Precise heterostructure diffraction optical element (DOE) design and processing method

Abstract

The invention relates to a design and processing method of a precise heterostructure diffraction optical element (DOE). The element structure is composed of a functional layer and a substrate layer, the functional layer is used for forming a phase-type diffraction structure, the substrate layer is used for bearing the functional layer, and the two layers of structures can be flexibly selected according to application. Based on a semiconductor processing technology, a design process and processing parameters are combined and considered, namely, the change condition of the refractive index of a functional layer material along with the temperature is measured and analyzed, the theoretical height of the diffraction structure at the post-baking temperature is accurately calculated, and the height of the diffraction structure is controlled by setting spin-coating parameters. A diffraction beam splitter and a beam shaper can be manufactured by using the design and processing technology. The heterostructure DOE design and processing method has the advantages of design accuracy, structure simplification, material selection flexibility, simple and efficient processing and the like, is helpful to meet the continuously increasing application requirements of DOE, and expands the application range of the DOE.

Description

Technical Field

[0001] The present invention relates to a precise heterogeneous structure diffractive optical element (DOE) structural design and processing method, specifically to a structural composition, design method and processing flow of a diffractive optical element using a substrate layer and a functional layer combination. This DOE has the advantages of precise design, simple structure, flexible material selection and simple processing. The application fields mainly include various types of beam shapers, beam homogenizers, laser beam splitters, etc. Technical Background

[0002] Different from the traditional refractive and reflective optical elements such as lenses, mirrors, and prisms commonly used in optical systems, the optical element that realizes the transformation and control of light distribution based on the diffraction principle is called a diffractive optical element (DOE). It has the advantages of light weight, small size, high diffraction efficiency, and low manufacturing cost. It can realize the functions of micro, array, integration, and arbitrary wavefront transformation that are difficult to accomplish with traditional optical elements. It is the future of miniaturization, integration, multi-function and low cost of optical systems.

[0003] In order to meet various requirements of practical applications, it is usually necessary to perform parameter control on the light emitted by the laser. According to the diffraction theory, we can manufacture micro-nano structures with specific morphology, refractive index, etc. on the substrate, so as to finely control the light wavefront passing through the micro-nano structure and produce diffraction phenomenon, and finally form the required light intensity distribution at a certain distance. The earliest diffractive optical element can be traced back to the amplitude-type Fresnel zone plate made by Rayleigh in 1871, and the binary optical element (BOE) proposed by the Lincoln Laboratory of the Massachusetts Institute of Technology in 1989 is the most mainstream diffractive optical element in the current application field. It is based on the theory of light wave diffraction, through computer-aided design, and using ultra-large-scale integrated circuit manufacturing technology (photolithography or micro-machining), a relief structure with two or more step depths is etched on the substrate to obtain a type of diffractive optical element with pure phase and high diffraction efficiency. This technology embodies the penetration and integration of optical and microelectronic technologies, and has been widely used in optical communications, information sensing, projection systems, precision testing, efficient scanning, entertainment consumption and other fields. The design of diffractive optical elements is an inverse problem of solving the control of diffractive structures with known input and target light field parameters. It is usually solved and calculated based on scalar diffraction theory and related algorithms (GS algorithm, Yang-Gu algorithm, etc.) or vector diffraction theory and related algorithms (coupled wave method, modal method, etc.). It is necessary to comprehensively consider factors such as design goals, structural feature dimensions, and calculation conditions to select a suitable design algorithm. However, no matter what theory and algorithm the diffractive optical element is designed based on, it ultimately needs to be manufactured into a finished product with a certain process in order to achieve actual results and meet the requirements of practicality and commercialization.

[0004] Most of the DOEs currently used are homogeneous structures, that is, they are made of a single material and are mainly made by mask etching, maskless direct writing and nanoimprinting. The traditional mask etching method has many processing steps, and the etching depth can usually only be controlled by etching time, so it is difficult to ensure the precise vertical height of the diffraction structure, and the processed sidewalls are relatively rough. These problems greatly limit the accuracy, reproducibility and efficiency of this process. Although the direct writing method has high accuracy, the equipment required is usually expensive, the processing speed is slow, and there is a problem that the contour depth and contour deformation of the etched pattern cannot be accurately controlled. It is usually more suitable for the high-precision production of single or small batch devices. In recent years, the emergence of nanoimprinting technology has realized the low-cost and batch preparation of DOEs of various sizes. The imprinting template can be reused indefinitely without damage, which greatly reduces the preparation cost and improves production efficiency. At present, more and more commercial mass-produced DOE products are manufactured using nanoimprinting, which also reflects the demand for simplified and efficient processing in this field.

