Research Projects

DEVeX – Divertor Edge and Vapor shielding eXperiment

Published in 2009

The plasma facing components in fusion reactors are subject to a number of high flux/high energy events during normal operation of the device. The Divertor Edge and Vapor shielding eXperiment (DEVeX) was developed to produce a pulsed, high density, high temperature plasmas capable of simulating the conditions in events such as type I Edge Localized Modes (ELMs). Several theoretical models predict that evaporated and ejected material from the surface of the PFCs will become ionized during these events forming a vapor cloud, which will absorb some of the incident plasma energy and reduce the erosion and heat loading on the surface of the PFCs. The end goal of DEVeX is to study the dynamics of the vapor cloud that is formed and provide a benchmark for the theoretical models.

    Lithium magnetron on DEVeX running in an argon atmosphere to deposit lithium on the target surface.

Lithium magnetron on DEVeX running in an argon atmosphere to deposit lithium on the target surface.

Previous work conducted on the ESP-gun experiment showed that in order to produce high density plasmas in a theta-pinch, a fast rise time (< 2 microseconds), high current capacitor bank is required. Subsequently, a capacitor bank is being designed and built that can operate up to 60 kV with 64 kJ of stored energy, which should be more than adequate to produce the desired plasmas.

Several diagnostics, such as a triple langmuir probe (TLP) and an optical spectrometer, are present in the target chamber to measure the density and temperature of the simulated ELM plasma. Additional diagnostics, such as an infrared temperature diagnostic, are planned as the experiment progresses. In order to investigate material erosion by exposure to a burst of high density plasma in a laboratory setting, plasma diagnostics and target analysis of a theta pinch device are carried out. A series of quadruple-probe diagnostics show that plasma sustains approximately for 100 μs at each pulse, with 1.0±0.2(10)21 /m3 plasma density and 12.5±2.5 eV electron temperature. However, when preionization takes place in conjunction with the main bank discharge, density and electron temperature show a few spikes at 1021-1022 /m3 and 30±10 eV. The temperature rise of the target is measured by thermocouple and corresponds to the energy deposition estimated by plasma parameters.

Based on the measurements of the incident plasma parameters, it is shown that the device produces 1021-1022 /m3 and 20-40 eV plasma parameters at various pressure ranges. The temperature changes measured by a thermocouple yield 8±0.3 oC and 33±5 oC for non-preionization and preionzation and they correspond to 3.5±0.1 J, 14±0.5 MW/m2 and 14±2.2 J, 58±8.7 MW/mrespectively. It is in agreement with the calculated energy delivery 2.5±1.7 J and 21±14 J predicted by from measured plasma parameters.

In the near future, DEVeX will be incorporated with the current generation SLiDE project where it will be used or the pushing of divertor/limiter plasma facing components. Such work is greatly important as in the edge and divertor region of the plasma, tokamaks experience off-normal events that cause intolerable damage to components of the reactors, which will become a much more serious issue in ITER. To simulate extreme events in a tokamak and provide a test-stand for liquid lithium plasma-facing components, a pulsed plasma source utilizing a theta pinch in conjunction with a coaxial plasma accelerator has been developed. The ThermoElectric-driven Liquid-metal plasma-facing Structures (TELS) device will provide fusion-relevant plasmas incident on targets with flowing liquid lithium surfaces.

TELS Diagram

TELS Diagram

Theta coil, Pyrex tube, optical diagnostics, guiding magnets

Theta coil, Pyrex tube, optical diagnostics, guiding magnets

(Updated June 2013)

Sn Cleaning with Reactive Ion Etching

Published in 2009

Cleaning Sn off from EUV optics by plasma etching method

Sn is more preferable among fuel materials for EUV light source since Sn has better conversion efficiency than other materials (Xe and Li). However, Sn is a condensable material so that it builds up on the EUV optics (collectors or mirrors). Debris build-up will roughen the surface and accordingly result in reflectivity loss. Therefore, cleaning is very essential step for HVM (high volume manufacture) level Sn EUV source.

The house modified chamber, GALAXY was first used for this study. With chlorine and argon plasma, etch rates of different material were measured and the optimal condition has been found so that Sn can be selectively removed from Ru mirror surface. Using a small mock-up collector in the GALAXY system, cleaning rate dependence on the distance from a plasma source was investigated as well.

