JP2005037236A - Thin film gas sensor and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CO-detecting thin film gas sensor having a large change rate (gradient) of sensor resistance to gas concentration, and having excellent output linearity in at least a CO detection concentration range (30 to 500 ppm CO/air). <P>SOLUTION: In this thin film gas sensor, a thin-film heater H is formed on a thin film-like support film having the periphery or both end parts supported by a Si substrate, the thin-film heater is coated with an electrical insulation film LI, gas-detecting electrodes E are formed thereon, a gas sensing film of a thin film mainly consisting of SnO2 is formed over the electrodes, and a selective combustion layer of porous alumina carrying a catalyst is formed such that the gas sensing film is completely coated. In the thin film gas sensor, a large number of step-like recessed and projecting parts (projecting part columnar crystals Sc and recessed parts Shg) are formed on the surface of a portion between the electrodes of the gas sensing film. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

    The present invention relates to a low power consumption thin film gas sensor suitable for battery driving.

  Generally, a gas sensor is a device that is used for a gas leak alarm, etc., and is a device that selectively reacts to a specific gas, for example, CO, CH4, C3H8, C2H5OH, etc. High responsiveness, high reliability, and low power consumption are indispensable.

  By the way, gas leak alarms that are widely used for household use include those for the purpose of detecting flammable gases for city gas and propane gas, and those for the purpose of detecting incomplete combustion gases in combustion equipment, or There are things that have both functions, but the penetration rate is not so high due to cost and installation problems. For this reason, in order to improve the diffusion rate, it is desired to improve the installation property, specifically, to be battery-driven and cordless.

  Low power consumption is the most important for realizing battery driving. However, in a catalytic combustion type or semiconductor type gas sensor, it is necessary to detect gas in a state heated to a high temperature of 100 ° C. to 500 ° C. From this point, in the conventional gas sensor in which powder such as SnO2 is sintered, there is a limit in reducing the thickness even if a method such as screen printing is used, and the heat capacity is too large to be used for battery driving. Therefore, development of a thin film gas sensor having a low heat capacity structure such as a diaphragm structure by a microfabrication process is underway.

  For example, Patent Document 1 proposes a specific element structure of a thin film gas sensor having the low heat capacity structure.

  FIG. 13 is a schematic cross-sectional view of an example of a thin film gas sensor having a diaphragm structure.

  A thin film heater H is formed on the thin support films L1, L2, and L3 whose outer periphery or both ends are supported by the Si substrate B. The thin film heater H is covered with the electrical insulating film LI, and is formed thereon. A gas sensing film electrode E is formed, and a gas sensing film S composed of a thin film mainly composed of SnO2 is formed between the electrodes, and the catalyst is supported so as to completely cover the gas sensing film. A selective combustion layer C made of alumina is formed. Usually, in the final process, Si having a diameter slightly larger than that of the sensor region such as a gas sensing film is completely removed by etching from the back surface of the Si substrate to form a diaphragm structure.

  Even in a gas leak alarm using a low power consumption thin film gas sensor with an ultra-low heat capacity structure such as a diaphragm structure, in order to have a life of 5 years or more without replacing the battery, pulse driving is also used in the thin film gas sensor. Required. Normally, the gas leak alarm needs to be detected once in a fixed period of 30 to 150 seconds, and the thin film gas sensor is heated from room temperature to a high temperature of 100 to 500 ° C. in accordance with this period. In order to respond to the life requirement of 5 years or more without replacing the battery, the heating time is targeted to be several hundred ms or less.

  Even in pulse-driven thin-film gas sensors, lowering the detection temperature, shortening the detection time, and prolonging the detection cycle (lengthening the time during which no current is supplied (hereinafter referred to as off time)) are required to reduce power consumption. is important. The detection temperature of the thin film gas sensor is ~ 100 ° C for the CO sensor due to detection sensitivity to the gas species, ~ 450 ° C for the CH4 sensor, the detection time is ~ 500msec due to the response of the sensor, the detection cycle is 30 seconds for the CH4 sensor, and the CO sensor 150 seconds.

  In addition, it is important to remove the moisture and other adsorbate adhering to the sensor surface during the Off time and clean the SnO2 surface in order to improve the stability over time of the battery-driven (pulse-driven) thin film gas sensor, Before detection, the sensor temperature is once heated to 400 ° C. to 500 ° C. (time to 100 msec), and immediately after that, gas detection is performed at the detection temperature of each gas.

  The gas detection principle of the SnO2 sensor is that the adsorbed oxygen (O2-) chemically adsorbed on the SnO2 surface reacts with the combustible gas such as CH4, H2, and CO (oxidation), and the electrons trapped in the adsorbed oxygen (O2-) This utilizes the fact that the resistance value changes (decreases) when injected into the SnO2 crystal as free electrons.

  The reaction formula with each detection gas is shown below.

CH4 + 4 O2- (ad) → CO2 + 2H2O + 8e ------- (1)
2H2 + O2- (ad) → H2O + 2e ------- (2)
CO + O2- (ad) → CO2 + 2e ------- (3)
The gas sensing film SnO2 of the thin film gas sensor has a smaller heat capacity because it has a lower heat capacity, which is advantageous for lower power consumption, but there are limitations such as long-term stability, microfabrication accuracy, and thin film deposition method / time. Therefore, the practical size is about several hundred μm square and the thickness is about 1 μm.

  As the SnO2 film of the thin film gas sensor, for example, as disclosed in Patent Document 2, SnO2 formed by sputtering is the most excellent in terms of sensor characteristics and mass productivity. FIG. 14 is a cross-sectional TEM photograph of the SnO2 layer formed by sputtering. From FIG. 14, it can be seen that SnO2 crystals grown in a columnar shape are densely grown perpendicular to the substrate.

