Linnik type wide spectrum measurement automatic focusing system and method

Abstract

The invention discloses a Linnik type wide spectrum measurement automatic focusing system and method. The Linnik type wide spectrum measurement automatic focusing system comprises a sample objective lens, a reference objective lens, a beam splitter prism, a beam splitter, an astigmatic lens group, a first detector, a barrel lens and a second detector. Light with sample morphology information enters the beam splitter prism through the sample objective lens, light reflected by the reference mirror enters the beam splitter prism through the reference objective lens, the light with the sample morphology information and the light reflected by the reference mirror are combined in the beam splitter prism and then enter the beam splitter, and the beam splitter divides the combined light into a first light beam and a second light beam. The first light beam enters the astigmatic lens group and irradiates the first detector, and the second light beam irradiates the second detector through the barrel lens. According to the invention, the focal plane can be searched in a large range, and the problem of short detection distance due to the coherent gap of the existing wide-spectrum interference measurement system is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of wide - spectrum interferometric measurement, and particularly relates to a Linnik - type wide - spectrum measurement autofocus system and method. Background Art

[0002] Interference distance measurement has the characteristics of high recognition accuracy and good repeatability and stability. Especially, a wide - spectrum interferometric measurement system can realize the confirmation of absolute position information and is widely used in fields such as 3D topography measurement and position location.

[0003] In the prior art, according to the calculation formula of the coherence length of wide - spectrum interference, the wide - spectrum interferometric measurement system can generate coherent signals only within the range of 1 - 2 μm. Therefore, the wide - spectrum interferometric measurement system has the problem of too short coherence length and small detection distance, and cannot meet the requirement of searching for the focal plane in a large range.

[0004] Based on this, a new technical solution is needed. Summary of the Invention

[0005] In view of this, embodiments of the present invention provide a Linnik - type wide - spectrum measurement autofocus system and method to at least solve the problem that the existing wide - spectrum interferometric measurement system cannot meet the requirement of searching for the focal plane in a large range.

[0006] Embodiments of the present invention provide the following technical solutions:

[0007] Embodiments of the present invention provide a Linnik - type wide - spectrum measurement autofocus system, including a sample objective lens, a reference objective lens, a beam - splitting prism, a beam splitter, an astigmatic lens group, a first detector, a tube lens, and a second detector;

[0008] Light carrying sample topography information enters the beam - splitting prism through the sample objective lens, and light reflected by the reference mirror enters the beam - splitting prism through the reference objective lens. The light carrying sample topography information and the light reflected by the reference mirror are combined in the beam - splitting prism and then enter the beam splitter. The beam splitter divides the combined light into a first beam of light and a second beam of light. The first beam of light enters the astigmatic lens group and irradiates the first detector, and the second beam of light irradiates the second detector through the tube lens;

[0009] The Linnik - type wide - spectrum measurement autofocus system is configured to roughly focus on the sample by the change in the spot shape of the light irradiated on the first detector through the astigmatic lens group; and based on the focusing result of the rough focusing, perform fine focusing on the sample using Linnik interference.

[0010] Further, a shutter is provided between the reference objective lens and the beam - splitting prism;

[0011] Under coherent conditions, the shutter opens to obtain interference information; when the shutter is closed, the second detector can obtain a clearer imaging quality.

[0012] Furthermore, the sample objective lens and the reference objective lens are high numerical aperture objective lenses of the same model, and the numerical aperture ranges from 0.7 to 0.9.

[0013] Furthermore, the reference mirror has a surface accuracy of λ / 10 or more, and the reflectivity matches the reflectivity of the sample surface.

[0014] Furthermore, the beam splitting prism is of a beam splitting cube structure, and the beam splitter is at least one of a beam splitting prism, a beam splitting cube, and a beam splitting film.

[0015] Furthermore, the astigmatic lens group includes at least one group of lenses for causing a difference in the focusing positions of the beams in the meridional plane and the sagittal plane.

[0016] Furthermore, the first detector is a two-dimensional array detector.

[0017] Furthermore, the spectral band range of the light source of the Linnik type wide-spectrum measurement autofocus system is 400 - 700 nm.

