Inverted vertical interferometer liquid level absolute test method and system

Abstract

The invention discloses an inverted vertical interferometer liquid level absolute test method and system, and the method comprises the steps: placing an optical flat I above an inverted vertical interferometer, forming an interference cavity with a reference optical flat TF of the interferometer, and measuring cavity test data W1; the optical flat II is used for constructing a light-transmitting liquid disc, establishing a liquid level reference and placing the liquid level reference in the interference cavity, so that the working surface of the optical flat I is parallel to the liquid level to obtain a wave surface test result W2 of the optical flat I and the liquid level; and subtracting the W2 from the W1 to obtain a liquid level absolute test result of the reference optical flat of the interferometer. The system comprises an inverted vertical interferometer, an optical flat I and an optical flat II, the optical flat I is placed above the interferometer, and an interference cavity is formed by the optical flat I and a reference optical flat TF of the interferometer; and the optical flat II is used for constructing a light-passing liquid disc and establishing a liquid level reference and is placed in the interference cavity. The in-situ test device can realize in-situ test of the reference optical flat of the inverted vertical interferometer, is not limited by a test aperture, and has the advantages of simple structure, high measurement precision and wide application range.

Description

Technical Field

[0001] The invention relates to the technical field of optical interference measurement, in particular to an absolute liquid level inspection method and system of an inverted vertical interferometer. Background Art

[0002] The inverted vertical interferometer adopts a system configuration in which the main unit is at the bottom and the outgoing light beam is perpendicular to the horizontal plane and upward. For example, the VTS (Vertical Test Station) "upward-looking" structure vertical laser interferometer launched by Zygo has the advantages of stable structure and high clamping efficiency, and has been widely used.

[0003] Absolute liquid surface inspection is an absolute test method that uses the liquid surface as a plane reference. Compared with the absolute inspection method based on a flat crystal, it can avoid complex operations such as rotation and translation and the alignment errors caused by them, and can realize in-situ testing of flat crystals. In theory, it is suitable for testing surfaces of any diameter. Ideally, the radius of curvature of the liquid surface is equal to that of the earth. If the radius of the earth is R and the diameter of the liquid surface is y, then the vertical arrow generated by gravity on the liquid surface is h = y. 2 / (8R). When the liquid surface diameter y = 300mm, the sag of the liquid surface is only λ / 340. Therefore, under the premise of ensuring the stability of the liquid surface, the liquid surface can be regarded as a standard plane, which can achieve high-precision absolute surface shape testing.

[0004] Stable high-precision liquid surface is particularly suitable for in-situ absolute inspection of upright vertical interferometers. For example, reference 1 (Miao Erlong, Su Dongqi, Peng Shijun. High-precision plane absolute calibration [J]. Laser and Optoelectronics Progress, 2014, 51(05): 114-120) uses methyl silicone oil with a thickness of 2-3 mm to calibrate the power term of the reference plane in the 300 mm aperture vertical interferometer alone, and combines other Zernike terms to obtain a complete plane calibration result; reference 2 (Ma Zhiyao, Chen Lei, Zheng Donghui, et al. System error calibration of Φ300 mm vertical Fizeau interferometer based on liquid reference plane [J]. Infrared and Laser Engineering, 2022, 51(2): 440-449) completes the system error calibration of the 300 mm aperture vertical Fizeau interferometer based on a large-aperture liquid surface reference, and guides the deformation error correction of the reference plane according to the calibration results. However, the liquid surface is always facing upward. When it is applied to the inverted vertical interferometer test, the interference cavity configuration needs to be redesigned, and the liquid disk must have light transmission performance. Reference 3 (Sanjib C, Pavan YK, Rishipal S, et al. Measurement of the surface form error of large aperture plane optical surfaces with a polarization phase-shifting liquid reference reflection Fizeau interferometer. [J]. Applied optics, 2016, 55 (2): 310-6) placed the liquid reference in a container with a glass window at the bottom, and completed the surface shape test of the flat crystal to be tested above the liquid under the condition that the system output beam is vertically upward. However, this work is limited to measuring the plane to be tested above the liquid surface, while in the inverted vertical interferometer, the interferometer reference flat crystal below the liquid surface reference needs to be measured. In this regard, a complete liquid surface absolute inspection solution has not yet been formed. Summary of the invention

[0005] The object of the present invention is to provide a method and system for absolute liquid level inspection of an inverted vertical interferometer, so as to realize high-precision absolute surface shape testing of a reference flat crystal in an inverted vertical interferometer.

