CN102095766B - Miniature integrated temperature control type CO2 gas sensor and manufacturing method thereof - Google Patents

Abstract

The invention discloses a miniature integrated temperature control type CO2 gas sensor and a manufacturing method thereof. The sensor comprises a miniature integrated temperature control type device and a high-temperature work film type solid electrolyte air-sensitive element and is characterized in that: the miniature integrated temperature control type device consists of a silicon substrate, a SiO2 layer, a Si3N4 layer, a Pt heater and a temperature measuring element which are processed by a micro electro mechanical system (MEMS) process; and on a miniature integrated temperature control structure, a film type solid electrolyte CO2 air-sensitive element is formed by a micromachining process deposition SiO2 layer, an Li3PO4 solid electrolyte film, a Pt metal electrode, a reaction electrode Li2CO3 and a reference electrode Li2TiO3/TiO2. A closed loop control system is formed by the feedback of the Pt heater and the temperature measuring element to make the temperature of the sensor reach the specified temperature required by working, keep the temperature constant and ensure the optimal performance of the CO2 gas sensor.

Description

Micro-integrated temperature-controlled CO2 gas sensor and preparation method

technical field

The invention relates to a structure and a preparation method of a micro-integrated temperature-controlled solid electrolyte _CO2 gas sensor.

Background technique

Solid electrolytes are widely used in gas sensors due to their good ionic conductivity. Since Faraday first discovered the law of the conductivity of _PbF2 changing with temperature in 1834, many research groups at home and abroad have begun to work on this aspect. Solid electrolytes Gas sensor has become one of the important development directions. At present, CO ₂ conventional solid electrolyte gas sensors have made great progress. There are CO ₂ sensor products prepared by traditional processing methods on the market. These sensors use traditional processing techniques such as sintering, bonding, etc. to combine solid electrolytes, electrodes, heating The sensor is integrated together, which makes the sensor have problems such as large size, high power consumption, poor stability and consistency. In addition, for the existing potentiometric _CO2 sensors made of solid electrolytes, the electrolyte materials need to work at high temperatures. For example, Dong-HyunKim, Ji-Young Yoon et al. mixed and sintered Li ₃ PO ₄ , Li ₂ CO ₃ , and Al ₂ O ₃ to a CO ₂ gas sensor, where the reference electrode was LiMn ₂ O ₄ , and the gas sensor At 370°C, it has better response characteristics to _CO2 gas, and is less disturbed by water vapor.

As mentioned above, the solid electrolyte _CO2 gas sensor with temperature control device produced by traditional technology has the disadvantages of large volume, high material consumption, poor consistency, high power consumption, etc., and the use of MEMS technology for miniaturization can overcome these shortcomings , is an inevitable trend in the development of solid electrolyte gas sensors. A device that integrates solid electrolyte sensors, heating and temperature control has been reported in the literature. For example, JF Currie designed a miniature solid electrolyte sensor with a structure of Pt|Na ₂ CO ₃ , Ba ₂ CO ₃ , AgSO ₄ |Ag, through The Pt heating structure is formed by the lift-off method, and the film structure is formed by wet etching. The main problem of this sensor is that the selectivity is not good. Yeung Bang et al. have developed a miniature all-thin film CO ₂ gas sensor. The sensor exhibits the measured electromotive force and gas concentration in different high-temperature environments according to the Nernst equation. But the device does not have integrated heating device and temperature measuring device. Due to the problems of material matching, system integration, preparation process, compensation method, etc., there is no micro-integrated temperature-controlled solid electrolyte _CO2 gas sensor that can work independently and be successfully applied.

Contents of the invention

The technical problem to be solved by the present invention is to improve the status quo of the micro-integrated temperature-controlled CO2 gas sensor in the background technology, and provide _a micro-integrated temperature-controlled _CO2 gas sensor based on MEMS technology, which has the functions of heating and temperature measurement integration On the temperature control device, the film-type solid electrolyte _CO2 gas sensor is prepared by micro-fabrication technology, so that the solid electrolyte film can reach the optimal temperature required for work.

