Calibrating Optical Edge Profilometry for Ultra Fine Circuit Conductors
Optical edge profilometry calibration requires step-height standards and numerical aperture matching to limit edge diffraction bias below half a micron.
Trace Sidewall Taper
Slope Morphology
Geometric trace characteristics describe the angle of the copper wall relative to the horizontal surface of the dielectric substrate. This trace sidewall taper is a byproduct of the chemical etching process where the liquid etchant removes material from the sides as well as the top of the copper foil. Measurement of this angle is necessary to calculate the true cross sectional area of a conductor.
Etchant Fluidics
Subtractive etching typically creates a trapezoidal shape because the top of the trace is exposed to the chemical spray for a longer duration than the base. The degree of trace sidewall taper depends on the etch factor, which is the ratio of the etch depth to the amount of lateral undercut. High pressure spray systems and specialized chemistry are used to minimize this effect and create walls that are as vertical as possible.
Liquid dynamics and the concentration of the dissolved copper in the chemical bath further influence the final slope angle.
Signal Integrity
Signal speed and impedance are influenced by the uneven distribution of current caused by the sloping walls. At very high frequencies, the skin effect concentrates the current at the edges, making the shape of the taper a critical factor in transmission line design. Excessive trace sidewall taper can also reduce the adhesion of the copper to the substrate by narrowing the contact area.