Interference objective lens with large scanning field of view and large image space numerical aperture for flying spot scanning interferometer

Abstract

The present invention discloses an interference objective lens with a large scanning field of view and a large image space numerical aperture for a flying spot scanning interferometer, comprising: a first spherical lens, a second spherical lens, a first spherical mirror, a second spherical mirror, a third spherical lens, and a fourth spherical lens; wherein, the first spherical lens, the second spherical lens, the first spherical mirror, the second spherical mirror, the third spherical lens, and the fourth spherical lens are sequentially arranged from the object to the image along the optical axis direction. The optical system in the present invention has a reasonable distribution of optical power, can effectively suppress aberrations, and by combining an expanding system with a coaxial two-mirror Cassegrain system, on the premise of maintaining the scanning field of view and the number of lenses of the interference objective lens unchanged, the maximum mirror reflection angle that the objective lens can receive is increased, and the alignment difficulty of the mirror can be greatly reduced.

Description

Technical Field

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

[0002] The flying-spot scanning interference technology is used for surface micro-nano detection of semiconductor wafers, especially for measuring parameters such as the thickness, warpage, holes, and surface coating thickness of wafers. Different from the monochromatic fringe measurement of traditional white-light interference technology, the flying-spot scanning interference adopts frequency-domain spectral interference technology. After steps such as light source collimation, prism beam splitting, galvanometer scanning, and diffraction spectral beam splitting, the spectral interference signal is finally analyzed to determine the surface topography. Without the need for optical axis direction scanning or the use of expensive high-precision displacement stages, the measurement efficiency and measurement accuracy can be improved.

[0003] Currently, in a flying-spot scanning interferometer, the broadband white light emitted by a superluminescent light-emitting diode (SLD light source) passes through an optical fiber coupler and is collimated and expanded by a collimator, and then enters a beam-splitting prism. One reflected light beam is focused onto a reflector by an interference objective lens as the reference arm, and the other transmitted light beam enters an XY scanning galvanometer. The light beam with a certain field of view angle reflected by the scanning galvanometer enters a scanning objective lens, and then is focused onto the sample by the scanning objective lens to complete the flying-spot scanning. The light reflected back from the reference arm and the sample arm interferes, and then is collimated by lens I and incident on a diffraction grating. After spectral beam splitting by the grating, it is focused onto a camera by lens II to collect the spectral interference signal.

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

[0005] For the flying-spot scanning interferometer of German company precitec on the market, the numerical apertures of its two interference objective lenses are NA = 0.0175 and NA = 0.05 respectively. When the central wavelength is 840 nm, the corresponding resolutions δ are 29.28 μm and 10.25 μm respectively, and the maximum allowable mirror reflection angles are ±1° and ±3° respectively, and the scanning field of view is Φ80 mm.

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

[0007] Therefore, on the premise of not reducing the scanning field of view Φ80mm, it is a common desire in actual operation to specifically optimize the design of the interferometric objective lens optical system, expand the numerical aperture, increase the maximum allowable mirror reflection angle, increase the adjustment redundancy of the mirror, reduce the difficulty of manufacturing and using the equipment, and improve the cost performance. Summary of the Invention

[0008] To solve the problem that the numerical aperture of the large-scanning-field interferometric objective lens of the existing flying-spot scanning interferometer is small, resulting in high installation requirements for the reference arm mirror, because the maximum allowable mirror reflection angle is too small, the reference arm cannot return the reference light and cannot interfere with the sample light, easily leading to measurement failure, the present invention proposes a large-scanning-field and large-image-space numerical aperture interferometric objective lens for a flying-spot scanning interferometer.

[0009] The specific technical solution of the present invention is as follows:

[0010] The large-scanning-field and large-image-space numerical aperture interferometric objective lens for a flying-spot scanning interferometer includes: a first spherical lens, a second spherical lens, a first spherical mirror, a second spherical mirror, a third spherical lens, and a fourth spherical lens;

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

[0012] Further, the first spherical lens has a negative focal power, the second spherical lens has a positive focal power, the first spherical mirror has a positive focal power, the second spherical mirror has a negative focal power, the third spherical lens has a positive focal power, and the fourth spherical lens has a negative focal power.

