A system for implementing wet holographic lithography using optical diffraction elements

Abstract

The present application relates to a kind of system for realizing wet holographic lithography using optical diffraction element, the optical path of the system includes ultraviolet light source, collimating light path, optical diffraction element, photoelectric switch, the substrate coated with photoresist and immersion area and electric displacement table.Here, wet holographic lithography system has four different selection schemes: amplitude diffraction element positive or reverse placement;Phase diffraction element positive or reverse placement, the above four selection schemes have different emphases.The present application generates the optical path of holographic image at a set distance by optical diffraction element, and simultaneously carries out holographic lithography in water, to realize smaller lithography feature size.Give full play to the advantages of high stability and high adaptability of optical diffraction element, promote the development of lithography field.

Description

Technical Field

[0001] This invention relates to a system for realizing wet holographic lithography using optical diffraction elements, belonging to the fields of computational holographic imaging and holographic lithography. Background Technology

[0002] Photolithography is closely related to people's daily lives. Chips play a crucial role in devices such as mobile phones, computers, and automobiles. Photolithography is the core technology in chip manufacturing and integrated circuit printing. High-end technology industries such as supercomputer manufacturing, artificial intelligence, and deep learning also heavily rely on photolithography technology. It can be said that photolithography plays an indispensable role in technological progress and development. Photolithography refers to the process of transferring pre-designed circuit board or pattern information onto a substrate coated with photoresist using a photomask under ultraviolet light, thus achieving large-scale integrated circuit manufacturing. Considering the limitations of Moore's Law on integrated circuits, the world today has increasingly higher requirements for chip feature sizes. Smaller feature sizes mean smaller logic circuits, smaller chip volume, and a significant increase in computing performance. The feature size of a chip is determined by photolithography technology and is limited by the lithographic feature size.

[0003] Based on the optical diffraction limit, methods to improve photolithography performance can be divided into two categories: reducing the photolithography wavelength and increasing the photolithography numerical aperture. Currently, the wavelengths used in photolithography technology range from 190nm to 450nm, with commonly used wavelengths being 436nm, 365nm, 248nm, and 193nm. Researchers are also continuously seeking mature extreme ultraviolet (EUV) lithography technology to significantly improve photolithography performance. Increasing the photolithography numerical aperture is often achieved through photolithography in a high-refractive-index medium. Compared to photolithography in air, photolithography in water can achieve smaller feature sizes. However, wet photolithography has a more complex optical path and higher requirements for the stability of optical components, which increases the difficulty of improving photolithography performance. Optical diffraction elements, on the other hand, are diffraction units constructed using etching processes. Each diffraction unit has a specific morphology and size, allowing for the control of the incident light wavefront distribution to form a specific light intensity distribution. They also possess good stability and can be used as photolithography pattern generating devices. Summary of the Invention

[0004] This invention addresses the technical problem of overcoming the shortcomings of existing technologies by providing a system for wet holographic lithography using optical diffraction elements. The optical path through which the optical diffraction element generates a holographic image at a specific distance is immersed in water, and holographic lithography is performed simultaneously in the water to achieve smaller lithographic feature sizes. This fully leverages the high stability and adaptability of optical diffraction elements, promoting the development of the lithography field.

[0005] The technical solution of this invention is a system for wet holographic lithography using optical diffraction elements. The optical path of the system includes an ultraviolet light source, a collimating optical path, optical diffraction elements, a photoelectric switch, a substrate coated with photoresist, a water-immersion area, and an electric displacement stage. The laser output from the ultraviolet light source, after passing through the collimating optical path, becomes a parallel beam that illuminates the optical diffraction elements. A photoelectric switch is used in front of the optical diffraction elements to control the lithography exposure time. The parallel light illuminating the optical diffraction elements carries additional phase or amplitude information and continues to diffract forward, forming a holographic image in the designated water-immersion area. The substrate, with photoresist uniformly coated, is then exposed on the electric displacement stage, thus realizing wet holographic lithography of the optical diffraction elements. Wet holographic lithography involves a water-immersion area, where the refractive index of water (1.34) is greater than that of air (1). According to the optical diffraction limit, the feature size of wet holographic lithography is three-quarters that of dry holographic lithography. If the immersion area is replaced with a medium with a higher refractive index (such as oil), the size of the photolithographic feature will be further reduced.

[0006] The optical diffraction element typically consists of a highly stable transparent substrate and a metal film. Introducing the optical diffraction element into a wet holographic lithography system can meet the high stability requirements of wet lithography, while simplifying the optical path and improving the system's integration.

