Off-axis illumination galvanometer scanning device, grating alignment system and off-axis illumination method

Abstract

An off-axis illumination galvanometer scanning device, a grating alignment system and an off-axis illumination method, the off-axis illumination galvanometer scanning device comprising: a first galvanometer (201), a second galvanometer (202), a third galvanometer (203) and a fourth galvanometer (204) arranged in sequence along the light path direction of an illumination light beam, the rotation axes of the first galvanometer (201) and the second galvanometer (202) are parallel to each other, forming a first galvanometer group, for controlling the illumination light beam to perform a first-dimensional scanning displacement on a target grating; the rotation axes of the third galvanometer (203) and the fourth galvanometer (204) are orthogonal to the rotation axis of the first galvanometer (201), forming a second galvanometer group, for controlling the illumination light beam to perform a second-dimensional scanning displacement on the target grating; the first galvanometer group and the second galvanometer group are cooperatively controlled, so that the illumination light beam emitted from the fourth galvanometer (204) can perform two-dimensional scanning on the target grating while keeping the off-axis incidence angle of the illumination light beam relative to the target grating unchanged.

Description

Technical Field

[0001] This disclosure relates to the field of optical precision measurement technology, and more specifically, to an off-axis illumination galvanometer scanning device, a grating alignment system, and an off-axis illumination method. Background Technology

[0002] In moiré fringe-based grating alignment techniques, existing technologies typically employ an off-axis illumination and vertical reception optical path architecture. Specifically, the incident angle of the illumination beam is adjusted using a mechanical rotary stage or similar device to ensure it illuminates the grating mark at a specific angle satisfying the Litterow condition. This causes the ±1st order diffracted light to return along a direction close to the normal, allowing for vertical collection by the subsequent imaging system. This method lays the optical foundation for achieving high signal-to-noise ratio detection.

[0003] However, this existing technology faces significant challenges in practical applications. First, its angle adjustment relies on mechanical motion, resulting in a slow response speed and difficulty in achieving rapid and precise reset of the incident angle when switching between different wavelengths or adapting to different grating periods, leading to insufficient system flexibility. Second, when scanning large target areas, existing methods struggle to maintain a constant off-axis incident angle and stable illumination spot size and shape during two-dimensional beam deflection scanning. Angle drift and aberrations during scanning directly cause the diffraction path to deviate from the perpendicular imaging direction, severely reducing signal quality and alignment accuracy.

[0004] Therefore, existing technologies inherently contradict each other in terms of the agility and accuracy of angle control, the stability of large-area scanning, and the convenience of multi-parameter adaptation, and cannot meet the requirements of modern precision manufacturing for high-efficiency, high-precision, and high-robust alignment. Summary of the Invention

[0005] In view of this, the present disclosure provides an off-axis illumination galvanometer scanning device, a grating alignment system, and an off-axis illumination method.

[0006] The first aspect of this disclosure provides an off-axis illumination galvanometer scanning device for use in a grating alignment system, comprising: a first galvanometer, a second galvanometer, a third galvanometer, and a fourth galvanometer arranged sequentially along the optical path direction of an illumination beam, wherein the rotation axes of the first galvanometer and the second galvanometer are parallel to each other; the third galvanometer and the fourth galvanometer constitute a first galvanometer group for controlling the illumination beam to perform a first-dimensional scanning displacement on a target grating; the rotation axes of the third galvanometer and the fourth galvanometer are parallel to each other and orthogonal to the rotation axis of the first galvanometer, constituting a second galvanometer group for controlling the illumination beam to perform a second-dimensional scanning displacement on the target grating and setting the off-axis incident angle of the illumination beam relative to the target grating; the first galvanometer group and the second galvanometer group are controlled in concert so that the illumination beam emitted from the fourth galvanometer can perform a two-dimensional scan on the target grating while maintaining its off-axis incident angle relative to the target grating unchanged.

[0007] According to an embodiment of this disclosure, the first galvanometer and the second galvanometer are arranged with their mirror surfaces facing each other and are configured to rotate synchronously. By rotating, the optical path of the illumination beam between the first galvanometer and the second galvanometer is changed, thereby driving the first-dimensional scanning displacement of the illumination beam.

[0008] According to an embodiment of this disclosure, the third galvanometer and the fourth galvanometer are arranged with their mirror surfaces facing each other and are configured to rotate collaboratively with an asymmetrical angular relationship. The collaborative rotation means that when the second-dimensional scanning displacement of the illumination beam is changed, the rotation angle of the third galvanometer and the fourth galvanometer is adjusted simultaneously to set and maintain the off-axis incident angle.

