WO2022233692A1

WO2022233692A1 - Transmission waveplate

Info

Publication number: WO2022233692A1
Application number: PCT/EP2022/061301
Authority: WO
Inventors: Jérôme NEAUPORT; Nicolas Bonod; Pierre Brianceau
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives; Centre National De La Recherche Scientifique; Universite D'aix Marseille; Ecole Centrale De Marseille
Priority date: 2021-05-03
Filing date: 2022-04-28
Publication date: 2022-11-10
Also published as: FR3122502B1; FR3122502A1

Abstract

Waveplate (1), operating in transmission, for modifying a polarization state of electromagnetic radiation possessing a dominant wavelength, denoted λ, comprised between 200 nm and 400 nm; the waveplate (1) comprising: - a substrate (2) that is transparent to the electromagnetic radiation and that comprises first and second opposite surfaces (20, 21) that are intended to be successively passed through by the electromagnetic radiation, which is preferably incident at normal incidence, the second surface (21) being planar; - features (200) produced in a dielectric that is transparent to the electromagnetic radiation, the dielectric having a refractive index, denoted n, lower than or equal to 1.6 at the dominant wavelength, the features (200) forming a transmission grating in the first surface (20) of the substrate (2), the features (200) extending in a first longitudinal direction (X), the features (200) being spaced apart periodically in a second longitudinal direction (Y) perpendicular to the first longitudinal direction (X), and with a spatial period, denoted p, respecting p < λ, the features (200) having a height, denoted h, along the normal (Z) to the first surface (20) respecting h ≥ 3 p, the height of the features (200) being adjusted depending on the spatial period of the features (200) so that the transmission grating shifts the phase of the electromagnetic radiation by π or π/2 in order to modify the polarization state.

Description

WAVE PLATE IN TRANSMISSION

Technical area

The invention relates to the technical field of waveplates, operating in transmission, to modify a state of polarization of electromagnetic radiation having a dominant wavelength between 200 nm and 900 nm.

The invention finds application in particular in the field of power lasers.

State of the art

A wave plate known from the state of the art comprises:

- a substrate, transparent to electromagnetic radiation, and comprising first and second opposite surfaces, intended to be traversed successively by the electromagnetic radiation, preferably at normal incidence;

- patterns, made of a dielectric material transparent to electromagnetic radiation, the material having a refractive index, denoted n, strictly greater than 1.6 at the dominant wavelength, the patterns forming a diffraction grating in transmission at the first surface of the substrate, the patterns extending along a first longitudinal direction; the patterns being periodically spaced along a second longitudinal direction, perpendicular to the first longitudinal direction, and according to a spatial period, denoted p, satisfying p<1; the patterns having a height, denoted h, along the normal to the first surface; the height of the patterns being adjusted according to the spatial period of the patterns so that the diffraction grating in transmission introduces a phase shift of p or p/2 to the electromagnetic radiation to modify the state of polarization.

For example, a so-called quarter-wave plate, introducing a phase shift of p/2, makes it possible to transform a state of linear polarization into a state of circular polarization.

The patterns, forming a transmission diffraction grating on the first surface of the substrate, make it possible to induce anisotropy. In other words, the transmission diffraction grating makes it possible to phase-shift two orthogonal components of a polarization state. The first longitudinal direction (between patterns) is the fast axis, while the second longitudinal direction is the slow axis.

The dielectric material of the patterns is chosen in the state of the art to present a high optical contrast (n>1.6; preferentially n>2; more preferentially n>3) with the surrounding medium. The surrounding environment is typically ambient air, or a medium under air vacuum, with a refractive index close to 1. The phase shift results from an electromagnetic interaction between the incident electromagnetic radiation and the diffraction grating in transmission, and depends on the optical contrast and the geometry of the periodic patterns.

