JPS6255620B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an electrochemical type exhaust gas oxygen sensor using a solid electrolyte.
electrochemical−type exhaust gas oxygen
(sensor) related. For more information, please refer to the vitried zirconia thimble for this type of sensor.
A sputtering process for depositing a platinum exhaust electrode on a zirconia thimble.

A typical automotive solid electrolyte exhaust gas oxygen sensor
An exhaust gas oxygen sensor is disclosed in Burgett et al., US Pat. No. 3,844,926. It is a tapered thimble.
Thimble-shaped zirconia sensing element (zirconia)
sensing element). One end is open and has a circumferential flange.
flange). The other end is closed and forms the most active part of the device. The interior and exterior of the thimble are provided with separate platinum porous electrode coatings. To determine the reference potential, the internal electrodes are exposed to an oxygen source such as air or mixed metal oxides. Generally, electrodes were made by coating a zirconia thimble with platinum ink, allowing the paint to dry, and then firing the coated thimble at a high temperature. The improved technology applicable to the thimble is disclosed in U.S. Patent Application No. 080449 (Japanese Patent Publication No.
10383) and is claimed as a patent.

An external electrode or "exhaust electrode" is exposed to the exhaust gas to determine a potential determined by the exhaust gas oxygen concentration. The outer electrode, like the inner electrode, is a thick porous membrane of platinum. However, this external electrode is not suitable for evaporation, sputtering, chemical vapor deposition.
Preferably, it is a thin film produced by deposition or other thin film deposition techniques. On the other hand,
It has been difficult to consistently reproduce desired properties such as porosity and electrical parameters in thin film electrodes. As a result, the yield of satisfactory electrode properties is limited and
Various auxiliary treatments have been developed to improve them. For example, U.S. Pat. No. 3,978,006 [Topp et al.] discloses that by heating a solid electrolyte body after electrode deposition, pores are formed in the electrode coating if the deposited electrode coating is not porous. Discloses that. US Pat. No. 4,136,000 (Davis et al.) discloses chemically and electrolytically treating an electrode coated sensor element to improve sensor properties.
Co-pending application of the present invention U.S. Patent Application No. 089264
No. 54-141667 discloses an improved sputtering method for producing porous platinum electrodes during deposition. Sensors with sputtering electrodes manufactured by the latter method remain unchanged throughout, especially if heated once or twice to 800°C for approximately 45 minutes at atmospheric pressure in pure nitrogen before use.
It exhibits a substantially stable response time of less than 600 milliseconds. This nitrogen aging treatment is described in co-pending U.S. Patent Application No. 030747 (Japanese Unexamined Patent Publication No. 55-141664).
Publication No.).

The present invention discloses a method of sputtering platinum electrodes onto a zirconia body by a still further improved method. The present invention provides improved consistency in electrode porosity and microstructure of the deposit. Either way, as molded, it provides a fast and substantially stable sensor. In order to obtain a fact acting sensor, post-treatment of the deposited electrode for activation and/or aging is unnecessary. In addition, rich to lean (rich
-to-lean) and lean-to-rich (lean-to-lean) and lean-to-rich (lean-to-lean)
-rich) is well balanced within the fabricated sensor,
Sensors have improved controllability
shows. On the other hand, aging is still advantageous, especially to improve the yield of fast-response sensors.

It is an object of the present invention to provide an improved sputtering method for depositing platinum exhaust electrodes on zirconia solid electrolyte bodies for electrochemical type exhaust gas oxygen sensors.

The present invention preferably involves depositing platinum on a zirconia thimble at a pressure of 10 to 20 mTorr (1.33322 to 2.66644 Pascals) in an atmosphere consisting essentially of 65 to 75 volume percent nitrogen and/or oxygen and the balance argon. This includes sputtering. A target-to-thimble spacing of 3.8 cm is used with a high sputtering power of 13 to 22 watts/cm ² of target area. Power requirements are described in the aforementioned U.S. Patent Application No.
It is the same as that described and patented in No. 089264.

The present invention is an improvement on the sputtering process described and claimed in the aforementioned US patent application Ser. No. 089,264. The main difference is that the sputtering atmosphere used is mainly nitrogen and/or oxygen. To achieve the advantages of the present invention, a range of 50 to 65 volume percent nitrogen and/or oxygen is required in the sputtering atmosphere. About 50 percent or more nitrogen and/or oxygen is required, and at about 65 percent,
The present invention appears to provide improved results throughout. However, to ensure that these results are consistently obtained, it is preferred to use 75 volume percent nitrogen and/or oxygen and 25 volume percent argon. Increasing the proportion of nitrogen and/or oxygen beyond 75% is unnecessary and has an adverse effect on productivity. Deposition rate decreases. Also, the adhesion of the membrane is reduced and/or the strength of the underlying zirconia is not optimal.

