Large-scanning-view-field large-image-space numerical aperture interference objective lens for flying-spot scanning interferometer

Abstract

The invention discloses a large-scanning-view-field large-image-space numerical aperture interference objective lens for a flying-spot scanning interferometer. The large-scanning-view-field large-image-space numerical aperture interference objective lens comprises a first spherical lens, a second spherical lens, a first spherical reflector, a second spherical reflector, a third spherical lens and a fourth spherical lens. Wherein the first spherical lens, the second spherical lens, the first spherical reflector, the second spherical reflector, the third spherical lens and the fourth spherical lens are sequentially arranged from an object to an image in the optical axis direction. The optical system provided by the invention has reasonable focal power distribution, can effectively suppress aberration, and by combining the beam expanding system with the coaxial double-reflector Cassegrain system, on the premise of keeping the scanning field of view and the number of interference objective lenses unchanged, the maximum mirror reflection angle which can be received by the objective lenses is increased, and the installation and adjustment difficulty of the reflectors can be greatly reduced.

Description

Technical Field

[0001] The invention belongs to the field of non-contact photoelectric micro-nano detection, and in particular relates to an interference objective lens with a large scanning field and a large image square numerical aperture for a flying spot scanning interferometer. Background Art

[0002] Flying spot scanning interferometry is used to measure the surface micro-nano detection of semiconductor wafers, especially the thickness, warpage, holes and surface coating thickness of wafers. Different from the monochrome fringe measurement of traditional white light interferometry, flying spot scanning interferometry uses frequency domain spectral interferometry. After light source collimation, prism splitting, galvanometer scanning, diffraction spectrum splitting and other steps, the spectral interference signal is finally analyzed to determine the surface morphology. It does not require scanning in the optical axis direction or the use of expensive high-precision translation stages, which can improve measurement efficiency and measurement accuracy.

[0003] At present, in the flying spot scanning interferometer, the wide-spectrum white light emitted by the superluminescent diode (SLD light source) passes through the fiber coupler, is collimated and expanded by the collimator, and then incident on the beam splitter prism. A beam of reflected light is focused on the reflector as the reference arm through the interference objective lens, and the other beam of transmitted light is incident on the XY scanning galvanometer. The scanning galvanometer reflects the light with a certain field angle and enters the scanning objective lens, which is then focused on the sample to complete the flying spot scanning. The light reflected from the reference arm and the sample arm interferes, and then collimated by lens I to the diffraction grating. After the grating spectrum is split, it is focused by lens II to the camera to collect the spectral interference signal.

[0004] Among them, the three conditions for the interference of two beams of light are: 1. The frequency is the same; 2. The vibration direction is the same; 3. The phase difference between the two beams of light at the intersection is constant. In order to ensure interference, the two interference lenses in the interferometer should be as identical as possible. These two interference lenses, the scanning lens of the sample arm and the interference lens of the reference arm, both adopt an image-space telecentric structure, with the same numerical aperture, the same number of lenses, and the same curvature of each lens, and are completely identical optical systems.

[0005] The flying spot scanning interferometer produced by the German company Precitec on the market has two interference objective lenses with numerical apertures of NA=0.0175 and NA=0.05 respectively. When the central wavelength is 840nm, the corresponding resolution δ is 29.28um and 10.25um respectively. The maximum allowable mirror reflection angles are ±1° and ±3° respectively, and the scanning field of view is Φ80mm.

[0006] For example, the Chinese patent with publication number CN114185152B relates to an image-side telecentric objective lens for a flying spot scanning interferometer. The objective lens is a flying spot scanning interferometer interferometer, with a large numerical aperture, NA of 0.4, 0.5 and 0.6 respectively; a high resolution of δ = 1.25um, 1um and 0.83um; and a large maximum allowable mirror reflection angle, θ = ± 23.5°, ± 30° and ± 36.87° respectively. However, due to the increase in numerical aperture, the scanning field of view is only Φ10mm.

