Chromatic confocal measurement device with splitter block

Abstract

The invention relates to a device (1) for the chromatic confocal measurement of a parameter of a surface (2) of a sample, the device comprising: a light source (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 from the surface (2) of the sample to a measurement arm (10), wherein the measurement device comprises a splitter block (20) configured to direct two reflections in a beam splitter (23) to what is referred to as a reflected beam from among the illumination light beam (6) and the backscattered light beam (9), and wherein an optical compensator is (24) arranged in the direct path of the other beam.

Description

[0001] Confocal chromatic measuring device with separator block

[0002] TECHNICAL FIELD

[0003] The present application generally relates to the field of optical inspection and control of a part, and more specifically relates to a device for confocal chromatic measurement of a parameter of a surface of a sample.

[0004] STATE OF THE ART

[0005] The measuring device is based on the principle of chromatic confocal microscopy, in which light, usually 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 clear image is obtained on the surface, which makes it possible to deduce the distance between the objective and the surface. Other parameters of the surface can be deduced by looking at other characteristics of the reflected light beam.

[0006] Patent applications FR3086748 and FR3006758 give examples of devices for chromatic confocal measurement of a parameter of a surface of a sample. The devices of the prior art may use a splitter cube whose role is to direct the illumination light beam towards the sample, and to direct a measurement light beam derived from the light beam backscattered by the sample towards a measuring device. The splitter cube comprises a semi-reflecting surface which makes it possible to intercept a portion of the backscattered light beam, forming the measurement light beam, and to redirect it, typically perpendicularly, towards the measuring device.

[0007] However, it has become apparent that such a configuration requires a very precise orientation of the splitter cube relative to a direct path through it along which the illumination beam and the backscattered light beam propagate. Any deviation in orientation can make the characteristics of the light beams non-rotational, and can therefore significantly deteriorate the quality of the measurements.

[0008] Furthermore, the light beams that are conveyed by optical fibers to the input and output of the splitter cube must have their ends perfectly conjugated with each other. They must be spatially stable not only with each other, but also with the splitter cube, which can be difficult, or make the device fragile and sensitive to mechanical shocks. PRESENTATION OF THE INVENTION

[0009] An objective of the present application is therefore to propose a device for confocal chromatic measurement of a parameter of a surface of a sample, which is mechanically robust, and of a design allowing greater measurement precision thanks to manufacturing and adjustment with fewer constraints.

[0010] For this purpose, a device is proposed for confocal chromatic measurement of a parameter of a surface of a sample, the device comprising:

[0011] - a light source configured to generate an illumination light beam;

[0012] - 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;

[0013] - a measuring arm configured for the backscattered light beam, the measuring arm comprising a device for measuring the parameter of the surface of the sample from the backscattered light beam; the measuring device comprising a splitter block configured to receive the illumination light beam through an input face and transmit it to the axial chromatism objective along a first path, receive in return the backscattered light beam and transmit via a second path the backscattered light beam to the measuring arm; the splitter block comprising:

[0014] - a beam splitter configured to subject a so-called reflected beam from among the illumination light beam and the backscattered light beam to a first reflection and a second reflection in the beam splitter, 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; and

[0015] - a compensator disposed in the direct path, the compensator being separated from the beam splitter by a gap along the direct path, the optical compensator defining for the direct path an optical length corresponding to the additional optical length made by the reflected beam in the beam splitter compared to the direct path in the beam splitter. Some preferred but non-limiting features of the measuring device are the following, taken individually or in combination:

[0016] - the splitter block comprises a support plate on which the beam splitter and the optical compensator are fixed;

[0017] - the splitter block comprises a locking plate fixed to the beam splitter and the optical compensator on a side opposite the support plate;

[0018] - the input face of the splitter block is provided with an opaque input mask pierced with at least one opening, and the output face of the splitter block is provided with an opaque output mask pierced with at least one opening, the opaque input and output masks forming conjugate spatial filters;

[0019] - one mask among the input opaque mask and the output opaque mask is fixed on the beam splitter;

[0020] - one mask among the input opaque mask and the output opaque mask is fixed on the optical compensator;

[0021] - the beam splitter comprises a semi-reflecting surface for performing one reflection, and a reflecting surface for performing the other reflection of the reflected beam;

[0022] - the beam splitter comprises a deflection prism inside which the first reflection and the second reflection occur, the direct path intersecting the indirect path, and the output face is perpendicular to the input face;

[0023] - the indirect path is distinct from the direct path in the beam splitter, and the output face is parallel to the input face;

[0024] - the measuring device is a spectral analysis device configured to determine a spectral distribution of the backscattered light beam, the parameter of the surface of the sample 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, the parameter of the surface of the sample being a reflectivity.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Other characteristics, aims and advantages of the present invention will appear better on reading the detailed description which follows, and with regard to the appended drawings given as non-limiting examples and in which: - figure 1 is an overall schematic view of a measuring device according to a possible embodiment of the invention;

[0027] - figure 2 is a top view of an example of a separator block according to a possible embodiment of the invention;

[0028] - figure 3 is a top view of an example of a separator block according to another possible embodiment of the invention.

