ALIGNMENT OF MASK PATTERNS TO CRYSTAL ORIENTATION

This work presents a method for aligning patterns on a lithography mas k parallel to the (110) direction with an accuracy of about 0.1 degree s. Wafers are oxidized or have silicon nitride deposited on them and w indows 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 pyramid s are misaligned to an extent determined by the alignment of the line joining their centres to the (110) direction. The proximity of the edg es at the bases of neighbouring pyramids is such that an accurate judg ement of the pyramids that are most closely aligned can be made using an optical microscope. Associated with each pyramid is a set of two al ignment marks also etched during the KOH etch. After finding the three most closely aligned pyramids, one chooses the alignment set associat ed with the central pyramid for alignment of the first device mask.