CN1431477A - Point diffraction interferometer for detecting surface shape - Google Patents

Abstract

A point diffraction interferometer for detecting surface shape includes an optical part and a data acquisition, processing and control part. The optical part includes a laser. On the optical axis along the direction of laser beam advance, a converging lens and a solid immersion lens coated with a super-resolution mask are arranged in sequence. The device to be tested is placed above the optical axis, and an imaging lens is arranged below; the data acquisition, processing and control part includes a CCD camera, a computer and a displacement controller. The key to the present invention is that by adopting the technology of solid immersion lens and super-resolution mask, a smaller small hole is obtained as the light source of the ideal spherical wave of the point diffraction interferometer. The small hole is not only adjustable in position and size, but also has high light transmittance and low requirements on the quality of the incident light beam. Therefore, the present invention is a device for detecting the surface shape of a device with high measurement accuracy and easy assembly and adjustment.

Description

Point Diffraction Interferometer for Surface Shape Detection

Technical field:

The invention relates to optical surface detection and is a device for detecting the surface shape of a device with high precision. In particular, a point-diffraction interferometer employing a solid immersion lens and a super-resolution mask.

Background technique:

The traditional method of inspecting the quality of optical surfaces such as spheres is to use Fizeau interferometer or Tyman-Green interferometer. These methods require an actual reference surface, which means that the traditional method of evaluating the quality of the surface shape of a device under test is to compare it with an actual reference surface that is regarded as ideal. For large optical devices, such as the mirrors of astronomical telescopes, which can be more than 1 meter in diameter, it is almost impossible to manufacture such a large reference surface. Moreover, the measurement accuracy of traditional methods cannot exceed the accuracy of the reference surface. For lenses that require high precision, such as lenses used in lithography machines, the accuracy is required to reach a few hundredths of a wavelength. To manufacture such a high-precision ideal reference On the surface, it is also impossible.

The point diffraction interferometer provides a feasible solution (see Interferometric apparatus and methods for measuring surface topography of a test surface, Gemma, et al.United States Patent: 6,344,898). The spherical wavefront diffracted by the point source provides the reference surface used in the inspection process, thereby avoiding the use of the actual reference surface. The key to this method is to manufacture small enough holes so that the light passing through the holes can be regarded as an ideal spherical wave, and the small holes are required to have a high light transmittance in order to obtain a certain intensity of spherical waves. In the prior art, methods such as etching are used to make small holes, which are difficult to make ideal small holes, so the diffracted light waves are no longer ideal spherical waves. This will seriously affect the accuracy of the measurement results, and the irradiation spot is not small enough, and the light transmittance of the small hole is low, which also affects the detection accuracy. At the same time, it is very difficult to make the light spot incident on the small hole with wavelength order, which increases the difficulty of instrument assembly, poor repeatability and high detection cost.

Invention content:

The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned prior art and provide a point diffraction interferometer for detecting surface shape, which has the advantages of high measurement accuracy and easy assembly and adjustment.

The basic idea of the present invention to solve the technical problem is that a smaller aperture can be used as an ideal spherical wave light source for a point diffraction interferometer by using a solid immersion lens and a super-resolution mask.

Concrete technical solution of the present invention is as follows:

A point diffraction interferometer for detecting surface shape, comprising an optical part and a data acquisition, processing and control part, characterized in that:

The optical part includes a laser, along the optical axis of the laser beam advancing direction, a converging lens, a solid immersion lens coated with a super-resolution mask are arranged in sequence, the device to be tested is placed above the optical axis, and an imaging lens is arranged below ;

The data acquisition, processing and control part includes CCD camera, computer and displacement controller;

Its position relationship is as follows: the solid immersion lens is fixed on the displacement control device, the computer is connected and controls the work of the CCD camera and the displacement controller, and the laser beam emitted by the laser passes through the converging lens, and its focus falls on the side of the solid immersion lens. In the middle of the super-resolution mask, a small hole is formed and the device under test is irradiated through the small hole. The measurement beam reflected by the surface TS of the device under test is focused on the surface of the super-resolution mask, and after being reflected by the super-resolution mask, The measurement beam and the reference beam from the aperture enter the CCD camera through the imaging lens.

The solid immersion lens is made of high refractive index glass, and the super-resolution mask is an antimony film.

