GB2046799A - Sputtered platinum exhaust gas oxygen sensor - Google Patents

Abstract

In sputtering platinum on to a vitrified zirconia thimble to form an exhaust electrode for an electrochemical- type exhaust gas oxygen sensor, sputtering is effected under an atmosphere consisting essentially of more than 50% oxygen and/or nitrogen by volume. Sensors having low symmetrical transition times are produced.

Description

SPECIFICATION Electrode sputtering process for exhaust gas oxygen sensor This invention relates to solid electrolyte electrochemical-type exhaust gas oxygen sensors.

It more particularly relates to a sputtering process for depositing a platinum exhaust gas electrode on to a vitrified zirconia thimble for such a sensor.

A typical automotive-type solid electrolyte exhaust gas oxygen sensor is disclosed in USPN 3,844,920 (Burgett et al). It has a zirconia sensing element shaped as a tapered thimble. One thimble end is open and has a circumferential flange. The other end is closed and forms the most active part of the element.

The interior and exterior of the thimble have separate porous electrode coatings of platinum. The inner electrode is exposed to a source of oxygen, such as air or mixed metal oxide, for establishing a reference potential.

The electrode has generally been formed by painting on a coating of platinum ink on to the zirconia thimble, drying the coating, and then firing the coated thimble at an elevated temperature. An improved technique by which it can be applied to the thimble is described and claimed in our co-pending United States patent application serial No. 080,449.

The outer electrode the "exhaust electrode'' is exposed to exhaust gas for establishing a potential determined by exhaust gas oxygen concentration. The outer electrode could be a porous thick film of platinum, like the inner electrode. However, it is preferred that this outer electrode be a thin film, applied by evaporation, sputtering, chemical vapor deposition or other such thin film deposition techniques. On the other hand, it has been difficult to reproduce consistently desirable properties, such as porosity and electrical parameters in the thin film electrodes. As a result, yields of satisfactory electrode properties have been limited, and various ancillary treatments have been developed to improve them.For example, United States patent No. 3,978,006 (Topp et al) discloses heating the solid electrolyte body after electrode deposition, to form pores in the electrode coating if it is not porous as deposited. United States patent No.

4,1 36,000 (Davis et al) discloses treating the electrode-coated sensor element chemically and electrolytically to enhance sensor properties. Our co-pending United States patent application serial No. 089,264 discloses an improved sputtering process for producing porous platinum electrodes as deposited. Sensors having sputtered electrodes produced by the latter process consistently exhibit substantially stable response times of less than 600 milliseconds, particularly if heated once or twice in pure nitrogen at atmospheric pressure to 800 C for about 45 minutes before they are used. This nitrogen aging treatment is disclosed in our co-pending United States patent application serial No 030,747.

The present invention discloses how to sputter platinum electrodes on to zirconia bodies in an even more improved way. The present invention provides improved consistency in electrode porosity and microstructure as deposited. In any event, it provides sensors that are fast and substantially stable as formed. Post-electrode deposition treatments for activation and/or aging are unnecessary to obtain fast acting sensors. Moreover, rich-tolean and lean-to-rich transition times are more balanced in the sensors as formed and the sensors exhibit improved controllability. On the other hand, aging can still be beneficial, particularly in increasing the yield of fast acting sensors.

An object of the present invention is to provide an improved sputtering process for depositing a platinum exhaust electrode on to a zirconia solid electrolyte body for an electrochemical-type exhaust gas oxygen sensor.

The invention involves sputtering platinum on to zirconia thimbles in an atmosphere preferably consisting essentially of 65-75% by volume nitrogen and/or oxygen and the balance argon at a pressure of 10-20 millitorr (1.33322-2.66644 Pa). A wide target-thimble spacing of 3.8 centimeters is used, along with a high sputtering power of 13-22 watts/cm2 of target area. These latter conditions are the same as those described and claimed in the aforementioned United States patent application 089,264.

