RU2532428C1 - Manufacturing method of gas sensor with nanostructure, and gas sensor on its basis - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to manufacture of gas sensors intended for detection of different gases. The invention proposes a gas sensor manufacturing method, in which a heterostructure is formed of different materials; a gas-sensitive layer is made in it; after that, it is fixed in the sensor housing, and contact pads are connected to terminals of the housing by means of contact conductors. The gas-sensitive layer is made in the form of a thin tread-like nanostructure (SiO₂)_20%(SnO₂)_80%, where 20% - mass fraction of SiO₂, and 80% - mass fraction of component SnO₂, by application of sol of orthosilicic acid, which contains stannum hydroxide, onto a silicone surface by means of a centrifuge with further annealing. An area with width of 1 mcm and depth of 200 nm is formed on the surface of the substrate surface by a method of local anodic oxidation. Sol is prepared at two stages: at the first stage, Tetraethoxysilane (TEOS) and ethyl alcohol (95%) is mixed in the ratio of 1:1.046 at room temperature, and the mixture is exposed during about 30 minutes, and at the second stage, to the obtained solution there introduced is distilled water in the ratio of 1:0.323; hydrochloric acid (HCl) in the ratio of 1:0.05; stannum chloride dihydrate (SnCl₂·2H₂O) in the ratio of 1:0.399, where TEOS volume is accepted as one, and stirred at least during 60 minutes. The invention also proposes a gas sensor with a nanostructure, which is made as per the proposed method.

EFFECT: increasing gas sensor sensitivity.

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Description

The present invention relates to measuring technique and can be used in the manufacture of gas sensors of a new generation, designed to detect various gases.

Currently, gas sensors are actively used in almost all industries, transport, as well as in agriculture and medicine. Gas sensors form the basis for creating fire and environmental safety systems. The most common gas sensors based on semiconductor metal oxides, such as tin oxide (SnO ₂ ). The mechanism of action of such devices is based on a change in the electrical conductivity of n-type semiconductors during chemical transformations occurring on their surface, for example, the interaction of a detected gas with chemisorbed oxygen. Sensors based on SnO _{2 are} characterized by low cost, good response time and a number of other advantages. Their typical drawbacks are a long time of dumping readings (due to the finite time of desorption of gases) and insufficient selectivity to the detected gases.

A known method of analyzing semiconductor sensors of a gas mixture containing combustible gases, such as CO and H ₂ . Tin dioxide doped with antimony is used as a gas-sensitive layer. The gas-sensitive SnO ₂ films obtained according to this invention showed high sensitivity to H ₂ and CO in atmospheres O ₂ / N ₂ and O ₂ / N ₂ / vapors-H ₂ O. The temperature range of sensitivity of the sensors obtained by this method is 200-550 ° C [1].

A sensor device for detecting CO is known, including an insulating substrate with measuring electrodes, a semiconductor oxide layer and a catalytic layer containing one of the following metals — Pt, Rh, Pd on an oxide support and a heating element. The specified device provides a relatively high sensitivity to CO at a moderate temperature of the heating element (120 ° C and below). The disadvantages of the proposed device are the low stability of the sensor caused by degradation of the structure of the sensitive layer of the semiconductor oxide.

Known sensor device for indicating CO, in which tin oxide with finely dispersed platinum is used as the material of the sensing element, additives of silicates such as feldspars and bentonite are used to create the optimal porous structure of the active layer [3]. The advantage of the device is the possibility of separate determination of carbon monoxide and hydrogen. The disadvantage of this device is the high electrical resistance of the sensitive layer, which complicates the measurement of the sensor signal and significantly complicates the design.

A known method of sensory analysis of a gas mixture containing reducing gases (CO and H ₂ ) and O ₂ . As catalysts for increasing the sensitivity of the gas-sensitive layer based on tin dioxide to CO and H ₂ , RuCl ₃ and PtCl _{2 are used} . The method shows that the optimal concentrations of RuCl ₃ and PtCl ₂ in SnO ₂ for the detection of CO and H ₂ are 1-5 mol.%. Ru and Pt, which were introduced into the matrix by impregnation of tin dioxide with chlorides of these elements. The resulting films based on these substances can be used in the temperature range 200-350 ° C [4].

