Chromatic confocal measuring device with beam splitter block

Abstract

A chromatic confocal measuring device (1) for a parameter of a surface (2) of a sample, the device comprising: a light source (4) configured to generate an illumination beam (6), an axially chromatic objective (8) configured to apply the illumination beam (6) to the surface (2) of the sample and to retransmit a backscattered beam from the surface (2) of the sample to a measuring arm (10), the measuring device comprising a beam splitter block (20) configured to make two reflections in a beam splitter (23) to a so-called reflected beam among the illumination beam (6) and the backscattered beam (9), an optical compensator (24) being disposed in the direct path of the other beam. Figure for the abbreviation: Fig. 1

Description

Title of the invention: Chromatic confocal measurement device with beam splitter block technical field

[0001] This application relates generally to the field of optical inspection and control of a part, and more specifically to a device for chromatic confocal measurement of a parameter of a sample surface. PRIOR TECHNOLOGY

[0002] The measuring device is based on the principle of chromatic confocal microscopy, in which a light, generally polychromatic, is focused onto the surface to be studied. The spectral distribution of the light beam is then analyzed to identify the wavelength of the light source for which a sharp image is obtained on the surface, thus allowing the distance between the objective lens and the surface to be deduced. Other surface parameters can be deduced by examining other characteristics of the reflected light beam.

[0003] Patent applications FR3086748 and FR3006758 give examples of devices for chromatic confocal measurement of a parameter of a sample surface. Prior art devices may use a beam splitter cube whose role is to direct the illumination beam towards the sample and to direct a measurement beam derived from the light beam backscattered by the sample towards a measuring instrument. The beam splitter cube includes a semi-reflective surface that intercepts a portion of the backscattered light beam, forming the measurement beam, and redirects it, typically perpendicularly, towards the measuring instrument.

[0004] However, it has become apparent that such a configuration requires a very precise orientation of the separator cube with respect to a direct path passing through it and along which the illumination beam and the backscattered light beam propagate. Any deviation in orientation can render the characteristics of the light beams non-invariant under rotation, and can therefore significantly degrade the quality of the measurements.

[0005] Furthermore, the light beams that are carried by optical fibers to the input and output of the beam splitter cube must have their ends perfectly aligned with each other. They must be spatially stable not only with each other, but also with the beam splitter cube, which can be difficult, or make the device fragile and sensitive to mechanical shocks. Presentation of the invention

[0006] One objective of the present application is therefore to propose a measurement device chromatic confocal of a parameter of a surface of a sample, which is mechanically robust, and of a design allowing greater measurement accuracy through manufacturing and adjustment with fewer constraints.

[0007] To this end, a chromatic confocal measurement device for a parameter of a sample surface is proposed, the device comprising: - a light source configured to generate a beam of illuminating light; - an axial chromatic objective configured to apply the illumination light beam to the surface of the sample and to retransmit a backscattered light beam from the surface of the sample; - a measuring arm configured for the backscattered light beam, the measuring arm including a device for measuring the surface parameter of the sample from the backscattered light beam; the measuring device comprising a separator block configured to receive the illumination light beam through an input face and transmit it to the axial chromatic objective along a first path, receive the backscattered light beam back and transmit the backscattered light beam to the measuring arm via a second path; the separator block comprising: - a beam splitter configured to subject a so-called reflected beam, among the illumination beam and the backscattered light beam, to a first and a second reflection within the beam splitter, the reflected beam then taking an indirect path among the first and second paths, the other path among the first and second paths being a direct path free from reflection in the beam splitter; and - a compensator disposed in the direct path, the compensator being separated from the beam splitter by a space along the direct path, the optical compensator defining for the direct path an optical length corresponding to the additional optical length made by the beam reflected in the beam splitter compared to the direct path in the beam splitter.

