METHOD FOR REFOCUSING AN OPTICAL ASSEMBLY

Abstract

A method refocuses on an optical assembly target surface, using at least one beam originating from a short-pulse optical source, having at least one optical system for focusing the beam on the surface. Refocusing occurs after learning reference conditions for which the assembly is considered as focused. A focusing signal is detected representing a time overlap of the pulses between a beam reflected and a reference beam not reflected by the surface and comes from the source, one of the beams delayed by a delay line, the beam optical path on which the delay line is placed is varied, on the basis of the reference conditions, to cause the focusing signal to reach or go beyond a predetermined threshold. The focus is adjusted on the basis of variation knowledge in the path between the reference conditions and the conditions for which the focusing signal reaches or goes beyond the threshold.

Claims

1-24. (canceled)

25. A method for refocusing an optical setup on a target surface, using at least one beam originating from a short-pulse optical source, comprising at least one optic for focusing the beam on the target surface, the refocusing being applied after knowing reference conditions for which the optical setup is considered to be focused, method wherein: a focusing signal is detected that is representative of a temporal overlapping of the pulses between a beam reflected by the target and a reference beam not reflected by the target surface and deriving from the source, one of the beams being delayed by a delay line, based on said reference conditions, the optical path of the beam on which the delay line is placed is made to vary so as to cause said focusing signal to reach or exceed a predefined threshold, and the focusing is readjusted on the basis of the knowledge of the variation of optical path between the reference conditions and the conditions for which the focusing signal reaches or exceeds said predefined threshold.

26. The method as claimed in claim 25, being applied to the analysis of a sample defining said target surface, using a probe beam and a pump beam, of which at least one originates from the short-pulse optical source, comprising at least one delay line placed on the trajectory of one of the beams, and at least one optic for focusing the pump and probe beams on the sample to be analyzed, the focusing signal representative of the temporal overlapping of the pulses being detected between a beam reflected by the sample, delayed by the delay line, and a reference beam not reflected by the sample.

27. The method as claimed in claim 25, wherein the optical path of the beam on which the delay line is placed is varied by moving the delay line relative to the target surface, or by moving the target surface relative to the delay line, notably using a mobile sample-holder on which the target surface is arranged.

28. The method as claimed in claim 25, wherein said predefined threshold is equal to zero.

29. The method as claimed in claim 25, wherein said predefined threshold is strictly greater than zero.

30. The method as claimed in claim 25, wherein the focusing signal representative of a temporal overlapping of the pulses is detected on the basis of the cross-correlation between a beam reflected by the target surface, and a reference beam not reflected by the target surface and deriving from the or one of the sources, one of the beams being delayed by the delay line.

31. The method as claimed in claim 25, wherein the optical setup comprises a splitter element for splitting the beam originating from the optical source in order to create the beam reflected by the target surface and the reference beam not reflected by the target surface.

32. The method as claimed in claim 26, wherein the optical analysis setup comprises a single optical source and a splitter element for splitting the beam originating from said optical source in order to create the pump and probe beams.

33. The method as claimed in claim 26, wherein the optical analysis setup comprises two optical sources respectively emitting the pump and probe beams.

34. The method as claimed in claim 26, wherein the delay line is placed on the trajectory of the pump beam.

35. The method as claimed in claim 26, wherein the reference beam not reflected by the sample is the pump beam not delayed by the delay line, or the probe beam or, in the case where the optical analysis setup comprises a single source and a splitter element, a beam deriving from the optical source captured before said splitter element.

36. The method as claimed in claim 25, wherein the focusing is readjusted by moving the focusing optic or optics relative to the target surface, or by moving the target surface relative to the focusing optic or optics, notably using a mobile sample-holder on which the target surface is arranged.

37. The method as claimed in claim 25, wherein the reference conditions correspond to a state wherein the focusing optic or optics are considered to be focused on a reference target surface, a reference length of the optical path between the delay line and the target surface being determined from said focusing state.

