Session DT2: Plasma Surface Interactions I

Negative-permittivity plasma generation in negative-permeability metamaterial space

Negative-permittivity plasma is generated in negative-permeability metamaterial space. Unlike cases of positive permeability, which is quite usual in almost all materials available so far, negative-permeability space realized in metamaterial structure [1] allows microwaves to propagate in negative-permittivity media. Our previous study [2, 3] verified that microwaves can propagate in a negative-permittivity plasma immersed in a negative-permeability metamaterial space, which indicates that a dynamic state of negative refractive index was successfully generated. In this study, negative-permeability space was prepared using metamaterial structure as well, and we investigated plasma generation by high-power microwaves in such a metamaterial structure. Langmuir probe measurement revealed that electron density is higher than the cutoff density, which means that permittivity is negative [4, 5]. We also confirmed in both model predictions [6] and experimental results [4, 5, 7] that nonlinear phenomena are key issues to understand underlying physics; they include bifurcations of permittivity or electron density in nonlinear dynamics and harmonic wave generation similar to that reported in nonlinear optics, and both phenomena are observed in experiments. \\[4pt] [1] J. B. Pendry et al., IEEE Trans. Microw. Theory Techniq. \textbf{47}, 2075 (1999).\\[0pt] [2] O. Sakai and K. Tachibana, Plasma Sources Sci. Technol. \textbf{21}, 013001 (2013).\\[0pt] [3] O. Sakai et al., Phys. Plasmas \textbf{20}, 073506 (2013).\\[0pt] [4] Y. Nakamura and O. Sakai, Jpn. J. Appl. Phys. \textbf{53}, 03DB04 (2014).\\[0pt] [5] Y. Nakamura, A. Iwai and O. Sakai, Plasma Sources Sci. Technol. \textbf{23}, 064009 (2014). \\[0pt] [6] O. Sakai, J. Appl. Phys. \textbf{109}, 084914 (2011).\\[0pt] [7] A. Iwai, N. Nakamura, A. Bambina and O. Sakai, Appl. Phys. Express \textbf{8}, 056201 (2015).   

Numerical modeling of plasma meta-materials for electromagnetic energy flow control

Meta-materials are a new and promising technology that could enable advances in several scientific fields -- especially in electromagnetic (EM) energy flow control. These materials though present a major drawback: They can only interact with a limited range of EM frequencies and their structure is pre-defined, rendering them non-tunable and non-reconfigurable. Instead of using structural crystal patterns as in common meta-materials, micro-plasma discharges can be used to control the EM energy propagation. Plasmas present resonant frequencies depending on their degree of ionization -- their charged particles density. By adjusting the plasma density, different EM wave frequencies can be manipulated -- controlled. In this article, we present 2D and 3D numerical results of plasma meta-materials and their interaction with high frequency (HF) EM waves. Maxwell's equations are coupled with the electron momentum equation and a quasi-neutral fluid description for the plasma dynamics. We study the interaction between a plasma array and HF EM waves demonstrating significant reduction in the transmitted EM energy. Remote ignition of the plasma micro-discharges by the EM waves is also numerically investigated in a simplified configuration.   

Carrier transport and trapping in a-Si:H films under plasma processing

Carrier transport is an important factor that determines the performances of solar cells and transistors [1]. It is often limited by carrier trapping, associated with various defects. The defects are created during fabrication processes using various plasmas; however the defect creation kinetics is not known. Here, we demonstrate the detection of the trapped carriers in a-Si:H films under plasma enhanced CVD, and discuss the carrier trapping and defect kinetics. Using an optically pump-probe technique, we detected the trapped carriers in an a-Si:H films during growth by plasma enhanced CVD [2]. An a-Si:H film growing on a glass substrate was illuminated with pump and probe light. The photocurrent induced by the pump was measured throughout the growth and postgrowth annealing [3]. An increment in the photocurrent induced by the pulsed probe was also measured. The trapped carrier density was determined from the increment since it originates from de-trapping of carriers. We found that the trapped carrier density was typically 10$^{18}$cm$^{-3}$. It was dependent on the growth temperature, and minimized at 473K. Interestingly, the detected trapped carriers were distributed uniformly in the direction of growth, and they were reduced during postgrowth annealing. \\[4pt] [1] S. Nunomura et. al., \textbf{Adv. Mater. 26}, 7555 (2014).\\[0pt] [2] S. Nunomura et. al. \textbf{AIP Advances 4}, 097110 (2014).\\[0pt] [3] S. Nunomura et. al. \textbf{Appl. Phys. Express. 6}, 126201 (2013).   

Damage formation mechanisms of Si and Ge substrates by ion bombardment

Recently the choice of materials for most advanced semiconductor devices, which typically have three dimensional (3D) structures rather than planer ones, has been changing from silicon to III-V compounds or germanium. Such changes have brought renewed interest in physical damages caused by ion bombardment because, in typical gate etching processes of 3D devices, the channel surfaces are exposed directly to the ion irradiation from the plasma. The angles of ion incidence on 3D device gates can be much larger than those on 2D planer devices. Therefore a better control of the damage layer formation on modern 3D devices requires a better understanding of the damage formation mechanisms on such new materials and structures. In this study, damage formation processes by energetic ion bombardment have been simulated for Si/Ge/SiGe substrates by molecular dynamics (MD) simulations and dependence of ion induced damages on species of incident ions and their doses has been examined. Based on the simulations results, damage formation mechanisms will be discussed and a semi-analytical model to predict the damage extent formed by ion bombardment will be also presented.   

Mechanisms of Hydrocarbon Based Polymer Etch

Dry etch of hydrocarbon based polymers is important for semiconductor device manufacturing. The etch mechanisms for oxygen rich plasma etch of hydrocarbon based polymers has been studied but the mechanism for lean chemistries has received little attention. We report on an experimental and analytic study of the mechanism for etching of a hydrocarbon based polymer using an Ar/O2 chemistry in a single frequency 13.56 MHz test bed. The experimental study employs an analysis of transients from sequential oxidation and Ar sputtering steps using OES and surface analytics to constrain conceptual models for the etch mechanism. The conceptual model is consistent with observations from MD studies and surface analysis performed by Vegh, et al. and Oehrlein, et al. [1,2] and other similar studies. Parameters of the model are fit using published data and the experimentally observed time scales.\\[4pt] [1] J.J. Vegh, D. Nest, D. B. Graves, R. Bruce, S. Englemann, T. Kwon, R. J. Phaneuf, G. S. Oehrlein, B. K. Long, and C. G. Willson, Jour. of Applied Physics \textbf{104}, 034308 (2008).\\[0pt] [2] G.S. Oehrlein, R. J. Phaneuf, D. G. Graves, J. Vac. Sci. Tech. B 29, 010801-1 (2011).   

The Effect of N2 Plasma on Atomic Hydrogen Surface Recombination

Remote plasma sources are increasingly being used with reducing chemistries for radical generation in on wafer applications. The repeatability of atomic hydrogen output from wafer to wafer and matching of performance between chambers is paramount and is a topic that receives a great deal of attention. The atomic radical recombination on plasma facing surfaces is known to have a strong impact on hydrogen radical output of remote plasma sources. Presented here are results showing that the source output can be attenuated by up to 40\% as a result of exposing the surface to nitrogen plasma, which chemically modified the wall resulting in a high recombination surface. It is also shown that subsequent hydrogen processing can convert the surface back to its low recombination state. Additionally this ``seasoning'' or ``conditioning'' effect is shown to be on a time scale of days. Measurements of radical concentrations were made with a calorimeter; surface analysis is done with XPS as well as high resolution SEM.