This invention is an improvement over the sputtering process described and claimed in the aforementioned United States patent ap plication 089,264. It differs primarily in that the sputtering atmosphere used is chiefly nitrogen and/or oxygen. Somewhere between 50% and 65% by volume nitrogen and/or oxygen is necessary in the sputtering atmosphere, to achieve the benefits of this invention. More than about 50% nitrogen and/or oxygen is needed, and about 65% appears to consistently provide the improved results of this invention. However, to ensure that these results are consistently obtained, it is preferable to employ about 75% by volume nitrogen and/or oxygen and about 25% by volume argon. Increasing the nitrogen and/or oxygen proportion beyond 75% is unnecessary and may even be counter-productive. The rate of deposition will decrease.Also, film adhesion may decrease and/or strength of the underlying zirconia may not be at an optimum.

Nitrogen and oxygen appear to be equally effective in producing the advantages of this invention. However, using oxygen requires that the apparatus, particularly the pumping system, be modified to accept it. Otherwise, seals and similar components can be attacked, creating safety problems. Accordingly, it is preferable to use nitrogen instead of oxygen.

Analogously, it is expected that other inert gases could be subsituted for argon. However, it is not considered practical to do so.

It is believed that the higher sputtering pressure of 10-20 millitorr (1.33322-2.66644 Pa) also ensures consistent platinum porosity and microstructure. Below a pressure of 10 millitorr (1.33322 Pa) in the deposition chamber, the coating appears to be less porous. Above 20 millitorr (2.66644 Pa), deposition rate decreases, deposition in unwanted areas commences, and formation of a desirable porous microstructure may be suppressed.

With the sputtering improvement of this invention over the sputtering process of the aforementioned United States patent application 089 264 the neutral atmosphere aging process of co-pending USSN 030,747 can be omitted. On the other hand, for reasons herei nafter more fully explained, it may not be preferable to omit such aging. One principal advantage of this invention over that of USSN 089 264 is an improved initial response time, regardless as to subsequent heat treatments.

Another advantage is in that this invention generally provides a narrower variation in sensor properties within a given batch of thimbles on which electrodes are deposited. This invention not only reduces mean response time of the sensor but also reduces the standard deviation from the response time, which improves yields of sensors. It is to be understood, however, that this invention is not known to preclude a gradual sensor deterioration normally attributable to extended sensor use.

Solid electrolyte thimble-like sensing elements for an automotive-type exhaust gas oxygen sensor can have electrodes deposited thereon in the following manner. The elements can be tapered thimbles of vitrified zirconia that is partially or fully stabilized in its cubic form by the inclusion of 4-8 mold percent yttria. Best results have been obtained using partially stabilized zirconia containing about 5 mole percent alumina and 5 mole percent yttria. The thimbles are of the same dimensions and have interior electrodes deposited therein in the same manner as described in the aforementioned co-pending USSN 080,499. In general, they are 3-5 cm long. Each thimble is preferably about 3.7 cm long and has a taper of about 3 degrees and 38 minutes. Its wider end is open and surrounded by a circumferential shoulder having an outer diameter of 1.32 cm.Its closed narrower end is rounded on its outer surface where it has a spherical radius of curvature of about 3 mm. Its diameter near the shoulder is 0.82 cm. Its diameter near the radius is about 0.4 cm. It is believed that it is most important to control the sputtering deposition on and near the rounded tip, which appears to be the most active part of the sensing element.

After the inner electrode is applied and fired to the thimble interior, the outer electrode is deposited. As is usual for any thin film deposition, the zirconia surface should be well cleaned before depositing the platinum electrode onto it. It is presumed that any of the normal and accepted cleaning procedures would work as preparation of the surface for sputtering in accordance with this invention.

For example, the sensors can be initially cleaned in an ultrasonic degreaser with Freon (Registered Trade Mark) -based solvent and then heated in air to a temperature of at least 600 C for 1 hour. The sensors can then be heated again at a temperature of only about 1 50' C for 45 minutes to 2 hours. The thus treated zirconia bodies can then be placed directly in a vacuum chamber for sputtering.

