Technical Approach - MUMPS Post-Processing

MUMPS is a three-layer polysilicon fabrication process with a top layer of metal. (The metal layer is incompatible with our post-processing steps, and is therefore not used in our devices.) The silicon layers are shaped and separated by intervening sacrificial layers of silicon dioxide. The oxide layers are removed with a hydrofluoric acid wet etch.

In our devices, the sacrificial oxide separates the scratch drives from the underlying electrodes. When the oxide is removed, the drives are suspended about 2 microns above the electrodes. We need to add a layer of insulator that is thick enough to prevent dielectric breakdown at the driving voltage of approximately 200 V, but that is not so thick that it fuses the scratch drives to the electrodes.

To do this, we grow thermal oxide on the silicon by heating in water vapor at C. (The relatively low oxidation temperature prevents thermal stress from deforming the thin scratch drive plates.) Since this process coats all silicon surfaces in insulator, there is then no way to make electrical contact with the substrate. So, the devices are lithographically patterned with a very low-resolution mask, and holes are opened up in the oxide above electrical contact pads.