Method for manufacturing a spatial light modulator for a holographic image projection device

Abstract

The present invention relates to a spatial light modulator intended to be illuminated by coherent light sources to project a holographic image and its manufacturing process (7). The spatial light modulator comprises a substrate on which at least one waveguide is formed. A coating layer (71) is deposited on the substrate and on the at least one waveguide to form a coated sample. The coated sample then undergoes first (72) and second (73) heating before the coating layer is thinned (74) to form a thinned sample. A plurality of nanoresonators is then deposited (75) onto the coating layer of the thinned sample, the nanoresonators being positioned on the coating layer directly above at least one waveguide, each nanoresonator being formed of at least one chain of nanoparticles and being optically activatable to define a pixel of the holographic image. Figure for the abstract: Figure 7

Description

Title of the invention: Method for manufacturing a spatial light modulator for a holographic image projection device technical field

[0001] The present invention is in the general field of dynamic holography, applicable to holographic image projection devices such as 3D displays or augmented reality systems embedded in vehicles, in particular autonomous vehicles. Technological background

[0002] Holography is a method of reproducing a 3D (three-dimensional) image based on interference patterns (commonly called holograms) and diffraction phenomena to reconstruct in 3D a wavefront emitted from an object towards an observer. The use of holography for the reconstruction and visualization of 3D images is a viable solution because it does not cause headaches or other discomfort that can be caused by other 3D technologies.

[0003] Augmented reality, or AR, which is increasingly found in the automotive field, is a technological evolution of Head-Up Display (HUD) systems. Essentially, an HUD system comprises a Picture Generation Unit (PGU) that generates an image. The HUD system also includes an optical system made up of several mirrors, possibly adjustable, and adapted so that an observer sees the image in an area beyond the windshield at a projection distance (approximately 2 meters in front of the driver) and at a certain angle α relative to the horizontal. An HUD system can display, for example, speed information, pictograms, or vehicle information in the driver's field of vision while the vehicle is being driven.

[0004] Augmented reality in the automotive field takes the form of one or more virtual images projected at a distance of approximately 7 meters in front of the windshield (or even more). Virtual reality images can be superimposed on images from a VTH system to, for example, highlight road features, virtually illuminate a GPS track, etc. [Fig. 1] illustrates an example of superimposing an image from a VTH system (part 1) with an image from an AR system (part 2).

[0005] Current AR systems adapted for the automotive field are based on the integration of several image generation units (or a single unit divided into two) and two optical paths implemented by refractive optics mirrors: one for the propagation of an image from a VTH system and the other for the propagation of an image from an AR system.

[0006] The main drawback of current systems is their size: AR systems based entirely on conventional optics have minimum volumes approaching 20 liters, while VTH systems have a volume of around 5 liters. Current VTH and AR systems are difficult to integrate without reducing their size.

[0007] 3D displays for the automotive sector are a trend in the vehicles of the future, both for integration into the cockpit (instrument cluster, central screen) and for integration as a conversational assistant. Human-Machine Interfaces would then be more intuitive because interaction with 3D interfaces is more intuitive compared to interaction with conventional 2D interfaces.

[0008] Existing 3D displays are based on autostereoscopy (with or without eye tracking) which produces a different image for the left and right eye of an observer or include multilayer displays which are equipped with several LCD panels to produce several image planes at different depths.

[0009] Autostereoscopic 3D displays suffer from the "vergence / accommodation" conflict, meaning that the observer focuses on the screen, but the 3D content pops out in front of it. The 3D experience can be degraded or even "rejected" by the observer. As for multilayer systems, they only allow display on several discrete planes in depth or pop-out.

[0010] Spatial light modulators, commonly called SLMs (from the English "Spatial Light Modulator"), can also be used as a holographic image display for an AR system or as a 3D display.

[0011] The modulation mechanism of a spatial light modulator relies on a method that modifies the properties of the modulation material and the way in which the spatial field of an incident light beam is modified. Various processes, including mechanical, electro-optical, thermo-optical, and magneto-optical effects, can be used to modify the properties of the modulation material of a spatial light modulator that interacts with incident light and transforms its spatial field distribution. One of the most commonly used modulation mechanisms today is the electro-optical spatial light modulator containing liquid crystals as the modulation material (LC-SLM, for "Liquid Crystal Spatial Light Modulator"). These LC-type modulators SLMs are based on a liquid crystal pixel array which, by modulating the phase of coherent light, generates a hologram at any given moment to produce 2D or 3D video at the output of an optical system. Current silicon / liquid crystal technologies for this type of modulator now allow for pixel sizes of 1 to 4 µm and image resolutions up to 4160 x 2464 pixels.

[0012] State-of-the-art AR systems that use spatial light modulators do not have sufficient resolution to allow the democratization of "dynamic" holography, i.e., holography controlled by a control unit, particularly in the automotive field.