FIG. 15 is a SEM image photograph of the surface of the SnO2 thin film, and FIG. 16 is an enlarged cross-sectional TEM image photograph of the outermost surface portion of the SnO2 thin film. The gap between the columnar SnO2 is as narrow as about 0.01 μm near the surface of the SnO2 thin film, and the gap is further narrowed toward the inside of the SnO2 thin film, and the columnar SnO2 crystal tends to become dense. Since the gas diffuses through the gap and penetrates into the SnO2 thin film, a larger gap is preferable for gas diffusion. From the evaluation of the crystallite diameter by X-ray diffraction, it is found that the columnar SnO2 crystal is a polycrystal composed of an aggregate of crystallites having a diameter of ~ 0.01 μm, and it can be seen in the cross-sectional TEM photograph of the columnar SnO2 crystal. However, there are finer pores between the crystallites. The sputtered columnar SnO2 crystal is a porous body having such a fine structure, and has a film quality suitable for a sensor having many active surfaces. Therefore, as described in Patent Document 2, it has high sensitivity to combustible gases such as CH4.
JP 2000-298108 A (page 4-7, FIG. 3) Japanese Examined Patent Publication No. 06-43978 (page 3-5, Fig. 1)

In the case of a gas sensor such as a CH4 sensor, the alarm concentration of the alarm device is set so that the detected gas concentration range (1000 ppm for CH4 sensors is CH4 to 4000 ppm).
Within CH4 / air), it is preferable that the sensor resistance changes linearly with respect to the gas concentration and that the rate of change (gradient) is high because a margin can be secured against changes with time in the sensor resistance.

  From the above viewpoint, the CH4 sensor disclosed in Patent Document 2 can be used without any problem. For sensors with a high detection temperature of 350 ° C to 500 ° C, such as the CH4 sensor, a sputter-deposited SnO2 thin film can be used without any problem, but with CO sensors that detect gas at a low temperature of 200 ° C or less, there is a problem. Come different.

  When the above sensor is transformed into a CO sensor and operated at 100 ° C., which is the detection temperature of CO, the following problems occur.

The CO detection concentration range is 30ppm to 500ppm CO / air,
The rate of change of sensor resistance with respect to CO gas concentration (concentration gradient of sensor resistance) decreases.

  The linearity of the sensor resistance with respect to the CO gas concentration decreases on the high CO concentration side, and the sensor resistance is saturated.

  These causes are presumed to be related to the small pore size of the SnO2 thin film formed by gas sputtering.

  The pore diameter and sensor sensitivity of the SnO2 thin film are described in detail in Non-Patent Document 1, but in the case of a SnO2 thin film formed by sputtering with many pores of ~ 0.01 μm or less, CO, H2, The diffusion of flammable gases such as CH4 is dominated by Knudsen diffusion, surface diffusion, etc., which diffuses while interacting with the pore tube wall, unlike ordinary molecular diffusion because the pore diameter is very small. .

As can be seen from the Knudsen diffusion coefficient Dk = 4r / 3 (2RT / πM) 1/2 (R: gas constant, r: pore diameter, T: temperature, M: molecular weight), Dk becomes smaller at low temperatures and enters the SnO2 thin film. The diffusion of combustible gases such as CO, H2, and CH4 is suppressed. This is particularly noticeable with high molecular weight CO. Although there is a difference depending on the reaction rate of each gas type, the reactions of the above formulas (1) to (3) occur near the outermost surface of the SnO2 thin film as compared with the CH4 sensor operating at a high temperature. That is, despite the presence of a large amount of detection gas in the gas phase, the reactions of the above formulas (1) to (3) hardly occur inside the SnO2 thin film. That is, although the resistance value decreases in the vicinity of the outermost surface of the SnO2 thin film due to the presence of the detection gas, the resistance value inside the SnO2 thin film hardly changes. The sensor resistance value is a parallel combined resistance of the SnO2 thin film resistance value near the outermost surface and the resistance value inside the SnO2 thin film due to its configuration, so the rate of change (gradient) of the sensor resistance with respect to the gas concentration decreases and becomes extreme. The rate of change is saturated, i.e. non-linear.
FIG. 17 is a schematic enlarged cross-sectional view of a conventional SnO2 thin film. The conventional SnO2 thin film has a structure in which SnO2 crystals Sc grown in a columnar shape of φ0.05μm are closely packed at intervals of 0.01μm, and there are almost no gas diffusion holes with large pore diameter (> 0.05μm). Detecting gases such as CO, H2, and CH4 in the phase diffuse through the 0.01 μm pore Shf by Knudsen diffusion / surface diffusion and penetrate into the SnO2 thin film. The SnO2 crystals Sc grown in a columnar shape are not independent from each other, but are partially connected to each other in the middle of the columnar crystal Sc. Although not shown, φ0.05 μm columnar SnO2 crystals are aggregates of microcrystals of φ0.01 μm, and although not shown in the figure, there are still more fine pores between the microcrystals. The detection gas penetrates into the columnar SnO2 crystal by surface diffusion / Knudsen diffusion. In such a dense columnar SnO2 crystal, the Knudsen diffusion and surface diffusion coefficients are low at low temperatures such as the CO sensor drive temperature, so the residence time becomes long and actually only near the outermost surface of SnO2. Oxidation reaction (sensing) does not occur.

It is an object of the present invention to solve the above problem, and the rate of change (gradient) of sensor resistance with respect to gas concentration is large, and at least the CO detection concentration range (30 ppm CO to 500 ppm)
CO / air) is to provide a CO sensor with good output linearity.
“Diffusion equation-based analysis of thin film semiconductor gas sensor-sensitivity dependence on film thickness and operating temperature” Go Sakai, Naoki Matsunaga, Kengo Shimanoe and Noboru Yamazoe TRANSDUCERS''01 EUROSENSORS (The 11th International Conference on Solid-State Sensors and Actuators , Munich, Germany, June10-14, 2001)

  In order to achieve the object of the present invention, a thin film heater is formed on a thin film-like support film supported at least at the outer periphery or both ends by a Si substrate, and the thin film heater is covered with an electrical insulating film, An electrode for the gas sensing film is formed on the electrode, and a gas sensing film composed of a thin film mainly composed of SnO2 is formed between the electrodes, and a porous material that supports the catalyst so as to completely cover the gas sensing film. In the thin film gas sensor in which the selective combustion layer made of alumina is formed, a large number of stepped irregularities are formed on the surface of the portion of the gas sensing film between the electrodes.

