A method and device for preparing a smooth wall surface of a microfluid channel based on mask projection lithography

Abstract

The application discloses a micro-flow channel smooth wall preparation method and device based on mask projection lithography, and belongs to the technical field of micro-fluidic chip preparation. The method comprises the following steps: substrate pretreatment, glue coating, mask alignment and projection exposure, development, PDMS reverse molding and post-processing, and can directly form three-dimensional micro-flow channels with high depth-width ratio and complex structure. The device is composed of a light source module, a mask module, a projection optical module and a substrate bearing module, and the core is to utilize the spatial frequency low-pass filtering characteristic of the projection objective lens: when exposed, the high-frequency details in the mask pattern beyond the optical cutoff frequency are filtered out, so that the micro-flow channel and chamber structure with smooth edges are directly formed on the photoresist. The characteristic reduces the stringent requirement for the edge processing precision of the mask, simplifies the mask preparation process, ensures the structural functionality of the micro-fluidic chip, and significantly reduces the manufacturing cost. The application introduces the semiconductor projection lithography technology into the micro-fluidic field, and provides a direct manufacturing solution for the micro-flow channel smooth wall, which is simple in process, high in precision and strong in flexibility.

Description

Technical Field

[0001] This invention relates to the field of microfluidic chip fabrication technology, and in particular to a method and apparatus for fabricating smooth walls of microfluidic channels based on mask projection lithography. Background Technology

[0002] Microfluidic chips, as important micro- and nano-devices, can precisely manipulate microfluidics at the micrometer to nanometer scale, thus showing broad application prospects in fields such as biomedical detection, chemical synthesis, environmental monitoring, drug screening, and tissue engineering. With the continued growth in demand for high-throughput, high-sensitivity, and low-cost detection in these fields, microfluidic chips are rapidly moving from laboratory research to practical applications, and the industrialization process is accelerating. Against this backdrop, how to achieve high-precision, low-cost, and high-efficiency fabrication of microfluidic channels has become a research hotspot in this field and one of the main bottlenecks restricting its industrialization.

[0003] Currently, the main methods for fabricating microfluidic channels include direct molding and 3D printing. Direct molding, represented by soft lithography, is the most widely used method. Its basic process involves first creating a male mold using photolithography, then pouring in polymer materials such as polydimethylsiloxane (PDMS), and finally peeling off the solidified layer to obtain a replica layer with the channel structure. While this method simplifies the process to some extent, it has significant drawbacks: due to the limited precision of the mold, the replicated channel pattern has low accuracy and poor dimensional uniformity. Especially in large-area chip fabrication, the channel aspect ratio and linewidth consistency are difficult to guarantee, severely affecting the chip's detection sensitivity and repeatability. In recent years, 3D printing technology has been attempted for the direct fabrication of microfluidic channels. This method involves building a three-dimensional channel structure through layer-by-layer stacking, or printing a master mold and then using polymer materials such as PDMS to cast a microfluidic chip. However, existing 3D printing methods suffer from low forming efficiency and long processing times per cycle; the types of printing materials available are limited; more importantly, due to the layer-by-layer stacking manufacturing principle, directly printing microfluidic channels or forming microfluidic channels by molding after printing a mold results in a significant step effect on the inner wall of the formed channel, leading to large wall roughness, which seriously affects fluid flow characteristics and detection accuracy. Furthermore, existing microfluidic channel fabrication devices related to photolithography also have many shortcomings. Current photolithography systems are mostly designed for semiconductor manufacturing, with dispersed structures and relatively independent functional modules, lacking a system integration design specifically for microfluidic chip fabrication.

[0004] This invention aims to overcome the shortcomings of the prior art by providing a method and apparatus for fabricating smooth walls of microfluidic channels based on mask projection lithography. The apparatus integrates a light source module, a mask module, a projection optics module, and a substrate support module. A 3D printing support is used to print a support, integrating all modules onto a vertical optical breadboard to achieve a high-precision, stable mask projection lithography system. This system projects a mask pattern onto a photosensitive material layer for exposure, and after development, a high-quality microfluidic channel structure master mold is directly obtained. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for fabricating smooth walls of microfluidic channels based on mask projection lithography.

