Centrosymmetric grating assembly, inclination angle measuring method and inclination eliminating method

Abstract

The invention relates to a centrosymmetric grating assembly, an inclination angle measuring method and an inclination eliminating method. According to the inclination angle measurement method based on the centrosymmetric grating assembly, moire fringes in at least one quadrant formed by a first grating and a second grating are obtained, the frequency components of the moire fringes in the two diagonal directions of the quadrant are obtained, and the included angle between the fringes and the original grating direction is calculated through the frequency components. And calculating an inclination angle between the first grating and the second grating through the included angle. In the scheme, the orthogonal component of the moire fringes can be extracted by decomposing the frequency of the moire fringes obtained after the gratings are inclined or the equivalent phase change of the moire fringes, the included angle of the corresponding moire fringes is obtained based on the orthogonal component, and the final inclination angle between the two gratings can be further obtained through the inclination angle of the moire fringes. Therefore, the angle between the mask and the wafer during photoetching can be corrected according to the obtained inclination angle.

Description

Technical Field

[0001] The invention belongs to the technical field of photolithography alignment, and in particular relates to a central symmetrical grating component, a tilt angle measurement method and a tilt elimination method. Background Art

[0002] Photolithography is a crucial precision processing technology in the semiconductor manufacturing process. Its accuracy directly determines the alignment quality and final yield of the wafer pattern. In the photolithography alignment process, the tilt error between the wafer and the mask is one of the key factors affecting the alignment accuracy. Traditional tilt error correction methods usually rely on linear moiré fringes generated by unidirectional gratings and resolve the tilt angle through phase calculation. However, due to the lack of symmetry of unidirectional gratings, the moiré fringes it generates can only provide tilt angle information in a single direction, which makes it susceptible to noise interference in practical applications and makes it difficult to ensure the stability and accuracy of the calculation results. Summary of the invention

[0003] In view of the problem that the existing technology of realizing photolithography tilt detection based on moiré fringes generated by unidirectional grating has limited stability and accuracy, the present invention provides a centrally symmetrical grating component, a tilt angle measurement method and a tilt elimination method.

[0004] The present application provides a central symmetrical grating component, comprising a first grating and a second grating, wherein the grating planes of the first grating and the second grating are perpendicular to an illumination light path, and the first grating and the second grating are arranged in sequence along the illumination light path; The first grating and the second grating are divided into first, second, third and fourth quadrants distributed in sequence in the circumferential direction by two orthogonal axes around the grating center; an L-shaped grating is formed in each sub-quadrant, and the L-shaped grating has a first grating segment and a second grating segment respectively parallel to the two orthogonal axes; wherein the first grating segment and the second grating segment are connected at one end to form a folded angle, and the folded angle extends outward along the diagonal line of the quadrant; The grating period of the L-shaped grating is repeated alternately with a first period P1 and a second period P2 in different quadrants, and P1≠P2. The grating period of the L-shaped grating in the corresponding quadrant of the second grating is complementary to the grating period of the L-shaped grating in the corresponding quadrant of the first grating.

[0005] Preferably, the relationship between the first period P1 and the second period P2 is P1=1.1P2.

[0006] The present application provides a method for measuring the tilt angle based on the above-mentioned central symmetrical grating assembly, obtaining the moiré fringes in at least one quadrant formed by the first grating and the second grating, obtaining the frequency components of the moiré fringes in the two diagonal directions of the quadrant, and calculating the angle between the fringes and the original grating direction through the frequency components. , through the angle Calculate the tilt angle between the first grating and the second grating .

[0007] Preferably, the angle between the stripes and the original grating direction is obtained by calculating the frequency component Middle, Angle Calculated by the following formula: , in, as well as are the frequency components in different diagonal directions respectively.

[0008] Preferably, by the angle Calculate the tilt angle between the first grating and the second grating By changing the angle Substituting into the following formula we get: .

[0009] Preferably, the angle between the stripes and the original grating direction is obtained by calculating the frequency component Middle, Angle Calculated by the following formula: , in, and is the phase difference in different diagonal directions.

[0010] Preferably, the phase difference is obtained by continuous phase estimation. During the continuous phase estimation process, the fringe image is converted from the spatial domain to the frequency domain by a two-dimensional Fourier transform, and the positive first-order spectrum is separated and restored to the time domain by an inverse Fourier transform to generate a phase distribution diagram.

[0011] Preferably, a quality graph-guided method is used to obtain phase unwrapping in the process of obtaining continuous phases; in the unwrapping process, the integral path with the highest reliability is selected through dynamic path planning, and the integral result of the path is used as the final unwrapping result.

