CDSEM Focus/Dose Monitor for Product Applications

摘要

Advanced 193 nm lithographic processes will require defocus control for product wafers in order to meet CD and profile requirements in the future. Dose control is already required. The interaction of product wafer materials with lithography requires additional controls beyond tool monitoring. While scatterometry has demonstrated excellent ability to extract effective defocus and dose information from monitor wafers, the addition of product film stacks introduces several issues for this technique. The additional complexity of model generation and the sensitivity to under-layer thickness and optical property variation are among these. A CDSEM technique for lithography focus monitoring overcomes these issues provided it has sufficient precision and relative accuracy. In this paper, we report on comparative studies of two CDSEM techniques. One technique uses angled e-beam to better view the sidewall for edge width measurement. The angle of the beam from normal incidence is considerably larger than previously explored thereby enabling sensitive measurements on shallower structures. The other technique introduces new target designs particularly suited to CDSEM measurement that have enhanced sensitivity to focus and dose. Implementation of these techniques requires expanded sampling during the course of a single measurement in order to suppress roughness. The small target size of these structures enables applications with targets in product kerf and embedded within the circuit. In summary, these methods enable the measurement of dose and focus variations on product wafers.