A method and system for measuring photoresist thickness based on metrology signals

Abstract

This invention discloses a method and system for measuring photoresist thickness based on measurement signals. The method includes: providing a wafer to be tested with a first patterned layer and overlay marks formed on its surface; coating the wafer surface with the photoresist to be tested and forming a second patterned layer; using a key-size scanning electron microscope to image the location of the overlay marks and extract coordinate information; guiding an overlay accuracy measurement device to the coordinate position to measure the overlay marks and obtain measurement signals; establishing a mapping relationship model between photoresist thickness and measurement signals; inputting the measurement signals into the mapping relationship model to calculate the photoresist thickness value at the location of the overlay marks. This invention utilizes the measurement signals generated during the overlay accuracy measurement process as the information carrier for photoresist thickness measurement, eliminating the need for a separate thickness measurement step and dedicated measurement station, thus achieving simultaneous acquisition of photoresist thickness information during overlay accuracy measurement.

Description

Technical Field

[0001] This invention relates to the field of semiconductor manufacturing technology, and in particular to the field of photoresist thickness measurement technology in semiconductor manufacturing processes. Specifically, it relates to a photoresist thickness measurement method and system based on measurement signals. Background Technology

[0002] In semiconductor manufacturing, photolithography is a core process that determines the feature size and pattern accuracy of integrated circuits. Photoresist, as the key medium material for pattern transfer in photolithography, has a thickness that significantly affects the resolution of the lithographic pattern, linewidth uniformity, and the effectiveness of subsequent etching or ion implantation processes. Currently, photoresist thickness is primarily measured using optical methods, such as ellipsometers or reflectance spectroscopy. Ellipsometry measures the change in polarization state of polarized light after reflection from the photoresist film surface and uses a fitting algorithm to calculate the film thickness. Reflectance spectroscopy analyzes the reflectance spectrum of the photoresist film to a wide range of light wavelengths and uses interference principles to invert the film thickness. While these methods provide sufficient accuracy for process control, they typically require separate measuring instruments and dedicated measurement stations, usually located in the wafer's testing area, separate from the product pattern area.

[0003] The use of the aforementioned independent measurement stations has the following drawbacks: Firstly, after photolithography exposure and development, the wafer needs to be transferred to a dedicated thickness measurement machine for offline measurement. This process increases the number of wafer transfers between devices and the waiting time, extending the production cycle and reducing production efficiency. Secondly, the thickness measurement stations occupy valuable testing area on the wafer. In advanced processes, as chip sizes continue to shrink, the testing area reduces the wafer area available for product patterning. This problem becomes increasingly prominent as chip sizes continue to shrink. Furthermore, since the measurement stations are located in the testing area of ​​the wafer rather than the product patterning area, the measurement results can only serve as an indirect reference for the photoresist thickness in the product patterning area and cannot reflect the actual thickness distribution in the product patterning area.

[0004] In actual production, the quality of the pattern after photolithography needs to be evaluated immediately and fed back to the front-end coating and developing machine for dynamic adjustment of process parameters. However, existing offline measurement methods cannot obtain photoresist thickness information at the same location and time while measuring the critical dimensions and overlay accuracy of the product pattern. This results in a spatial mismatch between thickness, critical dimension, and overlay accuracy information, hindering the timeliness and accuracy of closed-loop control in the photolithography process. Specifically, critical dimension measurements are typically performed in the product pattern area of ​​the wafer, while overlay accuracy measurements are performed at dedicated overlay marking locations, and thickness measurements are performed in a separate testing area. These three measurement locations are separated on the wafer, making it impossible to accurately correlate data from different measurement locations on the same wafer. When process deviations occur, it is difficult for process engineers to quickly and accurately pinpoint the root cause of the problem because it is impossible to determine whether the deviation originates from fluctuations in critical dimensions, misalignment of overlay, changes in photoresist thickness, or the coupling effect of these three factors. Summary of the Invention

[0005] This invention provides a photoresist thickness measurement method and system based on measurement signals, aiming to solve the technical problems in the prior art where photoresist thickness measurement requires an independent measuring machine and a dedicated measuring station, the thickness measurement results cannot be spatially correlated with the key dimensions and overlay accuracy measurement results, and the photoresist thickness information cannot be obtained simultaneously at the same location when measuring the key dimensions and overlay accuracy of the product pattern.

[0006] To address the above technical problems, this invention provides a method for measuring photoresist thickness based on measurement signals, comprising the following steps:

[0007] A wafer to be tested is provided, the surface of which has a first pattern layer and overlay marks formed on the surface. The surface of the wafer to be tested is coated with a photoresist to be tested, and the photoresist to be tested is formed into a second pattern layer by a photolithography process.

[0008] The location of the overlay mark on the wafer under test is imaged using a key-size scanning electron microscope to obtain a high-resolution electron microscope image, and the coordinate information and graphic morphology information of the location of the overlay mark are extracted from the high-resolution electron microscope image.