[0012] The present application provides a tilt elimination method, which cyclically obtains the real-time tilt angle between the wafer and the mask through any of the tilt angle measurement methods described above during the alignment process, and then inputs the selected real-time tilt angle into a high-precision rotating table to adjust the relative tilt angle between the wafer and the mask in real time, and repeats the above steps until the tilt error is completely eliminated or the preset accuracy conditions are met.

[0013] Preferably, in the step of obtaining the real-time tilt angle between the wafer and the mask, the real-time tilt angle is taken from one or more regions of the centrally symmetric grating component; when multiple regions of the centrally symmetric grating are used, the real-time tilt angle is taken from the average of the tilt angles of the multiple regions obtained or the value of the tilt angle with the highest accuracy.

[0014] The technical solution of the present application proposes a tilt angle measurement method based on a central symmetric grating assembly and a corresponding photolithography tilt elimination method. In this solution, the orthogonal component can be extracted by decomposing the frequency of the moiré fringes obtained after the grating is tilted or the equivalent phase change thereof, and the corresponding angle of the moiré fringes can be obtained based on the orthogonal component, and the final tilt angle between the two gratings can be further obtained by the tilt angle of the moiré fringes. Therefore, the angle between the mask and the wafer during photolithography can be corrected according to the obtained tilt angle. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a structural schematic diagram of the centrosymmetric grating assembly of the present invention; Figure 2 is a schematic structural diagram of the first grating 11 of the present invention; Figure 3 , Figure 4 They are and Moire fringe pattern when Figure 5 It is a schematic diagram of the tilt angle calculation of the present invention.

[0016] In the figure: 1: centrally symmetrical grating assembly; 11: first grating; 111: first grating segment; 112: second grating segment; 113: folded corner; 12: second grating; 13: L-shaped grating; D: straight line; O: grating center. DETAILED DESCRIPTION

[0017] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. In this specification, the size ratios in the drawings do not represent the actual size ratios, but are only used to reflect the relative position relationship and connection relationship between the components. Components with the same name or the same number represent similar or identical structures and are only for illustrative purposes.

[0018] In the description of this embodiment, the terms "upper", "lower", "left", "right" and other directions or positional relationships are based on the directions or positional relationships shown in the drawings, and are only for the convenience of description and simplification of operation, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operate in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are only used to distinguish in the description and have no special meaning.

[0019] The present invention first provides a central symmetrical grating component 1. Its structure is as follows Figure 1As shown, there are two sets of parallel central symmetrical gratings, namely the first grating 11 and the second grating 12. The grating planes of the first grating 11 and the second grating 12 are perpendicular to the illumination light path, and the first grating 11 and the second grating 12 are arranged in sequence along the illumination light path.

[0020] For the first grating 11 and the second grating 12, they are divided into first, second, third and fourth quadrants distributed in the circumferential direction in sequence by two orthogonal axes around the grating center O. Figure 2 As shown, an L-shaped grating 13 is formed in each sub-quadrant, and the L-shaped grating 13 has a first grating segment 111 and a second grating segment 112, each of which is parallel to two orthogonal axes. The first grating segment 111 and the second grating segment 112 are connected at one end to form a fold 113. In the L-shaped grating, the fold 113 extends outward along a straight line D in the quadrant. In some embodiments, the straight line D is the angle bisector of the quadrant.

[0021] For the L-shaped grating 13, its grating period is repeated alternately with the first period P1 and the second period P2 in different quadrants. In order to realize the generation of moiré fringes, it is required that the grating period of the L-shaped grating 13 in the corresponding quadrant of the second grating 12 is complementary to the grating period of the L-shaped grating 13 in the corresponding quadrant of the first grating 11. For example, for the first grating 11, in the first, second, third, and fourth quadrants, its grating period is P1, P2, P1, and P2 in sequence. At this time, the corresponding grating period of the second grating 12 in the first, second, third, and fourth quadrants is P2, P1, P2, and P1 in sequence.

[0022] In the actual photolithography alignment process, the first grating 11 and the second grating 12 can be formed on the wafer and the mask surface respectively. The two form a centrally symmetrical grating component 1 to ensure that the period design of the grating meets the requirements of central symmetry. Wherein: for the first grating 11, the grating period of the first quadrant and the third quadrant is set to P1, and the grating period of the second quadrant and the fourth quadrant is set to P2. P1≠P2, specifically set to P1=1.1P2. Through such a period design, a more significant moiré fringe amplification effect can be formed in the interference process.

