Surface Analysis of Photolithographic Patterns using ToF-SIMS and PCA.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a surface analysis technique well-suited to detect and identify trace surface species. With the latest analyzers, ion sources and data analysis methods, imaging ToF-SIMS provides detailed 2-D and 3-D surface reactivity maps. Coupling multivariate analysis methods such as principal component analysis (PCA) with ToF-SIMS provides a powerful method for differentiating spatial regions with different chemistries. ToF-SIMS and PCA are used in this study to image and analyze a two-component photolithograph-patterned surface chemistry currently published and commercialized for bioassays, bio-chips and cell-based biosensors. A widely used reactive surface coupling chemistry, N-hydroxysuccinimide (NHS), and 2-methoxyethylamine (MeO) were co-patterned into adjacent regions on a commercial microarray polymer coating using standard photolithography methods involving deposition, patterning and removal of a routinely used photoresist material. After routine processing, ToF-SIMS and PCA of the patterned surface revealed significant residual photoresist material remaining at the interface of the NHS/MeO patterns, as well as lower concentrations of residual photoresist material remaining within the MeO-containing regions, providing spatial mapping and residue analysis not evident from other characterization techniques. As detection of surface photoresist residue remains an inherent challenge in photolithographic processing of a wide array of materials, the use of ToF-SIMS coupled with PCA is shown to be a high-resolution characterization tool with the high sensitivity and specificity required for surface quality control measurements following photolithography and pattern development relevant to many current processes.