[0018] The present invention also provides a Linnik type wide-spectrum measurement autofocus method, including:

[0019] Moving the sample to the detection area and positioning the objective lens above the detection area;

[0020] Performing preliminary positioning using the astigmatic focusing method;

[0021] Positioning the sample to the center of the focal plane field of view through focal plane imaging;

[0022] Enabling the interference fine focusing mode, providing wide-spectrum light source illumination, and using a piezoelectric device to move the distance between the sample and the sample objective lens to generate an interference signal;

[0023] According to the interference signal, obtaining the focal plane position corresponding to the peak of the interference signal and taking the focal plane position as the optimal focal plane.

[0024] Furthermore, performing preliminary positioning using the astigmatic focusing method includes:

[0025] In the astigmatic focusing method, if the light intensity ratio between the meridional plane and the sagittal plane is close to 1, it indicates that it is near the focal plane to achieve a rough focusing effect.

[0026] Compared with the prior art, the beneficial effects that can be achieved by at least one of the above technical solutions adopted in the embodiments of the present invention at least include:

[0027] A Linnik-type wide-spectrum measurement autofocus system of the present invention can achieve coarse focusing on a sample by using an astigmatic lens group and a first detector, lock the focus near the depth of focus, and then enable the interference fine focusing mode. The best focal plane is obtained through the peak value of the interference signal, so as to achieve a large-range search for the focal plane and solve the problem that the existing wide-spectrum interference measurement system has a small detection distance due to a large coherence gap. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 It is an implementation environment diagram of a Linnik-type wide-spectrum measurement autofocus system of the present invention;

[0030] Figure 2 It is a schematic structural diagram of the autofocus system in a Linnik-type wide-spectrum measurement autofocus system of the present invention;

[0031] Figure 3 It is a schematic structural diagram of the illumination system in a Linnik-type wide-spectrum measurement autofocus system of the present invention;

[0032] Figure 4 It is a schematic diagram of the principle of using astigmatism for coarse focusing of the present invention;

[0033] Figure 5 It is the principle of using interference signals for fine focusing of the present invention;

[0034] Figure 6 It is a flowchart of the automatic focusing of a Linnik-type wide-spectrum measurement autofocus system of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] The embodiments of the present application will be described in detail below with reference to the drawings.

[0036] The following specific examples illustrate the implementation manners of the present application. Those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all embodiments. The present application can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present application. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope protected by the present application.

[0037] It should be noted that the following describes various aspects of the embodiments within the scope of the appended claims. It should be obvious that the aspects described herein can be embodied in a wide variety of forms, and any specific structure and / or function described herein is illustrative only. Based on the present application, those skilled in the art should understand that one aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number and aspects described herein can be used to implement the device and / or practice the method. Additionally, this device can be implemented and this method can be practiced using other structures and / or functionality in addition to one or more of the aspects described herein.

[0038] It also needs to be noted that the drawings provided in the following embodiments only schematically illustrate the basic concept of the present application. The drawings only show the components related to the present application and are not drawn according to the number, shape, and size of the components in actual implementation. The type, quantity, and ratio of each component in its actual implementation can be arbitrarily changed, and the component layout type may also be more complex.

[0039] In addition, in the following description, specific details are provided to facilitate a thorough understanding of the examples. However, those skilled in the art will understand that the examples can be practiced without these specific details.

[0040] A wide-spectrum interferometric measurement system utilizes the characteristic of measuring and positioning distance to solve the problem of accurately positioning the focal plane of a microscopic imaging measurement system.

[0041] The calculation formula for the coherence length of the wide-spectrum interference is as follows: L = λ^2 / Δλ

[0042] Among them, L is the coherence length, λ is the central wavelength of the spectrum, and Δλ is the spectral bandwidth. It can be seen from the formula that taking the white light spectrum as an example, when its central wavelength λ = 600 nm and bandwidth Δλ = 300 nm are selected, its coherence length ranges between 1 - 2 μm, that is, coherent signals are generated only within the range of 1 - 2 μm. Therefore, the wide-spectrum interference measurement system has the problem of too short coherence length and small detection distance, and cannot meet the requirement of searching the focal plane in a large range.

[0043] Based on this, the embodiments of this specification propose a processing solution: the Linnik-type wide-spectrum measurement autofocus system of the present invention, by introducing the astigmatism method, through the evaluation of the image effect, quickly finds the approximate position of the focal plane to be focused, and then reaches the range where the interference signal information can be read. On this basis, the interference signal is used to find a more accurate and optimal focal plane. The search range of the interference signal is shortened, and the focusing speed is improved.