[0008] Some preferred but not limiting features of the measuring device are the following, taken individually or in combination: - the splitter block includes a support plate on which the beam splitter and the optical compensator are fixed; - the splitter block includes a locking plate attached to the beam splitter and optical compensator on a side opposite the support plate; - the inlet face of the separator block is fitted with an opaque inlet mask pierced with at least one opening, and the outlet face of the separator block is fitted with an opaque outlet mask pierced with at least one opening, the opaque inlet and outlet masks output forming conjugate spatial filters; - a mask from among the opaque input mask and the opaque output mask is fixed on the beam splitter; - a mask, either the opaque inlet mask or the opaque outlet mask, is fixed to the optical compensator; - the beam splitter includes a semi-reflective surface to perform one reflection, and a reflective surface to perform the other reflection of the reflected beam; - the beam splitter includes a deflection prism inside which the first reflection and the second reflection occur, the direct path intersecting the indirect path, and the exit face is perpendicular to the entrance face; - the indirect path is distinct from the direct path in the beam splitter, and the exit face is parallel to the entrance face; - the measuring device is a spectral analysis device configured to determine a spectral distribution of the backscattered light beam, the sample surface parameter being a distance, or the measuring arm includes an imager configured to measure a light intensity of at least a part of the backscattered light beam, the sample surface parameter being a reflectivity. Brief description of the drawings

[0009] Other features, objectives and advantages of the present invention will become more apparent from the following detailed description and from the accompanying drawings, which are given by way of non-limiting examples and on which:

[0010] - Fig. 1 is a schematic overview view of a measuring device according to a possible embodiment of the invention;

[0011] - [Fig.2] is a top view of an example of a separator block according to a mode of possible realization of the invention;

[0012] - [Fig. 3] is a top view of an example of a separator block according to another possible embodiment of the invention.

[0013] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION

[0014] With reference to [Fig.1], the measuring device 1 includes an illumination arm 4 configured to generate a light beam of illumination 6, preferably polychromatic (as opposed to monochromatic illumination such as that produced by a laser), directed towards a surface 2 of the sample.

[0015] The implementation of a polychromatic illumination beam 6 makes it possible to perform chromatic confocal measurements and, in particular, to present axial chromaticism along the propagation axis, which allows for a measurement of distance without any movement of the measuring device 1. The illumination arm 4 includes a light source, and may in particular include a white light source, for example one or more light-emitting diodes. Furthermore, the illumination light beam 6 preferably has low coherence (spatial and temporal), as opposed to the laser beam which is spatially and temporally coherent.

[0016] The measuring device 1 includes an axially chromatic objective 8 configured to apply the illumination light beam 6 to the surface 2 of the sample. The axially chromatic objective 8 is also configured to reflect a backscattered light beam 9 from the surface 2 of the sample. The axially chromatic objective 8 comprises a series of lenses exhibiting known axial chromatic aberration, arranged within the objective such that their optical axes are coaxial, and which allow the chromatic dispersion of the illumination light beam 6 along its optical axis. This controlled optical aberration enables the encoding of the altitude or thickness of the sample surface 2 as a function of the wavelength in the reflected light beam 9.

[0017] The measuring device 1 comprises a beam splitter 20 configured to receive the illumination beam 6 through an entrance face 20a and transmit it to the axial chromatic objective 8 via a first path leading to a second face 20b. The second face 20b is here opposite the entrance face 20a, but could be another face, depending on the shape of the first path. The beam splitter is configured to receive the backscattered beam 9 from the sample surface 2, and to transmit the backscattered beam 9 to the measuring arm 10 via a second path.

[0018] The measuring arm 10 is configured to receive the backscattered light beam 9 and includes a measuring device 14 for the surface parameter 2 of the sample from the backscattered light beam 9. The measuring device 14 may be a spectral analyzer configured to determine a spectral distribution of the backscattered light beam 9, the surface parameter of the sample being a distance or an altitude. The measuring arm 10 may include an imager as a measuring device 14 configured to measure the light intensity of at least a portion of the backscattered light beam 9, the surface parameter 2 of the sample being a reflectivity.