There is a prejudice according to which the dielectric material of the patterns must have a high refractive index (n >1.6; preferably n >2; more preferably n > 3) in order to reduce the height h and therefore the ratio h/p for engraving convenience. This prejudice is manifest in particular in the documents DC Flanders, “Submicrometerperiodiâty gratings as artificial anisotropic dielectrics”, Appl. Phys. Lett. 42, 492 (1983); T.Isano et al., “Fabrication of Hafl-Wave Plates with Subwavelength Structures”, Jpn. J.Appl. Phys. 43, 5294 (2004); N. Passilly et al., “Polarization conversion in conical diffraction b _j metallic and dielectric subwavelength gratings”, Appl. Opt. 46(20) 4258-4265 (2007); SS Stafeev et al., “Subwavelength gratings for polarization conversion and focusing of laser light”, Photonics Nanostructures: Fundam. appl. 27, 32-41 (2017); T. Mori et al., “Periodic sub-wavelength structures with large phase retardation fabricated b _j glass nanoimprint”,]. Ceramic. Soc. Jpn. 117(1370), 1134-1137 (2009).

Such a wave plate of the state of the art is not entirely satisfactory insofar as the choice of the pattern material is restricted to dielectric materials having a high refractive index (n > 1.6). However, these materials generally have a very low damage threshold to withstand high power laser radiation (i.e. peak power greater than 100 mW), for example pulsed lasers.

Disclosure of Invention

The invention aims to remedy all or part of the aforementioned drawbacks. To this end, the subject of the invention is a wave plate, operating in transmission, to modify a state of polarization of electromagnetic radiation having a dominant wavelength, denoted l, between 200 nm and 900 nm; the wave plate comprising:

- a substrate, transparent to electromagnetic radiation, and comprising first and second opposite surfaces, intended to be traversed successively by the electromagnetic radiation, preferably at normal incidence; the second surface being planar;

- patterns, made of a dielectric material transparent to electromagnetic radiation, the material having a refractive index, denoted n, less than or equal to 1.6 at the dominant wavelength, the patterns forming a diffraction grating in transmission at the first surface of the substrate, the patterns extending along a first longitudinal direction; the patterns being periodically spaced along a second longitudinal direction, perpendicular to the first longitudinal direction, and according to a spatial period, denoted p, satisfying p<1; the patterns having a height, denoted h, following the normal to the first surface verifying h>3 p; the height of the patterns being adjusted as a function of the spatial period of the patterns so that the diffraction grating in transmission introduces a phase shift of p or p/2 to the electromagnetic radiation to modify the state of polarization.

An object of the invention is a wave plate, operating in transmission, to modify a state of polarization of electromagnetic radiation having a dominant wavelength, denoted l, between 200 nm and 400 nm; the wave plate comprising:

- patterns, made of a dielectric material transparent to electromagnetic radiation, the material having a refractive index, denoted n, less than or equal to 1.6 at the dominant wavelength, the patterns forming a diffraction grating in transmission at the first surface of the substrate, the patterns extending along a first longitudinal direction; the patterns being periodically spaced along a second longitudinal direction, perpendicular to the first longitudinal direction, and according to a spatial period, denoted p, satisfying p<1; the patterns having a height, denoted h, following the normal to the first surface verifying h>3 p; the height of the patterns being adjusted according to the spatial period of the patterns and verifies respectively 6 p £ h £ 12 p or 3 p £ h £ 6 p so that the diffraction grating in transmission introduces a phase shift of p or p/ 2 to electromagnetic radiation to change the state of polarization.

Definitions

- By "substrate" is meant a self-supporting physical medium.

- By "transparent substrate", we mean that the substrate has a transmission coefficient in intensity (or transmittance), averaged for the 2 polarizations TE (Transverse Electric) and TM (Transverse Magnetic), greater than 80%, preferably greater than 85 %, more preferably greater than 90%, even more preferably greater than 95% for the dominant wavelength of the electromagnetic radiation. - By "transparent dielectric material", it is meant that the transmission diffraction grating, formed by the patterns produced in the dielectric material, has an intensity transmission coefficient (or transmittance) of the zero diffraction order (called specular order ), averaged for the 2 TE and TM polarizations, greater than 80%, preferably greater than 85%, more preferably greater than 90%, even more preferably greater than 95%, for the dominant wavelength of the electromagnetic radiation.

- “it or i/2” means the phase shift in radians.

Thus, such a wave plate according to the invention allows a high damage threshold to resist high power laser radiation (peak power greater than 100 mW), thanks to the dielectric material of the patterns which has a lower refractive index or equal to 1.6. The inventors have found, surprisingly, that it is possible to carry out a correct etching of the patterns so as to obtain a high h/p ratio (³3) according to techniques compatible with CMOS technology ("Complementary Metal Oxide S emiconductor in English).