  The thickness of the concave portion of the gas sensing film is preferably 90% or less of the thickness of the convex portion.

  In the plan view of the gas sensing film, the area of the recess may be 20% to 80% of the area of the portion of the gas sensing film between the electrodes.

  At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the above thin film gas sensor manufacturing method, the sensing film process is performed in order to perform the sensing film process for forming the sensing film, and the sensing film process performs the SnO2 film formation by lift-off and before the SnO2 film formation, at the time of lift-off to the SnO2 film formation part. Including forming a gas sensing film having step-like irregularities in film thickness by interspersing the parts to be removed in an island shape and removing the lift-off material scattered in the island shape by lift-off after SnO2 film formation I will do it.

  At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the above thin film gas sensor manufacturing method including a sensing film process for forming a sensing film in order, the sensing film process includes performing SnO2 film formation by lift-off and before SnO2 film formation in the SnO2 film formation unit. After removing the lift-off material scattered in the island shape by lift-off after the SnO2 film formation, the portions removed by the lift-off are scattered in the island shape, and then on the SnO2 film formation part and the island-shaped part where the SnO2 is not formed. It is assumed that re-deposition of SnO2 to cover the entire top is included.

  At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the above thin film gas sensor manufacturing method including a sensing film process for forming a sensing film in order, the sensing film process includes performing SnO2 film formation by lift-off and before SnO2 film formation in the SnO2 film formation unit. The parts removed by lift-off are formed so as to be continuous with each other, and after removing the lift-off material of the continuum by lift-off after SnO 2 film formation, the SnO 2 on the SnO 2 film-forming portions scattered in islands and the SnO 2 It is assumed that SnO2 is formed again so as to cover the entire upper surface of the continuum which has not been formed.

  At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the above-described thin film gas sensor manufacturing method, which includes sequentially performing a sensing film process for forming a sensing film, the sensing film process includes etching after forming a flat SnO2 thin film and then finely processing island-shaped openings. It is assumed that an island-shaped recess is formed by etching after the mask is formed.

  At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the above-described thin film gas sensor manufacturing method, which includes sequentially performing a sensing film process for forming a sensing film, the sensing film process includes etching after forming a flat SnO2 thin film and then finely processing island-shaped openings. It is assumed that after the mask is formed, an island-shaped recess is formed by etching, and then SnO2 is formed again so as to cover the entire surface including the island-shaped recess.

  At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the above thin film gas sensor manufacturing method including a sensing film process for forming a sensing film in order, a continuous SnO2 thin film is formed in the sensing film process and then continuous openings are formed by microfabrication. It is assumed that, after forming the etching mask having, the recess is formed by etching, and then SnO2 is formed again so as to cover the entire surface including the recess.

  At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor, including a sensing film process for forming a sensing film in order, an insulating film treatment process for forming irregularities in the electrical insulating film is performed after the insulating film process, and then the sensing A film process is performed.

  In the above-described method for manufacturing a thin film gas sensor, the insulating film processing step is preferably a concavo-convex pattern formation by a photo process, followed by the electrode step and the sensing film step.

  In the above-described method for manufacturing a thin film gas sensor, the insulating film treatment process is a pattern formation by a photo process, and is preferably performed between the electrode process and the sensing film process.

  In the above-described method for manufacturing a thin film gas sensor, the insulating film processing step may be a sand blasting process, followed by the electrode step and the sensing film step.

  In the above-described method for manufacturing a thin film gas sensor, the insulating film processing step is a sand blasting process, and is preferably performed between the electrode step and the sensing film step.

  According to the present invention, a thin film heater is formed on a thin support film whose outer periphery or both ends are supported by a Si substrate, the thin film heater is covered with an electrical insulating film, and a gas sensing film is formed thereon. Is formed, and a gas sensing film composed of a thin film mainly composed of SnO2 is formed between the electrodes, and selective combustion composed of porous alumina carrying a catalyst so as to completely cover the gas sensing film In a thin film gas sensor with a layer formed, a large number of stepped irregularities are formed on the surface of the part between the electrodes of the gas sensing film, which facilitates gas diffusion into the SnO2 thin film that is the gas sensing film. CO gas penetrates into the inside of the SnO2 thin film quickly enough, the CO gas reacts with the entire SnO2 thin film, and the resistance of the entire SnO2 sensing film decreases uniformly. CO concentration As a result, a thin film gas sensor with a wide concentration range can be obtained in which the resistance change with respect to is large (the gradient is large) and the sensor resistance is not saturated at a high concentration.

  In addition, an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In a method of manufacturing a thin film gas sensor including a sensing film process for forming a sensing film in order, formation of irregularities by patterning in the sensing film process, and a sensing film process is performed after sandblasting is performed after the insulating film process, etc. The above thin film gas sensor can be obtained only by adding a relatively simple process to the conventional manufacturing method.

  The above technical problem can be solved by forming minute unevenness in the SnO2 thin film and creating a gas inlet capable of molecular diffusion in the SnO2 thin film with high density.

  FIG. 1 is a schematic enlarged sectional view of a SnO2 thin film of a thin film gas sensor according to the present invention. In the SnO2 thin film according to the present invention, detection gas such as gas phase CO, H2, CH4 can easily diffuse molecules, and relatively large pores Shg of 0.05 μm or more are formed in the cross-sectional direction of the SnO2 thin film by patterning. The solution of the technical problem is achieved by making the pore Shg the gas introduction hole and allowing the detection gas to easily penetrate into the SnO2 thin film. Since the molecular diffusion coefficient is large, the detection gas penetrates into the SnO2 thin film instantaneously through the gas introduction hole. The detection gas diffused through the gas introduction holes diffuses further into the columnar SnO2 crystal by Knudsen diffusion and surface diffusion from the lateral direction of the columnar crystal and penetrates into the SnO2 thin film.

