CN113218312B - Light needle type common-path interference confocal displacement measuring device and method - Google Patents
Light needle type common-path interference confocal displacement measuring device and method Download PDFInfo
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Abstract
A light-needle type common-light-path interference confocal displacement measuring device and a method belong to the technical field of ultra-precise three-dimensional measurement. The invention combines the laser interference technology and the confocal microscopic measurement technology, and provides a single-arm interference measurement structure constructed by splitting light by using an annular reflector; meanwhile, according to the interference and confocal measurement principles, a variable-area light intensity acquisition method is provided, so that the influence of signal coupling caused by the common light path of a confocal measurement system and an interference measurement system can be eliminated; in addition, an accurate displacement value is obtained through a difference and ratio data calculation method, and common mode additive noise and multiplicative noise in the system can be eliminated simultaneously. The method can effectively improve the measurement accuracy and the measurement stability of the system, is suitable for measuring the geometric parameters of the microstructure with large steps and high depth-to-width ratio and the three-dimensional shape of the rough surface, and has wide application prospect.
Description
Technical Field
The invention relates to the field of ultra-precise three-dimensional measurement, in particular to a light-needle type common-path interference confocal displacement measurement device and method capable of realizing wide range and high resolution.
Background
With the advance of nanotechnology, human beings are moving towards finer microscopic fields. The microstructure functional surface has been widely used in many industries such as micro-optical element, micro-electro-mechanical element and aircraft manufacturing, and the like, and the ultra-precision measurement of the surface appearance and the structural feature size of the product thereof has huge market demand, such as the detection of geometric features and film thickness in silicon crystal processing, the measurement of surface profiling and texture structure, and the processing and measurement of large-scale micro-structure functional surface in photoelectric products, and the like. In these fields, both interferometric techniques and confocal microscopy techniques are well-used. The phase-shifting interference technology has high measurement precision, but the measurement range is small, the influence of environmental interference is easy to occur, and the microstructure with large steps and high depth-to-width ratio is difficult to measure; the confocal microscopic measurement technology has unique three-dimensional chromatographic capacity and can realize measurement in a larger range, but the measurement precision is limited by the numerical aperture of an objective lens.
In order to overcome environmental vibration and realize large-range high-precision measurement, the phase-shifting interference technology and the confocal microscopy technology are combined, and the appearance measurement of the surface of the discontinuous microstructure can be completed by utilizing the zero point characteristic of the confocal microscopy technology and the periodic characteristic of an interference signal. Document "heterodyne confocal method for measuring height of large step" (china laser, 2005) proposes a dual-frequency laser heterodyne interference confocal microscope system, which determines the maximum value of confocal light intensity by means of Z-direction displacement scanning, and then determines the surface height by using the phase value of a dual-frequency laser interferometer. The whole system realizes high resolution and wide range in large-step measurement, but the method is limited by the precision and scanning range of a Z-direction scanning mechanism, and meanwhile, the system structure is complex and is not beneficial to interference suppression. The document 'synchronous phase-shifting interference confocal microscopic imaging technology research' proposes a measurement structure for realizing synchronous phase-shifting interference confocal by utilizing a polarization spectroscope, and the signal resolution of a four-way light intensity detector can realize the nanoscale measurement resolution in a larger range. However, the system is complex and has high installation and adjustment difficulty; meanwhile, the interference measurement system and the confocal microscope system share the same light intensity signal, and mutual interference can be generated during displacement signal calculation. The patent of 'pupil-splitting phase-shifting interference confocal micro-displacement measuring device' (patent publication No. ZL201610317208.5) proposes that four paths of interference confocal optical paths are integrated by utilizing a two-dimensional grating and a four-quadrant polarization phase-shifting, focus-shifting array and a soft pinhole array, so that the system structure is simplified and the problem of difficult system adjustment is solved while high-precision absolute position measurement is realized. However, the Michelson interference light path structure is still adopted, so that the reference arm and the measuring arm are separated, the anti-interference capability is insufficient, and the miniaturization is not facilitated; on the other hand, the interference subsystem and the confocal subsystem still share the same light intensity signal, and a signal coupling effect exists, so that the accuracy of measurement is influenced.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a light-needle type common-path interference confocal displacement measuring device and a method, the device has the characteristics of strong anti-interference capability, high resolution, large measuring range and high integration level, and simultaneously can remove the signal coupling effect between interference and common-focus subsystems and realize the three-dimensional accurate measurement of a discontinuous surface microstructure.
