CN118031793A - Self-calibration high-precision scanning white light interference system - Google Patents

Abstract

The self-calibration high-precision scanning white light interference system comprises a white light incidence unit, a laser incidence unit, a sample scanning unit, an imaging unit and a calibration unit. The white light incident beam generated by the white light incident unit and the laser incident beam generated by the laser incident unit are divided into a sample incident beam and a reference incident beam after being jointly used as a detection incident beam in the sample scanning unit. The sample incident light beam irradiates on the sample to reflect to form a sample reflected light beam, and the reference incident light beam irradiates on a reference reflector to reflect to form a reference reflected light beam; the sample reflected beam and the reference reflected beam are combined into a scanning beam in the sample scanning unit, and then separated into an imaging beam and a calibration beam. The imaging light beam enters the imaging unit to form an instant image of the sample at the current irradiation point, and the calibration light beam enters the calibration unit to calibrate the scanning step length of the white light. The self-calibration high-precision scanning white light interference system can precisely calibrate the scanning step length of the scanning white light.

Description

Self-calibration high-precision scanning white light interference system

Technical Field

The present invention relates to the field of interferometry, and in particular to self-calibrating interferometry systems.

Background

With the development of precision manufacturing industry, three-dimensional topography measurement is required for some objects such as MEMS devices, semiconductor chips and the like with surface height jump from hundreds of nanometers to hundreds of micrometers. White light scanning interferometry is widely applied to three-dimensional morphology measurement of microscopic and non-microscopic objects and object surface roughness measurement as a non-contact, non-destructive, high-resolution and high-precision three-dimensional morphology measurement method.

Please refer to fig. 1, which is a schematic diagram of an optical path of a conventional white light scanning interference system. The white light beam emitted from the white light source 11 is split into a sample incident light and a reference incident light by the beam splitter 32, the sample incident light is irradiated onto the sample by the sample objective lens 33, reflected by the sample surface to a sample reflected light, the reference incident light is irradiated onto the reference mirror 36 by the reference objective lens 35 to be reflected to a reference reflected light, the sample reflected light and the reference reflected light are combined into a beam Cheng Saomiao light by the beam splitter 32, and the scanned light is interference imaged on the imaging image sensor 41. When the scanning white light interferometer is used for measuring the surface morphology of an object, the object needs to be scanned along the optical axis direction, a sample is arranged on the sample stage 34, the sample stage 34 is controlled to drive the sample to move along the optical axis vertical direction, during the scanning process, the surface of the object gradually passes through the focal plane of the sample objective lens 33, a series of two-dimensional images corresponding to different scanning sites are recorded through the imaging image sensor 41 during the whole scanning process, three-dimensional interference image data are sequentially formed by stacking according to the optical path variation of the white light scanning at different sites of the sample, and then the surface morphology reconstruction is carried out pixel by pixel. The optical path variable of the white light scanning is defined as a scanning step length.

The experimental scanning white light interferometry system generally adopts a Linnik interferometer architecture, and the Linnik white light interferometry system has the advantage of long working distance, and is particularly reflected in the situation that a large numerical aperture objective lens is used for realizing high-resolution imaging. In the case that the lateral dimension of the surface features is greater than the lateral resolution of the system, the longitudinal measurement accuracy of the scanning white light interferometer depends on the analysis accuracy of interference fringes and the calibration accuracy of white light optical path change in the scanning process, and is not limited by the depth of field of the imaging system. The conventional method for calibrating the scanning step length of the scanning white light is to set a piezoelectric ceramic on a sample stage, take the displacement of the piezoelectric ceramic on the sample stage 34 as the optical path of white light scanning, measure the displacement of the piezoelectric ceramic multiple times in advance through interference phase shift and take the average value to determine a scanning step length standard curve.