  The gas introduction hole Shg has a diameter as small as possible for molecular diffusion, and a sensor with better characteristics can be obtained as the density is higher.

In this way, by adding minute irregularities to the SnO2 thin film and creating a high-density gas inlet capable of molecular diffusion in the SnO2 thin film, the surface area of the actual outermost surface is dramatically increased compared to the conventional one. In the CO mode, the resistance of almost all sections of the SnO2 thin film changes according to the gas concentration, so the rate of change (gradient) in sensor resistance with respect to the gas concentration is large, and the CO detection concentration Range (30ppmCO ~ 500ppm
CO / air) can provide a thin film gas sensor for CO detection with good output linearity.

  When unevenness is imparted by patterning, therefore, the unevenness interval is at least 0.5 to 1 μm. Accordingly, the width (W2) of the convex portion in FIG. 1 is necessarily 0.5 to 1 μm. As described above, since the diameter of the columnar SnO2 crystal is ~ 0.05 μm, it is an aggregate of 10 to 20 columnar SnO2 crystals in terms of the cross section of the convex portion (although it is two in FIG. 1 for easy understanding). ). The gap between the columnar SnO2 crystals is 0.01 μm or less, and the detection gas penetrates into the inside (lateral direction) by Knudsen diffusion and surface diffusion, but before reaching the SnO2 crystal at the center of the aggregate of columnar SnO2 crystals Most of the detected gas reacts with the gas. However, the surface of the underlying SiO2 insulating film on which the SnO2 gas sensing film is deposited is roughened by sandblasting, and after the high density and fine irregularities are formed on the SiO2 insulating film surface, the SnO2 gas sensing film is further improved. Is obtained. The pitch of fine irregularities may be about the same as the diameter of the columnar SnO2 crystal (˜0.05 μm). FIG. 2 shows the surface state of SiO2 obtained by subjecting smooth SiO2 to sand blasting according to the present invention, (a) is an SEM photograph showing the whole image (groove part and smooth part / sandblasted part), (b) is An enlarged SEM photograph of the unevenness of the groove, (c) is a histogram of unevenness of the unevenness. Sandblasting is performed under the same conditions as in the present invention. It can be seen from FIG. 2 that innumerable fine irregularities are formed and the average height difference of the irregularities is ˜0.45 μm.

  A description will be given of how a fine gas introduction hole is formed by forming a SnO2 gas sensing film on such a surface.

  FIG. 3 is a schematic cross-sectional view showing the growth state of SnO2 crystal on the surface of the SiO2 insulating film after sandblasting according to the present invention, (a) is the SiO2 insulating film after sandblasting, and (b) is the initial SnO2 film forming. , (C) is the middle stage of SnO2 film formation, and (d) the late stage of SnO2 film formation.

  In the uneven state (a) on the surface of the SiO2 insulating film after the sandblasting, unevenness with a height difference of ~ 0.5 μm is densely formed. An SnO2 gas sensing film formed thereon is deposited on the surface of the uneven SiO2 insulating film uniformly in the form of islands (reference Si) (b). The island-like SnO2 crystals are connected to each other, and the growth of columnar crystals has begun. It should be noted that the columnar crystal (reference symbol Sa) grown from the convex portion is larger than the columnar crystal (reference symbol Sb) grown from the concave portion (c). The probability that the sputtered SnO2 particles reach the substrate is almost the same in the initial stage of the growth, but gradually grows from the concave part due to the shielding effect of the columnar crystal that grows from the convex part as the columnar crystal grows. The probability of reaching (supplying) SnO2 to the columnar crystals decreases. Due to such a phenomenon, the columnar crystal grown from the convex portion grows larger than the columnar crystal grown from the concave portion in the middle of the film formation. As the film formation progresses, the shielding effect becomes more prominent, and the columnar crystal grown from the recess almost stops growing and has a cross-sectional shape as shown in (d). Gas introducing holes capable of molecular diffusion are formed at high density between the columnar crystals. In FIG. 3, since it is drawn in a two-dimensional plane, each columnar crystal appears to be coupled to each other only at the root, but in three dimensions, each columnar crystal is partially connected to each other during the growth, A network is formed. For reference, the cross-sectional structure of the columnar crystal of the conventional SnO2 gas sensing film is shown in FIG. 6, but the columnar crystal is closely packed and the gap is narrow.

Hereinafter, the thin film gas sensor and the manufacturing method thereof according to the present invention will be described in detail with reference to examples.
Example 1
The sectional view of the thin film gas sensor according to the present invention is the same as the figure.

The SiN film L2 and SiO2 film L3, which are the support layers of the diaphragm structure, are sequentially formed by plasma CVD on the surface of the Si substrate B with the 0.3 μm thick thermal oxide film LI formed on both sides, respectively. To do. On top of this, 0.05 μm of Ta was formed as the first bonding layer, and then PtW (Pt + 4 Wt%) was continuously formed as the metal layer for the heater H.
W) A 0.5 μm film is formed, and 0.05 μm of Ta is continuously formed as a second bonding layer (the bonding layer is not shown). Film formation is performed by normal sputtering using an RF magnetron sputtering apparatus. The film formation temperature is 100 ° C., the input power during film formation is 100 W, and the film formation pressure is 1 Pa.

  Thereafter, a heater pattern is formed by fine processing. As an etchant for wet etching, a mixed solution of sodium hydroxide and hydrogen peroxide was used for Ta and aqua regia was heated to 90 ° C. for Pt. Further, after forming the SiO2 insulating film LI by sputtering to 2.0 μm, the electrode pad portion of the heater is etched with HF and the window is opened after fine processing, although not shown by microfabrication. The second bonding layer Ta is removed with a mixed solution of sodium hydroxide and hydrogen peroxide. Next, an intermediate layer Ta for improving adhesion with the underlying SiO2 is formed to 0.05 μm, and then a Pt film for the sensing film electrode E is continuously formed to 0.2 μm. Film formation is performed by normal sputtering using an RF magnetron sputtering apparatus. The film forming temperature is 100 ° C., the film forming power is 100 W, and the film forming pressure is 1 Pa. Thereafter, the resistance measurement sensing film electrode E of the pair of sensing films SnO2 is patterned by wet etching in the same manner as the heater layer.