[0006] S1. Substrate pretreatment: Select a photolithography substrate material, and perform cleaning, drying and surface modification treatment on the substrate material to obtain a pretreated substrate.

[0007] S2. Photoresist coating process: Photoresist is coated on the surface of the pre-treated substrate, and a uniform photoresist film layer is formed by coating and pre-baking.

[0008] S3. Mask alignment: The mask with the microfluidic channel pattern is installed into the mask projection lithography system, and the relative position of the mask and the photoresist film layer is adjusted to achieve pattern alignment.

[0009] S4. Projection Exposure: Turn on the photolithography light source, project the microfluidic channel pattern on the photomask onto the photoresist film layer through the projection lens, and complete the exposure process according to the preset exposure time to obtain the exposed substrate;

[0010] S5. Development process: The exposed substrate is placed on a horizontal heating stage for post-baking. After cooling, the glass slide is placed in the developing solution to remove the photoresist in the unexposed areas and form a photoresist master mold corresponding to the microfluidic channel pattern.

[0011] S6. PDMS molding: The PDMS prepolymer and curing agent are mixed in a preset ratio, degassed, and then poured onto the surface of the photoresist master mold. After heating and curing, the mixture is peeled off to obtain a PDMS preform with a microfluidic channel cavity structure.

[0012] S7. Post-processing: The PDMS preform is cut and drilled, and the PDMS die is bonded and sealed to the matching substrate to finally obtain the microfluidic channel finished product.

[0013] This invention proposes a method for fabricating smooth walls of microfluidic channels based on mask projection lithography. Utilizing the spatial frequency low-pass filtering characteristics of the projection lithography optical system, high-frequency components in the mask pattern (corresponding to sharp edges) are effectively filtered out when passing through the projection lens, resulting in smooth edges and corners and reduced stress concentration points on the pattern projected onto the photoresist layer. The resulting photoresist master mold exhibits smooth, straight microfluidic channel sidewalls with clear contours and no microscopic sharp corner defects. The PDMS preform obtained by casting from this master mold has high-quality channel inner walls, which is beneficial for stable fluid transport and low-shear force manipulation in microfluidic chips. In summary, this invention achieves higher-quality microfluidic channel master molds and PDMS finished products with a simplified process flow, balancing manufacturing efficiency and product quality.

[0014] A second aspect of the present invention provides a mask projection lithography system apparatus for implementing the above-described method, comprising: Light source module: used to provide the light beam required for exposure, the light source module includes an LED light source, a filter and a vertically movable collimating and homogenizing lens. Mask module: Used to carry mask patterns. Projection optical module: used to project a mask pattern onto a substrate, the projection optical module includes a 45° beam splitter, an observation screen and an imaging lens. Substrate support module: used for placing and fixing the substrate, the substrate support module includes a precision displacement stage.

[0015] Preferably, in the light source module, the LED light source is an ultraviolet LED light source with a wavelength of 365nm; the filter is a narrow-band filter used to filter out stray light of non-target wavelengths in the emission spectrum of the LED light source; the collimating and homogenizing lens is an ultraviolet fused silica plano-convex lens used to shape the beam emitted by the LED light source into collimated parallel light and uniformly illuminate the mask surface. The position of the collimating and homogenizing lens in the vertical direction is adjustable to adapt to different sizes of illumination spots and different projection magnification requirements.

[0016] Preferably, the mask module includes a mask holder and a mask template, the mask template being printed using a 3D printer, and the printing material being an opaque photosensitive resin.

[0017] Preferably, the projection optical module is used to project a mask pattern onto a substrate. The projection optical module includes a 45° beam splitter, an observation screen, and an imaging objective lens. The 45° beam splitter is used to split the light beam from the mask module into transmitted light and reflected light, wherein the transmitted light enters the imaging objective lens, and the reflected light is directed to the observation screen; the observation screen is used to observe the imaging quality and alignment status of the mask pattern in real time; the imaging objective lens is used to project the mask pattern onto a photosensitive material layer on the surface of the substrate at a preset magnification.

[0018] Preferably, the substrate carrying module is used to place and fix the substrate, and the substrate carrying module includes a precision displacement stage. The precision displacement stage is a multi-degree-of-freedom displacement stage, including translational degrees of freedom along the X, Y, and Z axes, used to achieve precise positioning of the substrate in space.