However, in Linnik white light interferometry systems, how to accurately calibrate the scanning step, i.e., the optical path change, of the scanning white light remains a major challenge. Researchers at the national university of korea proposed to control the accurate scanning step length using analog feedback signals to solve the problem of high residual vibration of piezoelectric ceramics in rapid scanning, however, the system only considers the vibration influence of a scanning objective lens, and the vibration influence caused by other factors cannot be eliminated; tereschenko of the university of Carsel proposes to synchronously calibrate the actual scanning step length by using an infrared laser point detection system, but the precision of detecting sinusoidal interference signals by using a single-point detector is not high, and a calibration light path and an actual light path have more non-common path parts; the university of south China Zhou Yunfei et al propose to build an additional Michelson interference system on the other side of the piezoelectric ceramic, so that synchronous displacement calibration can be performed through the Michelson interference system when the piezoelectric ceramic performs white light scanning. The method has the limitation that the white light interference system and the calibration system are not in common path, and the disturbance received by the whole system is inconsistent with the disturbance received by the system during actual scanning white light measurement; on the other hand, vibration errors caused by factors other than PZT are not considered, and the repetition accuracy of the piezoelectric ceramic sample stage is difficult to ensure, so that certain errors exist between the calibrated scanning step length and the actual scanning step length.

Disclosure of Invention

Based on the above, the invention aims to provide a self-calibration high-precision scanning white light interference system which has the advantages of capability of precisely calibrating the scanning step length of the scanning white light and strong anti-interference capability.

The self-calibration high-precision scanning white light interference system of the invention comprises: the device comprises a white light incidence unit, a laser incidence unit, a sample scanning unit, an imaging unit and a calibration unit; the method comprises the steps that after a common-path combined beam of a white light incident beam generated by the white light incident unit and a laser incident beam generated by the laser incident unit is used as a detection incident beam in a sample scanning unit, the detection incident beam is divided into a sample incident beam and a reference incident beam; the sample incident light beam irradiates on the sample and is reflected to form a sample reflected light beam, and the reference incident light beam irradiates on a reference reflector and is reflected to form a reference reflected light beam; the sample reflected beam and the reference reflected beam are combined together again in the sample scanning unit to form a scanning beam, the scanning beam is divided into an imaging beam and a calibration beam again, the imaging beam enters the imaging unit to form an instant image of the sample at the current irradiation point, and the calibration beam enters the calibration unit to mark the scanning step length of white light.

Compared with the prior art, the self-calibration high-precision scanning white light interference system realizes the common-path structure of the white light scanning light path and the laser calibration light path, ensures the consistency of interference of laser and white light and the broad spectrum property of the white light, and can calibrate the actual scanning step length of the white light with high precision.

Further, the calibration unit comprises a Volaton prism, a focusing lens, a calibration polarizing plate and a calibration image sensor, the Volaton prism separates the calibration light beam into two light waves with the polarization directions being mutually perpendicular and a certain separation angle, the two light waves transmitted along different angles are converged on the calibration image sensor through the focusing lens, the two light waves after passing through the focusing lens are changed into the same polarization direction through the calibration polarizing plate, and finally interference imaging is carried out on the calibration image sensor to calibrate the scanning step length of white light.

Further, the white light incident unit comprises a white light source; the laser incidence unit comprises a laser light source; the sample scanning unit comprises a spectroscope and a beam splitter; the imaging unit includes an imaging image sensor; the method comprises the steps that after a white light incident beam generated by the white light source and a laser incident beam generated by the laser source are combined in a common way to be a detection incident beam, the detection incident beam is divided into a sample incident beam and a reference incident beam in the spectroscope; the sample reflected beam and the reference reflected beam are combined into a scanning beam in the spectroscope again in a common way, the scanning beam is divided into an imaging beam and a calibration beam again after passing through the beam splitter, and the imaging beam enters the imaging image sensor to form an instant image of the sample at a current irradiation point.

Further, the sample scanning unit further comprises a beam combining lens, a sample objective lens and a reference objective lens, wherein the beam combining lens is used for combining the white light incident beam and the laser incident beam together to form a detection incident beam, the sample objective lens is arranged between the spectroscope and the sample, and the reference objective lens is arranged between the spectroscope and the reference mirror.

Further, the sample scanning unit further comprises an incident tube lens and an emergent tube lens, wherein the incident tube lens is arranged between the beam combining lens and the spectroscope, the emergent tube lens is arranged between the beam splitting lens and the spectroscope, and the incident tube lens can focus and detect incident light beams, so that the influence of divergence after combining and sharing of white light incident light beams and laser incident light beams is reduced; the lens of the emergent tube is arranged to focus the scanning beam, so that the influence of divergence after the beam combination of the sample reflected beam and the reference reflected beam is reduced.

