Effect of film thickness on the dissolution rate behavior of photoresist polymer thin films

Abstract

Fabrication of future nanoscale electronic devices will likely require the use of ultra-thin resist films. It has been observed that film thickness, molecular weight, and substrate interactions can significantly affect the thermophysical properties of polymer thin films such as the glass transition temperature and coefficient of thermal expansion. Recently, film thickness has been reported to have a dramatic impact on the diffusion coefficient of small molecules in polymer ultra-thin films. Both of these factors, changes in either the polymer film thermophysical properties or diffusion behavior, can potentially have an impact of the lithographic performance of polymer thin film photoresists. As an extension to this previous work, it is desirable to understand the influence of film thickness on a variety of other lithographically important polymer properties. Dissolution rate is one such important physical property for photoresist polymer thin films that is of particular importance to the microelectronics industry. Simulation of lithographic processes relies to a great extent on knowledge of the dissolution or development behavior of photoresist thin films. Resist contrast is also known to be strongly affected by the dissolution behavior of the resist matrix polymer. So far, the possibility of film thickness significantly affecting the dissolution behavior of thin photoresist films has generally been ignored. This paper reports on work focused on determining the effect of film thickness on the dissolution behavior of a variety of resist polymers including novolac, polyhydroxystyrene (PHOST), and bis-trifluoromethyl carbinol substitute polynorbornene (HFAPNB). In the present work, Quartz Crystal Microbalance (QCM) methods were used to determine the dissolution rate of polymer thin films for thicknesses ranging from approximately 1 μm to 100 nm. It was observed that both poly(hydroxystyrene) (PHOST) and bis-trifluoromethyl carbinol substituted polynorbornene (HFAPNB) exhibit strong surface acceleration behavior as compared to the classic surface inhibition that has been extensively studied in novolac polymers. Most importantly, the dissolution rate of PHOST and HFAPNB thin films was found to depend strongly on the thickness of the polymer film for film thicknesses below a critical thickness value.