With the emergence of EUVL as the chosen technology for next-generation lithography (NGL) systems, significant effort has been spent in developing light sources consistent with the challenging requirements of the scanner manufacturers as well as meeting the aggressive demands of the end users for high-volume manufacturing (HVM). A light source with extremely high power and brightness is required for integration into a scanner tool based on reflective optics with multilayer (ML) coatings, since it needs to be designed for highest throughput at a wavelength of 13.5 nm. Over the past six years at Cymer Inc., we have pursued the research and development of DPPs to meet the demands for commercial HVM tools. A dense plasma focus (DPF) configuration was chosen because it provides an open geometry with large possible collection angle and can be operated over a wide parameter range. The main thrust of our research and development is devoted to achieving the challenging industry demands on light-source performance, requiring extremely high output power (<100 W), cleanliness, and component lifetime.
In this chapter we review the DPF development efforts at Cymer Inc. and discuss the various areas of investigation we have concentrated on during our efforts to optimize the performance of the light source. The main focus is on the characterization of the various EUV output parameters for different operating conditions. We also discuss thermal management, modeling efforts, debris mitigation, light collection, scalability, and risk areas. More extensive and detailed discussions of some particular aspects of the DPF source are described in our previous annual progress reports on source development.
The plasma-focus scheme was developed independently several decades ago by Mather and by Filippov et al. In a typical Mather-type DPF configuration, an annular sliding plasma discharge is first produced between coaxial electrodes in a so-called rundown phase. Strong magnetic forces created by the high discharge current then lead to a pinch event after plasma compression, with a hot dense microplasma zone developing on the axis near the end of the inner electrode. This highly ionized plasma is confined for a short time and emits intense EUV and soft x-ray radiation. Apart from the main application of large-scale DPF devices for fusion research (typically operated with single pulses), low-energy instruments (with less than ∼1-kJ stored energy) with submicrosecond pulse duration and sometimes higher repetition frequencies have also been investigated as radiation sources by several groups. Our choice of the DPF configuration was born out of related work on coaxial plasma guns and plasma thrusters for space applications. At Cymer Inc., we have adopted the DPF source technology because small-scale configurations can be constructed with very low discharge-circuit inductance and only moderate electrical power consumption while showing at the same time excellent prospects for reliable multikilohertz operation and high EUV generation efficiency.
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