Stick-on-mask for use with coulometric measuring instruments

Claim 1

For use in coulometric measurement apparatus for determining the thickness of a thin metal coating on a workpiece substrate through timed deplating thereof, said apparatus having an electrolyte container, grommet means dependent from the base of said electrolyte container and disposed in fluid communication with the interior thereof, said grommet means having a dependent end portion defining an aperture sized to perimetrically define a relatively large area of said workpiece substrate exposable to said electrolyte and a sealed perimetric interface thereabout, means for positioning said grommet means in operative abutting relation with said workpiece substrate, the improvement comprising, means for delivering a stream of electrolyte to the dependent end of said grommet means, means for connecting said electrolyte stream delivery means to a source of electrical potential of one polarity and said workpiece substrate to a source of electrical potential of opposite polarity to permit deplating of said portion of said thin metal coating exposed to said electrolyte from said workpiece substrate, a thin planar mask intermediate said workpiece substrate surface and the dependent end of said grommet means, said mask being of essentially non-compressible electrolyte impermeable material having an aperture therein of relatively small area compared to that of the aperture in said grommet means and perimetrically contoured to define an exposed area of said thin metal coating to be subjected to said electrolyte, and an electrolyte impermeable adhesive coating on one surface of said mask for removably securing said mask to the substrate with the perimetric edges of said mask aperture adhesively secured in electrolyte-tight relation with said thin metal layer to precisely define the area thereof to be exposed to said electrolyte, said mask being sized to be engaged by the dependent end portion of said grommet means at a location remote from said relatively small aperture therein to prevent escape of electrolyte from said electrolyte container.