A capacitive microfluidic pressure sensor, its fabrication method, and its microfluidic chip

Abstract

A capacitive microfluidic pressure sensor, its fabrication method, and a microfluidic chip are disclosed. The sensor comprises an upper, middle, and lower three-layer structure. The fabrication method includes: processing the upper, middle, and lower wafers to form a three-layer wafer bond; forming lead pads through magnetron sputtering; and finally dicing to fabricate the microfluidic pressure sensor. The upper, middle, and lower wafers are fabricated separately. This invention integrates with a microfluidic chip without disrupting the microchannel shape, enabling in-situ, real-time dynamic measurement of microfluidic pressure. The pressure sensor has a pressure measurement point diameter ranging from hundreds to thousands of micrometers, with a resolution of 1 Pa. This microfluidic pressure sensor is a silicon-based pressure sensor, using glass as the top cover to visualize the microfluidic chip. Compared to microfluidic pressure sensors used in polymer-based microfluidic chips such as PDMS, this sensor has lower hysteresis, shorter response time, and higher dimensional accuracy.

Description

Technical Field

[0001] This invention relates to an electronic component, its fabrication method, and its application, and particularly to a capacitive microfluidic pressure sensor, its fabrication method, and its microfluidic chip. Background Technology

[0002] Microfluidic chip technology is an important branch of MEMS (Micro-Mechanical, Microsystems, and Information) technology that studies how to miniaturize laboratory instruments onto chips to control, manipulate, and detect complex fluids at a microscale. With its advantages such as low time consumption, small sample size, high throughput, high precision, and ease of integration, microfluidic chip technology has demonstrated unique roles and enormous potential in fields such as drug analysis, chemical synthesis, microchannel heat exchange, and biosensoring. Microscale flow phenomena are fundamental to microfluidic chip research, and the control and measurement of flow parameters such as pressure and velocity are fundamental to this research.

[0003] For microscale flows, even minute pressure changes can significantly impact fluid flow, altering the experimental or analytical results of the entire microsystem. This necessitates that microfluidic pressure sensors meet stringent parameters such as resolution, accuracy, and response time. Furthermore, to achieve precise measurement of minute pressures and minimize external interference and pressure dissipation, the best approach for microfluidic pressure sensors is to integrate the pressure sensor with a microfluidic chip. This involves placing the pressure sensor on the wall of the microchannel (microfluidic flow path) for in-situ pressure measurement. Since the microchannels on the microfluidic chip are on the micrometer scale, the pressure measurement points are also limited to the micrometer level. In other words, the diameter of the diaphragm of the microfluidic pressure sensor integrated into the microfluidic chip is limited to the micrometer scale.

[0004] A diaphragm is a structure used in pressure sensors to sense pressure. When subjected to pressure, the diaphragm deforms, converting the mechanical signal into an electrical signal through principles such as piezoresistive and piezoelectric effects. For example, a conductive liquid can be injected into a pre-formed flexible cavity and then encapsulated. When the sensor deforms under pressure, the shape of the conductive liquid inside the cavity changes, leading to a change in the liquid's resistance. The magnitude of the applied force can be calculated by measuring this resistance change. Currently, diaphragms in microfluidic pressure sensors capable of in-situ measurements are often based on polymer materials such as PDMS. These materials have a low Young's modulus, often resulting in high hysteresis, longer response times, and poor dynamic characteristics during testing. This leads to severe distortion of test results in dynamic pressure measurements, significantly reducing the reliability of the device.

[0005] In addition to in-situ measurement methods, other approaches for measuring microfluidic pressure include opening holes along the microchannel to expose the microfluidic fluid for testing, and calculating pressure by observing the size of air bubbles in the fluid. However, these methods disrupt the original flow state of the microfluidic fluid, often resulting in inaccurate measurements and less convincing data.

[0006] Existing independent pressure sensors, such as patents CN101290255A and CN103278270A which use the piezoresistive principle, patent CN109141731A which uses the piezoelectric principle, and CN114459666A which uses the capacitive principle, all describe sensors as independent units. Their structures do not include microchannel structures, so they do not have the ability to be integrated with microchannels and cannot be placed on the walls of microchannels.

[0007] Existing microfluidic pressure sensors that can be integrated into microchannels, such as patents CN110132479A, CN111998985A, CN111024295A, and CN107860797A, use polymer materials such as PDMS as the diaphragm and walls of the microchannel. This often results in high hysteresis, longer response times, and poor dynamic characteristics during testing, leading to severe distortion of test results in dynamic pressure measurements and significantly reducing the reliability of the device.

