Deposition and patterning of electrodes on the vertical sidewalls of deep trenches

Abstract

A process able to produce at wafer scale patterned electrodes on the vertical sidewalls of trenches is described and investigated in detail. It is based on metal evaporation at oblique incidence through a shadow mask made in a dry photoresist film. A full model of the deposition and patterning process is established in order to map the shape as well as the thickness and density of patterned electrodes with or without wafer rotation. In the case of wafer rotation, the modelling is based on the analogy of each shadow mask edge with the gnomon of a sundial. The final electrode shape is then computed from the trajectories followed by the shadows of the gnomons. Offset of the substrate tilt axis with respect to substrate plane, and position on the wafer surface, are considered in the analysis, as well as the variation of film density with incidence angle. Advantages and limitations of the proposed process are discussed. It is shown that sidewall electrodes isolated or connected to top or bottom electrodes can be simply achieved by a suitable sizing and alignment of the shadow mask but that close electrodes on a given sidewall cannot be mutually isolated when the substrate is rotated. The patterning process is demonstrated in the case of Au/Cr electrodes on the sidewalls of through wafer trenches made in GaAs by deep reactive ion etching.