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Tin Etching in an EUV Source by Inherent and Surface Wave Plasma
ABSTRACT: The need for computing power grows everyday as the amount of data increases due to automation process and computing need which expands over all aspects of life. For over five decades, the semiconductor industry has continued to advance and follow Moore’s law by increasing the numerical aperture and reducing the wavelength of optical lithography. Extreme ultraviolet light (EUV) has replaced 193nm excimer laser as standard technique in lithography, thus decreasing the wavelength to 13.5nm. EUV light is generated by laser-produced plasma (LPP) when a laser beam hits molten tin, and it ionizes the tin to the 10+ state. A by-product of EUV plasma creation is tin debris which contaminates source surfaces, and it degrades the reflectivity of collector optics. Thus, tin mitigation is one of the main challenges for manufactures. Tin contamination lowers collector reflectivity, and it increases the time needed to process a wafer. A potential solution is to employ an in-situ cleaning method which will not require any source downtime.
Tin mitigation employs hydrogen radicals and ions, formed in a hydrogen plasma, to interact with tin to form tin hydride (SnH4) in the gaseous state, which can be removed through pumping system. Surface wave plasma (SWP) technology developed at University of Illinois generates high density hydrogen radicals and ions, resulting in tin etch rates that are high enough to keep extreme ultraviolet (EUV) lithographic tools clean. An advantage of an SWP antenna is the ability to generate a high density of hydrogen radicals and hydrogen ions directly at the desired etching location. In-situ etching of tin enables high availability EUV tools by maintaining high reflectivity of the multilayer mirror of the collector. Additionally, the SWP is characterized with low ion energies and low electron temperature, such that the multilayer mirror does not suffer any damage from sputtering or implantation of hydrogen ions during operation. Here, experiments elucidating the fundamental processes of tin removal are conducted by varying pressure, power, surface temperature, and gas flow rate in order to observe the etch rate behavior. Our results have shown that the presence of hydrogen ions increases etch rates because ion bombardment weakens Sn–Sn bonds, which, in turn, allows for a higher rate of chemical etching by the radicals. The ion bombardment reduces the number of radicals needed to etch a single tin atom to the range of 102 –103. The linear SWP antenna yields plasma densities on the order of 1016 to 1017 m-3 and radical densities on the order of 1018 to 1019 m-3, allowing for greater utilization of ion etch enhancement. Three different antenna configurations such button, linear and circular antennas embedded in the collector to test which geometry generates largest surface coverage and etch rate. Etch rates of up to 270 nm/min have been achieved. The surface temperature of the samples is a principal factor in the etching process such that the decrease in the surface temperature increases the etch rates and decreases the hydrogen desorption rates. In addition, a kinetic etch model is developed to explain the behavior of etch rates as a function of surface temperature. Furthermore, results from experiments performed in an Illinois NXE:3100 chamber will be discussed.
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