Hydrogen sensor and preparation method thereof

Abstract

The invention discloses a hydrogen sensor and a preparation method thereof. The hydrogen sensor comprises bonding glass; the bonding glass is bonded with the bottom of the SOI substrate to form a whole; the surface of the top of the SOI substrate is covered with a strain diaphragm; a palladium alloy film is arranged on the strain diaphragm; and a Wheatstone bridge for detecting the hydrogen concentration is integrated on the working crystal face of the strain diaphragm. The hydrogen sensor has the advantages of good selectivity, long service life, good long-term stability, high sensitivity and low measurement lower limit of a conventional palladium alloy film resistance sensor.

Description

Technical Field

[0001] This invention relates to hydrogen sensors, and more particularly to a hydrogen sensor and its preparation method. Background Technology

[0002] Currently, the main principles of hydrogen sensor technology include catalytic combustion, electrochemistry, metal oxide semiconductor, thermal conductivity, and palladium alloy thin film resistive, etc., and their advantages and disadvantages are shown in Table 1 below.

[0003]

[0004] As shown in the table above, existing conventional hydrogen sensors suffer from poor selectivity, short lifespan, poor long-term stability, low sensitivity, and insufficient measurement limits. Compared to other conventional hydrogen sensors, palladium alloy hydrogen sensors offer advantages such as long lifespan, good selectivity, and good long-term stability. However, they suffer from low sensitivity, making them unsuitable for low-concentration measurements, and their response time is relatively slow. Conventional palladium alloy thin-film resistance sensors work by depositing a palladium alloy thin-film resistance strip on a substrate. When hydrogen gas is present in the measurement medium, it decomposes into hydrogen atoms, which are adsorbed into the internal lattice of the palladium alloy thin-film resistance strip. This causes the strip to expand, resulting in a change in resistivity. The higher the hydrogen concentration, the greater the lattice expansion and the larger the resistivity change. The change in resistivity is used to identify the hydrogen concentration. However, at low concentrations, fewer hydrogen atoms are adsorbed into the internal lattice of the palladium alloy thin-film resistance strip, resulting in less lattice expansion and a smaller change in alloy resistivity. Furthermore, the slow response time fails to meet the requirements for low-concentration testing. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a hydrogen sensor and its preparation method that have the advantages of conventional palladium alloy thin film resistance sensors, such as good selectivity, long life and good long-term stability, as well as high sensitivity and low measurement limit, in order to overcome the shortcomings of the existing technology.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a hydrogen sensor, comprising bonded glass; characterized in that the bonded glass is bonded to the bottom of an SOI substrate to form an integral whole; a strain diaphragm is covered on the top surface of the SOI substrate; a palladium alloy thin film is disposed on the strain diaphragm; and a Wheatstone bridge for detecting hydrogen concentration is integrated on the working crystal surface of the strain diaphragm.

[0007] The resistors in the Wheatstone bridge are single-crystal silicon thin-film resistors.

[0008] The strain diaphragm is also equipped with a heating resistor and a temperature measuring resistor.

[0009] The temperature measuring resistor value is collected in real time, and the difference between the temperature measuring resistor value and the set value is used as the input of the PID controller to obtain an output value that is positively correlated with the current. The output value is applied to the heating resistor to realize the constant temperature control of the hydrogen sensor.

[0010] The thickness of the heating resistor and the temperature measuring resistor is 50nm-500nm.

[0011] The rate of change of resistance values ​​of each resistor in the Wheatstone bridge is:

[0012] ;

[0013] in, y , x These represent the longitudinal and transverse pressures at the strain gauge measurement points, respectively. R1, R2, R3, and R4 are the four resistors of a Wheatstone bridge. The connection point of R1 and R3 is connected to the positive terminal of the power supply, and the connection point of R2 and R4 is connected to the negative terminal of the power supply. π 44 This is the compression coefficient for monocrystalline silicon.

[0014] The thickness of the SOI substrate is 300um-1000um.

[0015] The strain diaphragm has a thickness of 20µm-100µm.

[0016] A method for preparing a hydrogen sensor according to any one of claims 1 to 8, characterized in that it comprises:

[0017] SOI wafers are selected, and the top single-crystal silicon is doped with high concentration of boron using ion implantation technology to obtain SOI substrates.