[0019] In this invention, projection lithography technology is used to replace traditional contact lithography, avoiding direct contact between the mask and the substrate, thus improving pattern fidelity and mask lifespan. Combined with a spectroscopic observation system, high-precision alignment of the transparent substrate is achieved. Furthermore, combined with a dedicated PDMS molding process, the invention features a short process flow, high pattern accuracy, good alignment accuracy, and high product consistency, making it suitable for the efficient fabrication of microfluidic chips. Attached Figure Description

[0020] Figure 1 is a complete flowchart of a method for fabricating smooth walls of microfluidic channels based on mask projection lithography proposed in this invention;

[0021] Figure 2 is a schematic diagram of the mask projection lithography system used in this invention. Detailed Implementation

[0022] As shown in Figures 1 and 2, Figure 1 is a complete flowchart of a method for fabricating a smooth wall of a microfluidic channel based on mask projection lithography according to the present invention, and Figure 2 is a schematic diagram of the structure of the mask projection lithography system of the present invention.

[0023] Referring to Figure 1, the present invention proposes a method for fabricating smooth walls of microfluidic channels based on mask projection lithography, comprising the following steps:

[0024] S1. Substrate pretreatment: Select a photolithography substrate material, and perform cleaning, drying and surface modification treatment on the substrate material to obtain a pretreated substrate.

[0025] Specifically, glass was selected as the substrate material. The glass slide was placed in anhydrous ethanol and deionized water, and ultrasonically cleaned for 1 minute each to remove surface organic contaminants and particulate impurities. After cleaning, it was dried with air and then placed in a plasma cleaner for 30 seconds at a power of 100W and an oxygen flow rate of 80 sccm to perform hydrophilic modification on the glass surface. After plasma treatment, the water contact angle on the glass surface decreased from approximately 70° before treatment to below 10°, which is beneficial for the uniform spreading and adhesion of subsequent photoresist.

[0026] S2. Photoresist coating process: Photoresist is coated on the surface of the pre-treated substrate, and a uniform photoresist film layer is formed by coating and pre-baking.

[0027] Specifically, the pretreated glass slide was placed on the platform of an automatic coating device, and an appropriate amount of SU-8 2075 negative photoresist was dropped onto the center of the glass slide. The coating speed was 1.5 cm / s to obtain a uniform photoresist film layer. After coating, the glass slide was placed on a horizontal hot plate for pre-baking: first baked at 65℃ for 10 minutes, then heated to 95℃ for 20 minutes to allow the solvent in the photoresist to fully evaporate. After pre-baking, the glass slide was transferred to a cooling stage to cool to room temperature. Microscopic calibration measurements showed that the thickness of the obtained photoresist film layer was 125 μm, and the thickness uniformity was better than ±5 μm across the entire field.

[0028] S3. Mask alignment: The mask with the microfluidic channel pattern is installed into the mask projection lithography system, and the relative position of the mask and the photoresist film layer is adjusted to achieve pattern alignment.

[0029] Specifically, the SU-8 photoresist film glass sheet is fixed on the substrate support module of the mask projection lithography apparatus. The mask template with microfluidic channel pattern is mounted on the mask stage. The displacement platform is adjusted, and the relative position of the mask template and the substrate is adjusted through the projection optics module and vision system until the mask pattern alignment mark coincides with the reference mark on the substrate, with an alignment accuracy better than 1μm.

[0030] S4. Projection Exposure: Turn on the photolithography light source, project the microfluidic channel pattern on the photomask onto the photoresist film layer through the projection lens, and complete the exposure process according to the preset exposure time to obtain the exposed substrate;

[0031] Specifically, the photolithography light source was turned on using a 365nm ultraviolet LED light source with an output power of 3W. The microfluidic channel pattern on the photomask was projected onto the SU-8 photoresist film layer through a projection lens. Based on the optimized conditions determined in advance through process window experiments, the exposure time was set to 12 seconds, corresponding to an exposure dose of approximately 250mJ / cm². After exposure, the light source was turned off, and the film was left to stand for 2 minutes to allow the heat generated during exposure to dissipate fully.