To achieve the above object, the present invention is achieved by taking the following technical solutions:

A micro-integrated temperature-controlled _CO2 gas sensor is characterized in that, on a silicon substrate (with _SiO2 on both sides), _Si3N4 layers are deposited on both sides, and one side of the silicon substrate is etched by dry method _. Etching off the Si ₃ N ₄ layer, and then wet etching out the heat dissipation window structure, using photolithography and lift-off processes to process the Pt heating electrode and Pt temperature measuring electrode on the Si ₃ N ₄ layer on the other side of the silicon substrate, and then Deposit a SiO ₂ layer to cover the Pt heating electrode and the Pt temperature measuring electrode, and expose the wiring pad; then deposit a Li ₃ PO ₄ solid electrolyte film on the SiO ₂ layer, and prepare two Pt conductive films on it, and finally in a A reaction electrode is prepared on a Pt conductive film, and a reference electrode is prepared on another Pt conductive film.

When in use, respectively connect the control wires for heating and temperature measurement on the wiring board, and connect the reaction electrode and the reference electrode to the output leads of the sensor. Through temperature control, the sensor works at the optimum temperature of solid electrolyte, 480°C. the

In the above solution, the Si ₃ N ₄ layer is a thin film of about 1 micron. The Pt heating electrode and the Pt temperature-measuring electrode are strip-shaped circuitous structures with a thickness of 90-110 nanometers. The thickness of the Li ₃ PO ₄ solid electrolyte film is 500-1000 nanometers. The Pt conductive thin film has two rectangular ring structures. The reaction electrode is a Li ₂ CO ₃ electrode; the reference electrode is a Li ₂ TiO ₃ /TiO ₂ electrode.

The aforementioned micro-integrated temperature-controlled CO _gas sensor preparation method comprises the following steps:

a. Deposit a layer of Si ₃ N ₄ film on both sides of the silicon substrate, etch the Si ₃ N ₄ layer on one side of the silicon substrate by dry method, and then wet etch the thermal window structure;

b. Using the photolithographic lift-off process, process the Pt heating electrode and the Pt temperature measuring electrode on the Si ₃ N ₄ layer on the other side of the silicon substrate;

c. Deposit a SiO ₂ layer on the Pt heating electrode and Pt temperature measuring electrode, and deposit a Li ₃ PO ₄ solid electrolyte film on it;

d. Shielded by a patterned mask, sputter two Pt conductive films on the Li ₃ PO ₄ solid electrolyte film, then prepare a reactive electrode on one Pt conductive film, and prepare a reference electrode on the other Pt conductive film. than the electrode.

Compared with prior art, the present invention has the following advantages:

1) The micro-integrated temperature control device is realized by MEMS technology, and the temperature control device is integrated with the gas sensitive element, so that the volume is reduced, the power consumption is reduced, the response speed is improved, and the packaging is simple. the

2) Using MEMS technology to integrate heating, dielectric layers, solid electrolyte materials, electrodes, etc. on the silicon substrate through a layer-by-layer deposition process, instead of using traditional methods such as bonding and clamping, the sensor process is stable and repeatable , Easy mass production, low cost. the

3) The heating and temperature measuring elements of the integrated temperature control device are processed at one time, which simplifies the processing technology. the

4) The gas sensor includes Li ₃ PO ₄ solid electrolyte film, Pt conductive film, reaction electrode Li ₂ CO ₃ and reference electrode Li ₂ TiO ₃ /TiO ₂ , because it works at the optimum temperature, the sensor maintains good sensitivity Under the premise, it has very good selectivity and response characteristics.

Description of drawings

Fig. 1 is a structural schematic diagram of the micro-integrated temperature-controlled solid electrolyte _CO2 gas sensor of the present invention.

Fig. 2 is a planar layout structure diagram of the Pt heating electrode and the Pt temperature measuring electrode pattern in Fig. 1 . the

Fig. 3 is a planar layout structure diagram of the reference electrode and the reaction electrode in Fig. 1 . the

In Fig. 1 to Fig. 3: 1. Silicon substrate (with SiO ₂ on both sides); 2. Si ₃ N ₄ layers; 3. Solid electrolyte film; 4. Pt conductive film; 5. Reaction electrode; 6. Reference Electrode; 7. SiO ₂ insulating protective layer; 8. Heat dissipation window; 9. Pt heating electrode; 10. Pt temperature measuring electrode; 11. Terminal board.