[0013] Further, the entrance pupil of the objective lens is on one side of the first spherical lens; when the chief ray of the field of view passes through the fourth spherical lens and is reflected by a mirror perpendicular to the axis, it returns along the original path and is used for the interference of the return light of the sample arm of the flying-spot interferometer; the entrance pupil is located at the position of the object-side focal plane of the interferometric objective lens, becoming an image-space telecentric optical system; among them, in the image-space telecentric optical system, the aperture stop of the optical path is located on the object-side focal plane of the optical system.

[0014] Furthermore, the first spherical lens and the second spherical lens form an afocal beam expander system for expanding light. The relationship between the incident beam diameter and the output beam diameter and the focal lengths of the first spherical lens and the second spherical lens is as follows:

[0015]

[0016] Wherein, D 入 and D 出 respectively represent the incident beam diameter and the output beam diameter, and f 1 and f 2 respectively represent the focal lengths of the first spherical lens and the second spherical lens.

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

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

[0019] Furthermore, the second spherical mirror shares the second surface of the second spherical lens, and a reflective film is plated in the central area of the second spherical lens to form the second spherical mirror.

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

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

[0022] The value range of the focal length f3 of the first spherical mirror is 1 ≤ f3 / f ≤ 5;

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

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

[0025] Wherein, 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. Without sacrificing the scanning field of view, the present invention expands the numerical aperture of the optical system, increases the maximum allowable mirror reflection angle, and reduces the difficulty of system alignment and the difficulty of using detection instruments.

[0028] 2. The present invention adopts a structure that combines and optimizes an expanding beam system and a two-mirror Cassegrain telescopic system. The optical system has a compact structure. By shortening the length of the interference arm, the height and volume of the interferometer can be reduced, its stability can be increased, space can be saved, and it is convenient for operators to operate. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0030] Figure 1 Schematic diagram of the structure of the large scanning field and large image space numerical aperture objective lens according to an embodiment of the present invention;

[0031] Figure 2 Schematic diagram of the principle of the flying spot scanning interferometer according to an embodiment of the present invention;

[0032] Figure 3 Optical path diagram of the large image space numerical aperture objective lens according to Embodiment 1 of the present invention;

[0033] Figure 4 MTF curve graph of the large image space numerical aperture objective lens according to Embodiment 1 of the present invention;

[0034] Figure 5 Spot diagram of the large image space numerical aperture objective lens according to Embodiment 1 of the present invention;

[0035] Figure 6 Imaging optical path diagram of the large image space numerical aperture objective lens according to Embodiment 2 of the present invention;

[0036] Figure 7 MTF curve graph of the large image space numerical aperture objective lens according to Embodiment 2 of the present invention;

[0037] Figure 8 Spot diagram of the large image space numerical aperture objective lens according to Embodiment 2 of the present invention;

[0038] Figure 9 Comparison diagram of the present invention's embodiment allowing the installation of the mirror to be tilted.

[0039] Among them, the reference numerals are as follows: 1: First spherical lens; 2: Second spherical lens; 3: First spherical mirror; 4: Second spherical mirror; 5: Third spherical lens; 6: Fourth spherical lens. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be 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 between 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 specular reflection angle θ of the objective lens and the numerical aperture NA is:

[0044] θ = ±asin(NA) (2)

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

[0046] As Figure 1 shown, the present invention is a further optimization of the reference arm interference objective lens in Figure 2 and discloses a large scanning field and large image space numerical aperture objective lens for a flying spot scanner. The optical structure of the large scanning field and large image space numerical aperture objective lens, the entire optical system of the objective lens, from left to right, is the entrance pupil, the first spherical lens 1 with negative focal power, the second spherical lens 2 with positive focal power. The first spherical lens 1 and the second spherical lens 2 are approximately afocal beam expanding systems, which do not bear the focal power of the system and are only used for beam expansion. 3 is the first spherical mirror with positive focal power, which is also the primary mirror of the Cassegrain system. The second spherical mirror 4 is a spherical mirror with negative focal length, and this mirror coincides with the rear surface of the second spherical lens 2, that is, the second spherical mirror 4 is the center region of the rear surface of the lens second spherical lens 2 coated with a reflective film. The first spherical mirror 3 and the second spherical mirror 4 together form a Cassegrain system. The third spherical lens 5 and the fourth spherical lens 6 are doublet lenses. The third spherical lens 5 has positive focal power, and the fourth spherical lens 6 has negative focal power. The doublet lens plays the role of converging light, increasing the numerical aperture, and reducing the structural size.