Further, the white light incidence unit further comprises a collimation beam expansion module and a small-hole filter, collimation of the white light incidence beam can be enhanced through the collimation beam expansion module, a plane white light beam with uniform light intensity distribution is formed, clutter such as filament artifacts in the white light incidence beam can be filtered through the small-hole filter, and uniformity of the plane white light beam is improved.

Further, a long-pass dichroic mirror is arranged between the imaging image sensor and the beam splitter, and the long-pass dichroic mirror can filter out laser parts with shorter wavelengths and pass through white light parts with longer wavelengths, so that interference imaging influence of the laser parts on the white light parts is prevented.

Further, an attenuation sheet is arranged between the laser light source and the beam combining lens, and the light intensity of the laser incident beam can be controlled through the attenuation sheet, so that other instruments in the light path are prevented from being damaged due to the fact that the light intensity is too high.

Further, the device also comprises an imaging processor, wherein the imaging processor receives an imaging image of the sample generated by the imaging image sensor at a scanning position and a laser calibration carrier frequency interference image generated by the calibration image sensor, and calibrates a white light scanning step length by using time sequence phase shift information in the laser calibration carrier frequency interference image to generate a sample surface appearance image.

Further, the sample scanning unit further comprises a micro-moving sample stage, and the micro-moving sample stage controls the sample to move within a certain required range according to a fixed direction perpendicular to the optical axis, so that the incident light beam of the sample can irradiate the whole surface of the sample within the required range, and surface information within the required range is obtained.

For a better understanding and implementation, the present invention is described in detail below with reference to the drawings.

Drawings

FIG. 1 is a schematic diagram of a conventional white light scanning interferometry system.

Fig. 2 is a schematic diagram of the optical path structure of the self-calibration high-precision scanning white light interference system of the present invention.

Fig. 3 is a white light wavelength diagram.

Fig. 4 is a laser wavelength diagram.

Fig. 5 is a wavelength diagram of white light passing through a long-pass dichroic mirror.

Fig. 6 is a wavelength chart of laser light passing through a long-pass dichroic mirror.

FIG. 7 is a schematic diagram of the light path of the calibration unit of FIG. 2.

FIG. 8 is a calibration carrier frequency fringe interference pattern acquired by the calibration image sensor of the present invention.

Detailed Description

The invention provides a self-calibration high-precision scanning white light interference system, which utilizes the polarization splitting characteristic of a Wolaston prism to construct a group of time sequence phase-shifted laser carrier frequency interference fringes by coupling a white light source for scanning with a laser light source for calibration, calculates the phase shift quantity of the laser carrier frequency interference fringes, and can be used for high-precision calibration of white light scanning step length and ensure the measurement precision of scanning white light interference because the phase shift quantity of the laser carrier frequency interference fringes and the white light scanning step length form a linear relation.

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

Referring to fig. 2, the self-calibration high-precision scanning white light interference system of the present invention includes a white light incidence unit 10, a laser incidence unit 20, a sample scanning unit 30, an imaging unit 40 and a calibration unit 50. The white light incident beam generated by the white light incident unit 10 and the laser incident beam generated by the laser incident unit 20 are combined together in the sample scanning unit 30 to form a detection incident beam, and then the detection incident beam is divided into a sample incident beam and a reference incident beam, wherein the sample incident beam irradiates the sample and is reflected to form a sample reflected beam, and the reference incident beam irradiates a reference mirror and is reflected to form a reference reflected beam. The reflected sample beam and the reflected reference beam are again co-combined into a scanning beam in the sample scanning unit, the scanning beam is then divided into an imaging beam and a calibration beam again, the imaging beam enters the imaging unit 40 to form an instant image of the sample at the lower irradiation point, the calibration beam enters the calibration unit 50 to calibrate the scanning step length of the white light, and the displacement of the sample during scanning can be accurately achieved.