[0013] Indeed, on the one hand, the field of view of a holographic image, due to the diffraction angle of the hologram, is linked to the size of a modulator pixel, and the number and size of the pixels play a role in the resolution of the holographic image. Furthermore, the product of the image size and the field of view is proportional to the number of pixels in the modulator. Thus, to obtain a hologram of 1Q12 pixels, which would allow for a satisfactory resolution of a holographic image, a field of view of 60° and an image size of 50 cm are necessary. However, current modulators only allow fields of view of approximately 20°, which results in insufficient resolution of the holographic images.

[0014] It is necessary to provide spatial light modulators that provide high-resolution holographic images and that provide fields of view beyond those produced today by spatial light modulators.

[0015] A compact yet high-resolution spatial light modulator is, for example, described in French patent application FR3124865, published on July 5, 2021. This spatial light modulator requires high manufacturing quality, notably requiring precise arrangement of the reflectors corresponding to nanoparticles, a particularly flat and clean surface on which these nanoparticles are arranged, and controlled material thicknesses to ensure control of each nanoparticle. Such a high level of precision in the manufacture of a spatial light modulator requires mastery of the manufacturing process that is difficult to achieve and repeat with conventional manufacturing methods. Summary of the present invention

[0016] One object of the present invention is to solve at least one of the problems of the technological background described above.

[0017] An object of the present invention is to enable the manufacture of a high-resolution spatial light modulator.

[0018] Another object of the present invention is to improve holographic image projection devices.

[0019] According to a first aspect, the present invention relates to a method for manufacturing a spatial light modulator intended to be illuminated by coherent light sources to project a holographic image, the spatial light modulator comprising: - a substrate on which at least one waveguide is formed; - a coating layer placed on the substrate and enveloping at least one waveguide; and - a plurality of nanoresonators placed on the coating layer directly above at least one waveguide, each nanoresonator being optically activatable and being formed of at least one chain of nanoparticles, each nanoresonator defining a pixel of the holographic image, the process being characterized in that it comprises the following steps: - deposition of the coating layer onto the substrate and onto at least one waveguide to form a coated sample, then - first heating of the coated sample to a first temperature, then - a second heating of the coated sample to a second temperature to form an annealed sample, then - thinning of the coating layer of the annealed sample to form a thinned sample, then - deposition of the plurality of nanoresonators on the coating layer of the thinned sample to obtain the spatial light modulator.

[0020] Such a manufacturing process makes it possible to have: • a coating layer made of a material similar to that of the substrate and having, in particular, similar optical properties, • of a surface of the coating layer ensuring good arrangement of each nanoresonator and their control via at least one optical wave circulating in a waveguide.

[0021] According to one variant of the manufacturing process, the coating layer is deposited by centrifugal coating. The use of centrifugal coating makes it possible to obtain a coating layer of the coated sample with a controlled and homogeneous thickness.

[0022] According to another variant of the manufacturing process, the first heating comprises a linear temperature increase up to the first temperature, the temperature increase being carried out over a first duration. The coated sample is thus gradually heated without undergoing thermal shock.

[0023] According to a further variant of the manufacturing process, the first duration is between 25 and 35 minutes and the first temperature is equal to 180 degrees Celsius. Such parameters guarantee the absence of thermal and mechanical stresses that could degrade the coated sample.

[0024] According to another variant of the manufacturing process, the second heating comprises a linear temperature increase to the second temperature followed by a linear temperature decrease to a third temperature, the second heating being carried out for a second duration. This second heating then allows for heat treatment of the coated sample.

[0025] According to yet another variant of the manufacturing process, the second heating is carried out in a nitrogen furnace, the second duration being between 160 and 200 minutes. The heat treatment is then controlled and carried out under a neutral gaseous atmosphere, thus avoiding any unexpected alteration of the materials composing the coated sample.

[0026] According to a further variant of the manufacturing process, the second temperature is between 570 and 630 degrees Celsius. This second temperature is defined so as to allow a transformation of the coating layer.

[0027] According to an additional variant of the manufacturing process, the coating layer is composed of hydrogenosilsesquioxanes (HSQ) transformed into silica (SiO2) during the second heating. This material then exhibits physical characteristics, particularly optical ones, close to those of the substrate.

[0028] According to yet another variant of the manufacturing process, the thinning step is carried out by reactive ion etching. Ion etching makes it possible to control the thickness of material removed while maintaining a clean surface.

[0029] According to another embodiment, the manufacturing process further includes a step of measuring the thickness of the coating layer of the annealed sample, the thinning being a function of the measured thickness. It is thus possible to continuously measure the height etched during the thinning step, for example using a laser, and to control this thinning in order to maintain a target coating layer thickness above at least one waveguide.

