Experiments

The experiment will involve the design and implementation of a vacuum oven to produce clean samples of SnLi alloys at multiple mass ratios. The samples’ material properties will then be examined, which include surface wetting and vapor pressure.

I am currently researching how tin ions are stopped as they travel through hydrogen plasma. Tin plasmas are generated during the production of extreme ultraviolet light (EUV) when molten microdroplets of tin are struck by a CO2 laser. The contamination of critical vessel components with tin is a serious concern for ASML, the project sponsor, and they are interested in learning more about how ions from the tin plasma are stopped by the background gases and plasmas in their vessel. I am currently using an ion beam installed on the chamber to measure how an ion beam attenuates as it travels through both hydrogen gas as well as hydrogen plasma generated by a helicon plasma source.

Extreme ultraviolet (EUV) lithography tools are the leading technologies responsible for the next generation of chip manufacturing. EUV has shown to accurately and precisely pattern wafers but still lags in the wafer production rate compared to previous high-volume manufacturing. Source availability is a key issue to be addressed in order to achieve a higher speed.1 In an EUV source, a plasma is generated when a CO2 pulsed laser hits molten droplets of tin, and it ionizes tin in the +8 to +12 range, which then emit 13.5 nm photons. A multilayer mirror (MLM) collector reflects the light toward a series of optic mirrors until EUV lights reaches the wafer.2 During de-excitation of the Sn plasma, droplets and atoms of Sn deposit on the inner surfaces of the source, including the walls and the collector mirror. Sn film deposits absorb 13.5nm photons, reducing the reflectivity that leads to severe drop in source power at the wafer. Conventional Sn cleaning is a multiday process, thus causing major downtime for the tool. An in situ tin etch method increases lifetimes of the EUV source including collector and plasma facing components. One advantage of using hydrogen gas inside an EUV source is that hydrogen radicals are produced and chemically etch Sn away to form tin hydride.3 Having shown previously the ability of tin etching in hydrogen plasmas4 and recovery of near total reflectivity,5 this study shows Sn removal from any surface in an EUV source as a function of surface temperature. Also, previous work has shown that the highest etch rates occur when plasma is in the reactive ion etch regime. A linear surface wave plasma (SWP) is used replicating actual source conditions (pressure, gas flow rate, and geometry) in the Illinois NXE:3100 system. SWP has high hydrogen radical and ion densities, while electron and ion energies are low in order to avoid damage to the MLM. Very high etch rates have been observed, up to 200nm/min with ion and radical densities on the order of 10^12 to10^13 cm3 and 10^14 cm3, respectively. A custom launcher and an antenna structure were built to interface with the present source geometry. At CPMI, we have tested semi-circular SWP antennas with etch rates as high at 270nm/min.

The Sealing Materials Degradation Chamber (SMDC) is designed to develop a deep and specialized knowledge around interactions of plasma with sealing materials. The SMDC is designed to test seals in both ion-rich and radical-rich environments, with the diagnostic capability to quantitatively understand these regimes. The IC fabrication industry requires seals that are robust over a long lifetime, and the knowledge gained in SMDC will help to push sealing technology forward.

Written by Nicholas Connolly

People Working on Experiment: Nicholas Connolly

The Sealing Materials Degradation Chamber (SMDC) is designed to develop a deep and specialized knowledge around interactions of plasma with sealing materials. The SMDC is designed to test seals in both ion-rich and radical-rich environments, with the diagnostic capability to quantitatively understand these regimes. The IC fabrication industry requires seals that are robust over a long lifetime, and the knowledge gained in SMDC will help to push sealing technology forward.

The Tungsten Fuzz Characterization by Helicon (TUFCON) chamber was originally used to investigate the formation of tungsten fuzz after helium plasma exposure. This work performed parametric sweeps over ion energy and fluence to determine the causes of the fuzz formation. Recently, the device has been used to determine the effect of hydrogen plasma parameters and radical density on the spitting of microdroplets from liquid metals (tin, lithium, tin-lithium).Upon radical or plasma exposure, hydrogen bubbles form in the molten metal, which eventually migrate to the surface and burst, releasing microdroplets of liquid metals. This could be extremely detrimental to fusion plasma operation, as droplets entering the core plasma would reduce the plasma performance. In an effort to better understand these droplets, the plasma in TUFCON is being fully characterized by Langmuir probe and radical probe to determine plasma temperature, plasma density, and radical density. After the plasma is fully characterized, a microdroplet size distribution will be made as a function of liquid temperature and plasma parameters. Eventually, the aim is to implement techniques which can mitigate the droplet emission or prevent the droplets from entering the core plasma.