  Next, the wafer subjected to the above patterning is set in a sputtering chamber, and a SnO2 thin film that is the gas sensing film S related to the present invention is formed.

  A SnO2 thin film having minute irregularities in the cross-sectional direction is formed by sputtering film formation according to the following procedure.

  The size of the sensing film S is 100 μm square. The deposition conditions for the SnO2 layer are a deposition power of 50 W, a deposition pressure of 1 Pa, a deposition atmosphere Ar + O2, a deposition temperature of 100 ° C., and under these conditions, the deposition rate of SnO2 is 5 nm / min.

  After the SnO2 thin film is finely processed before film formation, only the portion where the SnO2 sensing film S is formed is opened, and the other is coated with a resist, and then a SnO2 thin film with a predetermined thickness is formed by sputtering. At the same time as removing the resist, this is performed by a registry shift-off that removes SnO2 adhering to unnecessary portions (on the resist).

4A and 4B schematically show a resist pattern of the gas sensing film portion according to the present invention, wherein FIG. 4A is a plan view and FIG. 4B is a cross-sectional view taken along line AB in FIG. The resist R has a square shape and is arranged at square lattice points, and the portion without the resist has a lattice shape. A sensor resistance detection electrode E made of a pair of Pt having a width of 10 μm is formed on the base of the left and right ends of the gas sensing film region. The line width of the lattice is 1 μm, the resist R is square, and one side is similarly 1 μm. The resist thickness is 1.5μm, and LENOS (Latitude
Enhancement
In accordance with the Novel Single Layer Lithography) method, a cross-sectional condition with little sagging suitable for lift-off was used.

  SnO2 having a thickness of 0.5 μm with respect to the resist thickness is formed. SnO2 is also formed on the resist, but it is removed together with the resist by lifting off the resist, and the resist shown in FIG. An SnO2 thin film is formed in the lattice portion that was not present.

  FIG. 5 is a schematic cross-sectional view of a gas sensing film according to the present invention. Since the SnO 2 thin film after the lift-off is in a lattice shape, electrical continuity with the sensor electrode E is maintained.

  Here, the pattern of the gas sensing film of the present embodiment is a lattice shape, but it is only necessary to have a pattern that is electrically conductive with a pair of sensor electrodes, and has a pattern that causes unevenness in the SnO2 film formation cross section. Of course. In this embodiment, the lattice width and the lattice interval are made to coincide with each other. However, the lattice interval may be different, and the lattice interval may be short or long. The finer the lattice spacing / lattice width is, the better, but the minimum spacing is the region where the Knudsen diffusion takes place (0.05 μm or less).

  Next, a powder in which Pt and Pd catalysts are supported on alumina particles is used as a paste together with a binder, and is applied to the surface of the sensing film by screen printing and fired to form a selective combustion layer (catalyst filter) element C having a thickness of about 30 μm. The selective combustion layer C improves the sensitivity, gas type selectivity, and reliability of the gas sensor.

  Finally, Si is completely removed from the back surface of the Si substrate by dry etching to a size of 400 μm to form a diaphragm structure.

  For comparison, we also prototyped a thin-film gas sensor with a conventional structure (device name X) in which a SnO2 thin film with a thickness of 100 μm and a thickness of 0.5 μm was formed. The only difference from the thin film gas sensor (element name A) of this example is the SnO2 film formation pattern.

Here, the heater layer (Ta / PtW / Ta) and the sensing membrane electrode (Ta / P
When patterning Pt), it may be possible to follow a kind of lift-off method using two kinds of metal layers formed in the shape of a mushroom as a mask (see our company reference number 98P00568).
Example 2
The difference from the first embodiment is only the SnO2 formation step, and the steps before and after that are exactly the same as the first embodiment. In Example 1, since the space between the SnO2 lattices is a gap, the sensor resistance is slightly increased. In order to reduce the sensor resistance value, the second embodiment is preferable.

  FIG. 6 is a schematic cross-sectional view of a gas sensing film of another thin film gas sensor according to the present invention.

  After forming the same resist pattern as in Example 1, a first SnO2 (reference S1) is formed to a thickness of 0.4 μm.

  After the registry shift-off, the resist pattern is further formed on the 100 μm square opening without any lattice pattern by superimposing it on the 100 μm square gas sensing portion, and a second SnO 2 (reference S2) is formed to a thickness of 0.1 μm by sputtering. The second deposited SnO2 is laminated on the surface of the first lattice-like SnO2 of the base, and the SnO2 of the lattice portion has a thickness of 0.6 μm. Further, a second SnO2 film having a thickness of 0.1 μm is formed on the portion where there is no SnO2 between the lattices, and the cross-sectional structure as a whole is as shown in FIG. Although SnO2 is thinly attached to the side surface of the lattice-like SnO2 formed substantially perpendicular to the substrate, the lattice spacing becomes slightly narrow, but this is not shown in FIG. Although SnO2 coats the entire surface of the sensor detection portion of 100 μm square due to the re-deposition, the conductive path increases and the sensor resistance value decreases compared to the first embodiment. The thicker the second film, the smaller the film thickness ratio of the unevenness and the lower the resistance value, but the shallower the recessed part, the less preferable the CO concentration gradient characteristic of the resistance value. In the method of Example 2, there is no restriction on the pattern during the initial film formation. That is, even if SnO2 is an island-shaped film formation pattern at the time of the first film formation, it is electrically connected by re-filming. Therefore, the reverse pattern of Example 1 (SnO2 is formed in an island shape in the square part between the lattices) ) Is no problem.

  An element having such a gas sensing film was designated as element B, and the subsequent process was prototyped in the same process as in Example 1.