[0044] The following will describe the technical solutions provided by the embodiments of the present application with reference to the accompanying drawings.

[0045] Embodiment 1

[0046] As Figures 1 to 3 shown, the present invention provides a Linnik-type wide-spectrum measurement autofocus system, including a sample objective lens 32, a reference objective lens 33, a beam-splitting prism 35, a beam splitter 36, an astigmatic lens group 37, a first detector 38, a tube lens 39, and a second detector 40; the light carrying the morphological information of the sample 31 enters the beam-splitting prism 35 through the sample objective lens 32, and the light reflected by the reference mirror 34 enters the beam-splitting prism 35 through the reference objective lens 33. The light carrying the morphological information of the sample 31 and the light reflected by the reference mirror 34 are combined in the beam-splitting prism 35 and then enter the beam splitter 36. The beam splitter 36 divides the combined light into a first beam of light and a second beam of light. The first beam of light enters the astigmatic lens group 37 and irradiates the first detector 38, and the second beam of light irradiates the second detector 40 through the tube lens 39; the Linnik-type wide-spectrum measurement autofocus system is configured to roughly focus on the sample 31 through the change in the spot shape of the light irradiated on the first detector 38 by the astigmatic lens group 37; based on the focusing result of the rough focusing, the sample 31 is finely focused using Linnik interference.

[0047] As Figure 1 shown, the implementation environment of the present invention includes a camera 1, a light source 2, a piezoelectric device 3, an objective lens 4, a detector 5, a stage 6, etc. Among them, the light source 2 is used to provide Kohler illumination, the piezoelectric device 3 provides high-precision Z-axis displacement, the Linnik interference objective lens system forms an interference signal, the detector 5 reads and collects the interference signal, and the camera 2 measures the surface morphology of the sample 31; the stage 6 is used to carry the sample wafer.

[0048] As Figure 3As shown in the figure, it is a schematic diagram of the principle of the illumination and imaging optical path, which is based on the Kohler illumination system. The point light source forms a focused beam through the lens group, and then passes through the beam splitter prism and enters the sample objective lens 32 and the reference objective lens 33 respectively. After the two beams are reflected from the surfaces of the sample 31 and the reference mirror 34 respectively, the light with the morphological information of the sample 31 and the light reflected by the reference mirror 34 are formed, and the illumination light source has uniform illumination and good symmetry.

[0049] Among them, the point light source includes common light source forms such as a point light source composed of single-mode or multi-mode optical fibers, an LED light source, a halogen light source, a xenon light source, an LDLS light source, etc. The lens group is composed of one or more groups of lenses to ensure better light collection effect. The beam splitter prism is of a beam splitter cube structure, and its beam splitting ratio can be arbitrarily selected according to requirements. To ensure the interference signal intensity, between the sample objective lens 32 and the reference objective lens 33, it is preferably the same type of high-NA objective lens. The reference mirror 34 is preferably a mirror with a surface accuracy of more than λ / 10, and its reflectivity can be arbitrarily selected according to requirements, and it is kept equivalent to the reflectivity of the sample 31.

[0050] Further, a shutter is provided between the reference objective lens 33 and the beam splitter prism 35; under coherent conditions, the shutter is opened to obtain interference information; when the shutter is closed, the second detector 40 can obtain a clearer imaging quality.

[0051] Specifically, adding a shutter between the reference objective lens 33 and the beam splitter prism 35 in the interference avoidance can control whether the light beam passes through the interference arm end. Under coherent conditions, when the shutter is opened, interference information can be obtained; when the shutter is closed, the influence of the imaging optical path of the interference arm can be excluded, so that the second detector 40 can obtain a clearer imaging quality.

[0052] Further, between the sample objective lens 32 and the reference objective lens 33, it is preferably the same type of high-NA objective lens with a small wavefront deviation.

[0053] For example, between the sample objective lens 32 and the reference objective lens 33 is the same type of high numerical aperture objective lens, and the numerical aperture range is 0.7 - 0.9.

[0054] Further, the reference mirror 34 has a surface accuracy of more than λ / 10 and its reflectivity matches the surface reflectivity of the sample 31.

[0055] Further, the beam splitter prism 35 is of a beam splitter cube structure, and the beam splitter 36 is at least one of a beam splitter prism, a beam splitter cube, and a beam splitting film.

[0056] Further, the astigmatic lens group 37 is composed of at least one group or multiple groups of lenses, and is used to make the focusing positions of the light beams in the meridian plane and the sagittal plane different.

