Vijay Surla

An Investigation of Magnetic-Field-Assisted Material Removal in Micro-EDM for Nonmagnetic Materials

Previous magnetic-field-assisted microelectrical discharge machining (μ-EDM) techniques have been limited to use with magnetic materials. Therefore, a novel process has been developed and tested to improve material removal rate in magnetic-field-assisted μ-EDM for nonmagnetic materials. The workpiece electrodes were oriented to promote directionality in the current flowing through the workpiece, while an external magnetic field was applied in such a way as to produce a Lorentz force in the melt pool. Single-discharge events were carried out on nonmagnetic Grade 5 titanium workpieces to investigate the mechanical effects of the Lorentz force on material removal. Erosion efficiency, melt pool volume analysis, plasma temperature, electron density, and debris field characterization were used as the response metrics to quantify and explain the change in material removal with the applied Lorentz force. By orienting the Lorentz force to act in a direction pointing into the workpiece surface, volume of material removed was shown to in-crease by up to nearly 50%. Furthermore, erosion efficiency is observed to increase by over 54%. Plasma temperature is unaffected and electron density shows a slight decrease with the addition of the Lorentz force. The distribution of debris around the crater is shifted to greater distances from the discharge center with the Lorentz force. Taken together, these facts strongly suggest that the Lorentz force process developed produces a mechanical effect on the melt pool to aid in increasing material removal. The application of the Lorentz force is not found to negatively impact tool wear.

Characterization of an Atmospheric-Pressure Helium Plasma Generated by 2.45-GHz Microwave Power

An atmospheric-pressure helium plasma generated in a 2.45-GHz microwave-induced cylindrical plasma torch has been investigated. The atmospheric-pressure helium plasma can be ignited at a microwave power of less than 400 W and sustained at as low as 100 W, at a gas flow of 50 L/min. The electron temperature T_e, electron density n_e, and gas temperature T_g measured by optical emission spectroscopy are 0.28-0.5 eV, 10¹⁵-10¹⁶ cm^-3, and 1000-2000 K, respectively, at a power of 200-400 W and a gas flow rate of 20-50 L/min. In addition, it is noted that T_e ≈ T_v (vibrational temperature) >; T_g ≈ T_r (rotational temperature) for these experiment parameters. These results show that the helium plasma generated in this atmosphericpressure plasma torch is in partial local thermodynamic equilibrium (pLTE).

High-energy density beams and plasmas for micro- and nano-texturing of surfaces by rapid melting and solidification

Several advances in materials research have been made due to the wide array of tools currently available for the processing of materials: plasmas, electron beams, ion beams and lasers. The area of material science is fortunate to have seen the development of these tools over the years, be it for new bulk materials, coatings or for surface modification. Several applications have benefited and many more will in the future as the properties of the materials are altered on a micro/nanoscale. Currently, several techniques exist to modify the physical, chemical and biological properties of the material surface; however, this review limits itself to surface modification applications using the rapid thermal processing (RTP) technique. First, a brief overview of the existing surface modification methods using the principles of RTP is reviewed, and then a novel method to create micro/nanostructures on the surface using pulsed plasma exposure of materials is presented.

Removal of particles from lithographic masks through plasma-assisted cleaning by metastable atomic neutralization

For extreme ultraviolet lithography (EUVL) to become a high volume manufacturing technology for integrated circuit manufacturing, the cleanliness of the system, especially the photomask, is of high importance. For EUV photomasks, which cannot be protected from contamination by the use of a pellicle, an effective and quick cleaning technology needs to be ready in order to maintain wafer throughput. There are challenges to extend current wet cleaning technologies to meet the future needs for damage-free and high efficiency mask cleaning. Accordingly, a unique process for cleaning particulates from surfaces, specifically photomasks as well as wafers, has been evaluated at the University of Illinois Urbana-Champaign. The removal technique utilizes a high density plasma source as well as pulsed substrate biases to provide for removal. Helium is used as the primary gas in the plasma, which under ionization, provides for a large density of helium metastable atoms present in the plasma. These metastable helium atoms have on the order of 20 eV of energy which can transfer to particles on the substrate to be cleaned. When the substrate is under a small flux of ion bombardment, these bonds then remain broken and it is theorized that this allows the particles to be volatilized for their subsequent removal. 100 % particle removal efficiency has been obtained for 30 nm, 80 nm, and 200 nm polystyrene latex particles. In addition, removal rate has been correlated with helium metastable population density determined by optical emission spectroscopy.

