Method for improving overlay measurement stability

Abstract

The invention discloses a method for improving overlay measurement stability. The method is used for solving the problem of inaccurate measurement caused by inconsistent overlay mark morphology due to chemical mechanical polishing and other processes. The method comprises the following steps: providing a to-be-tested wafer provided with an overlay mark; dividing the measurement task of the overlay mark to obtain at least two measurement sub-tasks, and respectively configuring different measurement light sources for the measurement sub-tasks; measuring by adopting a measuring light source corresponding to each measuring sub-task so as to obtain overlay data of each measuring sub-task; and finally, merging and calculating the overlay data to obtain a final overlay measurement result. The division mode of the measurement task can be based on different areas on the wafer or based on different structural layers of a single overlay mark. According to the method, the optimal optical conditions are matched for the mark parts in different states, so that the interference of mark damage is effectively inhibited, and the stability and the accuracy of overlay measurement are remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor technology, and in particular to a method for improving the stability of overlay metrology. BACKGROUND

[0002] In the manufacturing process of semiconductor chips, multiple layers of complex circuit structures are formed on a wafer through multiple photolithography processes. In order to ensure that the circuit structures of different levels are accurately aligned, thereby forming good physical and electrical connections, accurate overlay metrology must be performed. Accurate control of overlay error is one of the key factors determining the performance and yield of chips.

[0003] Overlay metrology is usually achieved by using special overlay marks arranged on the wafer. A complete overlay mark is formed by at least two different photolithography processes, which define different structural layers of the mark. The image of the overlay mark is captured by an optical metrology device, and the relative offset between different structural layers, i.e. the overlay error, is calculated by using an image-based overlay algorithm.

[0004] However, between multiple photolithography processes, the wafer usually undergoes various process steps such as chemical mechanical polishing, etching, thin film deposition, etc. These processes, especially the chemical mechanical polishing process, can cause physical damage or changes to the topography of the previously formed overlay marks, such as blurred edges, reduced contrast, or asymmetry, etc.

[0005] When the topography of the overlay marks changes inconsistently, especially in the edge region of the wafer, the damage to the marks is usually more severe due to the edge effect of the chemical mechanical polishing process. The conventional overlay metrology method usually uses a single, fixed optical condition (e.g. using a single wavelength light source or a fixed illumination mode) to measure all the overlay marks on the entire wafer. This method can provide accurate results for marks with perfect topography, but when faced with damaged marks or marks with inconsistent topography in different regions of the wafer, the stability and accuracy of the measurement will be significantly reduced, and even cause the so-called "special pattern" phenomenon, i.e. the measurement results show a systematic error distribution that does not match the actual process deviation, which is caused by the combined effect of the measurement algorithm and the mark topography. This seriously affects the accurate monitoring and yield control of the photolithography process.

[0006] Therefore, there is an urgent need in the prior art for a method that can overcome the problem of measurement instability caused by inconsistent topography of overlay marks, especially due to damage caused by the chemical mechanical polishing process, in order to improve the accuracy and reliability of overlay metrology. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a method that can overcome the problem of measurement instability caused by inconsistent topography of overlay marks, thereby improving the accuracy and reliability of overlay metrology.

[0008] To achieve the above object and other related objects, the embodiment of the present application provides a method for improving overlay measurement stability, comprising:

[0009] Step one, providing a wafer to be measured, wherein the wafer to be measured is provided with an overlay mark;

[0010] Step two, dividing an overlay measurement task of the overlay mark to obtain at least two measurement sub-tasks, and configuring different measurement light sources for the at least two measurement sub-tasks respectively;

[0011] Step three, using the measurement light source corresponding to the measurement sub-task to execute the measurement sub-task on the overlay mark to obtain overlay data of each measurement sub-task;

[0012] Step four, merging and calculating the overlay data of each measurement sub-task to obtain an overlay measurement result of the overlay mark.

[0013] Preferably, in step two, the dividing of the overlay measurement task of the overlay mark comprises: dividing the wafer to be measured into at least two measurement regions according to the position of the overlay mark on the wafer to be measured; wherein the measurement sub-task is to measure the overlay mark in each measurement region.

[0014] Preferably, the at least two measurement regions comprise a wafer center region and a wafer edge region.

[0015] Preferably, in step two, the configuring of different measurement light sources for the at least two measurement sub-tasks comprises: configuring a white light source for the wafer center region and configuring a red light source for the wafer edge region.

