This work presents a method for aligning patterns on a lithography mask parallel to the (110) direction with an accuracy of about 0.1 degrees. Wafers are oxidized or have silicon nitride deposited on them and windows are etched into this using a dark-field mask which has a series of clear circular windows on it. Then, the wafer is etched in KOH for a time sufficient for an inverted etched pyramid to form beneath each of the circular windows. The bases of two neighbouring etched pyramids are misaligned to an extent determined by the alignment of the line joining their centres to the (110) direction. The proximity of the edges at the bases of neighbouring pyramids is such that an accurate judgement of the pyramids that are most closely aligned can be made using an optical microscope. Associated with each pyramid is a set of two alignment marks also etched during the KOH etch. After finding the three most closely aligned pyramids, one chooses the alignment set associated with the central pyramid for alignment of the first device mask.
