In optical-projection lithography, the patterns on wafers are reproductions of those on photomasks. The quality of the wafer patterns, as measured by linewidth control, overlay, and defects, is strongly affected by the quality of the corresponding parameters on the masks. Linewidth variations on the reticles ultimately result in linewidth variations on the wafer. Mask-registration errors contribute to overlay errors. Defects on the reticle may result in nonworking die. Consequently, masks are critical components of lithographic technology.
The impact from variations and defects on masks is particularly significant when wafer steppers are used, where the patterns of integrated circuits are formed on the wafers by the repeated imaging of reticles. This repetitive imaging process imposes stringent requirements on the reticles. Consider a reticle that contains four product dies. A single defect on the reticle capable of causing product failure reduces yield by 25%. Such a large yield loss from a single defect implies zero tolerance for defects on the reticle. Similarly, linewidth control and registration must be very good on reticles. Because reticle defects and variations are reproduced repetitively, photomask technology is an important aspect of lithography. Current and future mask requirements from the 2009 International Technology Roadmap for Semiconductors (ITRS) are shown in Table 7.1.
Photomasks are fabricated with techniques similar to those used in wafer processing. A photomask blank, consisting of an opaque film (usually chromiumor molybdenum-containing compounds) deposited on a glass substrate, is covered with resist. The resist is exposed according to the circuit pattern; the resist is then developed, and the exposed opaque material is etched. Photomask patterning is accomplished primarily by means of beam writers, which are tools that expose mask blanks according to suitably formatted circuit designs. Two types of beam writers-electron and optical-are used, and both are discussed in detail in this chapter.
At this early point in the discussion of photomasks, the strict difference between a mask and reticle is explained. In the early days of integrated circuits, the patterns of an integrated circuit were formed by transferring the patterns from masks to the wafers. For every layer, the transfer was 1:1 between the mask and wafer; the patterns for all of the dies on the wafer were contained on the mask.