Waveguide grating for realizing gradual change of diffraction efficiency, preparation method and equipment

Abstract

The invention discloses a waveguide grating for realizing diffraction efficiency gradual change, a preparation method and equipment. And the grating depth and the duty ratio are monotonically increased along the etching direction, so that the spatial gradient distribution of the diffraction efficiency is obtained. The method comprises the following steps of: firstly, forming periodic photoresist stripes on a substrate by utilizing an interference photoetching technology to obtain an initial grating pattern; and then, the substrate is placed in a plasma etching system, and the depth and duty ratio of the grating are monotonically increased along the etching direction by regulating and controlling the etching time or etching rate of different areas, so that the spatial gradient distribution of diffraction efficiency is realized. The waveguide grating disclosed by the invention effectively compensates attenuation of light in a waveguide transmission process, and the uniformity and imaging quality of AR display application and the like are remarkably improved. The problem of uneven brightness caused by light energy transmission attenuation of the waveguide grating in the prior art can be effectively solved, the manufacturing cost is low, the process expandability is high, and the waveguide grating is suitable for various waveguide optical display scenes such as mixed reality (MR) and virtual reality (VR).

Description

Technical Field

[0001] The present invention relates to waveguide grating technology, and particularly to a waveguide grating (including but not limited to an AR waveguide grating) with a gradually changing diffraction efficiency, a preparation method, and a device. Background Art

[0002] Augmented reality (AR) technology is reshaping the interaction paradigms in fields such as education, healthcare, and industrial manufacturing through the interactive experience of virtual-real fusion. With the breakthroughs in optical waveguide and Micro LED technologies, AR hardware is evolving towards lightweight and high light transmittance. Among them, diffractive optical waveguides have become the mainstream optical solution for consumer-grade AR devices due to their ultra-thin structure (thickness < 3 mm), light transmittance > 85%, and controllable mass production cost.

[0003] Diffractive optical waveguide technology has become the absolute mainstream of AR optical solutions due to its small size, low mass, high light transmittance, large viewing angle and eye movement range, and high mass production cost and process compatibility. Its core advantage lies in the integration of micro-nano optical design and advanced processes, which solves the contradiction between volume, light transmittance, and cost in traditional solutions and lays the foundation for the popularization of consumer-grade AR.

[0004] However, the core performance of diffractive optical waveguides - the field of view (FOV) and image uniformity - are directly restricted by the diffraction efficiency distribution accuracy of the surface grating structure. Currently, the main methods for manufacturing diffractive gratings are fixed mask etching, gray-scale lithography, and electron beam direct writing: 1) Although the fixed mask method has a simple process, it cannot achieve continuous gradation of the grating etching depth, resulting in a sharp drop in brightness at the edge of the field of view (measured attenuation > 30%); 2) Gray-scale lithography can adjust the etching depth through multi-level mask superposition, but it requires 5 - 7 mask alignments (overlay error up to ±0.5 μm), and the photoresist material is prone to develop residual after multiple exposures (surface roughness Ra > 10 nm), significantly reducing the consistency of the grating diffraction efficiency; 3) Although electron beam direct writing can achieve nanoscale accuracy (line width error < 5 nm), its writing speed is low (typical rate 0.1 mm 2 / s), and the processing time for a single device exceeds 24 hours, making it difficult to meet the requirements of large-scale production. The above methods have significant contradictions in the control of diffraction efficiency gradation, process complexity, and cost.

[0005] It should be noted that the information disclosed in the above background art section is only for understanding the background of the present application, and thus may include information that does not constitute the prior art known to those of ordinary skill in the art. Summary of the Invention

[0006] The main objective of the present invention is to overcome the defects existing in the above background art, and provide a waveguide grating, a preparation method, and a device for realizing a gradually changing diffraction efficiency.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] In the first aspect of the present invention, a method for preparing a waveguide grating with a gradually changing diffraction efficiency includes the following steps:

[0009] S1. Use interference lithography technology to form periodic photoresist stripes on a substrate;

[0010] S2. Place the substrate processed by interference lithography in a plasma etching system, and by adjusting the etching time or etching rate in different regions, make the grating depth and duty cycle increase monotonically along the etching direction, so as to obtain a spatial gradient distribution of the diffraction efficiency.

