Oxygenation plasma surface bonding method based on microfluidics

Abstract

The invention relates to an oxygenation plasma surface bonding method based on microfluidics. The silicon wafer and the PDMS substrate are subjected to plasma cleaning under the specific vacuum degree, silanization modification on the surface of the silicon wafer is achieved, and effective bonding can be formed between the silicon wafer and the PDMS substrate; meanwhile, hot plate heating and oven baking treatment are combined, the silicon wafer and the PDMS substrate which are subjected to plasma treatment and attached are cooperatively heated in all directions, the bonding effect is further improved, and therefore the bonding strength and the yield are remarkably improved under the non-ultra-clean room condition. The bonding method does not need to be carried out under an ultra-clean room condition, has the advantages of low cost, high bonding performance and high yield, and is beneficial to large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of microfluidic chips and relates to a microfluidic-based oxygen plasma surface bonding method. Background Technology

[0002] Wet cleaning and dry cleaning are the two main traditional cleaning methods for surface cleaning. Wet cleaning uses liquids as the cleaning medium, such as steam cleaning, solution immersion cleaning, and rotary spray cleaning. Liquid cleaning easily introduces new impurities, making it ineffective for materials like steel without the addition of other effective components. Dry cleaning, on the other hand, uses pressure and suction to clean material surfaces (Jia Caixia, Wang Qian, Pu Yongwei, Aerospace Manufacturing Technology, 2016, 59, 95–98). Commonly used methods include mechanical cleaning, ultrasonic cleaning, dry ice cleaning, and plasma cleaning. Unlike ultrasonic cleaning's dependence on a binding solution and dry ice's limitations for large-scale cleaning, plasma cleaning requires no chemical reagents, does not introduce new contaminants, and has advantages such as low operating costs (Zhang Guozhu, Du Haiwen, Liu Liqin, Electromechanical Components, 2001, 21, 31–34), and has been widely used for cleaning various materials (Zhang Cheng, Special Equipment for Electronic Industry, 2006, 35, 21–27).

[0003] Unlike the three known states of matter—solid, liquid, and gas—plasma is often defined as the fourth state of matter. Plasma is defined as an ionized gas containing ions, free electrons, atoms, photons, and positive ions. The principle of plasma cleaning is to generate high-energy, disordered plasma under certain pressure using a radio frequency power supply in a vacuum chamber. This plasma bombards the surface of the product being cleaned, achieving the cleaning purpose. Therefore, plasma can effectively remove dust, organic matter, and other contaminants from the surface of specific materials, and the surface properties (bonding and surface wettability, such as hydrophobicity and hydrophilicity) also change after cleaning. Based on this characteristic of plasma cleaning altering the surface properties of materials, chip fabrication, especially in the field of microfluidic chips, has seen significant development. This is reflected in the use of hydrophilic bonding processes and steps between polydimethylsiloxane (PDMS) and silicon wafers.

[0004] Microfluidic chips are currently mainly used in biological research, such as for the rapid calculation of cell numbers (Liu Shoukun, Su Xianzong, Jin Qinghui, Jing Fengxiang, Zhao Jianlong, Sensors & Microsystems, 2009, 28, 100–102). The material used to fabricate microfluidic channels is polydimethylsiloxane (PDMS), and the chip substrate is primarily glass (or silicon dioxide). Combining microfluidic technology with sensor fabrication requires hydrophilic treatment of the microfluidic chip surface material. The surface of a successfully constructed microfluidic chip is mainly composed of metallic materials and silicon dioxide. Changing the chip surface material necessitates adjusting relevant operational parameters of plasma cleaning, such as radio frequency intensity, heating temperature, and time, to strengthen the bonding strength between PDMS and the silicon-based material. This prevents sudden pressure increases during microfluidic liquid inflow, which could cause liquid sealing problems in the microfluidic chip.

