A crystallographic alignment method in silicon for deep, long microchannel fabrication

The aim of this work was to develop an alignment technique to be used in the production of long, deep, large area microchannel devices. The microchannel design specifications used for the investigation were 800 mu m deep channels of 100 mu m width, with a 200 mu m pitch, over an area of 40 mm x 40 mm. The device was fabricated with (110) orientated silicon, to take advantage of the large wet etch ratio between the {110} and {111} planes. Silicon nitride was used as the channel etchant mask, and was patterned by reactive ion etching. The channels were wet etched in a KOH 40 wt% solution at 80 degrees C to minimize undercut of the silicon nitride mask, while maintaining a reasonable etch rate of 2 mu m min(-1). The {111} crystal plane normal to the {110} wafer surface needed to be determined with high accuracy for the fabrication of microchannels of such a large size. Investigations of several established alignment techniques revealed only one suitable technique: the use of a wet etched alignment feature that is self-aligned to the {111} crystal planes. This resulted in an silicon nitride mask undercut of 10 mu m for channels 800 mu m deep and 45 mm in length.