[0019] The beam splitter block 20 includes a beam splitter 23 configured to subject a so-called reflected beam to a first and second reflection within the beam splitter 23. This reflected beam is one part of the illumination light beam 6 and the backscattered light beam 9, the other part of the reflected beam then taking an indirect path between the first and second paths. being a direct path free from reflection in the beam splitter. In the illustrated example, the beam splitter 23 is configured to partially reflect the light from the backscattered light beam 9 according to a first reflection, and to make the reflected light from the backscattered light beam 9 undergo a second reflection before the exit of the backscattered light beam 9 through an exit face 20c of the splitter block 20 towards the measuring arm 10.

[0020] Alternatively, for example by exchanging the positions of the illumination arm 4 and the measuring arm 10, the beam splitter 23 can be configured to reflect the light from the illumination light beam 6 coming from an input face 20c according to a first reflection, and to make the reflected light from the illumination light beam 6 undergo a second reflection before the exit of the illumination light beam 6 through an output face 20b of the splitter block 20 towards the axial chromatic objective 8.

[0021] The beam splitter block 20 also includes an optical compensator 24 disposed in the direct path and separated from the beam splitter 23 by a gap 30 along the direct path. In the illustrated example, the direct path is traversed by the illumination light beam 6, but the direct path could alternatively be traversed by the backscattered light beam 9.

[0022] In the example of [Fig. 2], the beam splitter 23 comprises a deflection prism, configured to deflect the light beam traveling the indirect path at an angle close to 90°, for example between 80° and 100°, relative to the direct path. In the illustrated example, this deflection prism is a pentaprism, which may, however, have only four sides, the fifth side being merely a chamfer created for ease of manufacture and not used optically. In the example of [Fig. 3], the beam splitter 23 comprises a first part 23a formed, for example, of a right prism with a right trapezoidal base, coupled to a second part 23b formed of a parallelepiped.

[0023] The optical compensator 24 has two faces parallel and perpendicular to the direct optical path. The optical compensator 24 typically has the shape of a rectangular parallelepiped, or cuboid. The beam splitter 23 and the optical compensator 24 are made of materials transparent to light in the range of wavelengths used, and are typically made of glass.

[0024] The inlet face 20a of the separator block 20 is provided with an opaque inlet mask 21a pierced with at least one inlet opening 22a through which the illumination light beam 6 enters. It is possible to provide several inlet openings 22a in the opaque inlet mask 21a in order to perform a multipoint measurement. The inlet openings 22a are preferably round. The inlet openings 22a are preferably aligned along a line, or even along several lines. However, in order to To avoid excessively increasing crosstalk between measurement points, the spacing between adjacent inlet apertures 22a is preferably greater than three times the diameter of the inlet apertures 22a, and preferably even greater than four times the diameter of the inlet apertures 22a. It is also possible for the inlet aperture 22a to be a continuous slit, in which case there is no longer any spatial filtering in the longitudinal direction in which the slit extends, but spatial filtering is retained for the transverse direction perpendicular to the longitudinal direction. In the examples in Figures 2 and 3, the opaque inlet mask 21a is fixed to the optical compensator 24, since it is the illumination beam 6 that follows the direct path. The second face 20b opposite the inlet face 20a is a face of the beam splitter 23.When it is the backscattered light beam 9 that travels the direct path, the opaque entrance mask can be fixed on a face 20c of the beam splitter 23.

[0025] In the examples illustrated and described below, the reflected beam is the backscattered light beam 9, and the indirect path is therefore the second path. The illumination light beam 6 travels along the direct path free of reflection in the beam splitter 23. Alternatively, the reflected beam can be the illumination light beam 6, and the indirect path can be the first path. The backscattered light beam can then travel along the direct path free of reflection in the beam splitter 23. Since light can travel the paths in both directions, the characteristics stated below apply to this alternative configuration, reversing the order of the light paths.