[0018] The resistor strip is etched using inductively coupled plasma dry etching technology to form a varistor strip with a single crystal silicon layer as the strain resistor, and the SiO2 insulating layer in the area other than the varistor strip is exposed.

[0019] A hydrogen-sensitive pattern was photolithographically etched on a SiO2 insulating layer, and a palladium alloy thin film was deposited as the hydrogen-sensitive pattern block.

[0020] Photolithography is used to create lead grooves and pads, and Ni-Au thin films are deposited as electrodes.

[0021] Photolithography etching windows are performed on the back of the SOI substrate, followed by dry etching of silicon cups.

[0022] An SOI substrate is electrostatically bonded to Pyrex 7740 glass, and then diced to obtain a hydrogen sensor.

[0023] The method of the present invention further includes:

[0024] A temperature-sensing resistance strip is photolithographically patterned on a SiO2 insulating layer, and a Ni thin film is deposited as a temperature-sensing resistor.

[0025] Heating resistance strips were photolithographically etched on a SiO2 insulating layer, and an Au thin film was deposited as the heating resistance.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1. Considering that the strain resistivity change of silicon is much higher than that of metals, and the strain factor is tens of times that of metals, the resistivity change will be tens of times higher under the same strain. The hydrogen absorption expansion material is also a palladium alloy thin film. Under the same hydrogen concentration, the strain remains unchanged. Therefore, this invention changes the strain resistor material from alloy material to semiconductor silicon, which significantly improves the resistivity change and increases the sensor output sensitivity by tens of times. This can solve the problem of low sensitivity of conventional palladium alloy resistance sensors under low concentration.

[0028] 2. Theoretically, the higher the sensor surface temperature, the faster the response time. To improve the sensor response time, constant temperature control of the sensor chip is required, with temperatures potentially reaching 150°C. Conventional bulk silicon strain gauge pressure sensors use p-type diffused resistors in their piezoresistive bridge and n-type silicon substrates for the elastic film. Isolation between resistors and between the resistors and the elastic film is achieved through a reverse-biased pn junction. However, when the operating temperature exceeds 120°C, the pn junction leakage current intensifies, causing severe sensor malfunction and limiting the sensor's ability to operate under high-temperature conditions. In this invention, the sensitive resistor of the high-temperature resistant pressure sensor with a single-crystal silicon SOI structure is fabricated on an insulating layer (the SOI substrate consists of three layers, from bottom to top: bottom silicon, insulating layer, and top silicon). Electrical isolation between resistors is achieved through the insulating layer, solving the failure problem of pn junction isolated pressure sensors operating above 120°C. Simultaneously, it effectively improves the nonlinearity issues caused by pn junction isolation.

[0029] 3. This invention utilizes the advantages of palladium alloy thin films, such as long lifespan, good selectivity, and good long-term stability, as well as the high sensitivity and fast response time of the SOI piezoresistive strain principle, to design a novel hydrogen sensor. It possesses the advantages of conventional palladium alloy thin film resistance sensors, such as good selectivity, long lifespan, and good long-term stability, as well as high sensitivity and low measurement limit, achieving full-range coverage from ppb to 100%vol. At the same time, by controlling the sensor's constant temperature, the sensor surface temperature is increased, significantly improving the sensor's response time and eliminating temperature drift. Attached Figure Description

[0030] Figure 1 This is a structural diagram of an embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram illustrating the working principle of the sensor strain according to an embodiment of the present invention;

[0032] Figure 3 This is a circuit diagram of the constant temperature control part in an embodiment of the present invention. Detailed Implementation

[0033] 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 and completely 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.

[0034] Example 1

[0035] Embodiment 1 of the present invention provides a hydrogen sensor, such as... Figure 1 As shown.