The displacement control device is a high-precision motor, which moves under instructions from a computer.

A compensator is also arranged between the small hole and the device under test.

The compensator can be a spherical transmissive optical device, a spherical reflective optical device, an aspheric transmissive optical device or an aspheric reflective optical device.

A beam splitter, a movable reflector and a fixed reflector are also arranged between the laser and the converging lens. The laser beam emitted by the laser is first divided into a measurement beam and a reference beam by the beam splitter, and the measurement beam is split into After being reflected by the fixed mirror, the reference beam is reflected by the beam splitter to the mirror after being reflected by the fixed mirror; while the reference beam reaches the movable mirror through the beam splitter, and then enters the Reflector. The mirror reflects the aforementioned incident reference and measurement beams to the converging lens.

The specific technical effects of the present invention are: the point diffraction interferometer of the present invention adopts the technology of combining a solid immersion lens and a super-resolution mask. The solid immersion lens has a good focusing effect on light beams, has a large numerical aperture and a small focus spot. When the diffraction-limited spot irradiates the super-resolution mask, due to the nonlinear effect of the mask, a temporary small hole (or scattering center) smaller than the incident spot will be generated at the irradiation spot, and the size of the small hole and the incident spot Intensity related. Combining a solid immersion lens with a super-resolution mask, that is, coating a layer of super-resolution mask on a solid immersion lens can produce a smaller pinhole. The solid immersion lens has a certain field of view, and can focus well on the beam of a certain angle range. The requirements for the incident beam will be relatively low, and the position of the generated spot can be adjusted by the incident beam, which makes the instrument assembly easier. The generated small hole also has the advantages of adjustable position, adjustable size and high light transmittance. Therefore, the disadvantages of the prior art can be overcome, and a device for detecting the surface shape of the device with high measurement accuracy and easy assembly and adjustment is provided.

Description of drawings:

Fig. 1 is a schematic diagram of a point diffraction interferometer for detecting surface shape in the present invention.

Fig. 2 is a schematic diagram of a solid immersion lens coated with a super-resolution mask.

Fig. 3 is a schematic diagram of a point diffraction interferometer for detecting the shape of an aspheric surface.

Figure 4 is a schematic diagram of a point diffraction interferometer for detecting surface shape with adjustable optical path difference

Detailed ways:

The present invention will be further described below in conjunction with the accompanying drawings.

Please refer to Fig. 1 first, it can be seen from the figure that the point diffraction interferometer for detecting surface shape of the present invention includes an optical part and a data acquisition, processing and control part. The optical part includes a laser 1, along the forward direction of the emitted light beam of the laser 1, on the optical axis oo, there are in turn a converging lens 2, a solid immersion lens 3 coated with a super-resolution mask, and above the optical axis oo, there is a device to be tested 4, Imaging lens 6 is arranged below;

The data acquisition, processing and control part includes a CCD camera 5 , a computer 7 and a displacement control device 8 .

The structure of the point diffraction interferometer of the present invention is shown in FIG. 1 . The optical part includes a laser 1, along the optical axis oo along the direction in which the beam emitted by the laser 1 advances, there is a converging lens 2, and a solid immersion lens 3 coated with a super-resolution mask. The orientation of the converging lens 2 should make its focal point Dropped in the middle of the super-resolution mask 3b on the side of the solid immersion lens 3a, so as to form the small hole 3c. In the forward direction of the spherical wave emitted through the small hole 3c, above the optical axis oo, there is a device to be tested 4, and the orientation of the device to be tested 4 should be such that the measuring beam reflected by its surface TS is focused on the super-resolution mask 3b Reflected by the surface of the super-resolution mask 3b, the measurement beam and the reference light emitted from the small hole 3c enter the CCD camera 5 through the imaging lens 6 below the optical axis oo. The orientation of the image of the imaging lens 6 is such that the object plane falls on the surface TS of the device under test 4 .

The data acquisition, processing and control part includes a CCD camera 5 , a computer 7 and a displacement control device 8 . The computer 7 is connected to the CCD camera 5 and the displacement control device 8 for controlling the position of the solid immersion lens 3 , and the solid immersion lens 3 is fixed on the displacement control device 8 .

