A CMOS MEMS integrated flow, gas / humidity sensor

Abstract

This invention relates to a CMOS MEMS integrated flow and gas / humidity sensor, specifically to a CMOS MEMS integrated flow and gas / humidity sensor and its fabrication method; belonging to the field of sensors. The purpose of this invention is to overcome the shortcomings of existing technologies, fill a gap in the field, and realize the function of simultaneously detecting gas flow and gas / humidity on a single chip, providing a CMOS MEMS integrated flow and gas / humidity sensor and its fabrication method. This sensor fully utilizes the heat from the heating resistor in the flow sensing part, greatly improving energy utilization, and has the advantages of low power consumption, high response speed, and low hysteresis. It employs a mask-free Post-CMOS process, avoiding complex photolithography steps, effectively reducing the complexity of post-processing for the entire device, and greatly improving the efficiency of the entire Post-CMOS post-processing.

Description

Technical Field

[0001] This invention relates to a CMOS MEMS integrated flow and gas / humidity sensor, specifically to a CMOS MEMS integrated flow and gas / humidity sensor and its fabrication method; it belongs to the field of sensors. Background Technology

[0002] Flow sensors play a crucial role in production, daily life, and scientific measurement, such as sensing environmental flow, monitoring industrial gas flow, flow sensing in biomedical applications, and marine hydrodynamic sensing. Advances in MEMS technology have made it possible to manufacture complex sensors. Numerous studies have demonstrated that MEMS processes can be successfully applied to manufacture various sensors for detecting flow rate and direction, and these sensors can be mass-produced at low cost. Today's flow sensors, in addition to requiring high sensitivity to fluid parameters (flow velocity, direction, etc.), also demand low power consumption, low cost, and long-term stability. In various applications, flow sensors are needed to measure the velocity and direction of liquid and gas flow, including flow pattern determination, wall shear stress measurement, viscosity and density measurement. The measured flow rate may be affected by the system's velocity, pressure, temperature, or chemical composition. Therefore, flow sensing devices are typically based on direct detection of volume, mass, velocity, or multiple parameters. MEMS flow sensors are miniature devices capable of high-resolution detection and have significant application prospects compared to conventional sensing devices, finding applications in high-precision fields such as underwater vehicle navigation and underwater target detection, biomedicine, and environmental monitoring.

[0003] MEMS flow sensors can be broadly classified into three categories based on their operating principles: thermal flow sensors, piezoresistive flow sensors, and piezoelectric flow sensors. Thermal flow sensors generally consist of two basic parts: a heater and a sensing element. The sensing element detects the heat transfer change between the heater and the fluid, and then uses the heat transfer intensity to determine the fluid velocity. Compared to other types of flow sensors, thermal flow sensors achieve high sensitivity and high accuracy measurement with low output signal drift. Furthermore, this type of sensor has the advantage of measuring flow velocity in miniature components without relying on any mechanical movement, which reduces device wear and extends service life. While MEMS technology allows for flexible fabrication processes and a wide range of materials, it is difficult to directly interconnect with circuitry, limiting the overall size of the test microsystem to some extent. Moreover, the fabrication process is relatively complex, making low-cost mass production difficult. With the rapid development of CMOS semiconductor technology, mature CMOS fabrication processes have demonstrated significant advantages in the sensor field, not only further reducing device size but also enabling low-cost mass production of sensors. Therefore, CMOS MEMS technology has been widely used in recent years to manufacture low-cost miniature flow sensors.

[0004] Gas sensors are devices used to detect gas concentrations and are widely used in food storage, electronics manufacturing, industrial automation, agriculture, textile production, home monitoring, and medical experiments. Resistive and capacitive gas sensors are the two most common types of gas sensors. A resistive gas sensor consists of a sensitive membrane, a heater, and electrodes. CMOS platforms provide available materials for manufacturing the heater and electrodes, and use additional materials (typically metal-oxide-semiconductor materials) as the sensitive membrane. The heater activates the sensitive membrane material, causing a change in resistivity when it adsorbs (or desorbs) gas molecules, which in turn allows for the detection of gas concentration through the change in resistance. Capacitive gas sensors are often used for humidity detection, specifically water vapor detection. A humidity-sensitive membrane material is filled into a capacitive humidity sensing structure. When it adsorbs (or desorbs) water molecules, it causes a change in the dielectric constant between the capacitor plates, which in turn allows for the detection of humidity through the change in capacitance.