[0029] Throughout the figures, similar elements have identical references.

[0030] DETAILED DESCRIPTION

[0031] With reference to Figure 1, the measuring device 1 comprises an illumination arm 4 configured to generate an illumination light beam 6, preferably polychromatic (as opposed to monochromatic illumination such as that produced by a laser), intended for a surface 2 of the sample.

[0032] The implementation of a polychromatic illumination light beam 6 in fact makes it possible to carry out chromatic confocal measurements and in particular to present an axial chromatism along the propagation axis, which allows a distance measurement without any movement of the measuring device 1. The illumination arm 4 comprises a light source, and may in particular comprise a white light source, for example one or more light-emitting diodes. Furthermore, the illumination light beam 6 preferably has a low coherence (spatial and temporal), as opposed to the laser beam which is spatially and temporally coherent.

[0033] The measuring device 1 comprises an axial chromatism objective 8 configured to apply the illumination light beam 6 to the surface 2 of the sample. The axial chromatism objective 8 is also configured to retransmit a backscattered light beam 9 from the surface 2 of the sample. The axial chromatism objective 8 comprises a series of lenses having a known axial chromatism which are arranged in 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 allows the coding of the altitude or thickness of the surface 2 of the sample as a function of the wavelength in the reflected light beam 9.The measuring device 1 comprises a 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 opening onto a second face 20b. The second face 20b is here opposite the input face 20a, but could be another face, depending on the shape of the first path. The splitter block is configured to receive in return the backscattered light beam 9 coming from the surface 2 of the sample, and transmit the backscattered light beam 9 to the measuring arm 10 via a second path.

[0034] The measuring arm 10 is configured to receive the backscattered light beam 9, and comprises a measuring apparatus 14 for measuring the parameter of the surface 2 of the sample from the backscattered light beam 9. The measuring apparatus 14 may be a spectral analysis apparatus configured to determine a spectral distribution of the backscattered light beam 9, the parameter of the surface of the sample being a distance or an altitude. The measuring arm 10 may comprise an imager as a measuring apparatus 14 configured to measure a light intensity of at least a portion of the backscattered light beam 9, the parameter of the surface 2 of the sample being a reflectivity.

[0035] The splitter block 20 comprises a beam splitter 23 configured to subject a so-called reflected beam from among the illumination light beam 6 and the backscattered light beam 9 to a first reflection and a second reflection in the beam splitter 23, 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 of 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 subject the reflected light from the backscattered light beam 9 to a second reflection before the backscattered light beam 9 exits through an exit face 20c of the splitter block 20 towards the measuring arm 10.

[0036] 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 of the illumination light beam 6 coming from an input face 20c according to a first reflection, and to subject the reflected light of the illumination light beam 6 to a second reflection before the illumination light beam 6 exits through an output face 20b of the splitter block 20 towards the axial chromatic objective 8.

[0037] The splitter block 20 also comprises an optical compensator 24 arranged in the direct path, and separated from the beam splitter 23 by a space 30 along the direct path. In the example illustrated, the direct path is traveled by the illumination light beam 6, but the direct path could alternatively be traveled by the backscattered light beam 9.

[0038] In the example of Figure 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 only a chamfer created for manufacturing convenience and which is not used optically. In the example of Figure 3, the beam splitter 23 comprises a first part 23a formed for example of a right prism based on a rectangular trapezoid, coupled to a second part 23b formed of a parallelepiped.

[0039] 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 right block. The beam splitter 23 and the optical compensator 24 are formed from materials transparent to light in the wavelength range used, and are typically made of glass.

[0040] The input face 20a of the separator block 20 is provided with an opaque input mask 21a pierced with at least one input opening 22a through which the illumination light beam 6 penetrates. It is possible to provide several input openings 22a in the opaque input mask 21a, in order to carry out a multipoint measurement. The input openings 22a are preferably round. The input openings 22a are preferably aligned along a line, or even along several lines. However, in order to avoid excessively increasing the crosstalk between the measurement points, the pitch between adjacent input openings 22a is preferably greater than 3 times the diameter of the input openings 22a, and more preferably greater than 4 times the diameter of the input openings 22a.It is also possible for the input 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 the spatial filtering is retained for the transverse direction perpendicular to the longitudinal direction. In the examples of Figures 2 and 3, the opaque input mask 21a is fixed to the optical compensator 24, since it is the illumination light beam 6 which travels the direct path. The second face 20b opposite the input face 20a is a face of the beam splitter 23. When it is the backscattered light beam 9 which travels the direct path, the opaque input mask can be fixed to a face 20c of the beam splitter 23.