[0007] Therefore, under the premise of maintaining the scanning field of view of Φ80mm, it is a common desire in actual operation to optimize the design of the interference objective optical system, expand the numerical aperture, increase the maximum allowable mirror reflection angle, increase the redundancy of the reflector adjustment, reduce the difficulty of manufacturing and using the equipment, and improve the cost performance. Summary of the invention

[0008] In order to solve the problem that the numerical aperture of the interference objective lens with a large scanning field of view of the existing flying spot scanning interferometer is small, resulting in high requirements for the installation of the reference arm reflector, and the reference arm cannot return the reference light or interfere with the sample light because the maximum receivable mirror reflection angle is too small, which easily leads to measurement failure, the present invention proposes an interference objective lens with a large scanning field of view and a large image side numerical aperture for a flying spot scanning interferometer.

[0009] The specific technical scheme of the present invention is:

[0010] An interference objective lens with a large scanning field and a large image square numerical aperture for a flying spot scanning interferometer comprises: a first spherical lens, a second spherical lens, a first spherical reflector, a second spherical reflector, a third spherical lens and a fourth spherical lens;

[0011] Among them, the first spherical lens, the second spherical lens, the first spherical reflector, the second spherical reflector, the third spherical lens and the fourth spherical lens are arranged in sequence from the object to the image along the direction of the light.

[0012] Further, the first spherical lens has negative optical power, the second spherical lens has positive optical power, the first spherical reflector has positive optical power, the second spherical reflector has negative optical power, the third spherical lens has positive optical power, and the fourth spherical lens has negative optical power.

[0013] Furthermore, the entrance pupil of the objective lens is on one side of the first spherical lens; after the main light of the field of view passes through the fourth spherical lens, it is reflected by the mirror perpendicular to the axis and returns along the original path to be used for interference with the return light of the sample arm of the flying spot interferometer; the entrance pupil is set at the position of the object focal plane of the interference objective lens, forming an image-side telecentric optical system; wherein, in the image-side telecentric optical system, the aperture stop of the optical path is located on the object focal plane of the optical system.

[0014] Furthermore, the first spherical lens and the second spherical lens form an afocal beam expansion system for expanding light, wherein the relationship between the incident beam diameter and the outgoing beam diameter and the focal length of the first spherical lens and the second spherical lens is as follows:

[0015]

[0016] Among them, D 入 and D 出 denote the incident beam diameter and the exit beam diameter, respectively, and f 1 and f 2 Represent the focal lengths of the first spherical lens and the second spherical lens respectively.

[0017] Furthermore, the second spherical reflector is located between the vertex and the focus of the first spherical reflector, forming a double-reflector optical system.

[0018] Furthermore, the third spherical lens and the fourth spherical lens form a doublet lens.

[0019] Furthermore, the second spherical reflector and the second spherical lens share the second surface of the second spherical lens, and a reflective film is plated on the central area of ​​the second spherical lens to form the second spherical reflector.

[0020] Further, the focal length f1 of the first spherical lens has a value range of -0.5≤f1 / f≤-0.1;

[0021] The focal length f2 of the second spherical lens has a value range of 0.5≤f2 / f≤5;

[0022] The focal length f3 of the first spherical reflector has a value range of 1≤f3 / f≤5;

[0023] The focal length f5 of the third spherical lens has a value range of 0.1≤f5 / f≤1;

[0024] The focal length f6 of the fourth spherical lens has a value range of -0.5≤f6 / f≤-0.1;

[0025] Where f is the focal length of the combined lens.

[0026] Furthermore, the numerical aperture NA of the objective lens has a value range of 0.05≤NA≤0.0845. Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1. The present invention expands the numerical aperture of the optical system without sacrificing the scanning field of view, increases the maximum allowable mirror reflection angle, and reduces the difficulty of system installation and adjustment and the difficulty of using the detection instrument.