When the dominant wavelength is between 200 nm and 400 nm, with a dielectric material of the patterns which has a refractive index less than or equal to 1.6, the height of the patterns is adjusted according to the spatial period of the patterns and check:

- 6 p £ h £ 12 p so that the diffraction grating in transmission introduces a phase shift of p;

- 3 p £ h £ 6 p so that the diffraction grating in transmission introduces a phase shift of p/ 2.

When h > 12 p (resp. h> 6p) for a phase shift of p (resp. p/2), a strong degradation of the tolerance to manufacturing errors of the waveplate was observed, in particular on the width patterns, as well as an unfavorable electric field distribution for laser damage performance.

The wave plate according to the invention may comprise one or more of the following characteristics.

According to a characteristic of the invention, the height of the patterns verifies:

3 p £ h £ 6 p, preferably 4 p £ h £ 5 p, so that the diffraction grating in transmission introduces a phase shift of p/2 to the electromagnetic radiation to modify the state of polarization. The "4 p £ h £ 5 p" relationship allows for improved manufacturing error tolerance of the waveplate, as well as better electric field distribution for laser damage performance.

6 p £ h £ 12 p, preferably 8 p £ h £ 10 p, so that the diffraction grating in transmission introduces a phase shift of p to the electromagnetic radiation to modify the state of polarization.

The “8p£h£10p” relationship improves the manufacturing error tolerance of the waveplate, as well as achieving better electric field distribution for laser damage performance.

According to a characteristic of the invention, the spatial period of the patterns verifies p £ l/h.

Thus, an advantage obtained is to avoid the dispersion of the energy of the electromagnetic radiation in several orders of diffraction. Such a spatial period makes it possible to structure the first surface of the substrate (when the substrate and the patterns are in one piece) without inducing the propagation of new diffraction orders. The phase shift brought to the zero order of diffraction in transmission is then associated with high energy efficiency. In other words, such a spatial period of the patterns achieves high transmission efficiency in the specular order for the waveplate, so as to obtain a high transmission waveplate.

According to one characteristic of the invention, the patterns have a width at mid-height, denoted c, in the second longitudinal direction verifying:

£0.5p c£0.7p

Thus, an advantage obtained is to authorize etching of the patterns according to techniques compatible with CMOS technology.

According to one characteristic of the invention, the patterns have a trapezoidal profile following the normal to the first surface.

The expression “trapezoidal profile” is understood within the usual tolerances linked to the experimental conditions of the manufacture of the patterns, and not perfectly in the mathematical sense of the term “trapezoidal”. According to one characteristic of the invention, the trapezoidal profile of the patterns forms an isosceles trapezium having:

- small and large bases, the large base being oriented towards the second surface;

- side edges connecting the small and large bases; the large base and the side edges forming an angle greater than or equal to 80° and less than or equal to 90°, preferably greater than or equal to 85° and less than or equal to 90°.

The large base and the side edges preferably form an angle greater than or equal to 87° and less than or equal to 90°.

When the large base and the side edges form an angle equal to 90°, then the isosceles trapezoid is a rectangle and the patterns are lamellar in shape.

According to one characteristic of the invention, the substrate and the patterns are made of a material chosen from among a glass, Si0 ₂ , CaF ₂ , MgF ₂ .

Thus, such materials have a refractive index lower than 1.6 and are transparent in the spectral range [200 nm, 900 nm]. In particular, an advantage provided by Si0 ₂ is its high transparency in the spectral range [200 nm, 900 nm] and its extremely low optical losses in this spectral range. In addition, Si0 ₂ has a very high damage threshold to withstand high peak power laser radiation (>100 mW). By way of non-limiting examples, the glass can be B ₂ C>3, CaO, etc.

According to one characteristic of the invention, the electromagnetic radiation is laser radiation having a peak power greater than 100 mW, the substrate and the patterns are made of a material which has a damage threshold suitable for resisting laser radiation.

According to a feature of the invention, the dominant wavelength is chosen from:

- a first spectral range comprised between 200 nm and 400 nm, preferably comprised between 315 nm and 400 nm;

- a second spectral range comprised between 400 nm and 900 nm, preferably comprised between 400 nm and 800 nm.