Said laser 1 can be a single-wavelength or multi-wavelength laser. When the laser 1 is a multi-wavelength laser, a filter needs to be added to output one wavelength each time for a detection. If necessary, another wavelength is used for detection, and the obtained two interference fringes are compared to obtain the device under test. Surface shape for more information.

The said converging lens 2 has a large numerical aperture.

The solid immersion lens 3 coated with a super-resolution mask (see FIG. 2 ) is composed of a solid immersion lens 3a and a super-resolution mask 3b. Solid wetted lenses are made of high refractive index glass. The super-resolution mask 3b adopts an antimony mask, and realizes the pinhole 3c by utilizing the sudden change of the laser transmittance caused by the transition between the crystalline state and the amorphous state of antimony. Its surface is coated with a reflective protective film. The super-resolution mask 3b may also be other non-linear film layers that further reduce the spot size. When the light beam is focused into the super-resolution mask 3b through the solid immersion lens 3a, a "small hole" 3c will be formed that is smaller than the spot produced by no film layer.

The lens 6 has the ability to correct distortion and aberration as large as possible.

The displacement control device 8 may be a high-precision motor.

The present invention has the above-mentioned structure, the beam emitted from the laser 1 converges to the solid wetting lens 3a through the lens 2, and then reaches the super-resolution mask 3b behind it through the solid wetting lens 3a. The energy of the beam is Gaussian distributed, and the temperature of the super-resolution mask 3b in the central area of the spot is higher than its melting point to make it change from a crystalline state to a molten state, because the super-resolution mask 3b in the molten state has a higher transmittance to incident light than its crystal state. The transmittance of the state, so that a small hole 3c smaller than the diameter of the original spot will be formed at the incident light spot. The light wave diffracted by the small hole 3c is regarded as an ideal diverging spherical wave SW. Part of the wavefront of the ideal spherical wave SW is directly incident on the surface TS of the device under test 4 as a measuring beam. The measuring beam is reflected by the surface TS of the device under test 4 and focused onto the surface of the super-resolution mask 3b. The device under test 4 plays a role of focusing the measuring beam. Therefore, the focus of the surface TS of the device under test 4 should be on the surface of the super-resolution mask 3b. The measuring light beam is reflected by the surface of the super-resolution mask 3b, and reaches the CCD camera 5 through the imaging lens 6.

Part of the wavefront of the ideal spherical wave SW is used as a reference beam, and reaches the CCD camera 5 through the imaging lens 6 , and the measurement beam forms interference fringes on the receiving surface of the CCD camera 5 . The output of the CCD camera 5 is sent to the computer 7 for analysis. Where the radius of curvature of the surface TS of the device under test 4 is consistent with that of the ideal spherical wavefront SW, the interference fringes are sparse, and the greater the difference in the radius of curvature, the denser the fringes. Therefore, the surface shape TS of the device under test 4 can be obtained from these interference fringe patterns.

Three specific examples are provided below.

Example 1

The structure schematic diagram of the device is shown in Fig. 1, the light source 1 adopts a semiconductor laser, and the wavelength λ=650nm. The converging lens 2 has a numerical aperture of 0.9 and a working distance of 3mm. The solid immersion lens 3 a has a radius of 0.764 mm, is made of glass with a refractive index of 1.8198 at 514.5 nm, and has a numerical aperture of 1.5. By adjusting the displacement control device 8, the focus spot falls on the super-resolution mask 3b. The device under test is placed on the piezoelectric ceramic base, and the distance between the device under test 4 and the solid wetting lens 3 is adjusted until clear stripes are obtained on the CCD camera 5 . Collect the data to the computer 7 to complete a measurement.

Example 2

A schematic diagram of the device structure is shown in FIG. 3 , and devices with the same numbers as those in Embodiment 1 will not be described in detail here. The compensator (NULL element) 9 is placed between the aperture 3c and the device under test 4 to generate a wavefront close to the TS on the surface TS of the device under test 4, to solve the problem that the surface TS of the device under test 4 is very different from the ideal spherical surface When the case is large, that is, the case of an aspheric surface, the entire measurement can be completed once. Its accuracy depends on the quality of the compensator 9 .

The compensator 9 may be a spherical transmissive optical device, a spherical reflective optical device, an aspheric transmissive optical device or an aspheric reflective optical device.