Although an example of one re-deposition is shown here, it goes without saying that the same repetition may be performed a plurality of times.
Example 3
The difference from the first and second embodiments is only the SnO2 formation step, and the steps before and after that are exactly the same as those of the first and second embodiments. In Examples 1 and 2, SnO2 is formed in a concavo-convex shape, whereas in Example 3 of the present invention, after SnO2 is formed by a flat conventional method having no concavo-convex shape, a resist is finely processed on the SnO2 surface. Patterning and etching are intended to give an uneven shape to SnO2. The pattern of the portion where the concave shape of SnO2 is to be formed is naturally open, and SnO2 is etched from the opening.
FIG. 7 schematically shows a resist pattern of a gas sensing film portion of another thin film gas sensor according to the present invention, where (a) is a plan view and (b) is a paragraph AB sectional view. FIG. 4 is a cross-sectional view after patterning. The resist portion is indicated by R. After forming a gas sensing film having a conventional structure in which SnO2 has a pattern of SnO2 having a thickness of 100 μm and a thickness of 0.5 μm (reference S3), a resist having a pattern as shown in FIG. 7 is patterned by fine processing. The line width of the resist is 1 μm, and the line interval is also 1 μm. A resist having a thickness of 1.5 μm and having a condition of obtaining a cross-sectional shape with little sagging suitable for lift-off according to the PEB method was used. Using the resist as a mask, SnO2 is etched by 0.5 μm from the resist opening by reverse sputtering.

FIG. 8 is a schematic cross-sectional view of a gas sensing film of another thin film gas sensor according to the present invention. Thereafter, the resist is removed to obtain a cross-sectional structure as shown in FIG. An element C in which the gas introduction hole Shg is formed in this way is referred to as element C.

In this embodiment, an example of etching by reverse sputtering (physical etching) is shown, but it is needless to say that chemical etching may be used.
Example 4
The difference from the first, second, and third embodiments is that unevenness is formed in advance on the SiO2 layer, which is the base, before SnO2 is formed. By depositing the SnO2 thin film on the uneven surface by the same method as the conventional method, the SnO2 thin film is automatically formed in the uneven shape. The steps before and after that are exactly the same as those in Examples 1, 2, and 3. The feature of Example 4 is that unevenness is formed on the SnO2 film formation base.

  FIG. 9 is a cross-sectional view of the SiO2 layer of the gas sensing film portion of another thin film gas sensor according to the present invention. FIG. 10 is a sectional view of a gas sensing film portion of another thin film gas sensor according to the present invention.

  After forming the sensor electrode E, a resist having the same pattern as that of the third embodiment (FIG. 7) is patterned on the SiO2 layer paragraph LI before forming the SnO2 thin film by fine processing. It is immersed in a buffered HF solution, and SiO2 in the resist opening is etched by about 0.4 μm to form an uneven shape as shown in FIG. 9 (after removing the resist) on the SiO2 surface. After removing the resist, when a flat conventional SnO2 thin film S4 (FIG. 10) is formed, SnO2 is formed along the underlying uneven shape, so that the SnO2 thin film S4 having the uneven shape is automatically formed. The In Example 4, SiO2 was processed by wet etching, but similar results can be obtained by dry etching using a fluorine-based gas. An element manufactured in this way is referred to as an element D.

  Subsequently, the dependence of the resistance values of the pulse-driven thin film gas sensor elements A, B, C, D, and X produced by the above method on the CO concentration was examined. The measurement conditions are 20 ° C., relative humidity 65%, and the CO concentration is 0, 30, 100, 300, 500, 1000 ppm CO / air.

  The pulse driving conditions and measurement conditions are as follows.

Detection cycle: 150 seconds Cleaning temperature x time = 450 ° C x 200 msec
CO detection temperature × detection timing = 100 ° C. × 100 ° C. After 500 msec after heating Table 1 shows the measurement results.

FIG. 11 is a graph showing the CO concentration dependence of the resistance values of the elements A to D of the present invention and the conventional element X.

  From FIG. 11, the change rate of the sensor resistance value (the slope of the curve) with respect to the change in the CO concentration within the range of 30 to 1000 ppm of CO is almost the same as the elements A, B, C, and D of the present invention. I understand. On the other hand, it can be seen that the change rate of the conventional element X is small and that the CO concentration is in the range of 300 to 1000 ppm.

  In the element of the conventional structure, the sputtered SnO2 thin film has almost no large (> 0.05 μm) gas diffusion holes, so the penetration of CO gas into the SnO2 thin film is caused by Knudsen diffusion and surface diffusion. Become the Lord. Knudsen diffusion and surface diffusion are slow and the interaction with SnO2 (collision, migration, etc.) increases dramatically, so that the catalytic oxidation of CO gas occurs before the gas diffuses into the SnO2 thin film. That is, the CO gas burns out near the outermost surface of the SnO2 sensing film, and the resistance decrease occurs only near the outermost surface. Since the sensor resistance is a combined resistance of the entire gas sensing film, it is considered that the resistance change with respect to the CO concentration is small (gradient is small). The above problem is also presumed to be caused by the saturation phenomenon of sensor resistance at a high concentration.

In contrast, in the element of the present invention, gas diffusion holes that allow easy gas diffusion are formed in the SnO2 thin film, which is a gas sensing film, at a high density, so that the CO gas penetrates into the SnO2 thin film sufficiently quickly, and the CO gas Reacts with the entire SnO2 thin film, and the resistance of the entire SnO2 sensing film decreases uniformly, so the resistance change with respect to the CO concentration becomes large (the gradient is large), and saturation of the sensor resistance at high concentration also occurs. There is nothing.
Example 5
The cross-sectional view of another thin film gas sensor according to the present invention is the same as that shown in FIG. 13, and the same reference numerals are used, but a surface treatment process for the SiO2 insulating film is added.

  Hereinafter, it explains in detail along the manufacturing process.

