NON-CONTACT OPTICAL METROLOGY SYSTEM TO MEASURE SIMULTANEOUSLY THE RELATIVE PISTON AND THE RELATIVE INCLINATION IN TWO AXES (TIP AND TILT) BETWEEN TWO REFLECTIVE SURFACES

Abstract

A non-contact optical metrology system to measure simultaneously the relative piston and the relative inclination in two axes between two low curvature reflective surfaces using partially coherent light interferometry. The system creates an interference pattern with partially coherent light from the linear phase change induced by a double prism system or equivalent, which allows the measurement of the relative piston and the inclination in two axes between the reflective surfaces. The relative piston between the mirrors is measured from the position of the interference pattern, the relative inclination in one axis from the distance between the fringes, and the relative inclination in the other axis from the inclination of the fringes. Relative piston measurements and relative inclination measurements in two axes are decoupled and can be extracted with simple morphological operations, without the need for marginal processing algorithms.

Claims

1. A non-contact optical metrology system to measure simultaneously a relative piston and a relative inclination in two axes between two reflective surfaces based on a use of a modified Mach-Zehnder interferometer pointing to two adjacent almost coplanar mirrors, with two light beams traveling along two different optical paths until converging in an interferogram image plane to form an interference pattern, the non-contact optical metrology system comprising: a partially coherent light beam that forms two light beams going in two different paths after hitting a first beam splitter: a first light beam that is transmitted towards a second beam splitter, aligned with the first beam splitter that illuminates an area of interest of a reference segment, the light is reflected in said segment and travels back to said second beam splitter, there it is reflected towards a third Beam Splitter where after reflection is transmitted through a fourth beam splitter to the image capture system, and a second light beam which is reflected by the beam splitter towards the fourth beam splitter and from this it is transmitted by the third beam splitter to illuminate the target mirror segment, which in turn reflects and transmits it through the beam splitters and to the image capture system that captures an interference between the second light beam and the first light beam; wherein: an optical component is interposed in a target arm through which the second light beam passes, which creates a linear optical path difference between two arms of the interferometer; and a compensation plate is interposed in a reference arm followed by the first light beam so that the phase gradient in the light is due solely to the change in the optical path introduced by the optical component; mechanisms that generate a fixed interference pattern modulated by an amplitude envelope, in which only fringes close to a region in which the optical path difference in the two arms of the interferometer is zero, will be visible; when projecting said interference pattern in a camera, three magnitudes measured are decoupled, so the three magnitudes measured are extracted with simple morphological operations and represented by: the relative piston by the position of the envelope of the interference pattern, the inclination in one axis by the thickness of the fringes and can therefore be measured from the width of a modulation pattern, and the inclination in another axis by the inclination of the fringes.

2. The non-contact optical metrology system according to claim 1, wherein the optical component that generates the optical path difference is a double prism, made up of two glasses with different but close refractive indices.

3. The non-contact optical metrology system according to claim 1, wherein the optical component that generates the optical path difference is a wedge prism.

4. The non-contact optical metrology system according to claim 1, wherein an effect of the optical component that generates the optical path difference is generated through the slight inclination of one or more of the beam splitters.

5. The non-contact optical metrology system according to claim 1, wherein the optical path difference induced by the optical component located in the target arm produces a displacement of the fringes proportional to the displacement of the piston present in the non-contact optical metrology system between the target and the reference surfaces.

6. The non-contact optical metrology system according to claim 1, wherein the measurement resolution values are a function of the degree of coherence of the light source used.

7. The non-contact optical metrology system according to claim 1, wherein the compensation plate has a stepped configuration in several different regions that introduce a constant path difference jump between said regions, in order to establish several correlative piston measurement zones in the detector, generating different measurement zones in the detector.

8. The non-contact optical metrology system according to claim 6, wherein the thickness di of each region is arranged in such a way that all possible piston values are visible in some region of the image.

9. The non-contact optical metrology system according to claim 6, wherein there is a certain amount of relative piston overlap between neighbouring regions to ensure that no piston value is omitted due to tolerances or other reasons at the edges of the field.

10. The non-contact optical metrology system according to claim 6, wherein the end result is an image with different horizontal bands along the X direction where the piston value linearly displaces the OPD=0 region and subsequently the interference pattern.

11. The non-contact optical metrology system according to claim 1, wherein the non-contact optical metrology system integrates an inclination sensor in the interferometer that uses the light transmitted by the two beam splitters, reflected by the reference segment and transmitted back by the second beam splitter and reflected in the first beam splitter that is focused on one detector per target, in order to be able to measure the absolute value of inclination in two axes.

Description

DESCRIPTION OF THE DRAWINGS

[0034] As a complement to the description being made, and for the purpose of helping to make the features of the invention more readily understandable, the present specification is accompanied by a set of drawings which, by way of illustration and not limitation, represent the following:

[0035] FIG. 1 is a schematic representation of a possible implementation of the invention.

[0036] FIGS. 2A and 2B show in pseudocolour the expected displacement of the fringe pattern caused by relative movement of the piston between the target and reference surfaces.

[0037] FIGS. 3A and 3B show in pseudocolour the expected broadening of the fringe pattern caused by a relative inclination in one axis (tip) between the target and reference surfaces.

[0038] FIGS. 4A and 4B show in pseudocolour the expected rotation of the fringe pattern caused by a relative inclination in the other axis (tilt) between the target and reference surfaces.

[0039] FIG. 5 is a schematic diagram showing the layout of a stepped compensator plate solution to establish five correlative piston measurement zones in the detector.

[0040] FIG. 6 shows the apparent piston error induced by the lack of perpendicularity between the interferometer and the reference mirror, as well as the implementation of an inclination sensor in the interferometer that provides a measure of the degree of perpendicularity in which it is located.

