A dry holographic lithography method and system using a phase diffraction element

Abstract

The application relates to a method and system for realizing dry holographic lithography by using a phase diffraction element, and the phase mask of the phase diffraction element is optimized based on the cost function of an output surface holographic image, and can be used for phase diffraction element processing; a lithography experiment light path system is built to realize holographic lithography. The application does not need an imaging lens, only the phase of the diffraction element is optimized, the light field distribution of the diffraction light field at a specific distance is used, a small-size holographic image can be generated, and the holographic image can be used for dry holographic lithography, the cost of the lithography process is reduced, and a brand-new lithography system is opened up.

Description

Technical Field

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

[0002] Photolithography, a common micro-nano patterning technology, is a crucial process in the semiconductor industry and integrated circuit printing. Photolithography can be broadly categorized into three types: electron beam lithography, proximity lithography, and optical projection lithography. Electron beam lithography, under computer control, scans the resist area using a focused electron beam according to different mask pattern requirements. Therefore, the solubility of the resist varies in different areas, resulting in the designed mask pattern after development. Proximity contact lithography involves direct contact or a very small gap between the photoresist-coated substrate and the mask, followed by exposure using ultraviolet light, directly transferring the mask pattern onto the photoresist. Optical projection lithography utilizes the principle of optical projection imaging, projecting the pattern on the mask onto the substrate using lenses or mirrors. However, these lithography techniques suffer from complex optical paths, high costs, large size, and the need for objective lenses. For example, in electron beam lithography, multiple magnetic lens groups are required to focus the electron beam to achieve a smaller spot size. In optical projection exposure technology, an objective lens is required to project the mask pattern proportionally to a smaller size for exposure. Therefore, a new photolithography technology that is low-cost, highly integrated, and does not require an objective lens will promote the development of the photolithography and semiconductor fields.

[0003] Since photolithography systems do not use objective lenses, additional optical elements (such as optical diffraction elements) are required to generate small-sized exposure patterns. Optical diffraction elements are diffraction units constructed using etching processes. Each diffraction unit has a specific morphology and size, allowing for control over the wavefront distribution of incident light. After passing through each diffraction unit, the incident light undergoes diffraction, forming a specific intensity distribution that can be used to generate small-sized holographic images. Summary of the Invention

[0004] The technical problem solved by this invention is to overcome the shortcomings of existing technologies and provide a method and system for dry holographic lithography using phase diffraction elements. This method eliminates the need for imaging lenses and generates small-sized holographic images by optimizing the phase of the diffraction elements and utilizing the light field distribution of their diffracted light field at a specific distance. These images can then be used in dry holographic lithography, reducing the cost of the lithography process and opening up a completely new lithography system.

[0005] Technical solution of the present invention:

[0006] In a first aspect, the present invention provides a method for optimizing a phase diffraction element, which is implemented as follows:

[0007] Randomly change any pixel (x) on the input plane of the phase diffraction element n ,y n The phase of the holographic image at the output surface of the phase diffraction element (x) changes from 0 to π or from π to 0. If the value of the cost function CF of the holographic image at the output surface of the phase diffraction element decreases, then the pixel (x) is updated. n ,y n The phase of the pixel (x) is updated; otherwise, the pixel (x) is not updated. n ,y n The phase of the phase diffraction element is determined by the cost function CF; the phase of all other positions of the phase diffraction element is then optimized using the same cost function CF, thus obtaining the optimized phase of the phase diffraction element and completing the optimization of the phase diffraction element. When the output holographic image changes, the same phase diffraction element optimization method can be used, and the phase of the phase diffraction element will also change accordingly.

[0008] The cost function CF of the holographic image on the output surface of the phase diffraction element is defined as CF = RMSE + k·SD, where RMSE is the root mean square error; SD is the standard deviation; and k is the weight of the standard deviation SD, varying between 0 and 3. When the overall size of the output holographic image is smaller than the overall size of the input surface, the value of k is correspondingly smaller and can vary between 0 and 1. 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.

[0009] Secondly, the present invention provides a method for dry holographic lithography using phase diffraction elements. A phase mask is obtained using the phase diffraction element optimization method and used for phase diffraction element processing. After holographic lithography and development, dry holographic lithography of the phase diffraction elements is realized.