[0028] 2. The present invention adopts a structure that combines a beam expansion system and a double reverse Cassegrain telescope system to optimize the structure. The optical system has a compact structure and shortens the length of the interference arm, which can reduce the height and volume of the interferometer, increase its stability, save space, and facilitate the operation of the operator. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0030] Figure 1 It is a schematic structural diagram of a large scanning field and large image numerical aperture objective lens according to an embodiment of the present invention;

[0031] Figure 2 A schematic diagram of the flying spot scanning interferometer principle of an embodiment of the present invention;

[0032] Figure 3 The optical path diagram of the large image-side numerical aperture objective lens of Embodiment 1 of the present invention;

[0033] Figure 4 The MTF curve diagram of the large image-side numerical aperture objective lens of Example 1 of the present invention;

[0034] Figure 5 This is a point diagram of a large image-side numerical aperture objective lens according to Embodiment 1 of the present invention;

[0035] Figure 6 The imaging optical path diagram of the large image-side numerical aperture objective lens of Embodiment 2 of the present invention;

[0036] Figure 7 The MTF curve diagram of the large image-side numerical aperture objective lens of Example 2 of the present invention;

[0037] Figure 8 This is a point diagram of a large image-side numerical aperture objective lens according to Embodiment 2 of the present invention;

[0038] Fig. 9 This is a comparison diagram of an embodiment of the present invention that allows the reflector to be installed tilted.

[0039] The reference numerals are as follows: 1: first spherical lens; 2: second spherical lens; 3: first spherical reflector; 4: second spherical reflector; 5: third spherical lens; 6: fourth spherical lens. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] The resolution of an objective lens refers to the minimum distance at which the objective lens can distinguish two adjacent objects. The numerical aperture is the angular range of light that the objective lens can collect. The relationship between the resolution δ and the numerical aperture NA is:

[0042] δ=0.61λ / NA (1)

[0043] The relationship between the maximum acceptable mirror reflection angle θ and the numerical aperture NA of the objective lens is:

[0044] θ=±asin(NA) (2)

[0045] Therefore, the numerical aperture NA of the objective lens determines the resolution δ and the maximum acceptable mirror reflection angle θ. The larger the numerical aperture, the smaller the resolution δ, the higher the resolution, the larger the maximum acceptable mirror reflection angle, and the larger the allowable tilt angle of the reference arm's reflector.

[0046] like Figure 1 As shown, the present invention is directed to Figure 2 Further optimization of the middle reference arm interference objective lens discloses a large scanning field of view and large image numerical aperture objective lens for flying spot scanners. The optical structure of the large scanning field of view and large image numerical aperture objective lens is as follows: the entire optical system of the objective lens is, from left to right, an entrance pupil, a first spherical lens 1 with negative optical focal length, a second spherical lens 2 with positive optical focal length, the first spherical lens 1 and the second spherical lens 2 are approximately afocal beam expansion system, which does not bear the optical focal length of the system and is only used for beam expansion of light. 3 is a first spherical reflector with positive optical focal length, which is also a Cassegrain system. The second spherical reflector 4 is a spherical reflector with a negative focal length, which coincides with the rear surface of the second spherical lens 2, that is, the second spherical reflector 4 is a lens, and a reflective film is plated on the central area of ​​the rear surface of the second spherical lens 2. The first spherical reflector 3 and the second spherical reflector 4 together form a Cassegrain system. The third spherical lens 5 and the fourth spherical lens 6 are double-cemented lenses. The third spherical lens 5 has a positive optical focal length, and the fourth spherical lens 6 has a negative optical focal length. The double-cemented lenses converge light, increase the numerical aperture, and reduce the structural size.

[0047] Example 1

[0048] like Figure 3 As shown, the numerical aperture NA of the image side is 0.0782, and the detailed design parameters are shown in Table 1. The resolution δ is calculated by equations (1) and (2) to be 6.56um, and the maximum allowable mirror reflection angle θ is ±4.48°. The MTF curve of the lens imaging quality is shown in Figure 4 As shown, the MTF is close to the diffraction limit under the full field of view, with high resolution; the lens point array is as follows Figure 5 shown.