[0011] In the second aspect of the present invention, a device for realizing a waveguide grating with a gradually changing diffraction efficiency based on a moving mask time-varying etching method includes:

[0012] A vacuum chamber for providing a high-vacuum environment;

[0013] An anode and a cathode, arranged in the vacuum chamber, with the substrate fixed on the cathode;

[0014] A radio frequency power supply, connected to the anode and the cathode, for applying a high-frequency voltage to excite a gas mixture containing SF6 and O2 to form a plasma;

[0015] A movable baffle, located between the substrate and the anode, covering a partial area of the substrate at the initial position and gradually exposing the substrate surface during horizontal movement;

[0016] A gas injection unit for introducing a mixed gas of argon, SF6 and O2 into the vacuum chamber and adjusting the system pressure through argon;

[0017] Wherein, the translation path and speed of the movable baffle are controlled by a controller, so that the etching time of each area of the substrate shows a gradient distribution with the movement of the baffle, and the gradient control of the grating depth and duty cycle is realized in the spatial dimension. Thus, the diffraction efficiency of the prepared waveguide grating gradually increases in the coupling-out direction, avoiding the problem of uneven brightness caused by the attenuation of light energy with the transmission distance in the AR waveguide, and achieving higher display uniformity and imaging quality.

[0018] In the third aspect of the present invention, a waveguide grating with a gradually changing diffraction efficiency, the grating depth and duty cycle increase monotonically along the etching direction, so as to obtain a spatial gradient distribution of the diffraction efficiency.

[0019] The present invention has the following beneficial effects:

[0020] The present invention provides a waveguide grating for realizing a gradual change in diffraction efficiency and a preparation method thereof. The grating depth and duty cycle of the waveguide grating increase monotonically along the etching direction, so as to obtain a spatial gradient distribution of diffraction efficiency. When used as an AR waveguide grating, it ingeniously solves the problem of uneven brightness caused by the attenuation of light energy transmission in the existing AR waveguide grating, realizes a gradual change in the diffraction efficiency of the output grating along a specified direction, and thus obtains uniform brightness within the entire eye movement range, significantly improving the uniformity and imaging quality of AR displays. In terms of the preparation process, compared with the traditional fixed mask etching method, the present invention breaks through the limitation that it cannot achieve a continuous gradual change in the grating etching depth, and avoids the problem of a sharp drop in brightness at the edge of the field of view; compared with the gray-scale lithography method, the present invention does not require multiple mask alignments, effectively reducing the overlay error and the problem of development residue caused by multiple exposures, and improving the consistency of the grating diffraction efficiency; compared with the electron beam direct writing method, the present invention greatly improves the processing efficiency, shortens the single-device processing time, reduces the manufacturing cost, and is more likely to meet the requirements of large-scale production. In addition, the present invention forms a high-precision grating pattern based on the interference lithography technology, and realizes a gradual change in the diffraction efficiency through the spatial differential design of the etching time, with the advantages of simple process, high efficiency, and strong scalability. It is not only applicable to various waveguide optical display scenarios such as mixed reality (MR) and virtual reality (VR), but also can be extended to the fields of optical communication and sensing detection, providing an innovative and practical solution for the mass production of high-uniformity and large-field-of-view optical waveguides.

[0021] Other beneficial effects in the embodiments of the present invention will be further described below. Description of the Drawings

[0022] Figure 1 It is a schematic diagram of the uneven field-of-view brightness phenomenon in a traditional AR optical waveguide system.

[0023] Figure 2A It is the output grating of the traditional scheme and its spatial distribution diagram of diffraction efficiency.

[0024] Figure 2B It is the gradient output grating of the present invention and its spatial distribution diagram of diffraction efficiency.

[0025] Figure 3 It is the relationship between the grating diffraction efficiency, depth, and duty cycle simulated by FDTD.

[0026] Figure 4 It is a schematic diagram of laser interference lithography in the embodiment of the present invention.

[0027] Figure 5 It is a schematic diagram of the gradient etching method in the embodiment of the present invention.

[0028] Figure 6 It is a cross-sectional SEM diagram of the photoresist mask in the embodiment of the present invention.