[0009] Guide the overlay accuracy measuring device to the position indicated by the coordinate information, control the overlay accuracy measuring device to measure the overlay mark, and obtain the measurement signal;

[0010] Establish a mapping relationship model between photoresist thickness and the measurement signal;

[0011] The measurement signal is input into the mapping model to calculate the photoresist thickness at the location of the overlay mark.

[0012] In some embodiments, a critical dimension scanning electron microscope is used to image the location of the overlay marks on the wafer under test, while simultaneously measuring the critical dimensions of the product pattern.

[0013] In some embodiments, the measurement signal is selected from at least one of the following signals: the contrast of the overlay mark image, the intensity of the characteristic signal obtained from spectral analysis, and the characteristic parameters of the signal waveform.

[0014] In some embodiments, establishing a mapping model between photoresist thickness and the measurement signal includes:

[0015] Based on a theoretical optical simulation model, the signal response of the overlay marks under different photoresist thicknesses is simulated to obtain the theoretical mapping relationship; or...

[0016] Based on the experimental data model, multiple standard samples with different known photoresist thicknesses are provided. The measurement signals of the overlaid marks on each of the standard samples are measured, and a dataset between the thickness value and the measurement signal is established and fitted to obtain the experimental mapping relationship.

[0017] In some embodiments, the mapping relationship model is established using a physical model based on thin-film interference theory. In this physical model, the relationship between the total reflectivity R and the photoresist thickness d is expressed by the multi-beam interference formula:

[0018] R = | (r1 + r2·e^(-i·2β)) / (1 + r1·r2·e^(-i·2β)) |²;

[0019] Where r1 is the reflection coefficient of the upper interface of the photoresist layer, r2 is the reflection coefficient of the lower interface of the photoresist layer, β = (2π / λ)·n·d·cosθ is the single-pass phase thickness of the photoresist layer, λ is the wavelength of the probe light in vacuum, n is the refractive index of the photoresist, and θ is the refraction angle of the light in the photoresist layer.

[0020] In some embodiments, the physical model is simplified to a cosine function form:

[0021] R(d) = R_avg + R_amp·cos(4πnd / λ);

[0022] Where R_avg is the average reflectance and R_amp is the amplitude of reflectance, R_avg and R_amp are obtained through experimental calibration.

[0023] In some embodiments, the photoresist thickness value is calculated according to the following formula:

[0024] d = (λ / (4πn))·arccos((R_meas - R_avg) / R_amp) + m·λ / (2n);

[0025] Where R_meas is the actual measured signal strength, m is the period number, and m is determined according to the process window of the photoresist thickness.

[0026] In some embodiments, the nominal value of the photoresist thickness d_nom is obtained, and m = round(d_nom / (λ / (2n))) is calculated, where round is the rounding function.

[0027] In some embodiments, R_avg and R_amp are obtained as follows: R_avg = (R_max + R_min) / 2, R_amp = (R_max - R_min) / 2, where R_max and R_min are the maximum and minimum values ​​of reflectivity, respectively.

[0028] On the other hand, the present invention also provides a photoresist thickness measurement system based on measurement signals for implementing the method, comprising:

[0029] The critical dimension scanning electron microscope module is used for high-resolution imaging and positioning of the wafer under test;

[0030] The overlay accuracy measurement module is used to measure the overlay marks at a specified location and acquire measurement signals;

[0031] The coordinate transfer and alignment module is used to transfer the coordinate information identified by the key dimension scanning electron microscope module to the overlay accuracy measurement module, and to ensure that the measurement position of the overlay accuracy measurement module is consistent with the imaging position of the key dimension scanning electron microscope module.

[0032] The data processing and analysis module has the mapping relationship model built in, which is used to receive the measurement signal and calculate and output the photoresist thickness;

[0033] The system control and integration platform is used to coordinate and control the various modules to perform their tasks in a predetermined sequence.

[0034] Compared with the prior art, the present invention has the following unexpected technical effects:

[0035] First, this invention utilizes the inherent signal generated during the overlay accuracy measurement process as the information carrier for photoresist thickness measurement. It achieves thickness measurement without the need for a separate thickness measurement step and a dedicated measurement station, expanding overlay accuracy measurement from a simple alignment deviation measurement to a comprehensive measurement method that also has thickness measurement functions. This effectively reduces measurement costs and shortens the production cycle.

[0036] Secondly, after achieving precise positioning through the key dimension scanning electron microscope module, the present invention guides the overlay accuracy measurement module to perform measurements at the same position through the coordinate transfer and alignment module, ensuring the consistency between the thickness measurement position and the key dimension measurement position and the overlay accuracy measurement position. This solves the technical problem of the separation of measurement positions of different parameters and the inability of data to achieve one-to-one correspondence in space in the prior art, and provides multi-dimensional data support for process defect analysis and uniformity control with precise spatial alignment.

[0037] Third, this invention establishes a mapping relationship model between measurement signals and photoresist thickness based on thin-film interference theory. This model has a clear physical basis and analytical expression, can accurately reflect the quantitative relationship between photoresist thickness and optical measurement signals, has good generalization ability and interpretability, and can maintain high measurement accuracy under different photoresist materials and process conditions.