38. The method as claimed in claim 37, wherein prior determination of the reference length is performed with a reference target surface other than that to be used, or with the target surface but at a reference point other than the point of the target surface having to be used.

39. The method as claimed in claim 25, wherein the length of the optical path between the delay line and the target surface for which the focusing signal reaches said predefined threshold is determined.

40. The method as claimed in claim 39, wherein the focusing is readjusted by a value dependent on the difference between said length and said reference length such that the focusing optic or optics are focused on the target surface.

41. A refocusing device intended to implement the method for refocusing an optical setup on a target surface as claimed in claim 25, said optical setup using at least one beam originating from a short-pulse optical source, comprising at least one optic for focusing the beam on the target surface, the device comprising a means for detecting a focusing signal representative of a temporal overlapping of the pulses between a beam reflected by the target surface, and a reference beam not reflected by the target surface and deriving from the source, one of the beams being delayed by a delay line, the device being configured to: vary, based on reference conditions for which the optical setup was considered to be focused, the optical path of the beam on which the delay line is placed so as to cause said focusing signal to reach or exceed a predefined threshold, and readjust the focusing based on the knowledge of the variation of optical path between said reference conditions and those for which the focusing signal reaches said predefined threshold.

42. The device as claimed in claim 41, being applied to the analysis of a sample defining said target surface, wherein the optical analysis setup uses a probe beam and a pump beam, of which at least one originates from the short-pulse optical source, comprises at least one delay line placed on the trajectory of one of the beams, and at least one optic for focusing the pump and probe beams on the sample to be analyzed, the focusing signal representative of the temporal overlapping of the pulses being detected between a beam reflected by the sample, delayed by the delay line, and a reference beam not reflected by the sample.

43. The device as claimed in claim 41, wherein the means for detecting said focusing signal comprises a nonlinear crystal or a two-photon photodiode.

44. The device as claimed in claim 41, wherein said at least one optical source is a short pulse laser, notably of between 10 fs and 10 ps.

45. The device as claimed in claim 41, wherein the optical delay line comprises a mirror and/or one or more total reflection prisms and/or a retro-reflector and/or an electro-optical modulator, notably borne by a mobile carriage.

46. The device as claimed in claim 41, wherein the optical delay line is incorporated in the means for detecting said focusing signal.

47. An assembly intended to implement a method for refocusing a target surface of an optical setup, using at least one beam originating from a short-pulse optical source, comprising at least one optic for focusing the beam on the target surface, the refocusing being applied after knowing reference conditions for which the optical setup is considered to be focused, method wherein: a focusing signal is detected that is representative of a temporal overlapping of the pulses between a beam reflected by the target and a reference beam not reflected by the target surface and deriving from the source, one of the beams being delayed by a delay line, based on said reference conditions, the optical path of the beam on which the delay line is placed is made to vary so as to cause said focusing signal to reach or exceed a predefined threshold, and the focusing is readjusted on the basis of the knowledge of the variation of optical path between the reference conditions and the conditions for which the focusing signal reaches or exceeds said predefined threshold, the assembly comprising: the refocusing device as claimed in claim 41, comprising at least one delay line and at least one optic for focusing beams on the target surface, and a reference target surface for the prior determination of said reference conditions for which the optical setup is considered to be focused on said reference target surface.

48. The assembly as claimed in claim 47, wherein the reference target surface is defined by a reference sample comprising a metal layer, notably of aluminum, and at least one layer of another material, notably silicon or glass, and is observed under normal incidence for the prior determination of said reference conditions.

Description

[0069] The invention will be able to be better understood on reading, the following detailed description, of nonlimiting exemplary implementations thereof, and on studying the attached drawing, in which:

[0070] FIG. 1, already described, represents a pump-probe optical setup according to the prior art,

[0071] FIG. 2 represents a device according to the invention for focusing an optical setup,

[0072] FIG. 3 represents a device according to the invention for focusing a pump-probe optical setup,

[0073] FIG. 4 represents the detection means according to the invention of the device of FIG. 3,

[0074] FIG. 5 illustrates steps in implementing the method according to the invention, and

[0075] FIG. 6 represents timing diagrams of signals obtained by applying the method according to the invention,

[0076] A device 1 for focusing a target surface 100 of an optical setup, intended to implement the refocusing method according to the invention, is represented in FIG. 2.