[0005] With the diversification of DOE types and design methods, heterogeneous structure DOE has gradually been proposed, that is, each layer of the element is composed of different materials. As an alternative implementation method of homogeneous structure DOE, heterogeneous structure DOE usually has the advantages of relatively simple processing technology, the finished product can accurately achieve the design height of the diffraction structure, and is easy to mass produce, which makes it possible to realize DOE more accurately and conveniently. In addition, heterogeneous structure DOE also has many advantages such as high diffraction efficiency, freedom of element design, and a wide range of material selection. It is convenient to design the structure and material selection according to different application scenarios, which can expand the application scope of DOE in different industries and meet the future development needs of this field.

[0006] Therefore, actively exploring the design of new heterostructure DOE and its process implementation methods, and adopting simpler, more economical and efficient processing technology to obtain usable heterostructure DOE will help meet the growing application needs and has great practical significance. Summary of the invention

[0007] The present invention discloses a precise structural composition and design method of a heterogeneous structure diffractive optical element. Unlike most homogeneous structure diffractive optical elements formed by etching, this element achieves its working effect by processing the functional layer on the surface of the substrate into a diffractive structure, which not only simplifies the process flow and avoids various errors caused by etching, but also makes it easy to flexibly select the materials of each layer according to application requirements. Its structural schematic diagram is shown in Figure 1 As shown, it includes a functional layer 101 and a substrate layer 102, and each layer is characterized by:

[0008] The main function of the functional layer 101 is to serve as a phase-type diffraction structure. This layer is made of photosensitive polymer material. The diffraction structure pattern on the mask is transferred to the material by spin coating the material on the surface of the substrate layer and then exposing and developing it. The layer is characterized by a binary step-type structure distribution. Its lateral distribution is derived from the phase calculation results in the design stage, while the vertical height is comprehensively determined by the design wavelength λ, the refractive index n1 of the functional layer material, the refractive index n2 of the medium, the baking temperature T after processing, and the designed phase difference. It is calculated. It is generally believed that the refractive index n1 of the functional layer material is related to the design wavelength, but in fact it is also affected by the post-baking temperature during the processing. Therefore, in order to accurately calculate the theoretical value of the longitudinal height, the influence of the post-baking temperature must be considered. In the design process, the present invention first measures the refractive index of the selected functional layer material at different wavelengths and temperatures to obtain its accurate refractive index under the conditions of the incident wavelength and post-baking temperature used, thereby realizing accurate calculation of the longitudinal height. The calculation formula is as follows.

[0009]

[0010] Where n1(T) represents the refractive index of the functional layer material. n1 is a variable affected by temperature under the condition of the design wavelength. When the surrounding medium is air, n2=1. When the incident light passes through the functional layer, the phase difference formed by the diffraction structure of the layer will impose phase modulation on the incident light, thereby obtaining the required modulated output light. In particular, for the diffraction structure of the second-order phase, that is, the π phase difference, since the phase is periodic, the odd multiples of the phase difference can still be equivalent to itself. Therefore, the present invention points out that the height of the functional layer can be increased by an odd multiple to achieve an equivalent working effect (i.e. ). This method can break through the coating thickness limit of the selected functional layer material by flexibly adjusting the height multiple. Once the required height is determined, the required thickness can be obtained by setting the spin coating program during the processing, and the height change is also accompanied by the adjustment of the subsequent exposure measurement. The design of this functional layer reflects the comprehensive consideration of the subsequent processing conditions.

[0011] The main function of the substrate layer 102 is to serve as a carrier of the functional layer. This layer uses standard commercial wafers, such as silicon wafers, fused quartz wafers, sapphire wafers, etc., according to actual needs, so as to adapt to general semiconductor processing equipment and conditions. For applications with visible light wavelengths, transparent wafers should be selected; for applications with invisible light wavelengths, opaque wafers can be selected. Usually, multiple functional layer diffraction structure units can be manufactured and carried on a wafer at the same time. For example, about 150 5×5mm diffraction elements can be stored on a 4-inch wafer, so mass production of elements can be achieved.