[0032] S5. Development process: The exposed substrate is placed on a horizontal heating stage for post-baking. After cooling, the glass slide is placed in the developing solution to remove the photoresist in the unexposed areas and form a photoresist master mold corresponding to the microfluidic channel pattern.

[0033] Specifically, the exposed glass slide was placed on a hot plate for post-baking: baked at 65℃ for 5 minutes, then heated to 95℃ for 15 minutes to ensure full cross-linking of the photoresist in the exposed areas. After post-baking, the glass slide was cooled to room temperature and immersed in SU-8 developer with gentle agitation for approximately 8 minutes. After development, the surface of the glass slide was rinsed with fresh developer, then rinsed with isopropanol and dried. Under an optical microscope, it was observed that the photoresist in the unexposed areas was completely removed, while the photoresist in the exposed areas remained, forming a clear photoresist master pattern. The microfluidic channel linewidth was 50 μm, with smooth and steep sidewalls and no residual photoresist at the bottom.

[0034] S6. PDMS molding: The PDMS prepolymer and curing agent are mixed in a preset ratio, degassed, and then poured onto the surface of the photoresist master mold. After heating and curing, the mixture is peeled off to obtain a PDMS preform with a microfluidic channel cavity structure.

[0035] Specifically, SYLGARD 184 PDMS prepolymer and curing agent were mixed at a mass ratio of 10:1 and stirred thoroughly with a glass rod for 10 minutes until homogeneous. The mixed PDMS prepolymer was placed in a vacuum desiccant for 30 minutes to degas until all bubbles were eliminated. The degassed PDMS prepolymer was slowly poured along the wall of a culture dish onto the surface of an SU-8 photoresist master mold, with a pouring thickness of approximately 2.5 mm. The culture dish was placed on a horizontal platform and allowed to stand for 5 minutes to allow the PDMS prepolymer to fully level. Then, it was transferred to a drying oven and cured at 65°C for 3 hours. After curing, the culture dish was removed and cooled to room temperature. The PDMS block was separated from the culture dish along the edge of the master mold using a cutting knife. The PDMS was carefully peeled off from the surface of the SU-8 master mold to obtain a PDMS preform with a microfluidic channel concave mold structure.

[0036] S7. Post-processing: The PDMS preform is cut and drilled, and the PDMS die is bonded and sealed to the matching substrate to finally obtain the microfluidic channel finished product.

[0037] Specifically, the PDMS preform is cut into single chips of the required size using a cutting tool. Holes are drilled at the inlet and outlet positions of the chips using a 1mm diameter punch to create through-holes. Another clean glass slide is ultrasonically cleaned with deionized water and dried. It is then placed in a plasma cleaner and treated at 50W for 1 minute, simultaneously activating the surface of the glass slide and the bonding surface of the PDMS chip. After treatment, the channel surface of the PDMS chip is quickly attached to the glass slide, and slight pressure is applied to ensure tight contact. The slide is then placed on a horizontal heating stage and heated at 120℃ for 20 minutes. This yields a sealed microfluidic channel. Deionized water is injected into the microfluidic channel, and observation under an optical microscope reveals no residual air bubbles, smooth liquid flow, and no leakage at the bonding interface, indicating a good bonding seal.

[0038] In this embodiment, the fabrication method is concise and efficient, eliminating the cumbersome intermediate steps of fabricating nickel and other metal molds in traditional soft lithography. The photoresist, after exposure and development, is directly used as the master mold for PDMS casting, significantly shortening the fabrication cycle and reducing process complexity and manufacturing costs. Utilizing the spatial frequency low-pass filtering characteristics of the projection lithography optical system, high-frequency components corresponding to sharp edges in the mask pattern are effectively filtered out when passing through the imaging objective, resulting in smooth edges and corners and fewer stress concentration points on the pattern projected onto the photoresist layer. The resulting photoresist master mold has smooth, straight microfluidic channel sidewalls with clear contours and no microscopic sharp corner defects. The PDMS preform obtained from this mold has high-quality channel inner walls, which is beneficial for stable fluid transport and low-shear force manipulation in microfluidic chips.

[0039] This embodiment provides a mask projection lithography system apparatus for implementing the microfluidic channel smooth wall fabrication method of the present invention.