Specifically, the white light incident unit 10 includes a white light source 11 and a collimator beam expanding module. The collimation and beam expansion module comprises a first lens 12 and a second lens 13, wherein the first lens 12 and the second lens 13 are two convex lenses with middle focuses at the same position. The white light incident beam from the white light source 11 passes through the first lens 12 and the second lens 13 to form a planar white light beam with uniformly distributed light intensity. In some embodiments, the white light incident unit 10 further includes a pinhole filter 14 disposed between the first lens 12 and the second lens 13, and the pinhole filter 14 can filter out clutter such as filament artifacts in the white light incident beam, so as to improve the uniformity of the planar white light beam.

The laser incidence unit 20 includes a laser source 21 for generating a laser incidence beam; in some embodiments, the laser light incident unit 20 further includes an attenuation sheet 22, and the light intensity of the laser light incident beam is controlled by the attenuation sheet 22.

The sample scanning unit 30 includes a beam combining mirror 31, a beam splitter 32, a sample objective 33, a sample stage 34, a reference objective 35, a reference mirror 36, and a beam splitter 37.

The white light incident beam generated by the white light incident unit 10 and the laser incident beam generated by the laser incident unit 20 are combined together in the beam combining mirror 31 to form a detection incident beam. In fact, if the white light incident beam generated by the white light incident unit 10 and the laser light incident beam generated by the laser light incident unit 20 are directly in the same optical path, the beam combining mirror may not be provided; however, if the white light incident beam generated by the white light incident unit 10 and the laser light incident beam generated by the laser light incident unit 20 are not in the same optical path, the two beams need to be combined together by the beam combiner 31. Preferably, the incident beam of white light and the incident beam of laser light are first made parallel, then a mirror 16 is provided in one of the beams so that one of the beams is perpendicular to the other beam, and the beams are combined in the beam combining mirror 31 to form the incident beam for detection. Specifically, in the arrangement of the present embodiment, the reflecting mirror 16 is disposed on the optical path of the incident white light beam at an angle of 45 ° so that the reflected incident white light beam perpendicularly intersects with the incident laser light beam; the white light incident beam is reflected in the beam combining lens, and the laser incident beam is transmitted in the beam combining lens, so that a combined detection incident beam is formed. In other embodiments, the reflection and transmission relationship between the white light incident beam and the laser incident beam in the beam combiner may be exchanged.

Both sides of the beam splitter 32 are semi-transmissive and semi-reflective structures. The detection incident beam is partially transmitted through the beam splitter 32 as a sample incident beam and partially reflected as a reference incident beam. The sample incident beam is irradiated onto a sample surface provided on a sample stage 34 through the sample objective lens 33, and reflected to form a sample reflected beam. The reference incident beam is irradiated on the reference mirror 36 through the reference objective 35, and reflected to form a reference reflected beam. The sample reflected beam is again reflected on the beam splitter 32 and combined with the reference reflected beam transmitted on the beam splitter 32 to form a scanning beam. The scanning beam is partially transmitted as an imaging beam into the imaging unit 40 and partially reflected as a calibration beam into the calibration unit 50 on the beam splitter 37.

Further, in other embodiments, an incident tube lens 38 is disposed on the optical path between the beam combiner 31 and the beam splitter 32 to converge the detected incident beam. An exit tube lens 39 is provided on the optical path between the beam splitter 32 and the beam splitter 37, and condenses the scanning beam exiting from the beam splitter 37.

The imaging unit 40 comprises an imaging image sensor 41, the imaging light beam being interferometrically imaged at the imaging image sensor 41. In other embodiments, since the imaging beam includes a white light portion and a laser portion, a long-pass dichroic mirror 42 is further disposed between the imaging image sensor 41 and the beam splitter 37 to filter out the laser light with a shorter wavelength in the imaging beam, and only the white light portion is left to perform interference imaging on the imaging image sensor 41, so that the influence of the laser beam on white light scanning imaging is avoided. Typically, the white light portion comprises a large range of wavelengths of light, primarily between 500nm and 800nm, as shown in FIG. 3; the laser section contains a narrow range of wavelengths of light, with light waves only around 532nm, as shown in fig. 4. Therefore, the filter parameter of the long-pass dichroic mirror 42 is set to 633nm, and when passing through the long-pass dichroic mirror, the white light portion also passes light waves of a larger wavelength range (as shown in fig. 5), and the laser portion has almost no light waves passing therethrough (as shown in fig. 6).