[0077] The optical setup uses a beam f.sub.u originating from an optical source 2. The optical source 2 is advantageously a short pulse laser, for example with a duration substantially equal to 100 fs, and tunable to wavelengths of between 680 nm and 1050 nm, which corresponds to the near infrared.

[0078] The optical setup comprises an optic 5 for focusing the beam f.sub.u on the target surface 100, and a delay line 14, incorporated in a detection means 17 described hereinbelow.

[0079] Prior to the implementation of the refocusing method according to the invention, a reference length d.sub.1 of the optical path between the delay line 14 and the target surface has been determined, in reference conditions in which the focusing optic 5 is considered to be focused on a reference target surface.

[0080] The prior determination of the reference length d.sub.1 may be performed with a reference target surface other than that to be used, or with the target surface 100 but in a reference area other than that having to be targeted.

[0081] A reference beam f.sub.ref not reflected by the target surface 100 is obtained by the arrangement of a splitter element 10 to capture a part of the beam f.sub.u and send it to a means 17 for detecting a focusing signal, using mirrors 18 and 19. The beam f.sub.r reflected by the target surface 100 and deriving from the beam f.sub.u is also sent to the means 17 for detecting a focusing signal. The reference beam f.sub.ref is delayed by the delay line 14.

[0082] A device 1 for focusing an optical setup for analyzing a sample 100 defining a target surface, intended to implement the method according to the invention, is represented in FIG. 3.

[0083] The sample 100 comprises, for example, a layer of aluminum, with a thickness equal to 10 nm, a layer of silicon nitride, with a thickness equal to 200 nm, and a layer of silicon, having the composition Al/SiN/Si. The invention is not however limited to a particular type of sample to be analyzed.

[0084] The optical analysis setup uses a pump beam f.sub.p and a probe beam f.sub.s originating from an initial beam f.sub.i emitted by a single optical source 2 and split into two pump and probe beams by a splitter element 3. In a variant not illustrated, the optical analysis setup comprises two optical sources respectively emitting the pump f.sub.p and probe f.sub.s beams.

[0085] A frequency doubler, not represented, may be arranged after the splitter element 3 in order to deliver, for the probe, pulses that may be tuned to wavelengths of between 350 nm and 520 nm, corresponding to the blue, without modifying the duration of the pulses. The invention is not however limited to a particular type of optical source or to particular types of pump and probe beams.

[0086] The optical analysis setup comprises a delay line 4 placed on the trajectory of the pump beam f.sub.p and a focusing optic 5 making it possible for the two pump f.sub.p and probe f.sub.s beams to be recombined on the sample to be analyzed 100. In a variant not illustrated, the optical analysis setup comprises a different focusing optic for each pump f.sub.p and probe f.sub.s beam. The focusing optic or optics 5 are for example lenses of 50 mm AC254-50-B or 60 mm Achro MG type.

[0087] Prior to the implementation of the refocusing method according to the invention, a reference length d.sub.1 of the optical path between the delay line 4 and the sample has been determined, in reference conditions in which the focusing optic 5 is considered to be focused on a reference sample.

[0088] The prior determination of the reference length d.sub.1 may be performed with a reference sample other than that to be analyzed, or with the sample to be analyzed 100 but in a reference area other than that having to be analyzed.

[0089] A reference probe beam f.sub.ref not reflected by the sample 100 is obtained by the arrangement of a splitter element 8 to capture a part of the probe beam and send it to a means 7 for detecting a focusing signal. The probe beam fs.sub.r reflected by the sample 100 is captured, in the example described, by a photodiode 6.