Based on the preliminary studies, more study about cleaning and plasma transport are in progress. For the more realistic experiment, a real commercial DPP EUV system (XTS 13-35) is used. By measuring the cleaning rates along the distance from the (cleaning) plasma source using a comparable size of mock-up collector to real ones, we investigate the means of cleaning Sn debris with plasma based method. Understanding plasma transportation through the mirror shells which have different gap width and modeling surface physics between Sn and chlorine plasma is a key contribution to science during this project.

https://youtube.com/watch?v=WGs2JlTYFJE%26amp%3Bhl%3Den%26amp%3Bfs%3D1

IIAX – The Ion-surface InterAction eXperiment

Published in 2009

Crucial to the understanding of plasma material interactions is the interaction of ions with surfaces. Acceleration of ions through the plasma sheath, as well as bombardment from high energy ions heated within the bulk plasma contributes heavily to the modification of surfaces exposed to plasma, particularly high energy plasmas, such as those seen in fusion.

Equipped with two different ion guns, the Ion-Surface Interaction Experiment has the ability to study a wide range of scenarios involving ion bombardment on surfaces. A Colutron ion gun allows for velocity and neutral filtered ion bombardment between 1 eV and 2 keV. A NTI lithium ion gun adds to the ion bombardment capability of the system. Magnetic sector and quadrupole residual gas analyzers as well as a quartz crystal microbalance are included to diagnose the interaction of accelerated ions with various surfaces.

The diagnostic suite allows for monitoring of sputtering yields as well as desorption of gaseous chemical sputtering products. IIAX in tandem with the Material Research Laboratory at the University of Illinois can also investigate changes in surface chemistry and morphology from ion bombardment.

(Updated 2013)

SHADE – Sputtering High-purity Atomic Deposition Experiment

Published in 2009

Sputtering High-purity Atomic Deposition Experiment (SHADE)

The SHADE chamber utilizes an ultra-high vacuum environment with a dual magnetron setup to deposit thin films and a secondary RF antenna in order to tailor the deposited film properties. SHADE is a valuable research tool allowing for a variety of sputtering targets to easily be exchanged and a loadlock system to transfer samples for deposition in and out without breaking vacuum.

Experimental Apparatus

SHADE_chamberSHADE is able to reach ultimate pressures in the 10-8 Torr range by using a turbomolecular pump as well as a cryo pump. However, operating pressures are typically between 1 and 20 mTorr with high stability through the use of two mass flow controllers (MFCs). Having two MFCs allow for multiple gases to be run simultaneously, a necessary ability for reactive sputtering.

Samples are transferred in and out of the chamber via a loadlock and gate valve system in order to maintain the integrity of the vacuum and to allow quick turnaround between experimental runs. Samples can be as large as two inches wide and still easily fit in the transfer mechanism. Once loaded into the chamber, the sample holder can be steadily rotated by a stepper motor in order to enhance uniformity during deposition.

SHADE_plasmaFilm thickness is monitored during deposition through the use of a dual quartz crystal microbalance (QCM). The QCM consists of two quartz crystals (one shielded and one exposed) which are oscillating at a very high frequency. As the exposed crystal receives deposition, the frequency slows down and the QCM interprets that as a film thickness. The shielded crystal is used as a check that any detected frequency change is due to actual deposition and not temperature change.

The plasma is monitored by an optical emission spectrometer which can detect wavelengths in the range of 300nm to 2000nm. The intensity of specific emission lines are monitored to ensure that the plasma remains stable and can be used to adjust processing parameters to achieve optimal results.

plasma-donutSHADE can be equipped with either two two-inch or three-inch magnetrons, depending on the size of the s

puttering target being used. Whereas the two-inch magnetrons are a balanced closed-field configuration,the three-inch magnetrons can be either balanced or unbalanced and open- or closed-field through the use of interchangeable magnet packs. This allows for flexibility in plasma density and confinement.

RF Antenna

When using the three-inch magnetrons, an RF antenna can be installed between them in order to create a secondary plasma besides the sputtering plasma. This secondary plasma increases the ionization of otherwise neutral gas and sputtered species which increases the sputtering rate and adds additional energy to the deposited film without significantly increasing the temperature of the sample. This additional energy allows for better film quality while maintaining a near room-temperature substrate. This high-quality, low-temperature deposition can therefore be used on plastic substrates which would otherwise melt at the higher temperatures required to achieve some high-quality films without a secondary plasma. Adjusting the RF power varies the power being delivered to the thin film and therefore the crystal structure of the film. Typical RF plasma power is 100-300W.