[0016] Preferably, in step two, the dividing of the overlay measurement task of the overlay mark comprises: dividing the measurement of a single overlay mark into at least two measurement sub-tasks according to the structure layering of the overlay mark, wherein the at least two measurement sub-tasks correspond to different structure layers of the overlay mark respectively.

[0017] Preferably, in step two, the different structure layers of the overlay mark comprise a current layer structure and at least one previous layer structure; and the measurement sub-tasks comprise: a measurement sub-task for collecting overlay data of the current layer structure, and a measurement sub-task for collecting overlay data of each of the at least one previous layer structure.

[0018] Preferably, the at least one front layer structure comprises a first front layer structure and a second front layer structure; in step two, the metrology sub-tasks comprise: collecting overlay data of the current layer structure, collecting overlay data of the first front layer structure, and collecting overlay data of the second front layer structure.

[0019] Preferably, the overlay marks are damaged in appearance due to the influence of the chemical mechanical polishing process.

[0020] Preferably, the overlay measurement is image-based overlay measurement.

[0021] As described above, the method for improving the stability of overlay measurement of the present application has the following beneficial effects:

[0022] The present application can specifically deal with the problem of inconsistent appearance of overlay marks caused by processes such as chemical mechanical polishing by dividing the overlay measurement task and configuring different measurement light sources for different measurement sub-tasks. Whether the division is based on different regions on the wafer or different structural levels of a single overlay mark, the optimal optical conditions can be matched for each measurement target, thereby effectively suppressing the interference caused by mark damage and improving the quality of the original signal. This method significantly improves the stability and accuracy of overlay measurement, eliminates error maps introduced by measurement, and provides a more reliable data basis for subsequent process control, which helps to improve the overall yield and performance of chips. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 A flowchart of a method for improving the stability of overlay measurement according to an embodiment of the present application is shown;

[0024] Figure 2 A schematic diagram of dividing a wafer into regions according to an embodiment of the present application is shown;

[0025] Figure 3 A schematic diagram of structurally layering a single overlay mark according to an embodiment of the present application is shown;

[0026] Figure 4 A schematic diagram of comparing overlay measurement results before and after the application of the method according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0027] The embodiments of the present application will be described in detail below with specific reference to specific examples. Those skilled in the art can easily understand other advantages and effects of the present application from the disclosure. The present application can also be implemented or applied in different specific embodiments, and the details in the specification can be modified or changed based on different views and applications without departing from the spirit of the present application.

[0028] Please refer to Figure 1This invention provides a method for improving the stability of overlay measurement. Specifically, this method effectively addresses the problem of inconsistent overlay mark morphology caused by processes such as chemical mechanical polishing, thereby improving the accuracy and reliability of overlay measurement. The method includes:

[0029] Step 1: In the semiconductor manufacturing process, a wafer to be tested is provided, which has overlay marks for interlayer alignment.

[0030] Overlay marks are patterned structures specifically designed to measure interlayer alignment errors. Their formation typically involves multiple process steps. First, a first pattern element of the overlay mark is formed on the wafer through a first photolithography process and subsequent etching. This element typically corresponds to the previous layer structure. Subsequently, the wafer undergoes a series of intermediate process steps, such as thin film deposition, chemical mechanical polishing, and etching. Finally, a second photolithography process forms a second pattern element of the overlay mark at a predetermined relative position to the first pattern element. This element corresponds to the current layer structure. Ideally, the second pattern element should be precisely overlaid on the first pattern element; any misalignment between them constitutes the overlay error to be measured.

[0031] Overlay markings have various specific design types, mainly divided into image-based markings and diffraction-based markings. Image-based markings are one of the most common types, with typical structures including "box-in-box" or "bar-in-bar" structures. For example, a "box-in-box" marking consists of a large box formed in the previous layer and a small box formed in the current layer. An image of the marking is captured using an optical microscope, and image processing algorithms are used to calculate the relative offset between the centers of the two boxes to obtain the overlay error. As technology continues to shrink, the requirements for measurement accuracy become increasingly stringent. Diffraction-based markings (also known as scattering measurement overlay markings) are widely used due to their higher precision. These markings typically consist of fine periodic grating structures, and their overlay error is not determined by direct imaging, but rather by emitting a beam of light onto the marking and analyzing the symmetry changes of its diffracted light for precise measurement.