Nitrogen and oxygen appear to be equally effective in producing the advantages of the present invention. However, the use of oxygen requires changes in equipment, especially the pump system. Otherwise, seals and similar components may be attacked, creating a safety hazard. Therefore, it is preferable to use nitrogen instead of oxygen. Similarly, it is envisioned that other inert gases may be used in place of argon. However, doing so is not very practical.

It is believed that high sputtering pressures of 10 to 20 mTorr (1.33322 to 2.66644 Pascals) ensure consistent platinum porosity and microstructure throughout. 10 mTorr (1.33322
At pressures below 100 Pascal), the coating appears to have poor porosity. Above 20 mTorr (2.66644 Pascal), the precipitation rate decreases and precipitation begins in unnecessary areas, suppressing the formation of the desired porous microstructure.

The improved sputtering of the present invention with respect to said US patent application Ser. No. 089,264 allows the neutral atmosphere aging step of the combined application US Pat. On the other hand, it is not suitable to omit such ripening for reasons explained in detail below. U.S. Patent Application Serial No. 089264
One major advantage of the present invention over the method of
Improved initial response time independent of subsequent heat treatment. Another advantage is that the present invention generally provides for a gradual change in sensor characteristics within a given batch of thimbles on which electrode plates are deposited. The present invention not only reduces the average response time of the sensor, but also reduces the standard deviation from the response time, increasing the yield of the sensor. However, it should be understood that the present invention is not known to prevent normal sagging sensor degradation due to long-term use.

A solid electrolyte thimble sensing element for an automobile exhaust gas oxygen sensor can be provided with a deposited electrode thereon in the following manner. The number of elements is 4 to 8.
A tapered thimble of fused zirconia partially or fully stabilized into a spherical shape by the inclusion of a mole percent of yttria. Best results are around 5
obtained when using partially stabilized zirconia containing mole percent alumina and 5 mole percent ittria. The thimble has the same dimensions as the aforementioned co-pending U.S. Patent Application No. 080449;
and has internal electrodes deposited in the same manner. Generally the length is 3 to 5 cm. Each thimble is preferably about 3.7 cm long and has a taper of about 3 degrees and 38 minutes. Its wide end is open and surrounded by a circular shoulder with an outer diameter of 1.32 cm. Its closed narrow end is rounded and its outer surface has a radius of curvature of approximately 3 mm. The diameter near the shoulder is 0.82cm. The diameter near the rounded end is 0.4 cm. Controlling sputtering deposition on and near the round tip is most important, as that appears to be the most active part of the sensing element.

After the internal electrode is applied inside the thimble and fired, the external electrode is deposited. Since any thin film deposition method can be used, the zirconium surface must be thoroughly cleaned before depositing the platinum electrode thereon. It is contemplated that any commonly available cleaning method may be used in making the sputtering surfaces of the present invention. For example, first clean the sensor in an ultrasonic degreaser using a Freon-based solvent, then clean it for at least 600 min in air.
Heat for 1 hour at a temperature of °C. The sensor is then heated again at a temperature of about 150° C. for 45 minutes to 2 hours. The zirconia body thus treated is placed directly into a vacuum chamber for sputtering. If the outer electrode is not sputtered within 72 hours after the last heating described above, the zirconia body may be
It must be heated again to 600°C and 150°C.

Generally, the electrodes are type MRC manufactured by Material Research Corporation, Orangeburg, New York.
- Sputtered by a 920DC magnetron sputtering apparatus. The apparatus includes an enlarged but fairly narrow preparation vacuum chamber for two fixed targets placed on a single anode that is larger than the target. Targets are described more fully below. Basically, however, the targets are two spaced apart objects oriented laterally to a rectangular stainless steel anode.
parallel strips. The anode is about 35 cm wide, about 50 cm long, and 2 to 3 cm thick. Water cooling of the anode is not required, but has the advantage of reducing cycle time. This device currently uses only one target. The target and anode are located in the main chamber adjacent to the antechamber, which is equipped with an elevator mechanism that allows two pallets to be stacked, and the antechamber is equipped with a load-lock. has been done. A movable sealing meaus separates the vestibule from the main chamber. A special carrier is provided for transporting pallets of substrates from the front chamber to the main chamber and back for sputtering and return. During sputtering, first a discharge is generated between the target and the anode.
Next, the carrier moves the pallet between them. Continue moving in the same direction from the bottom to the outside of the target, then discontinue the discharge. Adjust the carrier speed to obtain the desired coating thickness.

