A grating absolute line density detection system and method

Abstract

The present invention discloses a system and method for detecting the absolute linear density of a grating, belonging to the field of optical detection technology. By adding the functions of incident light wavelength modulation and wavelength monitoring, different diffraction angles at the same position of the grating are obtained by adjusting the wavelength of the incident light, thereby determining the absolute linear density at that position. Without changing the wavelength of the incident light, the relative detection accuracy of the LTP for the grating linear density is dN / N=5×10 ‑7 The present invention can achieve absolute detection of each position by only changing the wavelength of the incident light once, thus ensuring detection efficiency. After theoretical analysis and actual testing, the present invention has achieved an absolute detection accuracy of grating line density of dN / N=1×10 ‑6 .

Description

Technical Field

[0001] The present invention belongs to the technical field of optical detection, and relates to a grating absolute line density detection system and method. Background Art

[0002] With the construction of fourth-generation synchrotron radiation sources and the development of lithography systems, higher requirements are being placed on grating performance. Grating line density, a key metric for evaluating grating performance, influences its resolution and the reliability of optical information acquisition. To improve beam monochromaticity, enhance coherent diffraction imaging, and meet the high-precision positioning requirements of lithography system motion control platforms, more comprehensive grating line density measurement methods are required. Therefore, the development of precise grating line density measurement technology has important theoretical and practical significance.

[0003] Current methods for measuring line density include scanning electron microscopy, interferometry, diffraction, and moiré fringe methods. Among these, the long-range profilometer (LTP) method has the unique advantage of a large detection range and is the primary method for measuring the surface shape of large-aperture, large-curvature, and aspheric optical components. It also meets the detection requirements of long-range grating scales. LTP has achieved a high level of accuracy in measuring line density. Its detection principle is based on the grating equation. First, the incident light is perpendicularly incident, so that the incident light and diffracted light coincide. The grating base surface shape under zero-order diffraction light is obtained and the Littrow angle is derived. Then, under conditions close to the Littrow angle, first-order diffraction light is used for detection, obtaining dual data including the grating line density and base surface shape. Finally, the line density information of the grating is obtained by deducting the base surface shape.

[0004] However, this detection method can only obtain the relative change in grating line density by reading the change in diffraction angle, and cannot detect the absolute line density of the grating. Zero-point calibration of a grating for which only the relative change in line density is known, whether relying on processing data or supplemented by other detection methods, will introduce other errors and reduce positioning accuracy. A method that can obtain its absolute line density value while detecting the relative change in line density can avoid the introduction of calibration errors and improve positioning accuracy to a certain extent. The detection of absolute line density can also provide a standardized basis for the application of synchrotron radiation light sources, which is conducive to promoting the development and application of related technologies. Summary of the Invention

[0005] To address the above technical issues, the present invention proposes a grating absolute linear density detection system and method. By adjusting the wavelength of the incident light beam of the long-range profilometer, the relative linear density of the grating detected at different wavelengths is obtained, and the absolute linear density of each grating position is calculated, while maintaining high detection accuracy. This invention enables the long-range profilometer system to detect the absolute linear density of the grating, while significantly reducing the detection time while ensuring detection accuracy. It can achieve a relative detection error of grating linear density of dN / N=5×10 -7 , the absolute detection error is dN / N=1×10 -6 The detection accuracy meets the absolute detection requirements of high-precision grating line density. dN / N is the RMS value.

[0006] The present invention proposes a grating absolute linear density detection system, comprising a detector, a Fourier lens, a beam splitter, a laser, a collimator, a beam splitting optical fiber, a wavelength meter, a first plane reflector, a second plane reflector, and a grating to be measured; the detector, the Fourier lens, the beam splitter, the beam splitting optical fiber, the collimator, and the laser together constitute an LTP optical probe; when the wavelength is changed to detect the absolute linear density of the grating, the laser emits an incident light beam A, which first passes through the beam splitting optical fiber and then enters the wavelength meter to monitor the laser wavelength; the other part is reflected by the collimator and the beam splitter to the second plane reflector, and is reflected by the second plane reflector and the first plane reflector before being incident on the surface of the grating to be measured, generating a diffracted light beam B, which is then reflected by the first plane reflector and the second plane reflector and transmitted by the beam splitter and the Fourier lens, and finally imaged on the detector, thereby obtaining the light spot position information recorded by the detector at this time; thereafter, the wavelength of the incident light beam A is changed by regulating the laser, and the position information of the diffracted light beam C imaged on the detector is obtained.

