After nearly 40s years of innovation, the semiconductor industry is currently experiencing one of its largest paradigmatic shifts in history as lithographic processes are adapting extreme ultraviolet (EUV) light at 13.5 nm for manufacturing the most advanced integrated circuits (ICs). In EUV lithography, ionizing radiation from a tin-plasma source is imaged onto a photoresist, whereby the exposure changes its chemical nature allowing for a nanoscale pattern to be printed. This pattern can then be revealed and modified by processing steps that eventually form the nano and microstructures in advanced ICs. Despite the industry’s adoption of EUV lithography, the fundamental processes governing the chemical modification of resists upon exposure to EUV light are largely unknown. This knowledge gap is exacerbated by the difficulty in measuring the complex radiation chemistry, which requires both chemical and electron sensitive spectroscopies that can resolve the EUV-induced changes in real-time and to date do not exist. If, however, the complex radiation chemistry could be resolved, this knowledge could be leveraged to yield improved photoresists that can print ever smaller patterns, leading to ever fast, more efficient, and more powerful devices.
The goal of the ATTO-SPIE project was develop novel spectroscopies and microscopies that aimed at uncovering the critical processes occurring during EUV exposure in EUV photoresists. To this end, a multifaceted approach was formulated to shed light on three key interactions occurring during EUV exposure; 1) the initial absorption of EUV light by a photoresist, 2) the chemical transformations occurring during exposure, and 3) the kinetics of photoelectrons that are primarily responsible for driving the resist chemistry. These three thrusts formed the core of the three work packages (WPs) of the ATTO-SPIE project, with the ultimate objective to be able to relate these processes to the chemical nature of the photoresists themselves.
During the 22-month duration of this project, ATTO-SPIE was able to achieve many of the sub-objectives and goals as described in the three WPs. New beamlines and metrology tools were developed that enabled measurements of EUV absorption at 13.5 nm (WP1), chemical composition tracked via infrared spectroscopy (IR) and chemical changes occurring on EUV exposure (WP2) and first measurements on photoemission to study the effects of valence band structure on the resulting photoelectrons (WP3). In WP1, the overall objective was to perform measurements of EUV absorption on photoresists and correlate the absorption values to their chemical composition, with the goal of increasing the amount of EUV light absorbed. The beamline and sample chamber constructed and commissioned as part of this WP enabled such measurements and the system is now currently being used to perform absorption measurements on material supplied by leading photoresist vendors. In WP2, the goal was to track EUV-induced chemical changes in real time using time-resolved EUV-pump, mid-IR probe spectroscopy, which has never been demonstrated before on a photoresist system. The output of WP2 resulted in a new beamline that is capable of making such measurements, and this capability will be utilized by researchers at imec to further the technique. The main objective of WP3 was to measure photoelectrons generated during EUV exposure. A tool was commissioned using 30 nm EUV light showing the capability of photoemission spectroscopy to uncover different electron kinetics based on photoresist composition, and moving forward 13.5 nm excitation will be used to mimic the conditions in an EUV lithography scanner.