[0040] Referring to Figure 2, the device includes a light source module 100, a mask module 200, a projection optics module 300, a substrate support module 400, and an optical breadboard 500. All of the above modules are fixedly mounted on the optical breadboard 500 to ensure the relative position of each optical element remains stable during exposure, reducing the impact of external vibrations on image quality.

[0041] Specifically, the light source module 100 is located at the top of the device, and its function is to provide a highly uniform and collimated ultraviolet exposure beam. Referring to Figure 2, the light source module 100 includes, along the optical path, an LED light source 110, a filter 120, and a collimating and homogenizing lens 130. The LED light source 110 uses a high-power ultraviolet LED chip with an output wavelength of 365nm and a maximum output power of 3W. The filter 120 is a narrow-band interference filter with a center wavelength of 365nm. The collimating and homogenizing lens 130 is an ultraviolet fused silica plano-convex lens. The vertical position of the collimating and homogenizing lens 130 can be adjusted by a vertically adjustable latch (adjustment stroke of ±5cm) to accommodate different sizes of illumination spots.

[0042] Specifically, the mask module 200 is positioned below the light source module 100, located between the light source module 100 and the projection optics module 300 along the optical path. Referring to Figure 2, the mask module 200 includes a mask holder 210 and a mask template 220. The mask holder 210 is fixed to the main body of the device with screws. The mask template 220 is fixed to the mask holder 210 by mechanical clamping. The mask template 220 is made of opaque photosensitive resin material, and a microfluidic channel pattern is directly printed on the mask template using a Shining 3D printer.

[0043] The projection optics module 300 is positioned below the mask module 200. Its function is to project the microfluidic channel pattern on the mask 220 onto the photoresist film layer on the substrate surface at a preset magnification. Referring to Figure 2, the projection optics module 300 includes, in sequence along the optical path, a 45° beam splitter 310, an observation screen 320, and an imaging objective lens 330. The transmitted light beam (carrying mask pattern information) from the mask 220 is incident perpendicularly onto the 45° beam splitter 310. The 45° beam splitter 310 is a semi-transparent and semi-reflective beam splitter with a splitting ratio of 50:50. After passing through the 45° beam splitter 310, the incident light beam is split into two paths: one is a transmitted light beam, which continues to propagate downwards along its original direction and enters the imaging objective lens 330; the other is a reflected light beam, which turns 90° horizontally and propagates to the observation screen 320. An observation screen 320 is positioned on the horizontal reflected light path of the 45° beam splitter 310 and is used to observe the imaging status of the mask pattern in real time. The observation screen 320 employs a CCD image sensor. An imaging objective lens 330 is positioned below the 45° beam splitter 310 and is used to project the pattern on the mask 220 onto the substrate surface at a certain magnification. In this embodiment, the imaging objective lens 320 uses a commercially available fixed-focus industrial lens with the following specific optical parameters: resolution of 3MP, focal length of 50mm, image plane size of 2 / 3 inch (corresponding to a maximum imaging circle diameter of approximately 11mm), aperture of F-number of 1.4, optical distortion less than -0.30%, back focal length of 17.526mm, and closest working distance of 0.3m. This lens uses manual focusing; the image sharpness can be adjusted by rotating the focusing ring.

[0044] The substrate support module 400 is located below the imaging objective lens 320 and includes a three-dimensional manual displacement platform 410. This platform has three degrees of freedom: X-axis translation, Y-axis translation, and Z-axis lifting. The X-axis and Y-axis translation is used to position the substrate in the horizontal plane, with a stroke of 25 mm, which can be finely adjusted manually using a micrometer screw, with an adjustment accuracy of approximately 10 μm. The Z-axis lifting is used to adjust the distance between the substrate and the imaging objective lens to achieve clear focusing of the projected pattern on the substrate surface, with a stroke of 10 mm and an adjustment accuracy of approximately 5 μm. The substrate can be placed directly on the upper surface of the three-dimensional manual displacement platform 410, maintaining stability under its own weight, without the need for vacuum adsorption or additional clamping mechanisms. This fixing method is suitable for exposure operations on small-sized substrates (such as 25 mm × 75 mm standard glass slides), and is simple to operate and convenient to place and remove.