[0057] Further, the first detector 38 is a two-dimensional array detector, including but not limited to forms such as diode detectors, avalanche diode detectors, photomultiplier tube detectors, COMS, sCOMS, EMCCD, CCD, etc.

[0058] Further, the spectral band range of the light source of the Linnik-type wide-spectrum measurement autofocus system is 400 - 700 nm.

[0059] In some of these embodiments, the spectral band can be adjusted for the central wavelength and band using a filter according to requirements.

[0060] The present invention is based on a Linnik wide-spectrum interference focusing system. By introducing the astigmatism principle and using the astigmatic coarse focusing method, a fast automatic coarse focusing process is achieved. On the basis of ensuring high focusing accuracy and high stability, fast focus locking is realized. This system is applicable to a high-NA objective microscopic imaging system and is suitable for the semiconductor metrology field with high requirements for repeated focusing accuracy.

[0061] Embodiment 2

[0062] The present invention also provides a Linnik-type wide-spectrum measurement autofocus method, including:

[0063] Step S102: Move the sample 31 to the detection area and make the sample objective 32 located above the detection area;

[0064] Step S104: Use the astigmatic focusing method for preliminary positioning;

[0065] Step S106: Locate the sample 31 at the center of the focal plane field of view through focal plane imaging;

[0066] Step S108: Enable the interference fine focusing mode, provide wide-spectrum light source illumination, and use a piezoelectric device to move the distance between the sample 31 and the sample objective 32 to generate an interference signal;

[0067] Step S110: According to the interference signal, obtain the focal plane position corresponding to the peak of the interference signal and use the focal plane position as the optimal focal plane.

[0068] Specifically, as Figure 6As shown, the XY displacement stage provides XY-direction displacement for the area to be measured, positioning the sample objective lens 32 above the detection area. Then, using the method of astigmatic focusing, the Z-axis drives the entire optical path to achieve initial focal plane positioning. Through focal plane imaging, it can be confirmed whether the sample 31 to be measured is located at the center of the focal plane field of view. Moving the XY moving stage can achieve the operation of pulling the sample 31 to the center. Enable the interference fine focusing mode, provide broadband light source illumination, use piezoelectricity to move the distance between the sample 31 and the objective lens, thereby generating an interference signal. By calculating the peak position of the broadband interference, the focal plane is further locked, and image information acquisition and calculation are performed under this focal plane.

[0069] Further, the use of the astigmatic focusing method for preliminary positioning includes:

[0070] In the astigmatic focusing method, if the light intensity ratio between the meridional plane and the sagittal plane is close to 1, it indicates that it is near the focal plane to achieve the rough focusing effect.

[0071] Specifically, as Figure 4 shown, when using the astigmatic method to roughly focus on the sample 31, when moving the Z-axis to move the sample 31 up and down to search for the focal plane, at the near-focal position of the objective lens, the focusing beam angles of its sagittal plane and meridional plane are different, resulting in the imaging spot presenting an ellipse perpendicular to the meridional plane, to a short line perpendicular to the meridional plane, then to forming a circular spot, then to a short line perpendicular to the sagittal plane, to an ellipse perpendicular to the sagittal plane, alternating and changing.

[0072] The imaging effect at the first detector 38 end is a process from being clear in the meridional plane direction first, to being clearly common, and then to being clear in the sagittal plane direction. Therefore, the light intensities of the meridional plane and the sagittal plane can be monitored simultaneously through a two-dimensional array detector. When the light intensity of the meridional plane is greater than that of the sagittal plane, it indicates near-focal defocus; when the light intensity of the sagittal plane is greater than that of the meridional plane, it indicates off-axis defocus; when the light intensities of the meridional plane and the sagittal plane are approximately equal, it indicates near the focal plane, thereby achieving the rough focusing effect.

[0073] The present invention uses the astigmatic focusing method, and the accuracy can reach 1 / 4 of the depth of field range, meeting the requirements of preliminary focusing and reaching the position distance where the Linnik interference system can generate an interference signal. The astigmatic focusing method can judge the focusing effect through the strength of the light intensity, so it has a fast and efficient focusing effect.