[0029] Figure 7 This is the etching effect diagram of different etching times in the embodiments of the present invention. Detailed implementation manners

[0030] The following makes a detailed description of the implementation manners of the present invention. It should be emphasized that the following description is merely exemplary and not intended to limit the scope and application of the present invention.

[0031] It should be noted that when an element is referred to as "fixed to" or "disposed on" another element, it can be directly on the other element or indirectly on the other element. When an element is referred to as "connected to" another element, it can be directly connected to the other element or indirectly connected to the other element. In addition, the connection can be for a fixing function or for a coupling or communicating function.

[0032] It should be understood that the orientation or positional relationship indicated by the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the embodiments of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation of the present invention.

[0033] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the embodiments of the present invention, "a plurality" means two or more unless otherwise specifically defined.

[0034] The present invention aims to solve the problem of uneven brightness distribution of the coupling grating in waveguide (such as augmented reality (AR) waveguide) display. Specifically, in the prior art, when light travels in a waveguide, it will gradually attenuate, resulting in an uneven brightness distribution in the coupling region, such as the phenomenon that the left side is bright and the right side is dark. The present invention realizes a spatial gradient distribution of the diffraction efficiency of the grating through innovative design to compensate for the attenuation of the light intensity. When used as an AR waveguide grating, uniform brightness can be obtained within the entire eye movement range, improving the uniformity and imaging quality of AR display.

[0035] The embodiments of the present invention provide a preparation method for a waveguide grating realizing a gradual change in diffraction efficiency, including the following steps:

[0036] Step S1: Use interference lithography technology to form periodic photoresist stripes on a substrate; among them, periodic chemical reactions are generated in the photoresist through the interference of coherent light waves, and a photoresist stripe structure is formed after development.

[0037] Step S2: Place the substrate processed by interference lithography in a plasma etching system. By adjusting the etching time or etching rate of different regions and using gradient etching, the grating depth and duty cycle increase monotonically along the etching direction, thereby obtaining a spatial gradient distribution of diffraction efficiency.

[0038] Refer to Figure 2B , the waveguide grating prepared by this method realizes a gradient distribution of diffraction efficiency. In the prepared AR waveguide, after the incident light is coupled into the grating and enters the waveguide, it is transmitted to the output grating region through total internal reflection. Since the diffraction efficiency of the output grating shows a monotonic or gradient distribution along the specified direction, the light can compensate for the transmission loss during the output process, thereby achieving nearly uniform brightness within the entire field of view.

[0039] In some embodiments, in step S2, a movable baffle is used. By controlling the moving speed and position of the baffle, a gradient change in the effective etching time of different regions is realized; among them, initially, only part of the region is exposed to the plasma for etching through the shielding of the baffle; as the baffle moves, new regions are gradually exposed, and the etching time accumulates in sequence; finally, the entire substrate is exposed to the plasma for continuous etching for a period of time; through gradient etching, the depth and duty cycle of the grating change monotonically along the etching direction.

[0040] In some other embodiments, in step S2, at least one of the following methods is also used to adjust the etching time or etching rate of different regions. For example, segmented moving baffle: The baffle is divided into multiple pieces and is turned on sequentially or at different rhythms to achieve segmented etching time control, so that the depth and duty cycle of the grating increase monotonically along the etching direction. Another example is multiple exposure combined with multiple etching: Different exposure doses are used in the interference lithography stage to form multiple layers of photoresist stripes, and then etched for different durations respectively, so that the depth and duty cycle of the grating increase monotonically along the etching direction. Another example is additional mask layer control: A mask with a gradually variable thickness is first deposited on the surface of the substrate, and then uniform etching is carried out, so that the depth and duty cycle of the grating increase monotonically along the etching direction.

[0041] In some embodiments, in step S1, the interference lithography technology uses a helium-cadmium excimer laser. The laser beam forms a cylindrical plane wave after spatial filtering and collimating lenses, and an interference field is formed through the reflection of a Lloyd mirror to achieve exposure. As a preferred parameter configuration, the interference lithography technology uses a helium-cadmium excimer laser with a wavelength of 442 nm and a power of 180 mW. The laser beam forms a cylindrical plane wave with a diameter of 10 cm after spatial filtering and collimating lenses, and the exposure time is 40 seconds.

