Understanding the photoacid generator distribution at nanoscale using massive cluster secondary ion mass spectrometry

As the semiconductor industry continuously pursues smaller and more advanced device nodes, improving the lithographic performance of photoresists becomes more critical and challenging. The homogeneity of the photoresist formulation components within the thin film, such as the distribution of photoacid generator (PAG) molecules, are critical factors influencing resolving capability and the sidewall roughness after development. However, there is still lack of fundamental experimental approaches to probe the distribution of these components at the nanoscale throughout the photoresist film. Herein, we present the use of a new methodology, namely, massive cluster secondary ion mass spectrometry (MC-SIMS), to determine PAG homogeneity on a 10-15 nm scale within a photoresist film. In comparison to conventional SIMS, of which the detection spatial resolution is limited to large domains and the data is aggregated prior to analysis, MC-SIMS bombards the sample with a sequence of massive Au400 +4 nanoprojectiles, each separated in time and space, collecting and mass analyzing the co-emitted secondary ions from each projectile impact. Each sample is analyzed with a large quantity (106-107) of individual projectile impacts within an analysis area 125 μm in diameter. Analysis of co-emission of these independent 106-107 mass spectra allows for identification of co-localized molecules within nanodomains ~10-15 nm diameter and ~10 nm in depth (the emission area of a single impact) from the film surface. This unique method therefore reveals spatial distributions of molecules at the nanoscale. Using MC-SIMS methodology, we directly measured key factors influencing the PAG homogeneity at the nanoscale including (1) PAG concentration, (2) the nature of the polymer matrix, (3) the nature of the PAG, and (4) additives. We discovered that 85-95% of PAG salts aggregate at the nanoscale. The majority of the PAG aggregates are less than 10 nm in size and are highly homogeneously distributed within the polymer matrix in the film. Furthermore, the size of the PAG aggregates can be manipulated by additives through an ion-exchange mechanism.