Research

With such a flexible system there are a several areas of research which have been explored. Previous research has developed Gibbsian-segregated alloys for use in semiconductor manufacturing optics. High reflectivity mirrors are needed in order to bounce the extreme ultraviolet light without intensity loss. These mirrors are prone to damage but by incorporating these specialized coatings they can become “self-healing” so that when they are contaminated, the contamination segregates into the bulk of the film while leaving the surface structure intact, and therefore reflective.

raw_AZOCurrent research using SHADE involves deposition of transparent conducting oxides (TCOs) such as Indium Tin Oxide (ITO) and Aluminum-doped Zinc Oxide (AZO). TCOs are a class of materials with an increasing presence in digital displays and photovoltaic applications. They allow light to be transmitted through them while still being able to conduct electricity. The most commonly used TCO is ITO but it has the drawback of using Indium, an expensive material with uncertain world reserves. Therefore alternatives are being sought, such as AZO, which can maintain the desired material properties but at a lower price. SHADE is being used to deposit AZO with the secondary RF plasma to achieve low-temperature deposition of high-quality films on flexible plastic substrates. The goal is to develop flexible photovoltaic sheets for low-cost power production.

PACE – Plasma Assisted Cleaning by Electrostatics

Published in 2009

Objective Statement

The goal of PACE is to show how a helium plasma can reduce or eliminate organic contamination from materials used in extreme ultraviolet lithography.

Why PACE Has Taken on This Task

Reliability and speed in computing are two key aspects that continue to drive the economic and personal lives of individuals in today’s society. These two key aspects of computing are achieved through the continued development and refinement of the way in which integrated circuits (IC) are manufactured. In order to obtain more functionality out of a computer chip, more transistors are needed on that chip compared to previous generations. This proves to be a difficult hurdle to overcome, either the features (transistors) need to become smaller to fit more of them into the same amount of real-estate, or the chip needs to become physically larger and thus leads to larger devices. The trend has been to decrease the size of the transistors to increase the number of them per chip, however, this presents many complications for the design and fabrication engineers. In 1965, Gordon Moore made the observation that the number of transistors on a chip would double every two years [1]. To keep the trend now referred to as Moore’s law applicable, the shift to extreme ultraviolet lithography (EUVL) at 13.5 nm is seen as one solution in order to achieve this [2]. The idea of EUVL has led to a huge scientific undertaking by both private industry and Universities in order to produce the concepts and machines capable of making EUVL possible. There have been numerous challenges in both optical design, source design, photoresist sensitivity, and lithographic mask that have stood in the way as hurdles to EUVL’s implementation [2]. The subject of a lot of these hurdles has led to great advances in science and engineering, as well as aided numerous students to partake in research and development for both academic and industrial purposes. EUVL has the potential to be the next large step and revolutionize the way IC’s are manufactured. The ability to manufacture IC’s with a direct pattern technique instead of phase-shift masks or double patterning, as is the current path of IC manufacturing with current 193 immersion technology, will provide IC designers with more tools and abilities in the design of IC’s to further their applicability to the desire for increased computing power. One of the largest hurdles facing EUVL implementation is contamination in the EUVL system, particularly contamination of the photomask in the form of particulate defects and thin film contamination and carbon buildup and contamination of the collector optics. These key aspects need to be addressed and a high volume manufacturing solution determined before EUVL can be successfully implemented.

Contamination and Its Effect on EUVL

Integrated circuits are the combination of miniaturized electronic circuits and passive elements built into the surface of a semiconductor material such as silicon. There are several steps that are required to form an integrated circuit, these steps are: imaging, deposition, and etching [3]. To make a single device, several processing steps focused around the previously mentioned three are conducted. Supplementary steps such as cleaning and planarization[3] are also required to ensure the functionality of devices and to make the processing steps more precise.

EUV Lithography

As shown in Figure 1.1, there is no pellicle between the mask and the wafer due to the non-transparency of materials to 13.5 nm wavelength light. Also, the mask requires illumination from the side and at an angle due to the inability of a passthrough type design which is also due to the non-transparency of material to the 13.5 nm wavelength. Thus, whereas in an optical lithography system utilizing a transmissive mask, a pellicle can be used to reduce the number of defects that reach the mask, in an EUV system, this is less likely to be the case.