[0038] Fourth, this invention enables the simultaneous acquisition of thickness information while measuring conventional key dimensions and overlay accuracy. The acquisition of the three parameters is synchronized in time and at the same point in space, which significantly shortens the time cycle from the completion of photolithography to obtaining the evaluation results of all process parameters, thereby greatly improving the closed-loop control response speed of the photolithography process. Attached Figure Description

[0039] Figure 1 This is a schematic flowchart of the photoresist thickness measurement method based on measurement signals according to the present invention. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0041] To facilitate understanding of this invention, some terms used herein are first defined and explained. In this invention, the first patterning layer refers to a functional patterning layer already formed on the surface of the wafer under test, which can be a conductive or dielectric layer pattern formed after a certain process step in semiconductor device manufacturing. The second patterning layer refers to a pattern formed in the photoresist layer after the current photolithography process, corresponding to the design pattern to be transferred to the wafer surface. An overlay mark refers to a marking pattern with a specific geometric structure set on the wafer surface, used to measure the alignment deviation of the second patterning layer relative to the first patterning layer. The first patterning layer and the overlay mark can be formed simultaneously in the same process step.

[0042] This invention proposes a method and system for measuring photoresist thickness based on measurement signals. The core inventive concept lies in utilizing the physical correlation between the measurement signals acquired during overlay precision measurement and the photoresist thickness covering the overlay marks. By establishing a mapping model between the measurement signals and photoresist thickness, photoresist thickness information can be obtained simultaneously with overlay precision measurement. The physical basis of this mapping model is that overlay precision measurement equipment typically employs optical imaging or optical scattering measurement principles. The measurement signals acquired are affected by the optical interference effect of the photoresist film covering the overlay marks. Changes in photoresist thickness will lead to changes in interference conditions, thereby causing corresponding changes in the measurement signals. Therefore, by pre-establishing a quantitative mapping relationship between the measurement signals and photoresist thickness, the photoresist thickness at the measurement location can be deduced from the acquired measurement signals during actual measurement.

[0043] The present invention will now be described in detail through specific embodiments.

[0044] Figure 1 This is a schematic flowchart of the photoresist thickness measurement method based on measurement signals according to the present invention. (Refer to...) Figure 1 The photoresist thickness measurement method based on measurement signals provided by this invention includes the following steps:

[0045] Step S1: Provide a wafer to be tested with a first pattern layer and overlay marks formed on its surface. The surface of the wafer to be tested is coated with a photoresist to be tested, and the photoresist to be tested is formed with a second pattern layer by a photolithography process.

[0046] Step S2: Use a critical dimension scanning electron microscope to image the location of the overlay mark on the wafer to be tested, obtain a high-resolution electron microscope image, and extract the coordinate information and graphic morphology information of the location of the overlay mark based on the high-resolution electron microscope image.

[0047] Step S3: Guide the overlay accuracy measuring device to the position indicated by the coordinate information, control the overlay accuracy measuring device to measure the overlay mark, and obtain the measurement signal;

[0048] Step S4: Establish a mapping relationship model between photoresist thickness and the measurement signal;

[0049] Step S5: Input the measurement signal into the mapping relationship model to calculate the photoresist thickness value at the location of the overlay mark.

[0050] The following provides a detailed description of a photoresist thickness measurement method based on measurement signals provided in this embodiment.

[0051] First, step S1 is performed, providing a wafer to be tested with a first pattern layer and overlay marks already formed on its surface. The surface of the wafer to be tested is coated with a photoresist to be tested, and the photoresist to be tested is formed with a second pattern layer through a photolithography process.

[0052] In this step, a first patterning layer and overlay marks have already been formed on the surface of the wafer under test. The first patterning layer is a conductive or dielectric layer pattern formed during the semiconductor device manufacturing process, which can be formed through photolithography and etching processes. The overlay marks are used to measure the overlay deviation between the second patterning layer and the first patterning layer in subsequent processes. They are typically set as mark patterns with periodic structures, such as grating structures or box structures, to facilitate identification and positioning by overlay accuracy measurement equipment. The overlay marks and the first patterning layer can be formed simultaneously in the same process step, that is, the overlay marks and the first patterning layer use the same material and are formed in the same photolithography and etching steps. A photoresist layer is coated on the surface of the wafer, covering the first patterning layer and the overlay marks. The thickness of the photoresist layer is the target parameter to be measured. After the photoresist coating is completed, a second patterning layer is formed in the photoresist layer through exposure and development processes. The second patterning layer is the photoresist removal area, exposing at least a portion of the first patterning layer.