[0008] Patent CN111024295A sets a cavity in the microchannel, and patent CN104568288A sets a pressure measuring branch on the wall of the microchannel and inserts a capillary tube. This disrupts the original flow state of the microfluidic and may cause the experimental data to deviate from the actual situation.

[0009] Patent CN109738113 describes a method that injects immiscible microbubbles into the inlet of a microchannel. By observing the size and relative volume changes of the microbubbles during flow, the relative pressure changes at different locations within the channel are calculated using the gas law. Patent CN107930708A chooses to use a camera to capture the direction and distance of movement of the liquid interface, thus converting this into pressure changes. However, methods that calculate pressure by observing bubble size and liquid level changes are prone to errors and have low measurement accuracy.

[0010] For example, patent application number CN2013102209229, publication number CN103278270A, discloses a silicon-glass micro pressure sensor chip with an island-film self-encapsulation structure and its manufacturing method, relating to micro pressure sensors. It provides a silicon-glass micro pressure sensor chip with an island-film self-encapsulation structure and its manufacturing method that not only has high reliability but is also suitable for harsh environments such as humidity, acid and alkali, and static electricity. The silicon-glass micro pressure sensor chip with the island-film self-encapsulation structure has a pressure-sensing film and a base with a cavity; the pressure-sensing film is a front-side island-film composite structure, and four piezoresistors are provided in the area of ​​greatest stress concentration in the island-film composite structure. The four piezoresistors form a Wheatstone bridge through metal electrodes. The Wheatstone bridge is sealed in a closed pressure-absolute cavity using a silicon-glass anodic bonding process. The Wheatstone bridge is connected to an external testing device via metal leads, forming a complete pressure-sensing and measurement system. The process involves fabrication on an SOI wafer; substrate preparation; bonding and subsequent processes. Application No.: CN2011104439739, Publication No.: CN103185612A, discloses a single-silicon wafer microflow sensor suitable for surface mount packaging and its fabrication method. The single-silicon wafer microflow sensor includes a single-crystal silicon substrate, two pressure sensors, and a microfluidic channel with inlet / outlet ports. This invention uses a single-silicon wafer single-sided silicon micromachining method to fabricate the microfluidic channel, pressure tapping channel, and reference pressure cavity of the pressure sensor inside the single-crystal silicon substrate, and cleverly integrates the two pressure sensors and the inlet / outlet ports of the microfluidic channel on the same surface of the same single-crystal silicon substrate, resulting in a simple structure.

