Gap measurement method based on central symmetric grating modulation

Abstract

The present application relates to a gap measurement method based on central symmetric grating modulation. The method comprises constructing a first grating and a second grating which are complementary in period on two surfaces of a gap to be measured to form a central symmetric grating assembly; adjusting a light source to be aligned with the normal direction of the first grating, so that the light source is vertically incident on the central symmetric grating assembly to obtain a moire fringe pattern formed by interference of diffracted light; dividing the moire fringe pattern along a diagonal line into four regions, each region having misaligned phase regions, for at least one region, calculating a phase difference of the misaligned phase regions, and calculating a gap change amount according to the phase difference. The method estimates the gap between the two parallel gratings by using the phase difference of the diffracted fringes formed by the gratings of different structures in the same region of the diffracted fringes formed by the central symmetric grating assembly, can realize high-precision measurement of the gap between the mask and the wafer in a complex lithography environment, and has the characteristics of high precision and strong anti-interference ability.

Description

Technical Field

[0001] This invention belongs to the field of photolithography alignment technology, specifically relating to a gap measurement method based on centrally symmetric grating modulation. Background Technology

[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 lithography quality and accuracy, and improving the accuracy of gap measurement has become a core challenge facing the lithography process.

[0003] Traditional gap measurement methods, such as geometric projection, two-beam interferometry, and heterodyne interferometry, while having some application value in early low-resolution lithography, are limited by environmental vibrations, multiple reflections, and systematic errors in nanoscale lithography, making it difficult to meet the requirements of high precision and high stability. In recent years, novel methods based on interferometric spatial phase imaging, chirped grating diffraction imaging, and multi-wavelength frequency domain interferometry have gradually developed. These methods achieve nanoscale detection and control of gaps through high-precision interferometric fringe data processing and diffraction imaging techniques. 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 next-generation nanolithography technology. Summary of the Invention

[0004] In view of the limited stability and accuracy of existing techniques for photolithography tilt detection based on moiré fringes generated by unidirectional gratings, this invention provides a gap measurement method based on centrally symmetric grating modulation.

[0005] This application provides a gap measurement method based on centrosymmetric grating modulation, comprising the following steps:

[0006] S1. A first grating and a second grating with complementary periods are constructed on the two surfaces of the gap to be measured to form a centrally symmetric grating assembly;

[0007] 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 perpendicularly incident on the centrally symmetrical grating assembly, and the odd-order diffraction light interference of the first grating and the second grating (12) forms a moiré fringe pattern.

[0008] S3. Divide the moiré pattern into four regions along the diagonal. Each region contains misaligned phase regions. For at least one of these regions, calculate the phase difference between the misaligned phase regions. The gap change is calculated based on the phase difference. :

[0009] ,

[0010] in, , , The wavelength of the light source;

[0011] Both the first and second gratings are distributed circumferentially around the center of the grating in four quadrants: the first, second, third, and fourth quadrants. An L-shaped grating is formed in each quadrant. The grating period of the L-shaped grating repeats alternately with the first period P1 and the second period P2 in different quadrants, where P1 ≠ P2. In different quadrants, the grating periods of the first grating and the second grating are complementary.

[0012] Preferably, the gap change is calculated based on the phase difference. In this step, the change in gap in one region of the moiré fringe pattern is selected as the gap value of the result.

[0013] Alternatively, select four regions in the moiré fringe plot and calculate the gap variation for each region separately. The average value is taken as the gap value of the result.

[0014] Alternatively, select one of the four regions in the moiré fringe plot to measure the gap variation with the highest accuracy. The average value is taken as the gap value of the result.

[0015] Preferably, the first grating (11) and the second grating (12) are specifically set to P1=1.1P2, and the duty cycle of the first grating (11) and the second grating (12) is 0.5.

[0016] This application presents a gap measurement method based on centrosymmetric grating modulation. By utilizing the phase difference of diffraction fringes formed by gratings of different configurations within the same region of the diffraction fringes formed by a centrosymmetric grating assembly, the gap between two parallel gratings can be estimated. This method enables high-precision measurement of the mask-wafer gap in complex photolithography environments. Its core lies in generating moiré fringes through the self-interference of the centrosymmetric grating, combined with phase difference extraction and fringe displacement calculation, to achieve real-time monitoring and adjustment of gap changes. This method exhibits high precision, strong anti-interference capability, and excellent practicality. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the centrally symmetric grating assembly of the present invention;

[0018] Figure 2 This is a schematic diagram of the structure of the first grating of the present invention;

[0019] Figure 3This is a schematic diagram illustrating the principle of how the grating gap causes differences in diffraction fringes in this invention.

[0020] Figure 4 This is a schematic diagram of the moiré fringes of the present invention when the gap is stable;

[0021] Figure 5 This is a schematic diagram of the moiré fringes of the present invention when the gap changes.