[0053] In step S1 above, the purpose of providing the wafer to be tested is to establish the basic conditions for measuring the photoresist thickness. The surface of the wafer to be tested includes both a first patterning layer for product functionality and overlay marks for measuring overlay accuracy. Simultaneously, the photoresist layer covers all of these structures, enabling subsequent measurement steps to acquire both overlay accuracy and photoresist thickness parameters at the location of the overlay marks. Furthermore, during the imaging process, the critical dimensions of the product pattern are simultaneously measured using a critical dimension scanning electron microscope, thus establishing a spatial correlation between the three parameters. The structural feature of the photoresist layer covering the overlay marks requires the probe light to pass through the photoresist layer when the overlay accuracy measurement equipment acquires the overlay mark signal. This ensures that the acquired signal carries information about the photoresist thickness, providing a structural basis for measuring photoresist thickness based on the measurement signal.

[0054] Next, step S2 is performed, in which a key-size scanning electron microscope is used to image the location of the overlay mark on the wafer under test, and a high-resolution electron microscope image is obtained. The coordinate information and graphic morphology information of the location of the overlay mark are extracted based on the high-resolution electron microscope image.

[0055] In this embodiment, the imaging position of the critical dimension scanning electron microscope is selected as the location of the overlay mark. This position is usually located at the edge of the dicing area or product area on the wafer surface, and has a specific periodic geometry, which facilitates the identification and positioning of the critical dimension scanning electron microscope.

[0056] In this step, a critical dimension scanning electron microscope (CEM) with nanometer-level resolution and positioning accuracy enables high-precision imaging of the microstructure of the wafer surface. The CEM scans the wafer surface with a focused electron beam, acquiring secondary electron or backscattered electron signals to form an image. By analyzing and processing this image, the characteristic structure of the overlay markings can be identified, and their precise coordinate positions can be determined. This coordinate information provides a precise target location for subsequent overlay accuracy measurement equipment. Simultaneously, the CEM can also measure the critical dimensions of the product pattern, thereby obtaining critical dimension information during the imaging process and providing more comprehensive data support for subsequent process evaluation. Although the critical dimension measurement area and the location of the overlay markings may be slightly offset physically, the distance between them is known and fixed, and spatial correlation can be achieved through coordinate correction.

[0057] In step S2 above, the purpose of using a critical dimension scanning electron microscope to image and locate the overlay mark is twofold: firstly, to obtain the precise coordinates of the overlay mark, providing navigation for the overlay accuracy measurement equipment and ensuring that both measuring devices measure the same overlay mark; secondly, to simultaneously obtain the critical dimension data of the product graphic, allowing the critical dimension data to be spatially correlated with the photoresist thickness data through coordinate linkage in subsequent process evaluation. This step utilizes the high-precision positioning capability of the critical dimension scanning electron microscope to provide the overlay accuracy measurement equipment with accurate target position coordinates, ensuring the consistency and accuracy of subsequent measurements.

[0058] Next, step S3 is executed, guiding the overlay accuracy measuring device to the position indicated by the coordinate information, controlling the overlay accuracy measuring device to measure the overlay mark, and obtaining the measurement signal.

[0059] In this step, the overlay accuracy measurement device measures the overlay marks based on the precise coordinates provided in step S2. The overlay accuracy measurement device typically employs optical imaging principles, acquiring images of the overlay marks, identifying the edge positions of the marks, and calculating the alignment offset between the second and first graphic layers, i.e., the overlay deviation value. In addition to obtaining the overlay deviation value, this step also acquires auxiliary signals generated during the measurement process, including but not limited to the contrast of the overlay mark image, the spectral reflectance of the overlay mark area, the reflected light intensity distribution, and waveform characteristic parameters.

[0060] The physical mechanism by which the measurement signal is sensitive to photoresist thickness is as follows: When probe light is incident on the overlay mark surface covered with a photoresist layer, the light is reflected at the upper and lower interfaces of the photoresist layer, creating an optical path difference between the reflected light from the upper and lower interfaces. The two reflected beams interfere, and the interference condition depends on the relationship between the optical path difference and the wavelength. When the photoresist thickness changes, the optical path difference changes accordingly, the interference condition changes, and this ultimately manifests as a corresponding change in the measurement signal. Therefore, the measurement signal is sensitive to photoresist thickness, and there is a definite functional relationship between the two.

[0061] In step S3 above, guiding the overlay accuracy measurement equipment to the precise coordinate position for measurement aims to acquire auxiliary signals carrying photoresist thickness information while simultaneously measuring the overlay deviation. This step, without adding extra measurement steps or equipment, utilizes the inherent signals generated during the overlay accuracy measurement process as the information carrier for photoresist thickness measurement, thus expanding the measurement function. Furthermore, since the measurement position is completely consistent with the imaging position in step S2, a one-to-one spatial correspondence between the subsequently obtained thickness information and the overlay accuracy information is guaranteed.

[0062] Next, step S4 is executed to establish a mapping relationship model between the photoresist thickness and the measurement signal.

[0063] In this step, establishing a mapping relationship model is a crucial step in achieving thickness measurement in this embodiment. This model is established through one of the following two methods.