[0042] In some embodiments, in step S1, the cross-section of the developed photoresist stripes is thick in the middle and thin on both sides, which is formed by the combined action of the exposure time integral distribution, the development time gradient, the photoresist concentration gradient, and the development reaction slip effect.

[0043] In some embodiments, in step S2, the plasma etching is carried out in a high-vacuum environment. The etching medium is a gas mixture containing SF6 and O2, and the system pressure is adjusted by filling argon. The radio-frequency power supply excites the plasma to achieve etching.

[0044] In some embodiments, in step S2, the moving direction of the moving baffle is horizontal, and its moving speed and position are precisely controlled by the timing control system to match the preset gradient distribution of the grating diffraction efficiency. Further, the moving process of the baffle specifically includes: at the initial position, the baffle blocks most of the substrate area, and only a local area is exposed to the plasma; during the moving process, the baffle moves translationally at a constant speed or a variable speed, so that the etching time of each area of the substrate accumulates in the order of exposure; at the final position, the baffle completely moves out of the substrate range, and the whole substrate continues to be etched to ensure the continuity of the etching time.

[0045] In some embodiments, the spatial gradient distribution of the diffraction efficiency is in a monotonically increasing form, compensating for the linear attenuation of the light energy in the AR waveguide with the transmission distance, thereby achieving uniform brightness output.

[0046] In some embodiments, during the etching process, the groove depth and duty cycle of the grating are positively correlated with the etching time, and the synchronous increase of both is achieved through a single etching process.

[0047] Refer to Figure 5 , the embodiment of the present invention also provides a device for realizing a diffractive efficiency gradient waveguide grating, including: a vacuum chamber for providing a high-vacuum environment; an anode and a cathode arranged in the vacuum chamber, with the substrate fixed on the cathode; a radio-frequency power supply connected to the anode and the cathode for applying a high-frequency voltage to excite a gas mixture containing SF6 and O2 to form a plasma; a movable baffle located between the substrate and the anode, covering a local area of the substrate at the initial position and gradually exposing the substrate surface during the horizontal movement; a gas injection unit for introducing a mixed gas of argon, SF6, and O2 into the vacuum chamber and adjusting the system pressure by argon; wherein, the translation path and speed of the movable baffle are controlled by a controller, so that the etching time of each area of the substrate shows a gradient distribution with the movement of the baffle, and the gradient control of the grating depth and duty cycle is realized in the spatial dimension. Thus, the diffractive efficiency of the prepared waveguide grating gradually increases in the coupling-out direction, which can avoid the problem of uneven brightness caused by the attenuation of light energy with the transmission distance in the AR waveguide, thereby achieving higher display uniformity and imaging quality.

[0048] Refer to Figure 2B , an embodiment of the present invention further provides a diffractive efficiency gradient waveguide grating, where the grating depth and duty cycle increase monotonically along the etching direction, thereby obtaining a spatial gradient distribution of diffractive efficiency.

[0049] In some embodiments, the waveguide grating includes, but is not limited to, an AR waveguide grating.

[0050] By means of gradient etching, the embodiment of the present invention controls the grating depth and duty cycle to increase monotonically along the etching direction, realizing a spatial gradient distribution of diffractive efficiency, effectively solving the problem of uneven brightness caused by light intensity attenuation in the waveguide, improving the uniformity of display and imaging quality, and having the advantages of simple process, high efficiency, and good scalability, and is applicable to various waveguide optical display scenarios such as mixed reality (MR) and virtual reality (VR).

[0051] The principles, features, advantages, and experimental verification of specific embodiments of the present invention are further described below.

[0052] In an AR system, as Figure 1 shown, light from any point (denoted as A) on the display screen is converted into parallel light after passing through a collimating lens. When entering the coupling grating, the refractive angle of its positive first-order diffracted light exceeds the critical angle of total internal reflection, and thus can be transmitted in the waveguide by total internal reflection until it exits the grating region. At the same time, the negative first-order diffracted light maintains the same angle as the original incident light beam, so that parallel light with the same incident angle is formed at the output end. After refocusing on the human eye retina, these light rays visually present virtual images from an infinite distance.