The calibration unit 50 comprises an austenitic prism 51, a focusing lens 52, a calibration polarizer 53 and a calibration image sensor 54, wherein the calibration polarizer 53 is a linear polarizer, and the included angle between the polarization direction of incident light and the optical axis is 45 degrees. The austenitic prism 51 has polarization splitting characteristics, and has a first prism and a second prism. Referring to fig. 7, when the calibration beam split by the beam splitter 37 is incident on the surface of the first prism, the o-ray and the e-ray of the calibration beam in the first prism will propagate along the same direction without refraction at different speeds, respectively, because the optical axes of the two prisms are perpendicular to each other, when entering the second prism, the o-ray and the e-ray of the calibration beam in the second prism are interchanged with the o-ray and the e-ray in the first prism, so that the o-ray in the first prism is refracted at the interface of the two prisms with the relative refractive index ne/no, and the e-ray in the first prism is refracted at the relative refractive index no/ne; since the first prism and the second prism are negative crystals with refractive index no > ne, the e light in the second prism propagates away from the normal line of the interface between the first prism and the second prism, the o light in the second prism propagates close to the normal line of the interface between the first prism and the second prism, and finally is separated into two light wave outputs with mutually perpendicular polarization directions and a certain separation angle, the two light waves transmitted along different angles are converged on the calibration image sensor 54 through the focusing lens 52, and the two light waves after passing through the focusing lens 52 are changed into the same polarization direction through the calibration polarizer 53, and finally are interference imaged on the calibration image sensor 54.

Referring to fig. 8, since the coherence length of the laser portion is longer and the coherence length of the white light portion is shorter in the calibration beam, the laser portion interferes on the calibration image sensor 54 to form clear calibration carrier frequency fringes, so that a series of time-shifted laser calibration carrier frequency interferograms can be acquired by the calibration image sensor 54. In addition, due to the consistency of laser and white light phase shift, the laser phase shift calibration can be calculated through the laser calibration carrier frequency interference pattern, so that the high-precision white light scanning step length is obtained. The white light part does not interfere due to the short coherence length, so that the calibration accuracy is not affected.

Further, the self-calibration high-precision scanning white light interference system of the present invention further includes an imaging processor (not shown) that receives the imaging image of the sample generated by the imaging image sensor 41 at the scanning position, synchronously acquires the laser calibration carrier frequency interference image generated by the calibration image sensor 54, interprets the time sequence phase shift information from the laser calibration carrier frequency interference image, and calibrates the white light scanning step length by using the time sequence phase shift information to generate the sample surface appearance image.

Further, a linear polarizer 6 having an angle of 45 ° with respect to the optical axis with respect to the polarization direction of the incident light may be provided between the imaging image sensor 41 and the beam splitter 37; the linear polaroid 6 ensures that the polarization direction of the imaging light beam entering the imaging image sensor 41 is the same as that of the calibration light beam entering the calibration image sensor 54, so that the synchronism of the imaging light beam and the calibration light beam is improved, and the accuracy of the self calibration of the system is improved. In addition, a linear polarizer 6 can be further disposed on the light path of the incident beam detected between the beam combiner 31 and the beam splitter 32, and a linear polarizer 6 is disposed on the light path between the beam splitter 32 and the sample objective and the light path between the beam splitter 32 and the reference objective, so as to ensure that the polarization directions of the light beams of all the light paths are the same, and further improve the accuracy of self-calibration of the system.

Compared with the existing scanning white light interference system, the self-calibration high-precision scanning white light interference system forms a laser calibration light path coupled with a white light scanning imaging light path by arranging the laser incidence unit and the calibration unit, and further splits light and adds carrier frequency in the calibration unit through the Walsh prism, so that the common-path structure of the white light scanning light path and the laser calibration light path is realized, the consistency of interference of laser and white light and the wide spectrum of the white light are ensured, the noise immunity is strong, the actual scanning step length of the white light can be calibrated with high precision, the self-calibration high-precision scanning white light interference system can be widely applied to Linnik type white light scanning interferometry, and the measurement precision of Linnik type white light scanning interference is improved.

The above examples illustrate only a few embodiments of the invention, which are described in detail and are not to be construed as limiting the scope of the invention. It should be noted that modifications and improvements can be made by those skilled in the art without departing from the spirit of the invention, and the invention is intended to encompass such modifications and improvements.