[0011] Application No.: CN2013102205622, Publication No.: CN103335753A, discloses a silicon-glass substrate beam-film structure ultra-micro pressure sensor chip and its manufacturing method, relating to an ultra-micro pressure sensor. It provides a highly reliable silicon-glass substrate beam-film structure ultra-micro pressure sensor chip with a self-encapsulated structure, suitable for harsh environments such as humidity, acid and alkali, and static electricity, and its manufacturing method. The silicon-glass substrate beam-film structure ultra-micro pressure sensor chip has a box-shaped structure, with a substrate with a cavity and a pressure-sensitive film; the pressure-sensitive film has raised beam structures, forming a beam-film composite structure; the lower surface of the stress concentration area of ​​the pressure-sensitive film has four piezoresistors connected to form a Wheatstone bridge, and the piezoresistors are sealed in a vacuum pressure cavity through bonding between the substrate and the pressure-sensitive film; the Wheatstone bridge is electrically connected to the outside through pre-placed electrodes at the bonding interface. The process involves: first stage: fabrication on an SOI wafer; second stage: substrate preparation; third stage: bonding and subsequent processes. Application No.: CN2022101335276, Publication No.: CN114459666A, discloses a differential pressure sensor, its fabrication method, and its application. The sensor employs a three-layer silicon wafer bonding method. The upper and middle layers are both made of SOI wafers, while the lower layer is made of patterned doped intrinsic silicon wafers. The electrode lead pads are located on a three-step structure on one side of the sensor. Near-annular vias are formed around the upper and lower electrodes. This invention reduces the fixed capacitance in the upper capacitor's output capacitance signal by extending the path of the electric field lines in the fixed capacitor portion. The lower layer, made of intrinsic silicon, is doped only in the lower electrode and lead portion, allowing it to conduct electricity. By picking up the changes in two capacitances, the pressure difference between two different cavities is measured. The sensor's output sensitivity, resolution, and linearity are all improved. The external lead solder joints are arranged at the three-step structure, and the external circuitry is electrically connected to the sensor leads at the three-step structure, facilitating sensor integration and packaging. Application No.: CN200810024191x, Publication No. CN101290255A, discloses a 0-50pa monolithic silicon-based SOI ultra-low micro-pressure sensor. It has high sensitivity and linearity, high temperature resistance, simple process, and is suitable for large-scale production. Therefore, this invention also provides a method for processing the sensor. The chip includes a silicon substrate (1), with an insulating layer (2) covering both sides of the silicon substrate. A single crystal silicon thin film of about 0.3 micrometers is formed on the surface of the insulating layer (2) to generate a resistor (3), and an internal lead (4) is connected to the resistor (3). A back island (7) or a flat film without a back island is formed in the opening on the back of the chip. Between the back island and the frame is a composite elastic film of silicon nitride and silicon dioxide (8).Take a single crystal silicon as a substrate (1), and then etch an opening (6) on the back side of the substrate; perform oxygen ion implantation on the front side of the silicon wafer, and anneal the silicon wafer to form a silicon dioxide insulating layer and a single crystal silicon thin film. Thermally grow an oxide layer and a P-type conductive layer on the front side, and then form a resistive region (3) and an inner lead hot pressing foot region (4) on the front side of the silicon wafer, while the single crystal silicon thin film in other areas of the front side is etched away to expose the silicon dioxide layer; let the silicon nitride thin film deposited on the front and back sides of the silicon wafer and the silicon dioxide on the front side form a complementary composite insulating layer. For example, existing technologies such as those with publication numbers CN103185613A, CN105424261A, CN108168740A, CN107615031A, CN102723306A, CN107192967A, CN111564988A, CN111555654A, CN109141731A, CN113091989A, CN109060229A, and CN106586942A, while all involving differential pressure sensor structures and manufacturing methods, employ various methods for measuring microfluidic pressure. These methods, besides in-situ measurement of the microfluidic fluid, also involve methods such as opening holes along the microchannel to expose the microfluidic fluid for testing, and calculating pressure by observing the size of bubbles in the fluid. The use of these methods interferes with the original flow state of the microfluidic fluid, often resulting in insufficient measurement accuracy and unconvincing measurement data. Summary of the Invention

[0012] This invention proposes a capacitive microfluidic pressure sensor and its manufacturing method. This sensor can be integrated with a microfluidic chip with almost no disruption to the microchannel shape, enabling in-situ, real-time dynamic measurement of microfluidic pressure. The pressure sensor's measuring point diameter ranges from hundreds to thousands of micrometers, with a resolution reaching 1 Pa. This microfluidic pressure sensor is silicon-based and uses glass as the top cover to visualize the microfluidic chip. Compared to microfluidic pressure sensors used in polymer-based microfluidic chips such as PDMS, this sensor has low hysteresis, short response time, and higher dimensional accuracy. The manufacturing method of this pressure sensor is based on MEMS technology and features simple operation, high processing accuracy, and high yield. The technical solution of this invention is as follows:

[0013] A capacitive microfluidic pressure sensor comprises an upper, middle, and lower three-layer structure. The upper layer has a fluid microchannel inlet 14, a fluid microchannel outlet 2, and an upper lead hole 15 on its upper surface. The lower surface of the upper layer is disposed on the upper surface of the middle layer, forming a fluid microchannel with a fluid microchannel groove 12. The middle layer is disposed on the upper surface of the lower layer, and its lower surface is tightly connected to the upper surface of the lower layer. The middle layer has a fluid microchannel groove 12, an upper electrode lead pad 4, an upper electrode lead groove 5, and a middle layer lead hole 13. The lower layer has a lower lead hole 7, a lower electrode groove 8, a strip groove 9, a lower electrode lead groove 10, and a lower electrode lead pad 11. The lower electrode groove 8, strip groove 9, and lower electrode lead groove 10 achieve electrical conductivity through doping.

[0014] This invention discloses a method for fabricating a capacitive microfluidic pressure sensor, characterized by the following steps:

[0015] Step 1: First, the upper, middle and lower wafers are processed separately, the processed three-layer wafers are bonded together, and then lead pads are formed by magnetron sputtering. Finally, they are diced to realize the fabrication of the microfluidic pressure sensor.