[0023] The alignment marks on the mask are consistent with the alignment marks on the wafer, and their periodic parameters and central symmetry design are complementary to the wafer marks. The design of the centrally symmetric grating ensures that in the subsequent photolithography alignment process, the grating period of each quadrant cooperates to generate square moiré fringes, laying the foundation for the calculation of the tilt angle.

[0024] like Figure 1As shown in the figure, the optical path is built in a coaxial illumination manner, and a high-intensity, stable collimated light source (such as laser or LED) is selected. The light source is evenly expanded through a collimating beam expander lens to cover the entire alignment mark area on the wafer and mask. Aperture filtering: An aperture is set in the optical path to filter out stray light and unnecessary interference light and improve the signal-to-noise ratio of the optical path. The collimated light source is guided to pass through the alignment marks on the mask and wafer in sequence. The structure of the centrally symmetrical grating ensures that a four-way symmetrical diffraction beam is generated when the light source penetrates. By adjusting the relative position and angle of optical elements (such as reflectors and collimating lenses), it is ensured that the light source is vertically incident on the mask and wafer surfaces to ensure that the diffracted light forms stable square moiré fringes.

[0025] Corresponding to the above centrosymmetric grating assembly 1, the correction of the relative rotational tilt between the first grating 11 and the second grating 12 can be achieved.

[0026] Specifically, the collimated light is diffracted when passing through the first grating 11 and the second grating 12, and the diffracted light beam is modulated by superposition and interference to generate a period of The specific description is as follows: Moire fringe period calculation: When the wafer and the mask rotate, the tilt angle after rotation is When the moiré fringe period With tilt angle The relationship is:

[0027] That is, it is obvious that different tilt angles will lead to changes in the period of the moiré fringes.

[0028] Stripe changes caused by tilt: The size of the tilt angle not only affects the stripe period, but also causes the stripe direction to tilt. Figure 3 , Figure 4 As shown, they are and The moiré fringe pattern at different tilt angles leads to significant changes in fringe direction and density.

[0029] like Figure 5 As shown, the square moiré fringe is divided into left and right sides by straight line D. ,right ,superior and Four regions, each region of the stripes represent diffraction information in different directions. To improve the calculation accuracy, in each region, the stripes are further cut along the junction of the two L-shaped gratings 13. In any region, local stripe features can be extracted for phase calculation. The moire stripes in the region where any first grating segment 111 or any second grating segment 112 is located are extracted to calculate the tilt angle after the phase rotation between the first grating 11 and the second grating 12.

[0030] Taking the area where one of the first gate segments 111 is located as an example, the fringe intensity can be expressed as:

[0031] in, Represents pixel coordinates; and represent background intensity and amplitude intensity, respectively; and Represents the fringe frequency Decompose the frequency components on the X-axis and Y-axis; Represents the angle between the stripes and the original grating direction.

[0032] The fringe frequency Along (quadrant diagonal) direction and The two orthogonal axes in the (quadrant diagonal) direction are decomposed into and The frequency component can be expressed as: .

[0033] Then the angle between the stripes and the original grating direction can be calculated : .

[0034] Since the angle between the stripes and the original grating direction is With tilt angle The numerical relationship between them can be expressed as:

[0035] The calculated angle Substituting into the above formula, the inclination angle can be calculated The value of .

[0036] For example, a square moiré pattern is divided diagonally into ,right ,superior and There are four regions, and the stripes in each region represent diffraction information in different directions. To improve the calculation accuracy, the stripes are further cut along the center line in each region to extract local stripe features for phase calculation. The upper fringe intensity can be expressed as:

[0037] Represents pixel coordinates; and represent background intensity and amplitude intensity, respectively; and Represents the fringe frequency Decompose the frequency components on the X-axis and Y-axis; Indicates left The angle between the upper stripe of the region and the Y axis.

[0038] Left The angle between the upper stripe of the region and the Y axis With tilt angle The numerical relationship between them can be expressed as:

[0039] The fringe frequency Along Direction and The two orthogonal axes are decomposed into and The frequency component can be expressed as:

[0040] From this we can calculate the angle :

[0041] The calculated angle Substituting into the previous equation, we can determine the tilt angle The value of .

[0042] In a further embodiment, the frequency components can be calculated by continuous phase decomposition. and Directional decomposition, two frequency components and Calculated by the following formula: .

[0043] and Two orthogonal axes and The phase difference on and is the number of pixels in the corresponding direction. Usually, for the convenience of calculation, let . Tilt angle Calculated by the following formula: .

[0044] Obviously, the corresponding angle can be obtained based on this , and further the tilt angle .

