Etch Disparity
Physical vapor deposition processes remove material from a substrate surface at rate differentials determined by atomic mass and bond energy. Surface preparation steps during high-density interconnect fabrication encounter differential sputtering when inert argon ions bombard heterogeneous laminate interfaces. Metallic copper features etch at a substantially faster rate than surrounding epoxy resin or woven glass fibers.
The phenomenon occurs within vacuum plasma chambers during pre-sputter cleaning cycles prior to seed layer deposition, stopping at the boundary where physical ablation yields to chemical etching.
Interface Topography
Ion bombardment transfers momentum to exposed surface atoms, dislodging soft dielectric material more rapidly than crystalline metal structures. The differential erosion rate generates microscopic step heights and localized recessions at copper-resin boundaries. Recessed dielectric pockets collect excess sputtered metal, forming parasitic conductive paths or irregular seed layer thicknesses.
Controlling ion beam energy and incidence angle reduces topography formation across composite substrates. Process engineers adjust chamber pressure to equalize sputter yields across dissimilar materials.
Process Limit
Uncontrolled preferential removal degrades microvia wall angles and weakens adhesion strength of subsequent metallization layers. Reduced seed thickness over recessed dielectric steps creates voiding risks during subsequent electrolytic copper plating. Fabrication criteria restrict surface roughness variations to prevent signal dispersion at microwave frequencies.