[0009] According to an embodiment of this disclosure, the relationship between the rotation angle of the third galvanometer and the rotation angle of the fourth galvanometer is: θ3 = θ4 + 90° – α / 2, where θ3 is the rotation angle of the third galvanometer, θ4 is the rotation angle of the fourth galvanometer, α is the off-axis incident angle, which is the angle between the illumination beam and the normal of the target grating, 0≤α<90°, θ3 is the angle between the reflective surface of the third galvanometer and the normal, θ4 is the angle between the reflective surface of the fourth galvanometer and the normal, and the normal is perpendicular to the plane shown by the two-dimensional scan.

[0010] According to embodiments of this disclosure, the off-axis incident angle is determined based on the wavelength of the illumination beam and the period of the grating, and the formula for determining the off-axis incident angle is: Where α is the off-axis incident angle, λ is the wavelength of the illumination beam, and d is the period of the target grating.

[0011] According to an embodiment of this disclosure, the first galvanometer group and the second galvanometer group are configured to drive the illumination beam to scan the target grating, and the scanning step distance is smaller than the diameter of the light spot formed by the illumination beam on the target grating.

[0012] A second aspect of this disclosure provides a grating alignment system comprising: an off-axis illumination galvanometer scanning device as described in any of the first aspects; and an imaging module having its optical axis perpendicular to the target grating for receiving light signals diffracted from the target grating.

[0013] According to embodiments of this disclosure, it further includes: an adjustment module, configured to adjust the cooperative rotation angle of the third and fourth mirrors in the off-axis illumination galvanometer scanning device according to the period of the target grating or the change in the wavelength of the selected illumination beam, so as to dynamically set and maintain the off-axis incident angle of the illumination beam relative to the target grating.

[0014] A third aspect of this disclosure provides an off-axis illumination method employing the apparatus described in any one of the first aspects, comprising the steps of: determining, based on illumination requirements, the off-axis incident angle required for the illumination beam to incident on a target grating; coordinating the rotation of a first galvanometer group and a second galvanometer group so that the illumination beam is incident on a predetermined scanning position on the target grating at the off-axis incident angle; and, while keeping the off-axis incident angle constant, driving the illumination beam to perform a two-dimensional scan on the target grating along a preset path by changing the rotation state of the first galvanometer group and the second galvanometer group.

[0015] According to embodiments of this disclosure, by employing a four-mirror orthogonal configuration and coordinated control, the technical problems of difficulty in simultaneously achieving multi-wavelength adaptation, large-range two-dimensional scanning and stable off-axis incident angle in the prior art are at least partially overcome, thereby achieving the technical effects of high-precision and fast wavelength switching, large-range stable scanning and efficient vertical diffraction reception. Attached Figure Description

[0016] The above and other objects, features and advantages of this disclosure will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0017] Figure 1 A schematic three-dimensional view of an off-axis illumination galvanometer scanning apparatus according to an embodiment of the present disclosure is shown.

[0018] Figure 2A A schematic diagram of the optical path of the first galvanometer group according to an embodiment of the present disclosure is shown.

[0019] Figure 2B A schematic diagram of the optical path of the second galvanometer group according to an embodiment of the present disclosure is shown.

[0020] Figure 3 This schematic diagram illustrates the position of the light spot on the target grating according to an embodiment of the present disclosure;

[0021] Figure 4 A schematic diagram of the YZ cross-section of an off-axis illumination galvanometer scanning apparatus according to an embodiment of the present disclosure is shown.

[0022] Figure 5 A schematic diagram of a grating alignment system according to an embodiment of the present disclosure is shown. Detailed Implementation

[0023] The embodiments of the present disclosure will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the disclosure. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the present disclosure for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concepts of the present disclosure.

[0024] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0025] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0026] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0027] like Figure 1 As shown, the first embodiment of this disclosure provides an off-axis illumination galvanometer scanning device, which includes a first galvanometer 201, a second galvanometer 202, a third galvanometer 203 and a fourth galvanometer 204 arranged sequentially along the optical path direction of the illumination beam.

