The nature and characteristics of particles produced by EUV sources : exploration, prevention and mitigation

The demand for ever smaller and faster electronic devices is a drive for the IC and memory industry to make smaller and more complex features. Lithography is a crucial step in the production of these electronic components. In order to fulfill the demand of the market, the resolution of the features printed with lithography needs to improve and the imaging wavelength has to reduce. It is expected that lithography using extreme ultraviolet (EUV) radiation will be introduced to produce features smaller than 32 nm. This technology will make use of plasma light sources, which produce EUV radiation with a wavelength of 13.5 nm to project small-scale patterns onto wafers. Since various materials and gases are strongly absorbing for EUV, lithography systems require vacuum operation and in addition the optics should be reflective in nature as no material is transparent enough for EUV to make use of refractive optics. Currently one of the main challenges is to achieve and maintain sufficient in-band EUV power. In alpha-level EUV exposure tools, sources based on a Discharge Produced Plasma (DPP) of Sn have so far shown the highest EUV power. However, in addition to the desired EUV radiation these sources produce a significant amount of debris that can damage the collector optics. The lifetime of the collector optic in the source-collector assembly is one of the main challenges for EUV lithography to have high productivity. In addition to Sn deposition, a major factor which determines the lifetime is fast ion sputtering of the material at the collector surface. These ions are produced by the plasma itself and it is important to understand the mechanisms that are responsible for the creation of these ions. Generally the debris can be divided into three different groups: the particulate debris, the slow atomic/ionic debris and the fast ionic debris. The characteristics of the different kinds of debris were investigated such that measures can be taken to minimize the effect on the lifetime of the collector optic. The origin of the particulate debris was identified and the result of their impact upon different surfaces was studied. The ionic debris emitted from Sn-based DPP sources was characterized using a combination of different time-of-flights measurements. The amount of slow ionic debris contributing to deposition was experimentally defined. Additionally, fast Sn ions with energies up to 100 keV were identified. In order to discuss the production mechanisms of these suprathermal Sn ions, the z-pinch dynamics of the discharge plasma were studied. The crucial parameters for effective pinch formation were determined and solutions are provided to increase the EUV emission while minimizing the sputtering of the collector.