Alignment is based on specific alignment targets (a.k.a. alignment marks or keys) which are designed for this purpose, and they are completely independent of device structures (Figure 22.2). This procedure eliminates feature size and shape effects from the alignment process and enables copying of alignment structures from previous designs. Sometimes, an additional zero-level alignment key needs to be fabricated because, e.g., diffusion as the first step does produce contrasting structures that could be seen in optical microscopes. Usually, silicon or silicon dioxide etching is used to form the zero-level keys.
Fig. 22.2. Alignment keys: standard thin resist; for thick resist.
Even if alignment is correctly performed, overlay ...