[0007] The present invention also proposes a grating absolute line density detection method. When the wavelength of the incident light beam is λ, the grating equation is:

[0008] mNλ=sin(α)+sin(β) (1)

[0009] Where m is the diffraction order; λ is the wavelength of the detection beam; N is the grating line density; α and β are the incident angle and diffraction angle respectively; at the same position, the wavelength of the incident beam increases When , the grating equation is:

[0010] mN(λ+ )=sin(α)+sin(γ) (2)

[0011] γ is the wavelength of the incident beam increased After that, the diffraction angle when the incident angle is α; at the same position, the wavelength of the incident beam increases by 2 When , the grating equation is:

[0012] mN(λ+2 )=sin(α)+sin(δ) (3)

[0013] δ is the wavelength of the incident beam increased by 2 After that, the diffraction angle when the incident angle is α; the accurate values ​​of γ-β and δ-β can be obtained through the LTP optical probe 、 , that is, γ-β= ,δ-β= , so the combination of equations (1)(2)(3) can immediately solve the diffraction angle β:

[0014] β=arctan[(2 sin( )-sin( )) / (cos( )+1-2 cos( ))] (4)

[0015] Substitute it into formula (2)-(1):

[0016] mN =sin(β+ )-sin(β) (5)

[0017] That is, the absolute value N of the grating line density is obtained.

[0018] The present invention has the following beneficial effects:

[0019] The present invention uses the LTP system to detect the absolute linear density of the grating. The original LTP system is equipped with the functions of incident light wavelength modulation and wavelength monitoring. By adjusting the wavelength of the incident light, different diffraction angles are obtained at the same position of the grating, thereby determining the absolute linear density at that position. Without changing the wavelength of the incident light, the relative detection accuracy of the LTP for the grating linear density is dN / N=5×10 -7 The present invention can achieve absolute detection of each position by only changing the wavelength of the incident light twice, thus ensuring detection efficiency. After theoretical analysis and actual testing, the present invention has achieved an absolute detection accuracy of grating line density of dN / N=1×10 -6 . BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The structure and optical path diagram of the LTP variable wavelength detection system;

[0021] Figure 2 is the spot offset change diagram;

[0022] Figure 3is the relationship between the relative detection error of line density and the line density of the grating to be measured;

[0023] Figure 4 This is a diagram showing the relationship between line density detection error and the line density of the grating to be measured.

[0024] In the figure, the reference numerals are: detector 1, Fourier lens 2, beam splitter 3, laser 4, beam splitting fiber 5, collimator 6, wavelength meter 7, first plane reflector 8, second plane reflector 9, and grating to be measured 10. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other. To achieve the above-mentioned objectives, the present invention adopts the following technical solutions.

[0026] The present invention changes the laser wavelength of the LTP system, that is, changes the wavelength of the incident light, and calculates the absolute line density of the position by monitoring the change of the diffraction angle, such as Figure 1 As shown in the figure, a grating absolute linear density detection system is proposed, comprising a detector 1, a Fourier lens 2, a beam splitter 3, a laser 4, a beam splitting fiber 5, a collimator 6, a wavelength meter 7, a first plane mirror 8, a second plane mirror 9, and a grating to be measured 10. The detector 1, Fourier lens 2, beam splitter 3, laser 4, beam splitting fiber 5, and collimator 6 together constitute an LTP optical probe. When wavelength-changing grating absolute linear density detection is performed, the laser 4 emits an incident light beam A. A portion of the incident light beam A first passes through the beam splitter fiber 5 and enters the wavelength meter 7 to obtain the laser wavelength at that time. The remaining portion is reflected by the collimator 6 and beam splitter 3 onto the second plane mirror 9. After reflection from the second plane mirror 9 and the first plane mirror 8, the incident light beam B is incident on the surface of the grating to be measured 10, generating a diffracted light beam B. The diffracted light beam B is then reflected by the first plane mirror 8 and the second plane mirror 9, and transmitted by the beam splitter 3 and Fourier lens 2. Finally, the diffracted light beam B is imaged on the detector 1, obtaining the light spot position information recorded by the detector at that time. Then, by adjusting the laser 4, the wavelength of the incident beam A is changed to obtain the position information of the diffracted beam C imaged on the detector. The path of the incident beam does not change. When the wavelength of the incident beam is λ, the grating equation is:

[0027] mNλ=sin(α)+sin(β) (1)

[0028] Where m is the diffraction order; λ is the wavelength of the detection beam; N is the grating line density; α and β are the incident angle and diffraction angle respectively; at the same position, the wavelength of the incident beam increases When , the grating equation is:

[0029] mN(λ+ )=sin(α)+sin(γ) (2)

[0030] γ is the incident beam increase After that, the diffraction angle when the incident angle is α; at the same position, the wavelength of the incident beam increases by 2 When , the grating equation is:

[0031] mN(λ+2 )=sin(α)+sin(δ) (3)

[0032] δ is the incident beam increased by 2 After that, the diffraction angle when the incident angle is α; the accurate values ​​of γ-β and δ-β can be obtained through the LTP optical probe 、 , that is, γ-β= ,δ-β= , so the combination of equations (1)(2)(3) can immediately solve the exact diffraction angle β:

[0033] β=arctan[(2 sin( )-sin( )) / (cos( )+1-2 cos( ))] (4)

[0034] Substitute it into formula (2)-(1):

[0035] (5)

[0036] The absolute value N of the grating line density can be obtained.

[0037] The accuracy of using LTP to detect linear density is limited by the accuracy of the wavelength meter in monitoring the changing wavelength and the accuracy of LTP in angular resolution.

[0038] The accuracy of LTP on angular resolution mainly affects the relative detection of grating line density. Currently, its angular resolution accuracy is better than 50nrad, which is related to the line density detection error E N The relationship is:

[0039] N+E N =(sin(α)+sin(β+E β )) / m λ (6)

[0040] Substituting into formula (1) can be simplified to:

[0041] E N =(sin(β+ E β ) -sin(β)) / m λ;

[0042] Among them, E β is the LTP angle resolution accuracy, E N is the line density detection error. Assuming that the incident angle is the littrow angle and the angular resolution accuracy is 50nrad, the relationship between the line density relative detection error and the line density of the grating to be measured is as follows: Figure 2 As shown in the figure: the horizontal axis represents the grating line density to be measured, and the vertical axis represents the line density detection error. Figure 3 It can be seen that the influence of LTP resolution accuracy on the relative detection error of line density is about 5×10 -7 , the value is the RMS value.

[0043] The accuracy of the wavelength meter mainly affects the absolute detection of the grating line density. The accuracy of the wavelength meter used in the verification experiment is E λ =±0.0002nm, under the condition of near littrow angle, the relationship between line density detection error and wavelength meter accuracy is:

[0044] N+E N =(sin(α)+sin(β)) / m (λ+E λ ) (7)

[0045] N=(sin(α)+sin(β)) / m λ (8)

[0046] From the first two equations we can get:

[0047] E N = N λ / (λ+ E λ )-N (9)

[0048] From the above formula, it can be seen that the line density detection error is affected by both the line density of the grating to be measured and the accuracy of the wavelength meter. When the accuracy of the wavelength meter is ±0.0002nm, the relationship between the line density detection error and the line density of the grating to be measured is as follows: Figure 4 As shown in the figure, the horizontal axis represents the grating line density to be measured, and the vertical axis represents the line density detection error. Therefore, the influence of the wavelength meter accuracy on the line density detection error is about 1×10 -6 , the value is the RMS value.

[0049] Since the LTP angle resolution accuracy E β and wavelength meter accuracy E λAll are RMS values. RMS is the root mean square value, which is used to reflect the error distribution. Therefore, the calculated dN / N is also the RMS value.

[0050] The linear density detection error is related to the linear density of the grating to be measured. The evaluation index is dN / N. Since the linear density detection error in this system is mainly affected by the LTP angle resolution accuracy and the wavelength meter accuracy, after calculation, it can be determined that the linear density relative detection error is dN / N=5×10 -7 , the absolute detection error is dN / N=1×10 -6 .