If the outer electrodes are not sputtered within 72 hours after the last mentioned heating the zirconia bodies should be heated again to 600 C and 1 50' C, as already discussed.

The electrodes are currently sputtered with a Model MRC 902 DC magnetron sputtering apparatus obtained from Materials Research Corporation of Orangeburg, New York. This apparatus has an elongated but fairly shallow vacuum chamber with provision for two fixed targets disposed over a single anode that is much larger than the targets. The targets will hereinafter be more fully described. However, essentially they are two mutually spaced parallel strips oriented transverse to a rectangular stainless steel anode. The anode is about 35 cm wide, about 50 cm long, and 2-3 cm thick. Water cooling of the anode is not necessary but may be beneficial for reduced cycle time. Only one target is used at present in this apparatus.The targets and anode are in a main chamber adjacent an antechamber having an elevator mechanism that can stack two pallets, the antechamber being referred to as a load-lock. A movable sealing means separates the antechamber from the main chamber. A special carrier is provided for shuttling pallets of substrates from the antechamber into the main chamber for sputtering and then back again. In sputtering, a discharge is first estabished between a target and the anode.

Then the carrier moves the pallet between them. It continues to move in the same direction until out from under the target, whereupon the discharge is discontinued. Carrier speed is adjusted to attain the desired coating thickness.

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

In using the aformentioned apparatus the targets are substantially always maintained under sputtering atmosphere conditions or high vacuum, except for apparatus servicing.

To load the apparatus with substrates, a batch of the aforementioned zirconia thimbles are placed on a pallet in the antechamber while the antechamber is sealed from the main chamber and is at ambient pressure. The antechamber is then sealed to the ambient atmosphere and evacuated to a pressure of 100 millitorr (13.3322 Pa). After sputtering in the main chamber is discontinued, the seal between it and the antechamber is opened.

The pallet, if any, of workpieces just previously sputtered is shuttled to one level of the antechamber elevator, which picks it off the carrier. The elevator is moved to a next level, and the just-loaded pallet is placed on the pallet carrier. The pallet carrier then moves into the main chamber far enough to close the antechamber-main chamber seal.

After the main chamber is sealed from the antechamber, the antechamber can be backfilled with dry nitrogen to atmospheric pressure and opened to the ambient atmosphere for reloading. The main chamber is pumped down to below about 5 x 10-6 torr (133.322 x 10-6 Pa). A flow of 75% nitro gen-25% argon by volume is then introduced in to the main chamber at a rate of 75-100 cc per minute, whilst pumping continues.

Pumping is then throttled at a sufficient rate to dynamically maintain a pressure in the main chamber at 10-20 millitorr (1.33322-2.66644 Pa). Once pressure in the main chamber is stabilized, a discharge can be estabished between the target and anode.

Pressure is maintained at this level in this way during sputtering, as is usual.

It is preferred at present to coat the thimbles in batches of about 280 thimbles.

When the thimbles are placed on the pallet, they are all oriented vertically, and thus have parallel axes. Their closed ends are upwardly disposed, with their wider ends resting in recesses directly on the pallet. The pallet currently used is formed of two rectangular stainless steel plates about 32 cm long and about 32 cm wide. The bottom plate is about 0.4 cm thick. The top plate is about 0.6 cm thick and has an array of holes in it for receiving and uniformly closely spacing the thimbles. These holes, if closely fitting around the thimbles, can also serve as a mask to limit platinum deposition on the thimble flange radial surfaces. For example, the holes can be 1.33 cm in diameter, preferably in a uniform array of columns and orthogonal rows, both of which are spaced 0.5-0.6 cm apart from one another.In the present apparatus there are 1 5 holes per row and 18 holes per column. The lower plate also has a plurality of parallel grooves in its upper surface registered with the rows. The grooves may help to better evacuate the thimble interiors. The two plates of the pallet rest on a U-shaped pallet holder, which is supported by five alumina cylinders, each of which is about 1.3 cm long and 0.6 cm in diameter, which cylinders are referred to as "stand-offs". The stand-offs are, in turn, supported on a wheeled frame-like assembly that serves as a pallet carrier. The pallet carrier supports the pallet 0.5-1.5 cm above the sputtering anode.