[0026] The illumination light beam 6, after entering the separator block 20 through the entrance opening 22a formed in the opaque entrance mask 21a, passes through the optical compensator 24 located in the direct path of the illumination light beam 6, and through the beam splitter 23. Preferably, the illumination light beam 6 does not undergo reflection in the separator block 20, nor in the beam splitter 23, nor in the optical compensator 24. It is, in fact, a direct path. The illumination light beam 6 exits the separator block 20 through the second face 20b, and enters the axial chromatic lens 8 to reach the surface 2 of the sample.The illuminating light beam 6 interacts there with the surface 2 of the sample, and a backscattered light beam 9 is generated, which, by the principle of the reverse return of light, takes the same optical path as the illuminating light beam 6, and therefore passes through the axial chromatic lens 8 to enter the separating block 20 through its second face 20b. .

[0027] The backscattered light beam 9 then enters the beam splitter 23 and encounters a semi-reflective surface 25, which is inclined with respect to the perpendicular to the direct path. In a known manner, this semi-reflective surface 25 is for example, formed by a deposit of thin metallic layers or thin layers of dielectric materials. Part of the light from the backscattered light beam 9 is reflected by this semi-reflective surface 25, thus forming the reflected beam. The light from the reflected backscattered light beam 9 therefore undergoes its first reflection on this semi-reflective surface 25. From this semi-reflective surface 25, the backscattered light beam 9 propagates through the beam splitter 23 until it encounters a reflective surface 26 to perform the second reflection of the backscattered light beam 9. The reflective surface 26 is, for example, formed by a metallic deposit on one face of the beam splitter 23. In this example, the first and second reflections preferably have angles of reflection less than or equal to 45° with respect to the normal, and preferably even less than 30°.However, it is possible to predict reflection angles with other values, provided that two reflections take place in the beam splitter 23.

[0028] The reflective surface 26 is inclined so as to direct the backscattered light beam 9 towards an exit face 20c of the beam splitter 23, which the reflected beam passes through perpendicularly. As in [Fig. 2], the semi-reflective surface 25 and the reflective surface 26 are faces of the pentaprism, and the backscattered light beam 9 intersects the illumination light beam 6 perpendicularly after the second reflection, and the exit face 20c is then perpendicular to the entrance face 20a. The backscattered light beam 9 then exits the beam splitter 23 perpendicularly to the illumination light beam 6.

[0029] In the example of [Fig. 3], the backscattered light beam 9 does not intersect the illuminating light beam 6 after its first reflection. The semi-reflective surface 25 separates the first part 23a from the second part 23b of the beam splitter 2. The backscattered light beam 9 does not enter the first part 23a, but propagates through the second part 23b until it encounters the reflective surface 26 formed on the face of the second part 23b, and undergoes the second reflection there. In this case, the semi-reflective surface 25 and the reflective surface 26 are inclined at 45° to the optical axis through which the direct path passes. The exit face 20c is then parallel to the entrance face 20a, and the backscattered light beam 9 then exits the beam splitter 23 parallel to the illuminating light beam 6.

[0030] The outlet face 20c of the separator block 23 is provided with an opaque outlet mask 21c pierced with at least one opening 22c through which the backscattered light beam 9 passes. The opaque outlet mask 21c has the same shape as the opaque inlet mask 21a, and can therefore similarly be provided with several openings 22c, which may be aligned, or be provided with a slit. The opaque mask The input mask 21a and the output opaque mask 21c form conjugate spatial filters. The output opaque mask 21c is fixed to one face of the beam splitter block 20, formed, for example, by one face of the pentaprism in [Fig. 2] and one face of the second part of the beam splitter 23 in [Fig. 3]. The masks 21a and 21c are typically fixed with a suitable adhesive.

[0031] More generally, depending on the configurations, one mask from the opaque input mask 21a and the opaque output mask 21c is fixed on the beam splitter 23, while the other mask from the opaque input mask 21a and the opaque output mask 21c is fixed on the optical compensator 24.