Closest to the technical nature of the proposed solution is a method of manufacturing a sensitive element of a gas sensor using thin-film technology [5]. It consists in the fact that they form a heterostructure of various materials (dielectric substrate, platinum contact pads on one side of the substrate, a heater on the other side), in which a gas-sensitive layer is formed (a tin dioxide film 50 and 100 nm thick with an antimony content of 1.5 at .%). As a substrate, polycor plates with a thickness of 150 μm are used. The contacts to the tin dioxide layers and the heater on the reverse side are formed by sputtering platinum followed by photolithographic engraving prior to the deposition of tin dioxide (SnO ₂ ) films. Ultrathin layers of catalytic platinum are obtained by cathodic deposition. Finished samples are subjected to stabilizing annealing in air at 400 ° C for 24 hours. The disadvantage of this method is not sufficiently high sensitivity to various reducing gases (for example, ethyl alcohol vapors).

The technical result of the invention is to increase the sensitivity of the gas sensor.

This is achieved by the fact that in the known method of manufacturing a gas sensor with a nanostructure, which consists in forming a heterostructure of various materials in which a gas-sensitive layer is formed, after which it is fixed in the sensor housing, and the contact pads are connected to the housing leads using contact conductors in accordance with the invention gas-sensitive layer is formed as a thin filamentary nanostructures (SiO _{2) 20%} (SnO _{2) 80%,} where 20% - mass proportion of SiO ₂ and 80% - mass fraction of SnO ₂ component put m of applying an orthosilicic acid sol containing tin hydroxide onto a silicon substrate, on the surface of which a region of 1 μm wide, 200 nm deep is formed by local anodic oxidation using a centrifuge and subsequent annealing, the sol is prepared in two stages, tetraethoxysilane is mixed in the first stage (TEOS) and ethyl alcohol (95%) in a ratio of 1: 1.046 at room temperature and the mixture was kept for up to 30 minutes, then at the second stage distilled water in a ratio of 1: 0.323, hydrochloric acid (HCl) was introduced into the resulting solution ootnoshenii 1: 0.05, stannous chloride dihydrate (SnCl ₂ · 2H ₂ O) in a ratio of 1: 0.399, wherein the amount received per unit of TEOS and stirred for at least 60 minutes, the orthosilicic acid sol is applied on a substrate of silicon (Si) using a centrifuge using a dispenser at a centrifuge speed of 3000 rpm for 2 minutes, and annealing is carried out at a temperature of 600 ° C for 30 minutes in air.

Moreover, in a gas sensor with a nanostructure manufactured by the proposed method, comprising a housing, a heterogeneous structure made of thin films of materials formed thereon, formed on a semiconductor substrate, a gas-sensitive layer and contact pads formed in a heterogeneous structure, case leads and contact conductors connecting the pads with the findings of the housing, in accordance with the invention, the gas-sensitive layer is made in the form of a thin filiform nanostructure based on I of orthosilicic acid containing tin hydroxide on a silicon substrate on the surface of which a region of 1 μm wide, 200 nm deep was formed by local anodic oxidation using a centrifuge and subsequent annealing, the sol was prepared in two stages, tetraethoxysilane and ethyl were mixed in the first stage alcohol, and in a second step the resulting solution was administered distilled water, hydrochloric acid (HCl) and stannous chloride dihydrate (SnCl ₂ · H ₂ O), wherein tetraethoxysilane and ethyl alcohol in a ratio of 1: 1,046, with distilled water elations 1: 0.323, hydrochloric acid (HCl) in a ratio of 1: 0.05, stannous chloride dihydrate (SnCl ₂ · 2H ₂ O) in a ratio of 1: 1,597.

Figure 1 shows the design of a gas sensor, which is manufactured by the proposed methods. The gas sensor contains a housing 1 (Fig. 1), a heterogeneous structure 2 (from thin films of materials) in which a gas-sensitive layer 3 (thin filamentous nanostructure) is formed, contact pads 4, contact conductors 5, conclusions of the housing 6, fitting 7, insulators 8 substrate 9 (made of silicon, on the surface of which a region of a width of 1 μm and a depth of 200 nm was formed by local anodic oxidation).

