Pattern Alignment

PATTERN ALIGNMENT

Pattern alignment is a process in which the mask patterns is aligned along different crystallographic directions. It also refers to alignment of mask pattern with respect to the pattern that already exists on the wafer.

If the mask pattern is perfectly aligned with respect to the pre-etched pattern it is termed as perfect alignment. If it is not perfectly aligned then it is known as misalignment.

In the fabrication of MEMS (Micro Electro Mechanical System) components, Bulk and Surface micromachining processes basically involves several processing steps. Photolithography is one of the major steps. These involve transferring of required pattern on the substrate from the photo mask.

The exact structure of the desired pattern mainly depends on two things.

(i) How accurately the mask edge is aligned with the wafer flat.

(ii) How accurately the wafer flat is defined by the manufacturers. (Fig. 1.40).

Generally, (100), (110) and (111) silicon wafers with different flat orientations such as <100>, <110> and <111> are <110 and 11 a available in the market. These are used to fabricate different types of MEMS structures on wafer surfaces. A small degree of misalignment of the mask edge with the wafer flat results in significant change in the final structure.

Example

Consider a photolithography process that requires two layers of fabrication. The alignment marks used are a cross-type feature in the first layer, and a square pattern in the second layer. When properly aligned, the cross-type feature should fit inside the square pattern. (Fig. 1.41)