Resist plasma etching for 157 nm lithography

Summary form only given. International efforts are being conducted to develop the 157 nm lithography, which is expected to be the next generation optical lithography, allowing to meet the 70 nm node and possibly go beyond. We are involved in a European IST program, joining several national laboratories and industrials, to address resist challenges associated with the 157 nm lithography. Resist absorbance is a major problem at 157 nm, "usual" carbon containing resists having too high absorbance at this wavelength. A possible way to face this problem is to use silicon containing resists, which present a suitable absorbance at 157 nm. However, their too low resistance to chloro or fluorocarbon plasma etching does not allow to use them in a single layer scheme. They must be associated with a "usual" resist in a bilayer scheme. Our laboratory is concerned with this bilayer scheme. More precisely, we study the opening phase, following the silicon resist development and consisting in etching the under layer resist in O/sub 2/, SF/sub 6/ or CF/sub 4/ plasma. Our aims are to identify the etch mechanisms and to measure and understand the resist/resist selectivity, which is one of the most important parameter of the bilayer process. We have started our studies with the PDMS polymer (polydimethylsiloxane), which is a suitable resist for 157 nm lithography. Open field areas of PDMS over Novolac AZ5214 resist (carbon containing resist) have been etched in inductively coupled oxygen plasmas, for different bias power. Resist etch rates have been measured in real time by in-situ multi-wavelength ellipsometry. PDMS surface, after different plasma etching time, have been analysed by quasi in-situ X-ray photoelectron spectroscopy. We have shown that during the first seconds of oxygen plasma, a SiO/sub x/-like layer is formed onto PDMS and notably decreases its etch rate. Angular XPS analysis have shown that this effect is not a bulk effect, and only concerns first PDMS atomic layers. The SiO/sub x/-like layer protects PDMS and allows to reach high resist/resist selectivity.