Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
1995-06-05
2001-11-27
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S323000, C430S277100, C216S105000, C216S106000, C216S041000
Reexamination Certificate
active
06322955
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a solution for etching copper and its method of use to make fine-line copper elements where the etching is precise to close tolerance. The method is particularly suited for articles having complex curvature. The copper elements are commonly used in radomes.
BACKGROUND ART
Circuit board elements are often etched on flat, plated surfaces in conventional etching processes using photolithographic techniques to define lines to specific tolerances of about ±1.0 mil (i.e., 0.001 in). The initial line width of typically about 2-15 mils is patterned in the photoresist before introducing the etchant. Generally, a strong acid such as ferric chloride is used as the etchant. Ferric chloride penetrates quickly through the copper film, and, because of its vigorous etch rate, only modest tolerances are achievable. The etchant begins to etch laterally (i.e., undercut) as it simultaneously continues to etch into the metal. The initial photoresist pattern must define openings in the photoresist narrower than the desired line width in the metal, because undercutting will occur. The etchant will penetrate beneath the photoresist film. The circuit board etching process must be monitored closely so that etching can be halted at the precise time. With a conventional etching process, like those using ferric chloride, it is difficult, if not impossible, to achieve line widths ranging from 3-10 ±0.25 mils (0.003-0.010 in) even on flat boards, because of the etch rate and inherent process delays. Our applications require this close tolerance on large parts having complex curvature. Control of line width is more difficult if the film has a complex curvature. With large parts, it becomes more difficult to flush or to neutralize the etchant to stop the etching. Using
8
conventional circuit board etching processes, this precision is lost.
SUMMARY OF INVENTION
An etching solution especially valuable for precision etching of vapor deposited copper films of complex curvature comprises about 0.5 ft
3
etchant/ft
2
copper film (with a nominal thickness of about 0.1 mil), the solution being about 5 gm per liter of a mild copper etching acid, such as CuCl
2
, and at least about 125 gm per liter of NaCl or another suitable salt in deionized water. We reproducibly achieve line widths of about 3-10 mil ±0.25 mil.
REFERENCES:
patent: 3083129 (1963-03-01), Jones et al.
patent: 3650958 (1972-03-01), Shipley, Jr.
patent: 3753818 (1973-08-01), Poor et al.
patent: 4388388 (1983-06-01), Kornbau et al.
patent: 4696717 (1987-09-01), Bissinger
patent: 5298117 (1994-03-01), Hanson et al.
patent: 2149-684-A (1990-06-01), None
Missel et al, “Steady State Etching of Copper; Cupric Chloride-Sodium Chloride Baths”, Metal Finishing, Dec. 1969, pp. 47-52, 58, Dec. 1969.
Hamilton Cynthia
Hammer John C.
The Boeing Company
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