[0005] Traditional plasma bonding methods are typically performed in cleanrooms, resulting in high bonding costs. Furthermore, the use of a second gas (such as nitrogen, helium, neon, argon, or radon) for plasma treatment further increases costs and hinders large-scale production. Therefore, there is an urgent need for a plasma bonding method that can achieve excellent bonding strength, stability time, and yield while performing plasma bonding in non-cleanroom environments. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings and deficiencies of existing technologies and provide a microfluidic-based oxygenated plasma surface bonding method. This invention achieves silanolization modification of the silicon wafer surface by performing plasma cleaning on a silicon wafer and a PDMS substrate under a specific vacuum level, enabling effective bonding with the PDMS substrate. Simultaneously, by combining hot plate heating and oven baking, the plasma-treated and bonded silicon wafer and PDMS substrate are heated in all directions, further enhancing the bonding effect. This results in a significant improvement in bonding strength, bonding stability time, and yield even under non-cleanroom conditions. This bonding method does not require non-cleanroom conditions, offering advantages such as low cost, high bonding performance, and high yield, which is beneficial for large-scale production.

[0007] The objective of this invention can be achieved through the following methods: This invention provides a microfluidic-based oxygenation plasma surface bonding method, comprising the following steps: S1. Clean the silicon wafer and PDMS substrate separately (using nitrogen gas) and dry them, then seal and wrap them. S2. Under vacuum, the silicon wafer and PDMS substrate, after the seal packaging has been removed, are plasma cleaned and aligned for attachment. S3. The attached silicon wafer and PDMS substrate are heated by a hot plate and baked to obtain the microfluidic sensor.

[0008] As one embodiment of the present invention, in step S1, the silicon wafer includes a silicon substrate, a silicon dioxide layer, and a metal electrode arranged sequentially.

[0009] Furthermore, the thickness of the silicon dioxide layer is 100-200 nm; the metal electrode includes one of a platinum electrode, a nickel electrode, a titanium electrode, and a gold electrode, with a thickness of 100-200 nm.

[0010] Furthermore, the silicon wafer is prepared by the following steps: A1. Dry etching is performed on the surface of the silicon substrate to form a silicon dioxide layer; A2. Deposit hexamethyldisilazane on the surface of the silica layer and bake it to form a hexamethyldisilazane layer; A3. Spin-coating a forward-facing photosensitive material onto the surface of a hexamethyldisilazane layer; A4. A photomask with (three) through holes is used to cover the surface of the photosensitive material and then irradiated with ultraviolet light. A5. Remove the mask and perform plasma cleaning to remove the hexamethyldisilazane layer and residual positive photosensitive material in the areas not covered by the mask; then perform electron beam deposition to fill the pits formed after plasma cleaning, and use a stripping solution to remove the electrodes to obtain the metal electrodes, thus obtaining the silicon wafer.

[0011] Furthermore, in step A1, the dry etching temperature is 1000~1200 ℃, and the time is 4~6 h; the gas used for dry etching includes oxygen, and the oxygen flow rate is 15~25 sccm.

[0012] Furthermore, in step A2, the baking temperature is 120~130 ℃ and the time is 2~8 min.

[0013] Furthermore, in step A3, the spin coating includes a first spin coating of LOR10A forward photosensitive material and a second spin coating of Shipley1805 forward photosensitive material; wherein the spin coating rate for the first or second spin coating is 2500~3500 rpm / s and the time is 30~45 s.

[0014] Furthermore, in step A5, the stripping fluid includes SN905B (Kunshan Xingu Microelectronics Materials Co., Ltd., Suzhou, Jiangsu).

[0015] In one embodiment of the present invention, in step S1, the PDMS substrate is prepared by the following steps: B1. Negative photolithography is performed on the silicon wafer material to obtain a silicon wafer mold; B2. Mix dimethylsiloxane with a curing agent, remove air bubbles, pour into the silicon wafer mold, and cure to obtain a PDMS substrate.

[0016] Further, in step B1, the negative photolithography process includes: spin-coating photoresist on the surface of the silicon wafer material, covering the photoresist with a photomask containing microfluidic channels onto the photoresist, performing photolithography using a filter; then rinsing with a developer to obtain a silicon wafer mold after cleaning.