[0053] However, the prominent problem existing in the prior art is that when the light beam is transmitted to the output grating (eye movement range) in the waveguide, the light energy will gradually decrease during the output process, resulting in a continuous weakening of the residual light energy in the waveguide, and thus causing an uneven distribution of brightness in the output region (as Figure 1 shown, the common phenomenon is that the brightness on the left side is higher and the right side is darker). The inventor realizes that if uniform brightness is to be obtained within the entire eye movement range, the diffractive efficiency of the output grating needs to increase gradually from left to right. Only by achieving a gradient distribution of diffractive efficiency on the output grating can the uneven phenomenon caused by the sequential loss of light energy in the waveguide be eliminated, and ultimately a better AR display effect can be achieved.

[0054] When solving the problem of uneven brightness of the output grating, it is first necessary to clarify the relationship between the transmission intensity of light in the waveguide and the diffractive efficiency. As Figure 2AAs shown, the output light intensity I(x) at any position x in the waveguide depends on the product of the residual light intensity I′(x) in the waveguide and the grating diffraction efficiency η(x) at that position. When the grating has a constant diffraction efficiency, the light intensity I′(x) in the waveguide will continuously decay as the transmission distance increases, resulting in a decrease in the output light intensity I(x). Eventually, a light field distribution that gradually darkens from left to right is formed in the output region.

[0055] Figure 2B Shows the spatial distribution of the diffraction efficiency of the graded output grating of the present invention.

[0056] To obtain a uniform output light field, it is necessary to make the diffraction efficiency η(x) able to compensate for the linear loss of light intensity in the waveguide, that is, to make η(x) increase monotonically with the increase of x. Assuming that the ideal output light intensity I(x) is a constant I(0), according to the law of conservation of energy, the light intensity I′(x) in the waveguide can be approximately regarded as linearly decreasing with the distance, that is

[0057] I′(x) = I′(0) - kx

[0058] where k is a constant. It can be further deduced that

[0059]

[0060] It shows that the grating diffraction efficiency needs to increase with the increase of x according to this functional form in order to obtain a uniform output light intensity within the full range. For surface relief gratings, the diffraction efficiency is mainly determined by the grating depth and duty cycle, and within a certain range, both of them have a monotonic relationship with the diffraction efficiency. Therefore, as Figure 2B shown, the present invention simultaneously changes the depth and duty cycle in the spatial dimension to achieve the required graded diffraction efficiency distribution. This graded grating structure places extremely high requirements on traditional manufacturing processes. Based on this, the present invention improves on the laser interference lithography technology and precisely controls the depth and duty cycle of the grating at different positions by moving the mask and realizing time-graded etching, so as to meet the above-mentioned design requirements for graded diffraction efficiency.

[0061] Simulation experiment

[0062] In order to verify the correctness of the theoretical derivation, the present invention conducts a simulation analysis on the first-order diffraction efficiency of a one-dimensional rectangular grating under normal incidence based on the FDTD method. As follows Figure 3As shown, the abscissa is the duty cycle of the grating, and the ordinate is the grating depth. The simulation results show that within a certain range, the first-order diffraction efficiency shows a monotonically increasing trend with the increase of depth and duty cycle. That is to say, by changing the depth and duty cycle of the grating respectively in this region, a gradual change distribution of the diffraction efficiency can be achieved. This conclusion further verifies the correctness of the design idea and theoretical analysis of the grating structure in the present invention, and provides a feasible guiding basis for the subsequent preparation process.

[0063] Preparation process

[0064] Interference lithography

[0065] As follows Figure 4 As shown, interference lithography uses two coherent plane light waves to form periodic bright and dark fringes on the substrate plane, and then produces local chemical reactions in the photoresist sensitive to this wavelength band. After exposure, the photoresist is affected by the developer to form a photoresist stripe structure with the same period as the interference fringes. Compared with traditional processing methods such as mechanical scribing or electron beam exposure, interference lithography has the following advantages: high period consistency, high processing efficiency, adjustable period, can prepare one-dimensional and two-dimensional gratings, low cost, etc.