[0006] The technical solution to achieve the purpose of the present invention is: an inverted vertical interferometer liquid level absolute inspection method, comprising the following steps:

[0007] Step 1: Set up flat crystal I and place it above the inverted vertical interferometer to form an interferometric cavity with the reference flat crystal TF of the interferometer, and measure the cavity test data W. 1 ;

[0008] Step 2: Set up the flat crystal II to construct a transparent liquid plate and establish a liquid level reference;

[0009] Step 3: Place the constructed liquid surface reference in the interference cavity, and make fine adjustments after the liquid surface stabilizes to make the working surface of flat crystal I parallel to the liquid surface, and obtain the wave surface test result W of the working surface of flat crystal I and the liquid surface. 2 ;

[0010] Step 4: Subtract the test results in step 3 from those in step 1 to obtain the absolute test result of the liquid level of the inverted vertical interferometer reference flat crystal.

[0011] Furthermore, in step 1, a flat crystal I is set and placed above the inverted vertical interferometer to form an interferometric cavity with the reference flat crystal TF of the interferometer, and the cavity test data W is measured. 1 , as follows:

[0012] The working surface of the inverted vertical interferometer reference flat crystal TF is plane A, and the working surface of flat crystal I is plane C. Plane A and plane C are tested relative to obtain the cavity test data W 1 ;

[0013] W 1 (x,y)=A(x,y)+C(-x,y) (1)

[0014] In the formula, A(x, y) represents the surface shape of plane A, and C(-x, y) represents the surface shape of plane C.

[0015] Furthermore, in step 2, a flat crystal II is set to construct a transparent liquid plate and establish a liquid level reference, as follows:

[0016] Take a flat crystal II that is larger than the interferometer test aperture and use it as the bottom of the liquid container;

[0017] A closed ring is bonded on the side wall of the flat crystal II to form a liquid container wall, thereby constructing a liquid pan with light transmission function;

[0018] Pour dimethyl silicone oil with a viscosity greater than a set value into the constructed liquid pan to establish a stable liquid level reference.

[0019] Furthermore, in step 3, the constructed liquid surface reference is placed in the interference cavity, and after the liquid surface is stabilized, fine adjustment is performed to make the working surface of the flat crystal I parallel to the liquid surface, and the wave surface test result W of the working surface of the flat crystal I and the liquid surface is obtained. 2 , as follows:

[0020] Step 3.1, place the constructed liquid surface reference into the interference cavity, and wait for the liquid surface to stabilize;

[0021] Step 3.2, adjust the reflection point of plane A on the alignment plate out of the center of the field of view;

[0022] Step 3.3, fine-tune the center of gravity of the inverted vertical interferometer so that the outgoing light beam is parallel to the liquid surface, and at the same time fine-tune the position of flat crystal II so that there is a wedge angle between flat crystal II and the liquid surface to prevent the reflected light beam from the lower surface of flat crystal II from affecting the test results;

[0023] Step 3.4, the liquid surface is plane B, adjust the position of flat crystal I so that plane C is parallel to plane B, then the relative test result of plane B and plane C is W 2 ;

[0024] W 2 (x,y)=B(x,y)+C(-x,y) (2)

[0025] In the formula, B(x, y) represents the surface shape of plane B, and C(-x, y) represents the surface shape of plane C.

[0026] Furthermore, in step 4, the test results in step 3 and step 1 are subtracted to obtain the absolute test result of the liquid level of the inverted vertical interferometer reference flat crystal, which is as follows:

[0027] Subtracting equation (2) from equation (1), we can get the relative test result of plane B and plane A, which is the absolute shape of plane A to be measured:

[0028] W 2 (x,y)-W 1 (x,y)=B(x,y)-A(x,y)≈A(x,y) (3)

[0029] In the formula, B(x, y) represents the surface shape of plane B, and A(-x, y) represents the surface shape of plane A.