[0017] Furthermore, the photoresist includes SU-8 2010; the developer includes SU-8.

[0018] Furthermore, the spin coating comprises two stages; the first stage uses a rotation speed of 900-1100 rpm and an acceleration of 48-52 rpm / s for 6-10 seconds; the second stage uses an acceleration of 90-110 rpm / s to increase the rotation speed to 900-1100 rpm and maintains it for 25-35 seconds.

[0019] Furthermore, the photolithography time is 11-15 seconds. During the photolithography process, a filter is used to remove light waves below 340 nm, resulting in a final morphology with clean and sharp edges.

[0020] Furthermore, the elution time is 30-40 seconds; the cleaning is performed with isopropanol for 2-4 minutes, repeated 1-2 times. This is to remove residual SU-8 eluent from the surface.

[0021] Further, in step B2, the weight ratio of the dimethylsiloxane to the curing agent is 8:1-11:1, preferably 10:1.

[0022] As one embodiment of the present invention, in step S1, the cleaning process includes: cleaning with acetone, ethanol, isopropanol and deionized water in sequence.

[0023] In some embodiments, the sealing wrapping is performed using transparent tape or Scotch tape to prevent dust contamination.

[0024] In one embodiment of the present invention, in step S2, the vacuum degree is 30-50 Pa. In some embodiments, the vacuum degree is 50 Pa.

[0025] In one embodiment of the present invention, in step S2, the power of the plasma cleaning is 10.2-29.6 W and the time is 10-20 minutes.

[0026] This invention utilizes plasma cleaning under a specific vacuum level to achieve silanolization modification of the silicon wafer surface, forming Si-OH group clusters. This enables effective bonding with the polydimethylsiloxane surface ((CH3)3Si-O-Si(CH3)2-O-Si-(CH3)2-O-Si-(CH3)3), forming Si-O-Si chemical bonds, thereby improving the bonding effect and ultimately increasing the yield.

[0027] As one embodiment of the present invention, step S2 further includes: performing corona gun surface treatment on the silicon wafer and PDMS substrate after removing the sealing wrapping, and aligning and attaching them; the corona gun surface treatment is performed at 20-25°C for 8-12 seconds. In some embodiments, the corona gun surface treatment is performed at 1.013×10 5 Under Pascal ambient pressure and 20–25 °C temperature conditions, perform corona gun surface treatment for 10 seconds.

[0028] In one embodiment of the present invention, in step S3, the heating temperature of the hot plate is 90-120 ℃, and the heating time is 2-10 min. Too low a temperature will result in insufficient bonding; too high a temperature will cause deformation of the polydimethylsiloxane, leading to poor sealing.

[0029] In one embodiment of the present invention, in step S3, the baking temperature is 60-120 °C and the time is 60-72 h. Too low a temperature will result in insufficient bonding; too high a temperature will not significantly enhance the bonding effect, but will increase energy consumption.

[0030] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention achieves silanolization modification of the silicon wafer surface by performing plasma cleaning on the silicon wafer and PDMS substrate under a specific vacuum level, enabling effective bonding between the silicon wafer and the PDMS substrate, thereby improving the bonding strength between the two. Furthermore, this invention can also achieve effective bonding under non-cleanroom conditions by performing corona gun surface treatment on the silicon wafer and PDMS substrate.

[0031] 2. This invention creatively combines hot plate heating and oven baking to synergistically heat the plasma-treated and bonded silicon wafer and PDMS substrate in all directions, further improving the bonding effect. This results in a significant improvement in the bonding strength and yield between the silicon wafer and PDMS substrate under non-cleanroom conditions.