[0007] The optical diffraction elements can be divided into amplitude diffraction elements and phase diffraction elements, and can also be placed in either a forward or reverse orientation. Here, the wet holographic lithography system offers four different options: amplitude diffraction elements can be placed in either a forward or reverse orientation; phase diffraction elements can be placed in either a forward or reverse orientation.

[0008] When the amplitude or phase diffraction element is placed facing forward, the propagation distance of the holographic image needs to be greater than the substrate thickness. The reconstructed holographic image falls outside the substrate, enabling wet holographic lithography. However, when the amplitude or phase diffraction element is placed in reverse, there is no limitation on the imaging distance of the holographic image. When the phase diffraction element is placed in reverse, the refractive index of the substrate and the surrounding medium must be considered. When the difference in refractive index between the substrate and the surrounding medium is greater than 0.5, a phase difference of π can be achieved by etching the substrate to a shallow depth (less than one wavelength). The four selection schemes of the wet holographic lithography system have different focuses: to achieve high diffraction efficiency (greater than 10%), the holographic lithography system with the phase diffraction element placed facing forward is selected, with an efficiency of up to 40%. If the imaging distance of the holographic image is less than the substrate thickness (generally 500 μm), the holographic lithography system with the amplitude diffraction element placed in reverse is selected.

[0009] The advantages of this invention compared to the prior art are:

[0010] (1) In this invention, the optical path through which the optical diffraction element generates a holographic image at a set distance is immersed in water, and holographic lithography is performed in the water simultaneously. Compared with dry holographic lithography, the proposed wet holographic lithography system can reduce the lithographic feature size to three-quarters of the original size, which can give full play to the advantages of the diffraction element and promote the development of the lithography field.

[0011] (2) The present invention introduces optical diffraction elements into the wet holographic lithography system, which can improve the system stability, simplify the optical path of wet lithography, and improve the system integration.

[0012] (3) The wet holographic lithography system has high adaptability. Different experimental optical path systems can be built using phase or amplitude diffraction elements according to different holographic lithography requirements to achieve holographic lithography with high diffraction efficiency (up to 40%) or short imaging distance (less than the substrate thickness). Attached Figure Description

[0013] Figure 1 This is a flowchart illustrating the implementation of the method of the present invention;

[0014] Figure 2 This is a schematic diagram of wet holographic lithography for optical diffraction elements. In the diagram, a represents the amplitude diffraction element 6 placed facing forward; b represents the amplitude diffraction element 6 placed facing backward; c represents the phase diffraction element 10 placed facing forward; d represents the phase diffraction element 10 placed facing backward; 7 represents the immersion area; and 8 represents the photoresist.

[0015] Figure 3 The diagram shows the amplitude distribution of the input surface of the amplitude diffraction element and the holographic image of the output surface. In the diagram, 'a' represents the amplitude distribution of the input surface, and the right side shows a magnified view of the amplitude distribution. Black represents a pixel unit with an amplitude of 0, and white represents a pixel unit with an amplitude of 1. 'b' represents the intensity distribution of the holographic image of the output surface, and the right side shows a magnified view of the intensity.

[0016] Figure 4 This is a flowchart of the fabrication process for amplitude diffraction elements.

[0017] Figure 5 The diagram shows the optical path of a wet holographic lithography system, where 1 is the ultraviolet laser source, 2 is the first lens, 3 is the pinhole, 4 is the second lens, 5 is the photoelectric switch, 6 is the amplitude diffraction element, 7 is the immersion area, 8 is the photoresist, and 9 is the electric displacement stage. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0019] like Figure 1 As shown, wet holographic lithography using optical diffraction elements can be mainly divided into the following four steps: optimization of optical diffraction elements; fabrication of optical diffraction elements; construction of lithography experimental system; holographic lithography and development.

[0020] First, the amplitude or phase of the optical diffraction element needs to be optimized. A stochastic algorithm is used to optimize the binary amplitude (0 or 1) or binary phase (0 or π) of the optical diffraction element, and the diffraction from the input surface to the target surface is calculated using the Rayleigh-Sommerfeld diffraction integral. The cost function CF of the output holographic image is defined as CF = RMSE + k·SD, where RMSE is the root mean square error, which evaluates the similarity between the generated holographic image and the preset image; SD is the standard deviation, which can be used to enhance the uniformity of the holographic image; and k is the weight of the standard deviation SD, which can vary between 0 and 3. Since the overall size of the output holographic image is relatively small, k is set to 0.5 here. In the formula, I n Let N be the intensity at the nth pixel in the entire output holographic image, which has a total of N pixels; I represents the average intensity of the region with amplitude 1 in the holographic image. m I represents the intensity at the m-th pixel in the holographic image region, which has a total of M pixels; ideal The intensity of an ideal holographic image.

