0034] Figure 7 This is the 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 Implementation

[0035] The method of this invention is derived based on a detailed analysis of existing methods. Before describing the method of this invention in detail, the existing methods are further explained as follows:

[0036] One existing method is to use an FEM (Focal Analyzer) to monitor the focal length of a lithography machine, such as... Figure 1A The image shown is a SEM image of an existing photoresist FEM. The photoresist pattern on the wafer reflects the FEM, i.e., the matrix of the photoresist pattern corresponds to the FEM. As can be seen from Figure 1, the FEM wafer includes multiple exposure regions 101, and each exposure region 101 contains multiple photoresist patterns 102. The exposure regions 101 are arranged in a matrix structure. The focal lengths of the exposure regions 101 in the same row are the same, and the exposure energy, i.e., the exposure dose, will gradually change. The exposure energy of the exposure regions 101 in the same column is the same, and the focal length will gradually change.

[0037] The right side of Figure 1 shows multiple exposure focal lengths, where F = 0.0 μm represents the reference exposure focal length, and the specific values ​​of other exposure focal lengths are also given. The deviation between two adjacent exposure focal lengths is 0.2 μm.

[0038] like Figure 3 The figure shows the curves of various photoresist patterns in the photoresist FEM obtained by existing CDSEM imaging; Figure 3 In the diagram, the horizontal axis represents focal length, and the vertical axis represents key dimensions. It includes multiple curves, each corresponding to a specific exposure energy. Figure 6 The bottom of the image shows a total of 9 exposure energies, in mJ. Existing methods require... Figure 3 The corresponding curve is used to monitor the focal length, but the monitoring results are inaccurate and take a long time.

[0039] The applicant's analysis revealed that the morphology of the photoresist pattern 102 formed by exposure at different focal lengths in Figure 1 varies. For a clearer understanding, please refer to further references. Figure 1B As shown, Figure 1B yes Figure 1A Enlarged photographs of photoresist patterns at different focal lengths. Figure 1B Figure 1 shows the image of photoresist pattern 102a under negative defocus, photoresist pattern 102b under best focus, and photoresist pattern 102c under positive defocus. Negative defocus corresponds to a negative focal length F in Figure 1, such as one from -0.2μm to -0.8μm. Positive defocus corresponds to a positive focal length F in Figure 1, such as one from 0.2μm to 0.8μm. Best focus corresponds to F = 0μm.

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

[0041] like Figure 2A The image shown is a front view of the photoresist pattern in an existing CDSEM imaging. Figure 2A In the process, the photoresist pattern 201 has a narrow, trapezoidal cross-section structure at the top and symmetrical sides 201a and 201b. During CDSEM imaging, the electron beam 202 scans perpendicularly to the top surface of the wafer, i.e., the photoresist pattern 201.

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

[0043] like Figure 2C The image shown is a photograph corresponding to the front view of the photoresist pattern obtained by existing CDSEM imaging. The photograph of photoresist pattern 201 is represented by the label 201'.

[0044] like Figure 2D The image shown is a top view of the photoresist pattern obtained by existing CDSEM imaging; the image of the side view 201a is indicated separately by the label 201a'.

[0045] like Figure 4 The diagram shows a flowchart 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 includes the following steps:

[0046] Step 1: Provide a reference wafer and use a photolithography machine to expose and form at least one first photoresist pattern group on the reference wafer. The first photoresist pattern group includes multiple first photoresist patterns 301. Please refer to [reference needed for first photoresist pattern 301]. Figure 5A As shown.

[0047] The design values ​​of the key dimensions of each of the first photoresist patterns 301 are the same, as are the corresponding exposure energies.