[0032] Regardless of the specific structure of the overlay markings, the surface morphology, material reflectivity, or edge contour of the first pattern element formed on the previous layer may undergo unexpected changes or damage during intermediate processes such as chemical mechanical polishing. This is precisely the key problem that this invention aims to solve: how to obtain stable and reliable overlay measurement data even when the marking morphology is not ideal.

[0033] Step 2: Considering that different regions of the wafer or different structural layers of the overlay mark may exhibit different optical properties or damage states due to process influences, the measurement task of the overlay mark is divided into at least two measurement sub-tasks, and different measurement light sources are configured for each of the at least two measurement sub-tasks.

[0034] The fundamental reason for choosing different light sources lies in the fact that light sources with different characteristics (e.g., light sources with different wavelengths, polarization states, or illumination modes) interact differently with the overlay markers and the surrounding thin film stack. Specifically, the selection of light sources aims to maximize the effective signal from the target structural layer and suppress noise interference for each measurement subtask. For example, for markers with damaged surface morphology, a longer wavelength light source (such as red light) can be selected. The principle is that longer wavelength light has stronger penetrating power and is less easily scattered by microscopic defects on the surface, allowing it to penetrate or bypass the damaged area of ​​the surface, thereby more clearly capturing the contour information of the marker's underlying or core structure. For markers with intact morphology, a broad-spectrum light source (such as white light) can be used to obtain rich image details and a high signal-to-noise ratio. Therefore, by matching the most suitable light source for markers in different states or different parts of the marker, the quality of the original image can be significantly improved, laying a solid foundation for subsequent accurate calculation of overlay errors.

[0035] In some embodiments, step two involves dividing the measurement task of the overlay marks, including: dividing the wafer under test into at least two measurement regions based on the position of the overlay marks on the wafer under test; wherein the measurement sub-task is to measure the overlay marks in each measurement region. This division method is mainly aimed at the differences in mark morphology in different regions of the wafer caused by the center-edge effect of the process (such as chemical mechanical polishing).

[0036] In some embodiments, please refer to Figure 2 At least two measurement areas are required, including the wafer center region and the wafer edge region. Typically, the overlay marks in the wafer edge region are more susceptible to process damage.

[0037] In some embodiments, step two configures different measurement light sources for at least two measurement subtasks, including configuring a white light source for the wafer center region and a red light source for the wafer edge region. Practice shows that using a white light source to measure the well-shaped central region markers yields rich optical information, while using a red light source can effectively penetrate or reduce surface morphology interference caused by damage to the edge region markers, extracting more stable underlying structure signals, thereby improving the measurement stability of the edge region. This technical effect has been clearly verified in practical applications. For example, please refer to... Figure 4When measuring wafers with edge mark damage using a single light source, the overlay measurement results exhibit irregular and large-amplitude patterns in the outer region of the wafer. This is not a true process deviation, but rather an artifact caused by the interaction between the measurement method and the mark damage. However, by using the method of this invention, which uses white light for the inner region and red light for the outer region, the overlay measurement results are significantly improved. The error distribution is more regular and the amplitude is reduced, more accurately reflecting the actual process condition. The fundamental reason is that the red light source can effectively reduce the interference of mark morphology damage on the measurement signal, making the acquired mark image clearer and more stable, thereby fundamentally improving the accuracy of the measurement.

[0038] In some embodiments, step two involves dividing the measurement task of the overlay mark, including: dividing the measurement of a single overlay mark into at least two measurement sub-tasks based on the structural layering of the overlay mark, with each sub-task corresponding to a different structural layer of the overlay mark. This division method is suitable for situations where different structural layers of the overlay mark have different sensitivities to different light sources, or where different structural layers are affected by the process to varying degrees.

[0039] In some embodiments, the different structural layers of the overlay mark in step two include the current layer structure and at least one previous layer structure; the measurement subtask includes: a measurement subtask for collecting overlay data of the current layer structure, and a measurement subtask for collecting overlay data of each of the at least one previous layer structure.