[0005] However, most existing CMOS MEMS flow sensors or gas / humidity sensors can only detect flow or gas / humidity individually, and their power consumption is relatively high. In practical applications, it is often necessary to detect important parameters such as flow and gas / humidity simultaneously. The central heating resistor of a thermal gas flow sensor can activate the sensitive film material of the gas sensing structure. Therefore, based on the full utilization of CMOS MEMS technology, placing the gas sensor structure above the central heating resistor of the thermal flow sensor to achieve simultaneous detection of flow and gas / humidity is a feasible solution. Compared with previous CMOS MEMS thermal flow sensors, by precisely designing the parameters of the flow sensor heating resistor to achieve a higher temperature, not only can the performance of the flow sensor be improved, but the sensing efficiency of the gas sensor can also be improved. In addition, by fully utilizing the heat of the heating resistor and designing the parameters of the heating resistor to achieve a suitable response time, and using a square wave voltage signal with a frequency higher than the response frequency of the heating resistor to power it, the power consumption of the sensor is greatly reduced and the hysteresis of gas detection is reduced. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies, fill a gap in the field, and realize the function of simultaneously measuring flow rate and gas / humidity on a single chip. It provides a CMOS-MEMS monolithically integrated flow rate and gas / humidity sensor and its fabrication method. This sensor fully utilizes the heat from the heating resistor in the flow sensor, improving energy utilization. Furthermore, it employs a mask-free Post-CMOS process, avoiding complex photolithography steps and significantly improving the efficiency of the entire Post-CMOS post-processing. The sensing part adopts a suspended structure, offering advantages such as fast response speed and low heat dissipation.

[0007] The objective of this invention is achieved through the following technical solution.

[0008] A CMOS MEMS integrated flow and gas / humidity sensor includes a silicon substrate and a silicon dioxide structure layer, wherein the silicon dioxide structure layer is disposed above the silicon substrate. The silicon dioxide structure layer includes PAD regions disposed at both ends and a flow sensor region and a gas / humidity sensor region disposed in the center. In the PAD regions, silicon dioxide material and metal material are arranged in an overlapping manner, with the metal via penetrating through other metal layers except for the top metal layer. The flow sensor region includes a heating resistor and a first thermistor and a second thermistor distributed on both sides of the heating resistor. The heating resistor, the first thermistor, and the second thermistor are placed in the silicon dioxide material and are suspended above the cavity. The gas / humidity sensor region includes a heating resistor placed in the silicon dioxide material and a gas / humidity sensing electrode composed of the bottom metal layer located above the heating resistor, and a sensitive film material filled or deposited on the gas / humidity sensing electrode.

[0009] A method for fabricating a CMOS MEMS integrated flow and gas / humidity sensor, employing a mask-free Post-CMOS fabrication method, includes the following steps:

[0010] Step 1: Design a CMOS die using a 0.18µm 1P6M process. Design a suitable metal layer pattern and metal vias in the PAD region, filling the space between the top two metal layers with silicon dioxide material. This forms a natural mask for the PAD region during metal etching, protecting its structure. Design a suitable polysilicon layer pattern in the flow sensor region to form the heating resistor, first thermistor, and second thermistor of the flow sensor. Design a suitable metal layer pattern and metal vias in the flow sensor region, leaving a thin layer of silicon dioxide material above the heating resistor, first thermistor, and second thermistor, forming a natural mask for them. Simultaneously, the vias are directly connected to the silicon substrate, becoming channels for subsequent silicon etching. Utilize the bottom metal layer to design a suitable metal layer pattern and metal vias above the heating resistor of the flow sensor, forming the electrode structure of the gas / humidity sensor.

[0011] Step 2: Use RIE to etch the CMOS die obtained in Step 1 with SiO2 to form trenches and microcavities, exposing the top layer of aluminum metal and the substrate silicon.

[0012] Step 3: Etch aluminum onto the chip obtained in Step 2 to remove the top layer of aluminum.

[0013] Step 4: Use RIE to etch the chip obtained in Step 3 with SiO2 to remove the surface SiO2 and expose the electrode structure of the gas / humidity sensor located above the central heating resistor.

[0014] Step 5: Perform DRIE silicon etching on the chip obtained in Step 4. The SiO2 and metal layer mentioned above are used as a mask for silicon to form a silicon trench with a high aspect ratio.

[0015] Step 6: Perform XeF2 isotropic dry etching on the chip obtained in Step 5 to release the sensor levitation structure;

[0016] Step 7: Fill or deposit sensitive film material on the chip obtained in Step 6, so that it fills or deposits on the electrode structure of the gas / humidity sensor.

