Coating processes – Electrical product produced – Resistor for current control
Reexamination Certificate
1998-10-26
2001-01-30
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Resistor for current control
C427S103000, C427S126500, C427S380000, C029S610100, C029S620000
Reexamination Certificate
active
06180164
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to thick-film resistors used in hybrid electronic circuits, and to the processing of such resistors. More particularly, this invention relates to a method for forming a ruthenium-based thick-film resistor in combination with copper conductors that form terminations for the resistor.
BACKGROUND OF THE INVENTION
Thick-film resistors are employed in hybrid electronic circuits to provide a wide range of resistor values. Such resistors are printed on ceramic substrates using thick-film pastes, or inks, which are typically composed of an organic vehicle, a glass frit composition, and an electrically-conductive material. After printing, thick-film inks are typically dried and then sintered, or fired, to convert the ink into a solid film that adheres to the ceramic substrate. During firing, the ink is heated at a rate that is sufficiently slow to allow the organic vehicle of the ink to burn off, which generally begins at about 345° C. and is completed at about 400° C. to 450° C. with commercially available ink compositions. Peak firing temperatures are typically in the range of about 850° C. to 950° C. Both physical and chemical changes occur within the thick film during sintering, by which the conduction network or microstructure of the resistor is formed. Various additives may be used to achieve specific desired resistivity, stability and temperature characteristics.
Ruthenium-based resistors are widely recognized in the art for their reliability and stable resistance values. A limitation to ruthenium-based thick-film resistors is that their inks must be fired in oxidizing atmospheres in order to prevent the ruthenium compound, usually ruthenium dioxide (RuO2), from being reduced to metallic ruthenium. It has been reported that reduction of ruthenium dioxide begins at about 350° C. in a nitrogen atmosphere.
Thick-film conductors for hybrid circuits are also formed using thick-film inks, with thick-film copper conductors being widely used in view of their low bulk resistivity (sheet resistance about 3 milliohms per square). Thick-film copper inks are fired in a nitrogen atmosphere to avoid the metallic copper from being oxidized into copper oxide, which would prevent the resulting conductor from having high conductivity (low resistivity) and adequate solderability.
From the above, one can see that thick-film ruthenium-based resistors and copper conductors have conflicting processing requirements —ruthenium-based resistors require an oxidizing firing environment, while copper conductors require a non-oxidizing environment. Various solutions have been proposed to overcome this limitation and permit the simultaneous use of ruthenium-based resistors and copper conductors on the same hybrid circuit board. One solution is a process taught by Kuo,
Thick Film Copper Conductor and Ruthenium
-
Based Resistor System for Resistor Circuits,
The International Journal for Hybrid Microelectronics, International Microelectronics Symposium (1983), that requires a first firing in air at 850° C. to 950° C. for the thick-film copper conductor, a second firing in air for the ruthenium-based resistor, and then firing at about 260° C. to 400° C. in a hydrogen-nitrogen atmosphere to reduce the oxidized copper produced when the copper was fired in air. The copper conductors and ruthenium-based resistors produced by this process are disclosed as having desirable electrical properties.
Another process-related solution is to print and then fire a ruthenium-based thick-film ink in air at 850° C. to 950° C., followed by printing and firing a thick- film copper conductor ink at 600° C. in nitrogen. A significant drawback to this process is that the resulting resistors cannot be measured for resistance and temperature-related properties like TCR (temperature coefficient of resistance) until after the conductor had been printed and fired, resulting in scrappage that could be otherwise avoided.
Other suggested solutions have required composition changes to the ruthenium-based thick-film ink. One such solution taught by Hankey et al.,
Introduction of a Novel Copper Compatible Nitrogen Firing Resistor System,
IMC Proceedings (1986), p. 98-102, entails incorporating ruthenium dioxide in a perovskite structure to provide stability during firing in nitrogen. However, doing so significantly complicates the formulation process for obtaining a thick-film resistor of desired resistance value. Another alternative is to forego the advantages of ruthenium-based thick-film resistors, and instead employ base metal thick-film inks that can be fired in a nitrogen atmosphere so as to be compatible with copper conductors. Base metal (non-noble metal) base resistors are not as stable as ruthenium-based resistors, and generally require glass passivation to promote their stability.
From the above, it can be seen that present practices involving the processing of thick-film ruthenium-based resistors with copper conductors are generally complicated. Again, the incompatibility arises from the conventional wisdom that thick-film ruthenium-based resistors must be fired in an atmosphere that will adversely oxidize copper conductors. From the standpoint of cost and stability, it would be highly desirable if a less complicated process was available that enabled the production of thick-film ruthenium-based resistors with copper conductors.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a method for forming a ruthenium-based thick-film resistor having copper terminations, in which the thick-film resistor is fired in a non-oxidizing atmosphere so as not to oxidize the copper terminations yet without reducing the thick-film resistor to metallic ruthenium.
According to the present invention, a ruthenium-based thick-film resistor ink having a matrix material and an organic vehicle is deposited on a copper conductor that will form the terminations for the thick-film resistor formed by firing the ink. The organic vehicle of the ink is then burned out at a temperature of less than 350° C. in an oxidizing atmosphere, such as air. Thereafter, the ink is fired in a non-oxidizing atmosphere (e.g., nitrogen) at a temperature sufficient to sinter the matrix material and yield a ruthenium-based thick-film resistor with copper terminations formed by the copper layer.
From the above, it can be seen that the process of this invention is contrary to conventional wisdom that prohibits firing of a ruthenium-based resistor ink in anything other than an oxidizing atmosphere. The invention is also contrary to the prevailing opinion that the burnout of the organic portion of a thick-film ink must be done in the same atmosphere in which the ink is fired. Instead, it has been unexpectedly determined that a ruthenium-based thick-film ink can be fired in nitrogen or another non-oxidizing atmosphere if its organic constituents are removed prior to the ink being subjected to temperatures above about 350° C. At temperatures below 350° C., and particularly below 300° C., copper undergoes limited oxidation. By formulating the ruthenium-based thick-film ink to contain an organic vehicle with a lower burnout temperature than conventionally used, the organic vehicle can be removed in air with minimal detrimental effect on the copper terminations for the resistor.
Accordingly, a significant advantage of this invention is that a ruthenium-based thick-film resistor can be processed on a substrate with copper without complicated formulation and firing steps. As such, this invention makes possible an extremely stable thick-film resistor that is compatible with copper terminations, and therefore can benefit from the performance advantages associated with copper terminations.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
REFERENCES:
patent: 4316942 (1982-02-01), Kuo
patent: 4503090 (1985-03-01), Brown et al.
patent: 4949065 (1990-08-01), Watanabe et al.
patent: 5302412 (1994-04-01), Tamhankar et al.
patent: 5463367 (1995-10-01), El
Bowles Philip Harbaugh
Ellis Marion Edmond
Mobley Washington Morris
Delco Electronics Corporation
Funke Jimmy L.
Talbot Brian K.
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