[0049] Table 1 Optical system design parameters of Example 1

[0050]

[0051] According to formulas (1) and (2), the numerical aperture NA of the objective lens determines the resolution δ and the maximum acceptable mirror reflection angle θ. The larger the numerical aperture, the larger the maximum acceptable mirror reflection angle. Therefore, we give a certain tolerance when designing the interference objective lens, increase the numerical aperture, and allow more reflected light to enter the interference objective lens.

[0052] like Fig. 9 As shown in the figure, the dotted part is the image side aperture angle of the interference objective lens, which is also the maximum acceptable mirror reflection angle θ of the lens. The maximum acceptable mirror reflection angle θ of the objective lens on the left is small, which is equivalent to the half-angle β of the beam aperture after the parallel light is incident, so the reflector can hardly be tilted, and the installation requirements are relatively high. In the right figure, the maximum acceptable mirror reflection angle θ of the interference objective lens increases, which is much larger than the half-angle β of the beam aperture. In this way, even if the reflector is tilted, the reflected light is still within the image side aperture angle range.

[0053] The tilt angle α of the reflector:

[0054]

[0055] In Example 1, θ=±4.48°, assuming that the actual incident light aperture angle 2β=1.6°, that is, the half angle β=0.8°, the reflector can be tilted by ±1.84°. This means that the reflector can be installed within ±1.84° to achieve reflection, and the difficulty of installation and adjustment is reduced.

[0056] When θ=±1°, assuming the actual incident light aperture angle 2β=1.6°, the reflector can be tilted by ±0.1°. The installation is difficult and the adjustment time is long.

[0057] In order to make interference occur, the reflection arm and the sample arm are exactly the same. By increasing the numerical aperture, the reflection angle tolerance of the reflection arm increases, and the sample arm is also increased. The allowable error of the sample stage installation of the sample arm increases, and the difficulty of installation and adjustment is reduced. However, if the numerical aperture is greatly increased, the scanning field of view will be sharply reduced. Therefore, the numerical aperture should be appropriately increased while keeping the scanning field of view unchanged. In addition, the tilt angle of the reflector cannot be increased infinitely, because in the optical system of the entire flying spot detector, the reflected light will also be limited by the beam splitter, collimator, and coupler.

[0058] Example 2

[0059] The objective image square numerical aperture range is: 0.05≤NA≤0.0845, and the specific design parameters are shown in Table 2. The detailed design structure of the large scanning field and large image square numerical aperture objective lens is as follows: Figure 6As shown, the numerical aperture NA of the image side is 0.0845; the resolution δ is calculated by equations (1) and (2) to be 6.07um, and the maximum allowable mirror reflection angle θ is ±4.85°; the MTF curve of the lens imaging quality is shown in Figure 7 As shown; the lens point diagram is as follows Figure 8 shown.

[0060] Table 2 Design parameters of large scanning field and large image numerical aperture objective lens of Example 2

[0061]

[0062]

[0063] Compared with Example 1, Example 2 has a larger image-side numerical aperture NA and allows a larger maximum mirror reflection angle. If components such as the beam splitter, collimator, and coupler in the flying spot detector do not limit the light, Example 2 is suitable, otherwise Example 1 is selected.

[0064] The specific implementation of the present invention is an optimized design of the German interference objective lens of the prior art, and a large scanning field and large numerical aperture interference objective lens for a flying spot scanning interferometer is designed, wherein the lens group 1 is an afocal beam expansion system, which is used to increase the aperture of the small entrance pupil, that is, to increase the aperture of the Cassegrain double reflection system, thereby reducing the focal length of the whole system and increasing the image side numerical aperture of the whole system; the lens group 2 is a coaxial double reflection Cassegrain system, which is used to shorten the system structure size. The secondary mirror 4 of the double reflection Cassegrain system and the lens 2 share the same optical part, which can reduce the number of lenses of the system and ensure a compact structure.