[0028] This device can be applied to grating alignment systems. Grating alignment systems adjust the incident angle of the illumination beam to ensure it illuminates the target grating at a specific angle satisfying the Littorh condition. This causes the ±1st order diffracted light to return along a direction close to the normal, allowing subsequent imaging modules to collect it perpendicularly. According to the grating equations, the exit direction of the diffracted light strictly depends on the off-axis incident angle α. If α fluctuates during scanning, the diffracted light will deviate from the preset receiving direction (i.e., the normal direction of the grating) and cannot be stably and efficiently captured by the imaging module, resulting in signal loss or a decrease in the signal-to-noise ratio. Only by strictly maintaining the off-axis incident angle α constant and precisely matching the Littorh condition sinα = λ / d can the required diffraction order be ensured to return perpendicularly along the normal direction of the grating, where λ represents the wavelength of the illumination beam and d represents the period of the grating. The off-axis illumination galvanometer scanning device provided in this embodiment can ensure a constant off-axis incident angle α by coordinating the rotation of the first galvanometer 201, the second galvanometer 202, the third galvanometer 203, and the fourth galvanometer 204. This ensures that optimal diffraction efficiency and signal-to-noise ratio can be obtained at each wavelength, and also enables the position of the illumination beam incident on the grating to be displaced, allowing the illumination beam to perform a large-scale two-dimensional scan on the grating. This meets the detection requirements for efficient and high-precision grating alignment under different processing conditions, different mark positions, and different mark sizes.

[0029] refer to Figure 2A The first galvanometer 201 and the second galvanometer 202 are arranged with their mirror surfaces facing each other, and their rotation axes are parallel to the Y-axis, forming a first galvanometer group. The first galvanometer group is used to control the illumination beam to perform a first-dimensional scanning displacement on the target grating. In this embodiment, the first-dimensional direction is the X-axis direction. Specifically, the first galvanometer 201 and the second galvanometer 202 are configured to rotate synchronously, that is, they rotate at the same angle, θ1 = θ2, where θ1 is the angle between the first galvanometer 201 and the Z-axis direction, and θ2 is the angle between the second galvanometer 202 and the Z-axis direction. When they rotate synchronously, the optical path of the illumination beam between the first galvanometer 201 and the second galvanometer 202 is changed, thereby causing the beam emitted from the second galvanometer 202 to be translated in the X-direction, realizing the first-dimensional scanning displacement of the illumination beam. At the same time, this synchronous rotation does not change the directional component of the beam in the YZ plane.

[0030] refer to Figure 2BThe mirror surfaces of the third galvanometer 203 and the fourth galvanometer 204 are arranged opposite each other, rotating about an axis parallel to the X-axis, and orthogonal to the rotation axes of the first galvanometer 201 and the second galvanometer 202, forming a second galvanometer group. This second galvanometer group is used to control the illumination beam to perform a second-dimensional scanning displacement on the target grating and to set the off-axis incident angle α of the illumination beam relative to the target grating. In this embodiment, the second-dimensional direction is the Y-axis direction, and the off-axis incident angle is formed in the YZ plane. When the third galvanometer 203 and the fourth galvanometer 204 rotate together, the optical path of the illumination beam between the third galvanometer 203 and the fourth galvanometer 204 is changed, thereby causing the beam emitted from the fourth galvanometer 204 to be translated in the Y direction, realizing the Y-dimensional scanning displacement of the illumination beam.

[0031] The third galvanometer 203 and the fourth galvanometer 204 need to rotate together according to an asymmetric angular relationship. Specifically, taking the normal direction of the target grating (i.e., the direction perpendicular to the two-dimensional scanning plane, denoted as the Z-axis) as the angular reference, the directed angle between the reflective surface of the third galvanometer 203 and the Z-axis is defined as θ3, and the directed angle between the reflective surface of the fourth galvanometer 204 and the Z-axis is defined as θ4. The two must satisfy the following relationship:

[0032] θ3 = θ4 + 90° – α / 2

[0033] Where α is the desired off-axis incident angle, which is the angle between the illumination beam and the normal (Z-axis direction) of the target grating, 0 ≤ α < 90°. The value of α depends on the Littoral angle formula satisfying the wavelength λ of the illumination beam and the period d of the target grating. set up.

[0034] refer to Figure 2B To ensure that the off-axis angle α remains constant, θ4 needs to be changed accordingly when adjusting the illumination beam for Y-dimensional scanning displacement. θ3 is then adjusted according to the above formula to ensure that the off-axis angle α remains constant.

