Gap measurement method based on centrosymmetric grating modulation

Abstract

The invention relates to a gap measurement method based on centrosymmetric grating modulation. Comprising the following steps: constructing a first grating and a second grating which are periodically complementary on two surfaces of a to-be-measured gap to form a centrosymmetric grating assembly; the light source is adjusted to be aligned with the normal direction of the first grating, so that the light source vertically enters the centrosymmetric grating assembly, and a Moire fringe pattern formed by diffraction light interference is obtained; the moire fringe pattern is divided into four areas along diagonals, each area is internally provided with a staggered phase area, for at least one area, the phase difference # imgabs0 # of mutual dislocation is calculated, and the gap variation # imgabs1 # is calculated according to the phase difference. According to the method, the gap between the two parallel gratings is estimated by utilizing the phase difference of diffraction fringes formed by different gratings in the same area in the diffraction fringes formed by the centrally symmetric grating assembly, so that the high-precision measurement of the gap between the mask and the wafer can be realized in a complex photoetching environment; the method has the characteristics of high precision and strong anti-interference capability.

Description

Technical Field

[0001] The invention belongs to the technical field of photolithography alignment, and in particular relates to a gap measurement method based on central symmetric grating modulation. Background Art

[0002] With the continuous advancement of semiconductor manufacturing technology and the rapid development of nanotechnology, the feature size of integrated circuits continues to shrink, and high-resolution nanolithography technology has been widely used, such as nanoimprint lithography and zone plate array imaging lithography. However, in these lithography processes, the gap between the mask and the wafer has a crucial impact on the quality and accuracy of lithography, and improving the gap measurement accuracy has become a core challenge facing the lithography process.

[0003] Traditional gap measurement methods, such as geometric projection method, dual-beam interferometry and heterodyne interferometry, have certain application value in early low-resolution lithography, but they are limited by environmental vibration, multiple reflections and system errors in nano-level lithography, and it is difficult to meet the requirements of high precision and high stability. In recent years, new methods based on interferometric spatial phase imaging, chirped grating diffraction imaging and multi-wavelength frequency domain interferometry have gradually developed. These methods realize nano-level detection and control of gaps through high-precision interference fringe data processing and diffraction imaging technology. However, these methods still have room for optimization in terms of real-time performance, robustness and system cost in complex lithography environments. Therefore, developing a gap measurement method with high precision, strong adaptability and good stability has become an important research direction for realizing the next generation of nano-lithography technology. Summary of the invention

[0004] In view of the problem that the existing technology for realizing lithography tilt detection based on moiré fringes generated by unidirectional grating has limited stability and accuracy, the present invention provides a gap measurement method based on central symmetric grating modulation.

[0005] The present application provides a gap measurement method based on central symmetric grating modulation, comprising the following steps: S1, constructing a first grating and a second grating with complementary periods on two surfaces of the gap to be measured to form a centrally symmetrical grating assembly; S2, adjusting the optical axis of the light source to coincide with the normal direction of the first grating, so that the light source is vertically incident on the central symmetrical grating component, and the odd-order diffracted light of the first grating and the second grating (12) interfere to form a moiré fringe pattern; S3, dividing the moiré fringe pattern into four regions along the diagonal line, each region having a phase region with a misalignment, and for at least one of the regions, calculating the phase difference of the mutual misalignment , calculate the gap change according to the phase difference : , in, , , is the wavelength of the light source; The first grating and the second grating are arranged in four quadrants, namely the first, second, third and fourth quadrants, which are circumferentially distributed around the center of the grating in sequence. An L-shaped grating is formed in each 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. In different quadrants, the grating period of the first grating is complementary to the grating period of the second grating.

[0006] Preferably, the gap change is calculated based on the phase difference In the step of selecting the gap change in one area of ​​the moiré fringe diagram as the resulting gap value, Alternatively, select four regions in the moiré fringe pattern and calculate the gap change for each region , take the mean as the gap value of the result, Alternatively, select the gap variation with the highest measurement accuracy from the four areas in the moiré fringe pattern. , and take the mean as the resulting gap value.

[0007] Preferably, the first grating (11) and the second grating (12) are specifically set to P1=1.1P2, and the duty ratio of the first grating (11) and the second grating (12) is 0.5.