[0012] The present invention also discloses a precise processing technology for heterogeneous structure diffractive optical elements. The process is combined with design and improved based on semiconductor processing technology, such as Figure 2 As shown. According to the needs, a suitable substrate wafer is selected, and the functional layer material is spin-coated on the wafer surface and exposed and developed to form a micron-level diffraction structure. The process includes eight main processes: substrate drying, spin coating adhesion promoter, spin coating functional layer material, pre-baking, exposure, post-baking, development, and rinsing. Among them, the steps that need to be improved in combination with the design are as follows:

[0013] (1) Spin coating the functional layer material: This is the initial preparation of the functional layer. An appropriate amount of the selected functional layer material is added to the surface of the wafer coated with the tackifier, and the material is evenly spin coated using a coating machine. The parameters set in the spin coating program here directly determine the height of the final diffraction structure. Therefore, it is necessary to first combine the precise calculation method in the design process to obtain the required height h before spin coating, or select an odd multiple of the height kh for spin coating.

[0014] (2) Post-baking: Since the baking temperature in this process will affect the refractive index of the functional layer material, the post-baking temperature selected in this step should be based on the measured and analyzed variation of the refractive index of the functional layer material with temperature, taking into account the refractive index n1(T) of the functional layer material under this condition, and incorporating it into the calculation of the design process to obtain an accurate longitudinal structure height.

[0015] After all the process steps are completed, the sample is properly cleaned to obtain the finished product, and subsequent surface profile tests and surface step morphology tests are performed. The processing process is simple and easy to operate, and the mass production of the heterostructure DOE can be achieved in a short time. The process parameters are combined with the design process for analysis, and the heterostructure DOE is processed more accurately, and the diffraction structure height is less different from the theoretical value.

[0016] The present invention also relates to a beam splitter and a beam shaper realized by the precise heterostructure diffractive optical element, which are realized by the above-mentioned design method and processing technology. First, the required phase distribution of the beam splitter and the beam shaper needs to be designed according to the relevant algorithm, and then the required structure height, that is, the functional layer height, is calculated according to various parameters.

[0017] By combining design and setting appropriate process parameters and conditions, beam splitters and beam shapers can be manufactured for use. Figure 3As shown, the incident light 301 is generally a collimated laser, which irradiates the heterostructure diffractive optical element 302, and after phase modulation, presents the desired output result 303 at the working distance. In addition, according to the working band selected for different applications (such as visible light, invisible light), and the different diffraction working modes expected to be achieved (such as projection type, reflection type), different substrate layer and functional layer materials can be appropriately selected, and the corresponding process parameters can be modified. Due to the high precision processing of the diffraction structure, it is expected that the zero-order light will have a weak impact on the working result of the element. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the heterostructure diffractive optical element of the present invention

[0019] Figure 2 This is a process flow chart of the heterogeneous structure diffractive optical element of the present invention

[0020] Figure 3 This is the working principle diagram of the beam splitter and beam shaper realized by the heterogeneous structure diffractive optical element of the present invention

[0021] Figure 4 This is a graph showing the change in refractive index of the functional layer material as a function of wavelength as measured in an example of the present invention.

[0022] Figure 5 This is a graph showing the change in refractive index of the functional layer material as a function of temperature as measured in an example of the present invention.

[0023] Figure 6 This is a real picture of the diffractive optical element of the heterogeneous structure beam splitter made in the example of the present invention

[0024] Figure 7 This is a basic morphology observation diagram of the heterostructure beam splitter diffractive optical element sample produced in the example of the present invention.

[0025] Figure 8 This is a three-dimensional result diagram of the sample of the diffractive optical element of the quality structure beam splitter produced in the example of the present invention.

[0026] Fig. 9 This is a profile measurement result of a sample of a diffractive optical element of a high-quality structure beam splitter manufactured in an example of the present invention.

[0027] Fig.10 This is a step profile measurement result diagram of a sample of a diffractive optical element of a heterogeneous structure beam splitter manufactured in an example of the present invention.

[0028] Fig.11 This is a test result diagram of the working effect of the 3×3 beam splitter produced in the example of the present invention. DETAILED DESCRIPTION

[0029] To further illustrate the technical points of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and examples, but they are not intended to limit the scope of the present invention.