Removal of carbon and nanoparticles from lithographic materials by plasma assisted cleaning by metastable atom neutralization (PACMAN)

System cleanliness is a major issue facing the lithographic community as the prospects of integrating EUV lithography into integrated circuit manufacturing progress. Mask cleanliness, especially of particles in the sub-micron range, remains an issue for the implementation of EUV lithography since traditional mask cleaning processes are limited in their ability to remove nanometer scale contaminants. The result is lower wafer throughput due to errors in pattern transfer to the wafer from the particulate defects on the mask. Additionally, carbon contamination and growth on the collector optics due to energetic photon interactions degrade the mirror and shortens its functional life. Plasma cleaning of surfaces has been used for a variety of applications in the past, and now is being extended to cleaning surfaces for EUV, specifically the mask and collector optics, through a process developed in the Center for Plasma-Material Interactions (CPMI) called Plasma Assisted Cleaning by Metastable Atom Neutralization (PACMAN). This process uses energetic neutral atoms (metastables) in addition to a high-density plasma (Te ≈ 3 eV and ne ≈ 1017 m-3) to remove particles. The PACMAN process is a completely dry process and is carried out in a vacuum which makes it compatible with other EUV related processing steps. Experiments carried out on cleaning polystyrene latex (PSL) nanoparticles (30 nm to 500 nm) on silicon wafers, chrome coated mask blanks, and EUV mask blanks result in 100 % particle removal with a helium plasma and helium metastables. Removal rates greater than 20 nm/min have been achieved for PSL material. Similar removal rates have been achieved for the PACMAN cleaning of carbon from silicon wafers (simulating collector optic material) with 100% removal with helium plasma and helium metastables. The PACMAN cleaning technique has not caused any damage to the substrate type being cleaned either through roughening or surface sputtering. Current results of cleaning various particle types from surfaces through the PACMAN process are presented.

Sputtering and Thermal Evaporation Studies of Lithiated ATJ Graphite

Sputtering yields and thermal evaporation fluxes of lithium and lithiated ATJ graphite are studied. Sputtering yields are measured for the ATJ graphite by lithium ion bombardment at 45° incidence, with 700-2000 eV accelerations. Typically, 4 × 1013 ions/(cm2 · s) flux of Li ion beam is obtained from LiCl powder in a Colutron ion source. Sputtered particles are collected by a quartz crystal microbalance to determine sputtering yields. The experiment is repeated after Li is evaporated onto the ATJ graphite target. Suppressed amounts of sputtered particles are observed after Li treatment. Deuterium (D) saturation treatment for lithiated graphite is also done to simulate actual divertor conditions. The sputtering yield after D saturation does not show distinct difference with nonsaturated samples. In addition, thermal evaporation fluxes of Li on stainless steel (SS) and intercalated Li in the ATJ graphite are measured. An interesting finding is that Li in graphite shows a magnitudeless evaporation flux than Li on SS for surface temperatures ranging from 250°C to 500°C.

Characterizations on a 2.45 GHz microwave induced atmospheric pressure plasma torch

The Center for Plasma-Material Interactions (CPMI) at the University of Illinois at Urbana-Champaign has developed a greater than 10 mm diameter 2.45 GHz microwave-induced atmospheric pressure plasma torch (APPT) for use in various manufacturing applications. The APPT has the ability to generate various atmospheric pressure plasmas (helium, argon, nitrogen) with a gas temperature range from room temperature (30°C) to more than 3,000 °C at the ignition area. The diagnostics of the electron temperature T<inf>e</inf>, electron density n<inf>e</inf>, and the plasma gas temperature T<inf>g</inf> are based on the optical emission spectroscopy (OES) technique. The detailed dependence of the T<inf>e</inf>, n<inf>e</inf> and T<inf>g</inf> on the microwave power, radial distance referred to the ignition point, gas type and gas flow rate will be presented individually. The validity of the OES technique, including the local thermal equilibrium conditions and the model for H<inf>β</inf> calculation, has been verified and analyzed by the OES measured results. OES results has shown that T<inf>e</inf> is in the range of 0.5–1.5 eV, n<inf>e</inf> is in the range of 10¹⁴-10¹⁶ cm⁻³ and T<inf>g</inf> is in the range of 900–3000 K by varying the power from 200–3000 W, depending on the type and the mixture ratio of the plasma gases. The spatial dependence of T<inf>e</inf>, n<inf>e</inf> and T<inf>g</inf> has shown a drastic decrease in T<inf>g</inf> but relatively slower changes in T<inf>e</inf> and n<inf>e</inf>. This provides possibility on both thermal and non-thermal operation conditions for applications of selectable material processes.

