Method for determining optimal focus

Abstract

The invention provides a method for determining an optimal focus. The method comprises the following steps of: providing a wafer which is subjected to photoetching treatment under different focus settings; performing optical critical dimension measurement on the photoresist pattern on the wafer to obtain a plurality of side wall angle values respectively corresponding to different focus settings; based on the plurality of side wall angle values and different focus settings, establishing a relationship between the side wall angles and the focuses; and according to the relationship, determining a focus when the side wall angle is the preset target angle as an optimal focus. According to the method, by directly utilizing the side wall angle parameter strongly related to the focusing quality, the measurement interference of a traditional method is avoided, and the optimal focus of the photoetching process can be determined more accurately and reliably, so that the process window is widened, and the process stability and the product yield are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of semiconductor technology, and in particular to a method for determining an optimal focus. BACKGROUND

[0002] In the manufacturing process of semiconductor integrated circuits, photolithography is one of the key processes that determine the feature size and performance of the devices. The photolithography process forms a predetermined circuit pattern on a photoresist layer on the wafer surface through exposure and development. During the exposure process, the photolithography machine needs to accurately focus the pattern on the mask onto the photoresist on the wafer surface. Since the wafer itself or the thin film layers formed thereon can have uneven thickness or surface undulations, the flatness of the wafer surface is not ideal. Therefore, accurate control of the focus position of the exposure beam is crucial for forming a photoresist pattern with clear contours and precise dimensions.

[0003] In order to obtain consistent and optimal photolithography results across the entire wafer or between different batches of wafers, it is necessary to calibrate and determine the optimal focus of the photolithography machine periodically or when a new process is changed. One conventional method for determining the optimal focus is to use a focus exposure matrix (FEM) wafer. Specifically, a series of different focus and exposure energy combinations are set on a wafer for exposure, and then the critical dimension (CD) of the photoresist pattern under different conditions obtained in the after-development inspection (ADI) is measured. By analyzing the relationship between the focus and the CD, for example, by fitting a focus-CD curve to find the focus that can obtain the target CD value, it is determined as the optimal focus.

[0004] However, the inventors have found that the above method for determining the optimal focus based on critical dimension (CD) measurement has some inherent defects. First, the CD value is not only sensitive to the focus, but also sensitive to other process parameters such as exposure energy, and this coupling effect can interfere with the accurate determination of the optimal focus. Second, the measurement accuracy of the CD can be affected by factors such as measurement device noise, photoresist pattern contour irregularity (such as line edge roughness), etc., especially at the edge of the process window, the measurement error can be amplified. Therefore, the optimal focus obtained by CD fitting can deviate from its true optimal value, resulting in a narrower photolithography process window, affecting the stability of the process and the yield of the final product.

[0005] Therefore, there is an urgent need in the industry for a more direct, reliable and resistant to measurement interference method for determining the optimal focus to improve the stability of the photolithography process and the yield. SUMMARY

[0006] The technical problem to be solved by the present application is that the method for determining the optimal focus based on critical dimension (CD) measurement in the prior art is susceptible to the interference of factors such as measurement noise and process fluctuation, resulting in inaccurate determination of the optimal focus, and further affecting the stability of the photolithography process and the product yield.

[0007] To solve the above technical problem, the present application provides a method for determining the optimal focus, comprising:

[0008] Step one, providing a wafer subjected to photolithography processing under different focus settings;

[0009] Step two, performing optical critical dimension measurement on the photoresist patterns on the wafer to obtain a plurality of side wall angle values corresponding to the different focus settings respectively;

[0010] Step three, establishing the relationship between the side wall angle and the focus based on the plurality of side wall angle values and the different focus settings;

[0011] Step four, determining the focus at which the side wall angle is a preset target angle as the optimal focus according to the relationship.

[0012] Preferably, in step one, the step of providing the wafer subjected to photolithography processing under different focus settings comprises performing photolithography processing on a focus exposure matrix wafer.