While one batch of substrate is being coated in the main chamber, the other batch is removed from the antechamber, a new batch is placed in the antechamber, and the antechamber is returned to low pressure.

When using the apparatus described above, the target is virtually always kept under sputtering atmospheric conditions or high vacuum, except in the case of equipment repair. To load the substrate into the apparatus, the zirconia thimble is placed on a pallet in the vestibule while the vestibule is sealed from the main chamber and under atmospheric pressure. Next, the antechamber is sealed to normal pressure and evacuated to a pressure of 100 mTorr (13.3322 Pascals). After interrupting the sputtering in the main chamber, open the seal between the main chamber and the front chamber. The pallet of the workpiece that has just been sputtered is returned to one level in the front elevator and is transferred from the carrier to the elevator. Move the elevator to the next level and place the just loaded pallet onto the pallet carrier. The pallet carrier is then moved into the main chamber far enough to close the vestibule-to-main chamber seal.

After sealing the main chamber from the antechamber, the antechamber is brought back to normal pressure with dry nitrogen and opened to normal pressure for reloading.
The main chamber is 5×10 ^-6 Torr (133.322×10 ^-6 Pascal)
Exhaust with a pump below. Next, a flow of 75 volume percent nitrogen and 25 volume percent argon is applied, with continued pumping, from 75 to 100 c.c./
Introduced into the main room at a speed of 1 minute. The pump exhaust is then throttled to a rate sufficient to dynamically maintain the pressure within the main chamber between 10 and 20 millitorr (1.33322 and 2.66644 Pascals). At one end, the pressure in the main chamber is stabilized and a discharge is generated between the target and the anode. Typically, pressure is maintained at this level during sputtering in this manner.

It is currently preferred to coat the thimble in batches of about 280 thimble. When placing the thimbles on the pallet, they are all oriented vertically, so their axes are parallel. Place the closed end facing up and the wide end directly into the well in the pallet. The pallets currently in use are approximately
It is made from two square stainless steel plates measuring 32cm wide and approximately 32cm wide. The bottom plate is approximately 0.4 cm thick. The top plate is approximately 0.6cm thick.
It has an array of holes in the parent plate for receiving the thimbles and spacing them evenly and closely together. If these holes are tightly secured around the thimble, they also serve as a mask to limit the deposition of platinum onto the circumferential surface of the thimble flange. For example, the diameter of the holes is 1.33 cm, and preferably the holes are arranged in uniform columns and perpendicular rows, each row being 0.5 to 0.6 cm apart from each other. Current equipment has 15 holes per row and 18 holes per column. The lower plate also has a plurality of parallel grooves on its top surface, aligned with the rows. The group helps to better vent the inside of the thimble. The two plates of the pallet rest on a U-shaped pallet holder supported by five alumina cylinders, each approximately 1.3 cm long and diameter 0.6 cm and the cylinder is referred to as a "stand-off".
The standoff is then a wheeled frame-like assembly that serves as a pallet carrier.
assembly). The pallet carrier sputters the pallet onto the anode 0.5
Support to 1.5 cm.

The pallet carrier includes a metal wheel that rolls directly on a metal base plate in the sputtering chamber. The carrier wheel straddles the anode. The anode was located 3.01 cm above the chamber datum and was supported on metal standoffs, which in turn were supported on the chamber datum.

After opening the vestibule seal, the pallet is transferred onto the pallet carrier and the pallet carrier is returned to its initial position in front of the target. When positioned in this manner, the sealed end of the thimble is spaced approximately 3.8 cm below the platinum target surface of the cathode and the pallet is spaced 0.5 to 1.5 cm above the anode.

The sputtering target is a rectangular platinum sheet approximately 12 cm x 38 cm x 0.6 cm bonded to a supporting copper backing plate. For the present invention, the nature of the platinum target is no more critical than for any other platinum sputtering process. Targets are available from any commercial supplier and preferably provide a high purity platinum surface. In one example,
It has been recognized that it has been found desirable to include pure platinum in the platinum target along with small amounts of other metals, such as up to 5 weight percent palladium and/or rhodium. The target is assembled with a water cooler and a cathode containing a magnet array.