According to one characteristic of the invention, the dominant wavelength is between 315 nm and 400 nm, and the spatial period of the patterns is greater than or equal to 200 nm. According to one characteristic of the invention, the patterns have an entirely free surface.

According to one characteristic of the invention, the transmission diffraction grating formed by the patterns is the only diffraction grating of the waveplate. In other words, the waveplate has a single diffraction grating.

The invention also relates to an optical device, comprising:

- a laser source, suitable for emitting electromagnetic radiation, of the laser type, having a dominant wavelength, denoted l, between 200 nm and 900 nm;

- a wave plate according to the invention, the first and second surfaces being arranged so as to be traversed successively by the electromagnetic radiation, preferably at normal incidence.

The invention finally relates to a method for manufacturing a wave plate, operating in transmission, to modify a state of polarization of electromagnetic radiation having a dominant wavelength, denoted l, between 200 nm and 900nm; the method comprising the steps: a) providing a substrate, transparent to electromagnetic radiation, and having first and second opposite flat surfaces, intended to be traversed successively by the electromagnetic radiation, preferably at normal incidence; b) forming a transmission diffraction grating on the first surface of the substrate, the transmission diffraction grating comprising patterns:

- made of a dielectric material transparent to electromagnetic radiation, the material having a refractive index, denoted n, less than or equal to 1.6 at the dominant wavelength;

- extending in a first longitudinal direction;

- periodically spaced along a second longitudinal direction, perpendicular to the first longitudinal direction, and according to a spatial period, denoted p, verifying p < l;

- having a height, denoted h, along the normal to the first surface verifying h > 3 p, the height of the patterns being adjusted according to the spatial period of the patterns so that the diffraction grating in transmission introduces a phase shift of p or p/2 to electromagnetic radiation to modify the state of polarization. According to one characteristic of the invention, step b) comprises the steps: bi) forming a hard mask on the first surface of the substrate; b2) forming a photoresist on the hard mask; bs) patterning the photoresist by electron beam lithography; b4) etching the hard mask and the first surface of the substrate through the structured photoresist so as to structure the first surface of the substrate and form the transmission diffraction grating; bs) remove photoresist and hard mask.

Thus, an advantage obtained is to use engraving techniques compatible with CMOS technology. The substrate and the transmission diffraction grating are in one piece. The hard mask formed during the bi) state improves the control of the etching in order to obtain a quality etching with high h/p ratios (³3).

According to one characteristic of the invention, step bi) is preceded by a step bo) consisting in opacifying the substrate by forming, on the second surface of the substrate, at least one opaque layer.

Thus, an advantage provided by the opaque layer is to allow the detection of the substrate by microelectronic equipment (e.g. entrance sensor and exit sensor of the etching chamber, detector of the alignment notch (“notch align”) in English language) of the substrate). In addition, the opaque layer can make it possible to separate the substrate from an electrostatic holding plate (“electrostatic chuck” in English).

Brief description of the drawings

Other characteristics and advantages will appear in the detailed description of various embodiments of the invention, the description being accompanied by examples and references to the attached drawings.

Figure 1 is a schematic sectional view of a wave plate according to the invention. The inset is an enlarged scale view of two consecutive designs. The cut is made along the normal to the first surface (and to the second surface) of the substrate.

Figure 2 is a schematic perspective view of a wave plate according to the invention. The substrate and the transmission diffraction grating are in one piece.

Figure 3 includes schematic sectional views, illustrating a manufacturing method according to the invention. The cuts are made along the normal to the first surface (and to the second surface) of the substrate. It should be noted that the drawings described above are schematic, and are not to scale for the sake of readability and to simplify their understanding.

Detailed description of embodiments

Elements that are identical or provide the same function will bear the same references for the different embodiments, for the sake of simplification.