[0066] A holographic lithography system can be built using a helium-cadmium excimer laser with a wavelength of 442 nm and a power of 180 mW. After the laser beam is focused by a 40-fold microscope objective lens, spatial filtering is achieved through a small hole with a diameter of about 15 μm, and then after passing through the aperture and collimating lens, it is converted into a cylindrical plane wave with a diameter of 10 cm. An interference field is formed by reflection of the Lloyd mirror, and the measured average brightness is about 1.6 mW / cm 2 . In the experiment, after many attempts, it is determined that a 40-second exposure can obtain a one-dimensional photoresist grating with the best contrast. After development, observed by a super-depth-of-field microscope, the cross-section of the photoresist stripe is thick in the middle and thin on both sides. Its morphology is mainly affected by factors such as the integral distribution of the exposure time, the development time, the concentration gradient of the photoresist, and the reaction gradient slip effect during development.

[0067] In the subsequent etching process, due to the sacrificial effect of the mask, both the groove depth and the duty cycle of the grating are positively correlated with the etching time. By controlling the etching duration, the groove depth and the duty cycle can be synchronously increased, thereby further adjusting the diffraction efficiency distribution of the grating. Based on this, the present invention proposes a process scheme for realizing a gradual etching time by using a moving baffle to meet the processing requirements of a grating with a gradually changing diffraction efficiency.

[0068] Gradual etching

[0069] As Figure 5As shown in the figure, plasma etching was carried out in a high-vacuum environment: a small amount of argon gas was filled into the vacuum system to adjust the pressure, and a gas mixture containing SF6 and O2 was introduced as the etching medium. The radio-frequency power supply applied a high-frequency voltage to the gas to excite the formation of plasma; the charged particles in the plasma were accelerated by the electric field between the anode and the cathode to bombard the surface of the substrate, achieving the etching effect. On this basis, a movable baffle was introduced to precisely control the effective etching time at different positions on the substrate surface.

[0070] Its working process is as follows:

[0071] 1. Initial position: The movable baffle is located above the substrate, most of the area is blocked, and only part of the area is exposed to the plasma and begins to be bombarded by ions.

[0072] 2. Moving process: As the baffle moves horizontally, new areas on the substrate are gradually exposed to the plasma, and the etching time at each position accumulates in sequence according to the moving order.

[0073] 3. Final position: When the baffle moves to the limit position, the entire substrate is exposed to the plasma and continues to be etched for a period of time to ensure that the etching time of no area is zero.

[0074] Through this coupled control of the movable baffle and the etching time, a gradual change in the etching depth can be achieved at different positions, thereby changing the duty cycle and depth of the grating, making the diffraction efficiency monotonically or gradually distributed along the specified direction, so as to meet the design requirements of the grating with a gradually changing diffraction efficiency.

[0075] Example

[0076] The following is a specific example, which proves that this scheme is actually feasible. The morphology of the photoresist after development is mainly determined by the integral distribution of the exposure time, the development time, the concentration gradient diffusion of the photoresist, and the reaction gradient slip model during development. The finally generated exposure stripe cross-section is thick in the middle and thin on both sides. Its SEM image is as Figure 6 shown:

[0077] Due to the sacrificial process of the mask during the etching process, both the depth and the groove duty cycle of the finally etched grating are positively correlated with the etching time. As Figure 7 shown, using different etching doses, the groove duty cycle and the depth increase synchronously.

[0078] Alternative embodiment

[0079] The deformation scheme focuses on the core goal of achieving a gradual change in the diffraction efficiency on the grating structure, and is achieved by technical means of applying unequal etching times or etching rates to different regions during the etching stage.

[0080] Segmented moving baffle: The baffle is divided into multiple pieces and opened sequentially or at different rhythms to achieve segmented etching time control.

[0081] Multiple exposures combined with multiple etchings: Different exposure doses are used in the interference lithography stage to form multiple layers of photoresist stripes, and then etched for different durations respectively to achieve a gradient effect.

[0082] Additional mask layer regulation: A mask with a gradually variable thickness is first deposited on the substrate surface, and then a unified etching is performed to achieve local etching depth differences.

[0083] Devices or methods that can provide different etching conditions in space (such as moving baffles, segmented masks, or variable mask thicknesses, etc.). Based on the controllable changes in grating depth and duty cycle with position, a gradient distribution of diffraction efficiency is achieved.