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

[0049] In this embodiment of the invention, the reference wafer is directly an FEM wafer, and the first photoresist pattern group is a set of first photoresist patterns 301 corresponding to a selected exposure energy from the FEM wafer. An FEM wafer represents a wafer that has undergone photoresist exposure according to the FEM distribution. The photoresist on the wafer reflects the FEM. Referring to Figure 1, a SEM image of the photoresist FEM is shown, which includes multiple exposure regions 101, each containing multiple photoresist patterns 102. The exposure regions 101 are arranged in a matrix structure. In the same row, the focal lengths of the exposure regions 101 are the same, and the exposure energy (i.e., the exposure dose) gradually changes. In the same column, the exposure energy of the exposure regions 101 is the same, and the focal length gradually changes. When using an FEM wafer, the first photoresist pattern group can be formed by using multiple first photoresist patterns 301 continuously arranged on a column of the exposure regions 101.

[0050] In other embodiments, FEM wafers may not be used, that is, a matrix structure in which rows and columns change gradually is not required; only a gradually changing focal length is needed.

[0051] In this embodiment of the invention, the first photoresist pattern group includes a corresponding exposure focal length equal to the reference 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 reference exposure focal length. Preferably, the change in exposure focal length between two adjacent first photoresist patterns 301 includes 0.2 μm. For further understanding, please refer to Figure 1, where the right side of Figure 1 shows the change in exposure focal length, where F = 0.0 μm represents the reference exposure focal length, and the specific values ​​of other exposure focal lengths are also given. Please refer to Figure 1 for specific values.

[0052] Step 2: Using a CDSEM, 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. Then, establish a first relationship between the side width of the first photoresist pattern 301 and the exposure focal length. The electron beam scanning with the first angle θ utilizes the tilt function of the CDSEM.

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

[0054] like Figure 5B The image shown is a top view of the photoresist pattern obtained by CDSEM imaging in the method for monitoring the focal length of a lithography 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 view of the sides 301a and 301b of the first photoresist pattern 301 are not symmetrical. Specifically, the width of side 301a increases, which is beneficial for accurately measuring the side width of the first photoresist pattern 301. Figure 5C This is a photograph corresponding to the top view of the photoresist pattern in CDSEM imaging during the method for monitoring the focal length of a photolithography machine in this embodiment of the invention; Figure 5C In the middle, the side view photo 301a' is Figure 5B An image of the 301a from the middle side.

[0055] This ultimately increases the measurement accuracy of the side width of the first photoresist pattern 301 and the fitting accuracy of the first relational expression. For example... Figure 7 As shown, curve 401 is the relationship 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. Curve 401 corresponds to the first relationship.

[0056] like Figure 6 The figure shown is a curve of the key dimensions of each photoresist pattern obtained by CDSEM imaging in the method for monitoring the focal length of a lithography machine according to an embodiment of the present invention. Figure 6 In the diagram, the horizontal axis represents focal length, and the vertical axis represents key dimensions. It includes multiple curves, each corresponding to a specific exposure energy. Figure 6 The bottom of the image shows a total of 9 exposure energies, in mJ. In this embodiment of the invention, step three does not utilize... Figure 6 The actual exposure focal length is obtained from the curve, which requires the use of... Figure 7 The actual exposure focal length can be obtained by using curve 401 in the equation or by directly using the first relationship.

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

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

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

[0060] Step 3: Use the lithography machine to expose the online second wafer and form at least one second photoresist pattern.

[0061] In this embodiment of the invention, the second wafer is a product wafer. By directly using a product wafer, the actual exposure focal length of the lithography machine can be monitored in real time.

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

[0063] Step 4: Use 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 lithography machine during in-line exposure is obtained based on the side width of the second photoresist pattern and the first relationship.