Use of lithium as a plasma-facing component in fusion reactors has been shown to increase plasma performance. This is in part due to lithium’s ability to create a low-recycling regime inside the device. These low-recycling regimes are characterized by an absorbing wall that does not allow cold particles to escape back into the hot plasma. Lithium is able to getter any of the fusion fuel or impurity species that make it to the wall. However, the absorption of fusion fuel, specifically tritium poses some concerns for the long-term use of lithium in fusion devices.

At the Center for Plasma-Material Interactions, we are working to develop a variety of flowing lithium systems, along with hydrogen removal systems. The Hydrogen Desorption Experiment (HYDE) is working to develop a distillation column to use thermal desorption as a means to remove hydrogen species from the lithium bulk. This system has previously shown success at hydrogen removal in highly saturated lithium. Ongoing research is focusing on determining the efficiency of the device as a function of hydrogen saturation and developing specifications for its implementation into flowing system under reactor relevant conditions.

High energy neutrons (~14.1 MeV) are of interest in both fusion and industrial applications. Typically, these neutrons are generated through beam-target fusion with the use of tritium. Unfortunately, tritium is radioactive and strictly controlled, making it quite expensive to handle. The Compact Liquid Lithium Neutron Source (COLLINS) device at the Center for Plasma-Material Interactions (CPMI) aims to examine the production of these high energy neutrons through the D-Li7 reaction. A compact, floating source ion beam accelerates the deuterium and focuses it on a self-contained, flowing liquid lithium target. The flow is self-driven via thermo-electric magnetohydrodynamics (TEMHD) through the use of permanent magnets and the heat flux from the ion beam. This experiment aims to investigate the neutron output and energy spectrum as well as the heat flux handling of this target.

The Solid/Liquid Divertor Experiment (SLIDE) is a high vacuum experiment aimed to test a variety of divertor concepts developed at the Center for Plasma Material Interactions. An electron beam is used to replicate the heat flux produced in the divertor region of fusion devices. Beam shape and peak intensity can be controlled be changing the magnetic field in the device. SLIDE has tested a variety of divertor concepts and most recently has been used for testing the Liquid Metal Infused Trenches (LIMIT) technology. LIMIT utilizes thermo-electric magnetohydrodynamics to drive the flow when a heat flux (thermal gradient) is applied to the surface. This technology aims to provide a constant, clean liquid lithium surface to the fusion divertor which can handle the massive heat flux in fusion devices and consistently provide the plasma performance benefits of lithium. Recently, work has been done to test the heat flux handling of a variety of different tile geometries, including 3D printed trench designs. In the near future, SLIDE will be incorporated in the Actively Pumped Open-surface Lithium Loop (APOLLO) to demonstrate LIMIT’s ability to be used in a full flowing loop system.

In order to be able to fully utilize liquid metals in fusion devices, their corrosive attack on a variety of materials needs to be known. Particularly, lithium attack on the structural components of a device are of concern.

Previous work has looked at static lithium corrosion, often under extreme temperatures. The Spinning Lithium Attacking Potential Substrates (SLAPS) device at the Center for Plasma-Material Interactions investigates the effect of rotating lithium at relevant temperatures on the corrosion behavior of potential materials. SLAPS consists of a spindle with a variety of mounted samples which rotate in a bucket of molten lithium at operating temperatures (~300C) for multiple days (~100 hrs).Rotation allows the investigation of fluid shear on the corrosion dynamics. Mechanical properties of the materials are of prime concern for this project, so each material undergoes tensile testing before and after lithium exposure to determine how lithium has changed material performance. Additionally, 3D optical profilometry is employed to determine if microscopic surface changes have occurred. SLAPS will allow us to provide candidate materials for use in a variety of components in fusion devices.