[0013] Preferably, in step two, the optical critical dimension measurement is optical critical dimension measurement in post-lithography inspection.

[0014] Preferably, in step two, the optical critical dimension measurement comprises full map measurement on the wafer.

[0015] Preferably, in step three, the step of establishing the relationship comprises linear fitting of the plurality of side wall angle values and the different focus settings to obtain a fitting equation representing the relationship.

[0016] Preferably, in step four, the step of determining the optimal focus comprises substituting the preset target angle into the fitting equation to calculate the optimal focus.

[0017] Preferably, in step four, the preset target angle is 90°.

[0018] As described above, the method for determining the optimal focus of the present application has the following beneficial effects:

[0019] The present application measures the sidewall angle of photoresist pattern, which has stronger physical correlation with focus quality, and determines the optimal focus based on the direct relationship between the sidewall angle and the focus. This method avoids the interference and uncertainty caused by traditional critical dimension (CD) fitting, because the sidewall angle can more intuitively and stably reflect the focus effect and is less affected by measurement noise, so that the determined optimal focus is closer to the real optimal value. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A flowchart showing a method for determining the optimal focus according to an embodiment of the present application;

[0021] Figure 2 A diagram showing the change of the sidewall angle of photoresist pattern under different focus settings;

[0022] Figure 3 A diagram showing the sidewall angle measurement data of different points on the wafer under the same focus setting according to an embodiment of the present application;

[0023] Figure 4 A diagram showing the linear fitting of the relationship between the focus and the sidewall angle according to an embodiment of the present application. DETAILED DESCRIPTION

[0024] The present application will be described in detail below with specific reference to particular embodiments. Those skilled in the art will easily understand other advantages and effects of the present application from the content disclosed in this specification. The present application can also be implemented or applied in other different specific embodiments, and the details in this specification can be modified or changed in various ways based on different views and applications without departing from the spirit of the present application.

[0025] Reference will now be made to Figure 1 The present application provides a method for determining the optimal focus, characterized in that it comprises:

[0026] Step 1: Provide a wafer subjected to lithography processing under different focus settings. This step changes the focus, a key process parameter, systematically to prepare a set of samples containing focus variation information, providing a physical basis for establishing the correlation between the focus and the photoresist pattern morphology. By applying a series of gradient-changing focus values on a single wafer or multiple wafers for exposure, the impact of focus offset on the photoresist pattern can be comprehensively investigated.

[0027] In some embodiments, in step one, the step of providing a wafer subjected to lithography at different focus settings includes: subjecting a focus exposure matrix (FEM) wafer to lithography. Using a focus exposure matrix wafer is a high-efficiency way of generating samples, which divides multiple areas on one wafer, each area corresponding to a unique combination of focus and exposure energy. In this method, a fixed exposure energy can be selected and only the focus setting is changed, thereby excluding the influence of exposure energy changes on the side wall angle and more purely studying the relationship between the focus and the side wall angle.

[0028] In step two, the photoresist pattern on the wafer is subjected to optical critical dimension measurement to obtain multiple side wall angle values corresponding to different focus settings respectively. This step is to measure the geometric parameters that can directly reflect the focusing quality. Compared with the traditional method of only measuring the top view critical dimension (CD), this method directly obtains the cross-sectional profile parameter of the photoresist pattern, i.e., the side wall angle. The side wall angle intuitively represents the steepness of the photoresist side wall, and when the focus is at the best position, the light energy is concentrated and the most vertical side wall can be formed. This measurement can be completed by non-destructive means such as optical scatterometry (OCD), thereby quickly obtaining a large amount of pattern topography data. By directly focusing on the side wall angle, this method avoids the problem that the CD measurement in the traditional CD-based method is easily disturbed by factors such as measurement noise and pattern edge roughness, greatly improving the robustness and accuracy of focus judgment.