Claims

1. A grating absolute line density detection system, characterized in that: include: The detector, Fourier lens, beam splitter, laser, collimator, beam splitting fiber, wavelength meter, first plane mirror, and second plane mirror are composed of the detector, Fourier lens, beam splitter, laser, beam splitting fiber, and collimator. When the wavelength is changed to detect the absolute linear density of the grating, the laser emits an incident light beam A, a part of which first enters the wavelength meter through the beam splitting fiber to monitor the laser wavelength at this time, and the other part is reflected onto the second plane mirror through the collimator and the beam splitter. After reflection from the second plane mirror and the first plane mirror, the light beam is incident on the surface of the grating to be measured, generating a diffracted light beam B. The diffracted light beam B is then reflected by the first plane mirror and the second plane mirror and transmitted by the beam splitter and the Fourier lens, and finally imaged on the detector to obtain the light spot position information recorded by the detector at this time. Then, the wavelength of the incident light beam A is changed by adjusting the laser to obtain the position information of the diffracted light beam C imaged on the detector.

2. A method for detecting absolute line density of a grating using the grating absolute line density detection system according to claim 1, characterized in that: include: When the wavelength of the incident light beam is λ, the grating equation is: mNλ=sin(α)+sin(β) (1) Where m is the diffraction order; λ is the wavelength of the detection beam; N is the grating line density; α and β are the incident angle and diffraction angle respectively; at the same position, the wavelength of the incident beam increases When , the grating equation is: mN(λ+ )=sin(a)+sin(c) (2) γ is the incident beam increase After that, the diffraction angle when the incident angle is α; at the same position, the wavelength of the incident beam increases by 2 When , the grating equation is: mN(λ+2 )=sin(a)+sin(δ) (3) δ is the incident beam increased by 2 After that, the diffraction angle when the incident angle is α; the accurate values ​​of γ-β and δ-β can be obtained through the LTP optical probe 、 , that is, γ-β= ,δ-β= , so the diffraction angle β can be solved immediately by combining equations (1)(2)(3): β=arctan[(2 without( )-without( )) / (cos( )+1-2 cos( ))] (4) Substitute it into formula (2)-(1): mN =sin(β+ )-sin(β) (5) That is, the absolute value N of the grating line density is obtained.

3. The detection method according to claim 2, characterized in that The accuracy of the variable wavelength detection line density of the LTP optical probe depends on the accuracy of the wavelength meter in monitoring the variable wavelength and the accuracy of the LTP optical probe in angular resolution.

4. The detection method according to claim 3, characterized in that Angle resolution accuracy of LTP optical probe and line density detection error E N The relationship is: N+E N =(sin(α)+sin(β+E β )) / m l (6) Substituting into formula (1) and simplifying it into: E N =(sin(β+ E β )-sin(β)) / m l, Among them, E β is the angular resolution accuracy of the LTP optical probe, E N is the line density detection error.

5. The detection method according to claim 3, characterized in that The wavelength meter accuracy is E λ =±0.0002nm, under the condition of near littrow angle, the relationship between line density detection error and wavelength meter accuracy is: N+E N =(sin(a)+sin(b)) / m (λ+E λ ) (7) N=(sin(α)+sin(β)) / m l (8) E N = N λ / (λ+ E λ )-N (9).

6. The detection method according to claim 3, characterized in that Without changing the wavelength of the incident light, the relative detection accuracy of the grating line density is dN / N=5×10 -7 , where dN / N is the RMS value.

7. The detection method according to claim 3, characterized in that The absolute detection accuracy of grating line density reaches dN / N=1×10 -6 , where dN / N is the RMS value.

8. An electronic device, characterized in that: include: one or more processors; A memory for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors are enabled to implement the method according to any one of claims 2 to 6.

9. A computer-readable storage medium, characterized in that Executable instructions are stored thereon, and when the instructions are executed by a processor, the processor implements the method according to any one of claims 2 to 6.

10. An intelligent terminal comprising a processor, an input device, an output device, and a memory, wherein the processor, the input device, the output device, and the memory are interconnected, the memory is used to store a computer program, and the computer program includes program instructions, wherein: The processor is configured to call the program instructions and execute the method according to any one of claims 2 to 6.

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