[0030] Furthermore, in step 2, 500 cst of dimethyl silicone oil is poured into the constructed liquid tray, and the liquid surface thickness is set to 2 mm to construct a liquid surface reference with bottom light passing therethrough.

[0031] An inverted vertical interferometer liquid level absolute inspection system, the system is used to implement the inverted vertical interferometer liquid level absolute inspection method, the inspection system comprises an inverted vertical interferometer, a flat crystal I and a flat crystal II, wherein:

[0032] The flat crystal I is placed above the inverted vertical interferometer, and forms an interferometric cavity with the reference flat crystal TF of the interferometer;

[0033] The flat crystal II is used to construct a transparent liquid disk and establish a liquid level reference. The constructed liquid level reference is placed in the interference cavity, and the working surface of the flat crystal I is parallel to the liquid surface.

[0034] Furthermore, there is a wedge angle between the flat crystal II and the liquid surface, which is used to prevent the reflected light beam from the lower surface of the flat crystal II from affecting the test result.

[0035] Compared with the prior art, the present invention has the following significant advantages:

[0036] (1) It can realize in-situ testing of the reference plane of the inverted vertical interferometer, and can realize absolute surface shape testing with the help of another flat crystal, avoiding complex operations such as rotation and translation of the flat crystal and the errors caused by them. It is easy to operate and has high measurement accuracy.

[0037] (2) An additional flat crystal is used as the bottom of the liquid container, and the system output light beam can directly pass through the liquid to reach the liquid surface, avoiding the complex light path deflection structure and simplifying the interference cavity structure;

[0038] (3) The sag of the liquid level reference of a specific caliber is a fixed value, so under the condition of ensuring the stability of the liquid level, the test results have good reproducibility;

[0039] (4) The liquid level reference is not limited by the aperture and can be applied to the absolute inspection of inverted vertical interferometers of any aperture. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 The present invention is a schematic diagram of the interference cavity structure in an inverted vertical interferometer liquid level absolute inspection method.

[0041] Figure 2 It is a schematic diagram of the testing steps in the present invention.

[0042] Figure 3 Component and device diagrams in an embodiment of the present invention, wherein (a) is a liquid level reference constructed using a flat crystal as the bottom of a liquid pan, (b) is a flat crystal to be tested, and (c) is a device diagram for a simulation experiment in a Φ150mm upright vertical interferometer.

[0043] Figure 4 are two measurement results in the embodiment of the present invention, where (a) is the cavity test result W 1 , (b) is the relative test result W between flat crystal I and liquid surface 2 .

[0044] Figure 5 Surface shape distribution of the surface to be measured in the embodiment of the present invention. DETAILED DESCRIPTION

[0045] Combination Figure 1 , Figure 2 The present invention provides an inverted vertical interferometer liquid level absolute inspection method, comprising the following steps:

[0046] Step 1: Set up flat crystal I and place it above the inverted vertical interferometer to form an interferometric cavity with the reference flat crystal TF of the interferometer, and measure the cavity test data W. 1 ;

[0047] Step 2, setting the flat crystal II to construct a light-transmitting liquid disk, and using a high-viscosity liquid surface to establish a stable liquid surface reference;

[0048] Step 3: Place the constructed liquid surface reference in the interference cavity, and make fine adjustments after the liquid surface stabilizes to make the working surface of flat crystal I parallel to the liquid surface, and obtain the wave surface test result W of the two. 2 ;

[0049] Step 4: Subtract the two test results to obtain the absolute test result of the liquid level of the inverted vertical interferometer reference flat crystal.