In order to achieve the purpose, the invention is realized by the following technical scheme:
the invention discloses a light-needle type common-light-path interference confocal displacement measuring device, which comprises: a light source module consisting of a linearly polarized light source (101) and an 1/2 wave plate (102) emits parallel light, the polarization direction of the parallel light is consistent with the transmission axis (x direction) of a beam splitter Prism (PBS) (103), light beams are transmitted by the PBS (103) and then irradiated onto an annular reference reflector (105) through a first 1/4 wave plate (104), and a part of light is reflected by the annular reflector to form reference light and returns according to the original light path; a part of light passes through the annular reflector to form measuring light, forms a focusing light needle through the objective lens (106) to irradiate the surface (107) of the sample, and then returns according to the original light path after being reflected by the surface of the sample; because the measuring light and the reference light both pass through the first 1/4 wave plate (104) twice, the included angle between the optical axis of the wave plate and the x direction is 45 degrees, the polarization direction of the return light beam is changed into the y direction, and the return light is reflected by the PBS (103); the reference light passes through an annular 1/2 wave plate (108), the polarization direction is rotated by 90 degrees, and the measuring light passes through the center of the wave plate, and the polarization directions of the measuring light and the reference light are perpendicular to each other; then the reference light and the measuring light are respectively changed into left-handed circularly polarized light and right-handed circularly polarized light through a second 1/4 wave plate (109), and the reference light and the measuring light are subjected to phase shifting through a two-dimensional Ronchi grating (110) and a polarization phase shifting array (111); furthermore, the light respectively passes through 2 pairs of mutually conjugated focusing pupil arrays (112) to select four paths of (+/-1 and +/-1) diffraction light, wherein 2 beams of light are converged by a lens (113) after passing through a focusing pupil filter, the focal plane position of the light is positively focused, and the focal plane of the other 2 beams of light is negatively focused after passing through the conjugated focusing pupil filter; meanwhile, the measuring light and the reference light interfere with each other near the focal plane of the lens (113), the four paths of interference light are respectively detected by the variable-area light intensity detector (114), and a displacement measurement value is obtained after data processing.
The annular reflector (105) enables the central part of the light beam to pass through the element to form measuring light, and the outer ring area of the light beam is reflected to form reference light, so that the beam splitting function in the interference measurement is realized. Through structure size control, can guarantee that the area proportion that interference light and measuring light occupy is 1: 1, and does not affect the working distance of the objective lens.
The annular 1/2 wave plate (108) has an optical axis which forms an angle of 45 DEG with the polarization direction of the incident reference light, and changes the polarization direction of the reference light so that the polarization directions of the measurement light and the reference light are perpendicular to each other.
The included angle of the polarization axes of adjacent quadrants of the polarization phase shift array (111) is 45 degrees, and the phase shift of the measurement light and the reference light can be realized.
The structure of the focusing pupil array (112) comprises 4 sub-pupil areas which are symmetrical about the center, adjacent sub-pupils are phase conjugate pupils, each sub-pupil filter is a circularly symmetrical phase pupil and is of a two-ring or multi-ring structure, the outer contour radius of each sub-pupil is the same as that of an annular reference mirror, and the area outside each sub-pupil is opaque, so that four paths of (+/-1 and +/-1) diffracted light are selected, 2 beams of light are subjected to positive focusing shifting at the position of a rear focal plane of the collecting lens (113) after passing through the focusing pupil filter, and the other 2 beams of light are subjected to negative focusing shifting at the position of the rear focal plane of the collecting lens (113) after passing through the focusing pupil filter.
The further improvement of the invention is that the measuring light and the reference light are converged by a collecting lens (113), and then interfere near a focal plane, and then interference and confocal signal acquisition are realized by a variable-area light intensity detector.
The invention relates to a light needle type common-path interference confocal displacement measuring device, which comprises the following measuring methods:
four reference lights and four measuring lights obtained by the light path respectively interfere at the focal plane of the collecting lens, and the light field distribution of the four reference lights is as follows:
the four measuring light field distribution is as follows:
wherein,
and
the complex amplitudes of the reference light after positive focus shift and negative focus shift of the pupil respectively, and have
And
the complex amplitudes of the measuring light after pupil positive focusing and negative focusing are respectively,
is the phase difference between the reference light and the measurement light.
