Extreme ultraviolet (EUV) lithography sources expel Sn debris. This debris deposits on the collector optic used to focus the EUV light, lowering its reflectivity and EUV throughput to the wafer. Consequently, the collector must be cleaned, causing source downtime. To solve this, a hydrogen plasma source was developed to clean the collector in situ by using the collector as an antenna to create a hydrogen plasma and create H radicals, which etch Sn as SnH4. This technique has been shown to remove Sn from a 300-mm-diameter stainless steel dummy collector. The H radical density is of key importance in Sn etching. The effects of power, pressure, and flow on radical density are explored. A catalytic probe has been used to measure radical density, and a zero-dimensional model is used to provide the fundamental science behind radical creation and predict radical densities. Model predictions and experimental measurements are in good agreement. The trends observed in radical density, contrasted with measured Sn removal rates, show that radical density is not the limiting factor in this etching system; other factors, such as SnH4 redeposition and energetic ion bombardment, must be more fully understood in order to predict removal rates.
An in-situ hydrogen plasma cleaning technique to clean Sn off of EUV collector optics is studied in detail. The cleaning process uses hydrogen radicals (formed in the hydrogen plasma) to interact with Sn-coated surfaces, forming SnH4 and being pumped away. This technique has been used to clean a 300mm-diameter stainless steel dummy collector optic, and EUV reflectivity of multilayer mirror samples was restored after cleaning Sn from them, validating the potential of this technology. This method has the potential to significantly reduce downtime and increase source availability. However, net Sn removal is limited by decomposition of the SnH4 molecule upon impact with the collector and the resulting redeposition of Sn. This is true in all cleaning systems that make use of hydrogen radicals. Thus, to guide the design of effective cleaning systems, the transport of Sn in the chamber, and the fundamental processes affecting it, must be understood. Accordingly, an investigation into these processes Sn removal is being performed. These processes include the advection of gas through the chamber, the creation of hydrogen radicals, the etching of Sn by radicals, and the surface decomposition of SnH4. In this paper, experiments to determine the radical density are presented, along with a theoretical plasma chemistry model that explains the processes behind radical creation and validates the radical density measurements. Additionally, experiments are shown that provide an insight into the etching of Sn by hydrogen radicals, yielding calculations of etching probability as well as showing that Sn etching is very sensitive to oxygen contamination and surface morphology.
A theoretical model for describing the propagation and scattering of energetic species in an extreme ultraviolet (EUV) light lithography source is presented. An EUV light emitting XTREME XTS 13-35 Z-pinch plasma source is modeled with a focus on the effect of chamber pressure and buffer gas mass on energetic ion and neutral debris transport. The interactions of the energetic debris species, which is generated by the EUV light emitting plasma, with the buffer gas and chamber walls are considered as scattering events in the model, and the trajectories of the individual atomic species involved are traced using a Monte Carlo algorithm. This study aims to establish the means by which debris is transported to the intermediate focus with the intent to verify the various mitigation techniques currently employed to increase EUV lithography efficiency. The modeling is compared with an experimental investigation.
Extreme ultraviolet (EUV) lithography sources produce EUV photons by means of a hot, dense, highly-ionized Sn plasma. This plasma expels high-energy Sn ions and neutrals, which deposit on the collector optic used to focus the EUV light. This Sn deposition lowers the reflectivity of the collector optic, necessitating downtime for collector cleaning and replacement. A method is being developed to clean the collector with an in-situ hydrogen plasma, which provides hydrogen radicals that etch the Sn by forming gaseous SnH4. This method has the potential to significantly reduce collector-related source downtime. EUV reflectivity restoration and Sn cleaning have been demonstrated on multilayer mirror samples attached to a Sn-coated 300mm-diameter steel dummy collector driven at 300W RF power with 500sccm H2 and a pressure of 260mTorr. Use of the in-situ cleaning method is also being studied at industriallyapplicable high pressure (1.3 Torr). Plasma creation across the dummy collector surface has been demonstrated at 1.3 Torr with 1000sccm H2 flow, and etch rates have been measured. Additionally, etching has been demonstrated at higher flow rates up to 3200sccm. A catalytic probe has been used to measure radical density at various pressures and flows. The results lend further credence to the hypothesis that Sn removal is limited not by radical creation but by the removal of SnH4 from the plasma. Additionally, further progress has been made in an attempt to model the physical processes behind Sn removal.