[0016] Step 2: Next, prepare the upper wafer, the middle wafer, and the lower wafer respectively.

[0017] The present invention also discloses a microfluidic chip, characterized in that: the chip includes the capacitive microfluidic pressure sensor prepared by the above-described processing method of the capacitive microfluidic pressure sensor.

[0018] Beneficial effects

[0019] In the intermediate layer structure, the device layer and the bottom surface of the lower electrode groove 8 serve as two electrodes of different sizes, forming a capacitor. The external circuit connects this capacitor to the circuit via the upper electrode lead pad 4 and the lower electrode lead pad 11, respectively. The area of ​​the upper electrode and the lower surface of the groove 8 (which serves as the lower electrode) facing each other is the area of ​​the diaphragm 16 of the microfluidic pressure sensor.

[0020] The lower electrode groove 8 beneath the diaphragm of the microfluidic pressure sensor is connected to the external atmosphere via a strip groove 9 and a lower electrode lead groove 10. When the microfluidic fluid flows through the fluid microchannel groove 12, the fluid pressure acts on the upper surface of the diaphragm 16 of the microfluidic pressure sensor. The diaphragm 16, under the pressure difference between its upper and lower surfaces, deforms, causing a change in the lower surface of the lower electrode groove 8 and resulting in a change in capacitance. The external circuit detects this change in capacitance to analyze the pressure difference between the microfluidic fluid and the external atmosphere at the pressure measurement point.

[0021] By setting multiple pressure measurement points along the microfluidic path, that is, forming multiple microfluidic pressure sensors on the diaphragm 16, real-time pressure measurement along the microfluidic path can be achieved. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of a three-dimensional structure of a capacitive microfluidic pressure sensor provided in an embodiment of the present invention;

[0024] Figure 2 This is a schematic diagram of the planar structure of the fabrication result of the capacitive microfluidic pressure sensor in Example 1;

[0025] Figure 3 This is a diagram illustrating the materials used in the fabrication process of the capacitive microfluidic pressure sensor in Example 1.

[0026] Figure 4 This is a process flow diagram of the fabrication process of the upper wafer of the capacitive microfluidic pressure sensor in Example 1;

[0027] Figure 5 This is a flowchart of the first six steps in the fabrication process of the intermediate layer wafer for the capacitive microfluidic pressure sensor in Example 1;

[0028] Figure 6 This is a flowchart of the last six steps in the fabrication process of the intermediate layer wafer for the capacitive microfluidic pressure sensor in Example 1.

[0029] Figure 7 This is a flowchart illustrating the fabrication process of the lower wafer for the capacitive microfluidic pressure sensor in Example 1.

[0030] Figure 8 This is a process flow diagram of bonding and magnetron sputtering of the capacitive microfluidic pressure sensor in Example 1.

[0031] Figure label:

[0032] 1-Upper-level structure;

[0033] 2- Fluid microchannel outlet;

[0034] 3-Intermediate layer structure;

[0035] 4- Upper electrode lead pad;

[0036] 5-Upper electrode lead groove;

[0037] 6-Lower layer structure;

[0038] 7-Lower layer lead hole;

[0039] 8-Lower electrode groove;

[0040] 9-Striped groove;

[0041] 10-Lower electrode lead groove;

[0042] 11 - Lower electrode lead pad;

[0043] 12-Fluid microchannel groove;

[0044] 13 - Intermediate layer lead hole;

[0045] 14- Fluid microchannel inlet;

[0046] 15 - Upper layer lead hole;

[0047] 16-Diaphragm of microfluidic pressure sensor

[0048] 17 - UV adhesives used for bonding and bonding. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] Based on the patented invention, a design scheme for a capacitive microfluidic pressure sensor device is proposed, and the corresponding fabrication scheme is given.

[0051] Regarding the selection of wafer materials: the upper layer wafer is a 2mm thick glass wafer. The middle layer wafer is an SOI wafer with low device layer resistivity and double-sided polishing. The SOI wafer has a substrate layer thickness of 300 micrometers, a buried oxide layer thickness of 2 micrometers, a device layer thickness of 7 micrometers, and a device layer resistivity of less than 0.02 ohm-cm. The lower layer wafer is a double-sided polished silicon wafer with a silicon dioxide layer, with a silicon layer thickness of 500µm and a silicon dioxide layer thickness of 500nm. During the subsequent processing of the middle layer wafer, it is necessary to ensure that the thickness of the silicon dioxide layer formed on the lower surface is less than 2µm. This allows the lower surface of the fluid microchannel groove 12 to bend downwards during silicon-silicon bonding, resulting in a tighter bond.