[0029] In some embodiments, the optical critical dimension measurement is optical critical dimension measurement in after development inspection (ADI). The measurement is performed immediately after the photoresist development, which can directly evaluate the performance of the lithography process itself and obtain the direct influence of the focus on the photoresist pattern in time, thereby avoiding additional variables and errors that may be introduced by subsequent etching and other processes, and ensuring the originality and effectiveness of the feedback data.

[0030] In some embodiments, the optical critical dimension measurement includes full map measurement of the wafer. By performing multi-point or even full-surface measurement on the entire surface of the wafer, a large amount of data points can be obtained. This not only can capture the focus differences inside the wafer caused by wafer warping, uneven film thickness and other factors, but also can provide more abundant and statistically meaningful data support for subsequent fitting analysis, so that the relationship model finally established is more globally representative, effectively overcoming the deviation that may be caused by local sampling measurement, and the final optimal focus value is more suitable for the production of the entire wafer.

[0031] Specifically, please refer to Figure 3After optical critical dimension measurement in the inspection of a photoresist matrix wafer after lithography, a plurality of pattern parameters including bottom critical dimension (BCD), top critical dimension (TCD) and sidewall angle can be obtained. It has been proven that, within a stable process safety window, the sidewall angle values obtained at different sites of the wafer for the same focus setting show a high degree of consistency. For example, in a set of measurement data, the sidewall angle values at different sites are closely distributed around 86.5° with minimal fluctuation. This high repeatability indicates that the sidewall angle is a physical quantity that can stably and accurately reflect the lithography effect under a specific focus setting, providing a solid data foundation for subsequent establishment of a reliable focus-sidewall angle model.

[0032] Step three, establishing a relationship between the sidewall angle and the focus based on the plurality of sidewall angle values and the different focus settings. This step aims to convert discrete measurement data points into a continuous and predictable mathematical model. By analyzing the trend of the sidewall angle values obtained under different focus settings, it can be found that there is a strong correlation between them. Within a certain process window, the focus position and the sidewall angle of the photoresist show a good linear relationship. Establishing this relationship can simplify complex physical phenomena into clear mathematical expressions, providing a basis for accurate calculation of the optimal focus. Another advantage of this method is that the relationship model established can smooth out the measurement noise of individual data points, making the results more stable and reliable.

[0033] In some embodiments, in step three, establishing the relationship includes performing linear fitting on the plurality of sidewall angle values and the different focus settings to obtain a fitting equation representing the relationship. Linear fitting is a simple mathematical tool to calculate. Given the approximate linear physical relationship between focus offset and sidewall angle deviation from the ideal state (90°), linear fitting can quickly and accurately construct a fitting equation of the form "sidewall angle = a x focus + b". This equation intuitively quantifies the degree of influence of each unit change in focus on the sidewall angle, laying a foundation for subsequent accurate calculations.

[0034] As a specific example, refer to Figure 4 A series of different focus setting values are taken as independent variables (x-axis), and the corresponding sidewall angle measurement values are taken as dependent variables (y-axis), and a data scatter plot can be drawn. By performing linear fitting on these data points, a fitting straight line and the corresponding fitting equation can be obtained. For example, a linear equation of the form "y = -0.0037x + 0.3829" can be obtained, and the determination coefficient (R 2) can be as high as 0.9521. Such a high coefficient of determination value indicates that there is a very strong linear relationship between the focus and the sidewall angle, and the established fitting equation can very accurately describe the dependence between the two. The equation can then be used to accurately calculate the optimal focus x value required to achieve a target sidewall angle (e.g. 90°) by inputting the corresponding y value.

[0035] Step four, determine the focus corresponding to the preset target angle as the optimal focus according to the relationship. This step is the ultimate goal of the method, that is, to use the established mathematical model to inversely calculate the process parameters corresponding to the ideal process state. The preset target angle represents the most ideal photoresist pattern morphology, and through the model, the accurate focus value required to achieve this ideal morphology can be directly analyzed, which is the real and reliable optimal focus. Compared with the traditional method of indirectly determining the optimal focus by relying on the extreme value or inflection point of the CD fitting curve, the present method sets a clear and clear physical meaning target (ideal sidewall angle), making the determination process of the optimal focus more direct, objective and accurate, thereby significantly widening the process window and improving the stability of the process and the yield of the final product.