[0030] According to a second aspect, the present invention relates to a spatial light modulator intended to be illuminated by coherent light sources to project a holographic image, the spatial light modulator comprising: - a substrate on which is formed at least one waveguide; - a coating layer placed on the substrate and enveloping at least one waveguide; and - a plurality of nanoresonators placed on the coating layer directly above at least one waveguide, each nanoresonator being optically activatable and being formed of at least one chain of nanoparticles, each nanoresonator defining a pixel of the holographic image, the spatial light modulator being characterized in that it is obtained according to the manufacturing process described according to the first aspect of the invention.

[0031] Placing a plurality of optically activatable nanoresonators on the coating layer with high precision makes it possible to significantly increase the number of nanoresonators per waveguide intersection of the waveguide lattice. Since each nanoresonator can be optically activated independently of the others, each nanoresonator can correspond to a pixel of the holographic image. Thus, increasing the number of nanoresonators in the spatial light modulator increases the number of pixels and therefore the resolution of the projected holographic image without increasing the size of the optical component of a holographic image projection device such as an AR or VTH system or a 3D display.Furthermore, the high positioning accuracy of these nanoresonators is made possible by the flatness and cleanliness of the upper surface of the coating layer, thus enabling repeatable and high-quality mass production of this spatial light modulator.

[0032] Thus, the present invention makes it possible to obtain spatial light modulators with pixel sizes on the order of 500nm, whereas the pixel size of current spatial modulators is on the order of 1 to 4 pm, and this with optimal manufacturing quality.

[0033] According to a third aspect, the present invention relates to a holographic image projection device comprising at least one spatial light modulator according to the first aspect of the present invention.

[0034] According to a fourth aspect, the present invention relates to a vehicle comprising at least one holographic image projection device according to the third aspect of the present invention. Brief description of the figures

[0035] Other features and advantages of the present invention will become apparent from the description of the particular and non-limiting embodiments of the present invention below, with reference to the attached Figures 1 to 8, in which:

[0036] [Fig-1] schematically illustrates the projection of images by an augmented reality system and a Head Up system of a vehicle, according to the prior art;

[0037] [Fig.2] schematically illustrates the projection of images in a vehicle 10, according to a particular and non-limiting embodiment of the present invention;

[0038] [Fig.3] schematically illustrates a holographic image projection device for a vehicle, according to a particular and non-limiting embodiment of the present invention.

[0039] [Fig.4] schematically illustrates a spatial light modulator according to a particular and non-limiting embodiment of the present invention;

[0040] [Fig.5] schematically illustrates a cross-sectional view of the spatial light modulator of [Fig.4], according to a particular and non-limiting embodiment of the present invention;

[0041] [Fig.6] schematically illustrates a cross-sectional view of a nanoresonator of the spatial light modulator of [Fig.4], according to a particular and non-limiting embodiment of the present invention;

[0042] [Fig.7] illustrates a flowchart of the different stages of a manufacturing process for a spatial light modulator, according to a particular embodiment of the present invention; and

[0043] [Fig.8] illustrates an evolution of a sample to obtain a spatial light modulator according to the different stages of the process of [Fig.7], according to a particular embodiment of the present invention. Description of examples of achievements

[0044] A spatial light modulator, a method for manufacturing this spatial light modulator will now be described in what follows with joint reference to Figures 1 to 8. The same elements are identified with the same reference symbols throughout the description that follows.

[0045] The terms "first," "second" (or "firsts," "seconds"), etc., are used in this document by arbitrary convention to allow for the identification and distinction of different elements (such as operations, means, etc.) implemented in the embodiments described below. Such elements may be distinct or correspond to a single element, depending on the embodiment.

[0046] According to a particular and non-limiting embodiment, the present invention relates to a

[0047] The present invention relates to a spatial light modulator intended to be illuminated by coherent light sources to project a holographic image and its manufacturing process. The spatial light modulator comprises a substrate on which at least one waveguide is formed. An coating layer is deposited on the substrate and on the at least one waveguide to form a coated sample. The coated sample then undergoes first and second heating cycles before the coating layer is thinned to form a thinned sample. A plurality of nanoresonators is then deposited on the coating layer of the thinned sample, the nanoresonators being positioned on the coating layer directly above at least one waveguide. Each nanoresonator is formed of at least one chain of nanoparticles and is optically activatable to define a pixel of the holographic image.

[0048] This very high resolution spatial light modulator, which once illuminated by coherent light sources, for example a laser, can be integrated to create a 3D holographic screen for a 3D display application or inserted into an augmented reality system, known as an AR system, for example for the automotive industry.

[0049] Fig. 2 schematically illustrates the projection of images in a vehicle 10, according to a particular and non-limiting embodiment of the present invention.

[0050] Fig. 2 presents a view of the interior of a vehicle 10, in particular of the front part of the passenger compartment of the vehicle 10 including in particular the steering wheel, the dashboard, and the windscreen 100 of the vehicle 10.