[0084] In the deformation scheme, even if a continuously moving mask structure is not adopted, the spatial distribution adjustment of the grating diffraction efficiency can be achieved through a segmented mask or other gradient etching strategies similar to the principle of the present invention.

[0085] In summary, the present invention proposes a method and device for preparing an AR waveguide grating with a gradually changing diffraction efficiency. This method first uses interference lithography technology to form periodic photoresist stripes on the substrate to obtain an initial grating pattern. Subsequently, the substrate is placed in a plasma etching system, and by precisely controlling the etching time or rate of different regions, the gradient regulation of the grating depth and duty cycle is achieved in the spatial dimension, so that the diffraction efficiency gradually increases in the coupling-out direction. This process effectively solves the problem of uneven brightness caused by the attenuation of light energy with the transmission distance in the AR waveguide, and significantly improves the display uniformity and imaging quality. The manufacturing cost of the present invention is relatively low, and the process has strong scalability. It is not only applicable to various waveguide optical display scenarios such as mixed reality (MR) and virtual reality (VR), but also can be extended to the fields of optical communication and sensing detection.

[0086] The core of the present invention lies in proposing the innovative technical idea of "gradually changing diffraction efficiency grating", and precisely controlling the grating depth and duty cycle through a moving mask combined with time-varying etching, so as to achieve a continuous gradient distribution of diffraction efficiency from left to right in the waveguide, compensate for transmission losses, and ensure the uniformity of the output light field. This technology realizes the continuous gradient distribution of the grating etching depth in a single lithography process through the coordination of dynamic mask translation and time-modulated etching, breaking through the accuracy limitation of the prior art for the gradual change of diffraction efficiency, and providing an innovative solution for the mass production of high-uniformity and large-field-of-view AR optical waveguides.

[0087] The method of the present invention not only utilizes the advantage of interference lithography to form a high-precision grating pattern, but also realizes the gradual distribution of diffraction efficiency through the spatial differential design of etching time. It has the advantages of simple process, high efficiency, and good scalability, providing a practical technical approach for mass production of gratings with gradually varying diffraction efficiency. Specifically, the present invention first uses interference lithography to form periodic photoresist stripes on a substrate to obtain a preliminary surface relief grating topography. Then, by using a movable baffle in a plasma etching system, the etching time of different regions of the substrate is gradually controlled, so that the grating depth and groove duty cycle change monotonically with position, forming a gradient distribution of diffraction efficiency. In an AR waveguide, when the incident light enters the waveguide through the coupling grating and is transmitted to the coupling-out grating region by total internal reflection, it is coupled out by gratings with different diffraction efficiencies at each position, thereby achieving nearly uniform brightness within the entire field of view.

[0088] The present invention has significant advantages, including: high uniformity, which can effectively compensate for the attenuation of light intensity during waveguide transmission and achieve consistent brightness within the eye movement range; process controllability, the combination of a movable baffle and time-gradual etching has more flexible and precise parameter regulation capabilities, ensuring a smooth transition of the grating diffraction efficiency gradient; relatively low cost, compared with complex segmented schemes or multiple lithography methods, this method can be achieved with slight improvements based on traditional interference lithography and conventional plasma etching, and the process is simple and easy to implement.

[0089] The application scenarios of the present invention include (but are not limited to):

[0090] MR / VR headsets: Mixed reality and virtual reality devices that require large field of view or multi-focal plane displays.

[0091] Optical communication field: In devices such as waveguide couplers or beam splitters, higher-efficiency optical distribution is achieved through gradually varying diffraction efficiency.

[0092] Sensing and detection: Using a gradually varying grating to improve the sensitivity to specific angles or wavelengths, which can be used in the fields of biochemical or environmental monitoring.

[0093] Laser processing: In a processing system with high requirements for uniform laser energy distribution, energy regulation is achieved through waveguides and gradually varying diffraction gratings.

[0094] The above content is a further detailed description of the present invention in combination with specific / preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several alternatives or modifications can be made to these described embodiments, and these alternative or modified forms should all be regarded as belonging to the protection scope of the present invention. In the description of this specification, the description with reference to the terms "an embodiment", "some embodiments", "preferred embodiments", "examples", "specific examples", or "some examples", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. Without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the scope of protection of the patent application.