[0017] Beneficial effects:

[0018] 1. This invention achieves the function of simultaneously detecting flow rate and gas / humidity by placing the gas / humidity sensing structure above the central heating resistor of the flow sensing structure. It makes full use of the heat of the heating resistor to activate the sensitive membrane material of the gas / humidity sensor, which greatly improves the energy utilization rate.

[0019] 2. This invention achieves higher temperatures by precisely designing the parameters of the heating resistor in the flow sensor, which not only improves the performance of the flow sensor but also enhances the sensing efficiency of the gas / humidity sensor. Furthermore, by designing the parameters of the heating resistor to achieve a suitable response time and powering it with a square wave voltage signal at a frequency higher than the response frequency of the heating resistor, the power consumption of the sensor can be greatly reduced and the hysteresis of gas / humidity detection can be minimized.

[0020] 3. By designing a metal layer pattern to make it a mask of a natural polycrystalline silicon layer, the present invention can etch all the functional components of the sensor without a mask and without damage, thus avoiding complex photolithography steps.

[0021] 4. This invention achieves the integration of multiple functions into a single chip. The integrated design and subsequent simplified processing technology greatly reduce the cost of the sensor. This invention significantly improves energy utilization and has the advantages of high stability, high sensitivity, low heat dissipation, and low hysteresis. The single-chip flow and gas / humidity integrated sensor is small in size and highly integrated, thus effectively solving the problems of large size and poor adaptability caused by the simple stacking of current single-function sensors, as well as the problems of low energy utilization, high cost, large heat dissipation, and relatively limited sensitivity and response speed of gas flow and gas / humidity sensors available on the market. In addition, it is low in cost, simple in manufacturing and processing, and easy to mass-produce. Attached Figure Description

[0022] Figure 1This is a three-dimensional structural diagram of the CMOS MEMS integrated flow and gas / humidity sensor of the present invention; wherein (a) is a capacitive gas / humidity sensing electrode structure, and (b) is a resistive gas / humidity sensing electrode structure.

[0023] Figure 2 This is a flowchart of the fabrication process of the CMOS MEMS integrated flow and gas / humidity sensor of the present invention; wherein Figure a shows the designed CMOS die, Figure b shows the silicon dioxide etching of the CMOS die using RIE to form trenches and microcavities, Figure c shows the metal layer etching, Figure d shows the silicon dioxide etching to expose the electrode structure of the gas / humidity sensor, Figure e shows the silicon DRIE etching to form trenches on the substrate, Figure f shows the XeF2 etching to release the suspension structure, and Figure g shows the filling of the sensitive film material;

[0024] Figure 3 This is a cross-sectional view of the CMOS MEMS integrated flow and gas / humidity sensor of this invention.

[0025] Wherein: 1-Si substrate; 2-SiO2 structure layer; 3-first thermistor; 4-heating resistor; 5-second thermistor; 6-cavity; 7-gas / humidity sensing electrode structure; 8-sensitive film material. Detailed Implementation

[0026] To better illustrate the purpose and advantages of the present invention, the invention will be further described below in conjunction with the accompanying drawings and examples.

[0027] See Figure 1 , Figure 3 As shown: This embodiment discloses a CMOS MEMS integrated flow and gas / humidity sensor, including a silicon substrate layer 1 and a silicon dioxide structure layer 2, wherein the silicon dioxide structure layer 2 is disposed above the silicon substrate layer 1; the silicon dioxide structure layer 2 includes PAD regions disposed at both ends and a flow sensor region and a gas / humidity sensor region disposed in the center; in the PAD regions, silicon dioxide material and metal material are arranged in an overlapping manner, and the metal vias penetrate through other metal layers except for the top metal layer; the flow sensor region includes a heating resistor 4 and a first thermistor 3 and a second thermistor 5 distributed on both sides of the heating resistor 4, the heating resistor 4, the first thermistor 3 and the second thermistor 5 are placed in the silicon dioxide material, and are suspended above the cavity 6; the gas / humidity sensor region includes an electrode structure 7 of the gas / humidity sensor made of the bottom metal layer, placed above the heating resistor 4 in the silicon dioxide material, and a sensitive film material 8 filled or deposited on the electrode structure 7 of the gas / humidity sensor.

[0028] The substrate material is a silicon wafer.

[0029] The structural layer is made of silicon dioxide.

[0030] The heating resistor of the flow sensor and the first and second thermistors are made of polycrystalline silicon.