[0021] This example uses the amplitude optimization of an optical diffraction element. The amplitude diffraction element operates at a wavelength of 355nm. The input surface consists of 3001*3001 pixels, each with a pixel size of 323nm, resulting in a total input surface size of 969μm. The output surface consists of 551*551 pixels, each with a pixel size of 64.61nm, resulting in a total output surface size of 35.5μm. The propagation distance from the input to the output surface is 250μm. Randomly changing any pixel (x) on the input surface... n ,y n The amplitude of the cost function CF changes from 0 to 1 or from 1 to 0. If the value of the cost function CF decreases, the updated pixel (x) is retained. n ,y n The amplitude of the pixel (x) is updated; otherwise, the pixel (x) is not updated. n ,y n The amplitude of the holographic image is calculated. The same cost function (CF) is used to optimize the amplitude at all other locations until the amplitude of all pixel units on the entire input plane has been scanned. This global scan can be repeated multiple times to obtain a holographic image with uniformity and linewidth that meet the lithographic requirements.

[0022] Optical diffraction elements can be divided into amplitude diffraction elements and phase diffraction elements. In specific holographic lithography experiments, the orientation of the optical diffraction elements (forward or reverse) can also affect the experimental results. Figure 2Schematic diagrams illustrating four different wet holographic lithography methods for optical diffraction elements. In diagrams a and c, amplitude diffraction element 6 and phase diffraction element 10 are placed facing forward, respectively; in diagrams b and d, amplitude diffraction element 6 and phase diffraction element 10 are placed facing backward, respectively; 7 represents the immersion area; and 8 represents the photoresist. Because the elements in diagrams a and c are placed facing forward, the reconstructed holographic image must fall outside the substrate for holographic lithography to be performed. However, with the elements placed facing backward, the imaging distance issue for the holographic image is eliminated. In diagram d, the reverse placement of phase diffraction element 10 requires consideration of the refractive indices of the substrate and the surrounding medium. When the refractive index difference between the substrate and the surrounding medium is greater than 0.5, a shallow etching depth (less than one wavelength) is sufficient to achieve a phase difference of π. Here, the substrate is silicon oxide with a refractive index of 1.45; the surrounding medium is water with a refractive index of 1.34. Since their refractive indices are close, the holographic lithography method in diagram d is not recommended. In holographic lithography experiments, appropriate lithography methods can be selected based on different priorities. To achieve high diffraction efficiency (greater than 10%), Figure c can be selected; if the holographic image propagation distance is less than the substrate thickness (typically 500 μm), Figure b can be selected.

[0023] The above describes four different wet holographic lithography methods. The imaging distance of the holographic image is 250 μm, which is much smaller than the thickness of a commonly used silicon oxide substrate (500 μm), therefore, it can be used... Figure 2 The photolithography method of b is described, and its optimization and processing are introduced using the reverse placement of amplitude diffraction element 6 as an example. Figure 3 The optimized holographic images of the input and output surfaces of the diffraction element are shown. In the diagram, 'a' represents the amplitude distribution of the input surface, with a magnified view of the amplitude on the right. The overall size of the input surface is 969 μm, and the size of the right-hand region is 12.9 μm. 'b' represents the intensity distribution of the output surface holographic image, with a magnified view of the intensity on the right. The overall size of the output surface is 35.5 μm, and the size of the right-hand region is 7.8 μm. It can be seen that the holographic image generated using the amplitude distribution optimized by the random algorithm has a small linewidth (350 nm).

[0024] Once the amplitude of the amplitude diffraction element is known, the device needs to be fabricated. Since the pixel feature size of the diffraction element is 323nm, only electron beam lithography can be used for exposure. Figure 4 This is a flowchart of the fabrication process for an amplitude diffraction element. First, an aluminum film is deposited on a silicon oxide substrate to act as a metal mask and conductive layer. Next, an electron beam resist is uniformly coated onto the aluminum film. Then, an electron beam lithography machine is used to transfer the pattern onto the electron beam resist. After device development, a reactive ion etching machine is used to etch away the lower aluminum film until the bottom silicon oxide substrate is exposed. Finally, the remaining electron beam resist is removed to obtain the desired amplitude diffraction element.

