In-situ Metrology for Deep Ultraviolet Lithography Process Control

Submicron Deep Ultraviolet (DUV) photolithographic processes present significant manufacturing challenges due to the relatively small process windows often associated with these technologies. The sensitivity of the process to small upstream variations in incoming film reflectivity, photoresist coat and softbake steps as well as the bake plate temperature can result in the final critical dimension (CD) going out of specifications and more importantly, not being identified until the end of the lot. In this work, we have identified the pre-exposure film stack reflectance and the pre-develop exposed area photoresist thickness loss as wafer state quantities, related to the final photoresist critical dimension. A control model integrating equipment and wafer state parameters has been constructed, using a designed experiment, to facilitate control of the photolithography sequence. We have also installed a scanning head, broadband reflectance spectrometer as an in-situ sensor to measure the film stack reflectance for feed-forward corrections to the exposure dose, and to measure the resist thickness loss as a uniformity measure of the combined exposure and the post-exposure bake steps and hence of the final photoresist CD.