[0051] Vehicle 10 corresponds, for example, to a vehicle with an internal combustion engine, with electric motor(s), or even a hybrid vehicle with an internal combustion engine and one or more electric motors. Vehicle 1 thus corresponds, for example, to a land vehicle, for example a car, a truck, a bus.

[0052] The vehicle 10 includes, for example, a set of driver assistance systems, called AD AS systems (from the English "Advanced Driver-Assistance System" or in French "Système d'aide à la conduite avancé") and an infotainment system, called IVI system (from the English "In-Vehicle Infotainment" or in French "Info-divertissement étoilé").

[0053] The vehicle 10 advantageously incorporates a holographic image projection device. The holographic image projection device includes, for example, one or more projectors integrated, for example, into the dashboard. Such a holographic image or graphic content projection device corresponds, for example, to a 3D display or an AR or VTH system, which allows virtual objects to be superimposed into the field of vision of a driver of the vehicle 10, for example, onto the windshield 100 of the vehicle 10, so as to overlay the virtual objects onto the real road scene. The projection of the holographic image is, for example, controlled by one or more computers of the vehicle 10's onboard system, for example, by the computer of the vehicle 10's IVI system.

[0054] The holographic image is advantageously displayed on a surface 101 by projecting the image data in the form of a light beam reflected by a spatial light modulator. The surface 101 corresponds, for example, to an opaque strip arranged on the dashboard of the vehicle 10, for example above and behind the steering wheel with respect to a viewpoint corresponding to the viewpoint from which the driver of the vehicle 10 is supposed to look at the road ahead when driving the vehicle 10.

[0055] The holographic image advantageously comprises a set of graphic objects, each representing a parameter or operating state of the vehicle 10. By way of example, the set of graphic objects represents parameters or state vehicle operating information 10 corresponding to all or part of the following information, according to all possible combinations: - information representative of instantaneous speed; and / or - information representing the rotation of a drive shaft (also called tachometer information); and / or - information representing a distance traveled; and / or - information representing a fuel level or the battery charge level in the case of an electric vehicle; and / or - information representative of the temperature of an engine coolant; and / or - information representative of a warning light (e.g., battery charge indicator, engine oil pressure indicator, engine oil or coolant temperature indicator, brake failure indicator, etc.), corresponding, for example, to a pictogram that appears or changes color in the event of a warning; and / or - information representative of a warning indicator (for example, engine oil level indicator, airbag indicator (also called inflatable cushion), brake pad wear indicator, etc.), corresponding for example to a pictogram that appears or changes color when a warning is given; and / or - information representative of an indicator for the operation of an on-board system (positioning light indicator, dipped or main beam indicator, hazard warning light indicator, rear window defogger indicator, etc.), corresponding for example to a pictogram that appears or changes color when the on-board system is activated.

[0056] The set of graphic objects can also represent contextual or environmental information of the vehicle 10, corresponding to all or part of the following information, according to all possible combinations: - representative mapping information of the environment of vehicle 10, for example obtained from a navigation system embedded in the vehicle or from a navigation system installed on a mobile communication device (for example a smartphone connected wirelessly to vehicle 10, or more specifically to the vehicle's embedded system 10 including the computer(s) responsible for controlling the projection of the first and second images; and / or - representative vehicle navigation information, for example a trace of the route to be followed by the vehicle, the vehicle's current position, the vehicle's instantaneous speed, the speed limit applicable to the section of road on which the vehicle is travelling 10; this information is obtained for example from the navigation system built into the vehicle or navigation system installed on a mobile communication device; and / or - information representative of an object detected in the environment, for example the presence of a vehicle preceding vehicle 10, the presence of a pedestrian or an animal on the road in front of vehicle 10, the presence of a road sign, the presence of a stationary object on the road, the presence of a tunnel entrance, etc.; this information is obtained, for example, from one or more object detection sensors on board vehicle 10 and, for example, associated with one or more AD AS systems of vehicle 10.

[0057] Of course, the list of information displayed via the holographic image is given by way of example and is not limited to the examples above.

[0058] Fig. 3 schematically illustrates a holographic image projection device for a vehicle 10, according to a particular and non-limiting embodiment of the present invention.

[0059] The holographic image projection device 3 includes a control unit 31 configured for processing data received from sensors of the vehicle 10 and / or systems embedded in the vehicle 10. The control unit 31 is advantageously configured to provide information necessary for the generation of one or more holographic images from the received data.

[0060] The holographic image projection device 3 includes a spatial light modulator 32 configured to reflect an incident light beam, a reflected light beam being representative of a holographic image generated from information from the control unit 31. The incident light beam is generated by at least one coherent light source (for example laser) 34 formed, for example, of an RGB laser diode (from the English "Red, Green, Blue", in French Rouge, Vert Bleu).

[0061] According to a particular and non-limiting embodiment, the coherent light sources are adapted to emit near-infrared light waves (700nm to 1600nm).