According to the proposed method, the sol of orthosilicic acid containing tin hydroxide is prepared in two stages for applying to the substrate 9 of silicon (figure 1). At the first stage, tetraethoxysilane and ethyl alcohol are mixed, the mixture is kept for up to 30 minutes before proceeding to the second stage. The exposure time was established based on the time of the reaction of the exchange interaction between tetraethoxysilane and ethyl alcohol, as a result of which ethyl ether of orthosilicic acid is formed. In the second stage, after the introduction of distilled water, hydrochloric acid (HCl) and tin chloride (SnCl ₂ · 2H ₂ O), the mixture is stirred for at least 60 minutes. The process time was established based on the time of the reaction of hydrolysis of the ether, as a result of which orthosilicic acid is formed. And also based on the fact that during this time tin hydroxide (Sn (OH) ₂ ) is formed at this stage and the polycondensation reaction of orthosilicic acid proceeds.

A sol of orthosilicic acid containing tin hydroxide is deposited on a substrate 9 (Fig. 1) made of silicon (Si), on the surface of which a region of a width of 1 μm and a depth of 200 nm is formed by local anodic oxidation using a centrifuge using a dispenser with a centrifuge speed of 3000 rpm for 2 minutes. The use of such centrifuge modes allows to achieve uniform distribution of the sol, as well as partially remove the solvent from this film.

As a substrate of silicon (Si), KEF (111) silicon wafers with a thickness of 200-300 μm, industrially oxidized in oxygen, having an SiO ₂ oxide layer with a thickness of about 800 nm can be used. A region with a width of 1 μm and a depth of 200 nm is formed on the surface of the substrate by the method of local anodic oxidation, in which a gas-sensitive layer is formed in the form of a thin whisker nanostructure (SiO ₂ ) of _20% (SnO ₂ ) _80% , where 20% is the mass fraction of SiO ₂ , and 80% is the mass fraction of the SnO ₂ component. Figure 2 shows the surface morphology of the gas-sensitive layer 3 obtained by scanning electron microscopy (SEM), with a mass fraction of tin dioxide (SnO ₂ ) of 80% (Fig. 2a - an increase of 5,000 times, Fig. 2b - an increase of 300,000 time).

Annealing is carried out at a temperature of 600 ° C for 30 minutes in air. The use of such process parameters allows the final removal of solvent from pores on the surface and in the bulk of the film, as well as the decomposition of orthosilicic acid (Si (OH) ₄ ) to silicon dioxide (SiO ₂ ) and tin hydroxide (Sn (OH) ₄ ) to tin dioxide (SnO ₂ ). The contact pads 4 to the gas sensitive layer of Ag are formed by annealing at a temperature of 600 ° C.

The gas sensor operates as follows. The gas-sensitive layer 3 using the conclusions of the housing 6 is included in the bridge measuring circuit (bridge) as one of its shoulders, using a trimming resistor (not shown), the bridge is balanced (the readings of the measuring device are set to zero in the absence of gas). The interaction of a gas with a gas-sensitive layer leads to a change in its electrical conductivity during chemical transformations occurring on the surface, for example, the interaction of a detected gas with chemisorbed oxygen. Since the gas-sensitive layer 3 is included in the bridge measuring circuit, then with a change in the gas concentration, its imbalance occurs, which is a function of concentration.

