X-RAY PHOTOELECTRON-SPECTROSCOPY AND INFRARED STUDY OF THE PROCESSING OF A SILYLATED POSITIVE PHOTORESIST

Liftoff processes are being used in microelectronics technology for various metallization steps. In a typical application, a photoresist is spun on top of a soluble polymer, exposed, developed, silylated, and cured. Subsequently, the silylated resist acts as a stencil mask for O2 reactive ion etching (RIE) pattern transfer into the underlying polymer. The generation of debris, leading to etch residues ("grass"), is a major concern during this process step. The goal of this work is to follow the various process steps that the resist is subjected to, and in particular to determine the effect of ultraviolet (uv) hardening on the silylated resist. We used the complementary techniques of x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrophotometry (FTIR) to obtain information on the chemical composition in the surface region and in the bulk of the film. In contrast to conventional baking, uv hardening appears to induce a substantial carbon depletion in the top layer. The aromatic structure of the novolac-type resist is diminished and the surface exhibits higher siloxane concentration. It is inferred that under O2 RIE patterning, this layer is quickly converted to an etch resistant (sub) oxide and less silicon containing debris is generated in the process than in the case of a baked silylated layer.