[0050] Furthermore, the setting flat crystal I described in step 1 is placed above the inverted vertical interferometer to form an interferometric cavity with the reference flat crystal TF of the interferometer, and the cavity test data W is measured. 1 , specifically:

[0051] The working surface of the inverted vertical interferometer reference flat crystal TF is plane A, and the working surface of flat crystal I is plane C. The two are tested relative to each other to obtain the cavity test data W 1 ;

[0052] W 1 (x,y)=A(x,y)+C(-x,y) (1)

[0053] In the formula, A(x, y) represents the surface shape of plane A, and C(-x, y) represents the surface shape of plane C.

[0054] Furthermore, the flat crystal II described in step 2 is used to construct a light-transmitting liquid disk, and a high-viscosity liquid surface is used to establish a stable liquid surface reference, specifically:

[0055] Take a flat crystal II that is slightly larger than the interferometer test aperture and use it as the bottom of the liquid container; bond a closed ring on the side wall of the flat crystal to form the liquid container wall, thereby constructing a liquid pan with light transmission function. Pour high-viscosity dimethyl silicone oil into the constructed liquid pan to establish a high-precision stable liquid level reference. To ensure the stability of the liquid level, 500cSt dimethyl silicone oil is selected, and the liquid level thickness is set to 2mm.

[0056] Furthermore, the constructed liquid surface reference is placed in the interference cavity as described in step 3, and after the liquid surface is stabilized, fine adjustment is performed to make the working surface of the flat crystal I parallel to the liquid surface, and the wave surface test result W of the two is obtained. 2 , specifically:

[0057] Step 3.1, place the constructed liquid surface reference into the interference cavity, and wait for the liquid surface to stabilize;

[0058] Step 3.2, adjust the reflection point of plane A on the alignment plate out of the center of the field of view;

[0059] Step 3.3, fine-tune the center of gravity of the inverted vertical interferometer so that the system output beam is parallel to the liquid surface, and at the same time fine-tune the position of flat crystal II so that there is a wedge angle between it and the liquid surface to prevent the reflected beam from the lower surface of flat crystal II from affecting the test results;

[0060] Step 3.4, adjust the position of flat crystal I so that its working surface is parallel to the liquid surface. The liquid surface is plane B, and the relative test result of plane B and plane C is W 2 ;

[0061] W 2 (x,y)=B(x,y)+C(-x,y) (2)

[0062] In the formula, B(x, y) represents the surface shape of plane B, and C(-x, y) represents the surface shape of plane C.

[0063] Furthermore, the difference between the two test results in step 4 is used to obtain the absolute test result of the liquid level of the inverted vertical interferometer reference flat crystal, which is specifically:

[0064] Subtract equation (2) from equation (1) to get the relative test result of plane B and plane A. Since plane B is a high-precision liquid surface, its sag is much smaller than the surface shape accuracy of the flat crystal to be measured. Therefore, under the condition of ensuring the stability of the liquid surface, plane B can be considered as an ideal standard plane. Therefore, the relative test result of plane B and plane A is the absolute surface shape of plane A to be measured.

[0065] W 2 (x,y)-W 1 (x,y)=B(x,y)-A(x,y)≈A(x,y) (3)

[0066] In the formula, B(x, y) represents the surface shape of plane B, and A(-x, y) represents the surface shape of plane A.

[0067] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0068] Example

[0069] This embodiment uses a Φ150mm upright vertical interferometer to simulate the test and solution process of the inverted vertical interferometer liquid level absolute inspection. In the simulation experiment, the flat crystal clamped in the interferometer is used as flat crystal I; Figure 3The flat crystal shown in (a) is used as the reference flat crystal TF of the inverted vertical interferometer, and its upper surface is the surface to be measured. The Φ150mm upright vertical interferometer laser operating wavelength is λ=632.8nm, and the laboratory temperature is 20℃.

[0070] Step 1: Set up flat crystal I and place it above the inverted vertical interferometer to form an interferometric cavity with the reference flat crystal TF of the interferometer, and measure the cavity test data W. 1 , as follows:

[0071] Clamp the flat crystal I in the interferometer, with its lower surface as C; and Figure 3 The flat crystal shown in (a) is placed on the test table of the interferometer, and its upper surface is A. The two form an interferometer cavity, and the cavity test result W is measured. 1 like Figure 4 As shown in (a), PV = 34.48nm, RMS = 6.40nm.