An object of the invention is a wave plate 1, operating in transmission, to modify a state of polarization of electromagnetic radiation E (illustrated in FIG. 2) having a dominant wavelength, denoted l, comprised between 200nm and 900nm; the wave plate 1 comprising:

- A substrate 2, transparent to electromagnetic radiation E, and comprising first and second surfaces 20, 21 opposite each other, intended to be traversed successively by electromagnetic radiation E, preferably at normal incidence; the second surface 21 being planar;

- patterns 200, made of a dielectric material transparent to electromagnetic radiation E, the material having a refractive index, denoted n, less than or equal to 1.6 at the dominant wavelength, the patterns 200 forming a diffraction grating in transmission to the first surface 20 of the substrate 2, the patterns 200 extending along a first longitudinal direction X; the patterns 200 being periodically spaced along a second longitudinal direction Y, perpendicular to the first longitudinal direction X, and according to a spatial period, denoted p, satisfying p<1; the patterns 200 having a height, denoted h, along the normal Z to the first surface 20 verifying h>3 p; the height of the patterns 200 being adjusted as a function of the spatial period of the patterns 200 so that the diffraction grating in transmission introduces a phase shift of p or p/2 to the electromagnetic radiation E to modify the state of polarization.

Waveplate 1 is shown in Figures 1 and 2.

Incident electromagnetic radiation

The electromagnetic radiation E may be laser radiation having a peak power greater than 100 mW.

The dominant wavelength is advantageously chosen from:

- a first spectral range between 200 nm and 400 nm, preferably between 315 nm and 400 nm, more preferably between 315 nm and 380 nm; - a second spectral range comprised between 400 nm and 900 nm, preferably comprised between 400 nm and 800 nm.

The electromagnetic radiation E is intended to propagate along a direction D (illustrated in FIG. 2) corresponding to normal incidence, or close to normal incidence (± 10° relative to the normal Z to the first surface 20 of the substrate 2).

Substrate material

The material of the substrate 2 is advantageously a dielectric material.

The material of the substrate 2 is advantageously chosen from among a glass, Si0 ₂ , CaF ₂ , MgF ₂ .

The material of the substrate 2 advantageously has a damage threshold (LIDT for “Laser Induced Damage Threshold” in English) suitable for resisting laser radiation having a peak power greater than 100 mW. Laser damage threshold test methods are defined in ISO 21254-1:2011 and ISO 21254-2:2011. The material of the substrate 2 advantageously has a LIDT of between 10 J/cm ² and 15 J/cm ² , in particular at the dominant wavelength of 351 nm for laser pulses with a duration of 3 ns.

By way of non-limiting example, the substrate 2 may be a wafer, which may have a diameter of 200 mm. Substrate 2 may have a thickness of the order of 725 μm.

Grounds

The dielectric material, in which the patterns 200 are made, is advantageously chosen from among a glass, Si0 ₂ , CaF ₂ , MgF ₂ . The dielectric material advantageously has a damage threshold (LIDT for “Laser Induced Damage Threshold” in English) suitable for resisting laser radiation having a peak power greater than 100 mW. Laser damage threshold test methods are defined in ISO 21254-1:2011 and ISO 21254-2:2011. The dielectric material advantageously has a LIDT of between 10 J/cm ² and 15 J/cm ² , in particular at the dominant wavelength of 351 nm for laser pulses with a duration of 3 ns.

The patterns 200, forming the transmission diffraction grating, and the substrate 2 are advantageously made in one piece. Patterns 200 and substrate 2 are then made from the same dielectric material, preferably silica. The first surface 20 of the substrate 2 is structured and comprises the patterns 200 forming the transmission diffraction grating. By “structured surface”, it is meant that the first surface 20 of the substrate 2 is non-flat (break in flatness), and delimits a set of patterns 200 (reliefs). Patterns 200, forming the transmission diffraction grating, and substrate 2 may not be in one piece. The dielectric material of the patterns 200 may belong to a dielectric layer which is formed on the substrate 2. The dielectric layer is etched to obtain the patterns 200 forming the transmission diffraction grating. By way of non-limiting example, the substrate 2 can be made of glass, and the dielectric layer can be made of silica.

According to a first embodiment, the height of the patterns 200 verifies:

3 p £ h £ 6 p, preferably 4 p £ h £ 5 p, so that the diffraction grating in transmission introduces a phase shift of p/2 to the electromagnetic radiation E to modify the state of polarization. The inventors found that it was both possible to introduce a phase shift of p/2 and to obtain a transparency of the patterns 200 of the substrate 2, defined by the intensity transmission coefficient (or transmittance) of the order diffraction zero, averaged for the 2 TE and TM polarizations, which was greater than 90% when the material of the patterns 200 is made of silica, in particular when the dominant wavelength is between 315 nm and 400 nm, for example 351 nm.