[0035] According to the aforementioned apparatus, the first and second galvanometer groups are configured to drive the illumination beam to scan the target grating, with the scanning step distance being smaller than the diameter of the light spot formed by the illumination beam on the target grating. The first galvanometer group controls the X-axis displacement of the illumination beam, and the second galvanometer group controls the Y-axis displacement of the illumination beam and the off-axis incident angle α. The first and second galvanometer groups are controlled collaboratively, allowing the light spot of the illumination beam falling on the target grating to move along the X and Y axes, thereby performing a large-scale two-dimensional scan on the target grating. While maintaining a constant off-axis incident angle, the collaborative control of the first and second galvanometer groups ensures that the illumination beam emitted from the fourth galvanometer 204 maintains a constant off-axis incident angle relative to the target grating. This guarantees that the ±1st order diffracted light generated by the illumination beam on the target grating can efficiently enter the imaging module along the designed direction, which is crucial for obtaining high-quality moiré fringes.

[0036] It should be noted that in the above embodiments, the reference directions defined by "X-axis," "Y-axis," and "Z-axis" are for ease of description and understanding. Those skilled in the art should understand that the "first-dimensional direction," "second-dimensional direction," and their corresponding scanning control functions, as well as the plane containing the "off-axis incident angle," essentially refer to the fact that the rotation axes of the first and second galvanometer groups are orthogonal to each other, respectively achieving displacement control of the beam in two orthogonal directions on the target surface and setting of the off-axis incident angle. Therefore, any equivalent transformation of the coordinate axis names or spatial orientation, as long as it retains the orthogonal relationship and functional division between the aforementioned galvanometer groups, should fall within the protection scope of this disclosure.

[0037] like Figure 3 As shown, this simulation assumes an off-axis incident angle of 19° for the illumination beam. By adjusting the first and second galvanometer groups, the beam spot is scanned at multiple points on the target grating. The specific mapping relationship between the rotation angles of the first and second galvanometer groups and the four positions of the target grating is shown in Table 1.

[0038] Table 1

[0039] Center-of-mass galvanometer （-6.69，7） （7，7） （-6.69，-7） （7，-7） 201 38 51 38 51 202 38 51 38 51 203 -41 -41 -54 -54 204 39.5 39.5 26.5 26.5

[0040] As can be seen from Table 1, through comparison Figure 3 a and b and comparison Figure 3 The positions of the light spots in c and d are shown. The Y-coordinate of the light spots in a and b is 7, and the Y-coordinate of the light spots in c and d is -7. When the Y-coordinate is fixed, only the angles of the first set of galvanometers (201 and 202) change from 38° to 51°, while the angles of the second set of galvanometers (203 and 204) remain completely unchanged. That is, the synchronous rotation of the first set of galvanometers (201, 202) can independently and uniquely drive the light spot to move in the X direction. By comparison... Figure 3 The positions of the light spots in points a and c, and points b and d are shown. The X-coordinate of the light spots in points a and c is -6.69, and the X-coordinate of the light spots in points b and d is 7. When the X-coordinate is fixed, only the angles of the second set of galvanometers (203 and 204) change. The third galvanometer 203 changes from -41° to -54°, ​​and the fourth galvanometer 204 changes from 39.5° to 26.5°. That is, the coordinated rotation of the second set of galvanometers (203, 204) independently and uniquely drives the light spot to move in the Y direction.

[0041] Based on the above analysis, it can be seen that when either the first or second set of galvanometers is fixed, changing the rotation angle of the other set can achieve linear scanning along the corresponding axis. Through the synergistic effect of the first and second sets of galvanometers, global scanning of the target grating can be achieved.

[0042] Litterow configuration condition that the off-axis incident angle α must precisely satisfy To ensure that the diffracted light energy returns efficiently along the vertical direction and is received by the imaging module, the off-axis incident angle α needs to be determined based on the wavelength of the illumination beam and the period of the target grating. For example, the illumination source can be switched between blue light (wavelength 450nm), green light (wavelength 532nm), and red light (wavelength 620nm), corresponding to Littoral angles of 13.18°, 15.18°, and 18.05°, respectively, to adapt to the structure with a grating period of 2μm.

[0043] like Figure 4 As shown, a and b represent the YZ profile optical path states of the second galvanometer group when the off-axis incident angles are 15° and 19°, respectively. Different off-axis incident angles of the illumination beam can be achieved by adjusting the angle difference of the second set of galvanometers. Table 2 lists the galvanometer rotation angles required to achieve off-axis incident angles of 15° and 19°.