Fig. 4 is the corresponding curve relationship between the sensitivity of the Li ₃ PO ₄ solid electrolyte film CO ₂ gas sensor and the heating temperature in the range of 420°C to 480°C. The curve shows that the sensitivity will not increase significantly when the temperature is higher than 480°C.

Detailed ways

As shown in Figure 1, a micro-integrated temperature-controlled solid electrolyte CO ₂ gas sensor based on MEMS technology, deposited Si ₃ N ₄ layers 2 on both sides of a silicon substrate (with SiO ₂ on both sides) 1, and dry-etched Etch away the Si ₃ N ₄ layer, process the heat dissipation window 8 structure by wet etching process, and process the Pt heating electrode 9 and the temperature measuring electrode 10 on the other side of the silicon substrate Si ₃ N ₄ layer by photolithography stripping process, and then Deposit the SiO ₂ insulating protection layer 7 to cover the Pt heating electrode and the Pt temperature measuring electrode, and expose the junction pad 11, and then deposit the Li ₃ PO ₄ solid electrolyte film 3 on the SiO ₂ layer 7, and prepare two Pt conductive electrodes on it. film 4, and finally prepare a Li ₂ CO ₃ reaction electrode 5 and a Li ₂ TiO ₃ /TiO ₂ reference electrode 6 on the two conductive films respectively.

As shown in Figure 2, the patterns of the Pt heating electrode and the Pt temperature-measuring electrode are, including two independent strip-shaped winding film structures with a micron-scale scale, with a thickness of about 100 nanometers, and two ends of each film structure. Independent wiring board 11. Connect the control wires for heating and temperature measurement to the wiring board, and connect the reaction electrode and reference electrode to the output leads of the sensor. The resistance change of the Pt temperature measuring electrode has a corresponding relationship with the temperature, and the resistance signal conversion circuit is converted into a voltage signal, which is used as a feedback and connected to the control circuit to control the driving voltage applied to the Pt heating electrode. the

The change of Pt resistance with temperature conforms to the formula

R _t =R ₀ (1+At+Bt ² ) (1)

In the formula: A=3.90802×10 ^-3 /°C; B=-5.80195 ^{×10 -7} /°C; R _t and R ₀ are the resistance values of Pt at t°C and 0°C, respectively.

The input signal loads the voltage on both ends of the Pt heating electrode through the control circuit, so that the solid electrolyte of the sensor can work at the optimal temperature of 480°C. Since the target temperature is adjustable, the sensor can also work at a certain temperature required, so the integrated device can also be extended to other high-temperature solid electrolyte gas sensors

As shown in FIG. 3 , the Li ₃ PO ₄ solid electrolyte film 3 in the figure has a square film structure with a thickness of 680 nanometers. The Pt conductive thin film 4 is a rectangular annular thin film structure, on which are the thick film sintered Li ₂ CO ₃ reaction electrode 5 and the Li ₂ TiO ₃ /TiO ₂ reference electrode 6 with a thickness of tens of microns. The voltage change between the reaction electrode and the reference electrode has a corresponding relationship with the measured CO ₂ gas concentration when the temperature is constant. On the Li ₂ CO ₃ reaction electrode 5, CO ₂ undergoes the following chemical reaction:

Li ⁺ , e ⁻ are generated and reach the reference electrode 6 through the conduction of Pt and Li ₃ PO ₄ . On the Li ₂ TiO ₃ /TiO ₂ reference electrode, the reaction takes place:

A potential difference is formed between the reaction electrode and the reference electrode, the potential difference equation

E＝Eo-RT/nF Ln p(CO ₂ ) (4)

Among them, the electromotive force at a given gas concentration p(CO ₂ ) under Eo standard conditions;

R-gas constant (8.314J K ^-1 mol ^-1 );

T-temperature (K);

The number of electrons gained and lost in n-electrode reactions;

F-Faraday's constant (96485C·mol ^-1 ).

By measuring the electromotive force between the reaction electrode and the reference electrode and the heating temperature, according to formula (4), the measured gas concentration can be read. the

Taking the response characteristics of the structural sensor to _CO2 gas as an example, the sensor is calibrated at a room temperature of 20°C, that is, in an environment with a temperature of 20°C and a solid electrolyte at 480°C, when the measured environment _CO2 concentration is 500ppm , the voltage value of the measured reaction gas concentration is 260mV, and the _E0 measured under standard conditions is 253.25mV, which can be obtained from equation (4):

E=253.25×10 ^-3 -8.314×753/2×96485×Ln(500×10 ^-6 )=257.25×10 ^-3 V is basically consistent with the measured results.