[0032] Thanks to the fact that the opaque inlet mask 21a and the opaque outlet mask 21c are fixed to the separator block 20, the positioning of these masks 21a, 21c is perfectly controlled, resulting in higher quality spatial filtering. Furthermore, the masks are integral with the separator block 20, and there is no risk of misalignment of the masks 21a, 21c. This is a significant improvement compared to devices where the masks would be mounted on another mechanical reference, for example, linked to optical transport fibers or a homogenizer.

[0033] The inlet face 20a, the second face 20b, and the outlet face 20c of the separator block 20 can advantageously be treated to be anti-reflective, for example with a deposition of thin films of dielectric materials.

[0034] Thanks to the two reflections experienced by the reflected light beam traveling the indirect path in the beam splitter 23, the measuring device 1 is much more robust to positioning imperfections of the beam splitter 23, and in particular makes it possible to render the characteristics of the light beams invariant under rotation. This results, on the one hand, in improved measurement quality since a source of defect is eliminated, and on the other hand, in easier manufacturing of the measuring device 1, since the positioning constraints are reduced.

[0035] However, the double reflection experienced by the reflected light beam traveling the indirect path in the beam splitter 23 means that the reflected beam travels a longer optical path than the light traveling the direct path in the beam splitter 23. The optical compensator 24, located in the direct path, compensates for the increase in the optical path length experienced by the reflected beam. The optical compensator 24 thus defines, for the direct path, an optical length corresponding to the optical length traveled by the reflected beam in the beam splitter 23. The optical length is the distance traveled by a light ray multiplied by the refractive index encountered by the ray during its path.To simplify, when the optical compensator 24 is in the same material as the beam splitter 23, the length of the optical compensator 24 along the optical path corresponds to the distance traveled by the reflected beam between its first. reflection, for example on the semi-reflective surface 25, and its exit through the exit face 21c.

[0036] The optical compensator 24 is distinct from the beam splitter 23 with respect to the light beams, in that the optical compensator 24 is separated from the beam splitter by a gap 30 along the direct path. The gap 30 is typically an air gap. To ensure mechanical stability between the beam splitter 23 and the optical compensator 24, the splitter block 20 includes a support plate 28 on which the beam splitter 23 and the optical compensator 24 are fixed. Preferably, the material of the support plate 28 has mechanical characteristics similar to those of the materials of the beam splitter 23 and the optical compensator 24, and in particular has a very similar coefficient of thermal expansion. Simply put, the support plate 28, the beam splitter 23, and the optical compensator 24 are made of the same material, typically glass.The beam splitter 23 and the optical compensator 24 can be glued to the support slab 28.

[0037] To further improve the mechanical strength of the elements of the beam splitter block 20, the beam splitter block 20 may include a locking plate attached to the beam splitter 23 and the optical compensator 24 on a side opposite the support plate 28, for example by bonding. The beam splitter 23 and the optical compensator 24 are then held in position by both the support plate 28 and the locking plate, thus ensuring excellent mechanical strength.

[0038] The positioning of the optical compensator 24 compensates for any manufacturing imperfections in the beam splitter 23. During assembly, the beam splitter 23 is fixed to the support plate 28. The optical compensator 24 is positioned on the support plate 28 in the direct path, with a layer of adhesive whose polymerization is controlled, for example, by exposure to ultraviolet light. Controllable actuators can hold the optical compensator 24 in place. Measurements are then taken, for example, with axially chromatic aberration objective devices placed on the second face 20b of the splitter block 20, preferably at several points on the faces of the splitter block 20. Initial measurements are first taken at the output face 20c by masking the direct path with a mask placed in the air gap 30, and then second measurements are taken at the output face 20c without masking in the air gap 30.By subtracting the first measurements from the second, we obtain third measurements. In both the first and third measurements, an intensity peak indicates the length of the optical path. The positioning of the optical compensator 24 is adjusted, for example using controllable actuators, to align the intensity peaks. Once this alignment is achieved, the position of the optical compensator 24 is fixed. thanks to bonding, for example by polymerizing the adhesive with ultraviolet light. The locking plate can further solidify the beam splitter block 20. A non-uniform space 30 between the beam splitter 23 and the optical compensator 24 allows for compensation of any geometric or positioning errors. This also ensures that the masks 21a and 21c are perfectly aligned.