[0031] The gas / humidity sensing structure is made of aluminum and a sensitive membrane material that is filled or deposited on it.

[0032] See Figure 2 As shown: This invention also provides a method for fabricating a CMOS MEMS integrated flow and gas / humidity sensor. Figure 2 This is a flowchart illustrating the fabrication process of the CMOS MEMS integrated flow and gas / humidity sensor in an embodiment of the present invention, including the following steps:

[0033] Step 1: Design a CMOS die using a 0.18µm 1P6M process. Design a suitable metal layer pattern and metal vias in the PAD region, filling the space between the top two metal layers with silicon dioxide. This forms a natural mask for the PAD region during metal etching, protecting its structure. Design a suitable polysilicon layer pattern in the gas flow sensor region to form the heating resistor, first thermistor, and second thermistor of the flow sensor. Design a suitable metal layer pattern and metal vias in the flow sensor region, leaving a thin layer of silicon dioxide above the heating resistor, first thermistor, and second thermistor, forming a natural mask for them. Simultaneously, the vias connect directly to the silicon substrate, becoming channels for subsequent silicon etching. Utilize the bottom metal layer to design a suitable metal layer pattern and metal vias above the heating resistor of the flow sensor, forming the electrode structure of the gas / humidity sensor. For example, design an interdigital capacitive humidity sensing structure.

[0034] Step 2: Use ion reactive etching (RIE) to etch the CMOS die from Step 1 with SiO2 to form trenches and microcavities, exposing the sixth layer of aluminum and the substrate silicon. For example, use CF4 and SF6 for anisotropic dry etching.

[0035] Step 3: Etch aluminum onto the chip obtained in Step 2 to remove the top layer of aluminum. For example, use Cl2 and BCl3 for anisotropic dry etching.

[0036] Step 4: Perform SiO2 etching on the chip obtained in Step 3 using reactive ion etching (RIE) to remove the surface SiO2 and expose the electrode structure of the gas / humidity sensor located above the central heating resistor. For example, anisotropic dry etching can be performed using CF4 and SF6.

[0037] Step 5: Perform deep reactive ion etching (DRIE) on the chip obtained in Step 4. The SiO2 and metal layer mentioned above serve as a mask for the silicon, forming high aspect ratio silicon trenches. DRIE, also known as deep reactive ion etching, is a high aspect ratio dry etching process for silicon based on fluorine-based gases. For example, SF6 and C4F8 are used for etching and passivation respectively in an HSE200S (deep silicon etching machine).

[0038] Step 6: Perform isotropic dry etching with XeF2 gas on the chip obtained in Step 5 to release the sensor suspension structure. Using XeF2 gas for dry isotropic etching allows simultaneous etching to both sides and the bottom of the silicon trench, thereby forming the suspension structure of the heating resistor and two thermistors.

[0039] Step 7: Fill or deposit a sensitive film material onto the chip obtained in Step 6, so that it fills or deposits onto the electrode structure of the gas / humidity sensor. For example, fill the electrode structure with polyimide by dispensing and cure the polyimide at 150°C for 3 hours.

[0040] The practical application of the CMOS MEMS integrated flow and gas / humidity sensor provided by this invention can include: attaching the sensor to a carrier (such as a PCB board), leading out wires, connecting to an external signal processing module, and simultaneously detecting the flow and gas / humidity in the environment.

[0041] This invention fully utilizes the heat from a heating resistor to activate the sensitive membrane material of a gas / humidity sensor, improving energy utilization. Furthermore, by precisely designing the parameters of the heating resistor in the flow sensor, it can reach higher temperatures, which not only improves the performance of the flow sensor but also enhances the sensing efficiency of the gas sensor. For humidity sensors using polyimide as the humidity-sensitive material, this significantly reduces sensing hysteresis. In addition, by designing the heating resistor parameters to achieve a suitable response time and powering it with a square wave voltage signal at a frequency higher than the heating resistor's response frequency, the power consumption of the sensor can be greatly reduced, and the hysteresis of gas / humidity detection can be minimized.