[0090] As may be seen in FIGS. 2 and 4, the means 7, 17 for detecting the focusing signal is advantageously a two-photon photodiode, for example of silicon carbide (SiC), particularly advantageous in the case of a pump-probe setup, when the wavelengths of the pump f.sub.p and probe f.sub.s beams are different.

[0091] In some variants, the means 7 for detecting the focusing signal is a two-photon photodiode of gallium arsenide phosphide (GaAsP) or gallium phosphide (GaP), or comprises a nonlinear crystal, for example a crystal of barium beta borate (BBO). The invention is not however limited to a particular type of means for detecting said focusing signal.

[0092] In the examples described, the delay line 4, 14 comprises a mirror borne by a mobile carriage. In some variants, the delay line 4, 14 comprises one or more total reflection prisms and/or a retro-reflector and/or an electro-optical modulator.

[0093] A microcontroller electronic circuit, not represented, may be present to drive the movements of the different mobile elements of the optical setup, notably the delay line 4, 14.

[0094] The refocusing method according to the invention may be implemented automatically using a control loop, an example of which is represented in FIG. 5.

[0095] Upon a change of target surface or change of observed area, a change represented by the step 11 in FIG. 5, a new focusing of the optical setup is required.

[0096] The delay line 4, 14 is initially placed at the value corresponding to d.sub.1−d, in the step 12. The value d is the maximum offset between the delay line 4, 14 and the target surface 100 permitted by the optical analysis setup, and d may be between 0 and 25 mm. The delay line 4, 14 is progressively moved from the position corresponding to the optical path d.sub.1−d, in a step 13, in order to increase the optical path of the beam on which the delay line 4, 14 is placed.

[0097] On each movement increment, in a step 14, a detection of the focusing signal representative of a temporal overlapping of the pulses between the beam f.sub.r reflected by the sample 100, and the reference beam f.sub.ref, not reflected by the sample 100, is performed. The length of the optical path is incremented by moving the delay line until a focusing signal above a predefined threshold is detected.

[0098] In a step 15, the corresponding length d.sub.2 is stored. In a variant not illustrated, the target surface 100 is moved relative to the delay line 4, 14 in order to make vary the optical path of the beam, for example using a sample-holder on which the target surface is arranged.

[0099] The detection of the focusing signal is done advantageously, as described above, by the cross-correlation between the beam f.sub.r reflected by the sample 100, and the reference beam f.sub.ref, not reflected by the sample 100.

[0100] In a step 16, the delay line 4, 14 is repositioned at the reference zero, which corresponds to the reference length d.sub.1 of the optical path.

[0101] In a step 17, the focusing is readjusted by moving the focusing, optic 5 by a value d.sub.2−d.sub.1, such that the focusing optic is once again correctly focused on the target surface 100.

[0102] The focusing may be readjusted by moving the focusing optic 5 relative to the target surface 100 by the value d.sub.2−d.sub.1. In a variant, the focusing is readjusted by moving the target surface 100 relative to the focusing optic 5, for example by moving the sample-holder on which the target surface is arranged, the focusing optic 5 remaining fixed.

[0103] In the case where the optical setup is configured for one of the beams to pass several times through the delay line 4, 14, the focusing readjustment value depends on the number of go and return passes.

[0104] The analysis of the sample defining the target surface 100, or the machining of the target surface 100, may then begin in the step 18.

[0105] FIG. 6 represents timing diagrams of signals obtained by applying the method according to the invention in the case of an optical analysis setup of the pump-probe type.

[0106] The timing diagram 6(a) represents an example of focusing signals obtained by cross-correlation between the pump beam f.sub.r reflected by the sample 100 and delayed by the delay line 4, and a reference probe beam f.sub.ref, not reflected by the sample 100. The timing diagram 6(b) represents a smoothed curve passing through the measured values.

[0107] The timing diagrams 6(c) and 6(d) represent different overlappings according to the movement of the delay line 4.

[0108] A kit comprising the refocusing device according to the invention and a reference sample for the prior determination of the reference conditions may be proposed.

[0109] The expression “comprising a” should be understood to mean “comprising at least one”, unless specified otherwise.