[0032] 3. The bonding method of the present invention does not require the use of non-cleanroom conditions, and has the advantages of low cost, high bonding performance and high yield, which is conducive to large-scale production. Attached Figure Description

[0033] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram illustrating the working principle of the bonding between the silicon wafer and the PDMS surface in this invention. Figure 2 This is a schematic diagram of the step-by-step drilling of the PDMS substrate according to the present invention; Figure 3 These are the front view and top view of the metal electrode on the silicon wafer surface of the present invention, wherein the left view is the front view and the right view is the top view; Figure 4 These are schematic diagrams illustrating the cleaning of silicon wafers and PDMS substrates in Examples 1-4; Figure 5 These are schematic diagrams illustrating the sealing and encapsulation of the silicon wafer and PDMS substrate in Examples 1-4; Figure 6 Schematic diagrams of plasma-cleaned silicon wafers and PDMS substrates in Examples 1-4; Figure 7 This is a schematic diagram of the sensor after it has been attached to the oven during baking in Examples 1-4. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The following examples are implemented under the premise of the technical solution of the present invention, providing detailed implementation methods and specific operating procedures, which will help those skilled in the art to further understand the present invention. It should be noted that the scope of protection of the present invention is not limited to the following embodiments; any adjustments and improvements made under the concept of the present invention are all within the scope of protection of the present invention.

[0035] The silicon wafers and PDMS substrates in the embodiments and comparative examples of this invention were prepared using the following methods: 1. Preparation of PDMS substrate First, the silicon wafer mold is made using the following steps: A silicon wafer mold for fabricating a PDMS substrate with grooved surfaces is obtained by negative photolithography on silicon wafer material. The negative photolithography method is as follows: First, photoresist is spin-coated onto the surface of the silicon wafer material. In the first stage, SU-82010 photoresist is used at a rotation speed of 1000 rpm and an acceleration of 50 rpm / s for 8 seconds. In the second stage, the acceleration is increased from 100 rpm / s to 1000 rpm and maintained for 30 seconds. Next, a pre-designed microfluidic channel photomask is applied to the photoresist and photolithography is performed for 13 seconds. During the photolithography process, a filter is used to remove light waves below 340 nm, resulting in a clean and sharp edge of the final morphology. Then, SU-8 developer is used for elution for 40 seconds, followed by isopropanol cleaning for 4 minutes. If necessary, the isopropanol can be replaced 1-2 times to remove residual SU-8 eluent from the surface, thus obtaining the silicon wafer mold.

[0036] Secondly, a PDMS substrate with grooves is prepared using the aforementioned silicon wafer mold.

[0037] Mix dimethylsiloxane and curing agent (Dow Corning SYLGARD 184 elastic monomer, Dow Corning elastic curing agent, order code: 01673921) in a 10:1 ratio, stirring thoroughly to remove air bubbles. After thorough mixing, place in a vacuum drying oven at room temperature (25°C) and evacuate until all air bubbles are removed from the liquid surface. Remove the mixture and pour it into a silicon wafer mold obtained through negative photolithography. The pouring process should be slow and concentrated at the center point, and the liquid surface should be kept balanced after pouring. Remove any air bubbles generated after pouring using a 23-gauge syringe, then place in an oven at 65°C overnight. Remove the overnight cured PDMS and use a scalpel to cut and demold it according to the microfluidic channels (2:1 aspect ratio) designed in the mold. Based on the inlet and outlet of the channel design, use a punch with a smaller aperture than the designed inlet size to vertically drill holes. To avoid damaging the designed pipes during drilling, the drill bit is used to drill a hole on one side of the PDMS to 3 / 4 of the solid surface. Then, the drill bit is flipped over to align with the inlet / outlet and drilled, thus completing the vertical drilling of the pipe inlet / outlet (drilling steps are as follows). Figure 2 As shown in the figure, a PDMS substrate with a thickness of 100 nanometers was obtained.