According to a second embodiment, the height of the patterns 200 verifies:

6 p £ h £ 12 p, preferably 8 p £ h £ 10 p, so that the diffraction grating in transmission introduces a phase shift of p to the electromagnetic radiation E to modify the state of polarization.

When h > 12 p (resp. h> 6p) for a phase shift of p (resp. p/2), an unfavorable electric field distribution for laser damage performance was found. The laser damage performance of an optical component can be related to the variation of the electric field. In TE bias, areas of high electric field intensity are distributed within 200 patterns, and the peak electric field intensity is 10% lower at 3£h/p£6 ratios compared to h/p > 6 in the case of a quarter wave plate. In TE polarization, the zones of high intensity of the electric field are located at the base of the patterns 200 for h/p ratios > 6 in the case of a quarter-wave plate, which is unfavorable for the performance of laser damage.

The spatial period of the patterns 200 advantageously verifies p £ l/h. When the dominant wavelength is between 315 nm and 400 nm, the spatial period of the patterns 200 is advantageously greater than or equal to 200 nm.

The patterns 200 advantageously have a width at mid-height, denoted c, along the second longitudinal direction Y verifying:

£0.5p c£0.7p The patterns 200 may have a trapezoidal profile along the normal Z to the first surface 20. The trapezoidal profile of the patterns 200 may form an isosceles trapezoid having:

- small and large bases, the large base being oriented towards the second surface 21;

- Side edges 201 connecting the small and large bases; the large base and the side edges 201 forming an angle a (illustrated in FIG. 1) greater than or equal to 80° and less than or equal to 90°, preferably greater than or equal to 85° and less than or equal to 90°. When the large base and the lateral edges 201 form an angle α of 90°, the patterns 200 are lamellar in shape.

When h > 12 p (resp. h> 6p) for a phase shift of p (resp. p/2), a strong degradation of the tolerance to manufacturing errors of the waveplate was observed. More specifically, failure to control the angle a formed between the large base and the lateral edges 201 (ideally tending towards 90°) has a strong impact on the duty cycle at mid-height (1-c/p) and therefore degrades the function optical.

The patterns 200 advantageously have an entirely free surface, that is to say that no element is formed on the surface of the patterns 200. In particular, the surface of the patterns 200 is devoid of additional patterns. The transmission diffraction grating formed by the patterns 200 advantageously constitutes the single diffraction grating of the wave plate 1. In other words, the wave plate 1 advantageously comprises a single and unique diffraction grating.

Optical device

An object of the invention is an optical device, comprising:

- a laser source (not shown), suitable for emitting electromagnetic radiation E, of the laser type, having a dominant wavelength, denoted l, of between 200 nm and 900 nm;

- a wave plate 1, in accordance with the invention, the first and second surfaces 20, 21 being arranged so as to be traversed successively by the electromagnetic radiation E, preferably at normal incidence.

The laser source can be continuous or pulsed. The laser source may comprise a laser such as a continuous laser or a pulsed laser (e.g. nanosecond laser) whose power may be high. The laser source may comprise high-power laser chains, such as the Megajoule Laser chains or the laser chains entitled “National Ignition Fadlity”.

The medium M surrounding the laser source and the wave plate 1 is preferably ambient air, or a medium M under vacuum. Manufacturing process

As illustrated in Figure 3, an object of the invention is a method of manufacturing a wave plate 1, operating in transmission, to modify a state of polarization of electromagnetic radiation E having a dominant wavelength , denoted l, between 200 nm and 900 nm; the method comprising the steps: a) providing a substrate 2, transparent to the electromagnetic radiation E, and having first and second opposite flat surfaces 20, 21, intended to be traversed successively by the electromagnetic radiation E, preferably at normal incidence; b) forming a transmission diffraction grating on the first surface 20 of the substrate 2, the transmission diffraction grating comprising patterns 200:

- made of a dielectric material transparent to electromagnetic radiation E, the material having a refractive index, denoted n, less than or equal to 1.6 at the dominant wavelength;

- extending along a first longitudinal direction X;

- Periodically spaced along a second longitudinal direction Y, perpendicular to the first longitudinal direction X, and according to a spatial period, denoted p, verifying p < l;

- having a height, denoted h, along the normal Z to the first surface 20 satisfying h>3 p, the height of the patterns 200 being adjusted according to the spatial period of the patterns 200 so that the diffraction grating in transmission introduces a phase shift of p or p/2 to the electromagnetic radiation E to modify the state of polarization.