A SiN film L2 and a SiO2 film L3, which serve as a support layer of the diaphragm structure, are sequentially formed by plasma CVD on the surface of the Si substrate B on which both sides of the thermal oxide film LI are formed to 0.3 .mu.m, by plasma CVD. On top of this, 0.05 μm of Ta was formed as the first bonding layer, and then continuously, PtW (Pt + 4
m% W) film is formed to a thickness of 0.5 μm, and then Ta is continuously formed to a thickness of 0.05 μm as the second bonding layer. Film formation is performed by normal sputtering using an RF magnetron sputtering apparatus. The film forming temperature is 100 ° C., the film forming power is 100 W, and the film forming pressure is 1 Pa.

  Thereafter, a pattern of the heater H is formed by fine processing. As an etchant for wet etching, a mixed solution of sodium hydroxide and hydrogen peroxide was used for Ta and aqua regia was heated to 90 ° C. for Pt. Further, after forming the SiO2 insulating film LI by sputtering to 3.0 μm, the electrode pad portion of the heater is etched with HF and the window is opened after fine processing, although not shown by fine processing. The second bonding layer Ta is removed with a mixed solution of sodium hydroxide and hydrogen peroxide.

Thereafter, a sandblasting process according to the present invention is performed to form a fine and high-density unevenness on the surface of the SiO2 insulating film LI. A dry film (for example, ORDYL-BF-405 manufactured by Tokyo Ohka) is laminated on the surface of the processed wafer, and the part where the SnO2 gas sensing film S is formed by fine processing is opened, and the other parts are covered. Pattern is formed. As an abrasive, silicon carbide # 800 (average particle size
20
μm) and an injection spot diameter of φ10 mm was used. Sandblasting conditions were as follows: sandblasting was performed on the entire surface of a φ4 inch wafer at an abrasive supply rate of 140 g / min, an injection pressure of 0.15 MPa (air), and a moving speed of 200 mm / sec.

  Thereafter, the surface of the SiO2 insulating film that has been subjected to the sandblasting treatment is immersed in a 10% aqueous solution of HF for 20 seconds to remove the strain layer generated by the sandblasting treatment on the outermost surface of SiO2. At this point, irregularities are formed on the surface of SiO2. Next, the dry film is removed.

  Next, after forming 0.05 μm of an intermediate layer Ta for improving adhesion to the underlying SiO 2, a Pt sensing film electrode is continuously formed to a thickness of 0.2 μm. Film formation is performed by normal sputtering using an RF magnetron sputtering apparatus. The film forming temperature is 100 ° C., the film forming power is 100 W, and the film forming pressure is 1 Pa. Thereafter, the resistance measurement sensing film electrode E of the pair of sensing films SnO2 is patterned by wet etching in the same manner as the heater layer.

  Note that after the sensing film electrode is formed, the sensing film electrode portion may be covered with a resist, and the underlying SiO2 may be sandblasted under the same conditions.

  Next, the wafer subjected to the above patterning is set in a sputtering chamber, and a SnO2 thin film as a gas sensing film is formed.

  The size of the sensing film is 100 μm square. The deposition conditions for the SnO2 layer are a deposition power of 50 W, a deposition pressure of 1 Pa, a deposition atmosphere Ar + O2, a deposition temperature of 100 ° C., and under these conditions, the deposition rate of SnO2 is 5 nm / min.

  The prototype of the present invention element is designated as element E.

  For comparison, an element having a conventional structure not subjected to sandblasting was also prototyped (element X). The only difference from the element of the present invention (element E) is the SnO2 film formation pattern.

  Next, a powder in which Pt and Pd catalyst are supported on alumina particles is used as a paste together with a binder, and is applied to the surface of the sensing film by screen printing and fired to form a selective combustion layer (catalyst filter) C having a thickness of about 30 μm. The selective combustion layer C improves the sensitivity, gas type selectivity, and reliability of the gas sensor. Finally, Si is completely removed from the back surface of the substrate by dry etching to a diameter of 400 μm to form a diaphragm structure.

  Subsequently, the CO concentration dependence of the resistance values of the pulse-driven thin film gas sensor elements A and B produced by the above method was examined. The measurement conditions are 20 ° C., relative humidity 65%, and the CO concentration is 0, 30, 100, 300, 500, 1000 ppm CO / air.

  The pulse driving conditions and measurement conditions are as follows.

Detection cycle = 150 seconds Cleaning temperature x time = 450 ° C x 200 msec
CO detection temperature × detection timing = 100 ° C. × 100 ° C. After heating for 500 msec Table 2 shows the measurement results. The unit of resistance value is KΩ.

FIG. 12 is a graph showing the CO concentration dependence of the resistance values of the element E according to the present invention and the conventional element X.

  From FIG. 12, it can be seen that the change rate of the sensor resistance value (the slope of the curve) with respect to the change of the CO concentration is a constant value in the element E of the present invention in the range of the CO concentration of 30 to 1000 ppm. On the other hand, it can be seen that the change rate of the conventional element X is small and that the CO concentration is in the range of 300 to 1000 ppm.

It is a model expanded sectional view of the SnO2 thin film of the thin film gas sensor which concerns on this invention. According to the present invention, the surface state of SiO2 obtained by subjecting smooth SiO2 to sand blasting is shown, (a) is an SEM photograph showing the whole image (groove part and smooth part / no sandblasting part), and (b) is an uneven groove part. An enlarged SEM photograph, (c) is a histogram of unevenness of the unevenness. It is a schematic cross section showing the growth state of SnO2 crystal on the SiO2 insulating film surface after sandblasting according to the present invention, (a) is the SiO2 insulating film after sandblasting, (b) is the initial stage of SnO2 film formation, (c ) Is the middle stage of SnO2 film formation, and (d) the late stage of SnO2 film formation. The pattern of the resist of the gas sensing film part which concerns on this invention is shown typically, (a) is a top view, (b) is AB sectional drawing in (a). 1 is a schematic cross-sectional view of a gas sensing film according to the present invention. It is a schematic cross section of the gas sensing film | membrane of the other thin film gas sensor which concerns on this invention. The resist pattern of the gas sensing film part of another thin film gas sensor which concerns on this invention is shown typically, (a) is a top view, (b) is AB sectional drawing. It is a schematic cross section of the gas sensing film of another thin film gas sensor concerning the present invention. It is sectional drawing of the SiO2 layer of the gas sensing film | membrane part of another thin film gas sensor which concerns on this invention. It is sectional drawing of the gas sensing film | membrane part of another thin film gas sensor which concerns on this invention. It is a graph which shows CO density | concentration dependence of the resistance value of the elements AD of this invention, and the conventional element X. FIG. It is a graph which shows the CO density | concentration dependence of the resistance value of the element E which concerns on this invention, and the conventional element X. FIG. It is a schematic cross section of an example of the thin film gas sensor of a diaphragm structure. 2 is a cross-sectional TEM photograph of a SnO2 layer formed by sputtering. It is a SEM image photograph of the surface of a SnO2 thin film. 3 is an enlarged cross-sectional TEM image photograph of the outermost surface portion of the SnO2 thin film. It is a model expanded sectional view of the conventional SnO2 thin film.