[0008] The gap measurement method based on central symmetric grating modulation of the present application estimates the gap between two parallel gratings by utilizing the phase difference of diffraction fringes formed by gratings of different compositions in the same area of ​​the diffraction fringes formed by the central symmetric grating component, and can achieve high-precision measurement of the gap between the mask and the wafer in a complex lithography environment. The core of the method is to generate moiré fringes through the self-interference of the central symmetric grating, and combine the phase difference extraction and fringing displacement calculation to achieve real-time monitoring and adjustment of the gap change, with high precision, strong anti-interference ability and excellent practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] 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 a first grating of the present invention; Figure 3 A schematic diagram of the principle of the grating gap causing the difference in diffraction fringes of the present invention; Figure 4 A schematic diagram of the moiré fringes of the present invention when the gap is stable; Figure 5 It is a schematic diagram of the moiré fringes of the present invention when the gap changes.

[0010] 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

[0011] 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.

[0012] 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.

[0013] The steps of the gap measurement method based on central symmetric grating modulation of the present application are as follows.

[0014] S1. A first grating 11 and a second grating 12 with complementary periods are constructed on two surfaces of a gap to be measured to form a centrally symmetrical grating assembly 1. During the photolithography alignment process, the first grating 11 and the second grating 12 are formed on the wafer surface and the mask surface, respectively, or on the mask surface and the wafer surface, respectively.

[0015] 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.

[0016] 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.

[0017] In the actual photolithography alignment process, the first grating 11 and the second grating 12 can be etched 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. The duty cycle of the two gratings is 0.5 by default.

[0018] This design ensures that during the optical diffraction process, the grating can generate moiré fringes with the optimal structure and improve the sensitivity of gap measurement. The design of the grating structure is achieved through electron beam etching technology or photolithography technology to ensure high-precision control of the grating period and duty cycle. The grating period of each quadrant is precisely adjusted to avoid the influence of processing errors on the moiré magnification effect. Figure 1 As shown, the centrally symmetric grating structure uses the diagonal as the symmetry axis, which ensures the symmetry of the optical path and helps to reduce system errors in the subsequent interference process.

[0019] S2, S2, adjust the optical axis of the light source to coincide with the normal direction of the first grating, so that the light source is vertically incident on the central symmetrical grating component, and the odd-order diffraction light of the first grating and the second grating (12) interfere to form a moiré fringe pattern with a wavelength of The light source is vertically incident on the central symmetrical grating component 1, generating multi-order diffraction beams, such as Figure 2 As shown, a moiré fringe pattern formed by interference of diffracted light is obtained.

[0020] The light source uses a single-frequency laser or a quasi-single-frequency light source to ensure the stability of the wavelength, such as The collimation of the light source is optimized by a beam collimator to ensure that the incident angle is precisely 0° and to reduce the influence of the tilt angle on the formation of the interference pattern. Figure 3 As shown, the incident light is diffracted by the centrally symmetrical grating to form four diffracted light beams, which are distributed in the left, right, upper and lower quadrants respectively, providing a basis for the subsequent generation of interference fringes.

[0021] S3, divide the moiré fringe pattern into four areas along the diagonal line on the left , right , above , below Four regions. For at least one of the regions, the fringe pattern phase is extracted by two-dimensional fast Fourier transform, and the fringe phase is expanded using weighted least squares method to obtain continuous phase. In each region, , , , The phase difference on both sides of the midline of the region can be expressed as: .

[0022] From this, the gap change can be inferred : .

[0023] Preferably, four regions in the moiré fringe diagram can be selected to calculate their respective gap changes. , take the average value, or take the set of values ​​with the highest measurement accuracy. Finally, high-precision measurement of the gap between the mask and the wafer is achieved.

[0024] Specifically, if Figure 4 As shown in Figure 2, when the gap does not change, the position and distribution of the moiré fringes remain stable. , , , The regional fringe intensity can be expressed as:

[0025] In formula (1) , , , It is a beam , , , Strength; represents the spatial frequency of the interference fringes; It is a grating and The diffraction angle of and Represents the pixel points of the fringe pattern; is the initial phase difference.

[0026] When the gap between the mask and the wafer changes When , the optical path length of the diffracted light returning from the wafer changes. The optical path length change of the grating is , from the period The optical path length change of the grating is This will cause the phase distribution of the moiré fringe to change, resulting in the displacement of the fringe along the center line, forming a gap-adjusted moiré fringe pattern, such as Figure 5 At this time, the intensity formula of the stripes in each area is adjusted to: The fringe intensity of the region can be expressed as:

[0027] The stripes of the region The fringe movement directions of the regions are opposite, the measurement sensitivity increases, and the fringe intensity can be expressed as:

[0028] The fringe intensity of the region can be expressed as:

[0029] The stripes of the region The fringe movement directions of the regions are opposite, the measurement sensitivity increases, and the fringe intensity can be expressed as:

[0030] The fringe pattern phase is extracted by two-dimensional fast Fourier transform, and the fringe phase is expanded using weighted least squares method to obtain continuous phase. , , , The phase difference on both sides of the midline of the region can be expressed as:

[0031] From this, the gap change can be inferred :

[0032] By selecting one set of measurement values, or taking their average, or taking the set of values ​​with the highest measurement accuracy, high-precision measurement of the gap between the mask and the wafer can be achieved.

