Indirect nanoscale characterization of polymer photoresist wetting using ultra-high frequency acoustic waves

Abstract The wetting of surfaces with patterns in the order of a hundred nanometers is often a complex phenomenon to analyze and control. In the semiconductor industry, whether it is during the surface cleaning steps or the deposition of the protective mask (photosensitive liquid resin that is then cross-linked), the conformity of the deposit of the liquid layer on the patterned surface must be perfect or else the functionality of the targeted electronic component will be compromised. Thus, understanding the surface wetting of these liquids allows the implementation of optimized processes. In this paper, we present a method of indirect wetting characterization of a photoresist based on ultra-high frequency (# GHz) acoustic waves. This resin is a commercial product called GKR 4602 (belonging to the KrF series of positive photoresists), which is coated in two different ways: either directly onto the surface of a patterned silicon wafer, or after application of a solvent, Propylene Glycol Ethyl Ether (PGEE), which then acts as a pre-wetting layer. The patterned wafer, playing the role of electrical insulation (Deep Trench Isolation, DTI) are 200 nm wide, deep trenches with a high aspect ratio (> 50). The originality of this paper lies in the validation of the acoustic characterization by direct observation of the wetting of the cross-linked resin. To do so, we used a FIB (Focused Ion Beam) microscope which allowed us to make cuts and capture localized images of the wetting state of the photoresist. Moreover, all the results obtained (resins and patterned silicon surfaces) are directly from the microelectronics industry (STMicroelectronics), showing that our method is fully compatible with an industrial approach.