Claims

1. A method for preparing a waveguide grating with a gradual change in diffraction efficiency, characterized in that: The following steps are involved: S1. Forming periodic photoresist stripes on a substrate using interference lithography technology; S2. Place the substrate processed by interference lithography in a plasma etching system, and adjust the etching time or etching rate in different areas to achieve a monotonically increasing grating depth and duty cycle along the etching direction, thereby obtaining a spatial gradient distribution of diffraction efficiency.

2. The preparation method according to claim 1, characterized in that: In step S2, a movable baffle is used to control the moving speed and position of the baffle to achieve a gradual change of the effective etching time of different areas; wherein, initially, only part of the area is exposed to the plasma to start etching due to the shielding of the baffle; as the baffle moves, new areas are gradually exposed, and the etching time accumulates sequentially; finally, the entire substrate is exposed to the plasma and continues to be etched for a period of time; through gradual etching, the depth and duty cycle of the grating are monotonically changed along the etching direction.

3. The preparation method according to claim 1, characterized in that: In step S2, the etching time or etching rate of different regions is regulated by at least one of the following methods: Segmented moving baffle: The baffle is divided into multiple pieces and opened in sequence or at different rhythms to achieve segmented etching time control, so that the depth and duty cycle of the grating increase monotonically along the etching direction; Multiple exposure combined with multiple etching: In the interference lithography stage, different exposure doses are used to form multiple layers of photoresist stripes, and then etching is performed for different lengths of time, so that the depth and duty cycle of the grating increase monotonically along the etching direction; Additional mask layer control: a mask with gradually variable thickness is first deposited on the substrate surface, and then uniformly etched so that the depth and duty cycle of the grating increase monotonically along the etching direction.

4. The preparation method according to claim 1, characterized in that: In step S1, the interference lithography technology uses a helium-cadmium excimer laser, and the laser beam is spatially filtered and collimated by a lens to form a cylindrical plane wave, which is then reflected by a Loey mirror to form an interference field to achieve exposure.

5. The preparation method according to claim 1, characterized in that: In step S1 , the cross section of the developed photoresist stripe is thick in the middle and thin on both sides, which is formed by the combined effects of the integral distribution of exposure time, the gradient of development time, the gradient of photoresist concentration and the slip effect of development reaction.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step S2, plasma etching is performed in a high vacuum environment, the etching medium is a gas mixture containing SF6 and O2, and the system pressure is adjusted by filling argon gas, and the radio frequency power supply excites the plasma to achieve etching.

7. The preparation method according to any one of claims 1 to 6, characterized in that: In step S2, the moving direction of the movable baffle is horizontal, and its moving speed and position are precisely controlled by the timing control system to match the preset gradient distribution of the grating diffraction efficiency.

8. The preparation method according to any one of claims 1 to 7, characterized in that: In step S2, the movement process of the baffle includes: In the initial position, the baffle shields most of the substrate, and only part of it is exposed to the plasma; During the movement, the baffle plate is translated at a uniform or variable speed, so that the etching time of each area of ​​the substrate is accumulated in the order of exposure; At the final position, the baffle is completely moved out of the substrate range, and the entire substrate continues to be etched to ensure the continuity of the etching time.

9. A device for realizing a diffraction efficiency gradient waveguide grating, characterized in that: include: A vacuum chamber, used to provide a high vacuum environment; An anode and a cathode are arranged in a vacuum chamber, and a substrate is fixed on the cathode; A radio frequency power source, connected to the anode and the cathode, for applying a high frequency voltage to excite the gas mixture containing SF6 and O2 to form plasma; A movable baffle is located between the substrate and the anode, covers a local area of ​​the substrate in an initial position, and gradually exposes the substrate surface during horizontal movement; The gas injection unit is used to introduce a mixed gas of argon, SF6 and O2 into the vacuum chamber and adjust the system pressure through argon; The translation path and speed of the movable baffle are controlled by a controller, so that the etching time of each area of ​​the substrate is distributed in a gradient as the baffle moves, thereby realizing gradient control of the grating depth and duty cycle in the spatial dimension.

10. A diffraction efficiency gradient waveguide grating, characterized in that: The grating depth and duty cycle increase monotonically along the etching direction, thereby obtaining a spatial gradient distribution of diffraction efficiency.

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