For the measurement light, the intensity of the light field on the detector is expressed as follows from the confocal microscopy imaging principle:
where (v, u) is a point on the surface of the object to be measured, v and u represent normalized radial and axial optical coordinates, h 1 (v，u)，h 2 (v, u) are complex amplitude point spread functions of the illumination arm and the detection arm, respectively, and D (v) is a detection region. Since the focus-shifting pupil is added to the detection arm, I (v, u) ═ h when the detection area is infinite 1 (v,u)| 2 At this time, the effect of the shift pupil on the detection light disappears.
Therefore, when the reference light and the measurement light interfere in the vicinity of the focal plane of the collecting lens, the interference light is collected in a large area, and the phase difference between the measurement light and the reference light can be obtained by the formula (1)
And its corresponding axial optical path difference z:
further, the light intensity obtained by the four-way point pinhole detector is calculated by the following formula to obtain the confocal subsystem output I CMS (u)，
Therefore, a rough measurement value u corresponding to the displacement can be obtained by the measurement characteristics of the confocal subsystem, and further the axial defocusing position can be obtained
Where α is the objective numerical aperture.
In the method, the light intensity is collected in a larger area, and the accurate interference phase is obtained by the formula (1), so that the alternating current part of the interference light intensity can be reserved, and the coupling effect of the focusing pupil on the measurement of the interference phase is eliminated; in the confocal measurement, point pinhole detection and formula (2) calculation are utilized, so that the direct current part of interference light intensity can be reserved, and the influence of the alternating current part of the interference light intensity on the measurement of the confocal subsystem is eliminated.
During actual measurement, firstly, a measured object surface is placed near the focal plane of the objective lens, and the current a position of the object surface is measured by the measuring device to obtain
And I CMS (u) to obtain its corresponding position z a And l a (ii) a Then, by moving the displacement table to the b position in the lateral direction, z can be obtained accordingly b And l b (ii) a Calculating the height difference H of the object plane by using the formula (3) ab ：
H ab ＝kT+z b -z a (3)
Where T ═ λ/2 is the measurement period of the interferometric subsystem, and k is l b -l a The number of interferometric periods involved is an integer.
The invention has the following remarkable characteristics and beneficial effects:
1. the invention adopts a common-light path structure, namely, the measuring light and the reference light are separated by utilizing the annular reflector, and simultaneously, the interference measurement and the confocal measurement are combined in the same light path, so that the system integration level can be obviously improved, and the common-mode interference resistance of the system can be effectively improved;
2. interference and confocal signals are decoupled by a method of acquiring light intensity in a variable region, so that the coupling effect in the system is effectively inhibited;
3. in data processing, the stability of the system can be further improved and common mode additive noise and multiplicative noise can be eliminated by adopting difference value and ratio value calculation processing.
The method can be applied to the measurement of the geometric parameters of the microstructure with large steps and high depth-to-width ratio and the three-dimensional shape of the rough surface.
Drawings
FIG. 1 is a schematic structural diagram of a light-needle type common-path interference confocal displacement measuring device.
FIG. 2 is a schematic view of the structure of the ring reflector (105)
FIG. 3 is a structural diagram of a ring-shaped 1/2 wave plate (108)
FIG. 4 is a schematic diagram of the polarization phase shift array (111) and the focus shift pupil array (112) components
FIG. 5 is a characteristic diagram of phase-shifting interference axial response (a) and confocal axial response (b)
FIG. 6 is a schematic structural diagram of an example of a light-needle type common-path interference confocal displacement measuring device.
Detailed Description
The present invention will be described in further detail below with reference to examples and the accompanying drawings.
Example of implementation
Fig. 1 is a schematic structural diagram of a light-needle type common-path interference confocal displacement measurement device, and is also a schematic structural diagram of a light path in an embodiment of the present invention. It can be seen from the figure that the optical-needle type common-path interference confocal displacement measuring device of the invention comprises:
the device comprises a linear polarization laser (101), an 1/2 wave plate (102), a polarization beam splitter Prism (PBS) (103), a first 1/4 wave plate (104), an annular reference reflector (105), an objective lens (106), a measured surface (107), an annular 1/2 wave plate (108), a second 1/4 wave plate (109), a two-dimensional Ronchi grating (110), a polarization phase shift array (111), a focus shift pupil array (112), a collecting lens (113), an imaging tube mirror (115) and a CCD (116).
Wherein the linear polarization laser (101) and the 1/2 wave plate (102) form a light source module of the measuring system.
The annular reflector (105) enables the central part of the light beam to pass through the element to form measuring light, and the outer ring area of the light beam is reflected to form reference light, so that the beam splitting function in the interference measurement is realized. The structure is shown in fig. 2, the normalized radius R of the center hole is 0.707, and the normalized radius R of the outer circle is 1.