[0036] The hydrogen sensor in this embodiment includes a bonded glass 1, a silicon cup 2 (SOI substrate), a strain gauge 3, a silicon strain gauge strip 4, a palladium alloy film 5, a heating resistor 6, a temperature sensing resistor 7, and a wire pad 8. The bonded glass is bonded to the bottom of the silicon cup to form a single unit, providing support, and its thickness is typically 300µm-1000µm. The strain gauge 3 is located at the top of the silicon cup, and its thickness is typically 20µm-100µm. The silicon strain gauge strip, palladium alloy film, heating resistor, temperature sensing resistor, and wire pad are all located on the strain gauge 3. The silicon strain gauge strip is typically 0.2µm-10µm thick, and the palladium alloy film, heating resistor, temperature sensing resistor, and wire pad are typically 50nm-500nm thick. The core principle is: when the palladium alloy film adsorbs hydrogen, the lattice expands, and the strain is transferred to the strain gauge 3 of the silicon cup. When the resistances R1, R2, R3, and R4 of the two arms of the Wheatstone bridge integrated on the working crystal surface of the silicon 3D film (100) change, their rate of change... The sign of R / R change is determined by the sign of the stress difference. For R1 and R3, the longitudinal stress... l = y Transverse stress t = x For R2 and R4, longitudinal stress l= x, transverse stress t= y. Longitudinal piezoresistive coefficient π l =1 / 2π 44 lateral piezoresistive coefficient π t=-1 / 2π44, when the resistance changes, the rates of change of the resistance values ​​of each resistor on the Wheatstone bridge are as follows:

[0037]

[0038] in: y , x These represent the longitudinal and lateral pressures at the measurement points of the elastic element, respectively.

[0039] When stress is generated, two resistors increase and two decrease. The Wheatstone bridge, composed of four resistors, can sensitively respond to the resistance changes caused by stress and effectively eliminate the non-uniformity of the diffusion resistance itself and the influence of the temperature coefficient of resistance. Under constant power supply excitation, the Wheatstone bridge outputs an electrical signal positively correlated with the hydrogen concentration, thus yielding the hydrogen concentration value. Figure 2 As shown.

[0040] The Wheatstone bridge structure is commonly used in the design of pressure and gas sensors, significantly improving their sensitivity and accuracy. In this embodiment, the palladium alloy thin film deforms after absorbing hydrogen, causing deformation of the silicon strain gauge beneath it. This deformation of the strain gauge leads to deformation of the silicon strain resistor strip. Silicon's resistivity change is much higher than that of metals, with a strain factor tens of times greater. Under the same strain, the resistivity change is tens of times higher. While the hydrogen absorption expansion material is also a palladium alloy thin film, the strain remains constant at the same hydrogen concentration. However, by changing the strain resistor material from an alloy to semiconductor silicon, the resistivity change is significantly increased, resulting in a tens-fold increase in sensor output sensitivity. Therefore, the sensor's sensitivity at low concentrations is much higher than that of the palladium alloy resistor sensor in the comparative document, leading to a lower detection limit for hydrogen concentration. The Wheatstone bridge structure in this embodiment consists of four single-crystal silicon thin-film resistors.

[0041] Furthermore, since the hydrogen absorption energy and lattice expansion of the palladium alloy thin film are temperature-dependent, platinum heating resistors and platinum temperature sensing resistors are etched on the sensor surface to eliminate the influence of temperature variables on the sensor. Combined with external circuitry and digital PID algorithm control, the temperature sensing resistor value is acquired in real time, and the difference is calculated with the set value. Then, through proportional amplification, integration, and differentiation, a value positively correlated with the current is obtained. This current is then applied to the heating resistor to achieve constant temperature control of the sensor chip. Partial circuit diagram is shown below. Figure 3 . Figure 3In this circuit, the constant current source detects a current of approximately 300mA, with an output voltage of 1.5V and an input voltage of 0-1.5V. The control current accuracy is 0.6mA. R_heat_DAC is the real-time output voltage of the PID loop (which also serves as the input voltage of the controllable constant current source). U12, N2, and external resistors and capacitors constitute the constant current heating circuit. The input voltage R_heat_DAC and resistor R32 determine the heating current, I = R_heat_DAC / R32. U13, sampling resistor R29, and external capacitors constitute the current sampling circuit. By acquiring the real-time heating current, overcurrent software protection can be set to prevent excessive heating current from damaging the sensor.