[0064] The first approach is based on a theoretical optical simulation model. Specifically, an optical simulation model is established based on the geometric parameters of the overlay marks, the optical constants of the photoresist, the material parameters of the wafer surface film system, and the optical system parameters of the overlay accuracy measurement equipment. Electromagnetic field numerical calculation methods such as rigorous coupled-wave analysis and the finite-difference time-domain method can be used to calculate the theoretical measurement signal values ​​under different photoresist thicknesses, thereby obtaining the thickness-signal mapping relationship. This method can obtain the mapping relationship without the need for preparing standard samples, making it suitable for situations where standard samples are unavailable or for rapid evaluation stages.

[0065] The second approach is based on an experimental data model. Specifically, multiple standard wafers with different photoresist thicknesses are prepared. Using the same precision measurement equipment under identical measurement conditions, measurement signals from the markings on each standard wafer are acquired, obtaining thickness-signal data pairs. Then, a fitting algorithm is used to establish a quantitative mapping model between thickness and signal. The fitting algorithm can employ polynomial fitting, spline interpolation fitting, least squares fitting, or machine learning algorithms. This approach, based on actual measurement data, offers high accuracy and reliability.

[0066] The mapping relationship models established by the theoretical optical simulation model and the experimental data model described above can both be expressed in a unified mathematical form: R = f(d), where d represents the photoresist thickness, R represents the reflectivity signal intensity acquired by the overlay accuracy measurement equipment, and f is the mapping function. Within a certain thickness range, the photoresist thickness and reflectivity signal intensity exhibit an approximately periodic relationship, and this periodicity originates from the optical interference effect of the photoresist thin film.

[0067] As a preferred implementation based on a theoretical optical simulation model, a mapping relationship model is established in step S4 using a physical model based on thin-film interference theory to replace the pure numerical simulation method, thereby providing an analytical expression with clear physical meaning. This physical model treats the photoresist layer on the overlay mark as a transparent thin film covering the substrate. When the probe light from the overlay accuracy measurement device is incident perpendicularly or obliquely onto the photoresist surface, the light is reflected at the upper and lower interfaces of the photoresist layer, respectively. An optical path difference is generated between the two reflected beams, thus forming interference. Changes in the photoresist thickness alter the optical path difference between the two reflected beams, thereby changing the interference conditions and ultimately leading to a change in the total reflectivity.

[0068] Based on the above physical model, a three-layer dielectric model is first established, simplifying the structure on the wafer into three dielectric layers: the incident medium is air (medium 0), the intermediate layer is a photoresist film (medium 1), and the substrate is the wafer (medium 2). Probe light is incident perpendicularly from air onto the photoresist surface, and reflection and transmission occur at the upper and lower interfaces of the photoresist layer, respectively.

[0069] According to thin-film interference theory, for perpendicular incidence, the reflection coefficients r1 of the upper interface (air-photoresist interface) and r2 of the lower interface (photoresist-substrate interface) of the photoresist layer are respectively:

[0070] r1 = |(n0 - n) / (n0 + n)|;

[0071] r2= |(n - n_sub) / (n + n_sub)|;

[0072] Where n0 is the refractive index of air, n is the refractive index of the photoresist, and n_sub is the refractive index of the substrate.

[0073] For a single-layer thin-film structure, the total reflection coefficient r is given by the multi-beam interference formula:

[0074] r = (r1 + r2·e -i·2 β) / (1 + r1·r2·e -i·2 β);

[0075] Where β = (2π / λ)·n·d·cosθ is the single-pass phase thickness of the photoresist layer, 2β is the phase change of the light beam during one round trip in the thin film, λ is the wavelength of the probe light in vacuum, and θ is the refraction angle of the light in the photoresist layer.

[0076] The total reflectivity R is the square of the modulus of the total reflectance, i.e., R = |r|². Substituting the expression for the total reflectance r into this equation, and combining it with β = (2π / λ)·n·d·cosθ, we can obtain the aforementioned relationship between the total reflectivity R and the photoresist thickness d. It is evident that the total reflectivity R is a periodic function of the photoresist thickness d, with a period of λ / (2n). When changes in the photoresist thickness lead to changes in the phase thickness β, the interference conditions change, and the total reflectivity exhibits periodic oscillations, manifesting as a cosine function. This physical model clarifies the quantitative relationship between the measured signal and the photoresist thickness, providing a theoretical foundation for establishing a mapping relationship model.

[0077] In practical photoresist-wafer systems, the reflection coefficient r1 of the interface on the photoresist layer (air-photoresist interface) is usually much smaller than 1. Therefore, the contribution of the multiple reflection terms in the multi-beam interference formula is weak, and a two-beam approximation can be used for simplification.

[0078] Under the two-beam approximation, neglecting multiple reflections of the beam within the photoresist layer and considering only the interference between the reflected light from the upper and lower interfaces, the total reflectivity R can be approximated as:

[0079] R ≈ r1 2 + r2 2 + 2·r1·r2·cos(2β);

[0080] Where 2β is the phase difference between the two reflected beams, and the cos(2β) term reflects the influence of the interference effect on the reflectivity.