The annular 1/2 wave plate (108) has an optical axis which forms an angle of 45 DEG with the polarization direction of the incident reference light, and changes the polarization direction of the reference light so that the polarization directions of the measurement light and the reference light are perpendicular to each other. The structure is shown in fig. 3, the normalized radius R of the center hole is 0.707, and the normalized radius R of the outer circle is 1.
The structure of the polarization phase shift array (111) and the focusing pupil array (112) is shown in figure 4. The included angle of the polarization axes of adjacent quadrants of the polarization phase shift array is 45 degrees, and the phase pupils of the adjacent quadrants of the focusing pupil array are conjugated with each other.
Optical path: a light source module consisting of a linearly polarized light source (101) and an 1/2 wave plate (102) emits parallel light, the polarization direction of the parallel light is consistent with the transmission axis (x direction) of a beam splitter Prism (PBS) (103), light beams are transmitted by the PBS (103) and then are irradiated onto an annular reference reflecting mirror (105) through a first 1/4 wave plate (104), and the reference light returns according to the original light path; the measuring light forms a focusing light needle through an objective lens (106) to irradiate the surface (107) of the sample, and then returns according to the original light path through the reflection of the surface of the sample; because the measuring light and the reference light both pass through the first 1/4 wave plate (104) twice, the included angle between the optical axis of the wave plate and the x direction is 45 degrees, the polarization direction of the return light beam is changed into the y direction, and the return light is reflected by the PBS (103); the reference light passes through an annular 1/2 wave plate (108), the polarization direction is rotated by 90 degrees, and the measuring light passes through the center of the wave plate, and the polarization directions of the measuring light and the reference light are perpendicular to each other; then the reference light and the measuring light are respectively changed into left-handed circularly polarized light and right-handed circularly polarized light through a second 1/4 wave plate (109), and the reference light and the measuring light are subjected to phase shifting through a two-dimensional Ronchi grating (110) and a polarization phase shifting array (111); furthermore, the light respectively passes through 2 pairs of mutually conjugated focusing pupil arrays (112) to select four paths of (+/-1 and +/-1) diffraction light, wherein 2 beams of light are converged by a lens (113) after passing through a focusing pupil filter, the focal plane position of the light is positively focused, and the focal plane of the other 2 beams of light is negatively focused after passing through the conjugated focusing pupil filter; meanwhile, the measuring light and the reference light interfere with each other near the focal plane of the lens (113), the four paths of interference light are respectively detected by the variable-area light intensity detector (114), and a displacement measurement value is obtained after data processing.
The invention further improves the method and the device and also comprises a variable-area light intensity detector (114) which can be composed of an imaging tube lens (115) and a CCD (116). Light beams interfere near a focal plane, then are amplified through an imaging tube lens (115), four paths of interference confocal light spots located on the focal plane are imaged on a CCD image plane (116), and interference and confocal signals are respectively collected through a variable-area light intensity detection method.
When the surface of a measured object is measured, the measuring process and the data processing steps are as follows:
Example two
The imaging tube lens (115) and the CCD (116) in the first embodiment are replaced by the detection structure in FIG. 6, so that the second embodiment is formed. The detection structure includes: the optical lens comprises a semi-reflecting and semi-transparent lens (601), a pinhole array (602) formed by 4 pinholes, a four-quadrant photoelectric detector PSD (603) which is a light intensity detector of a confocal subsystem, and a four-quadrant photoelectric detector PSD (604) which is a light intensity detector of an interference subsystem. The rest of the measuring method and the measuring device are the same as the first embodiment.
While the invention has been described with reference to specific embodiments, these descriptions should not be construed as limiting the scope of the invention.
The scope of the invention is defined by the appended claims, and any modification based on the claims is intended to be within the scope of the invention.