Figures 1 to 3 are based on MEMS technology, and the micro-integrated temperature-controlled solid electrolyte _CO2 gas sensor is mainly realized by the following process steps:

a. Deposit a layer of Si ₃ N ₄ film 2 on both sides of the silicon substrate (with SiO ₂ on both sides) 1, one side of the silicon substrate is etched away by dry method Si ₃ N ₄ layer, and then wet etched out Heat dissipation window 8;

b. Utilize the photolithographic stripping process to process the Pt heating electrode 9 and the Pt temperature measuring electrode 10 of the micro-thin film structure on the Si ₃ N ₄ layer on the other side of the silicon substrate;

c. Sputtering SiO ₂ insulating protective layer 7 on the Pt heating electrode and Pt temperature measuring electrode, and preparing Li ₃ PO ₄ solid electrolyte film 3 by thermal evaporation coating process thereon;

d. Shielded by a patterned mask, two Pt conductive films 4 are sputtered on the Li ₃ PO ₄ solid electrolyte film, and then the reaction electrode 5 and the reference electrode 6 are respectively prepared on the two conductive films by thick film technology.

The temperature control compensation method of sensor of the present invention comprises the steps:

a, as shown in Figure 2, apply a voltage across the Pt heating electrode 9, detect the resistance change of the Pt temperature measuring electrode 10, convert the resistance change into a voltage signal through a signal processing circuit, and then compare it with the applied voltage as a feedback signal to form Closed loop temperature control system. the

b. Since the Pt resistance change has a definite relationship with the temperature change, a specific temperature is targeted, and the resistance change of the temperature measuring element is kept constant through the control circuit. the

c. As shown in FIG. 3 , detect the electromotive force change between the reaction electrode 5 and the reference electrode 6 to obtain the CO ₂ gas concentration in the environment.

Claims (2)

1. miniature integrated Temperature Control Type CO ₂Gas sensor is characterized in that, on silicon chip, and double-sided deposition Si ₃N ₄Layer falls Si in the wherein one side of silicon chip by dry etching ₃N ₄Layer, then wet method etches the radiator window structure is at the Si of the another side of silicon chip ₃N ₄Adopt photoetching, stripping technology to process Pt heating electrode and Pt thermometric electrode on layer, then deposit SiO ₂Insulating protective layer covers Pt heating electrode and Pt thermometric electrode, and exposes patching panel; Then at SiO ₂Deposit Li on insulating protective layer ₃PO ₄Solid electrolyte film, and prepare thereon two Pt conductive films, prepare reaction electrode at last on a Pt conductive film, prepare contrast electrode on another Pt conductive film; Wherein, Pt heating electrode and Pt thermometric electrode are banded circuitous configuration, and thickness is the 90-110 nanometer; Li ₃PO ₄Solid electrolyte film thickness is the 500-1000 nanometer; The Pt conductive film is two rectangular ring structures; Reaction electrode is Li ₂CO ₃Electrode; Contrast electrode is Li ₂TiO ₃/ TiO ₂Electrode.

2. miniature integrated Temperature Control Type CO as claimed in claim 1 ₂The preparation method of gas sensor is characterized in that, comprises the steps:

A. at silicon chip double-sided deposition one deck Si ₃N ₄Film falls Si in the wherein one side of silicon chip by dry etching ₃N ₄Layer, then wet method etches the radiator window structure;

B. utilize lithography stripping technique, at the Si of the another side of silicon chip ₃N ₄Process Pt heating electrode and Pt thermometric electrode on layer;

C. deposit SiO on Pt heating electrode and Pt thermometric electrode ₂Layer, and deposit Li thereon ₃PO ₄Solid electrolyte film;

D. by being with figuratum mask plate to cover, at Li ₃PO ₄On solid electrolyte film, two Pt conductive films of sputter, then prepare reaction electrode on a Pt conductive film, prepares contrast electrode on another Pt conductive film.

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