A method for patterning a metal substrate includes a series of surface treatments to control tunnel initiation at a micron or sub-micron level. In particular, the series of surface treatments include forming a hydration layer which acts as a mask while etching the surface of the metal substrate. The hydration layer mask enables control of the tunnel initiation on a micron or sub-micron level because the etching does not undercut the interface between the metal substrate and the hydration layer. As a result, the tunnels can be initiated in an orthogonal direction and closer together, thereby increasing the tunnel density.