Characterizing acid mobility in chemically amplified resists via spectroscopic methods

Abstract

State-of-the-art microlithographic processes used to make features smaller than 0.25 microns are based upon deep-UV lithography and chemically amplified resists (CARs). In these resists, photoacid generated during exposure initiates cascading deprotection reactions during post exposure bake (PEB) to form a developable image. Reaction may not be limited to the illuminated areas since the photo-generated protons may diffuse outside this region; therefore, it is important to understand the diffusional characteristics of the photoacid. In this contribution, the photoacid generation during exposure was characterized using acid- sensitive fluorescence of fluorescein, and the acid mobility and concentration during PEB were studied using crystal violet. These probes were added to the Shipley Apex-E 2408 DUV photoresist, and measurements were carried out in situ on quartz substrates. For fluorescein, characteristic changes in the fluorescence peaks intensities were used to determine the acid concentration during exposure. Crystal violet is a triphenylmethane 'propeller' molecule whose excited state lifetime increases as the local free volume decreases, and molecular rotation is hindered. This feature can be used to characterize the free volume in the photoresists using ground state recovery experiments. In addition, crystal violet has three protolytic forms, each with a unique absorption spectrum, and calibration of the absorption spectrum as a function of acid concentration was used to measure the acid concentration during PEB. These studies illustrate the utility of spectroscopic techniques based upon carefully selected molecular probes to characterize CARs in situ.