[0044] Table 2

[0045] Off-axis incident angle galvanometer 15 degrees 19 degrees 201 any any 202 any any 203 -41 -41 204 41.5 39.5

[0046] In embodiments of this disclosure, the first and second galvanometer groups are configured to enable the illumination beam to perform a full-coverage scan of a scanning area of ​​at least 16mm × 16mm on the target grating. The illumination beam is collimated, forming a uniform and stable spot on the target grating with a diameter no greater than 3mm. By controlling the galvanometer groups, the 3mm spot is moved within the target area in steps smaller than its diameter, achieving complete coverage illumination without omissions and ensuring the acquisition of complete alignment information.

[0047] like Figure 5As shown, a second embodiment of this disclosure provides a grating alignment system, including a light source 101, an off-axis illumination galvanometer scanning device 102, an imaging module 103, and a target grating 104. The light source 101 provides a monochromatic illumination beam. The off-axis illumination galvanometer scanning device 102, as described above, processes the illumination beam emitted by the light source 101. The imaging module 103 has its optical axis perpendicular to the plane of the target grating 104, i.e., its optical axis is parallel to the Z-axis. Because the illumination precisely satisfies the Littoral condition, specific diffraction orders of the grating (e.g., +1 order) will return strictly along the Z-axis direction and be efficiently captured by the imaging module 103, thereby obtaining a high-contrast alignment signal. The captured diffracted light forms an image on a sensor (e.g., CCD or CMOS) of the imaging module. When the illumination spot covers multiple periods of the target grating, the diffracted light interference forms a pattern of alternating bright and dark moiré fringes. The phase of the moiré fringes directly corresponds to the sub-pixel-level positional offset of the target grating. The absolute phase of the moiré fringe pattern directly and precisely corresponds to the lateral positional shift (ΔX, ΔY) of the target grating at the sub-pixel level. When the target grating undergoes a tiny displacement d within the illumination area, it causes a phase shift φ in the overall moiré fringe pattern, and the two satisfy a strict proportional relationship (φ = 2π × d / P_m, where P_m is the equivalent period of the moiré fringe). Since the moiré fringe period P_m is much larger than the original grating period and the sensor pixel size, this method can amplify and transform nanometer-scale physical displacements into measurable changes in the phase of larger-scale fringes, thereby achieving sub-pixel or even sub-nanometer resolution position detection.

[0048] In some embodiments of this disclosure, the grating alignment system further includes an adjustment module. The function of this adjustment module is to automatically calculate and drive the third mirror 203 and the fourth mirror 204 in the off-axis illumination galvanometer scanning device to perform specific coordinated rotations in response to changes in the target grating period d or the illumination beam wavelength λ, thereby dynamically setting and maintaining the required off-axis incident angle α of the illumination beam.

[0049] When the system needs to switch the illumination wavelength λ or align with gratings of different periods d, the adjustment module calculates the new off-axis angle α of the target according to the formula sinα = λ / d. Based on the angular relationship between the third galvanometer 203 and the fourth galvanometer 204, θ3 = θ4 + 90° – α / 2, the adjustment module drives the third and fourth galvanometers to rotate synchronously to the calculated new angle (θ3, θ4), thereby precisely setting the new off-axis incident angle α. During subsequent two-dimensional scanning, the adjustment module controls the coordinated movement of the third galvanometer 203 and the fourth galvanometer 204 in real time, ensuring that the angular difference between them constantly satisfies the above relationship at any scanning position, thus achieving a stable off-axis angle α throughout the scanning process. The adjustment module enables the grating alignment system to automatically and quickly adapt to different process conditions, realizing intelligent operation to maintain the off-axis incident angle.

[0050] The third embodiment of this disclosure provides an off-axis illumination method using the above-described off-axis illumination galvanometer scanning device, including steps S1 to S3.

[0051] S1. Based on the lighting requirements, determine the off-axis incident angle α required for the lighting beam to enter the target grating according to the formula sinα = λ / d.

[0052] S2, based on the calculated α, according to the formula θ3 = θ4 + 90° – α / 2, coordinately control the rotation of the third galvanometer 203 and the fourth galvanometer 204 of the second galvanometer group, so that the illumination beam is incident on the predetermined scanning position on the target grating at an off-axis incident angle α.

[0053] S3, while keeping the off-axis incident angle constant, by changing the rotation state of the first galvanometer group and the second galvanometer group, including always keeping θ3 and θ4 of the second galvanometer group in accordance with the above relationship, the α angle is locked. The X-axis coordinate of the illumination beam spot is controlled by synchronously rotating the first galvanometer group (201, 202), and the Y-axis coordinate is controlled by cooperating in rotating the second galvanometer group (203, 204). The illumination beam is driven to perform two-dimensional scanning on the target grating according to a preset path.