[0062] The holographic image projection device 3 also includes a reflective device 33 associated with the spatial light modulator 32. The reflective device 33 is configured to direct the light beam reflected by the spatial light modulator 32 towards the surface 300, corresponding for example to the surface 101 previously described, for display of the holographic image in the field of vision of the viewpoint 310 corresponding for example to the position of an eye of a user, for example the driver of the vehicle 10 of [Fig.2].

[0063] According to one variant, the holographic image projection device 3 does not include such reflective device(s), the spatial light modulator 32 being arranged so as to project the light beam directly onto the surface 101.

[0064] According to an optional embodiment, the holographic image projection device 3 further includes optical means (for example an arrangement of optical lenses not shown) configured to focus the holographic image in the image plane.

[0065] Figure 4 schematically illustrates a spatial light modulator according to a particular and non-limiting embodiment of the present invention.

[0066] The spatial light modulator 32 is intended to be illuminated by coherent light sources 34 to project a holographic image by reflection of the coherent light. These coherent light sources 34 are controlled (activated / deactivated) by information from the control unit 31 to generate and project a holographic image. The holographic image corresponds to a set of pixels, very often arranged in the form of a lattice, with each pixel associated with information, for example, gray levels coded on 8, 10, or 12 bits for each color channel of a given color space, for example, RGB.

[0067] The holographic image is for example generated to graphically represent information on the operating state of the vehicle 10 or on the context or environment in which the vehicle 10 is operating. This information is for example received from sensors and / or systems embedded in the vehicle and measuring control parameters of the vehicle 10.

[0068] The holographic image from the spatial light modulator 32 is projected in the form of a light beam onto the surface 101 by the spatial light modulator, either directly or via an optical system and / or a reflective device such as the reflective device 33 of [Fig.4].

[0069] Of course, the generation and projection are not limited to a single image but extend to the generation and projection of a sequence of images, for example at an image frequency equal to, for example, 24, 30 or 60 images per second.

[0070] The spatial light modulator 32 comprises a substrate 321, for example made of silica (SiO2). Waveguides 322 are positioned on this substrate 321 in the form of a lattice, the waveguides intersecting and forming intersections. By waveguide lattice 322, we mean an array of waveguides arranged to form at least one intersection between any two waveguides 322, the lattice being structured, in particular, to form a geometric pattern. According to a particular embodiment, the width of each waveguide 322 is on the order of 500 nm and the spacing between the waveguides is on the order of 500 nm, or even 300 nm, i.e., a distance corresponding to a center-to-center spacing of the waveguides between 800 nm and 1 pm. According to another particular and non-limiting example of embodiment, the waveguides are formed from silicon (Si) or silicon nitride (Si3N4).

[0071] The spatial light modulator 32 illustrated in this [Fig. 4] comprises a lattice of six waveguides 322 extending in the same plane, three waveguides 322 extending in a first direction and three waveguides 322 extending in a second direction, the second direction being, for example, normal to the first direction. The waveguides 322 form a matrix comprising, according to this example, nine intersections.

[0072] The substrate 321 and the waveguides 322 are coated with an encapsulating layer 323. This encapsulating layer 323 is thus placed on the substrate 321 and envelops each waveguide 322. According to a particular embodiment, this encapsulating layer 323 is composed of hydrogen silsesquioxanes (HSQ). This resin, once annealed, yields a material whose optical index is very close to that of the substrate 321, the annealed encapsulating layer being composed of silica (SiO2).

[0073] Four nanoresonators 324 are positioned directly above each intersection on a surface of the coating layer 323 defining an upper plane 'P', the upper plane 'P' being the plane defined by the surface of the coating layer 323 opposite the substrate 321, also called the distal surface of the substrate 321. The four nanoresonators 324 are arranged in groups 325 of nanoresonators, each nanoresonator being located near a corner of a surface of the coating layer 323 corresponding to an intersection. Each nanoresonator 324 then corresponds to a pixel of a holographic image.

[0074] The spatial light modulator 32 according to the present invention is not limited to the example given in [Fig. 4]. Indeed, the spatial light modulator may comprise more than one waveguide lattice, more than six waveguides 322 per lattice, any number of nanoresonators 324 per intersection without departing from the scope of the present invention.

[0075] Placing four nanoresonators 324 at each intersection of waveguides 322 doubles the number of nanoresonators 324 in each direction of waveguide 322. Thus, for the same waveguide width 322 and the same distance separating two waveguides 322, the image resolution is multiplied by 4 compared to a solution offering a single nanoresonator 324 at each intersection of waveguides 322. It should be noted that the number of nanoresonators 324 at each intersection of waveguides 322 varies between 0 and n, where n represents the maximum number of nanoresonators 324 present at the same intersection.

[0076] For example, the nanoresonators 324 have a size of 100 to 250 nm. The spatial light modulator 32 makes it possible to obtain pixel sizes less than 500nm leading to fields of view of more than 60° and very high holographic image resolutions, much higher than those of current holographic images.

