LATENT IMAGE-FORMATION - NANOSCALE TOPOGRAPHY AND CALORIMETRIC MEASUREMENTS IN CHEMICALLY AMPLIFIED RESISTS

The characterization of photoresists during the image formation proces s has traditionally relied on bulk methods during or after development . This article shows that it is not necessary to develop the photoresi st in order to obtain significant information about the image formatio n process in x-ray, electron beam, and UV lithography. The characteriz ation of the image formation process in chemically amplified photoresi sts prior to development is difficult due to their sensitivity to elec trons used in scanning electron microscopy and the weak changes in die lectric constant needed for optical microscopy. The advent and develop ment of the atomic force microscope (AFM) have allowed local surface m easurements of exposure induced changed in photoresists with sub-mu m scale resolution and negligible modification of the sample. A series o f chemically amplified resists, positive and negative, have been studi ed with a variety of exposure radiation (electron beam, x-ray, and UV) . The results discussed in this article focus on two of these resists: SAL 605 (-), and TOK 010 (+). Both positive and negative chemically a mplified resists exhibit significant changes in topography and calorim etric properties during the image formation process. Interpretation of the resist topography in negative resists is given by a semi-empirica l model that assumes Fickian diffusion of resist material during post- exposure bake (FEB) and verified with AFM data. The glass transition t emperature, Tg, of wafer spun thin films of SAL 605 has been measured, prior to exposure, to be 75+/-3 degrees C with an ellipsometric techn ique. The FEB temperature, 105 degrees C, is much greater than the Tg of SAL 605, thereby providing some justification for our Fickian diffu sion model. These results are preliminary until a chemical map can be done on the resist at sub-mu m resolution, which at present time is no t possible. The need for detecting intrinsic material property changes due to the image formation process motivates the exploration of calor imetric properties of the resist. Interpretation of the calorimetric d ata is still unclear. Both cases of positive and negative photoresists exhibit an increase of heat absorbed in the exposed regions. Photoaci d ions produced by exposure to radiation may be considered responsible for such an increase. (C) 1996 American Vacuum Society.