[0052] In terms of structural dimensions: the radius of the upper lead hole 15 is 1mm, the radius of the middle lead hole 13 is 2.5mm, and the radius of the lower electrode lead groove is 3mm. This prevents non-target areas from becoming electrically connected to the lower electrode lead pad 11 due to insufficient operational precision during magnetron sputtering fabrication of the lower electrode lead pad 11. Similarly, the diameter of the lower lead hole 7 is 4mm, and the diameter of the upper electrode lead groove is 6mm, which also prevents other unnecessary parts from short-circuiting with the upper electrode lead pad. The diameters of the fluid microchannel inlet 14 and the fluid microchannel outlet 2 are both 2mm, facilitating fluid entry and exit and sealing. The radius of the lower electrode groove 8 is 400um, meaning that the diaphragm radius of the microfluidic pressure sensor is 400um, and the radius of the pressure measurement point is 400um. The depth of the lower electrode groove 8 is 2um.

[0053] 1. Processing flow of the upper structure (e.g.) Figure 4 )

[0054] Step 1: Prepare the wafer;

[0055] Step 2: Laser drilling to form fluid microchannel outlet 2, fluid microchannel inlet 14, upper lead hole 15, alignment marks and cutting marks, clean the wafer, and prepare for bonding.

[0056] 2. Processing flow of the intermediate layer structure (e.g.) Figure 5 , 6 )

[0057] Step 1: Prepare SOI wafers with low device layer resistivity and double-sided polishing;

[0058] Step 2: Dry thermal oxidation, forming a silicon dioxide layer with a thickness of 600 nanometers on the upper and lower surfaces of the wafer;

[0059] Step 3: Using the PECVD process, silicon nitride is deposited on the lower surface of the wafer to form a silicon nitride layer with a thickness of 500 nanometers;

[0060] Step 4: Perform photolithography on the upper and lower surfaces of the wafer. Spin-coat an 8-micrometer thick layer of AZ4620 photoresist on the upper and lower surfaces of the wafer. After drying, exposure, and development, the photoresist is patterned, exposing the fluid microchannel groove 12, the two corresponding parts of the intermediate layer lead hole 13 on the upper surface of the wafer, and the alignment marks used for bonding on the upper surface. On the lower surface of the wafer, the electrode lead groove 5, the two corresponding parts of the intermediate layer lead hole 13, and the alignment marks used for bonding on the lower surface are also exposed.

[0061] Step 5: Dry etching of silicon nitride to complete the patterning of silicon nitride and expose the silicon dioxide underneath;

[0062] Step 6: Use BOE solution to etch the exposed silica. At this point, the intermediate layer groove 5 has been formed.

[0063] Step 7: Apply photoresist to the electrode lead groove 5 section on the lower surface, and dry it to prevent this section from being etched;

[0064] Step 8: Dry etching the lower surface of the wafer so that the portion corresponding to the intermediate layer lead hole 13 is etched to the silicon dioxide layer in the middle of the SOI wafer;

[0065] Step 9: Use BOE solution to etch the exposed silica.

[0066] Step 10: Dry etch the portion of the upper surface of the wafer corresponding to the fluid microchannel groove 12 and the intermediate layer lead hole 13, etching down to the silicon dioxide layer of the SOI wafer;

[0067] Step 11: Dissolve the photoresist using acetone;

[0068] Step 12: Immerse the wafer in concentrated phosphoric acid heated to 60°C to etch the silicon nitride layer on the wafer surface, preparing for bonding.

[0069] 3. Processing flow of the lower structure (e.g.) Figure 7 )

[0070] Step 1: Prepare a double-sided polished silicon wafer with a silicon dioxide layer on its surface;

[0071] Step 2: Perform photolithography on the upper and lower surfaces of the wafer. Spin-coat a 2µm thick layer of S1813 photoresist on the upper and lower surfaces of the wafer. After drying, exposure, and development, the photoresist is patterned, exposing the four parts corresponding to the lower electrode groove 8, the strip groove 9, the lower electrode lead groove 10, and the lower lead hole 7 on the upper surface of the wafer, as well as the alignment marks used for bonding later.

[0072] Step 3: Use BOE solution to etch the exposed silica.

