Session FT1: Plasma Applications

The role of atomic physics in collisional-radiative modeling of tin plasmas for lithography

We report on our continuing efforts to understand the extreme ultra-violet (EUV) emission from tin plasmas of interest to nanolithography. The intense and narrow emission of EUV radiation centered at 13.5 nm is of interest to the micro-electronics industry where it has much potential for etching smaller features on micro-processors.

The tin plasma of relevance (at a temperature around 30 eV and electron density around $10^{21}$ cm$^{-3}$) is interesting from an atomic physics perspective, since it requires highly accurate atomic structure and transition information from complex open 4p and 4d subshells of tin ions from around Sn$^{8+}$ to Sn$^{15+}$. The opacity spectra of Sn plasma in this regime are calculated, under local thermodynamic equilibrium conditions, using the Los Alamos suite of atomic codes and the opacity and plasma modeling code ATOMIC. The detailed atomic structure calculations of the complex Sn ions show an unexpectedly large contribution to EUV emission from transitions between highly-excited states, up to around 90% of the total opacity, with the more well-known EUV transitions to the ground manifold contributing only 10%. The transitions between doubly- and triply-excited states are shown to be serendipitously aligned around 13.5 nm, the wavelength of relevance in EUV light sources. Our calculations are in excellent agreement with the emissivity measured from a Nd:YAG laser-produced plasma. We also explore the properties of CO2 laser-produced tin plasmas, where non-LTE effects are more important.   

Profiling of High-Pressure DC Microdischarge for excimer emission.

UV excimer emission from microdischarges is of enormous interest, with numerous publications in this area.  Microdischarges have been shown to have increased excimer UV emission at higher pressures up to 1 atm.  The higher pressure operation is desirable due to higher plasma density and stronger excimer emission. This work focuses on the micro discharges operating in the 1–2 atm. the range for a possibly stronger UV emission. 

The microfabricated device uses a planar micro discharge configuration with a thick electroplated copper film as the electrode material. This device shows stable operation at higher pressures due to good heat transfer from the thick copper. Devices utilizing thin-film copper are seen to arc very easily and thus unstable. In this study, electrode gaps are varied from 25µm to 50µm.  The devices tend to arc if the current is not limited, but by using appropriate biasing, we observe that it is possible to maintain stable glow discharges in Argon in the pressure range of 1-2 atm.  (The 2 atm limitation comes from the test setup, not the plasma so operation at higher pressures is possible). For a 50 µm spacing device, the breakdown voltage varies linearly between 400 V at 1atm and 480 V at 2 atm. The glow is less diffuse as the pressure increases, but the plasma impedance remains the same (~120 KΩ) even at higher pressure. UV-VIS spectra taken with an ocean-optics HR4000 spectrometer show glow discharge characteristics with strong emission from the usual Argon lines. VUV characterization is ongoing.    

Bridging the gap between fluid and kinetic plasma simulations for industrial plasma sources

Development of advanced plasma tools for ion implantation and etching segments of semiconductor processing industry requires deep and detailed understanding of underlying physical processes. Comprehensive plasma modeling is typically used to address these problems. In most cases, fluid or hybrid models provide satisfactory results. However, with development of supercomputing facilities, Particle-in-Cell (PIC) methods show substantial value in addressing industrial problems. In this presentation, two types of industrial plasma sources will be explored using fluid and PIC approaches. First one is hot cathode magnetized ion source used in ion implantation. This source operates in mTorr range pressure with ~100G magnetic field confining plasmas with 10¹¹ - 10¹² cm^-3 densities. The detailed analysis of source operation using hybrid and PIC plasma models will be presented and specific problems each numerical approach can address will be discussed. Second one is low pressure ICP source for ribbon beam plasma processing. This source also operate in mTorr pressure range with or without external magnetic field. The talk will address plasma diffusion using fluid and PIC simulations and discuss effects of the gas pressure, power and source geometry. Benchmarking against experimental data will also be discussed.