[0042] This invention employs a 0.18µm 1P6M CMOS semiconductor process to design the required sensor structure in a single step, including a silicon substrate layer 1 and a silicon dioxide structure layer 2, as well as various functional components contained within the silicon dioxide structure layer. Then, a mask-free post-CMOS process is used to obtain a monolithically integrated CMOS-MEMS flow and gas / humidity sensor with a suspended structure. Unlike ordinary CMOS-MEMS flow or gas / humidity sensors, this invention innovatively proposes a method for fabricating a single-chip integrated flow and gas / humidity sensor. By placing the gas / humidity sensing structure on the central heating resistor of the flow sensor, simultaneous measurement of gas / humidity and flow rate is achieved. A metal layer pattern is designed to serve as a mask for silicon dioxide and polysilicon, allowing for maskless and non-destructive etching of the sensor's heater, temperature sensing area, and gas / humidity detection structure, avoiding complex photolithography steps. Unlike ordinary flow sensors, this invention uses the DRIE anisotropic silicon etching method and the XeF2 isotropic silicon etching method to form a thermally oriented flow sensor with a suspended structure, offering advantages such as low heat dissipation and fast response speed.

[0043] This invention provides a design and fabrication method for a CMOS-MEMS monolithically integrated flow and gas / humidity sensor, achieving the integration of multiple functions into a single chip. The integrated design and subsequent simplified fabrication process reduce the cost of the sensor. This invention significantly improves energy utilization and has the advantages of high stability, high sensitivity, low heat dissipation, and low hysteresis. The monolithically integrated flow and gas / humidity sensor is small in size and highly integrated, thus effectively solving the problems of large size and poor adaptability caused by the simple stacking of current single-function sensors, as well as the problems of low energy utilization, high cost, large heat dissipation, and relatively limited sensitivity and response speed of commercially available gas flow and gas / humidity sensors. Furthermore, it is low in cost, simple to fabricate and process, and easy to mass-produce.

[0044] The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the scope of protection of this application. Other structures and principles are the same as those in the prior art and will not be described in detail here.

Claims

1. A method for fabricating a CMOS MEMS integrated flow and gas / humidity sensor, characterized in that: The Post-CMOS method, which does not require mask manipulation, includes the following steps: Step 1: CMOS die: A metal layer pattern and metal vias are formed in the PAD area, filling the space between the top two metal layers with silicon dioxide material to create a natural mask for the PAD area during metal etching. A polysilicon layer pattern is formed in the gas flow sensor area to constitute the heating resistor, first thermistor, and second thermistor of the flow sensor. A metal layer pattern and metal vias are formed in the gas flow sensor area, leaving a thin layer of silicon dioxide above the heating resistor, first thermistor, and second thermistor, forming a natural mask for them. Simultaneously, the vias extend through the silicon substrate, becoming channels for subsequent silicon etching. A metal layer pattern and metal vias are formed above the heating resistor of the gas flow sensor using the bottom metal layer to create the electrode structure of the gas / humidity sensor. Step 2: Use RIE to etch the CMOS die from Step 1 with SiO2 to form trenches and microcavities, exposing the top layer of aluminum metal and the substrate silicon. Step 3: Etch aluminum onto the chip obtained in Step 2 to remove the top layer of aluminum. Step 4: Use RIE to etch the chip obtained in Step 3 with SiO2 to remove the surface SiO2 and expose the electrode structure of the gas / humidity sensor located above the central heating resistor. Step 5: Perform DRIE silicon etching on the chip obtained in Step 4, using SiO2 and a metal layer as a mask for silicon to form a silicon trench with a high aspect ratio. Step 6: Perform XeF2 isotropic dry etching on the chip obtained in Step 5 to release the sensor levitation structure; Step 7: Fill or deposit sensitive film material on the chip obtained in Step 6, so that it fills or deposits on the electrode structure of the gas / humidity sensor.

2. A CMOS MEMS integrated flow and gas / humidity sensor that can be manufactured using the fabrication method described in claim 1, characterized in that: The system includes a silicon substrate layer (1) and a silicon dioxide structure layer (2); the silicon dioxide structure layer (2) is disposed above the silicon substrate layer (1); the silicon dioxide structure layer (2) includes PAD regions disposed at both ends and a gas flow sensor region and a gas humidity sensor region disposed in the center; in the PAD regions, silicon dioxide material and metal material are arranged in an overlapping manner, and the metal via penetrates through other metal layers except for the top metal layer; the gas flow sensor region includes a heating resistor (4) and a first thermistor (3) and a second thermistor (5) distributed on both sides of the heating resistor (4), the heating resistor (4), the first thermistor (3) and the second thermistor (5) are placed in the silicon dioxide material and are suspended above the cavity (6); the gas / humidity sensor region includes a heating resistor placed in the silicon dioxide material and a gas / humidity sensing electrode composed of the bottom metal layer located above the heating resistor and a sensitive film material filled or deposited on the gas / humidity sensing electrode.

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