[0041] FIG. 7 represents the magnitudes: directions of the relative piston, tip and tilt, in relation to the system.

EMBODIMENT OF THE INVENTION

[0042] As has already been indicated, a preferential implementation of the measurement system of the invention is based on the use of a modified Mach-Zehnder interferometer pointing to two adjacent almost coplanar reflective optical surfaces (RS) and (TS). As in any interferometer, this interferometer uses two light beams traveling along two different optical paths, through a system of prisms or mirrors that converge in an image plane (IIP) to form an interference pattern. In this case, a partially coherent light beam (LS) hits a first beam splitter (BS1), which splits the light into two light beams (TP, RP) going in different directions; a first light beam (RP) (represented with dashed lines) is transmitted towards a second beam splitter (BS2), aligned with the previous one, where it is also transmitted and illuminates the area of interest of the reference segment (RS). From the segment, the light is reflected back to the second beam splitter (BS2) where it is reflected towards (BS3), from where it is transmitted by (BS4) to the image capture system (MD).

[0043] Furthermore, the second beam (TP) of the same light source (represented with solid lines) is reflected by the beam splitter (BS1) towards a fourth beam splitter (BS4) and from this it is transmitted by a third beam splitter (BS3) to illuminate the target mirror segment (TS), which in turn reflects and transmits it through the beam splitters (BS3) and (BS4) to the same image capture system (MD) that captures the interference between the two beams (TP, RP).

[0044] The key modification over the classical Mach-Zehnder interferometer in the proposed system is the insertion of: [0045] an optical component (DP) that generates a linear optical path difference (measurement tilt) in the target arm through which the beam passes (TP), and [0046] a compensation plate (CP) in the reference path followed by the beam (RP), in order to balance the two arms of the interferometer by centring the interference pattern.

[0047] In a preferred embodiment, a double prism (DP) made with two optical glasses with different but close refractive indices is used in order to generate the desired optical path gradient. However, some other alternative solutions are possible for introducing that linear gradient, for example using a wedge prism or slightly inclining one or more of the beam splitters (BS1 to BS4) relative to each other.

[0048] In this way, a fixed interference pattern modulated by an amplitude envelope appears due to the use of partially coherent light, so that only the fringes close to the region in which the optical path difference in the two arms of the interferometer is zero (OPD=0) will be visible. Said interference pattern is projected onto a camera (MD) preferably mounted behind an afocal system.

[0049] As can be seen in FIG. 2B, the phase shift induced by the wave plate (DP) located in the target arm produces a displacement of the fringes proportional to the displacement of the piston present in the system, between the target and the reference surfaces.

[0050] Different measurement resolution values can be achieved by adapting the degree of coherence of the light source used.

[0051] In this measurement system the relative piston is represented by the position of the fringe envelope. The inclination in one axis (tip) is represented by the thickness of the fringes and can therefore be measured from the width of the modulation pattern. In turn, the inclination in the other axis (tilt) is represented by the inclination of the fringes. As these three magnitudes are shown decoupled, they can be extracted with simple morphological operations, without the need to use complex fringe processing algorithms.

[0052] In addition, this system is capable of measuring, from a single interferogram, not only the relative piston between its two arms, but also the relative inclination in two axes (tip and tilt).

[0053] In a preferred embodiment and in order to accommodate the combination of range and resolution required in the system, the double material prism (DP) is combined with a stepped compensation plate (CP) in several different regions that introduce a constant path difference jump between them, in order to establish several correlative piston measurement zones in the detector. The measurement range can thereby be easily extended, if necessary, by using a stepped compensation plate (CP) to generate different measurement zones in the detector. (see FIG. 5). The number of zones varies depending on system specifications.

[0054] In this case, depending on the number of relative piston values, fringes will be visible through one of the regions or the other, as long as the OPD=0 position is set by the piston value being measured. The thickness di of each region is arranged in such a way that all possible piston values are visible in some region of the image. A certain amount of relative piston overlap between neighbouring regions can be allowed to ensure that no piston value is omitted due to tolerances or other reasons at the edges of the field. The final result is an image with different horizontal bands along the X direction where the piston value linearly displaces the OPD=0 region and subsequently the interference pattern.

[0055] The interferometer design can be adapted to the geometries, dimensions and positions of the detection areas on the reference and target surfaces.

[0056] The most significant advantage of this configuration is that the relative piston measurement is performed without the need to perform a fringe reconstruction, but only from the position of the partial coherence interferogram in the detector. This enables fast and reliable image processing algorithms, eliminates the need for complex fringe processing, is less prone to be affected by noise, and reduces piston measurement to morphological operations on the image.

[0057] The interferometer described in the previous lines is self-referenced, which enables several of its described measurement properties. However, the system lacks the ability to measure absolute inclination values which, if present, can lead to errors in the measurement of relative piston values. FIG. 6 schematically represents the apparent piston (measurement error) induced by the lack of perpendicularity between the interferometer and the reference mirror. However, this problem, which is common to all phase measurement methods, can be easily solved by integrating a two-axis absolute inclination (tip and tilt) sensor in the proposed interferometer.

[0058] A possible solution to integrate inclination sensor in the interferometer is based on the use of the collimated light transmitted by the two beam splitters (BS1 and BS2) in front of the illumination system to the reference segment and transmitted back by the second beam splitter (BS2) and reflected in the first beam splitter (BS1) being focused on one detector per target.

[0059] FIG. 6 is a schematic diagram showing the implementation of a tip inclination (SC) sensor in the interferometer, as well as a lens and an image detector that forms the red dot light, which gives us an idea of how perpendicular it is.

[0060] It is stated for the appropriate purposes that the materials, shape, size and arrangement of the elements described may be modified, as long as this does not imply an alteration of the essential features of the invention that are claimed below.