[0047] Example 1

[0048] As Figure 3 shown, the numerical aperture NA in the image space = 0.0782, and the detailed design parameters are shown in Table 1; the resolution δ = 6.56um and the allowable maximum specular reflection angle θ = ±4.48° are calculated from equations (1) and (2); the MTF curve graph of the lens imaging quality is as Figure 4 shown. The MTF is close to the diffraction limit in the full field of view, having high resolution; the spot diagram of the lens is as 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 specular reflection angle θ. The larger the numerical aperture, the larger the maximum acceptable specular reflection angle. Therefore, when designing the interference objective lens, a certain tolerance is given to increase the numerical aperture so that more reflected light can enter the interference objective lens.

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

[0053] Tilt angle α of the mirror:

[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°, then the mirror can be tilted by ±1.84°. It shows that as long as the installation position of the mirror is within ±1.84°, reflection can be achieved, and the alignment difficulty is reduced.

[0056] When θ = ±1°, assuming that the actual incident light aperture angle 2β = 1.6°, then the mirror can be tilted by ±0.1°. The installation difficulty is large and the alignment time is long.

[0057] In order to generate interference, the reflection arm and the sample arm are exactly the same. By increasing the numerical aperture, the tolerance of the reflection angle of the reflection arm increases, and then the sample arm also increases accordingly. The allowable error of the sample stage installation of the sample arm increases, and the alignment difficulty is reduced. However, if the numerical aperture is increased significantly, the scanning field of view will decrease sharply. Therefore, the numerical aperture should be appropriately increased on the premise of keeping the scanning field of view unchanged. In addition, the tilt angle of the mirror 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 prism, collimator, and coupler.

[0058] Example 2

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

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

[0061]

[0062]

[0063] Compared with Example 1, the numerical aperture NA of the image space in Example 2 is larger, and the maximum allowable mirror reflection angle is also larger. If the components such as the beam splitter prism, collimator, and coupler in the flying spot detector do not limit the light, it is suitable to choose Example 2; otherwise, choose Example 1.

[0064] The specific implementation of the present invention is an optimized design of the existing German interference objective lens. A large-scanning field and large-numerical aperture interference objective lens for a flying spot scanner is designed. Among them, lens group 1 is a afocal beam expander system, whose function is to increase the light passing aperture of the small entrance pupil diameter, that is, to increase the light passing aperture of the Cassegrain double reflection system, thereby reducing the focal length of the whole system and increasing the numerical aperture of the image space of the whole system; lens group 2 is a coaxial double reflection Cassegrain system, whose function is to shorten the structural size of the system. Among them, the secondary mirror 4 of the double reflection Cassegrain system shares the same optical part with lens 2, which can reduce the number of lenses in the system and ensure a compact structure.

[0065] The optical system in the present invention has a reasonable distribution of optical power and can effectively suppress aberrations. By combining the beam expander system with the coaxial double reflection Cassegrain system, on the premise of maintaining the scanning field and the number of interference objective lens elements unchanged, the maximum allowable mirror reflection angle of the objective lens is increased from ±1° to ±4.48°, or even larger, such as ±4.85° in Example 2. This can greatly reduce the alignment difficulty of the mirror.

[0066] Compared with the German lens in the prior art, the large-scanning field and large-image-space numerical aperture interference lens disclosed in the present invention has high resolution, strong operability, low manufacturing cost, high cost performance, and better meets the actual use requirements under the premise of a large scanning field.

[0067] The embodiments described in the present invention are only descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design idea of the present invention, various deformations and improvements made by those skilled in the art to the technical solutions of the present invention should 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; 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 arranged in sequence from the object to the image along the direction of the light; The focal length f1 of the first spherical lens ranges from ; The focal length f2 of the second spherical lens ranges from ; The focal length f3 of the first spherical reflector ranges from ; The focal length f5 of the third spherical lens ranges from ; The focal length f6 of the fourth spherical lens has a range of ; Where f is the focal length of the combined lens.

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 returning 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: （1） in, and denote the incident beam diameter and the exit beam diameter, respectively. and 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 numerical aperture NA of the objective lens has a value range of 0.05≤NA≤0.0845.

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