[0077] Each nanoresonator 324 comprises two nanoparticle chains, a first chain 324a arranged along the first direction along which waveguides 322 extend and a second chain 324b arranged along the second direction along which waveguides 322 extend. The length of a nanoparticle chain is defined so as to activate the first chain 324a or second chain 324b as a function of a defined wavelength.

[0078] Figure 6 schematically illustrates a cross-sectional view of a nanoresonator 324 according to a particular, non-limiting embodiment of the present invention. Each chain of nanoparticles 324a, 324b of the nanoresonator 324 is composed of a phase-change material 3242 deposited between two layers of a plasmonic material 3241, 3243. The layer 3243 is in contact with the coating layer 323. According to a particular, non-limiting embodiment, the plasmonic material is gold or silver and the phase-change material is vanadium dioxide (VO2), a germanium-antimony-tellurium alloy (GeSbTe), or antimony trisulfide (SbS3).

[0079] The activation lengths are defined by the lengths L of the lower layer 3243 of the nanoparticle chains. Thus, to activate a nanoresonator 324, the first nanoparticle chain 324a of this nanoresonator 324 must be illuminated by a first light wave with an activation wavelength equal to the first length Li of the first nanoparticle chain 324a, and the second nanoparticle chain 324b of this nanoresonator 324 must be illuminated by a second light wave with an activation wavelength equal to the second length L2 of the nanoparticle chain 324b. The first Li and second L2 lengths of the first 324a and second 324b nanoparticle chains of the same nanoresonator 324 are equal or different depending on the application.Thus, any chain of nanoparticles oriented along the longitudinal direction of a waveguide 322 and placed on this waveguide is activated when this waveguide 322 is traversed by a light wave whose wavelength corresponds to the activation wavelength associated with the length L of this chain of nanoparticles.

[0080] Arranging a plurality of nanoresonators 324 at the intersection of waveguides 322 increases the number of nanoresonators 324 in each longitudinal direction of the waveguide 322. Thus, for the same waveguide width 322 and the same distance separating two waveguides 322, the resolution of an image is increased compared to a solution offering a single 324 nanoresonators are placed at each intersection of 322 waveguides. The maximum number of 324 nanoresonators can vary depending on the intersections of a waveguide lattice to create a holographic image comprising spatial regions with different resolutions. Alternatively, the same number of 324 nanoresonators are placed directly above each intersection of a waveguide lattice.

[0081] A nanoresonator 324 is activated when its two chains 324a and 324b are activated. A first light wave with a wavelength sufficient to activate the first chain 324a of the nanoresonator 324 is then emitted into the first waveguide 322 extending in the first direction and forming the intersection at which the nanoresonator 324 is located. A second light wave with a wavelength sufficient to activate the second chain 324a of the nanoresonator 324 is then emitted into the second waveguide 322 extending in the second direction and forming the intersection at which the nanoresonator 324 is located. The first and second waves are generated simultaneously. The first and second light waves are called activation waves.The nanoresonator 324 of the spatial light modulator 32 is then activated or excited so as to modify the angle of reflection of an incident light ray and thus generate a pixel of the holographic image by the reflected light ray.

[0082] In other words, the first and second emitted light waves propagate respectively along the longitudinal direction of the first and second waveguides and interact with chains of nanoparticles aligned along the longitudinal directions of these waveguides. Only the chains of nanoparticles aligned along the first and second waveguides whose lengths correspond to the (activation) wavelengths of the emitted light waves are then activated (or excited).

[0083] Each pixel of a holographic image can thus be illuminated. The propagation of two light waves of specific wavelengths makes it possible to address each pixel of the holographic image. This principle of individual pixel addressing is controlled by the wavelength values ​​of the light waves emitted along longitudinal directions of the waveguides 322, that is to say, by wavelength values ​​that correspond to the resonances of chains of nanoparticles 324a, 324b forming nanoresonators 324 placed directly above the intersections of the waveguide lattice.

[0084] To enable the activation of each chain 324a, 324b, it is necessary to control the thickness of the material between a waveguide 322 and a nanoresonator 324. Figure 5 schematically illustrates a cross-sectional view of the spatial light modulator of Figure 4, according to a particular and non-limiting embodiment of the present invention. This thickness 'e' of material in the coating layer 323 is in particular determined so that the waves emitted through the waveguides 322 can interact with the nanoresonators 324 arranged on the coating layer 323. Too great a thickness 'e' of material would in particular attenuate the effect of an activation wave on a chain of nanoparticles 324a, 324b intended to receive it and would thus disrupt the operation of the spatial light modulator 32. The thickness of the coating layer 323 between a waveguide 322 and a nanoresonator 324 is for example between 30 and 100nm, such a thickness thus allowing a light wave traveling through a waveguide 322 to activate a chain of nanoparticles 324a, 324b of a nanoresonator 324 placed above this waveguide 322.