1.1

Figure 1.1: A diagram of a particle falling onto an EUV lithography mask. If the particle remains on the mask, then it will block light reflecting off of the mask and appear to be another feature not intended by the mask design which will cause errors and device failures.

The alternating light/dark layers beneath the dense features are the multilayers. The multi-layers are specifically designed to reflect EUV light. NOTE: the size of the wafer/resist are not to size but have been enlarged for clarity and the incoming light is not at as extreme an angle.

EUV Mask Fabrication

There is an additional issue for particle contamination when considering EUV lithography. In forming the alternating multilayers for the reflective properties of the EUV mask, particle control is important. As seen in Figure 1.2, if a particle falls onto the surface during multilayer deposition, the overall smoothness of the mask is affected. Thus, by effective particle and contamination control or an effective cleaning process, the mask smoothness before patterning can be maintained to provide a defect free mask during fabrication.

1.2

Figure 1.2: A diagram of a non-patterned EUV mask. To fabricate the mask, a blank 6.35 mm thick quartz blank is used to alternatively deposit the reflective layers. If a particle is present, it will perturb the smoothness and effect the mask properties.

EUV Optics Degradation Due to Carbon Contamination

In addition to the mask degradation due to particulate contamination outlined in section 1.3, the high intensity photon irradiation of other components of the system, namely the EUV mirrors which compose the collector array, can become contaminated. It has been shown that the high energy photon interaction with collector mirrors in the presence of background hydrocarbons in the vacuum of the proposed EUV lithography tool will result in a buildup of carbon layers on the mirror [4]. This resulting carbon layer can be cleaned by atomic hydrogen, plasma etching, or through use of molecular oxygen [4]. However, these cleaning techniques can lead to damage of the surface through either physical damage, chemical attack, or oxidation [4].

How PACE Works

When a plasma is formed, energetic ions and electrons are created in the plasma and are typically what are important to the engineer creating the plasma. However, there is a strong energetic neutral component in plasmas too in the form of metastable atoms. Metastable atoms are those that are stuck in a quantum state and are forbidden through conservation of momentum from decaying to the ground state (such as 2s to 1s transitions being forbidden). Thus, any electron entering the 2s state will be hypothetically `stuck.’ In general, the lifetime of these metastable atoms is low, so they don’t often play a significant role. There are two metastable states in helium, singlet 2s and triplet 2s, with 20.616 eV and 19.820 eV of energy each as seen in Figure 3.1 [28, 29] with lifetimes on the order of seconds. Helium plasma has been used for the current work due to its low sputtering threshold on lithographic material (due to its low mass), in order to achieve particle removal. Under some conditions of prior work, namely the electrostatic removal process associated with PACE, a pulsed DC bias was applied to the sample [24]. This same type of processing has been carried forward to the current work due to the pulsed power supply that was previously used able to draw more current. The pulse typically operated between +10 volts and -70 volts at a frequency of 100 Hz at a duty cycle of 90 % positive. This leads to sputtering of the sample at -70 volts for 10 % of the processing time. Section 3.1 outlines the expected removal of three material types from pure sputtering alone which when compared with the preliminary results, does not explain the removal mechanism. Through optical emission spectroscopy, it is possible to monitor the 501.6 nm and 388.9 nm transition that populate the 2s singlet and 2s triplet state respectively. As shown in Figure 3.2 and Figure 3.3, one is able to affect the density of the metastable states in the plasma through both bias to the sample as well as overall plasma source power.

3.1

Figure 3.1: A diagram of the energy levels of the helium atom. The two circled levels represent the metastable energy levels which are quantum mechanically forbidden from decaying to the ground state energy due to the conservation of angular momentum. Diagram from Sasaki et. al. [28]

Sputtering of Material

Sputtering yields for incident ions on a variety of surfaces can be determined through simulation via SRIM/TRIM [30]. Figure 3.4 shows the results of this simulation, and specific sputtering yields for 70 eV helium ions are given in Table 3.1 for various materials. The ion current being drawn through the sample is measured to be 1.6 Ǻcm-2. Read More>>

SLIP – Study of Lithium Infusion in Porous media

Published in 2009

Innovative materials are created every year as candidates for fusion first-walls and the like. One such material is a porous molybdenum foam specifically proposed to hold a volume of liquid lithium within a fusion experiment. The SLIP experiment was created to examine the wetting behavior of this foam. Its follow-up experiment, SLIP2X, is a plasma source designed to impinge a porous metal target with infused lithium and examine the changes in bulk plasma properties.