[0073] Step 4: Dry etching of the upper surface of the wafer to a depth of about 1.5 micrometers to form grooves;

[0074] Step 5: Ion implantation is performed on all the grooves on the upper surface. Then, the photoresist is removed using acetone solution and the area is annealed in an annealing furnace to complete the patterning doping of the lower surface of all the grooves, which is used to form the lower electrode and lead portion.

[0075] Step Six: Re-perform the photolithography process on the lower surface of the wafer. Re-coat the lower surface of the wafer with 8-micron thick AZ4620 photoresist, and complete the patterning of the photoresist through drying, exposure, and development.

[0076] Step 7: Use ICP etching technology to dry etch the lower surface of the wafer to form a through hole in the corresponding part of the lower layer lead hole 7;

[0077] Step 8: Dissolve the photoresist using acetone;

[0078] Step 9: Use BOE solution to etch the silicon dioxide layer on the wafer surface and wait for bonding.

[0079] 4. Overall processing (e.g.) Figure 8 )

[0080] After processing each wafer layer separately, a direct silicon-silicon bonding process is used to bond the intermediate layer wafer to the lower layer wafer. Using an adhesive process, a uniform layer of UV adhesive 17 with a thickness of approximately 2 micrometers is applied to the upper surface of the intermediate layer wafer using a roller. The upper glass wafer is then aligned with the intermediate layer wafer and placed on the upper surface of the intermediate layer wafer. A pressure of 3 kg is applied using weights, and the adhesive is cured by irradiating the coated area with ultraviolet light, completing the bonding between the upper and intermediate layer wafers. Subsequently, sputtering masks are applied to the upper and lower surfaces of the bonded multilayer wafers, and titanium (200 nm), platinum (200 nm), and gold (300 nm) are sputtered sequentially, respectively. Finally, the wafers are diced to form independent microfluidic pressure sensors.

[0081] In terms of wafer material selection, the preferred material for the upper layer structure is a glass wafer, enabling visualization of microfluidic flow. The preferred material for the intermediate layer structure is an SOI wafer with low resistivity and double-sided polishing, facilitating the formation of a diaphragm for a microfluidic pressure sensor with controllable thickness and a smooth surface, as well as subsequent lead conductivity. Due to thermal oxidation during processing, silicon dioxide layers are generated and retained on both the upper and lower surfaces of the SOI wafer. From a material distribution perspective, the SOI wafer consists of, from bottom to top, a silicon dioxide layer on the lower surface, a device layer, a buried oxide layer, a substrate layer, and a silicon dioxide layer on the upper surface. Therefore, the intermediate layer structure also consists of five layers: a silicon dioxide layer on the lower surface, a device layer (silicon layer), a buried oxide layer (silicon dioxide layer), a substrate layer (silicon layer), and a silicon dioxide layer on the upper surface. The substrate layer of the intermediate layer forms the fluid microchannel groove 12, the device layer forms the upper electrode of the variable capacitor, and the buried oxide layer (silicon dioxide layer) serves as the bottom surface of the fluid microchannel groove 12, providing electrical insulation between the fluid microchannel and the variable capacitor. The bottom surface of the upper electrode lead groove 5 is the device layer of the intermediate layer. Therefore, when an external circuit probe contacts the upper electrode lead pad 4, electrical conduction can be achieved with the device layer portion of the intermediate layer structure. The material used to fabricate the lower layer structure is preferably a double-sided polished silicon wafer with a silicon dioxide layer on its surface.

[0082] Structurally, the microfluidic pressure sensor of this invention is a superposition of three layers: an upper structure 1, an intermediate structure 3, and a lower structure 6. The fluid microchannel groove 12 located in the intermediate structure 3 forms a fluid microchannel with the lower bottom surface of the upper structure 1. Fluid can enter the fluid microchannel from the fluid microchannel inlet 14 and exit from the fluid microchannel outlet 2. Except for the portion directly above the lower electrode groove 8 of the lower structure, the lower bottom surface of the fluid microchannel groove 12 is supported and fixed by the upper end surface of the lower structure, with only the portion directly above the lower electrode groove 8 suspended. This suspended portion can deform under the pressure difference on both sides, serving as the diaphragm 16 of the microfluidic pressure sensor. The lower bottom surfaces of the lower electrode groove 8, the strip groove 9, and the lower electrode lead groove 10 are electrically conductive through doping. The lead probe of the external circuit is inserted into the upper lead hole 15, passes through the intermediate lead hole 13, and contacts the lower electrode lead pad 11, achieving electrical conductivity between the external circuit and the doped portion of the bottom surface of the lower electrode groove 8. The lead probes of the external circuit are inserted into the lower lead hole 7 and contact the upper electrode lead pad 4 to achieve electrical conduction with the middle layer structure device layer.

