Thick-film copper firing process

The firing process for thick-film copper is different from tirat of otirer thick-film conductors. Because copper readily oxidizes in tire presence of oxygen, the standard thick-film firing process must be modified to prevent tire oxidization. Instead of air flowing through the furnace, dry nitrogen with less than 10 ppm of oxygen is used [5]. 

Thick-film copper conductor inks are not pure copper. The ink consists of a functional material of copper, a solvent, a temporary binder, and a permanent binder. The permanent binder tailors the CTE to that of the substrate. It also aids in the adhesion of either the substrate or the dielectric. The thick-film firing process in nitrogen bums out the solvent and temporary binders. This leaves just the copper and the permanent binder. In addition to tailoring the CTE, the permanent binder significantly reduces both the electrical and thermal conductivities of the conductor. A typical fired conductor thickness is 15-18 pm. If it were pure copper, it would have a sheet resistivity of 0.94-1.13 mQ/Q. The 9924 thick-film copper conductor material from El DuPont Electronics specifies a sheet resistivity of 1.9-4.8 mD/ for the same thickness. This results in the published resistivity of this thick-film material being only 23% of the resistivity of pure copper. 

V. P. Suita and R. J. Bacher, Firing Process-Related Failure Mechanisms in Thick Film Copper Multilayers, Proc. 36th Elec. Comp. Conf, Seattle, pp. 471 80, 1986. 

Belt furnaces having independently controlled heated zones through which a belt travels at a constant speed are commonly used for firing thick films. By adjusting the zone temperature and the belt speed, a variety of time vs. temperature profiles can be achieved. The furnace must also have provisions for atmosphere control during the firing process to prevent reduction (or, in the case of copper, oxidation) of the constituents. 

To obtain finer lines and smaller vias, one can use photoimageable thick-film process for dielectric and conductors and diffusion patterning. The photoimageable thick-film process involves the use of a photoactive paste printed on a substrate and exposed through artwork or a mask to define circuif characteristics, lines, and vias. The materials are developed in an aqueous process and then fired using the conventional thick-film fechnique. Copper, silver, and gold metallizations are used, and layer coxmts of up to 10 circuit layers are possible. 

A copper thick-film conductor, 0.2 in. long and 0.01 in. wide, is fabricated using the standard screen-printing process and is shown in Figure 8.3. The ink manufacturer s data sheet specifies a resistivity of 1.9 to 4.8 mfl/Q for a 13-pm fired thickness [5]. Using the worst-case value of 4.8 mfl/Q, the resistance of the conductor is calculated using Equation 8.6. 

Copper conductors can be deposited and defined in either one of two processes thick-film screen printing and etched thick film. In the former process, thick-film inks are forced through a screen in a definite pattern, dried, and fired. In the latter process, a blanket coat of conductor ink is applied to the ceramic with a screen and fired. It is then etched to the final pattern using classical photolithographic techniques. Line widtirs in tiiis etch process can be as narrow as 0.001 in., whereas the classical screen-printed conductor can only be as narrow as 0.004 in. 

To lower the electrical resistivity of a thick-film conductor, some manufacturers print multiple printings of the same copper conductor ink using toe same pattern. This process can provide a fired-ink thickness of 20 to 25 pm and a resistivity of approximately 1 to 2 mO/n. 

J. P. Bradley, Copper Thick Film Nitrogen Atmos[4iere Furnace Design and Firing Process Considerations, Proc. Inti. Symp. Microelec., pp. 435-453, 1985. 

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