Experiments conducted on a number of fusion machines around the world have shown the benefits of deploying lithium plasma facing surfaces (PFCs) in the machines. The list includes TFTR, T11-M, CDX-U, FTU, JT-II and NSTX. Of these machines, TFTR and JT-II have only applied lithium via pellet injection or evaporation. T11-M, CDX-U(early phases) and FTU have utilized a porous metal mesh with liquid lithium wicked into the pores by capillary action. This method restrains the liquid lithium from ejection into the plasma and machine while still maintaining a liquid lithium surface to the plasma. CDX-U has implemented a fully liquid lithium limiter and NSTX will implement a fully liquid lithium divertor.

In order to prevent ejection of the liquid lithium into the machine (as was seen on the Li-DIMES DIII-D experiment), a porous material has been proposed. One such material, fabricated by ULTRAMET was provided to UIUC for examination. Testing was carried out to determine the wetting characteristics of the porous molybdenum mesh and its re-usability. The SLIP experiment was created to examine this and results were recently published in the Journal of Nuclear Materials.

A follow-up experiment has been constructed to diagnose a fusion relevant plasma in contact with a liquid lithium PFC. This experiment has been dubbed “SLIP2X”. It is a magnetized plasma gun with an internal solenoid field similar to a planar spheromak gun (cf. Bellan’s research group at CalTech). The machine pulses energy into the plasma heating it to 10s of eVs at densities above 10^19 m^-3. Probe diagnostics and magnetic diagnostics are utilized to characterize the plasma. SLIP2X is primarily intended for development of diagnostics before implementation on the DEVeX plasma source and as an educational tool.

XCEED – XTREME Commercial EUV Exposure Diagnostic

Published in 2009

An extreme ultraviolet light source is investigated at the University of Illinois. The source is a z-pinch plasma using Xenon gas with a short pulse width (~1 ms). As the plasma compresses, high energy photons in the extreme ultraviolet range are released and available for EUV lithography. The light emission is followed by ejection of multiply charged ions (+8-+10 Xe) which can significantly damage nearby mirror surfaces. After the fast ions pass, a slower moving cloud of ions and neutrals including Xenon and electrode material spread from the source causing continued coating and sputtering of the mirror surfaces.

The XCEED experiment serves to characterize the ejecta from a commercial extreme ultraviolet light source, investigate the damage mechanisms that affect nearby optics lifetime, and evaluate debris mitigation techniques that may be used to increase the lifetime of these mirror optics.

Characterization of the ejecta is performed through several diagnostics. A spherical sector energy analyzer (ESA) is used to diagnose fast ion species by energy-to-charge ratio using ion time of flight (ITOF) analysis. This instrument is capable of characterizing up to 15 keV ions emitted from the source. For neutral characterization, a set of Burle microchannel plates is placed directly in line with the pinch debris and a deflecting potential is used to divert ions from the detector. Signals from particle incidence on a Faraday Cup are also observed.

The effects of particle flux on mirror samples is investigated through exposure experiments and surface analysis. Samples are placed at normal and grazing incidences to the incoming particles and exposed for varying timeframes to the source. Photodiodes measure reflectivity degradation over time. The samples are removed and examined using Scanning Electron Microscopy (SEM), Atomic Force Microscopy, and other methods to analyze surface quality and particle deposition.

Recent modifications of the XCEED system also allow for the investigation of laser assisted discharge produced plasmas (LADPP) with a solid Sn electrode as the EUV fuel source. The laser ablates Sn atoms into the pinch produced plasma, where is it becomes highly ionized and emits a spectrum of light. A small portion of this light is transmitted in the EUV spectrum. The shift to Sn is ongoing within industry due to the increase in EUV emission conversion efficiency seen by Sn over Li and Xe. Laser ablation is carried out in the modified XCEED system using an Nd:YAG laser (100 Hz, 325 mJ/pulse, ~10ns pulse, capable of frequency doubling to 532nm and 266nm).

https://youtube.com/watch?v=8id5wzIbBWs%26amp%3Bhl%3Den%26amp%3Bfs%3D1

https://youtube.com/watch?v=QuwwO1iradI%26amp%3Bhl%3Den%26amp%3Bfs%3D1