[0072] Step 2: Set up the flat crystal II to construct a transparent liquid plate and establish a high-precision stable liquid level reference, as follows:

[0073] Take flat crystal II as the bottom of the liquid container; glue a circle of closed rings on the side wall of the flat crystal to form the liquid container wall, thereby constructing a liquid pan with light-transmitting function. Pour high-viscosity dimethyl silicone oil into the constructed liquid pan to establish a high-precision stable liquid level reference. To ensure the stability of the liquid level, 500cSt dimethyl silicone oil is selected, and the liquid level thickness is set to 2mm. The constructed bottom light-transmitting liquid level reference is as follows Figure 3 As shown in (b) in .

[0074] Step 3: Place the constructed liquid surface reference in the interference cavity, and make fine adjustments after the liquid surface stabilizes to make the working surface of flat crystal I parallel to the liquid surface, and obtain the wave surface test result W of the two. 2 , as follows:

[0075] Step 3.1, place a two-dimensional frame on the test bench and place the constructed liquid level reference on the two-dimensional frame. Figure 3 As shown in (c), wait for the liquid surface to stabilize;

[0076] Step 3.2, adjust the reflection point A on the upper surface of the alignment plate out of the center of the field of view;

[0077] Step 3.3, fine-tune the center of gravity of the interferometer as a whole so that the system output beam is parallel to the liquid surface, and at the same time fine-tune the position of flat crystal II so that there is a wedge angle between it and the liquid surface to prevent the reflected beam from the lower surface of flat crystal II from affecting the test results;

[0078] Step 3.4, adjust the position of flat crystal I by the fine adjustment knob inside the interferometer so that its working surface is parallel to the liquid surface. The liquid surface is B, then the relative test result of plane B and plane C is W 2 like Figure 4 As shown in (b), PV = 33.79nm, RMS = 4.43nm;

[0079] Step 4: Subtract the two test results to obtain the absolute test result of the liquid level of the inverted vertical interferometer reference flat crystal, as follows:

[0080] The difference between the above two test results is obtained to obtain the relative test result of plane B and plane A. Since plane B is a high-precision liquid surface, its sag is much smaller than the surface shape accuracy of the flat crystal to be tested. Therefore, under the condition of ensuring the stability of the liquid surface, plane B can be considered as an ideal standard plane. Therefore, the relative test result of plane B and plane A is the absolute surface shape of plane A to be tested, such as Figure 5 As shown, PV = 37.38nm, RMS = 4.87nm.

[0081] In summary, the inverted vertical interferometer liquid level absolute inspection method of the present invention applies the liquid level absolute inspection technology to the absolute test of the reference plane of the inverted vertical interferometer, which can realize the in-situ test of the reference plane, and can realize the absolute surface shape test with the help of only another flat crystal, avoiding the complex operations such as rotation and translation of the flat crystal, and improving the test accuracy. An additional flat crystal is used as the bottom of the liquid container, avoiding the complex optical path deflection structure, and the interference cavity structure is simple. In addition, the present invention is suitable for the absolute inspection of inverted vertical interferometers of any caliber.

Claims

1. An inverted vertical interferometer liquid level absolute inspection method, characterized in that: The following steps are involved: Step 1, set the flat crystal I, place it on the inverted vertical interferometer, form an interferometric cavity with the reference flat crystal TF of the interferometer, and measure the cavity test data W1; Step 2, set up the flat crystal II to construct a transparent liquid plate and establish a liquid level reference; Step 3, placing the constructed liquid surface reference in the interference cavity, and performing fine adjustment after the liquid surface is stable, so that the working surface of the flat crystal I is parallel to the liquid surface, and obtaining the wave surface test result W2 of the working surface of the flat crystal I and the liquid surface; Step 4: Subtract the test results in step 3 from those in step 1 to obtain the absolute test result of the liquid level of the inverted vertical interferometer reference flat crystal.