[0045] Among them, in order to obtain a continuous phase estimate. The technical solution of the present application first uses a two-dimensional fast Fourier transform (2D-FFT) algorithm to process the moiré fringe pattern to achieve high-precision calculation of phase extraction and phase expansion. First, the fringe image is converted from the spatial domain to the frequency domain by a two-dimensional Fourier transform to separate the positive first-order spectrum containing phase information. In the frequency domain processing process, a two-dimensional Hamming bandpass filter is designed and applied to filter out zero-frequency and high-frequency noise components while reducing spectrum leakage. The main lobe width and sidelobe attenuation rate of the filter are optimized to ensure the integrity of the positive first-order spectrum and effectively suppress noise interference. Subsequently, the filtered positive first-order spectrum is restored to the time domain by an inverse Fourier transform (IFT) to generate a phase distribution diagram.

[0046] In order to further obtain the continuous phase, the present invention adopts the quality map-guided method in the path tracking algorithm for phase unwrapping. This method generates a quality map to evaluate the reliability of each pixel in the wrapped phase, and preferentially unwraps the pixels in the high-quality area, avoiding the low-quality areas caused by noise or distortion, thereby significantly improving the robustness and accuracy of the unwrapping. During the unwrapping process, the most reliable integral path is selected through dynamic path planning, and the integral result of the path is used as the final unwrapping result. In order to further optimize the unwrapping effect, the present invention pre-processes the stripe image in combination with Gaussian filtering before unwrapping to reduce the influence of random noise, and adopts the regional weighted averaging method to reduce errors when processing boundary points.

[0047] Based on the above-mentioned centrally symmetrical grating component, we provide a method for eliminating lithography tilt. During the alignment process, the real-time tilt angle between the wafer and the mask is cyclically obtained through the above-mentioned centrally symmetrical grating component, and then the selected real-time tilt angle value is input into the high-precision rotating stage to adjust the relative tilt angle between the wafer and the mask in real time, and the above steps are repeated until the tilt error is completely eliminated or the preset accuracy condition is met. In the step of obtaining the real-time tilt angle between the wafer and the mask through the above-mentioned centrally symmetrical grating component, the real-time tilt angle is taken from one or more areas of the centrally symmetrical grating component. When using multiple areas of the centrally symmetrical grating, the real-time tilt angle can be the average of the tilt angles of the multiple areas obtained or the value of the tilt angle with the highest accuracy.

[0048] The combination of centrally symmetric grating and moiré fringe technology provides a new solution for high-precision correction of lithography tilt errors. The unique structure of the centrally symmetric grating can generate square moiré fringes, which is significantly different from traditional linear moiré fringes. The square moiré fringes have independent phase distributions in four directions, so that the tilt angle information can be extracted from four directions respectively, thereby greatly enhancing the anti-interference ability. By comparing the calculation results of the tilt angles in the four directions, the tilt angle value with the smallest error and the highest accuracy is further selected as the feedback input high-precision rotation stage for adjustment. This multi-directional calculation and selection mechanism significantly improves the robustness and accuracy of tilt correction, and provides a more stable and reliable technical guarantee for lithography alignment in complex application scenarios.

[0049] Although centrosymmetric gratings show great advantages in theory, the current research on lithography tilt elimination technology based on square moiré fringes is still incomplete. There are still many technical difficulties in the specific implementation of fast extraction of moiré fringe phase, decomposition of orthogonal phase components, and accurate calculation of tilt angle. Therefore, there is an urgent need for an innovative method that can give full play to the four-way characteristics of centrosymmetric gratings, efficiently extract tilt angle information, and achieve high-precision tilt correction between wafer and mask.

[0050] The beneficial technical effects of the present invention are: 1. Significantly improved correction accuracy: The present invention realizes the calculation of multi-directional tilt angle information by extracting and decomposing the four independent phases of the square moiré fringes. Compared with the linear moiré fringes generated by the traditional unidirectional grating, the present invention can select the tilt angle with the smallest error and the highest accuracy for feedback adjustment, greatly improving the accuracy of correction.

[0051] 2. Stronger anti-interference ability: Traditional linear moiré fringes are easily affected by noise and optical path interference, resulting in instability in the calculation results of the tilt angle. The present invention utilizes the structural characteristics of the central symmetrical grating to generate square moiré fringes, which have good symmetry and anti-interference ability, and can still ensure the reliability of correction under complex optical path conditions.