Example 3

The structure of the device is shown in Figure 4, and the devices with the same numbers as those in Embodiment 1 will not be described in detail here. Light emitted from the light source 1 is split by a beam splitter 12 into a measurement beam and a reference beam. The reference beam passes through a beam splitter 12 to a movable mirror 10 . The optical path difference between the reference beam and the measurement beam can be adjusted by moving the mirror 10 . The reference beam passes through the beam splitter 12 again, is reflected by the mirror 13 to the converging lens 2 , and then is focused to the solid immersion lens 3 by the converging lens 2 . At the same time, the measuring beam is reflected by the beam splitter 12 and encounters the fixed mirror 11 . The measuring beam is reflected back to the beam splitter 12 . Then reflected to the mirror 13, the mirror 13 reflects the measuring beam to the converging lens 2, and is focused by the converging lens 2 to the solid immersion lens 3, together with the reference beam to form a small hole 3c in the super-resolution mask 3b. The measuring beam is irradiated onto the surface TS of the device under test 4 through the small hole 3c. The measuring beam is reflected by the surface TS of the device under test 4 and then focused onto the surface of the super-resolution mask 3b. After being reflected by the surface of the super-resolution mask 3 b, it passes through the imaging lens 6 and reaches the CCD camera 5 . The reference beam directly reaches the imaging lens 6 through the small hole 3c, and then reaches the CCD camera 5 through the imaging lens 6. The reference beam and the measurement beam form interference fringes on the receiving surface of the CCD camera 5 .

The advantage of this embodiment is that the optical path difference between the two light beams can be adjusted by moving the reflector 11, thereby adjusting the contrast of the interference fringes and improving the measurement accuracy.

This implementation manner can also use the method in Embodiment 2, that is, FIG. 3 , that is, use the compensator 9 to solve the detection problem of the aspheric surface.

Claims (6)

1. a point-diffraction interferometer that detects surface configuration comprises opticator and data acquisition, processing and control section, it is characterized in that:

Described opticator comprises laser instrument (1), along on the optical axis of laser beam working direction, be provided with convergent lens (2) successively, the solid that is coated with mask layer (3b) soaks into lens (3), place device under test (4) above optical axis, the below is provided with imaging len (6);

Described data acquisition, processing and control section comprise ccd video camera (5), computing machine (7) and displacement controller (8);

The position relation of above-mentioned each components and parts is as follows: this solid soaks into lens (3) and is fixed on this displacement control device (8), computing machine (7) connects and controls the work of ccd video camera (5) and displacement controller (8), the laser beam that laser instrument (1) sends is behind convergent lens (2), its focus drops in the middle of the mask layer (3b) of solid infiltration lens (3) one sides, form aperture and shine device under test (4) through this aperture (3c), the measuring beam of the surperficial TS reflection of this device under test (4) focuses on the surface of this mask layer (3b), after the reflection of this mask layer (3b), this measuring beam and enter ccd video camera (5) through imaging len (6) together from the reference beam that aperture (3c) sends.

2. point-diffraction interferometer according to claim 1 is characterized in that described solid soaks into lens (3a) and made by glass of high refractive index, and described mask layer (3b) is the antimony film.

3. point-diffraction interferometer according to claim 1 is characterized in that described displacement control device (8) is a high-precision motor, accepts the instruction of computing machine (7) and moves.

4. point-diffraction interferometer according to claim 1 is characterized in that also being provided with compensator (9) between described aperture (3c) and the device under test (4).

5. point-diffraction interferometer according to claim 4 is characterized in that described compensator (9) can be Homology of Sphere optical device, spheric reflection optical device, aspheric transmitting optical device or aspheric surface reflective optical device.

6. point-diffraction interferometer according to claim 1, it is characterized in that also being provided with between described laser instrument (1) and the convergent lens (2) beam splitter (12), removable catoptron (10) and stationary mirror (11), the laser beam that laser instrument (1) is launched at first is divided into measuring beam and reference beam by beam splitter (12), and this measuring beam is reflexed to catoptron (13) by beam splitter (12) after being run into the reflection of stationary mirror (11) after beam splitter (12) reflection again; And reference beam arrives removable catoptron (10) by beam splitter (12), by inciding catoptron (13) by beam splitter (12) once more after the reflection of removable catoptron (10), this catoptron (13) reflexes to convergent lens (2) with the reference beam and the measuring beam of above-mentioned incident.

Images