[0022] In the picture:

[0023] 1: Centrally symmetric 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 Implementation

[0024] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. In this specification, the dimensions of the drawings do not represent the actual dimensions. They are only used to illustrate the relative positional and connection relationships between the components. Components with the same name or the same reference numeral represent similar or identical structures and are limited to illustrative purposes.

[0025] In the description of this embodiment, the terms "upper," "lower," "left," and "right," etc., refer to the orientation or positional relationship shown in the accompanying drawings. They are used only for ease of description and simplification of operation, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention. In addition, the terms "first" and "second" are used only for distinction in description and have no special meaning.

[0026] The steps of the gap measurement method based on centrally symmetric grating modulation in this application are as follows.

[0027] S1. A first grating 11 and a second grating 12 with complementary periods are constructed on the two surfaces of the gap to be measured to form a centrally symmetric 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.

[0028] For the first grating 11 and the second grating 12, they are divided into four quadrants—the first, second, third, and fourth—circumferentially distributed around the grating center O by two orthogonal axes. For example... Figure 2As shown, an L-shaped grating 13 is formed in each sub-quadrant. Each L-shaped grating 13 has a first grating segment 111 and a second grating segment 112, each parallel to two orthogonal axes. The first grating segment 111 and the second grating segment 112 are connected at one end to form a bend 113. In the L-shaped grating, the bend 113 extends outward along a straight line D within that quadrant. In some embodiments, the straight line D is the angle bisector of that quadrant.

[0029] For the L-shaped grating 13, its grating period repeats alternately with a first period P1 and a second period P2 in different quadrants. To achieve moiré fringe generation, the grating period of the L-shaped grating 13 in the corresponding quadrant of the second grating 12 must be 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, its grating period in the first, second, third, and fourth quadrants is P1, P2, P1, P2, respectively. Correspondingly, on the second grating 12, its grating period in the first, second, third, and fourth quadrants is P2, P1, P2, P1, respectively.

[0030] In the actual photolithography alignment process, the first grating 11 and the second grating 12 can be etched onto the wafer and the mask surface, respectively. Together, they form a centrally symmetric grating assembly 1, ensuring that the period design of the gratings meets the requirement of central symmetry. Specifically, for the first grating 11, the grating period in the first and third quadrants is set to P1, and the grating period in the second and fourth quadrants is set to P2. P1 ≠ P2, specifically set to P1 = 1.1P2. This period design allows for a more significant moiré fringe amplification effect during interference. The duty cycle of both gratings is 0.5 by default.

[0031] This design ensures that the grating generates moiré fringes with optimal structure during optical diffraction and improves the sensitivity of gap measurements. The grating structure is designed using electron beam etching or photolithography to ensure high-precision control of the grating period and duty cycle. The grating period in each quadrant is precisely adjusted to avoid the influence of manufacturing errors on the moiré amplification effect. Figure 1 As shown, the centrally symmetric grating structure, with the diagonal as the axis of symmetry, ensures symmetry along the optical path, which helps to reduce systematic errors in subsequent interference processes.

[0032] 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 perpendicularly incident on the centrally symmetrical grating assembly. The odd-order diffracted light of the first grating and the second grating (12) interferes to form a moiré fringe pattern with a wavelength of A light source is incident perpendicularly onto the centrally symmetrical grating assembly 1, generating multi-level diffracted beams, such as... Figure 2 As shown, the moiré fringe pattern formed by the interference of diffracted light is obtained.

[0033] The light source uses a single-frequency laser or a quasi-single-frequency light source to ensure wavelength stability, for example... The laser source is optimized using a beam collimator to ensure an incident angle of precisely 0°, reducing the impact of the tilt angle on the formation of the interferogram. For example... Figure 3 As shown, the incident light is diffracted by the centrally symmetric grating to form four diffracted beams, which are distributed in the left, right, upper and lower quadrants, respectively, providing the basis for the generation of subsequent interference fringes.

[0034] S3. Divide the moiré fringe pattern into four regions along the diagonal on the left side. right side Above Below Four regions. For at least one of these regions, the fringe phase is extracted using a two-dimensional fast Fourier transform, and the fringe phase is expanded using weighted least squares to obtain a continuous phase. In each region, , , , The phase difference between the two sides of the midline of the region can be expressed as:

[0035] .

[0036] From this, the change in gap can be deduced. :

[0037] .

[0038] Preferably, the gap variation can be calculated separately for each of the four regions in the moiré fringe pattern. The average value is taken, or the set of values ​​with the highest measurement accuracy is selected. This ultimately achieves high-precision measurement of the gap between the mask and the wafer.

[0039] Specifically, such as Figure 4 As shown, when the gap remains unchanged, the position and distribution of the moiré fringes remain stable. , , , The intensity of the regional fringes can be expressed as:

[0040]

[0041] In formula (1) , , , It is a beam of light , , , The strength; Indicates the spatial frequency of the interference fringes; It is a grating and The diffraction angle; and The pixels representing the stripe pattern; It is the initial phase difference.