[0041] 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 the direct reflection-free path in the beam splitter 23. Alternatively, the reflected beam may be the illumination light beam 6, and the indirect path may be the first path. The backscattered light beam may then travel the direct reflection-free path in the beam splitter 23. Since light can travel the paths in both directions, the characteristics set out below apply to this alternative configuration, reversing the order of the light paths.

[0042] The illumination light beam 6, after having entered the splitter block 20 through the inlet opening 22a provided in the opaque inlet mask 21a, passes through the optical compensator 24 arranged in the direct path of the illumination light beam 6, and passes through the beam splitter 23. Preferably, the illumination light beam 6 does not undergo reflection in the splitter block 20: neither in the beam splitter 23, nor in the optical compensator 24. This is in fact a direct path. The illumination light beam 6 exits the splitter block 20 via the second face 20b, and enters the axial chromatism objective 8 to reach the surface 2 of the sample.The illumination 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 illumination light beam 6, and therefore passes through the axial chromatism objective 8 to penetrate into the separator block 20 via its second face 20b.

[0043] The backscattered light beam 9 then enters the beam splitter 23, and encounters a semi-reflecting surface 25, which is inclined relative to the perpendicular to the direct path. In a known manner, this semi-reflecting surface 25 is for example formed by a deposition of thin metal layers or thin thin layers of dielectric materials. A portion of the light of the backscattered light beam 9 is reflected by this semi-reflected surface 25, and then forms the reflected beam. The light of the reflected backscattered light beam 9 therefore undergoes its first reflection on this semi-reflecting surface 25. From this semi-reflecting surface 25, the backscattered light beam 9 propagates in the beam splitter 23 until it encounters a reflecting surface 26 to carry out the second reflection of the backscattered light beam 9.The reflecting surface 26 is for example formed by a metal deposit on one face of the beam splitter 23. In this example, the first reflection and the second reflection preferably have reflection angles less than or equal to 45° relative to the normal, and preferably still less than 30°. It is however possible to provide reflection angles with other values, since two reflections take place in the beam splitter 23.

[0044] The reflecting surface 26 is inclined so as to direct the backscattered light beam 9 towards an exit face 20c of the splitter block 23, which the reflected beam passes through perpendicularly. As in Figure 2, the semi-reflecting surface 25 and the reflecting 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 entry face 20a. The backscattered light beam 9 then exits the beam splitter 23 perpendicular to the illumination light beam 6.

[0045] In the example of Figure 3, the backscattered light beam 9 does not intersect the illumination light beam 6 after its first reflection. The semi-reflecting surface 25 separates the first portion 23a from the second portion 23b of the beam splitter 2. The backscattered light beam 9 does not enter the first portion 23a, but propagates in the second portion 23b until it encounters the reflecting surface 26 formed on the face of the second portion 23b, and undergoes the second reflection there. In this case, the semi-reflecting surface 25 and the reflecting surface 26 are inclined by 45° relative to the optical axis through which the direct path passes. The exit face 20c is then parallel to the entry face 20a, and the backscattered light beam 9 then exits the beam splitter 23 parallel to the illumination light beam 6.

[0046] The output face 20c of the splitter block 23 is provided with an opaque output mask 21c pierced with at least one opening 22c through which the backscattered light beam 9 passes. The opaque output mask 21c has the same conformation as the opaque input mask 21, and can therefore be similarly provided with several openings 22c, which can be aligned, or be provided with a slit. The opaque input mask 21a and the opaque output mask 21c form conjugate spatial filters. The opaque output mask 21c is fixed to a face of the splitter block 20, formed for example by a face of the pentaprism in Figure 2 and a face of the second part of the beam splitter 23 in Figure 3. The fixing of the masks 21a, 21c is typically carried out with a suitable glue.More generally, depending on the configurations, one mask among the opaque input mask 21a and the opaque output mask 21c is fixed on the beam splitter 23, while the other mask among the opaque input mask 21a and the opaque output mask 21c is fixed on the optical compensator 24.

[0047] Due to the fact that the input opaque mask 21a and the output opaque mask 21c are fixed to the splitter block 20, the positioning of these masks 21a, 21c is perfectly controlled, which results in higher quality spatial filtering. In addition, the masks are integral with the splitter block 20, and there is no risk of misalignment of the masks 21a, 21c. This is a great improvement over devices where the masks would be carried on another mechanical reference, for example linked to optical transport fibers or a homogenizer.

[0048] The input face 20a, the second face 20b, and the output face 20c of the separator block 20 can advantageously be anti-reflective treated, for example with a deposition of thin layers of dielectric materials.