In extreme ultraviolet (EUV) lithography, plasmas are used to generate EUV light. Unfortunately, these plasmas expel high-energy ions and neutrals which damage the collector optic used to collect and focus the EUV light. One of the main problems facing EUV source manufacturers is the necessity to mitigate this debris. A magnetic mitigation system to deflect ionic debris by use of a strong permanent magnet is proposed and investigated. A detailed computational model of magnetic mitigation is presented, and experimental results from an EUV source confirm both the correctness of the model and the viability of magnetic mitigation as a successful means of deflecting ionic debris.
The presence of Sn on the collector optic of an extreme ultraviolet (EUV) light lithography tool continues to be a
concern for source manufacturers. A mere nanometers deposition results in reduction of EUV light reflectivity to
unacceptable levels. It has been shown previously that hydrogen radical etching of Sn provides a promising technique for
in-situ cleaning of the collector optic. One concern in this technique is the redeposition by radicalized SnH4 breaking
apart after making contact with a surface. To address this concern, large scale etching measurements were made using a
metallic antenna as the substrate. Optimized etch rates approaching 7.5±1 nm/min have been achieved with a flow rate
of 500 sccm at a pressure of 80 mTorr. The effect of variations in the Sn cleaning environment will be investigated with
respect to temperature increases as well as air, oxygen, and methane contamination gasses. Furthermore, the effect of Sn
located away from the cleaning location will also be presented.
The emission of species that can chemically or physically alter the surface of post intermediate-focus optics will increase the cost of ownership of such an EUV lithography tool past the point of cost effectiveness. To address this
concern, the Center for Plasma-Material Interactions has developed the Sn Intermediate Focus Flux Emission Detector (SNIFFED). The effects of increasing buffer gas, increasing pressure, and chosen buffer gas species will be presented. Furthermore the presence of a secondary plasma, generated by EUV light will be analyzed and exposed as a potential issue in the strive for a contaminant free intermediate focus.
Extreme Ultraviolet (EUV) lithography sources expel high-energy ions and neutral particles, which degrade the quality of the collector optic. The mitigation of this debris is one of the main problems facing potential manufacturers of EUV sources. The use of magnetic fields to deflect ionic debris has been proposed and is investigated here. In this paper, we present a detailed computational model of magnetic mitigation, along with experimental results that confirm the correctness of the model. Using a strong permanent magnet, it is experimentally shown that, using high enough fields, magnetic mitigation can be a successful method of deflecting ionic debris from an EUV source. For example, through an orifice centered at 0° from the pinch, we saw a flux of 1.65×108 +/- 1.5×107 ions/(m2*pulse*eV) of 4keV ions without deflection and a negligible flux with deflection.
With the orifice at a 35° angle from the pinch, a negligible 4keV flux was seen without deflection. However, with
magnetic deflection, a 4keV flux of 1.03x108 +/- 9.4x106 ions/(m2*pulse*eV) were seen. The half-angle spread of the orifice was .047° with a tolerance of .008°.
One of the main challenges in extreme ultraviolet lithography (EUVL) is the development of a method for cleaning
collector optics without inhibiting cost-effectiveness. Cost-effectiveness of EUV methods can be increased by in-situ
processes for removing debris placed on the collector optic. This paper focuses on the use of a hydrogen plasma to
remove Sn, a common EUV fuel, from Si surfaces. Sn was deposited on both large and small Si samples via magnetron
sputtering, and optimized hydrogen plasma selectively etched the Sn. Deposition uniformity and thickness are
measured, as are Sn etch rates and cleaning uniformity. Positive results indicate the potential of this method for use in
cleaning EUV mirrors.
The Center for Plasma-Material Interactions has developed a detector capable of diagnosing the energetic ion
and neutral spectrums emanating from extreme ultraviolet light sources. This tool has been used in the past for
high-power output sources, but it is readily evident that actinic inspections tools require the use of debris
mitigation analyzers. Using this tool, manufacturers can optimize the use of debris mitigation techniques, as
well as analyze the effects brightness increases have on tool lifetime.