[0038] 2. Silicon wafer fabrication Forward etching of silicon dioxide surface: A silicon dioxide layer was formed on the silicon wafer surface by dry etching at 1200℃ and 25 sccm flow rate for 6 hours, followed by deposition of a hexamethyldisilazane layer and baking at 130℃ for 8 minutes. The forward photosensitive material LOR10A was used for the first spin coating at 3500 rpm / s for 45 seconds after baking. A second spin coating was performed using Shipley1805 at 3500 rpm / s for 45 seconds. The forward photosensitive material was then covered by a photomask with a pre-designed mask (a template with 3 holes) on the surface of the two spin-coated photosensitive materials and irradiated with ultraviolet light to remove it. After removing the mask, plasma cleaning was used to clean the areas not covered by the mask, removing the hexamethyldisilazane layer and residual forward photosensitive material. Electron beam deposition was then used to fill the pits formed after oxygen plasma treatment, thus forming a gold capping layer on the surface of the substrate. Finally, an SN905B (Kunshan Xingu Microelectronics Materials Co., Ltd., Suzhou, Jiangsu) stripping solution was used for electrode stripping to obtain... Figure 3 The electrodes shown are used to obtain a silicon wafer with a thickness of 100 nanometers.

[0039] Examples 1-4 Step 1: Place the silicon wafer and PDMS substrate into two separate beakers, and clean them sequentially in an ultrasonic cleaner with acetone, ethanol, isopropanol, and deionized water. Figure 4 ( ), sonicate for 5 minutes. After cleaning, dry the silicon wafer and PDMS substrate with nitrogen gas. Then, as follows Figure 5 As shown, the two parts are quickly wrapped with transparent tape to prevent dust from contaminating them.

[0040] Step 2: Place the PDMS and silicon wafer into a plasma cleaner (remove the tape before placing them into the cleaner), set the pressure to 30-50 Pa (i.e., vacuum), and continue for 10-20 minutes at a setting of 29.6 watts (i.e., high power). Figure 6 PDMS and silicon wafers will be processed in a plasma cleaner. Figure 1 The reaction shown involves rapidly degassing the equipment (3 seconds) after plasma cleaning, removing the silicon wafer and PDMS (2 seconds), and then aligning the PDMS with the silicon dioxide wafer surface within 6 seconds for attachment. The device is then heated on a hot plate at 90°C for 3 minutes. The attached sensor is then maintained in an oven at 100°C for 72 hours. Figure 7 For example, after baking for 72 hours, the microfluidic sensor can be removed and allowed to cool to room temperature before further use.

[0041] Comparative Example 1 The oxygenated plasma surface bonding method in this comparative example is basically the same as that in Example 3, except that the vacuum degree of the plasma cleaner is set to 100 Pa.

[0042] Comparative Example 2 The oxygenated plasma surface bonding method in this comparative example is basically the same as that in Example 3, except that the vacuum degree of the plasma cleaner is set to 20 Pa.

[0043] Comparative Example 3 The oxygenated plasma surface bonding method in this comparative example is basically the same as that in Example 3, except that the oven baking process is omitted.

[0044] Comparative Example 4 The oxygen plasma surface bonding method in this comparative example is basically the same as that in Example 3, except that the hot plate heating treatment is omitted.

[0045] The specific parameters of Examples 1-4 and Comparative Examples 1-4, as well as the corresponding test data on bond strength, bond stability time, and yield, are shown in Table 1.

[0046] Table 1. Comparison of different bonding parameters for bond pull test

[0047] *** indicates the strongest bond strength (greater than 29 N), allowing for control of liquid flow through the microfluidic sensor using different flow rates.

[0048] ** indicates that the bonding strength stability is generally low. If the liquid velocity through the microfluidic channel is higher than 5 μL / min, the bonding strength will decrease and leakage is likely to occur.

[0049] * indicates low bonding strength, meaning that PDMS and silicon wafers have already separated before the liquid is introduced.

[0050] As shown in Table 1, compared with Example 3, Comparative Examples 1 and 2 set excessively high and excessively low vacuum levels in the plasma cleaning machine, respectively, resulting in a weakening effect of silanolization modification on the silicon wafer surface, thereby reducing the bonding strength. Comparative Example 3 omitted oven baking, making it impossible to heat all orientations of the bonded silicon wafer and PDMS substrate, resulting in a significant decrease in its bonding strength and yield. Comparative Example 4 omitted hot plate heating, leading to a decrease in bonding strength and yield. It can be seen that hot plate heating plays a crucial role in ensuring the final yield within the critical 3 minutes after the first bonding step. Without hot plate heating, the bonding strength will be affected.