Figure 3 shows the dependence of the sensor signal (5) of the gas-sensitive layer 3 on the concentration of the detected gas — ethanol vapor (s): curve 1 — gas-sensitive layer in the form of a continuous SnO ₂ film, curve 2 — gas-sensitive layer in the form of a thin filamentous nanostructure (SiO ₂ ) _20% (SnO ₂ ) _80% , where 20% is the mass fraction of SiO ₂ , and 80% is the mass fraction of the SnO ₂ component. It is seen that in the presence of a thin whisker nanostructure (SiO ₂ ) _20% (SnO ₂ ) _80% (curve 2), the sensor signal at the same gas concentration is much larger than in the absence of it (curve 1). The thin filamentous nanostructure (SiO ₂ ) _20% (SnO ₂ ) _80% , obtained in the framework of the sol-gel technology, is a percalation structure. This structure has maximum sensitivity due to the following circumstance. When a structure is found in air, chemisorbed oxygen creates a depleted layer near the bulkheads of the grains; therefore, such a structure has a high resistance (R). Under the influence of reducing gases (ethanol vapor) on a thin whisker nanostructure (SiO ₂ ) _20% (SnO ₂ ) _80% , various chemical reactions occur during a certain time (t), including the binding of chemisorbed oxygen. When this depletion disappears and the resistance (R) is significantly reduced (figure 4). The gas-sensitive layer in the form of a continuous SnO ₂ film has a structure corresponding to spinodal decomposition. Such a structure is much less sensitive to gases than the percolation structure.

Thanks to the distinguishing features of the invention, the sensitivity of the gas sensor is increased.

As a result of tests of experimental samples of gas sensors made in accordance with the claims, it was found that the sensitivity of gas sensors is significantly increased.

The proposed method of manufacturing a gas sensor with a nanostructure and a gas sensor based on it compares favorably with the known ones and can find wide application in the manufacture of gas sensors.

Information sources

1. U.S. Pat. No. 4614669, 09/30/1986.

2. US patent 4792433, MKI G01N 20/16, 1988.

3. UK patent 2249179, MKI G01N 27/12, 1992.

4. U.S. Pat. No. 4397888, 08/09/1983.

5. Anisimov O. V., Maksimova N. K., Filonov N. G. Features of the response of thin Pt / SnO2: Sb films to CO exposure // Journal of Physical Chemistry, 2004 - T.78. - No. 10. - S. 1907-1912.

Claims (2)

1. A method of manufacturing a gas sensor with a nanostructure, which consists in the fact that they form a heterostructure of various materials, which form a gas-sensitive layer, after which it is fixed in the sensor housing, and the contact pads are connected to the leads of the housing using contact conductors, characterized in that gas-sensitive layer is formed as a thin filamentary nanostructures (SiO _{2) 20%} (SnO _{2) 80%,} where 20% - mass proportion of SiO ₂ and 80% - mass fraction of SnO ₂ component, by applying a sol orthosilicic acid comprising guide tin oxide, on a silicon substrate, on the surface of which a region of 1 μm wide, 200 nm deep is formed by local anodic oxidation using a centrifuge and subsequent annealing, the sol is prepared in two stages, at the first stage tetraethoxysilane (TEOS) and ethyl alcohol are mixed ( 95%) in a ratio of 1: 1.046 at room temperature and the mixture is kept for up to 30 minutes, then, at the second stage, distilled water in a ratio of 1: 0.323, hydrochloric acid (HCl) in a ratio of 1: 0.05, and two-water tin chloride are added to the resulting solution (SnCl ₂ · 2H ₂ O) in the aspect ratio shenii 1: 0,399, which passed per unit volume of TEOS and stirred for at least 60 minutes. 2. A gas sensor with a nanostructure made according to claim 1, comprising a housing, a heterogeneous structure made of thin films of materials formed on it, formed on a semiconductor substrate, a gas-sensitive layer and contact pads formed in a heterogeneous structure, body leads and contact conductors connecting the contact pads with the leads of the case, the gas-sensitive layer is made in the form of a thin whisker nanostructure based on the sol of orthosilicic acid containing tin hydroxide on a substrate of cre an opinion on the surface of which a region 1 μm wide and 200 nm deep was formed by local anodic oxidation using a centrifuge and subsequent annealing, which was prepared in two stages, tetraethoxysilane and ethyl alcohol were mixed in the first stage, and distilled distilled into the resulting solution water, hydrochloric acid (HCl) and two-water tin chloride (SnCl ₂ · 2H ₂ O), with tetraethoxysilane and ethyl alcohol in a ratio of 1: 1,046, distilled water in a ratio of 1: 0,323, hydrochloric acid (HCl) in a ratio of 1: 0, 05, two-water chlorine tin (SnCl ₂ · 2H ₂ O) in a ratio of 1: 1,597.

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