The pallet carrier has metal wheels which roll directly on the metal base plate of the sputtering chamber. The wheels of the carrier straddle the anode. The anode is 3.01 cm above the chamber base plate and is supported on metal stand-offs which, in turn, are supported on the chamber base plate.

After the antechamber seal is opened, the pallet is transferred on to the pallet carrier and the pallet carrier shuttled to its initial position in front of the target. When so positioned, the thimble closed ends are spaced about 3.8 cm below a planar platinum target surface of a cathode and the pallet is spaced 0.5-1.5 cm above its anode.

The sputtering target is a rectangular platinum sheet about 12 cm by 38 cm by 0.6 cm bonded to a supporting copper backing plate.

The nature of the platinum target is no more critical to this invention than it is to any other sputtering process for platinum. The target can be obtained by any commercial source, and preferably provides a high purity platinum surface. It is recognized that in some instances it may prove to be desirable to include minor amounts of other metals in the platinum target along with the pure platinum, as, for example, up to about 5% by weight palladium and/or rhodium. The target is assembled with a cathode that includes water cooling means and a magnet array.

A DC voltage of 500-800 volts is applied between the target and the anode. The sputtering power supply is then adjusted to provide a DC power between the target and the anode of approximately 13-22 watts/cm2 of target area. No special means are used in the pallet, pallet carrier or anode to heat or cool the thimbles during sputtering. Also, no electrical bias or grounding is used on the pallet assembly. The pallet assembly is allowed to electrically float i.e. it is not specifically electrically connected to the rest of the sputtering apparatus. After the plasma discharge has stabilized, the pallet carrier moves the pallet through the plasma discharge at a uniform rate of 4 to 5 cm/min. As previously mentioned, the carrier movement rate is adjusted to obtain the desired coating thickness.

Under the foregoing conditions, a porous platinum coating 1.0-1.5 micrometers thick on the upper ends of the zirconia thimbles will be formed. The thimble closed end will obviously get the greatest thickness of platinum deposit. Side walls on the element will get a correspondingly less thick platinum deposit.

Sputtering is carried on long enough to produce a coating 0.65-1.0 micrometer thick at a point about 0.5 centimeter distant from the thimble closed end, along with a coating 0.3-0.55 micrometer thick about 2 centimeters distant from the thimble closed end.

As mentioned in the process claimed in the aforementioned USSN 089,264, it is preferable to heat the zirconia thimble after the platinum electrode has been sputtered, to increase electrode adhesion. Adhesion can be increased by heating the coated thimble at 800 C in air at atmospheric pressure for about 1 hour. This increase in adhesive can be obtained by heat treatments in air over a rather wide temperature range extending from 600 C to 1 200' C. However, it should be recognized that heat treatments above 800 C tend to open large pores in the coating, 0.5-5 micrometer in width. If extensive, such heating can sinter the electrode film to such an extent that isolated platinum islands are formed. This is obviously objectionable.Also, heat treating reduces apparent electrode surface area to a value much closer to its geometric surface area. By apparent area is meant the surface area as determined by gas adsorption techniques. By geometric surface area, is meant the surface area as calculated from the drawing of the thimble. As discussed in the aforementioned USSN 089,264 and USSN 030,747, reducing the high apparent surface area to a lower value by a post-electrode deposition heat treatment somehow does not destroy the advantages of the initial high apparent surface area, so long as the heat treatment is not severe.

Then a porous coating of magnesium-aluminate spinel is flame sprayed on to the platinum electrode, leaving a portion of the electrode uncovered for making of a low resistance electrode connection. The thimble is then directly incorporated into a working sensor without any further treatment, and can be used as assembled.