[0083] In principle, the microfluidic pressure sensor of this invention is similar to a capacitive pressure sensor for measuring gauge pressure. Its technical principle involves a diaphragm acting as an electrode, forming a variable capacitor with another electrode. The gap in the variable capacitor is open to the atmosphere. The pressure difference between the measured pressure and atmospheric pressure causes the diaphragm to deform, thus changing the variable capacitor. The intermediate layer structure is the device layer in the intermediate layer structure obtained from SOI wafer processing. It, along with the bottom surface of the circular groove 8 in the lower layer structure 6, serves as the upper and lower electrodes, forming a variable capacitor. The main effective part of this capacitor is the capacitance between the device layer of the intermediate layer structure and the portion directly opposite the bottom surface of the circular groove 8, that is, the capacitance formed by the device layer of the diaphragm 16 of the microfluidic pressure sensor and the bottom surface of the groove 8. The gap in the variable capacitor is open to the external gas through a strip-shaped groove 9. When the microfluidic fluid flows through, a pressure difference is created between the microfluidic fluid and the external gas. Under the action of this pressure difference, the diaphragm 16 of the microfluidic pressure sensor deforms, changing the distance between the upper and lower electrodes, and thus changing the capacitance value of the variable capacitor.

[0084] From a manufacturing perspective, the stacking of the three-layer structure of the microfluidic pressure sensor is based on MEMS multilayer bonding technology. In practice, the upper, middle, and lower wafers need to be processed separately to form their own microstructures such as vias and grooves. A bonding process is then used to bond the three wafers together. Next, magnetron sputtering of metal forms lead pads, and finally, the multilayer wafers are diced to form multiple individual structures, such as... Figure 1 The microfluidic pressure sensor shown.