[0074] After the result of rough focusing based on astigmatism, then use Linnik interference to perform fine focusing on the system. As Figure 5As shown, the X-axis represents the relative displacement between the objective lens and the sample 31, and Y represents the interference intensity signal. It can be seen from the figure that the range of the wave packet formed by the interference intensity fluctuation is the coherence length L. Point A in the figure is the strongest peak of the interference envelope formed by the superposition of broad spectra, and it appears only once in the objective lens focusing system. Therefore, based on the fixed relationship between point A and the relative displacement, the relative position between the objective lens and the sample 31 can be obtained, and then the focusing position can be obtained.

[0075] The Linnik interference system of the present invention is applicable to high-NA objective lens systems and is applicable to the field of semiconductor metrology. Through the combination of astigmatic coarse focusing and interference fine focusing, the focusing search range and applicability are improved, the focusing speed and focusing accuracy are guaranteed, and fast and accurate focusing is achieved. Compared with an independent focusing signal acquisition system, the detection efficiency is improved.

[0076] In this specification, the same or similar parts among the various embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments. In particular, for the product embodiments described later, since they correspond to the methods, the descriptions are relatively simple, and the relevant parts can be referred to the partial descriptions of the system embodiments.

[0077] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be covered by the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A Linnik type wide spectrum measurement autofocus system, characterized in that: It includes a sample objective lens, a reference objective lens, a beam splitter prism, a beam splitter, an astigmatism lens group, a first detector, a tube lens, and a second detector; The light with sample morphology information enters the beam splitter prism through the sample objective lens, and the light reflected by the reference lens enters the beam splitter prism through the reference objective lens. The light with sample morphology information and the light reflected by the reference lens are combined in the beam splitter prism and enter the beam splitter. The beam splitter divides the combined light into a first beam of light and a second beam of light. The first beam of light enters the astigmatic lens group and irradiates the first detector, and the second beam of light passes through the tube lens and irradiates the second detector. The Linnik type wide spectrum measurement automatic focusing system is configured to coarsely focus the sample by changing the shape of the light spot irradiated on the first detector by the astigmatic lens group; Based on the focusing result of the coarse focusing, the sample is finely focused using Linnik interferometry.

2. The Linnik type wide spectrum measurement autofocus system according to claim 1, characterized in that: A shutter is provided between the reference objective lens and the beam splitter prism; Under coherent conditions, the shutter is opened to obtain interference information; when the shutter is closed, the second detector can obtain clearer imaging quality.

3. The Linnik type wide spectrum measurement autofocus system according to claim 1, characterized in that: The sample objective lens and the reference objective lens are high numerical aperture objective lenses of the same model, and the numerical aperture range is 0.7-0.

9.

4. The Linnik wide spectrum measurement autofocus system according to claim 1, characterized in that: The reference mirror has a surface accuracy of λ / 10 or more, and its reflectivity matches that of the sample surface.

5. The Linnik type wide spectrum measurement autofocus system according to claim 1, characterized in that: The beam splitter is a beam splitter cube structure, and the beam splitter is at least one of a beam splitter prism, a beam splitter cube, and a beam splitter film.

6. The Linnik type wide spectrum measurement autofocus system according to claim 1, characterized in that: The astigmatic lens group includes at least one group of lenses, which is used to make the focusing position of the light beam on the meridian plane and the sagittal plane different.

7. The Linnik type wide spectrum measurement autofocus system according to claim 6, characterized in that: The first detector is a two-dimensional array detector.

8. The Linnik type wide spectrum measurement autofocus system according to claim 1, characterized in that: The light source spectrum band of the Linnik type wide spectrum measurement automatic focusing system ranges from 400 to 700 nm.

9. A Linnik-type wide spectrum measurement autofocus method, characterized in that: include: Moving the sample to a detection area and positioning the objective lens above the detection area; Use astigmatic focusing method for preliminary positioning; Positioning the sample at the center of the focal plane field of view by focal plane imaging; Enable interferometric fine focusing mode and provide broad spectrum light source illumination, using piezoelectric to move the distance between the sample and the sample objective lens to generate an interference signal; According to the interference signal, a focal plane position corresponding to a peak value of the interference signal is acquired, and the focal plane position is used as an optimal focal plane.

10. The Linnik type wide spectrum measurement automatic focusing method according to claim 9, characterized in that: Initial positioning using astigmatic focusing methods includes: In the astigmatic focusing method, if the light intensity ratio between the meridian plane and the sagittal plane is close to 1, it means that it is near the focal plane, so as to achieve a coarse focusing effect.

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