[0065] The optical system of the present invention has a reasonable distribution of optical focal length, which can effectively suppress aberrations. By combining the beam expansion system with the coaxial double-reflection Cassegrain system, the maximum mirror reflection angle that can be received by the objective lens is increased from ±1° to ±4.48°, or even larger, such as ±4.85° in 2, while maintaining the scanning field of view and the number of interference objective lenses unchanged. This can greatly reduce the difficulty of installing and adjusting the reflector.

[0066] Compared with the German lens in the prior art, the large scanning field and large image numerical aperture interference lens disclosed in the present invention has high resolution, strong operability, low manufacturing cost, and high cost performance under the premise of a large scanning field, which is more in line with our actual use needs.

[0067] The examples described in the present invention are merely descriptions of the preferred implementation modes of the present invention, and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should all fall within the protection scope of the present invention.

Claims

1. A large scanning field and large image numerical aperture interference objective lens for a flying spot scanning interferometer, characterized in that: include: A first spherical lens, a second spherical lens, a first spherical reflector, a second spherical reflector, a third spherical lens and a fourth spherical lens; Among them, the first spherical lens, the second spherical lens, the first spherical reflector, the second spherical reflector, the third spherical lens and the fourth spherical lens are arranged in sequence from the object to the image along the direction of the light.

2. The objective lens according to claim 1, characterized in that The first spherical lens has negative optical power, the second spherical lens has positive optical power, the first spherical reflector has positive optical power, the second spherical reflector has negative optical power, the third spherical lens has positive optical power, and the fourth spherical lens has negative optical power.

3. The objective lens according to claim 1, characterized in that The entrance pupil of the objective lens is on one side of the first spherical lens; after the main light of the field of view passes through the fourth spherical lens, it is reflected by the mirror perpendicular to the axis and returns along the original path to be used for interference with the return light of the sample arm of the flying spot interferometer; The entrance pupil is set at the position of the object focal plane of the interference objective lens, forming an image-side telecentric optical system; wherein, in the image-side telecentric optical system, the aperture stop of the optical path is located on the object focal plane of the optical system.

4. The objective lens according to claim 1, characterized in that The first spherical lens and the second spherical lens form an afocal beam expansion system for expanding light, wherein the relationship between the incident beam diameter and the outgoing beam diameter and the focal length of the first spherical lens and the second spherical lens is as follows: Among them, D 入 and D 出 They represent the incident beam diameter and the exit beam diameter respectively, and f1 and f2 represent the focal lengths of the first spherical lens and the second spherical lens respectively.

5. The objective lens according to claim 1, characterized in that The second spherical reflector is located between the vertex and the focus of the first spherical reflector, forming a double-reflector optical system.

6. The objective lens according to claim 1, characterized in that The third spherical lens and the fourth spherical lens form a doublet lens.

7. The objective lens according to claim 1, characterized in that The second spherical reflector and the second spherical lens share the second surface of the second spherical lens, and a reflective film is plated on the central area of ​​the second spherical lens to form the second spherical reflector.

8. The objective lens according to claim 1, characterized in that The focal length f1 of the first spherical lens has a value range of -0.5≤f1 / f≤-0.1; The focal length f2 of the second spherical lens has a value range of 0.5≤f2 / f≤5; The focal length f3 of the first spherical reflector has a value range of 1≤f3 / f≤5; The focal length f5 of the third spherical lens has a value range of 0.1≤f5 / f≤1; The focal length f6 of the fourth spherical lens has a value range of -0.5≤f6 / f≤-0.1; Where f is the focal length of the combined lens.

9. The objective lens according to claim 1, characterized in that The numerical aperture NA of the objective lens has a value range of 0.05≤NA≤0.0845.

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