The technical characteristics described above (incident electromagnetic radiation E, material of substrate 2, patterns 200 of substrate 2) apply for the manufacturing method according to the invention.

Step b) advantageously comprises the steps: bi) forming a hard mask 3 on the first surface 20 of the substrate 2; bz) forming a photoresist 4 on the hard mask 3; bs) structuring photoresist 4 by electron beam lithography; b^ etching the hard mask 3 and the first surface 20 of the substrate 2 through the structured photoresist 4 so as to structure the first surface 20 of the substrate 2 and form the transmission diffraction grating.

When the material of the substrate 2 is made of silica (S1O2), the hard mask 3 formed during step bi) is advantageously made of titanium. The titanium hard mask 3 advantageously has a thickness of between 40 nm and 60 nm. The Titanium Hard Mask 3 formed during step bi) makes it possible to evacuate the electrons present on the first surface 20 of the substrate 2. The hard titanium mask 3 formed during state bi) makes it possible to improve the control of the etching of the silica in order to obtain a quality etching with high h/p ratios (³3), in particular for dominant wavelengths of the incident electromagnetic radiation E between 315 nm and 400 nm, which made it possible to overcome the prejudice of the state of the art.

When the material of the substrate 2 is made of silica (S1O2), the photosensitive resin 4 (eg NEB22™ marketed by the company Sumitomo) formed during step b 2) advantageously has a thickness of between 300 nm and 400 nm. The minimum spatial period of the patterns 200 is a critical dimension, denoted DC, linked to the experimental conditions of step bs), that is to say the structuring of the photoresist, and is in particular linked to the optical resolution of electron beam lithography:

Where :

- ki is a factor linking the Rayleigh criterion with the actual behavior of a given manufacturing process (aimed at printing the smallest possible patterns);

- l is the dominant wavelength of the incident electromagnetic radiation E;

- NA is the numerical aperture of the focusing system used for electron beam lithography.

When the material of the substrate 2 is made of silica (S1O ₂ ), step bi) can comprise the steps: b4o) etching the hard mask 3 in titanium by plasma etching, for example an inductively coupled plasma; b«) etching the first surface 20 of the substrate 2 in silica by plasma etching, for example a capacitively coupled plasma.

The inductively coupled plasma executed during step b^) can comprise as reactive gases BCI3 (to etch a possible superficial titanium oxide) and Cb/HBr to etch the hard titanium mask 3 with good selectivity (approximately 3) against S1O2 and photosensitive resin 4 (of the NEB22™ type).

The capacitively coupled plasma executed during step b 41) can comprise C4F8 as reactive gas to etch the silica with good selectivity (approximately 5) with respect to the photosensitive resin 4 (of the NEB22™ type). Step bi) is advantageously preceded by a step bo) consisting in opacifying the substrate 2 by forming, on the second surface 21 of the substrate 2, at least one opaque layer 5. Thus, an advantage procured by the opaque layer 5 is to authorize the detection of the substrate 2 by microelectronic equipment (eg entry sensor and exit sensor of the etching chamber, detector of the alignment notch (“notch align” in English language) of the substrate). Opaque layer 5 is opaque to the electromagnetic radiation used by microelectronic equipment for detecting substrate 2.

When the material of the substrate 2 is made of silica (S1O2), step bo) may consist in successively forming a layer 6 of titanium (Ti) and a layer 5 of silicon nitride (SiN) on the second surface 21 of the substrate 2. Layer 5 of silicon nitride formed during step bo) makes it possible to opacify substrate 2. Layer 6 of titanium formed during step bo) makes it possible to separate substrate 2 from a holding plate electrostatic (not shown). The titanium layer 6 formed during step bo) advantageously has a thickness of between 80 nm and 120 nm. The silicon nitride layer 5 formed during step bo) advantageously has a thickness of between 450 nm and 550 nm.

