Photoresist latent and developer images as probed by neutron reflectivity methods
Vivek M. Prabhu, Shuhui Kang, David L. VanderHart, Eric K. Lin, Wen-Li Wu
Photoresist materials enable the fabrication of advanced integrated circuits with ever decreasing feature sizes. As next-generation light sources are developed, using extreme ultraviolet light of wavelength 13.5 nm, these highly-tuned formulations must meet strict image-fidelity criteria in order to maintain the expected performance to feature-size gains. However, polymer photoresists appear to be reaching resolution limits as the feature sizes are reduced to below 32 nm. The observed resolution limits are due to coupled optical resolution and materials properties. In order to bridge a knowledge gap in materials understanding, and separate optical effects, advancements in measurements of the in situ formed solid/solid and solid/liquid interface is necessary. This review describes the chemical and physical structure of the lithographic feature edge with chemically amplified photoresists at the length scales between 1 nm and 100 nm using specular and off-specular neutron reflectivity. These methods provide nanometer-scale composition profiling of the chemical latent image at an ideal lithographic line-edge that separates optical resolution effects from materials processing. Photoresist structure-resolution relationships are developed to provide insight into ultimate materials resolution. Four-generations of advanced photoresist formulations were examined over the course of seven years in extensive collaborations with semiconductor industry leaders to quantify specifically photoresist/photoacid and photoresist/developer interactions on the fidelity of lithographic features. Of significance was the measurement of the shape and spatial extent of the photoacid catalyzed reaction-diffusion front and systematic control by photoacid generator size, photoresist polymer chemical composition, and critical amine-base quenchers. Secondly, the spatial extent of photoresist swelling and hydroxide base penetration at the line-edge highlights a fraction of the photoresist line that is infused by aqueous base developer. This developer/resist interface may be minimized by control of the latent image or process treatment. Neutron reflectivity determined nanometer length scales and composition profiles through contrast variant approaches. The outcome of these measurements complement traditional resist design criteria by providing relationships between the impacts of the photoresist and processing on the feature fidelity. These physical effects are also described in the context of novel resist architectures under consideration for next-generation photolithography with extreme-ultraviolet radiation.