Thin Film Oxygen Sensor With Microheater

The present invention relates to a thin film oxygen sensor incorporating an oxide semiconductor and having a structure wherein a sensor element is integrally formed with a heater element.

An exhaust gas cleaning system has been known as a method for improving fuel consumption of an internal combustion engine of an automobile and for decreasing toxic components in an exhaust gas, wherein the concentration of oxygen contained in the exhaust gas is detected, and the air or fuel quantity is controlled in accordance with the detection signal. In particular, the present invention relates to an improvement in an oxygen sensor (especially, of a thin film oxygen sensor of an oxide semiconductor) used for such an exhaust gas cleaning system.

Conventional problems are described hereinafter. Oxide elements such as TiO.sub.2, CoO, CeO.sub.2, Nb.sub.2 O.sub.5 and SnO.sub.2 are used for oxide semiconductor type ceramic oxygen sensors (of resistance change type) which carry a catalyst such as Pt. When the characteristics of these oxygen sensors are actually examined by being placed in an exhaust gas from an internal combustion engine, and changes in resistances of the senors are measured when the air-fuel ratio changes, the characteristics of such changes in resistance are greatly different in the following cases: (1) when the engine is started; (2) when the exhaust gas is warmed; and (3) when the exhaust gas is sufficiently heated to a high temperature. The operation state of the internal combustion engine varies; the engine is operated at random in a deceleration, acceleration or constant speed mode. For this reason, the temperature and flow rate of the exhaust gas greatly vary, so that the components of the exhaust gas reaching the oxygen sensor greatly vary.

(a) Application of Oxygen Sensor

A resistance-change type oxygen sensor (resistance R.sub.2) using an oxide semiconductor is connected in series with a reference resistor (resistance R.sub.1). A voltage V.sub.1 is applied from a constant voltage source to the series circuit of the oxygen sensor and the reference resistor. An output voltage V.sub.2 appearing across two ends of the sensor having the resistance R.sub.2 is given as V.sub.2 ={R.sub.2 /(R.sub.1 +R.sub.2)}V.sub.1 in accordance with a change in the sensor resistance R.sub.2. The output voltage V.sub.2 is entered as data in an air-fuel ratio control computer and is compared with a reference voltage. The computer determines that the air-fuel ratio represents a "lean" (insufficience fuel) state when the output voltage is higher than the reference voltage. Otherwise, the computer determines that the air-fuel ratio represents a "rich" (excessive fuel) state. The fuel injection quantity is controlled in accordance with the determination result so as to always combust the fuel at the theoretical air-fuel ratio, thereby improving combustion efficiency.

The following methods are considered to improve precision in controlling the air-fuel ratio of the resistance-change type oxygen sensor:

(1) A thermistor is inserted in series with the sensor to compensate for temperature dependence.

(2) The sensor is heated to and kept at a predetermined temperature to decrease temperature dependency of control precision.

(3) Since the resistance of the oxide semiconductor greatly changes in accordance with an oxygen partial pressure, a catalyst is carried on the surface of and inside the sensor so as to completely combust a noncombusted gas, thereby greatly changing the oxygen partial pressure in the vicinity of the sensor.

(b) Response Control

In order to accurately detect a change in the air-fuel ratio of the engine, a thin film method unlike the conventional sintering method was used by the present inventors to manufacture a sensor in such a way as to improve the response characteristics of the sensor. As a result, the thin film sensor had a high speed response about 5 to 10 times that of the conventional sintered sensor.

When the sensor was mounted in an automobile driven in a test run, it was found that no problems occurred provided the engine speed was not less than 4,000 [rpm], but that controllability of the engine was degraded when the engine was operated at a low speed. This is caused by the fact that any slight difference between the cylinder injectors which control the fuel injection quantity results in variation in the air-fuel ratios of fuel quantities injected into the respective cylinders at a low speed of not more than 1,000 [rpm]. For example, when the quantity of fuel injected into the first cylinder differs from that injected into the second cylinder, the air-fuel ratio with respect to the first cylinder differs from that of the second cylinder since the quantity of air supplied to the first cylinder is the same as that supplied to the second cylinder. As a result, this difference is detected as a change in the air-fuel ratio by the sensor disposed in the common portion of the intake manifold, and the sensor having high speed response detects it as the variation of the air fuel ratio thus resulting in degradation of controllability.

In general, the sensor must have good sensitivity characteristics for accurately detecting a change in air-fuel ratio. However, when the response characteristics of the sensor are improved, it has an increased tendencey to be influenced by a distrubing signal, resulting in erroneous operation. Therefore, the response characteristics of the sensor must have a proper response range. An engine control sensor preferably has a response range from about 50 msec to 1000 msec.

(c) Cause of Poor Reproducibility and Low Stability

The present inventors examined why a resistance-change type oxygen sensor incorporating an oxide semiconductor had low stability and poor reproducibility.

A sintered TiO.sub.2 sensor was mounted in an automobile which was subjected to a test run. This sensor was heated to and kept at by a built-in sensor a temperature of 700.degree. C. to 800.degree. C. and had a sufficient catalyst to increase activity.

The following phenomenon occasionally occurs when an engine is started and operated at a steady state and changes in resistance of the sensor are examined. Even if the resistance of the sensor is very small and the air-fuel ratio is given as lean, the sensor detects a similar result as if the air-fuel ratio of the engine was changed to a rich ratio.

It was found that noncombusted components such as soot or tar become attached to the surface of the sensor when the air-fuel ratio was set to a richer ratio (excess air factor .lambda.=0.5 or less), thereby impairing stability and reproducibility. Further, even if the catalyst of the sensor is highly active, the gasoline cannot be completely vaporized, and can only be partially combusted at the start of engine operation. As a result, an unacceptably rich state can be obtained. Furthermore, since the temperature of the exhaust gas is low, incomplete combustion occurs. Therefore, even if the sensor is kept at a predetermined temperature, the noncombusted components become attached to the surface of the sensor when the sensor is exposed in a rich atmosphere. Although the noncombusted components attached on the surface of the sensor can be partially combusted when the sensor is heated, the noncombusted components may not be completely combusted. When some of the noncombusted components are left on the sensor, such residual components have a higher conductivity than TiO.sub.2, which constitutes the sintered sensor. As a result, even if the resistance between sensor terminals is low, a similar signal is generated as if the rich atmosphere obtains.

In the conventional resistance-change type oxygen sensor having the structure wherein noncombusted components become attached to the surface of the sensor, it is found that poor reproducibility and low stability disables practical application.

The present invention has been made to solve the conventional problems described above, and has for its object to provide a compact, low-cost oxide semiconductor type thin film oxygen sensor with a microheater which has good reproducibility and high stability, wherein a resistance immediately changes in response to the amount of oxygen in the surrounding atmosphere.

In order to achieve the above object of the present invention, there is provided an oxygen sensor incorporating an oxide semiconductor and having an integral structure consisting of a sensor portion and a heater portion, wherein a porous catalyst layer is formed on a surface of said sensor portion to prevent noncombusted components from becoming attached to said surface of said sensor portion, the thickness of said catalyst layer and the average diameter of pores thereof being determined such that the ratio of said thickness of said catalyst layer to said average diameter of the pores thereof falls within a range from 50 to 3,000.

A catalyst component carried on the catalyst layer may be a material selected from the group consisting of palladium (Pd), rhodium (Rh), platinum (Pt) and a mixture of at least two thereof, and carried in an amount falling within a range from 0.001 to 50 [wt %].

An oxide semiconductor of the oxygen sensor comprises a thin layer formed of a material selected from the group consisting of TiO.sub.2, Nb.sub.2 O.sub.5, CeO.sub.2 and SnO.sub.2.

FIGS. 1 to 3 show part of an oxygen sensor having an integral structure consisting of a heater portion and a sensor portion according to the present invention, in which FIG. 1 is a plan view thereof so as to show the sensor portion, FIG. 2 is a side view thereof, and FIG. 3 is a bottom view thereof so as to show the heater portion;

FIG. 4 is a perspective view of an oxygen sensor mounted on an Al.sub.2 O.sub.3 holder;

FIG. 5 is a graph showing the characteristics of the oxygen sensor when a heating temperature is used as a parameter;

FIG. 6 is a graph showing the change in sensor resistance when the sensor is exposed at the excess air factor .lambda. of 0.55 and at an exhaust gas temperature of 200.degree. C., and the reproducibility of the sensor resistance when the excess air factor .lambda. is changed to 1.55;

FIGS. 7 and 8, respectively, are graphs for explaining the degree of variation in the sensor resistance as a function of catalyst types and amounts in the respective catalyst layers coated on the sensor portions;

FIGS. 9. to 12 are graphs for explaining changes in resistance as a function of changes in the excess air factor when respective oxide semiconductors comprise CeO.sub.2, SnO.sub.2, Nb.sub.2 O.sub.5 and ZnO, respectively; and

FIG. 13 is a graph for explaining the thickness of the catalyst coating as a function of the response time.

An oxygen sensor embodying the present invention preferably comprises: a layer carrying a catalyst on a porous coating base; electrodes; an oxide semiconductor type thin film whose resistance changes in accordance with the air-fuel ratio; an alumina substrate; a thin film heater for heating the sensor portion to a temperature up to about 800.degree. C.; and a dense protective layer.

A method for manufacturing the oxygen sensor described above will be briefly described below. A thin TiO.sub.2 film (thickness of 1,000 .ANG. to 5 .mu.m is formed by a high-rate RF sputtering apparatus on one major surface of an alumina substrate (thickness of 0.1 to 0.3 mm and area of 30 mm .times.30 mm). A Pt electrode film is deposited on the thin TiO.sub.2 film to a thickness of about 1 .mu.m. A heater material such as Pt which has good heat resistance characteristics is sputtered on the other major surface of the alumina substrate. The interdigital Pt electrodes and the heater electrodes are connected to Pt lead wires, respectively. Thereafter, a porous ceramic film carrying the catalyst is formed on a prospective sensor portion of the thin oxide film to a thickness of about 20 to 5,000 .mu.m. The porosity of the porous ceramic film is given to be 20 to 80%. On the other hand, a dense alumina adhesive is applied to cover the heater surface to a thickness of 50 to 1,000 .mu.m so as to protect the heater.

The thin film oxygen sensor with the microcomputer according to the present invention is normally kept at a predetermined temperature by the heater. Even if changes in the temperature and flow rate of the exhaust gas occur, the change in sensor temperature deviates by as little as within the range of 0.1 %. Noncombusted molecules and combustible materials in the noncombusted state which reach the sensor are constrained to pass through the porous ceramic layer which carries the catalyst, so that the noncombusted components are subject to gas reaction and hence combustion. Therefore, only molecules which have completed gas reaction and combustion reach the surface of the oxide semiconductor. For this reason, the gas reaching the sensor surface will not still be in the noncombusted state. As a result, the sensor can immediately react to changes in the composition of the ambient gas. Furthermore, the sensor itself can be micropatterned in accordance with a thin film micropatterning technique so as to obtain a sensor having dimensions of, for example, 1.5.times.1.55 .times.0.2 mm. As a result, power required for sensor operation is as low as within 5 W. Furthermore, the microheater is built into the oxygen sensor, so that the sensor of this type can be normally operated across the range from a low temperature unlike the conventional oxygen sensor which is not operated at the start of the engine or in idling to a high temperature.

The present invention will be described in detail by way of examples.

A thin TiO.sub.2 film was formed by a bipolar RF sputtering apparatus on an Al.sub.2 O.sub.3 substrate having dimensions of 30.times.30.times.0.2 mm. The TiO.sub.2 film consisted of a rutile type TiO.sub.2 sintered body of 99.9% or more % purity as a target having a diameter of 110 mm and a thickness of 5 mm. The sputtering conditions were given as follows:

Vacuum pressure: 2.times.10.sup.-6 TORR

Vacuum pressure in Ar atmosphere: 3 to 5.times.10.sup.-3 TORR

Input power: 200 W

Reflected power: 20 W

Film thickness: 2,000 to 3,000 .ANG.

Sputtering time: 45 minutes.

After the thin TiO.sub.2 film was formed on the Al.sub.2 O.sub.3 substrate, the resultant structure was heated in the air at a temperature of 1,000.degree. C. for about one hour.

A photoconductive resin was applied to the upper and lower surfaces of the Al.sub.2 O.sub.3 substrate. By using the photoconductive resin patterns as masks, an interdigital electrode pattern was formed on the surface which had the TiO.sub.2 film, and the heater pattern was formed on the opposing surface. Thereafter, Pt was sputtered for about one hour under the same sputtering conditions as described above. In this case, a deposited Pt film had a thickness of about 1 .mu.m. When the photoconductive resin pattern was removed, the Pt electrodes and Pt heater were prepared. Thereafter, the resultant structure was cut by a dicing cutter into dimensions of 1.55.times.1.50 mm, and Pt wires each having a diameter of 50 .mu.m were bonded to the electrodes, thereby obtaining lead wires. FIGS. 1 to 3 show the sensor manufactured as described above. FIG. 1 is a plan view of the sensor which shows the Pt electrodes; FIG. 2 is a side view of the sensor; and FIG. 3 is a bottom view of the sensor which shows the heater portion. Referring to FIGS. 1 to 3, reference numeral 1 denotes interdigital Pt electrodes of the sensor; 2, a TiO.sub.2 film; 3, an Al.sub.2 O.sub.3 substrate; 4 and 4', Pt lead wires leading from the Pt electrodes 1, respectively; 5, a Pt heater; and 6 and 6', lead wires connected to the Pt heater 5.

The oxygen detection portion and the lead wire portions of the sensor having the structure described above were coated with a porous ceramic coating agent having a catalyst as an additive. Meanwhile, a dense coating agent (e.g., alumina adhesive) was coated on the heater portion so as to prevent the heater portion from receiving any influence from the atmospheric gas.

The sensor obtained as described above was fixed on an Al.sub.2 O.sub.3 holder 9 connected to Pt lead wires 8, as shown in FIG. 4. Referring to FIG. 4, reference numeral 7 denotes a porous ceramic layer which contains a catalyst.

The characteristics of the sensor as described above were examined. FIG. 5 is a graph for explaining sensor resistance R as a function of excess air factor .lambda. when the temperature of the heater for heating the sensor is used as a parameter. The excess air factor .lambda. is plotted along the axis of abscissa, and the logarithm of the resistance log.sub.10 R is plotted along the axis of ordinate.

In order to examine the influence of the noncombusted components, sensors having different carried catalysts were exposed in an atmosphere at an excess air factor .lambda. of 0.55 and at an exhaust gas temperature of 200.degree. C. Under these conditions, the changes in sensor resistance was examined. Furthermore, the excess air factor .lambda. was changed to 1.55, and the reproducibility of the changes in sensor resistance was examined. FIG. 6 is a graph for explaining the resistance and the excess air factor as a function of time. Test results are summarized in Table 1 below.

According to the results shown in Table 1, the degree of variation in the sensor resistance at a gas temperature of 200.degree. C. does not greatly vary according to the types of catalysts carried on the porous layer. However, when the stability of the resistance of the sensor exposed in a very rich (.lambda.=0.55) atmosphere and the reproducibility of changes in the sensor resistance when the rich atmosphere (.lambda.=0.55) was changed to a lean atmosphere (.lambda.=1.55) are examined, the influence of the noncombusted gas components can be completely eliminated when a catalyst such as Pd, Rh, Pt or a mixture of at least two thereof is used.

The thin TiO.sub.2 film was formed on the Al.sub.2 O.sub.3 substrate in the same manner as in Example 1. A thin film heater was formed on one major surface. On the other hand, a porous ceramic film which carried a catalyst was formed on a sensor portion so as to prevent the sensor portion from being influenced by noncombusted components. The reproducibility and stability of the resistance of the sensor differs in accordance with the type of catalyst. The present inventors found that catalysts such as Pd, Rh, Pt and a mixture of at least two thereof effectively activated the noncombusted components. The present inventors then examined a proper content of the catalyst.

The change in resistance with respect to the change in the air-fuel ratio was examined with the catalyst content used as a parameter (i.e., the air-fuel ratio was changed from a rich (.lambda.=0.95) ratio to a lean ratio (.lambda.=1.05). Results are shown in FIG. 7 and 8.

As may be apparent from the graphs shown in FIGS. 7 and 8, it is preferred that a catalyst content falls within the range of 0.001 wt % to 50 wt %. When the catalyst content is smaller than 0.001 wt %, the degree of variation in the resistance is reduced, and the sensitivity is degraded. However, when the catalyst content is greater than 50 wt %, the conductivity of the ceramic film is greatly increased by the catalyst, resulting in an extremely low resistance similar to that obtained in a short circuit. The degree of variation in the resistance is slight.

Thin CeO.sub.2, Nb.sub.2 O.sub.5, SnO.sub.2, and ZnO films were respectively formed on Al.sub.2 O.sub.3 substrates under the same sputtering conditions as in Example 1 so as to examine possible effective materials other than TiO.sub.2 for the oxygen sensor. Each thin film had a thickness of about 2,000 to 5000 .ANG.. Thus, thin film oxygen sensors with various heaters were respectively prepared. Changes in resistance with respect to changes in the excess air factor were examined.

Results are shown in FIGS. 9 to 12. As may be apparent from FIGS. 9 to 12, CeO.sub.2, Nb.sub.2 O.sub.5 and SnO.sub.2 besides TiO.sub.2 show great changes in resistance without any hysteresis when the excess air factor .lambda. is changed in an order of 1.5, 0.5 and 1.5. However, the thin ZnO film sensor has a significant hysteresis and poor reproducibility when the excess air factor changes. It is found that Nb.sub.2 O.sub.5, CeO.sub.2 and SnO.sub.2 besides TiO.sub.2 are suitable as thin film oxygen sensors.

As described in Examples 1 to 3, the porous ceramic film (catalyst layer) which carries the catalyst for controlling the gas reaching the surface of the sensor is the main feature of the present invention. The average diameter of the pores in ceramic films according to various coating methods are shown in Table 2 below.

Sensors were prepared having porous ceramic films (each carrying 5 wt % of Pd) of varying film thickness in accordance with the coating methods described above. These sensors were exposed in a lean atmosphere (.lambda.=1.5) at a temperature of 600.degree. C. for 5 minutes. The sensors were then placed in a rich (.lambda.=0.5) exhaust gas atmosphere for one hour so as to examine the relationship between the response time and the film thickness. The results are shown in FIG. 13. The relationship between the film thickness and the response time is summarized in Table 3 when the various coating methods are respectively used as parameters.

The ratio t/r of the film thickness t to the pore diameter r preferably falls within the range of 50 to 3,000. The time required for the sensor resistance to change by 50% falls within the range of 50 msec to 5 sec.