After step b^, the method includes steps bs) of cleaning the surfaces, in order to remove the photosensitive resin 4, the hard mask 3, and the opaque layers 5, for example using plasma treatments. The invention is not limited to the disclosed embodiments. The person skilled in the art is able to consider their technically effective combinations, and to substitute them with equivalents.

Claims

1. Wave plate (1), operating in transmission, to modify a state of polarization of electromagnetic radiation (E) having a dominant wavelength, denoted l, between 200 nm and 400 nm; the wave plate (1) comprising:

- a substrate (2), transparent to electromagnetic radiation (E), and comprising first and second opposite surfaces (20, 21), intended to be traversed successively by electromagnetic radiation (E), preferably at normal incidence; the second surface (21) being planar;

- patterns (200), made of a dielectric material transparent to electromagnetic radiation (E), the material having a refractive index, noted n, less than or equal to 1.6 at the dominant wavelength, the patterns (200 ) forming a transmission diffraction grating on the first surface (20) of the substrate (2), the patterns (200) extending along a first longitudinal direction (X); the patterns (200) being periodically spaced along a second longitudinal direction (Y), perpendicular to the first longitudinal direction (X), and according to a spatial period, denoted p, satisfying p<1; the patterns (200) having a height, denoted h, along the normal (Z) to the first surface (20) satisfying h>3 p; the height of the patterns (200) being adjusted according to the spatial period of the patterns (200) and verifies respectively 6 p £ h £ 12 p or 3 p £ h £ 6 p so that the diffraction grating in transmission introduces a phase shift of p or p/2 to the electromagnetic radiation (E) to modify the state of polarization.

2. Wave plate (1) according to claim 1, in which the height of the patterns (200) verifies:

4 p £ h £ 5 p, so the transmission diffraction grating introduces a p/2 phase shift to the electromagnetic (E) radiation to change the polarization state.

3. Wave plate (1) according to claim 1, in which the height of the patterns (200) verifies:

8 p £ h £ 10 p, so the transmission diffraction grating introduces a p phase shift to the electromagnetic (E) radiation to change the polarization state.

4. Wave plate (1) according to one of claims 1 to 3, in which the spatial period of the patterns (200) verifies p £ l/h.

5. wave plate (1) according to one of claims 1 to 4, wherein the patterns (200) have a width at mid-height, denoted c, in the second longitudinal direction (Y) verifying: 0.5 p £ c £ 0.7 p.

6. Wave plate (1) according to one of claims 1 to 5, in which the patterns (200) have a trapezoidal profile along the normal (Z) to the first surface (20). 7. Wave plate (1) according to claim 6, in which the trapezoidal profile of the patterns

(200) forms an isosceles trapezium with:

- small and large bases, the large base being oriented towards the second surface (21);

- side edges (201) connecting the small and large bases; the large base and the lateral edges (201) forming an angle (a) greater than or equal to 80° and less than or equal to 90°, preferably greater than or equal to 85° and less than or equal to 90°.

8. Waveplate (1) according to one of claims 1 to 7, wherein the substrate (2) and the patterns (200) are made of a material selected from glass, Si0 ₂ , CaF ₂ , MgF ₂ . 9. Wave plate (1) according to one of claims 1 to 8, in which the electromagnetic radiation (E) is laser radiation having a peak power greater than 100 mW, the substrate (2) and the patterns (200 ) are made of a material which has a damage threshold suitable for resisting laser radiation. 10. Waveplate (1) according to one of claims 1 to 9, in which the dominant wavelength is chosen in a first spectral range between 315 nm and 400 nm.

11. Wave plate (1) according to one of claims 1 to 10, in which the dominant wavelength is between 315 nm and 400 nm, and the spatial period of the patterns (200) is greater than or equal to 200nm.

12. Waveplate (1) according to one of claims 1 to 11, wherein the patterns (200) have an entirely free surface.

13. Wave plate (1) according to one of claims 1 to 12, in which the transmission diffraction grating formed by the patterns (200) is the only diffraction grating of the wave plate (1) . 14. Optical device, comprising:

- a laser source, suitable for emitting electromagnetic radiation (E), of the laser type, having a dominant wavelength, denoted l, between 200 nm and 400 nm;

- a waveplate (1), according to one of claims 1 to 13, the first and second surfaces (20, 21) being arranged so as to be crossed successively by the electromagnetic radiation (E), preferably in incidence normal.