[0085] Controlling these dimensions and ensuring the proper functioning of a spatial light modulator as described above requires specific expertise in the manufacturing process of such a spatial light modulator. Figure 7 illustrates a flowchart of the various stages of a manufacturing process for a spatial light modulator, according to a particular embodiment of the present invention, while Figure 8 illustrates the evolution of a sample to obtain a spatial light modulator, following the various stages of the process in Figure 7, according to a particular embodiment of the present invention.

[0086] The manufacturing process for a spatial light modulator as described above requires following the steps below, which are followed in the same order as that developed in this description.

[0087] The manufacturing process begins with an initial sample 32A comprising a substrate 321 on which at least one waveguide 322 is arranged. The dimensions of the substrate 321 and the number of light guides 322 are not limiting and correspond for example to the particular embodiments described above.

[0088] In a step 71, a coating layer 323 is deposited on the substrate 321 and on at least one waveguide 322 to form a coated sample 32B.

[0089] According to one particular embodiment, the coating layer 323 is deposited by spin coating, commonly known as spin coating. The coating layer 323 is, for example, made of hydrogen silsesquioxane (HSQ) resin, which is known to be a planarizing material and to transform into silica (SiO2) upon annealing (see step 73 below). Other materials are obviously possible, such as other types of resins like polymethyl methacrylate (PMMA), in particular.

[0090] In a step 72, the coated sample 32B undergoes a first heating, the first heating comprising, for example, a linear temperature increase to a first temperature over a first duration. According to an embodiment In particular, the first duration is between 25 and 35 minutes, preferably 30 minutes, and the first temperature is equal to 180 degrees Celsius.

[0091] For this step 72, the coated sample 32B is, for example, placed on a hot plate for 30 min with a temperature ramp reaching 180°C.

[0092] This second step allows the coated sample 32B to be heated progressively, thus avoiding any mechanical stress due to differential expansion of the different materials composing the coated sample 32B.

[0093] Once the coated sample 32B has been heated to a temperature equal to the first temperature, it undergoes, in a step 73, a second heating to a second temperature to obtain an annealed sample 32C.

[0094] The second heating comprises, for example, a linear temperature increase to the second temperature followed by a linear temperature decrease to a third temperature, the second heating 73 being carried out for a second duration. For example, the second heating is carried out in a tube furnace under nitrogen, the second duration being between 160 and 200 minutes and the second temperature being between 570 and 630, for example equal to 600 degrees Celsius.

[0095] Thus, the coated sample 32B is gradually heated to 600 degrees Celsius, for example linearly, allowing the coating layer 323, composed of hydrogen silsesquioxanes (HSQ) if applicable, to transform into silica (SiO2), which is the same material as the substrate 321 according to a particular embodiment. It should be noted that the second temperature is maintained for a sufficient duration for the transformation to occur throughout the entire material of the coating layer 323, or even for a longer duration to prevent any process drift and make it more robust. The sample is then an annealed sample 32C. Its temperature is then lowered to a third temperature, for example, the third temperature being equal to ambient temperature, i.e., a temperature of approximately 20 degrees Celsius.

[0096] The coating layer 323 has an initial thickness 'E' intentionally greater than the thickness 'e' required for the proper functioning of the spatial light modulator. Indeed, it is not possible, without rectification, to obtain a sufficiently controlled thickness and sufficient flatness to directly place the nanoresonators 324 on the surface 'S' of the coating layer 323 distal to the substrate 321.

[0097] Thus, in a step 74, the coating layer 323 of the annealed sample 32C is thinned to form a thinned sample 32D, that is to say that its initial thickness 'E' is reduced to the thickness 'e' required for the proper functioning of the spatial light modulator by rectifying the distal surface 'S' of the substrate 321.

[0098] Such thinning, also called rectification, is achieved, for example, by reactive ion etching, that is, by plasma etching by inductively coupled plasma-reactive ion etching, also known as ICP-RIE (from the English "Inductively Coupled Plasma-Reactive Ion Etching"). The gas used for this etching is preferably trifluoromethane or fluoroform (CHF3), although this can also be replaced by sulfur hexafluoride (SF6) or by a mixture of several gases. This plasma etching mechanism avoids contaminating the surface of the coating layer with chemical species other than those contained on the sample.

[0099] This type of process is preferred to other known industrial processes such as a chemical mechanical polishing process called CMP (from the English "Chemical Mechanical Process"). This technique uses a machine consisting of a polishing pad that is often much larger than the sample. The sample is periodically brought close to the pad, which rotates and onto which a liquid containing various chemical species is deposited to chemically etch the sample. However, this solution has the disadvantage of requiring direct contact with the sample, which can cause mechanical stresses within the sample, potentially damaging or even destroying it, or at least less dense materials such as more fragile dielectric materials. Furthermore, there is no real-time monitoring of the amount of material removed.Thus, even when the process is calibrated, checks are still necessary during this stage to ensure the thickness of material removed. Finally, the surface obtained after this process is often dirty, as the chemical solution used contaminates the surface and residues may have accumulated. It is therefore necessary to integrate an additional surface cleaning step, which can be complex, time-consuming, and costly.

