PHOTOELECTRON EFFECTS IN X-RAY MASK REPLICATION

In this paper some experimental observations of the interference of photoelectrons in an x-ray mask replicating process are reported on. High resolution low contrast (i.e., producing absorbed dose ratios of less than 2) x-ray masks have been used to produce lines smaller than 0.1-mu-m in poly(methlmethacrylate) (PMMA) on a silicon substrate with an aspect ratio of about 2. When the experiment was repeated on a gold plated substrate, anomalous adhesion problems were encountered. Lines smaller than 0.2-mu-m, for example, had poor adhesion, and those smaller than 0.1-mu-m had entirely lifted off. A higher contrast mask, with a dose contrast of about 6, and with smaller features was tried on the same two substrates. Using this mask, 800 angstrom lines in PMMA in 0.5-mu-m thick resist have been demonstrated, with good adhesion. These effects are consistent with the assumption that the x rays that penetrate the mask's absorber, and the resist, but are absorbed in the plating base, generating photoelectrons, which then propagate across the interface. These extra photoelectrons create a thin layer of increased exposure in the resist near the interface that is responsible for loss of adhesion for ultrasmall structures. A low-Z photoelectron block layer can be used to eliminate this problem. Experimental evidence in conjuncture with computer simulation were done to investigate this assumption.