Claims (10)

1. A self-calibrating high precision scanning white light interferometry system, comprising: the device comprises a white light incidence unit, a laser incidence unit, a sample scanning unit, an imaging unit and a calibration unit; the method comprises the steps that after a common-path combined beam of a white light incident beam generated by the white light incident unit and a laser incident beam generated by the laser incident unit is used as a detection incident beam in a sample scanning unit, the detection incident beam is divided into a sample incident beam and a reference incident beam; the sample incident light beam irradiates on the sample and is reflected to form a sample reflected light beam, and the reference incident light beam irradiates on a reference reflector and is reflected to form a reference reflected light beam; the sample reflected beam and the reference reflected beam are combined together again in the sample scanning unit to form a scanning beam, the scanning beam is divided into an imaging beam and a calibration beam again, the imaging beam enters the imaging unit to form an instant image of the sample at the current irradiation point, and the calibration beam enters the calibration unit to mark the scanning step length of white light.

2. The self-calibrating high accuracy scanning white light interferometry system of claim 1, wherein: the calibration unit comprises a Voronoi prism, a focusing lens, a calibration polarizing plate and a calibration image sensor, wherein the Voronoi prism separates the calibration light beam into two light waves with the polarization directions being mutually perpendicular and a certain separation angle, the two light waves transmitted along different angles are converged on the calibration image sensor through the focusing lens, the two light waves after passing through the focusing lens are changed into the same polarization direction through the calibration polarizing plate, and finally, the two light waves are subjected to interference imaging on the calibration image sensor to calibrate the scanning step length of white light.

3. The self-calibrating high accuracy scanning white light interferometry system of claim 2, wherein: the white light incidence unit comprises a white light source; the laser incidence unit comprises a laser light source; the sample scanning unit comprises a spectroscope and a beam splitter; the imaging unit includes an imaging image sensor; the method comprises the steps that after a white light incident beam generated by the white light source and a laser incident beam generated by the laser source are combined in a common way to be a detection incident beam, the detection incident beam is divided into a sample incident beam and a reference incident beam in the spectroscope; the sample reflected beam and the reference reflected beam are combined into a scanning beam in the spectroscope again in a common way, the scanning beam is divided into an imaging beam and a calibration beam again after passing through the beam splitter, and the imaging beam enters the imaging image sensor to form an instant image of the sample at a current irradiation point.

4. A self-calibrating high accuracy scanning white light interferometry system according to claim 3, wherein: the sample scanning unit further comprises a beam combining lens, a sample objective lens and a reference objective lens, wherein the beam combining lens is used for combining the white light incident beam and the laser incident beam to form a detection incident beam, the sample objective lens is arranged between the spectroscope and the sample, and the reference objective lens is arranged between the spectroscope and the reference reflecting mirror.

5. The self-calibrating high accuracy scanning white light interferometry system of claim 4, wherein: the sample scanning unit further comprises an incidence tube lens and an emergent tube lens, wherein the incidence tube lens is arranged between the beam combining lens and the spectroscope, and the emergent tube lens is arranged between the beam splitting lens and the spectroscope.

6. The self-calibrating high accuracy scanning white light interferometry system of claim 5, wherein: the white light incidence unit also comprises a collimation beam expansion module and a small-hole filter.

7. The self-calibrating high accuracy scanning white light interferometry system of claim 6, wherein: a long-pass dichroic mirror is arranged between the imaging image sensor and the beam splitter.

8. The self-calibrating high accuracy scanning white light interferometry system of claim 7, wherein: an attenuation sheet is arranged between the laser light source and the beam combining lens.

9. The self-calibrating high accuracy scanning white light interferometry system of claim 8, wherein: the imaging processor receives an imaging image of the sample generated by the imaging image sensor at a scanning position and a laser calibration carrier frequency interference image generated by the calibration image sensor, and calibrates a white light scanning step length by using time sequence phase shift information in the laser calibration carrier frequency interference image to generate a sample surface appearance image.

10. The self-calibrating high accuracy scanning white light interferometry system of claim 9, wherein: the sample scanning unit further comprises a micro-moving sample stage.