[0081] In practical photoresist systems, because the refractive index of the photoresist is between that of air and the wafer, the light reflected from the upper interface experiences a half-wave loss (phase flip π) relative to the incident light, while the light reflected from the lower interface does not experience phase flip. Therefore, the actual phase difference between the two reflected beams is 2β+π, and the interference term is actually -2·r1·r2·cos(2β). However, in practical engineering applications, R_avg and R_amp are not calculated from the theoretical values ​​of r1 and r2, but are directly obtained through experimentally calibrated R_max and R_min (R_avg=(R_max+R_min) / 2, R_amp= (R_max-R_min) / 2). This calibration process already includes all the phase information in the actual system. Therefore, regardless of the sign of the interference term, the final cosine function can be uniformly expressed as R(d) = R_avg + R_amp·cos(4πnd / λ).

[0082] This simplified form significantly reduces the computational complexity of thickness inversion while maintaining the accuracy of the physical model, making it easier to implement real-time thickness calculations in actual measurement equipment.

[0083] Based on this simplified model, the actual thickness d is analytically obtained by the following formula:

[0084] d = (λ / (4πn))·arccos((R_meas - R_avg) / R_amp) + m·λ / (2n);

[0085] Where R_meas is the signal strength value (i.e., reflectivity intensity) collected by the overlay accuracy measurement equipment during actual measurement, R_avg is the average reflectivity, R_amp is the amplitude of reflectivity, and m is the period ordinal number. The value of m needs to be determined according to the process window of the photoresist thickness. R_avg and R_amp are obtained through experimental calibration. Specifically, R_avg = (R_max + R_min) / 2, R_amp = (R_max - R_min) / 2, where R_max and R_min are the maximum and minimum values ​​of reflectivity, respectively.

[0086] Substituting the definitions of R_avg and R_amp into this formula, we obtain an equivalent expression:

[0087] d = (λ / (4πn))·arccos((2R_meas - R_max - R_min) / (R_max - R_min)) +m·λ / (2n);

[0088] The two expressions described above are mathematically equivalent. The latter directly uses R_max and R_min as calibration parameters, making it easier to directly substitute experimental data. In practical applications, either form can be chosen depending on the type of calibration data.

[0089] The method for determining the period ordinal m is as follows: obtain the nominal value d_nom of the photoresist thickness, and calculate m = round(d_nom / (λ / (2n))), that is, take the integer closest to the period ordinal corresponding to the nominal value. Since the photoresist thickness fluctuates around the nominal value in actual processes, and the fluctuation range is usually much smaller than half a period λ / (2n), the period ordinal m can be uniquely determined by the above method, thereby obtaining a unique thickness value d.

[0090] In step S4 above, the purpose of establishing the mapping relationship model is to provide a quantitative calculation basis for thickness inversion during the actual measurement process. By obtaining the functional relationship between thickness and measurement signal in advance through theoretical simulation or experimental calibration, the corresponding photoresist thickness value can be quickly and accurately calculated simply by acquiring the measurement signal during subsequent actual measurements, without the need for complex optical fitting or offline analysis.

[0091] Next, step S5 is executed, in which the measurement signal is input into the mapping relationship model to calculate the photoresist thickness value at the location of the overlay mark.

[0092] In this step, the actual acquired measurement signal intensity R_meas is substituted into the mapping relationship model R = f(d), and the corresponding photoresist thickness value d_calc = f⁻¹(R_meas) can be obtained through inverse function calculation. To ensure the accuracy of thickness inversion, when the mapping relationship is a periodic function, it is necessary to combine the nominal value of the photoresist thickness or the process window range to determine the unique correct period ordinal number, thereby obtaining a unique thickness value.

[0093] In step S5 above, the purpose of calculating the actual photoresist thickness value is to obtain quantitative data on the photoresist thickness at the location of the overlay mark. By inputting the actual measurement signal into the mapping relationship model, quantitative measurement results of the photoresist thickness are obtained at the same location while simultaneously measuring the overlay accuracy. This thickness value is spatially correlated with the key dimension data obtained in step S2 through coordinate correction, and it belongs to the same spatial location as the overlay deviation data obtained in step S3, providing multi-dimensional and accurate data support for subsequent process analysis and closed-loop control.

[0094] The following provides a specific example of thickness inversion calculation to further illustrate the technical solution of the present invention.

[0095] Assuming the probe light wavelength used in the overlay accuracy measurement equipment is λ=633nm, the refractive index of the photoresist is n=1.7, and the probe light is incident perpendicularly. The extreme reflectivity parameters are obtained through experimental calibration: R_max=0.3, R_min=0.1. In actual measurement, the acquired measurement signal intensity R_meas=0.2. The nominal photoresist thickness is d_nom=200nm, and the process window is 200±20nm.