[0010] Thirdly, the present invention provides a system for dry holographic lithography using a phase diffraction element. The optical path of the system includes an ultraviolet light source, a collimated optical path, the optimized phase diffraction element, a photoelectric switch, a substrate coated with photoresist, and an electric displacement stage. The laser output from the ultraviolet light source is collimated into a parallel beam after passing through the collimated optical path and irradiates the phase diffraction element. The photoelectric switch is used to control the lithography exposure time in front of the phase diffraction element. The parallel light irradiating the phase diffraction element carries additional phase information and continues to diffract and propagate forward, forming a holographic image in a set Fresnel diffraction region. The substrate with uniformly coated photoresist fixed on the electric displacement stage is exposed to achieve dry holographic lithography of the phase diffraction element.

[0011] The advantages of this invention compared to the prior art are:

[0012] (1) The optimization method of phase diffraction element is to introduce the standard deviation weighting factor into the cost function, which can be adjusted according to the overall size of the output holographic image, thereby improving the uniformity of the holographic image and ensuring the quality of holographic lithography.

[0013] (2) Phase diffraction elements have high diffraction efficiency, simple optical path, and flexible use. Different phase diffraction elements can generate different holographic images for holographic lithography, expanding the practical application scenarios of optical diffraction elements.

[0014] (3) This invention provides a novel photolithography technology. Compared with traditional photolithography, the proposed holographic photolithography does not require the use of an objective lens system, which improves the system integration and reduces the cost of the photolithography process. Attached Figure Description

[0015] Figure 1 This is a flowchart illustrating the implementation of the method of the present invention;

[0016] Figure 2 The images are holographic images of the phase of the input surface and the output surface of the phase diffraction element; where a is the phase distribution of the input surface and the right side is a magnified view of the phase, and b is the intensity distribution of the holographic image of the output surface and the right side is a magnified view of the intensity.

[0017] Figure 3 This is a flowchart of the fabrication process for a phase diffraction element.

[0018] Figure 4 A schematic diagram of dry holographic lithography using a phase diffraction element, where 6 is the phase diffraction element and 7 is the photoresist;

[0019] Figure 5 The optical path diagram of the holographic lithography system is shown, where 1 is the laser source, 2 is the first lens, 3 is the pinhole, 4 is the second lens, 5 is the photoelectric switch, 6 is the phase diffraction element, 7 is the photoresist, and 8 is the electric displacement stage. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] like Figure 1 As shown, dry holographic lithography using phase diffraction elements can be mainly divided into the following four steps: phase optimization of diffraction elements; fabrication of phase diffraction elements; construction of the optical path for lithography experiments; holographic lithography and development.

[0022] First, the phase of the diffraction element needs to be optimized. A stochastic algorithm is used to optimize the phase of the optical diffraction element, where the phase of the diffraction element is a binary phase (0 or π). 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 = 0.5 is set 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.

[0023] The phase diffraction element operates at a wavelength of 355 nm. The input surface consists of 3001 x 3001 pixels, each with a pixel size of 323 nm, resulting in a total input surface size of 969 μm. The output surface consists of 1501 x 1501 pixels, each with a pixel size of 64.61 nm, resulting in a total output surface size of 96.9 μm. The propagation distance from the input to the output surface is 250 μm. Randomly changing any pixel (x) on the input surface... n ,y n The phase of the pixel (x) changes from 0 to π or from π to 0. If the value of the cost function CF decreases, the pixel (x) is updated. n ,y n The phase of the pixel (x) is updated; otherwise, the pixel (x) is not updated. n ,y n The phase of the holographic image is then optimized using the same cost function CF at all other locations until the holographic image quality meets the lithographic requirements.

[0024] Figure 2 The image shows the optimized phase distribution of the diffraction element's input surface and the holographic image of its output surface. In Figure a, a represents the phase distribution of the input surface, with a magnified view of the phase on the right. The overall size of the input surface is 969 μm, and the size of the right region is 12.9 μm. In Figure b, 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 96.9 μm, and the size of the right region is 7.8 μm. It can be seen that the holographic image generated using the phase distribution optimized by the random algorithm has a small linewidth (350 nm).

[0025] Once the phase of the phase diffraction element is known, the device needs to be fabricated. Since the pixel feature size of the diffraction element is 323nm, it can only be exposed using electron beam lithography. Figure 3 This is a flowchart of the fabrication process for a phase diffraction element. First, a chromium film is deposited on a silicon oxide substrate to act as a conductive layer. Then, an electron beam resist is uniformly coated on top of the chromium film. Next, 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 chromium film, exposing the bottom silicon oxide substrate. Using the upper electron beam resist and chromium film as a mask, the silicon oxide is etched to the appropriate depth according to the wavelength. To achieve a phase difference of π. In the formula, λ = 355 nm is the operating wavelength of the diffraction element; n is the refractive index of the substrate silicon oxide. Finally, the residual electron beam adhesive and metallic chromium film are removed to obtain the desired phase diffraction element.