[0049] Thanks to the two reflections undergone by the reflected light beam traveling the indirect path in the beam splitter 23, the measuring device 1 is much more robust to the imperfections of positioning of the beam splitter 23, and in particular makes it possible to make the characteristics of the light beams invariant by rotation. This results on the one hand in an improvement in the quality of the measurements 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 alleviated.

[0050] However, the double reflection experienced by the reflected light beam traveling the indirect path in the beam splitter 23 implies that the reflected beam travels a longer optical path than the light on the direct path in the beam splitter 23. The optical compensator 24, arranged in the direct path, makes it possible to compensate for the increase in the optical path 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 that the ray encountered during its path.To simplify, when the optical compensator 24 is made of 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-reflecting surface 25, and its exit through the exit face 21c.

[0051] The optical compensator 24 is distinct from the beam splitter 23 from the point of view of the light beams, in that the optical compensator 24 is separated from the beam splitter by a space 30 along the direct path. The space 30 is typically an air gap. In order to guarantee mechanical stability between the beam splitter 23 and the optical compensator 24, the splitter block 20 comprises 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 close coefficient of thermal expansion. To put it simply, 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 plate 28.

[0052] In order to further improve the mechanical strength of the elements of the splitter block 20, the splitter block 20 may comprise a locking plate fixed to the beam splitter 23 and to the optical compensator 24 on a side opposite the support plate 28, for example by gluing. The beam splitter 23 and the optical compensator 24 are then held in position by both the support plate 28 and by the locking plate, thus ensuring excellent mechanical strength.

[0053] The positioning of the optical compensator 24 makes it possible to compensate 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 arranged on the support plate 28 in the direct path, with a layer of glue whose polymerization is controlled, for example via exposure to ultraviolet light. Controllable actuators can hold the optical compensator 24. Measurements are then made for example with axial chromatism 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. First measurements are first collected at the output face 20c by masking the direct path with a mask placed in the air gap 30, then second measurements are collected at the output face 20c without masking in the air gap 30.By taking the difference between the second measurements and the first measurements, third measurements are obtained. In the first measurements and the third measurements, an intensity peak indicates the length of the optical path. The positioning of the optical compensator 24 is modified, for example using controllable actuators, in order to make the intensity peaks coincide. Once this coincidence is achieved, the position of the optical compensator 24 is made definitive by gluing, for example by polymerization of the glue by ultraviolet radiation. The locking plate can further solidify the splitter block 20. It is thus possible, by a space 30 of non-uniform size between the beam splitter 23 and the optical compensator 24, to compensate for any geometric or positioning defects. This also ensures that the masks 21a, 21c are perfectly conjugated.

[0054] The invention is defined by the claims, and is not limited to the embodiments described and shown in the attached figures. Modifications remain possible, in particular from the point of view of the constitution of the various elements or by substitution of technical equivalents, without departing from the scope of protection of the invention.

Claims

CLAIMS 1. Device (1) for confocal chromatic measurement of 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 objective (8) configured to apply the illumination light beam (6) to the surface (2) of the sample and to retransmit a backscattered light beam (9) coming 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 device (14) for the parameter of the surface (2) of the sample from the backscattered light beam (9); - a 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 splitter block (20) comprises: - a beam splitter (23) configured to subject a so-called reflected beam from among the illumination light beam (6) and the backscattered light beam (9) to a first reflection and a second reflection in the beam splitter (23), 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) arranged in the direct path, the compensator (24) being separated from the beam splitter (23) by a gap (30) along the direct path, the optical compensator (24) defining for the direct path an optical length corresponding to the additional optical length carried out by the reflected beam in the beam splitter (23) compared to the direct path in the beam splitter (23).

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

4. Device according to one of the preceding claims, in which the input face (20a) of the separator block (20) is provided with an opaque input mask (21a) pierced with at least one opening (22a), and the output face (20c) of the separator block (20) is provided with an opaque output mask (21c) pierced with at least one opening (22c), the opaque input and output masks forming conjugate spatial filters.

5. Device according to claim 4, wherein one of the opaque input mask (21a) and the opaque output mask (21c) is fixed on the beam splitter (24).

6. Device according to claim 4 or 5, wherein one of the opaque input mask (21a) and the opaque output mask (21c) is fixed on the optical compensator (24).

7. A device according to any preceding claim, wherein the beam splitter (23) comprises a semi-reflecting surface (25) for effecting one reflection, and a reflecting surface (26) for effecting the other reflection of the reflected beam.

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

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

10. Device according to any one of the preceding claims, in which the measuring apparatus (14) is a spectral analysis apparatus configured to determine a spectral distribution of the backscattered light beam (9), the parameter of the sample surface 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 parameter of the sample surface being a reflectivity.

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