[0096] First, calculate the periodic ordinal number m:

[0097] The period T = λ / (2n) = 633 / (2×1.7) ≈ 186.2nm;

[0098] m = round(d_nom / T) = round(200 / 186.2) = round(1.074) = 1;

[0099] Then calculate the principal thickness d1:

[0100] R_avg = (R_max + R_min) / 2 = (0.3 + 0.1) / 2 = 0.2;

[0101] R_amp=(R_max-R_min) / 2=(0.3-0.1) / 2=0.1;

[0102] d1=(λ / (4πn))·arccos((R_meas-R_avg) / R_amp)=(633 / (4×3.1416×1.7))·arccos((0.2 - 0.2) / 0.1)= (633 / 21.3629)·arccos(0)≈29.63×(π / 2)≈ 46.5nm.

[0103] Finally, the actual photoresist thickness was calculated as follows: d = d1 + m·T = 46.5 + 1 × 186.2 = 232.7 nm.

[0104] The calculated value of d=232.7nm falls within the process window range for photoresist thickness, consistent with expectations, indicating high reliability of the measurement results. This calculation process can be embedded into the data processing unit of an overlay accuracy measurement device, enabling real-time thickness calculation and output without manual intervention.

[0105] Based on the above method, when establishing the mapping relationship model in step S4, a fitting model based on experimental data can also be used as an alternative. Specifically, firstly, a set of standard samples is prepared, which includes at least five wafers with different photoresist thicknesses, the photoresist thickness being uniformly distributed within the process window. Using the same overlay accuracy measurement equipment as the subsequent actual measurement, the measurement signals of the overlay marks on each standard sample are acquired under the same measurement conditions. The thickness value d_i of each standard sample and its corresponding measurement signal feature value R_i are recorded, forming a dataset {(d_i, R_i)}. Then, the least squares method is used to fit the dataset, and the form of the fitting function is set as follows:

[0106] R (d) = A + B·cos(C·d + D);

[0107] In this model, A, B, C, and D are the parameters to be fitted. A nonlinear least squares algorithm is used to iteratively optimize and minimize the sum of squared residuals between the fitted function and the experimental data, thus determining the optimal values ​​for each parameter. After fitting, in actual measurements, the acquired measurement signal intensity R_meas is substituted into the fitted function, and the photoresist thickness value is obtained by numerically solving the inverse function. This experimental fitting method can automatically compensate for system differences between equipment and the influence of process fluctuations, exhibiting high actual measurement accuracy.

[0108] The photoresist thickness measurement method based on measurement signals provided by this invention can be widely applied to photolithography process monitoring and quality control in the semiconductor manufacturing field. This invention utilizes existing critical-dimensional scanning electron microscopes and overlay precision measurement equipment already configured in semiconductor manufacturing lines. By establishing a mapping relationship model between measurement signals and photoresist thickness, in-situ measurement of photoresist thickness can be achieved without adding additional hardware or measurement steps. This method is applicable not only to photolithography processes in logic device manufacturing but also to the manufacturing processes of various semiconductor devices such as memory devices, power devices, and analog devices. Therefore, this invention has broad industrial applicability and promising prospects for widespread application.

[0109] Based on the above method, the present invention also provides a photoresist thickness measurement system based on measurement signals to implement the above method. The system includes a critical dimension scanning electron microscope module, an overlay accuracy measurement module, a coordinate transfer and alignment module, a data processing and analysis module, and a system control and integration platform.

[0110] The critical dimension scanning electron microscope module is used for high-resolution imaging and positioning of wafers. This module includes an electron optics system, a scanning control system, and an image acquisition and processing system. It is capable of imaging the microstructure of the wafer surface at nanometer resolution and extracting the precise coordinates of the measurement location. This module also has critical dimension measurement capabilities, enabling simultaneous measurement of dimensional parameters such as linewidth of the product pattern during imaging.

[0111] The overlay accuracy measurement module is used to acquire signals from overlay marks at designated locations. This module includes an optical measurement system, a signal acquisition system, and a data processing system. It can perform optical measurements on the overlay marks at specified coordinate locations, acquire measurement signals, and obtain overlay deviation values. This module has signal acquisition capabilities sensitive to photoresist thickness; the acquired measurement signals are used not only for overlay deviation calculations but also as raw data for photoresist thickness measurement.

[0112] The coordinate transfer and alignment module is used to transfer the precise coordinates identified by the critical dimension scanning electron microscope module to the overlay accuracy measurement module, ensuring consistent measurement positions. This module includes a coordinate data interface, a coordinate transformation unit, and an alignment control unit. It enables precise conversion and matching between the critical dimension scanning electron microscope coordinate system and the overlay accuracy measurement coordinate system, ensuring that the overlay accuracy measurement module can accurately move to the target position identified by the critical dimension scanning electron microscope for measurement.

[0113] The data processing and analysis module incorporates the mapping model to receive measurement signals and calculate the photoresist thickness. This module includes a data receiving unit, a model calling unit, and a thickness calculation unit. It receives measurement signal data from the overlay accuracy measurement module, calls the pre-stored mapping model, and calculates and outputs the photoresist thickness value at the measurement location. The module also has data storage capabilities, enabling it to associate and store thickness measurement results with corresponding key dimension data and overlay accuracy data, forming a multi-dimensional process parameter dataset.