[0039] The invention is defined by the claims and is not limited to the embodiments described and shown in the accompanying figures. Modifications remain possible, particularly with regard to the composition of the various elements or by substitution of technical equivalents, without departing from the scope of protection of the invention.

Claims

Demands

1. A chromatic confocal measuring device (1) for a parameter of a surface (2) of a sample, the device comprising: - an illumination arm (4) configured to generate an illumination light beam (6); - an axial chromatic lens (8) configured to apply the illumination light beam (6) to the surface (2) of the sample and to retransmit a backscattered light beam (9) from the surface (2) of the sample; - a measuring arm (10) configured for the backscattered light beam (9), the measuring arm (10) comprising a measuring apparatus (14) for the parameter of the surface (2) of the sample from the backscattered light beam (9);- a beam splitter block (20) configured to receive the illumination light beam (6) through an input face (20a) and transmit it to the axial chromatic objective (8) along a first path, receive in return the backscattered light beam (9) and transmit by a second path the backscattered light beam (9) to the measuring arm through an output face (20c); characterized in that the beam splitter block (20) comprises: - a beam splitter (23) configured to make a beam said to be reflected undergo a first reflection and a second reflection in the beam splitter (23) among the illumination light beam (6) and the backscattered light beam (9), the reflected beam then making an indirect path among the first path and the second path, the other path among the first path and the second path being a direct path free from reflection in the beam splitter;- a compensator (24) disposed in the direct path, the compensator (24) being separated from the beam splitter (23) by a space (30) along the direct path, the optical compensator (24) defining for the direct path an optical length corresponding to the additional optical length made by the beam reflected in the beam splitter (23) compared to the direct path in the beam splitter (23).

2. Device according to claim 1, wherein the splitter block (20) comprises a support plate (28) on which the beam splitter (23) and the optical compensator (24) are fixed.

3. Device according to claim 2, wherein the splitter block (20) includes a locking plate fixed to the beam splitter (23) and the optical compensator (24) on a side opposite to the support plate (28).

4. A device according to any one of the preceding claims, wherein the inlet face (20a) of the separator block (20) is provided with an opaque inlet mask (21a) pierced with at least one opening (22a), and the outlet face (20c) of the separator block (20) is provided with an opaque outlet mask (21c) pierced with at least one opening (22c), the opaque inlet and outlet masks forming conjugate spatial filters.

5. Device according to claim 4, wherein a mask among the opaque input mask (21a) and the opaque output mask (21c) is fixed on the beam splitter (23).

6. Device according to claim 4, wherein a mask among the opaque inlet mask (21a) and the opaque outlet mask (21c) is fixed on the optical compensator (24).

7. Device according to any one of the preceding claims, wherein the beam splitter (23) comprises a semi-reflective surface (25) for performing one reflection, and a reflective surface (26) for performing the other reflection of the reflected beam.

8. Device according to any one of the preceding claims, wherein the beam splitter (23) comprises a deflection prism within which the first reflection and the second reflection occur, the direct path intersecting the indirect path, and the exit face (20c) is perpendicular to the entrance face (20a).

9. Device according to any one of claims 1 to 7, wherein the indirect path is distinct from the direct path in the beam splitter (23), and the exit face (20a) is parallel to the inlet face (20a).

10. Device according to any one of the preceding claims, wherein the measuring apparatus (14) is a spectral analysis apparatus configured to determine a spectral distribution of the backscattered light beam (9), the sample surface parameter being a distance, or the measuring arm comprises an imager configured to measure a light intensity of at least a portion of the backscattered light beam (9), the sample surface parameter being a reflectivity.

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