[0100] According to one embodiment, the manufacturing process 7 includes an additional step of measuring the thickness of the coating layer 323 of the annealed sample 32C, the thinning being a function of this thickness. During this step, it is possible to continuously monitor the engraved height and / or the remaining material thickness using a laser positioned above the annealed sample and illuminating it at an angle of incidence normal to the surface 'S' to be ground, the laser emitting a light beam with a wavelength of 679 nm. The thinning of the annealed sample 32C is then controlled according to the measured material thickness to achieve, at the end of the step, a target thickness corresponding to the previously determined thickness 'e'.

[0101] At the end of step 74, the sample is a thinned sample 32D, the thickness of material above at least one waveguide 322 is equal to the thickness 'e' defined previously.

[0102] In a step 75, the plurality of nanoresonators 324 is deposited on the coating layer 323 of the annealed sample to obtain said spatial light modulator 32, in particular in the positions previously defined, for example directly above at least one waveguide 322, directly above the intersections of waveguides 322 when a lattice of waveguides 322 is present.

[0103] Such a process has the advantage of being simple to implement, as the machines used are often located in a cleanroom. The thinning step is carried out without direct contact with the sample and does not damage it. Finally, it is possible to monitor the etched height during the thinning step to precisely control the remaining material thickness and thus guarantee the proper functioning of the nanoresonators.

[0104] According to another embodiment, the coating layer is thinned locally, or even in such a way as to obtain different thicknesses in specific areas through the use of localized masks. Such a process is known to those skilled in the art and is, for example, used in reactive-ion etching, also known as RIE (from the English "Reactive-Ion Etching").

[0105] Of course, the present invention is not limited to the embodiments described above but extends to a spatial light modulator which would include secondary elements without going out of the scope of the present invention as well as to a method of manufacturing such a spatial light modulator.

[0106] The present invention also relates to a holographic image projection device comprising at least one spatial light modulator 32 obtained by the manufacturing process described above.

[0107] The present invention also relates to a vehicle, for example an automobile or more generally an autonomous land-powered vehicle, comprising at least one holographic image projection device described above.

Claims

Demands

1. Method of manufacturing (7) a spatial light modulator (32) intended to be illuminated by coherent light sources to project a holographic image, said spatial light modulator (32) comprising: - a substrate (321) on which is formed at least one waveguide (32A); - an encapsulation layer (323) placed on said substrate (321) and enveloping said at least one waveguide; and - a plurality of nanoresonators placed on the coating layer directly above said at least one waveguide (322), each nanoresonator (324) being optically activatable and being formed of at least one chain of nanoparticles, each nanoresonator (324) defining a pixel of said holographic image, said process being characterized in that it comprises the following steps: - deposition (71) of said coating layer (323) on said substrate (321) and on the at least one waveguide (322) to form a coated sample (32B),then - first heating (72) of said coated sample (32B) to a first temperature, then - second heating (73) of said coated sample (32B) to a second temperature to form an annealed sample (32C), then - thinning (74) of said coating layer (323) of said annealed sample (32C) to form a thinned sample (32D), then - deposition (75) of said plurality of nanoresonators on said coating layer (323) of said thinned sample (32D) to obtain said spatial light modulator (32).

2. Manufacturing method (7) according to claim 1, wherein said coating layer (323) is deposited (71) by centrifugal coating.

3. A manufacturing method (7) according to any one of claims 1 to 2, wherein said first heating (72) comprises a linear temperature rise to said first temperature, said temperature rise being carried out over a first duration.

4. Manufacturing method (7) according to claim 3, wherein said first time is between 25 and 35 minutes and said first temperature is equal to 180 degrees Celsius.

5. A manufacturing method (7) according to any one of claims 1 to 4, wherein said second heating (72) comprises a linear temperature rise to said second temperature followed by a linear temperature fall to a third temperature, said second heating (73) being carried out for a second duration.

6. Manufacturing method (7) according to claim 5, wherein said second heating is carried out in a nitrogen furnace, said second duration being between 160 and 200 minutes.

7. A manufacturing method (7) according to any one of claims 1 to 6, wherein said second temperature is between 570 and 630 degrees Celsius.

8. Manufacturing process (7) according to claim 7, wherein said coating layer (323) is composed of hydrogenosilsesquioxanes (HSQ) transformed into silica (SiO2) during said second heating (73).

9. Manufacturing method (7) according to any one of claims 1 to 8, wherein the thinning step (74) is carried out by reactive ion etching.

10. A manufacturing method (7) according to claim 9, further comprising a step of measuring the thickness of said coating layer (323) of said annealed sample, the thinning (74) being a function of said thickness.

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