[0026] Figure 4 This is a schematic diagram illustrating the use of a fabricated phase diffraction element to achieve dry holographic lithography. Ultraviolet light with a wavelength of 355 nm passes through the phase diffraction element 6 designed in this invention, forming a uniform holographic image in the Fresnel diffraction region. This image is then projected onto the photoresist 7, which can be used to achieve holographic lithography.

[0027] Figure 5 The 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 phase diffraction element 6. A photoelectric switch 5 is used in the optical path before the phase diffraction element 6. Turning the photoelectric switch 5 on and off controls the photolithography exposure time. Excessive exposure time leads to overexposure, blurring the edges of the lithographic pattern; insufficient exposure time results in photoresist residue and an uneven lithographic pattern. The parallel light illuminating the phase diffraction element 6 carries additional phase information and continues diffraction forward, forming a predetermined holographic image in the Fresnel diffraction region 250μm from the surface. The final part of the optical path is an electric displacement stage 8, on which a silicon oxide substrate uniformly coated with photoresist 7 is fixed. The electric displacement stage 8 controls the longitudinal movement of the substrate and the lateral movement of the exposure area. By carefully adjusting the surface of the photoresist substrate to coincide with the imaging surface of the holographic image using the electric displacement stage 8, the holographic image can be transferred onto the photoresist 7. At the same time, the optimal exposure time can be determined using the photoelectric switch 5, thus realizing dry holographic lithography of the phase diffraction element.

[0028] Here, the present invention provides a method and system for dry holographic lithography using phase diffraction elements. Without the need for imaging lenses, small-sized holographic images can be generated by optimizing the phase of the diffraction elements and utilizing the light field distribution of their diffracted light field at a specific distance. These images can then be used for dry holographic lithography, reducing the cost of the lithography process and opening up a completely new lithography system.

[0029] 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 method for optimizing a phase diffraction element, characterized in that, The implementation is as follows: Randomly change any pixel on the input surface of the phase diffraction element ( The phase of the phase diffraction element, i.e., the phase changes from 0 to π or from π to 0, is related to the cost function of the holographic image output from the phase diffraction element. If the value decreases, then update the pixel ( The phase of the pixel is updated, otherwise the pixel is not updated. The phase of ); using the same cost function The phase of the phase diffraction element is optimized at all other positions to obtain the optimized phase of the phase diffraction element, thus completing the optimization of the phase diffraction element. When the holographic image of the output surface changes, the phase of the phase diffraction element will also change accordingly using the same optimization method. The cost function of the holographic image output from the phase diffraction element Defined as In the formula The root mean square error, ), Standard deviation In the formula, For the entire output holographic image, the first The intensity at each pixel is totaled. 1 pixel; The average intensity of the region with amplitude 1 in the holographic image. For the holographic image region The intensity at each pixel is totaled. 1 pixel; The intensity of an ideal holographic image; Standard deviation The weights; The It varies between 0 and 3; when the overall size of the output holographic image is smaller than the overall size of the input image. The value is relatively small, varying between 0 and 1.

2. A method for dry holographic lithography using phase diffraction elements, characterized in that: A phase mask is obtained using the phase diffraction element optimization method according to claim 1, which is used for phase diffraction element processing. After holographic lithography and development, dry holographic lithography of the phase diffraction element is realized.

3. A system for realizing dry holographic lithography using phase diffraction elements, characterized in that: The optical path of the system includes an ultraviolet light source, a collimated optical path, an optimized phase diffraction element as described in claim 1, a photoelectric switch, a substrate coated with photoresist, and an electric displacement stage. The laser output from the ultraviolet light source is collimated into a parallel beam after passing through the collimated optical path and irradiates the phase diffraction element. The photoelectric switch is used in front of the phase diffraction element to control the photolithography exposure time. The parallel light irradiating the phase diffraction element carries additional phase information and continues to diffract and propagate forward, forming a holographic image in the set Fresnel diffraction region. The substrate with uniformly coated photoresist fixed on the electric displacement stage is exposed to achieve dry holographic lithography of the phase diffraction element.

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