[0052] 3. Higher computational efficiency: By extracting the fringe phase through 2D-FFT and combining orthogonal axis decomposition and fast phase unwrapping technology, the present invention greatly simplifies the complexity of tilt angle calculation. Compared with existing methods, it achieves higher computational efficiency while ensuring accuracy.

[0053] 4. Wider application scope: The present invention is applicable to a variety of lithography alignment scenarios, especially in application environments where tilt error correction is extremely required in semiconductor manufacturing. It has good applicability and can effectively meet the needs of high-precision lithography alignment.

[0054] In summary, the present invention overcomes the defects of the prior art such as insufficient correction accuracy, poor anti-interference ability and high computational complexity, and provides a tilt error correction method with high accuracy, high efficiency and strong stability.

[0055] The above content only describes the preferred implementation mode of the present invention, and does not limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solution of the present invention by ordinary technicians in this field should fall within the protection scope determined by the claims of the present invention.

Claims

1. A centrosymmetric grating assembly, characterized in that: It comprises a first grating (11) and a second grating (12), wherein the grating planes of the first grating (11) and the second grating (12) are perpendicular to the illumination light path, and the first grating (11) and the second grating (12) are arranged in sequence along the illumination light path; The first grating (11) and the second grating (12) are divided into first, second, third and fourth quadrants distributed in sequence in the circumferential direction by two orthogonal axes around the grating center (O); an L-shaped grating (13) is formed in each sub-quadrant, and the L-shaped grating (13) has a first grating segment (111) and a second grating segment (112) respectively parallel to the two orthogonal axes; wherein the first grating segment (111) and the second grating segment (112) are connected at one end to form a folded angle (113), and the folded angle (113) extends outward along the diagonal line of the quadrant; The grating period of the L-shaped grating (13) is repeated alternately with a first period P1 and a second period P2 in different quadrants, and P1≠P2. The grating period of the L-shaped grating (13) in the corresponding quadrant of the second grating (12) is complementary to the grating period of the L-shaped grating (13) in the corresponding quadrant of the first grating (11).

2. The centrosymmetric grating assembly according to claim 1, characterized in that: The relationship between the first period P1 and the second period P2 is P1=1.1P2.

3. A method for measuring the tilt angle based on a centrally symmetrical grating assembly according to any one of claims 1 to 2, characterized in that: Obtaining moiré fringes in at least one quadrant formed by the first grating (11) and the second grating (12), obtaining frequency components of the moiré fringes in two diagonal directions of the quadrant, and calculating the angle between the fringes and the original grating direction through the frequency components , through the angle Calculate the tilt angle between the first grating (11) and the second grating (12) .

4. The inclination angle measurement method according to claim 3, characterized in that: The angle between the stripes and the original grating direction is calculated by the frequency component Middle, Angle Calculated by the following formula: ， in, as well as are the frequency components in different diagonal directions respectively.

5. The inclination angle measurement method according to claim 3, characterized in that: Through the angle Calculate the tilt angle between the first grating (11) and the second grating (12) By changing the angle Substituting into the following formula we get: 。 6. The inclination angle measurement method according to claim 3, characterized in that: The angle between the stripes and the original grating direction is calculated by the frequency component Middle, Angle Calculated by the following formula: ， in, and is the phase difference in different diagonal directions.

7. The inclination angle measurement method according to claim 6, characterized in that: The phase difference is obtained through continuous phase estimation. During the continuous phase estimation process, the fringe image is converted from the spatial domain to the frequency domain through a two-dimensional Fourier transform, the positive first-order spectrum is separated, and the positive first-order spectrum is restored to the time domain through an inverse Fourier transform to generate a phase distribution diagram.

8. The inclination angle measurement method according to claim 7, characterized in that: The quality graph-guided method is used to obtain phase unwrapping in the continuous phase process; during the unwrapping process, the most reliable integral path is selected through dynamic path planning, and the integral result of the path is used as the final unwrapping result.

9. A tilt elimination method, characterized in that: During the alignment process, the real-time tilt angle between the wafer and the mask is obtained cyclically by the tilt angle measurement method described in any one of claims 3-8, and then the selected real-time tilt angle is input into the rotating table to adjust the relative tilt angle between the wafer and the mask in real time, and the above steps are repeated until the tilt error is completely eliminated or the preset accuracy condition is met.

10. The tilt elimination method according to claim 8, characterized in that: In the step of obtaining the real-time tilt angle between the wafer and the mask, the real-time tilt angle is taken from one or more regions of the centrally symmetric grating component; when multiple regions of the centrally symmetric grating are used, the real-time tilt angle is taken from the average of the tilt angles of the multiple regions or the value of the tilt angle with the highest accuracy.

Images