[0042] Example 2

[0043] Embodiment 2 of the present invention provides a method for fabricating the hydrogen sensor described in Embodiment 1 above. It employs semiconductor MEMS technology, combining conventional MEMS pressure sensor technology with palladium alloy thin-film sensor alloy coating technology. SOI wafers are selected, and the top single-crystal silicon is first doped with a high concentration of boron (B doping concentration not less than 1*10⁻⁶) using ion implantation technology. 16 The process involves etching resistance strips using inductively coupled plasma dry etching technology to form piezoresistive strips with a single-crystal silicon layer as the strain resistor, exposing the SiO2 insulating layer in the area excluding the piezoresistive strips; photolithographically etching temperature-sensing resistance strips on the SiO2 insulating layer and depositing a Ni thin film as the temperature-sensing resistor; photolithographically etching heating resistance strips on the SiO2 insulating layer and depositing an Au thin film as the heating resistor; photolithographically etching hydrogen-sensitive patterns on the SiO2 insulating layer and depositing a palladium alloy thin film as the hydrogen-sensitive pattern; photolithographically etching lead grooves and pads and depositing a Ni-Au thin film as the electrode; photolithographically etching etching windows on the back of the SOI substrate and dry etching the silicon cup; finally, electrostatically sealing the SOI substrate with Pyrex 7740 glass and dicing to obtain the hydrogen sensor of Example 1.

[0044] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0045] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A hydrogen sensor, comprising bonded glass; characterized in that, The bonding glass is bonded to the bottom of the SOI substrate to form an integral whole; the top surface of the SOI substrate is covered with a strain diaphragm; a palladium alloy thin film is disposed on the strain diaphragm; a Wheatstone bridge for detecting hydrogen concentration is integrated on the working crystal surface of the strain diaphragm.

2. The hydrogen sensor according to claim 1, characterized in that, The resistors in the Wheatstone bridge are single-crystal silicon thin-film resistors.

3. The hydrogen sensor according to claim 1, characterized in that, The strain diaphragm is also equipped with a heating resistor and a temperature measuring resistor.

4. The hydrogen sensor according to claim 3, characterized in that, The temperature measuring resistor value is collected in real time, and the difference between the temperature measuring resistor value and the set value is used as the input of the PID controller to obtain an output value that is positively correlated with the current. The output value is applied to the heating resistor to realize the constant temperature control of the hydrogen sensor.

5. The hydrogen sensor according to claim 3, characterized in that, The thickness of the heating resistor and the temperature measuring resistor is 50nm-500nm.

6. The hydrogen sensor according to claim 1, characterized in that, The rate of change of resistance values ​​of each resistor in the Wheatstone bridge is: ； in, y , x These represent the longitudinal and transverse pressures at the strain gauge measurement points, respectively. R1, R2, R3, and R4 are the four resistors of a Wheatstone bridge. The connection point of R1 and R3 is connected to the positive terminal of the power supply, and the connection point of R2 and R4 is connected to the negative terminal of the power supply. π 44 This is the compression coefficient for monocrystalline silicon.

7. The hydrogen sensor according to any one of claims 1 to 6, characterized in that, The thickness of the SOI substrate is 300um-1000um.

8. The hydrogen sensor according to any one of claims 1 to 6, characterized in that, The strain diaphragm has a thickness of 20µm-100µm.

9. A method for preparing a hydrogen sensor according to any one of claims 1 to 8, characterized in that, include: SOI wafers are selected, and the top single-crystal silicon is doped with high concentration of boron using ion implantation technology to obtain SOI substrates. The resistor strip is etched using inductively coupled plasma dry etching technology to form a varistor strip with a single crystal silicon layer as the strain resistor, and the SiO2 insulating layer in the area other than the varistor strip is exposed. A hydrogen-sensitive pattern was photolithographically etched on a SiO2 insulating layer, and a palladium alloy thin film was deposited as the hydrogen-sensitive pattern block. Photolithography is used to create lead grooves and pads, and Ni-Au thin films are deposited as electrodes. Photolithography etching windows are performed on the back of the SOI substrate, followed by dry etching of silicon cups. An SOI substrate is electrostatically bonded to Pyrex 7740 glass, and then diced to obtain a hydrogen sensor.

10. The preparation method according to claim 6, characterized in that, Also includes: A temperature-sensing resistance strip is photolithographically patterned on a SiO2 insulating layer, and a Ni thin film is deposited as a temperature-sensing resistor. Heating resistance strips were photolithographically etched on a SiO2 insulating layer, and an Au thin film was deposited as the heating resistance.

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