It is recognized that the flame spraying of a ceramic overcoat on to the thin electrode may drastically alter the physical appearance of the thin film. On the other hand, it does not appear to deleteriously affect controllability of switching response times obtained by this invention. Electrical characteristics attributable to the nature of the electrode as it was initially deposited apparent still remain. The resultant sensor as assembled consistently exhibits fast switching transition times that are similar for rich-to-lean and lean-to-rich and also exhibits a controllability close to stoichiometry. For example, a rich-to-lean time response of less than 600 milliseconds is regularly obtained in the sensors as assembled. Moreover, more than half of these sensors will consistently have an assembled rich-to-lean response times of less than 200 milliseconds.In recent tests, sensors were made that exhibited switching transition times having a means value of 120 milliseconds as assembled. These sensors had rich-to-lean response times no greater than 1.5 times the lean-to-rich response times.

Controllability for example is regularly obtained to within less than 0.1 air/fuel ratios on the lean side of stoichiometry. Stoichiometry is 14.55:1 under standard conditions.

Hence, the sensors control at about 14.65 as assembled. Moreover, the sensor switching transition times and controllability do not appear to significantly change when the sensor is initially used. Accordingly, sensors produced in accordance with this invention do not need to be operationally or artifically aged in order to achieve stability, low response time or better controllability.

The benefits of this invention should be obtained whether or not the electrode-coated thimble is heated in air to increase electrode adhesion and whether or not the electrode is given a porous overcoat. Still further, other porous overcoats can probably be used, as for example, the gamma alumina coatings disclosed in USPN 4,116,883 (Rhodes). Also, it may be desirable to use a platinum cermet stripe on the outer surface of the zirconia thimble under the sputtered electrode, at least where the porous overcoat does not cover it, to improve durability. If so, the cermet stripe can be fired at the same time as the inner electrode. Then, the sputtered exhaust electrode would be applied using the method of the present invention.

Further, it is expected that the results of this invention should be obtainable regardless as to the nature of the reference electrode or its method of application. Analogously, they should be obtainable on partially or fully stabilized zirconia having other stabilizing agents than hereinbefore described and even with other solid electrolytes. Further, the method of this invention should be equally applicable to RF sputtering and to DC sputtering other than magnetron sputtering. Still further, the sputtering conditions of this invention should be useful in a single batch type apparatus having no antechamber, or in a continuous processing apparatus that would include one or more controlled atmosphere chambers before and after the sputtering chamber.

Still further, it was hereinbefore explained that is invention produces a higher yield of sensors having a lower average response time as formed, over sensors produced in accordance with the method of USSN 089,264.

On the other hand, even faster average switching transition times and great reproducibility may be available, if sensors formed in accordance with this invention are also given a nitrogen aging treatment such as described and claimed in USSN 030,747.

In this latter connection, it should be recognized that when hundreds of elements are simultaneously electrode-coated in a batch, some are not as good as others. The vast majority of elements will form very good sensors, but some will not. The aforementioned aging treatment of USSN 030,747 will proba bly help those sensors which are marginal, and may even improve those that are only slightly better than marginal. However, it is difficult to discern which sensors will benefit from such a treatment and which will not.

Since the aging treatment is relatively inexpensive, it is preferable to age all of the sensors and thus ensure that optimum characteristics are attained with all of the sensors.

As already mentioned, the pallet is left electrically floating, since there does not appear to be any benefit in applying a special separate electrical bias to it. On the other hand, it has been noted that yields of improved sensor elements decreased when stain- less steel stand-offs were substituted for alumina stand-offs.