[0114] The system control and integration platform is used to coordinate and control the sequential operation of each module. The platform includes a scheduling and control unit, a communication interface, and a user interface. It can coordinate the sequential execution of the critical dimension scanning electron microscope module, the overlay accuracy measurement module, the coordinate transfer and alignment module, and the data processing and analysis module according to a preset workflow, realizing automated process control from wafer loading, measurement and positioning, signal acquisition to thickness calculation.

[0115] Those skilled in the art should understand that the specific values, material types, and process conditions described in the above embodiments are merely illustrative and do not constitute a limitation on the scope of protection of this invention. In practical applications, appropriate parameters and models can be selected based on the specific photoresist material, overlay mark design, and process conditions.

[0116] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for measuring photoresist thickness based on measurement signals, characterized in that, Includes the following steps: A wafer to be tested is provided, the surface of which has a first pattern layer and overlay marks formed on the surface. The surface of the wafer to be tested is coated with a photoresist to be tested, and the photoresist to be tested is formed into a second pattern layer by a photolithography process. The location of the overlay mark on the wafer under test is imaged using a key-size scanning electron microscope to obtain a high-resolution electron microscope image, and the coordinate information and graphic morphology information of the location of the overlay mark are extracted from the high-resolution electron microscope image. Guide the overlay accuracy measuring device to the position indicated by the coordinate information, control the overlay accuracy measuring device to measure the overlay mark, and obtain the measurement signal; Establish a mapping relationship model between photoresist thickness and the measurement signal; The measurement signal is input into the mapping model to calculate the photoresist thickness at the location of the overlay mark.

2. The method according to claim 1, characterized in that, While imaging the location of the overlay marks on the wafer under test using a critical dimension scanning electron microscope, the critical dimensions of the product pattern are measured.

3. The method according to claim 1, characterized in that, The measurement signal is selected from at least one of the following signals: the contrast of the overlay mark image, the intensity of the characteristic signal obtained from spectral analysis, and the characteristic parameters of the signal waveform.

4. The method according to claim 1, characterized in that, Establishing a mapping model between photoresist thickness and the measurement signal includes: Based on a theoretical optical simulation model, the signal response of the overlay marks under different photoresist thicknesses is simulated to obtain the theoretical mapping relationship; or... Based on the experimental data model, multiple standard samples with different known photoresist thicknesses are provided. The measurement signals of the overlaid marks on each of the standard samples are measured, and a dataset between the thickness value and the measurement signal is established and fitted to obtain the experimental mapping relationship.

5. The method according to claim 4, characterized in that, The mapping relationship model is established using a physical model based on thin-film interference theory. In this physical model, the relationship between the total reflectivity R and the photoresist thickness d is expressed by the multi-beam interference formula: R = | (r1 + r2·e^(-i·2β)) / (1 + r1·r2·e^(-i·2β)) |²; Where r1 is the reflection coefficient of the upper interface of the photoresist layer, r2 is the reflection coefficient of the lower interface of the photoresist layer, β = (2π / λ)·n·d·cosθ is the single-pass phase thickness of the photoresist layer, λ is the wavelength of the probe light in vacuum, n is the refractive index of the photoresist, and θ is the refraction angle of the light in the photoresist layer.

6. The method according to claim 5, characterized in that, The physical model is simplified to a cosine function form: R(d) = R_avg + R_amp·cos(4πnd / λ); Where R_avg is the average reflectance and R_amp is the amplitude of reflectance, R_avg and R_amp are obtained through experimental calibration.

7. The method according to claim 6, characterized in that, Calculate the photoresist thickness using the following formula: d = (λ / (4πn))·arccos((R_meas - R_avg) / R_amp) + m·λ / (2n); Where R_meas is the actual measured signal strength, m is the period number, and m is determined according to the process window of the photoresist thickness.

8. The method according to claim 7, characterized in that, Obtain the nominal value of the photoresist thickness d_nom, and calculate m =round(d_nom / (λ / (2n))), where round is the rounding function.

9. The method according to claim 7, characterized in that, The R_avg and R_amp are obtained as follows: R_avg = (R_max + R_min) / 2, R_amp = (R_max - R_min) / 2, where R_max and R_min are the maximum and minimum values ​​of reflectivity, respectively.

10. A photoresist thickness measurement system based on measurement signals, used to implement the method according to any one of claims 1 to 9, characterized in that, include: The critical dimension scanning electron microscope module is used for high-resolution imaging and positioning of the wafer under test; The overlay accuracy measurement module is used to measure the overlay marks at a specified location and acquire measurement signals; The coordinate transfer and alignment module is used to transfer the coordinate information identified by the key dimension scanning electron microscope module to the overlay accuracy measurement module, and to ensure that the measurement position of the overlay accuracy measurement module is consistent with the imaging position of the key dimension scanning electron microscope module. The data processing and analysis module has the mapping relationship model built in, which is used to receive the measurement signal and calculate and output the photoresist thickness; The system control and integration platform is used to coordinate and control the various modules to perform their tasks in a predetermined sequence.

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