[0042] When the gap between the mask and the wafer changes At that time, the optical path length of the diffracted light returning from the wafer changes. (The image shows a period of...) The optical path change of the grating is From a period of The optical path change of the grating is This causes a change in the phase distribution of the moiré fringes, resulting in a displacement of the fringes along the midline, forming a moiré fringe pattern with adjusted gaps, such as... Figure 5 As shown. At this point, the intensity formula for the stripes in each region is adjusted to:

[0043] The fringe intensity of the region can be expressed as follows:

[0044]

[0045] The stripes of the area and The fringes in the region shift in opposite directions, increasing the measurement sensitivity. The fringe intensities can be expressed as follows:

[0046]

[0047] The fringe intensity of the region can be expressed as follows:

[0048]

[0049] The stripes of the area and The fringes in the region shift in opposite directions, increasing the measurement sensitivity. The fringe intensities can be expressed as follows:

[0050]

[0051] The fringe pattern phase is extracted using a two-dimensional fast Fourier transform, and the continuous phase is obtained by expanding the fringe phase using the weighted least squares method. , , , The phase difference between the two sides of the midline of the region can be expressed as:

[0052]

[0053] From this, the change in gap can be deduced. :

[0054]

[0055] 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.

[0056] Through the above steps, the gap measurement method proposed in this invention can achieve high-precision measurement of the gap between the mask and the wafer in complex photolithography environments. Its core lies in generating moiré fringes through the self-interference of a centrally symmetric grating, and combining phase difference extraction and fringe displacement calculation to achieve real-time monitoring and adjustment of gap changes. This method possesses high precision, strong anti-interference capability, and excellent practicality.

[0057] The advantages of this invention compared to the prior art are:

[0058] (1) High-precision measurement: This 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 the traditional geometric projection method and interferometry, this invention has higher measurement accuracy and can meet the stringent requirements of modern nanolithography processes.

[0059] (2) Strong anti-interference capability: The present invention adopts a centrally symmetrical grating design, which can effectively reduce the systematic errors introduced by environmental vibration, multiple reflections of photoresist and marking contamination. Compared with traditional two-beam interference and heterodyne interference methods, it has better anti-interference performance and is suitable for complex process environments.

[0060] (3) Real-time performance and high sensitivity: By quickly extracting the phase distribution of moiré fringes and calculating the phase difference, the present invention can realize real-time monitoring and adjustment of the gap, with extremely high measurement sensitivity, which can significantly improve the efficiency and stability of the photolithography process.

[0061] (4) Structural design optimization: The period and duty cycle of the centrally symmetric grating are precisely designed to amplify the spatial frequency variation of the moiré fringes, further improving the resolution and sensitivity of gap measurement, which has significant advantages over the traditional grating method.

[0062] (5) Strong applicability: The method of the present invention can not only meet the precise measurement requirements of the gap between the mask and the wafer in the high-resolution nanolithography process, but also has strong environmental adaptability. It can maintain stable measurement performance under complex conditions such as vibration and pollution, and has broad industrial application potential.

[0063] (6) Low cost and simplified optical path: The present invention adopts a simple centrally symmetric grating structure design, with 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.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A gap measurement method based on centrosymmetric grating modulation, characterized in that, Includes the following steps: S1. A first grating (11) and a second grating (12) with complementary periods are constructed on the two surfaces of the gap to be measured to form a centrally symmetric grating assembly (1); S2. Adjust the optical axis of the light source to coincide with the normal direction of the first grating (11), so that the light source is perpendicularly incident on the centrally symmetrical grating assembly (1), and the odd-order diffraction light of the first grating (11) and the second grating (12) interferes to form a moiré fringe pattern. S3. Divide the moiré fringe pattern into four regions along the diagonal. Each region contains misaligned phase regions. For at least one region, extract the fringe phase using a two-dimensional fast Fourier transform. Expand the fringe phase using weighted least squares to obtain continuous phase, and calculate the phase difference between the misaligned regions. The gap change is calculated based on the phase difference. : in, , , The wavelength of the light source; the amount of gap change is calculated based on the phase difference. In this step, the gap variation of each of the four regions in the moiré fringe pattern is calculated. The average value is taken as the gap value of the result; the first grating (11) and the second grating (12) are circumferentially distributed around the grating center (0) in the first, second, third and fourth quadrants, and an L-shaped grating (13) is formed in each quadrant. The L-shaped grating (13) has a first grating segment (111) and a second grating segment (112) parallel to two orthogonal axes. The first grating segment (111) and the second grating segment (112) are connected at one end to form a bend (113). In the L-shaped grating, the bend (113) extends outward along a straight line (D) in the quadrant. The straight line (D) is the angle bisector of the quadrant. The grating period of the L-shaped grating (13) is the first period in different quadrants. Second cycle Alternate repetition, 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 centrosymmetric grating modulation as described in claim 1, characterized in that, The first grating (11) and the second grating (12) are specifically set to P1=1.1P2, and the duty cycle of the first grating (11) and the second grating (12) is 0.5.

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