A DC voltage of 500 to 800 volts is applied between the target and the anode. Next, adjust the sputtering power supply to approximately
Provides 13 to 22 watts/cm ² of DC power between the target and the anode. During sputtering, no special equipment is used in the anode to heat or cool the pallet carrier or thimble. Additionally, an electrical bias is applied to the pallet assembly.
or not using grounding. The pallet assembly is allowed to float electrically, ie, has no specific electrical connection with the rest of the sputtering apparatus. After the plasma discharge has stabilized, the pallet
A carrier moves the pallet through the plasma discharge at a uniform speed of 4 to 5 cm/min. As described above, the speed of carrier movement is adjusted to obtain the desired coating thickness.

Under the above conditions, a porous platinum coating of 1.0 to 1.5 micrometers is formed on the top of the zirconia thimble. The closed end of the thimble clearly obtains the greatest thickness of platinum deposit. The side walls of the element obtain a correspondingly less thick platinum deposit. At a point approximately 0.5 cm from the sealed end of the thimble.
Apply a coating 0.65 to 1.0 micrometers thick at a distance of approximately 2 cm from the sealed end of the thimble.
Sputtering is carried out long enough to produce a coating with a thickness of 0.3 to 0.55 micrometers.

After sputtering the platinum electrode to increase electrode adhesion, as described in the claimed method of the aforementioned U.S. Patent Application No. 089,264,
It is preferred to heat the zirconium thimble. Adhesion is increased by heating the coated thimble at 800° C. in air under atmospheric pressure for about 1 hour. This increase in adhesion is obtained by heat treatment in air over a rather wide temperature range from 600°C to 1200°C. but,
It must be acknowledged that temperatures above 800°C tend to create large pores of 0.5 to 5 micrometers in diameter in the coating. If excessively heat treated, such heating will sinter the electrode film to the extent that independent platinum islands are formed. This is clearly not desirable. Heat treatment also reduces the apparent electrode surface area to near its geometric surface area. Apparent area means the surface area measured by gas adsorption methods. Geometric surface area means the surface area calculated from the drawing of the thimble. As discussed in the aforementioned U.S. patent application Ser. without compromising the advantages of

Magnesium-aluminate spinel is then applied, leaving uncovered electrode portions to create a low resistance electrode connection.
A porous paint of spinel is flame sprayed onto the platinum electrode. The thimble is then directly incorporated into the actuation sensor and used as an assembly without further processing.

Ceramic protective coating on thin film electrodes (ceramic protective coating on thin film electrodes)
Flame spraying (overcoat) drastically changes the physical appearance of the thin film. However, this does not appear to have a detrimental effect on the controllability of switching response time obtained with the present invention. The electrode characteristics that result from the nature of the electrode when it was first deposited still remain evident. The assembled sensor has a fast switching transition time that does not change from beginning to end.
time) and rich-to-lean (rich-to-lean).
-lean) and lean-to-rich (lean-to-)
rich) and exhibits controllability close to stoichiometry. For example, a rich-to-lean time response of 600 ms or less
is obtained in the assembled sensor. Additionally, more than half of the sensors collectively have a consistent rich-to-lean response time of less than 200 milliseconds. In recent exams,
Sensors were created that, when assembled, exhibited switching response times with an average value of 120 milliseconds. The rich-to-lean response time of these sensors was not greater than 1.5 times the lean-to-rich response time. For example, controllability is 0.1 air/fuel on the lean side of stoichiometry.
(ratio) can be obtained regularly within the following range. The stoichiometry is 14.55:1 under standard conditions. Therefore, the sensors collectively control at about 14.65. Furthermore, the switching time and controllability of the sensor do not exhibit significant changes even during the first use of the sensor. Therefore, the sensor of the invention does not need to be aged under operating or artificial conditions to obtain stability, low response time or better controllability.

The advantages of the present invention can be obtained whether or not the electrode coated thimble is heated in air to increase electrode adhesion and whether or not the electrode is provided with a porous coating. Still further, e.g.
Other porous overcoats can also be used, such as the gamma alumina coating disclosed in Rodes No. 4,116,883. At least in areas where the porous protective coating does not cover the thimble, it is desirable to use a platinum cermet stripe on the outer surface of the zirconia thimble below the sputtered electrode to improve durability. . If so, the cermet stripes are fired at the same time as the inner electrodes.
Next, the method of the invention is applied to the sputtered exhaust electrode.