Claims (7)
1. The utility model provides a confocal displacement measurement device of light needle formula common light path interference which characterized in that: a light source module consisting of a linearly polarized light source (101) and an 1/2 wave plate (102) emits parallel light, the polarization direction of the parallel light is consistent with the x direction of a light transmission axis of a beam splitter prism PBS (103), light beams are transmitted by the PBS (103) and then are irradiated onto an annular reference reflecting mirror (105) through a first 1/4 wave plate (104), and reference light returns according to an original light path; the measuring light forms a focusing light needle through an objective lens (106) to irradiate the surface (107) of the sample, and then returns back along the original light path through the reflection of the surface of the sample; since the measurement light and the reference light both pass through the first 1/4 wave plate (104) twice, the included angle between the optical axis of the first 1/4 wave plate (104) and the x direction is 45 degrees, the polarization direction of the return light beam is changed into the y direction, and then the return light is reflected by the PBS (103); the reference light passes through an annular 1/2 wave plate (108), the polarization direction is rotated by 90 degrees, and the measurement light passes through the center of the annular 1/2 wave plate (108), and the polarization directions of the measurement light and the reference light are perpendicular to each other; then the reference light and the measuring light are respectively changed into left-handed circularly polarized light and right-handed circularly polarized light through a second 1/4 wave plate (109), and the reference light and the measuring light are subjected to phase shifting through a two-dimensional Ronchi grating (110) and a polarization phase shifting array (111); furthermore, the light respectively passes through 2 pairs of mutually conjugated focusing pupil arrays (112) to select four paths of [ +/-1, +/-1 ] diffraction light, wherein 2 beams of light are converged by a lens (113) after passing through a focusing pupil filter, the focal plane position of the light can generate positive focusing, and the focal plane of the other 2 beams of light can generate negative focusing after passing through the conjugated focusing pupil filter; meanwhile, the measuring light and the reference light interfere with each other near the focal plane of the lens (113), the four paths of interference light are respectively detected by the variable-area light intensity detector (114), and a displacement measurement value is obtained after data processing.
2. The optical-pin type common-path interference confocal displacement measurement device according to claim 1, wherein: an annular reference mirror (105) having a central aperture with the same area as the outer annular area is placed in front of the objective lens.
3. The optical-needle type common-path interference confocal displacement measuring device according to claim 1, characterized in that: and the outer contour radius and the inner hole radius of the annular 1/2 wave plate (108) are the same as those of the annular reference reflector (105), and the fast/slow axis of the wave plate forms an angle of 45 degrees with the polarization direction of the reference light.
4. The optical-needle type common-path interference confocal displacement measuring device according to claim 1, characterized in that: a focusing pupil array (112) which comprises 4 sub-pupil regions symmetrical about the center, each sub-pupil filter is a circularly symmetrical phase pupil and has a two-ring or multi-ring structure, the outer contour radius of the sub-pupil is the same as that of the annular reference mirror, and the region outside the sub-pupil is opaque, which is used for selecting four paths of +/-1 diffraction light; and adjacent sub-pupils are phase conjugate pupils and can respectively realize positive focus shifting and negative focus shifting.
5. The optical-needle type common-path interference confocal displacement measuring device according to claim 1, characterized in that: the variable-area light intensity detector (114) can be composed of an imaging tube lens (115) and a CCD (116).
6. The optical-needle type common-path interference confocal displacement measuring device according to claim 1, characterized in that: the variable-area light intensity detector (114) can be composed of a half-reflecting and half-transmitting lens (601), a pinhole array (602) formed by 4 pinholes, a first four-quadrant photodetector PSD (603) and a second four-quadrant photodetector PSD (604), wherein the first four-quadrant photodetector PSD (603) behind the pinholes is a confocal subsystem detector, and the second four-quadrant photodetector PSD (604) is an interference subsystem detector.
7. A measurement method using the apparatus of claim 1, characterized in that: when light intensity is collected, the gravity centers of four interference light spots are respectively used as centers, and the output of an interference subsystem, namely I, is obtained by collecting the light intensities in 4 set large areas A ，I B ，I C ，I D (ii) a Correspondingly, the light intensity in the set 4 small needle holes is collected to obtain the confocal subsystem output, namely: i' A ，I' B ，I' C ，I' D And by formula (1):
the phase difference between the measuring light and the reference light can be obtained
And its corresponding axial optical path difference z;
calculating the confocal subsystem output I by equation (2) CMS (u)：
And obtaining a rough measurement value u of corresponding displacement by the axial measurement characteristic of the confocal subsystem, wherein u is a normalized axial optical coordinate; further obtaining the axial out-of-focus position
Wherein α is the objective numerical aperture;
measuring the current a position and b position of the object plane by using the measuring device so as to obtain the corresponding position z a 、l a And z b 、l b (ii) a Finally, calculating the height difference H of the object plane by using a formula (3) ab ：
H ab ＝kT+z b -z a (3)
Where T ═ λ/2 is the measurement period of the interferometric subsystem, and k is l b -l a The number of interferometric periods involved is an integer.
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