[0025] Figure 5The experimental optical path for photolithography is demonstrated. A 355nm laser beam emitted from light source 1 passes through a collimated optical path composed of a first lens 2, a pinhole 3, and a second lens 4, becoming a parallel beam that illuminates the amplitude diffraction element 6. A photoelectric switch 5 is used in the optical path before the amplitude diffraction element to control the photolithography exposure time. Excessive exposure time will lead to overexposure, blurring the edges of the photolithographic pattern; insufficient exposure time will result in photoresist residue, leading to an uneven photolithographic pattern. The beam enters from the back of the amplitude diffraction element 6, and after passing through it, forms a uniform holographic image in the water-immersed region 7, 250μm from the surface. The stability of the water-immersed region 7 is crucial to the optical path setup. After passing through the amplitude diffraction element, the ultraviolet light needs to propagate in water and reconstruct the holographic image underwater. This means that the area between the lower surface of the amplitude diffraction element 6 and the upper surface of the photoresist 8 needs to be submerged in water to achieve a smaller linewidth holographic image. The final part of the optical path is the electric displacement stage 9, on which a silicon oxide substrate with photoresist 8 uniformly coated is fixed. The electric displacement stage 9 is used to control the longitudinal movement of the substrate and the lateral movement of the exposure area. By adjusting the surface of the photoresist substrate to coincide with the imaging surface of the holographic image using the electric displacement stage 9, the holographic image can be transferred onto the photoresist, realizing wet holographic lithography.

[0026] In summary, this invention provides a system for wet holographic lithography using optical diffraction elements. The optical path through which the optical diffraction element generates a holographic image at a specific distance is immersed in water, and holographic lithography is performed simultaneously in the water to achieve smaller lithographic feature sizes. This fully leverages the high stability and adaptability of optical diffraction elements, promoting the development of the lithography field.

[0027] The above embodiments are provided merely for the purpose of describing the present invention and are not intended to limit the scope of the invention. The scope of the invention is defined by the appended claims. Various equivalent substitutions and modifications made without departing from the spirit and principles of the invention should be covered within the scope of the invention.

Claims

1. A system for realizing wet holographic lithography using optical diffraction elements, characterized in that: The optical path of the system includes an ultraviolet light source, a collimating optical path, an optical diffraction element, a photoelectric switch, a substrate coated with photoresist, a water immersion area, and an electric displacement stage. The laser output from the ultraviolet light source, after passing through the collimating optical path, becomes a parallel beam that illuminates the optical diffraction element. A photoelectric switch controls the photolithography exposure time in front of the optical diffraction element. The parallel light illuminating the optical diffraction element carries additional phase or amplitude information and continues to diffract forward, forming a holographic image in the designated water immersion area. The substrate, fixed on the electric displacement stage and coated with photoresist, is then exposed, achieving wet holographic lithography of the optical diffraction element. The feature size of the wet holographic lithography is three-quarters that of the dry holographic lithography. The optical diffraction element is composed of a highly stable transparent substrate and a metal film; The optical diffraction elements are placed in both forward and reverse orientations; Optical diffraction elements are divided into amplitude diffraction elements and phase diffraction elements. There are four different options for this wet holographic lithography system: amplitude diffraction elements can be placed forward or backward, and phase diffraction elements can be placed forward or backward. When the amplitude or phase diffraction element is placed facing forward, the propagation distance of the holographic image needs to be greater than the substrate thickness, and the reconstructed holographic image falls outside the substrate, enabling wet holographic lithography. However, when the amplitude or phase diffraction element is placed in reverse, there is no limitation on the imaging distance of the holographic image. When the phase diffraction element is placed in reverse, the refractive index of the substrate and the surrounding medium must be considered. When the difference in refractive index between the substrate and the surrounding medium is greater than 0.5, a phase difference of π can be achieved by etching the substrate only shallowly, i.e., less than one wavelength. The four selection schemes of the wet holographic lithography system have different focuses: if a high diffraction efficiency of greater than 10% is to be achieved, the holographic lithography system with the phase diffraction element placed facing forward is the most efficient; if the imaging distance of the holographic image is less than the substrate thickness, the holographic lithography system with the amplitude diffraction element placed in reverse is the most efficient.

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