[0085] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A capacitive microfluidic pressure sensor comprising an upper layer, a middle layer and a lower layer; characterized in that: the upper end surface of the structure is configured with a fluid microchannel inlet, a fluid microchannel outlet and an upper layer lead hole; the lower end surface of the upper layer structure is arranged on the upper end surface of the middle layer structure and forms a fluid microchannel with the fluid microchannel groove; the middle layer structure is arranged on the upper end surface of the lower layer structure, and the lower end surface of the middle layer structure is tightly connected with the upper end surface of the lower layer structure; the middle layer structure is configured with a fluid microchannel groove, an upper electrode lead pad, an upper electrode lead groove and a middle layer lead hole; the upper surface of the lower layer structure is configured with a lower layer lead hole, a lower electrode groove, a belt-shaped groove, a lower electrode lead groove and a lower electrode lead pad; the lower electrode groove, the belt-shaped groove and the lower electrode lead groove are electrically connected by doping; the middle layer structure is made of an SOI wafer with small device layer resistivity and double-sided polishing; the middle layer structure comprises a lower surface silicon dioxide layer, a device layer, a buried oxygen layer, a substrate layer and an upper surface silicon dioxide layer; the substrate layer and the buried oxygen layer are used to form the fluid microchannel groove, the lower bottom surface of the fluid microchannel groove is the upper surface of the buried oxygen layer, and the buried oxygen layer is used to realize electrical insulation between the device layer and the substrate layer; the bottom surface of the upper electrode lead groove is the device layer of the middle layer; the device layer and the lower bottom surface of the lower electrode groove form a capacitor; after the middle layer structure is tightly connected with the lower layer structure, the lower bottom surface of the fluid microchannel groove is supported and fixed by the upper end surface of the lower layer structure except for the part located directly above the lower electrode groove of the lower layer structure, and only the part located directly above the lower electrode groove of the lower layer structure is suspended; the suspended part serves as a diaphragm of the microfluidic pressure sensor and is used to sense the pressure difference between the microfluid and the lower electrode groove; when the diaphragm of the microfluidic pressure sensor is deformed by the pressure difference, the capacitance value of the capacitor formed by the device layer of the middle layer and the lower bottom surface of the lower electrode groove changes. upper layer The method comprises the following steps: Step 1: firstly, the upper layer, the middle layer and the lower layer wafers are processed respectively, the three wafers are bonded after processing, lead pads are formed by magnetron sputtering, and the microfluidic pressure sensor is prepared after cutting; Step 2: secondly, the upper layer wafer, the middle layer wafer and the lower layer wafer are prepared respectively. The processing steps of the upper layer wafer include: preparing a wafer; laser drilling to form a fluid microchannel outlet, a fluid microchannel inlet and an upper layer lead hole; and cleaning the surface of the wafer for bonding. The processing steps of the middle layer wafer include: Step 1: preparing an SOI wafer with small device layer resistivity and double-sided polishing; 2. A method for processing a capacitive microfluidic pressure sensor, based on a capacitive microfluidic pressure sensor according to claim 1, characterized in that: Step 2: dry thermal oxidation to form a 600-nanometer-thick silicon dioxide layer on the upper and lower surfaces of the wafer; Step 3: depositing silicon nitride on the lower surface of the wafer by PECVD process to form a 500-nanometer-thick silicon nitride layer; ​ 3. The method of claim 2, wherein the method is characterized by: ​ 4. The method of claim 3, wherein the method is characterized by: ​ ​ ​ ​ Step four: carry out photoetching process on the upper and lower surfaces of the wafer: spin 8 microns thick AZ4620 photoresist on the upper and lower surfaces of the wafer, and after drying, exposure and development steps, complete the patterning of the photoresist, so that the two parts corresponding to the fluid microchannel groove and the middle layer lead hole on the upper surface of the wafer and the alignment mark for bonding on the upper surface are exposed, and the two parts corresponding to the electrode lead groove and the middle layer lead hole on the lower surface of the wafer and the alignment mark for bonding on the lower surface are exposed; Step five: dry etching of silicon nitride, complete the patterning of silicon nitride, expose the silicon dioxide under the silicon nitride; Step six: use BOE solution to etch the exposed part of the silicon dioxide, at this time the middle layer groove has been formed; Step seven: use photoresist to smear the electrode lead groove part on the lower surface, dry to avoid this part being etched; Step eight: dry etching of the lower surface of the wafer, so that the part corresponding to the middle layer lead hole is etched to the silicon dioxide layer in the middle of the SOI wafer; Step nine: use BOE solution to etch the exposed part of the silicon dioxide; Step ten: dry etching of the upper surface of the wafer and the part corresponding to the fluid microchannel groove and the middle layer lead hole, etching to the silicon dioxide layer of the SOI wafer; Step eleven: use acetone reagent to dissolve the photoresist; Step twelve: soak the wafer in concentrated phosphoric acid heated to 60°C to etch the silicon nitride layer on the surface of the wafer, ready for bonding.

5. The method of claim 4, wherein the step of depositing a second layer of electrically conductive material is performed by sputtering a layer of electrically conductive material onto the substrate. The process steps of the lower wafer include: Step one: prepare a double-sided polished silicon wafer with a silicon dioxide layer on the surface; Step two: carry out photoetching process on the upper and lower surfaces of the wafer: spin 2um thick S1813 photoresist on the upper and lower surfaces of the wafer, and after drying, exposure and development steps, complete the patterning of the photoresist, so that the four parts corresponding to the lower electrode groove, belt groove, lower electrode lead groove and lower layer lead hole on the upper surface of the wafer and the alignment mark for bonding later are exposed; Step three: use BOE solution to etch the exposed part of the silicon dioxide; Step four: dry etching of the upper surface of the wafer, the etching depth is about 1.5 microns, forming a groove; Step five: ion implantation is performed on all the groove parts on the upper surface, then acetone solution is used to remove the photoresist, and the wafer is placed in an annealing furnace for annealing, completing the patterning doping of the lower surface of all the grooves, which is used to form the lower electrode and the lead part; Step six: carry out photoetching process on the lower surface of the wafer again: re-coat the wafer with photoresist, spin 8 microns thick AZ4620 photoresist, and after drying, exposure and development steps, complete the patterning of the photoresist; Step seven: use ICP etching technology to dry etch the lower surface of the wafer, so that the part corresponding to the lower layer lead hole forms a through hole; Step eight: use acetone reagent to dissolve the photoresist; Step nine: use BOE solution to etch the silicon dioxide layer on the surface of the wafer, and wait for bonding.

6. A microfluidic chip, characterized by: The chip comprises the capacitive microfluid pressure sensor of claim 1 prepared by the processing method of the capacitive microfluid pressure sensor of any one of claims 2-5.

Images