[0036] In some embodiments, in step four, the step of determining the optimal focus includes: substituting the preset target angle into the fitting equation to calculate the optimal focus. This is a direct solution. For example, after obtaining the equation "sidewall angle = a x focus + b", substitute the preset target sidewall angle value into the left side of the equation, and the corresponding optimal focus value can be accurately calculated through simple algebraic operation (focus = (target sidewall angle - b) / a). The entire process has high automation degree and strong repeatability.

[0037] In some embodiments, in step four, the preset target angle is 90°. Setting the preset target angle to 90° has a clear physical meaning. A sidewall angle of 90° means that the sidewall of the photoresist pattern is completely perpendicular to the wafer surface, which represents the most perfect focusing of the exposure energy in the vertical direction, the smallest pattern distortion, and the most ideal mask profile for subsequent etching or ion implantation steps. Therefore, the focus calculated by taking 90° sidewall angle as the target is the "optimal focus" in the physical sense, which can maximize the fidelity of pattern transfer.

[0038] Please refer to Figure 2In a photolithography process, the position of the focal point of the exposure beam in the direction of the photoresist (PR) thickness (i.e. Z direction) directly determines the size of the sidewall angle of the photoresist pattern after development. Specifically, when the focal point is at the ideal optimal position, for example, the center region of the photoresist layer, the exposure energy distribution is the most concentrated and vertical, and after development, a nearly vertical sidewall can be formed, and the sidewall angle is 90°. When the focal point position deviates from the optimal position upwards or downwards, the exposure energy will diverge, resulting in a tilted (conical) or undercut sidewall profile, and the sidewall angle will deviate from 90°.

[0039] It should be noted that the diagrams provided in the embodiments only schematically illustrate the basic concept of the present application, and only the components related to the present application are shown in the diagrams, rather than being drawn according to the number, shape and size of the components when actually implemented. The actual implementation of each component can be arbitrarily changed in terms of type, number and ratio, and the component layout pattern can be more complex.

[0040] The above embodiments only illustratively explain the principles and effects of the present application, and are not intended to limit the present application. Any person skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed by the present application should be covered by the claims of the present application.

Claims

1. A method of determining an optimal focus, characterized by, At least comprising: Step one, providing a wafer subjected to photolithography under different focus settings; Step two, performing optical critical dimension measurement on a photoresist pattern on the wafer to obtain a plurality of sidewall angle values corresponding to the different focus settings respectively; Step three, establishing a relationship between the sidewall angle and the focus based on the plurality of sidewall angle values and the different focus settings; Step four, determining the focus at which the sidewall angle is a preset target angle as the optimal focus according to the relationship.

2. The method of determining the best focus point of claim 1, wherein: In step one, the step of providing the wafer subjected to photolithography under different focus settings comprises performing photolithography on a focus exposure matrix wafer.

3. The method of determining the best focus point according to claim 1 or 2, characterized in that: In step two, the optical critical dimension measurement is optical critical dimension measurement in post-lithography inspection.

4. The method of determining the best focus point of claim 1, wherein: In step two, the optical critical dimension measurement comprises full map measurement on the wafer.

5. The method of determining the best focus point of claim 1, wherein: In step three, the step of establishing the relationship comprises linear fitting of the plurality of sidewall angle values and the different focus settings to obtain a fitting equation representing the relationship.

6. The method of determining the best focus point of claim 5, wherein: In step four, the step of determining the optimal focus comprises substituting the preset target angle into the fitting equation to calculate the optimal focus.

7. The method of determining the best focus point of claim 1, wherein: In step four, the preset target angle is 90°.

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