Explanation of symbols

B Si substrate
L1 thermal oxide film
L2 nitride film
L3 silicon dioxide film
LI insulation film
H heater
E electrode
S SnO2 film
C selective combustion layer
S1 SnO2 film
S2 SnO2 film
S3 SnO2 film
S4 SnO2 film
Sa SnO2 film
Sb SnO2 film
Sc Columnar SnO2 film
Shf micropore
Shg gas introduction hole SR resist

Claims (14)

A thin film heater is formed on a thin film-like support film supported at least at the outer periphery or both ends by a Si substrate. The thin film heater is covered with an electrical insulating film, and an electrode for a gas sensing film is formed thereon. In addition, a gas sensing film composed of a thin film mainly composed of SnO2 is formed between the electrodes, and a selective combustion layer composed of porous alumina supporting a catalyst is formed so as to completely cover the gas sensing film. In the thin film gas sensor, a plurality of stepped irregularities are formed on the surface of the portion between the electrodes of the gas sensing film. 2. The thin film gas sensor according to claim 1, wherein the thickness of the concave portion of the gas sensing film is 90% or less of the thickness of the convex portion. 3. The thin film gas sensor according to claim 1, wherein the area of the recess is 20% to 80% of the area of the portion of the gas sensing film between the electrodes in the plan view of the gas sensing film. Production method. At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor including a sensing film process for forming a sensing film in order, SnO2 film formation is performed in the sensing film process by lift-off, and before SnO2 film formation, at the time of lift-off to the SnO2 film formation unit Including forming a gas sensing film having step-like irregularities in film thickness by interspersing the parts to be removed in an island shape and removing the lift-off material scattered in the island shape by lift-off after SnO2 film formation The method for manufacturing a thin film gas sensor according to any one of claims 1 to 3. At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor, which includes sequentially performing a sensing film process for forming a sensing film, the sensing film process includes performing a lift-off of SnO2 film formation and lifting off to the SnO2 film forming unit before SnO2 film formation. After removing the lift-off material scattered in the island shape by lift-off after SnO2 film formation, the parts removed in step 1 are scattered on the island shape, and then on the SnO2 film formation part and on the island-like part where the SnO2 is not formed. 4. The method of manufacturing a thin film gas sensor according to claim 1, further comprising re-depositing SnO2 so as to cover the whole. At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor including a sensing film process for forming a sensing film in order, SnO2 film formation is performed in a lift-off process before the SnO2 film formation process. After the SnO2 film is formed, the lift-off material is removed by lift-off after the SnO2 film is formed, and the SnO2 film is deposited on the SnO2 film-forming parts scattered in islands. 4. The method of manufacturing a thin film gas sensor according to claim 1, further comprising re-depositing SnO2 so as to cover the entire upper surface of the continuous body. At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor, which includes sequentially performing a sensing film process for forming a sensing film, the sensing film process includes forming an etching mask having island-shaped openings by microfabrication after forming a flat SnO2 thin film. 4. The method of manufacturing a thin film gas sensor according to claim 1, further comprising forming an island-shaped recess by etching after the formation. At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor, which includes sequentially performing a sensing film process for forming a sensing film, the sensing film process includes forming an etching mask having island-shaped openings by microfabrication after forming a flat SnO2 thin film. 4. An island-shaped recess is formed by etching after the formation, and then SnO2 is formed again so as to cover the entire surface including the island-shaped recess. The manufacturing method of the thin film gas sensor in any one of. At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor, which includes sequentially performing a sensing film process for forming a sensing film, the sensing film process includes forming a flat SnO2 thin film and then performing etching with continuous openings that are continuous with each other by microfabrication. 4. The method according to claim 1, further comprising: forming a mask, forming a recess by etching, and then re-depositing SnO2 so as to cover the entire surface including the recess. The manufacturing method of the thin film gas sensor of description. At least an insulating film process for forming an electrical insulating film covering a thin film heater, an electrode process for forming an electrode for a gas sensing film thereon, and a gas comprising a thin film mainly composed of SnO2 between the electrodes In the method of manufacturing a thin film gas sensor, including a sensing film process for forming a sensing film in order, an insulating film treatment process for forming irregularities on the electrical insulating film is performed after the insulating film process, and then the sensing film process The method of manufacturing a thin film gas sensor according to claim 1, wherein: 11. The method of manufacturing a thin film gas sensor according to claim 10, wherein the insulating film processing step is pattern formation by a photo process, and subsequently the electrode step and the sensing film step are performed. 4. The method of manufacturing a thin film gas sensor according to claim 1, wherein the insulating film processing step is a pattern formation of unevenness by a photo process, and is performed between the electrode step and the sensing film step. . 11. The method of manufacturing a thin film gas sensor according to claim 10, wherein the insulating film processing step is a sand blasting process, and subsequently the electrode step and the sensing film step are performed. 4. The method of manufacturing a thin film gas sensor according to claim 1, wherein the insulating film treatment step is a sandblast treatment, and is performed between the electrode step and the sensing film step.