[0030] like Figure 1 The heterogeneous structure diffractive optical element shown in the figure provides an embodiment of the present invention, that is, using the method of the present invention to realize a beam splitter at a wavelength of 1550nm.

[0031] (1) Design

[0032] Since this requirement is applied to the invisible light band, the substrate layer 102 uses a 4-inch double-polished silicon wafer. To achieve a micron-level diffraction structure, the functional layer 101 uses AZ 12XT-20PL-10 photoresist. The designed phase is second order, the phase difference is π, and the feature size is 1μm. Figure 4 The measurement results show that the refractive index of photoresist AZ 12XT-20PL-10 at a wavelength of 1550nm is about n1 = 1.564. Considering that the post-baking temperature required for this process is 90°C, and according to the measured relationship between the refractive index and temperature, as shown in Figure 5 As shown, its refractive index under this temperature condition is about n1(T) = 1.554, and the surrounding medium is air. Therefore, according to the formula

[0033]

[0034] That is, the functional layer height h=1398.92nm corresponding to the phase difference π can be accurately calculated. However, since the application thickness of the selected AZ12XT-20PL-10 photoresist is limited to about 6-10μm, it is impossible to spin-coat a single layer of photoresist with a height of h. Using the principle of odd-number multiples of height being equivalent, h=6.995μm (i.e., 5h corresponding to height) is taken in this example. The height of the processed diffraction structure can be accurately controlled by the spin-coating program during the processing.

[0035] (2) Processing

[0036] The specific processing flow of this component is as follows

[0037] 1. Bake and dry the silicon wafer on a constant temperature heating table for about 1 minute.

[0038] 2. Place the silicon wafer on the glue spreader, add about 3 ml of HMDS on the wafer surface for spin coating, and after the HMDS is spin coated, place the wafer in the vacuum plate of the glue spreader, and use the vacuum mode to bake at 110°C for 120s. After taking it out, let it stand and cool appropriately.

[0039] 3. According to the relationship between the spin coating thickness and the rotation speed of the photoresist used, when the spin coating speed is 4000rpm, the photoresist thickness is about 6.5μm, which is close to the expected thickness, so adjust the appropriate spin coating speed at about 4000rpm. Place the silicon wafer on the glue machine again, add about 3ml of photoresist on the wafer surface, and perform a two-step spin coating procedure: step 1 (speed 500rpm, acceleration 100rpm / s, time 5s), step 2 (speed about 4000rpm, acceleration 9000rpm / s, time 30s).

[0040] 4. Place the wafer with the photoresist spun on it into the vacuum hot plate of the (spin coating machine), and bake it at 110°C for 120 seconds in vacuum mode. After taking it out, let it stand and cool down appropriately.

[0041] 5. Place the processed wafer in the photolithography machine for exposure. From the thickness of the spin-coated photoresist, it can be seen that the required exposure dose is 105mJ / cm 2 , and then the automatic exposure time can be calculated based on the actual light intensity measured before this exposure. The exposure method is vacuum contact exposure.

[0042] 6. The exposed wafer is post-baked at 90°C for 60 seconds using a constant temperature heating table and then left to cool.

[0043] 7. Place the wafer on the coating machine, first quickly and evenly add about 15 ml of AZ 300MIF developer on the wafer surface at a low speed (speed 40 rpm, acceleration 900 rpm / s, time 5 s), then develop for 20 s, and finally shake off the developer at a high speed (speed 500 rpm, acceleration 900 rpm / s, time 5 s). This process needs to be repeated twice.

[0044] 8. Continue to use the glue spreader, first quickly and evenly add pure water to the wafer surface at a low speed (speed 40rpm, acceleration 900rpm / s, time 5s), let it stand for 20s, and finally shake off the pure water at a high speed (speed 500rpm, acceleration 900rpm / s, time 5s). This process needs to be repeated twice.

[0045] Figure 6 The figure shows a heterostructure beam splitter DOE for 1550nm incident light made by this process. The functional layer diffraction structure developed on the surface of the silicon substrate can be clearly seen by the naked eye, indicating that this process is feasible.

[0046] (3) Sample testing

[0047] Figure 7What is shown is the result of basic morphological observation of the sample using a confocal microscope. It can be seen that the shape and distribution of the diffraction structure of the functional layer on the surface of the silicon substrate layer are very clear, without obvious defects, indicating that the processed phase distribution is in good agreement with the design results.