[0045] In this embodiment, the device has a simple and compact structure, consisting only of an LED light source, a filter, a collimating and homogenizing lens, a mask holder, a 45° beam splitter, an imaging objective lens, an observation screen, and a three-dimensional manual displacement platform. It requires no complex automatic control system or precision-machined components, resulting in low cost and suitability for laboratory and small-scale production scenarios. During focusing, a beam splitting observation design is employed, with the observation optical path coaxial with the exposure optical path. The image on the observation screen accurately reflects the imaging state of the substrate surface, making operation intuitive. The alignment accuracy meets the requirements for fabricating microfluidic channels (linewidth ≥ 50 μm).

Claims

1. A method for fabricating smooth walls of microfluidic channels based on mask projection lithography, characterized in that, Includes the following steps: S1. Substrate pretreatment: Select a photolithography substrate material, and perform cleaning, drying and surface modification treatment on the substrate material to obtain a pretreated substrate. S2. Photoresist coating process: Photoresist is coated on the surface of the pre-treated substrate, and a uniform photoresist film layer is formed by coating and pre-baking. S3. Mask alignment: The mask with the microfluidic channel pattern is installed into the projection lithography system, and the relative position of the mask and the photoresist film layer is adjusted to achieve pattern alignment. S4. Projection Exposure: Turn on the photolithography light source, project the microfluidic channel pattern on the photomask onto the photoresist film layer through the projection lens, and complete the exposure process according to the preset exposure time to obtain the exposed substrate; S5. Development process: The exposed substrate is placed on a horizontal heating stage for post-baking. After cooling, the glass slide is placed in the developing solution to remove the photoresist in the unexposed areas and form a photoresist master mold corresponding to the microfluidic channel pattern. S6. PDMS molding: The PDMS prepolymer and curing agent are mixed in a preset ratio, degassed, and then poured onto the surface of the photoresist master mold. After heating and curing, the mixture is peeled off to obtain a PDMS preform with a microfluidic channel cavity structure. S7. Post-processing: The PDMS preform is cut and drilled, and the PDMS die is bonded and sealed to the matching substrate to finally obtain the microfluidic channel finished product.

2. An apparatus for fabricating smooth walls of microfluidic channels based on mask projection lithography to implement the method of claim 1, characterized in that, include: Light source module, mask module, projection optics module, and substrate support module. Light source module: used to provide the light beam required for exposure, the light source module includes an LED light source, a filter and a vertically movable collimating and homogenizing lens. Mask module: Used to carry mask patterns. Projection optical module: used to project a mask pattern onto a substrate, the projection optical module includes a 45° beam splitter, an observation screen and an imaging lens. Substrate support module: used for placing and fixing the substrate, the substrate support module includes a three-dimensional precision displacement stage.

3. The preparation method according to claim 1, characterized in that, The substrate material in step S1 is glass, and the surface modification treatment is plasma treatment.

4. The preparation method according to claim 1, characterized in that, The photoresist mentioned in step S2 is a negative photoresist with high viscosity. It is applied using an automatic scraping device, and after the scraping is completed, it is pre-baked in segments.

5. The preparation method according to claim 1, characterized in that, The wavelength of the photolithography light source in step S4 is 365nm; exposure is performed according to the optimized conditions determined in advance through process window experiments, and the exposure time meets the requirements for complete photosensitive reaction of the photoresist, so that the photoresist master pattern formed after development is complete and without residue.

6. The preparation method according to claim 1, characterized in that, The bonding seal described in step S7 is a plasma bonding followed by heating-assisted bonding.

7. The preparation apparatus according to claim 2, characterized in that, All modules of the device are assembled by vertically fixing self-printed brackets onto the same optical breadboard.

8. The preparation apparatus according to claim 2, characterized in that, The light source module of the device uses a 365nm wavelength ultraviolet light source, which has low spatial coherence and can achieve partially coherent illumination, effectively suppressing speckle noise and ensuring exposure uniformity.

9. The preparation apparatus according to claim 2, characterized in that, The mask module of the device is made using a 3D printer.

10. The preparation apparatus according to claim 2, characterized in that, The imaging objective is a fixed-focus industrial lens with an imaging field of view diameter of not less than 10mm.

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