[0051] Example 5 The oxygen plasma surface bonding method in this embodiment is basically the same as that in Embodiment 3, except that the plasma cleaning in step 2 is replaced by corona gun surface treatment. The specific steps are as follows: Used to perform conditions without vacuum (at a laboratory pressure of 1.013 × 10⁻⁶). 5 Under Pascal conditions and at 20–25 °C, PDMS and silicon wafers were subjected to corona gun surface treatment for 10 seconds, and then the PDMS was placed and aligned onto the silicon dioxide wafer surface within 6 seconds for attachment. The attached sensor was then maintained in an oven at 100 °C for 72 hours, and surface bonding and pull tests were performed according to the same procedure. Specific parameters and test data are shown in Table 2.

[0052] Table 2

[0053] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A microfluidic-based oxygen plasma surface bonding method, comprising the following steps: S1, respectively, clean the silicon wafer and the PDMS substrate, and blow dry, then seal and wrap; S2, under vacuum, plasma clean the silicon wafer and the PDMS substrate after removing the seal and wrap, and align and attach; the vacuum degree of the vacuum is 30-50 Pa; S3, heat the attached silicon wafer and PDMS substrate on a hot plate, and after baking, obtain a microfluidic sensor.

2. The oxygenated plasma surface bonding method of claim 1, wherein, In step S1, the silicon wafer comprises a silicon substrate, a silicon dioxide layer, and a metal electrode arranged in sequence.

3. The oxygenated plasma surface bonding method of claim 2, wherein, The thickness of the silicon dioxide layer is 100-200 nm; the metal electrode comprises one of a platinum electrode, a nickel electrode, a titanium electrode, and a gold electrode, and the thickness is 100-200 nm.

4. The oxygenated plasma surface bonding method of claim 2, wherein, The silicon wafer is prepared by the following steps: A1, dry etching the surface of the silicon substrate to form a silicon dioxide layer; A2, depositing hexamethyldisilazane on the surface of the silicon dioxide layer and baking to form a hexamethyldisilazane layer; A3, spin-coating a positive photosensitive material on the surface of the hexamethyldisilazane layer; A4, covering the surface of the positive photosensitive material with a mask having a through hole, and irradiating with ultraviolet light; A5, removing the mask, performing plasma cleaning, and removing the hexamethyldisilazane layer and residual positive photosensitive material in the area not covered by the mask; Then, electron beam deposition of metal is performed to fill the pits formed after plasma cleaning, and electrode stripping is performed using a stripping solution to obtain a metal electrode, thereby obtaining the silicon wafer.

5. The oxygenated plasma surface bonding method of claim 1, wherein, In step S1, the PDMS substrate is prepared by the following steps: B1, negative photoetching of a silicon wafer material to obtain a silicon wafer mold; B2, mixing dimethylsiloxane with a curing agent, excluding air bubbles, pouring into the silicon wafer mold, and curing to obtain a PDMS substrate.

6. The oxygenated plasma surface bonding method of claim 1, wherein, In step S1, the cleaning process comprises: sequentially cleaning with acetone, isopropyl alcohol, and deionized water.

7. The oxygenated plasma surface bonding method of claim 1, wherein, In step S2, the power of the plasma cleaning is 10.2-29.6 W, and the time is 10-20 minutes.

8. The oxygenated plasma surface bonding method of claim 1, wherein, In step S3, the temperature of the hot plate heating is 90-120 ℃, and the time is 2-10 min.

9. The oxygenated plasma surface bonding method of claim 1, wherein, In step S3, the baking temperature is 60-120 ℃, and the time is 60-72 h. 10.A microfluidic sensor prepared by the oxygen plasma surface bonding method according to any one of claims 1-9.

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