[0065] This invention employs a CDSEM to scan the side widths of each first photoresist pattern 301 in a first photoresist pattern group with sequentially varying exposure focal lengths using an electron beam scan at a first angle θ. This yields a first relationship between the side width of the first photoresist pattern 301 and the exposure focal length. Since the electron beam scan at the first angle θ magnifies one side dimension of the first photoresist pattern 301 in the CDSEM image, it improves the measurement accuracy of the side width of the first photoresist pattern 301, thus enhancing the accuracy of the first relationship. After exposing the second wafer, the actual exposure focal length during lithography exposure can be calculated by measuring the side width of the second photoresist pattern and applying the first relationship. Therefore, this invention's CDSEM-based monitoring of online products allows for timely monitoring of the lithography machine's focal length. The CDSEM measurement of the side width of the photoresist pattern improves the accuracy of focal length monitoring. Compared to existing methods using FEM to monitor focal length, this invention's method is simpler, improves monitoring efficiency, and ultimately increases product yield. Furthermore, the embodiments of the present invention use CDSEM to monitor the actual focal length during the online product lithography process. Therefore, the embodiments of the present invention can use CDSEM to simultaneously monitor the linewidth of the photoresist pattern and the focal length of the lithography machine.

[0066] Measurement is the eye, and its accuracy is crucial. Only with sufficient accuracy can problems be accurately reflected, providing a reasonable basis for subsequent optimization. Therefore, better measurement accuracy is a goal that lithography and semiconductor professionals strive for. Currently, CDSEM is mainly used to monitor linewidth and does not obtain other information. This invention utilizes inline measurement of PRwidth to simultaneously monitor scanner focus, thereby monitoring inline product focus and improving product yield.

[0067] In this embodiment of the invention, the CDSEM tilt function is used to monitor the photoresist profile, i.e., the side width, which yields a clearer photoresist signal and saves more measurement time. By measuring the line width and edge width (EW) of the photoresist pattern, i.e., the side width, using the CDSEM tilt function, the side width SW combined with the critical dimension (CD) of the line width can distinguish different focal lengths. Therefore, this embodiment of the invention can obtain the real-time state of the photoresist focal length based on the tilt measurement results.

[0068] The present invention has been described in detail above through specific embodiments, but these are not intended to limit the invention. Many modifications and improvements can be made by those skilled in the art without departing from the principles of the invention, and these should also be considered within the scope of protection of the present invention.

Claims

1. A method for monitoring the focal length of a lithography machine, characterized in that, Includes the following steps: Step 1: Provide a reference wafer and use a photolithography machine to expose and form at least one first photoresist pattern group on the reference wafer. The first photoresist pattern group includes multiple first photoresist patterns. The design values ​​of the key dimensions of each of the first photoresist patterns are the same, as are the corresponding exposure energies. The exposure focal length of each of the first photoresist patterns changes sequentially, and each of the first photoresist patterns corresponds to one of the exposure focal lengths; Step 2: Use 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. Then establish a first relationship between the side width of the first photoresist pattern and the exposure focal length. The electron beam scanning at the first angle can magnify the side dimension of the first photoresist pattern in the CDSEM image by a certain proportion, thereby improving the measurement accuracy of the side width of the first photoresist pattern. Step 3: Use the aforementioned photolithography machine to expose the online second wafer and form at least one second photoresist pattern; Step 4: Use CDSEM to perform electron beam scanning with 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 lithography machine during in-line exposure is obtained based on the side width of the second photoresist pattern and the first relationship.

2. The method for monitoring the focal length of a lithography machine as described in claim 1, characterized in that: The reference wafer is directly adopted as an FEM wafer, and the first photoresist pattern group adopts a group of first photoresist patterns from the FEM wafer corresponding to a selected exposure energy.

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

4. The method for monitoring the focal length of a lithography machine as described in claim 3, characterized in that: The minimum side width of the first photoresist pattern is 2nm to 3nm.

5. The method for monitoring the focal length of a lithography machine as described in claim 3, characterized in that: The first angle includes 5 degrees.

6. The method for monitoring the focal length of a lithography machine as described in claim 1, characterized in that: The second wafer is a product wafer.

7. The method for monitoring the focal length of a lithography machine as described in claim 1, characterized in that: The first photoresist pattern group includes a corresponding exposure focal length equal to the reference exposure focal length of the lithography 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 as described in claim 7, characterized in that: In step one, the change in exposure focal length between two adjacent first photoresist patterns includes 0.2 μm.

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