Sn debris cleaning by plasma in DPP EUV source systems for HVM

The tin (Sn) debris contamination is one of the technical challenges for the development of high power EUV light source with Sn fuel with a long lifetime of EUV collectors. The debris mitigation techniques (DMTs) can considerably minimize the Sn debris coming out of the source thereby reducing the need or effort for cleaning. However, for HVM, which requires higher EUV power output than today, it is questionable if the DMTs alone will completely eliminate the Sn contamination. Besides, at abnormal instances, we also need to clean thick Sn debris from the mirror surface. For this purpose, the Center for Plasma-Material Interactions (CPMI) at University of Illinois at Urbana-Champaign has developed a plasma-based Sn cleaning method using chlorine plasma with densities and temperature around ~9×109/cm3 and ~ 4 eV respectively. From the previous studies at CPMI, it was shown that chlorine plasma etching can remove Sn debris from Ru mirror surface in a fast (> 400 nm/min) and in situ manner. In this study, we applied the same method to clean Sn contamination on the mock-up collector in our XTS13-35 DPP EUV source system. The mock-up is made of two shells with different gap widths (4 cm, 7.5 cm and 10 cm) in similar size with the actual collector optic. The cleaning rate at different locations on the mockup was experimentally investigated, and it was found that the cleaning rates vary largely with the distance from the chlorine plasma in the range of 20 - 100 nm/min. In addition, a simple analytical model to predict the cleaning rate was developed based on the plasma-surface reactions and the plasma transport. The model describes how plasma transport, chlorine radical distribution and pumping flow affect the Sn cleaning rate with chlorine plasma. Finally, the model is then compared to the experimental results and validated. Based on the knowledge of chlorine plasma and Sn interactions obtained in this study, a remote plasma cleaning technique was also investigated and the results obtained therein are presented. The experimental results along with the model predictions will help design an integrated cleaning system for collector optic in the high power EUV source system for HVM.

Debris Measurement at the Intermediate Focus of a Laser-Assisted Discharge-Produced Plasma Light Source

For extreme ultraviolet light lithography to be a viable process for the future development of computer chips, it is necessary that clean photons are produced at the intermediate focus (IF). To measure the flux at the IF, the Center for Plasma-Material Interactiosn (CPMI) at the University of Illinois at Urbana-Champaign has developed a Sn IF flux emission detector (SNIFFED) apparatus that is capable of measuring charged and neutral particle flux at the IF. Results will be presented that diagnose debris produced at the IF, as well as methods by which this debris can be mitigated. Advanced Materials Research Center, AMRC, International SEMATECH Manufacturing Initiative, and ISMI are servicemarks of SEMATECH, Inc. SEMATECH, and the SEMATECH logo are registered servicemarks of SEMATECH, Inc. All other servicemarks and trademarks are the property of their respective owners.

Sputtering studies of lithiated ATJ graphite by lithium ion bombardment

Sputtering yields of Li+ on ATJ graphite at 45 degree incidence with 700 eV-2000 eV accelerations were studied. Typically, 4 × 1013 ions/(cm2 s) flux of Li+ beam was obtained from LiCl powder in a Colutron ion source. Sputtered particles were collected by a quartz crystal microbalance to determine sputtering yields. The experiment was repeated after lithium was evaporated onto the ATJ graphite target. Suppressed amounts of sputtered particles were observed after lithium treatment. Deuterium saturation treatment for lithiated graphite was also done to simulate actual divertor conditions. The sputtering yield after deuterium saturation does not show distinct differences with non-saturated samples.