2. The inverted vertical interferometer liquid level absolute inspection method according to claim 1, characterized in that: In step 1, flat crystal I is set and placed above the inverted vertical interferometer to form an interferometric cavity with the reference flat crystal TF of the interferometer, and the cavity test data W1 is measured, as follows: The working surface of the inverted vertical interferometer reference flat crystal TF is plane A, and the working surface of flat crystal I is plane C. Plane A and plane C are tested relative to obtain cavity test data W1; W1(x,y)=A(x,y)+C(-x,y) (1) In the formula, A(x, y) represents the surface shape of plane A, and C(-x, y) represents the surface shape of plane C.

3. The inverted vertical interferometer liquid level absolute inspection method according to claim 2, characterized in that: In step 2, flat crystal II is set to construct a transparent liquid plate and establish a liquid level reference, as follows: Take a flat crystal II that is larger than the interferometer test aperture and use it as the bottom of the liquid container; A closed ring is bonded on the side wall of the flat crystal II to form a liquid container wall, thereby constructing a liquid pan with light transmission function; Pour dimethyl silicone oil with a viscosity greater than a set value into the constructed liquid pan to establish a stable liquid level reference.

4. The inverted vertical interferometer liquid level absolute inspection method according to claim 3, characterized in that: In step 3, the constructed liquid surface reference is placed in the interference cavity, and after the liquid surface is stabilized, fine adjustment is performed to make the working surface of the flat crystal I parallel to the liquid surface, and the wave surface test result W2 of the working surface of the flat crystal I and the liquid surface is obtained, which is as follows: Step 3.1, place the constructed liquid surface reference into the interference cavity, and wait for the liquid surface to stabilize; Step 3.2, adjust the reflection point of plane A on the alignment plate out of the center of the field of view; Step 3.3, fine-tune the center of gravity of the inverted vertical interferometer so that the outgoing light beam is parallel to the liquid surface, and at the same time fine-tune the position of flat crystal II so that there is a wedge angle between flat crystal II and the liquid surface to prevent the reflected light beam from the lower surface of flat crystal II from affecting the test results; Step 3.4, the liquid surface is plane B, adjust the position of flat crystal I so that plane C is parallel to plane B, then the relative test result of plane B and plane C is W2; W2(x,y)=B(x,y)+C(-x,y) (2) In the formula, B(x, y) represents the shape of plane B, and C(-x, y) represents the shape of plane C.

5. The inverted vertical interferometer liquid level absolute inspection method according to claim 4, characterized in that: In step 4, the test results in step 3 and step 1 are subtracted to obtain the absolute test result of the liquid level of the inverted vertical interferometer reference flat crystal, which is as follows: Subtracting equation (2) from equation (1), we can get the relative test result of plane B and plane A, which is the absolute surface shape of plane A to be measured: W2(x,y)-W1(x,y)=B(x,y)-A(x,y)≈A(x,y) (3) In the formula, B(x, y) represents the surface shape of plane B, and A(-x, y) represents the surface shape of plane A.

6. The inverted vertical interferometer liquid level absolute inspection method according to claim 5, characterized in that: In step 2, 500 cst of dimethyl silicone oil is poured into the constructed liquid tray, and the liquid surface thickness is set to 2 mm to construct a liquid surface reference with bottom light passing therethrough.

7. An inverted vertical interferometer liquid level absolute inspection system, characterized in that: The system is used to implement the inverted vertical interferometer liquid level absolute inspection method according to any one of claims 1 to 6, and the inspection system comprises an inverted vertical interferometer, a flat crystal I and a flat crystal II, wherein: The flat crystal I is placed above the inverted vertical interferometer, and forms an interferometric cavity with the reference flat crystal TF of the interferometer; The flat crystal II is used to construct a transparent liquid disk and establish a liquid level reference. The constructed liquid level reference is placed in the interference cavity, and the working surface of the flat crystal I is parallel to the liquid surface.

8. The inverted vertical interferometer liquid level absolute inspection system according to claim 7, characterized in that: There is a wedge angle between flat crystal II and the liquid surface to prevent the reflected light beam from the lower surface of flat crystal II from affecting the test results.

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