[0033] Through the above steps, the gap measurement method proposed in the present invention can achieve high-precision measurement of the gap between the mask and the wafer in a complex lithography environment. The core is to generate moiré fringes through the self-interference of the central symmetric grating, and combine phase difference extraction and fringe displacement calculation to achieve real-time monitoring and adjustment of gap changes, with high precision, strong anti-interference ability and excellent practicality.

[0034] The advantages of the present invention compared with the prior art are: (1) High-precision measurement: The present invention utilizes the self-interference of a centrally symmetric grating to form moiré fringes. By analyzing the phase difference of the fringes, it is possible to achieve nanometer-level precise measurement of the gap between the mask and the wafer. Compared with traditional geometric projection and interference methods, the present invention has higher measurement accuracy and can meet the strict requirements of modern nanolithography processes.

[0035] (2) Strong anti-interference ability: The present invention adopts a centrally symmetrical grating design, which can effectively reduce the system errors introduced by factors such as environmental vibration, multiple reflections of photoresist and mark contamination. Compared with traditional dual-beam interference and heterodyne interference methods, it has better anti-interference performance and is suitable for complex process environments.

[0036] (3) Real-time and high sensitivity: By quickly extracting the phase distribution of the moiré fringe and calculating the phase difference, the present invention can realize real-time monitoring and adjustment of the gap, has extremely high measurement sensitivity, and can significantly improve the efficiency and stability of the lithography process.

[0037] (4) Structural design optimization: The period and duty cycle of the centrally symmetrical grating are precisely designed to amplify the spatial frequency variation of the moiré fringes, further improving the resolution and sensitivity of the gap measurement, which has significant advantages over the traditional grating method.

[0038] (5) Strong applicability: The method of the present invention can not only meet the demand for accurate measurement of the gap between the mask and the wafer in high-resolution nanolithography process, but also has strong environmental adaptability and can maintain stable measurement performance under complex conditions such as vibration and pollution, thus possessing broad industrial application potential.

[0039] (6) Low cost and simplified optical path: The present invention adopts a simple centrally symmetrical grating structure design, and has low requirements for light source alignment. Compared with the multi-wavelength frequency domain interference method, it has a lower implementation cost and a simpler optical path system design, which is convenient for practical engineering applications.

[0040] 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 gap measurement method based on central symmetric grating modulation, characterized in that: The steps include: S1, constructing a first grating (11) and a second grating (12) with complementary periods on two surfaces of the gap to be measured to form a centrally symmetrical grating assembly (1); S2, adjusting the optical axis of the light source to coincide with the normal direction of the first grating (11), so that the light source is vertically incident on the central symmetrical grating component (1), and the odd-order diffracted light of the first grating (11) and the second grating (12) interfere to form a moiré fringe pattern; S3, dividing the moiré fringe pattern into four regions along the diagonal line, each region having a phase region with a misalignment, and for at least one of the regions, calculating the phase difference of the mutual misalignment , calculate the gap change according to the phase difference : ， in, , , is the wavelength of the light source; The first grating (11) and the second grating (12) are arranged in the first, second, third and fourth quadrants which are sequentially distributed circumferentially around the grating center (O), and an L-shaped grating (13) is formed in each 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. In different quadrants, the grating period of the first grating (11) is complementary to the grating period of the second grating (12).

2. The gap measurement method based on central symmetric grating modulation according to claim 1, characterized in that: Calculate the gap change based on the phase difference In the step of selecting the gap change in one area of ​​the moiré fringe diagram as the resulting gap value, Alternatively, select four regions in the moiré fringe pattern and calculate the gap change for each region , take the mean as the gap value of the result, Alternatively, select the gap variation with the highest measurement accuracy from the four areas in the moiré fringe pattern. , as the resulting gap value.

3. The gap measurement method based on central symmetric grating modulation according to claim 1, characterized in that: The first grating (11) and the second grating (12) are specifically set to P1=1.1P2, and the duty ratio of the first grating (11) and the second grating (12) is 0.5.

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