[0054] The embodiments of this disclosure have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of this disclosure. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of this disclosure, and all such substitutions and modifications should fall within the scope of this disclosure.

Claims

1. An off-axis illumination galvanometer scanning device, applied to a grating alignment system, characterized in that, include: A first galvanometer (201), a second galvanometer (202), a third galvanometer (203), and a fourth galvanometer (204) are arranged sequentially along the optical path of the illumination beam. The rotation axes of the first galvanometer (201) and the second galvanometer (202) are parallel to each other, forming a first galvanometer group, which is used to control the illumination beam to perform a first-dimensional scanning displacement on the target grating; The rotation axes of the third galvanometer (203) and the fourth galvanometer (204) are parallel to each other and orthogonal to the rotation axis of the first galvanometer (201); the third galvanometer (203) and the fourth galvanometer (204) constitute a second galvanometer group, which is used to control the illumination beam to perform a second-dimensional scanning displacement on the target grating and to set the off-axis incident angle of the illumination beam relative to the target grating; The first and second galvanometer groups are controlled in a coordinated manner so that the illumination beam emitted from the fourth galvanometer (204) can perform a two-dimensional scan on the target grating while maintaining its off-axis incident angle relative to the target grating.

2. The apparatus according to claim 1, characterized in that, The mirror surfaces of the first galvanometer (201) and the second galvanometer (202) are arranged opposite each other and are configured to rotate synchronously. By rotating, the optical path of the illumination beam between the first galvanometer (201) and the second galvanometer (202) is changed, thereby driving the first-dimensional scanning displacement of the illumination beam.

3. The apparatus according to claim 1, characterized in that, The third galvanometer (203) and the fourth galvanometer (204) are arranged with their mirror surfaces facing each other and are configured to rotate in a coordinated manner with an asymmetrical angular relationship. The coordinated rotation means that when the second-dimensional scanning displacement of the illumination beam is changed, the rotation angle of the third galvanometer (203) and the fourth galvanometer (204) is adjusted simultaneously to set and maintain the off-axis incident angle.

4. The apparatus according to claim 3, characterized in that, The relationship between the rotation angle of the third galvanometer (203) and the rotation angle of the fourth galvanometer (204) is as follows: θ3 = θ4 + 90° – α / 2 Where θ3 is the rotation angle of the third galvanometer (203), θ4 is the rotation angle of the fourth galvanometer (204), α is the off-axis incident angle, which is the angle between the illumination beam and the normal of the target grating, 0≤α<90°, θ3 is the angle between the reflective surface of the third galvanometer (203) and the normal, θ4 is the angle between the reflective surface of the fourth galvanometer (204) and the normal, and the normal is perpendicular to the plane shown by the two-dimensional scan. —Does this require a 0° azimuth angle? 5. The apparatus according to claim 1, characterized in that, The off-axis incident angle is determined based on the wavelength of the illumination beam and the period of the grating. The formula for determining the off-axis incident angle is: Wherein, α is the off-axis incident angle, λ is the wavelength of the illumination beam, and d is the period of the target grating.

6. The apparatus according to claim 1, characterized in that, The first and second galvanometer groups are configured to drive the illumination beam to scan the target grating, and the scanning step distance is smaller than the diameter of the light spot formed by the illumination beam on the target grating.

7. A grating alignment system, characterized in that, include: The off-axis illumination galvanometer scanning apparatus as described in any one of claims 1 to 6; An imaging module, whose optical axis is perpendicular to the target grating, is used to receive light signals diffracted from the target grating.

8. The system according to claim 7, characterized in that, Also includes: An adjustment module is used to adjust the cooperative rotation angle of the third mirror (203) and the fourth mirror (204) in the off-axis illumination galvanometer scanning device according to the period of the target grating or the change in the wavelength of the selected illumination beam, so as to dynamically set and maintain the off-axis incident angle of the illumination beam relative to the target grating.

9. An off-axis illumination method, characterized in that, Using the apparatus as described in any one of claims 1 to 6, the method includes the following steps: Determine the off-axis incident angle required for the illumination beam to enter the target grating based on the illumination requirements; The first and second galvanometer groups are rotated in coordination to cause the illumination beam to be incident on the predetermined scanning position on the target grating at the off-axis incident angle. While keeping the off-axis incident angle constant, the illumination beam is driven to perform a two-dimensional scan on the target grating along a preset path by changing the rotation state of the first galvanometer group and the second galvanometer group.

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