[0040] In some embodiments, at least one front-layer structure includes a first front-layer structure and a second front-layer structure; the measurement subtask in step two includes: acquiring overlay data of the current layer structure, acquiring overlay data of the first front-layer structure, and acquiring overlay data of the second front-layer structure. For example, in a specific application scenario, the measurement of an overlay mark can be subdivided into independent acquisition of multiple structural layers. Specifically, please refer to... Figure 3 The inner box-like structure of the marker can be defined as the current layer structure and equipped with a dedicated light source A for data acquisition. Simultaneously, the outer frame-like structure of the marker can be further divided into two groups. For example, the horizontal strip-like structure can be defined as the first front layer structure and equipped with light source B, while the vertical strip-like structure can be defined as the second front layer structure and equipped with light source C. This refined division and light source configuration strategy can specifically address the different optical characteristics exhibited by different structural layers or structures in different directions due to process variations, thereby acquiring the optimal image signal for each part and significantly improving the measurement accuracy and stability of complex markers.

[0041] Step 3: Using pre-configured measurement light sources corresponding to each measurement sub-task, perform measurement sub-tasks on the overlay marks using optical measurement equipment to obtain overlay data for each sub-task. This method ensures that high-precision data is obtained using conventional light sources for parts with intact morphology, while specific light sources are used for parts with damaged morphology to suppress the negative impact of the damage, thereby optimizing the data acquisition quality of each sub-task.

[0042] Step 4: Merge and calculate the overlay data from each measurement subtask to obtain the overlay measurement result for the overlay marks. Finally, the optimized data obtained from different regions or layers are integrated and calculated to obtain a comprehensive measurement result that accurately reflects the true overlay error of the entire wafer or the entire mark. This method effectively eliminates the "special pattern" phenomenon caused by local mark damage, making the final overlay measurement result smoother and more realistic. This provides a more reliable basis for subsequent photolithography process correction, thereby improving chip yield and performance.

[0043] In some embodiments, the overlay marks suffer morphological damage due to the chemical mechanical polishing process. The method of the present invention is an effective solution to this key technical problem.

[0044] In some embodiments, overlay measurement is image-based overlay measurement. This method directly improves the quality of the original image input to the image processing algorithm by optimizing the light source configuration, thereby fundamentally improving the accuracy and stability of the final result of image-based overlay measurement.

[0045] It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0046] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for improving the stability of overlay measurement, characterized in that, include: Step 1: Provide a wafer to be tested, on which overlay marks are provided; Step 2: Divide the measurement task of the overlay mark into at least two measurement sub-tasks, and configure different measurement light sources for the at least two measurement sub-tasks respectively; Step 3: Using the measurement light source corresponding to the measurement sub-task, perform the measurement sub-task on the overlay mark to obtain the overlay data of each measurement sub-task; Step 4: Combine and calculate the overlay data of each measurement subtask to obtain the overlay measurement result of the overlay mark.

2. The method for improving the stability of overlay measurement according to claim 1, characterized in that: In step two, the measurement task of the overlay mark is divided into: dividing the wafer under test into at least two measurement regions according to the position of the overlay mark on the wafer under test; wherein the measurement sub-task is to measure the overlay mark in each measurement region.

3. The method for improving the stability of overlay measurement according to claim 2, characterized in that: The at least two measurement regions include the wafer center region and the wafer edge region.

4. The method for improving the stability of overlay measurement according to claim 3, characterized in that: In step two, configuring different measurement light sources for the at least two measurement subtasks includes configuring a white light source for the center region of the wafer and a red light source for the edge region of the wafer.

5. The method for improving the stability of overlay measurement according to claim 1, characterized in that: In step two, the measurement task of the overlay mark is divided into: according to the structural layering of the overlay mark, the measurement of a single overlay mark is divided into at least two measurement sub-tasks, and the at least two measurement sub-tasks correspond to different structural layers of the overlay mark.

6. The method for improving the stability of overlay measurement according to claim 5, characterized in that: In step two, the different structural layers of the overlay mark include a current layer structure and at least one previous layer structure; The measurement subtasks include: a measurement subtask for collecting overlay data of the current layer structure, and a measurement subtask for collecting overlay data of each of the at least one preceding layer structure.

7. The method for improving the stability of overlay measurement according to claim 6, characterized in that: The at least one front-layer structure includes a first front-layer structure and a second front-layer structure; In step two, the measurement subtask includes: collecting overlay data of the current layer structure, collecting overlay data of the first preceding layer structure, and collecting overlay data of the second preceding layer structure.

8. The method for improving the stability of overlay measurement according to claim 1, characterized in that: The overlay marks suffered morphological damage due to the chemical mechanical polishing process.

9. The method for improving the stability of overlay measurement according to claim 1, characterized in that: The overlay measurement is an image-based overlay measurement.

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