[0048] Figure 8 Shown is the three-dimensional result of measuring the top surface of the sample using a confocal microscope. It can be seen that the three-dimensional shape of the diffraction structure of the functional layer is regular, with obvious protrusions and steep edges, indicating that the processed functional layer meets expectations.

[0049] Fig. 9 What is shown is the result of profile measurement of the sample using a confocal microscope. It can be seen that the outline of the diffraction structure of the functional layer is clear and the measured lateral line width of the structure is consistent with the design.

[0050] Fig.10 The results of step profile measurement of the sample using a step profiler show that the functional layer has steeper steps than the substrate layer. The measured height is 6.965μm, which is only 30nm different from the theoretical value of 6.995μm. This shows that the height of the processed functional layer steps meets the design requirements.

[0051] (4) Work performance test

[0052] Fig.11 The figure shows the working effect test of the 3×3 beam splitter designed and processed by the method of the present invention. It can be seen that the beam splitter makes the energy distribution of each point uniform. And because the height error is small, the zero-order light is weak, indicating that a good working effect has been achieved.

Claims

1. A precise design method for a heterogeneous structure diffractive optical element, wherein the longitudinal height design of the element diffractive structure is obtained by combining the parameter analysis of wavelength λ, refractive index n1 of the structural material, refractive index n2 of the medium, post-processing baking temperature T, and designed phase difference φ, that is, to achieve the required phase difference φ, the theoretical value of the longitudinal height is calculated according to the formula h=φ·{λ / 2п·[n1(T)-n2]}. The design method measures the refractive index of the selected structural material at different temperatures to obtain its accurate refractive index under the post-baking temperature conditions used, thereby realizing the precise calculation of the longitudinal height. The lateral phase design of the element diffraction structure is derived from the algorithm calculation results under the corresponding optical requirements.

2. A precise heterogeneous structure diffractive optical element, the element structure comprising: The functional layer is an upper structure and is used as a phase-type diffraction structure to modulate the phase of the incident light. The substrate layer is the lower structure used to carry the functional layer.

3. The functional layer as described in claim 2 is made of photosensitive polymer (such as commercial standard photoresist, etc.) according to actual needs to achieve a precise height structure that fits the design. The layer is characterized by a binary stepped structural distribution, which can realize various diffraction functions. In addition, by utilizing the periodicity of the phase, the calculated theoretical value of the longitudinal height can be increased by an odd number of times to equivalently achieve the expected design results under the restrictions of the use of different materials.

4. The substrate layer as described in claim 2 shall select commercial standard wafers according to actual needs. For applications with visible light wavelengths, transparent wafers (such as fused quartz wafers, sapphire wafers, etc.) shall be selected. For applications with invisible light wavelengths, opaque wafers (such as silicon wafers, etc.) shall be selected. A single wafer shall carry a large number of the functional layer diffraction structure units at the same time.

5. The diffractive optical element as described in claims 2 to 4 is realized by the design method as described in claim 1 and manufactured by the processing technology as described in claims 6 to 9.

6. A precise processing technology for heterostructure diffractive optical devices, which is improved based on the traditional photolithography process flow, and the main steps include: Substrate drying, spin coating adhesion promoter, spin coating functional layer material, pre-baking, exposure, post-baking, development, and rinsing.

7. The post-baking step as described in claim 6 requires first testing and analyzing the effect of baking temperature on the refractive index of the selected functional layer material, calculating its refractive index at the selected post-baking temperature, and then combining other parameters to accurately design the theoretical value of the longitudinal height of the diffraction structure at the post-baking temperature.

8. The step of spin coating the functional layer material as described in claim 6, using a coating machine to evenly spin coat the selected functional layer material on the surface of the wafer coated with the adhesion promoter, and controlling the spin coating speed to process the designed theoretical longitudinal height or increase the equivalent height.

9. The processing technology as described in claims 6 to 8, wherein various parameters of the process will be reasonably modified in combination with actual material selection and changes in processing objectives, and after the processing is completed, the sample surface profile test and step morphology test will be carried out to determine whether the finished product meets the theoretical design requirements and whether the process parameters need to be modified.

10. A beam splitter and beam shaper based on a precise heterostructure diffractive optical element, which is implemented by the design method of claim 1, implemented by the structure of claims 2 to 4, manufactured by the processing technology of claims 6 to 8, and quality inspected by the testing method of claim 9.

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