As hereinbefore mentioned, it is believed that the deposition conditions at the closed upper end of the zirconia thimble are most important. It appears that if the deposition conditions are adjusted to obtain a thin black deposit on the zirconia closed end, the fastest response sensors are obtained. It is thought that the black deposit is or contains chemically reduced zirconia. The deposit should only extend 2-4 mm distant from the closed end and be only about 0.5 mm in to the original zirconia surface. The effect this black layer has on long term, i.e. 50,000 mile, durability of the sensors has not yet been determined. On the other hand, it is recognized that if the thimble is blackened completely through its thickness and/or completely along its length, poorer response sensors are produced. Changing the stand-offs from ceramic to metal appears to cause too much of this black deposit to be formed. This indicates that inherently there is a controlled amount of current leakage from the pallet during sputtering, notwithstanding the alimina stand-offs. Accordingly, for other apparatus than that disclosed herein, it may be preferable to add a controlled resistance path to ground from the pallet, apply an electrical bias, or increase electrical isolation or order to attain just the right amount of blackened tip.

In retrospect, it may be that the sputtering conditions recited herein coincidentally produce a suitable but as yet undefined electrode microstructure, artifically age the electrode in situ, and controllably reduce the zirconia just enough to improve sensor performance after the air annealing stage.

Claims (5)

1. A method of sputtering platinum on to a solid electrolyte body to form an exhaust gas electrode for an electrochemical-type exhaust gas oxygen sensor, in which the platinum is sputtered under an atmosphere consisting essentially of more than 50% by volume of at least one member selected from the group consisting of nitrogen and oxygen and less than 50% of an inert gas, said atmosphere proportion being effective to provide low sensor rich-to-lean and lean-to-rich switch- ing response times of less than 600 milliseconds with the electrode as deposited.

2. A method of sputtering platinum according to claim 1, in which an exhaust gas electrode is deposited on a vitrified zirconia body for an electrochemical-type exhaust gas oxygen sensor, characterised in that the sputtering is performed using an argon atmosphere that contains at least 65% by volume of nitrogen or oxygen.

3. A method of sputtering according to claim 1, to deposit platinum exhaust gas electrode on to a vitrified zirconia thimble for an electrochemical-type exhaust gas oxygen sensor, in which a generally planar platinum target is oriented normal to the axis of said zirconia thimble, the target is spaced at least 3.0 cm from a closed end on the thimble, and the target is sputtered under an argon atmosphere at a pressure of 10-20 millitorr (1.33322-2.66644 Pa) and a power of 13-22 watts/cm2 of target area, characterised in that the atmosphere contains more than 65% by volume of at least one member selected from the group consisting of nitrogen and oxygen and the pressure is 10-20 millitorr (1.33322-2.66644 Pa) so as to increase sensor controllablility and to reproducibly obtain sensor rich-to-lean and lean-to-rich switch- ing response times below 250 milliseconds.

4. A method of sputtering according to claim 2, to deposit a porous platinum exhaust gas electrode on to a vitrified zirconia thimble for an electrochemical-type exhaust gas oxygen sensor, which a generally planar platinum target is oriented normal to the axis of said zirconia thimble, the target is spaced 3.0-5.0 cm from a closed end on the thimble, a coating thickness of at least 0.65 micrometer is applied to the thimble end, and the target is sputtered under argon at a pressure of 1 0-20 millitorr (1.33322-2.66644 Pa) and a DC power of 13-22 watts/cm2 of target area, characterised in that the platinum is sputtered in an atmosphere consisting essentially of 65-75% by volume nitrogen and the balance argon at a pressure of 10-20 millitorr (1 ,33322-2.66644 Pa) so as to increase sensor controllability and to reproducibly obtain low and symmetrical sensor rich-to-lean and lean-to-rich switching response times below 250 milliseconds without further electrolytic or heat treatments after electrode deposition.

5. A method of sputtering platinum on to a solid electrolyte body according to claim 1, to form an exhaust gas electrode for a zirconia, solid-electrolyte exhaust gas oxygen sensor, in which the platinum is sputtered under an atmosphere predominantly of nitrogen and at a target spacing, deposition rate and chamber pressure sufficient to provide on the zirconia body a higher porous platinum deposit and a partially blackened zirconia surface under the platinum deposit, whereby sensor switching response time is significantly reduced.