Furthermore, the results of the present invention are expected to be obtained independent of the nature of the reference electrode or the method of its application. Similarly, the results of the invention are obtained on partially or fully stabilized zirconia with stabilizers other than those mentioned above, and even for other solid electrolytes. Furthermore, the method of the present invention
It is equally applicable to high frequency sputtering or direct current sputtering other than magnetron and sputtering. Furthermore, the sputtering conditions of the present invention may be applied to a single-batch apparatus without a prechamber or with a single batch before or after the sputtering chamber.
It is also useful in continuous systems containing one or more controlled zero atmosphere chambers.

Additionally, the present invention is disclosed in U.S. Patent Application No.
It has been previously described that a high yield of sensors are produced that have lower average response times as formed than the sensors produced by the method of the '089264 patent. However, if the sensor is formed with the method of the present invention and subjected to a nitrogen aging process as described and claimed in U.S. Patent Application Ser. It is possible to obtain it.

In this regard, when hundreds of devices were coated on the electrode simultaneously in a batch, some were not as good as others. Most devices make very good sensors, but some do not. The aging process described above in US patent application Ser. However, it is difficult to tell whether the sensor will become better or not better after such treatment. Since the aging process is relatively inexpensive, it is preferable to age all sensors, thus ensuring that the best characteristics are obtained for all sensors.

As previously mentioned, there appears to be no benefit to applying a special separate electrical bias to the pallet, so the pallet remains electrically floating. However, it has been noticed that replacing stainless steel standoffs with alumina standoffs reduces the yield of improved sensor elements.

As mentioned above, it is believed that the deposition conditions at the closed end of the zirconia thimble are most important. The fastest response time appears to be obtained if the deposition conditions are adjusted to obtain a thin black deposit on the closed end of the zirconia. The black material appears to be or contain chemically reduced zirconia. The precipitate extends a distance of only 2 to 4 mm from the closed end and extends only about 0.5 mm into the original zirconia plane. The effect of this black layer lasts for a long time, approximately 80,450 km (50,000 miles).
However, the durability of the sensor has not yet been measured. However, it has been observed that if the thimble is completely blackened through its thickness and/or completely in its axial direction, a sensor with poor response is produced. Changing the standoff from ceramic to metal appears to produce too much of this black deposit. This essentially indicates that despite the alumina standoff, the amount of current shunting from the pallet during sputtering is controlled. Therefore, for other devices other than those disclosed herein, it is necessary to add a controlled resistive path from the pallet to ground, apply an electrical bias, or obtain the correct amount of black tip. Therefore, it is preferable to increase the electrical insulation. In retrospect, the sputtering conditions cited here simultaneously produce a suitable but as yet undefined electrode microstructure, artificially age the electrode in situ, and allow the sensor to mature after the air annealing process. The goal appears to be to manageably reduce the zirconia just enough to improve performance.

Claims (1)

[Claims] 1 Rich-to-lean and lean-to-lean
A method of sputtering platinum onto a fused zirconia solid electrolyte body for producing an exhaust gas electrode for an electrochemical type exhaust gas oxygen sensor with a rich switching response time of less than 600 milliseconds, comprising: Using a platinum target separated by 10 to 20 mTorr (1.33322 to
2.66644 Pascals) and an atmosphere consisting essentially of more than 50 volume percent of at least one gas selected from the group consisting of nitrogen and oxygen and less than 50 volume percent of an inert gas, Sputtering power per target area
A method characterized by sputtering at 13 to 22 watts/cm ² . 2. A method according to claim 1, characterized in that sputtering is carried out using an argon atmosphere containing at least 65 volume percent nitrogen or oxygen. 3. the solid electrolyte body is a thimble, a substantially flat platinum target is oriented perpendicular to the axis of the zirconia thimble, and the target is at least 3.0 cm from the closed end of the thimble, thereby increasing the controllability of the sensor; Rich time shorter than 250 ms
A method according to claim 1, characterized in that lean-to-lean and lean-to-rich switching response times are obtained with good reproducibility. 4 Place the target 3.0 to 5.0 cm from the sealed end of the thimble and sputter with DC power in an atmosphere consisting essentially of 65 to 75 volume percent nitrogen and the remainder argon to reduce the film thickness at the thimble end to at least 0.65 micrometer, which increases the controllability of the sensor and allows rich-to-lean and lean-to-lean measurements of low, symmetrical sensors in less than 250 ms after electrode deposition without further electrochemical or heat treatment. 4. A method according to claim 3, characterized in that the time-to-rich switching response time is obtained with good reproducibility. 5 Sputtering is carried out in an atmosphere consisting mainly of nitrogen and an inert gas, whereby the highly porous platinum precipitates and the partially blackened zirconia surface under the platinum precipitates are transferred onto the zirconia body. A method according to claim 1, characterized in that it provides: