Session FT1: Optical Emission Spectroscopy

UV/VUV photon fluxes from cylindrical ICPs at 1 MHz in hydrogen.

The photon fluxes of molecular hydrogen bands and atomic hydrogen lines in the wavelength range from 120 nm to 300 nm in a cylindrical ICP are quantified by means of VUV emission spectroscopy. The molecular analysis is supported by ro-vibrationally resolved corona models taking into account cascading as well. The latter is of particular importance for the interpretation of the ro-vibrational population of the first electronic state (B-state) in the singlet system resulting in the resonant Lyman band (B-X transition). For the analysis of the atomic lines a collisional radiative model is used in which opacity effects are considered. Optical emission spectroscopy is applied as well, allowing for the comparison of the UV/VUV radiation with the VIS radiation. The latter is also used to deduce the plasma parameters such as the electron density and temperature, the degree of dissociation and the atomic particle flux. Langmuir probe measurements yield spatially resolved measurements and allow for quantification of the ion fluxes. The pressure range from 0.3 Pa to 10 Pa for RF powers up to 1 kW is investigated, revealing that the photon fluxes are comparable to the ion fluxes, both of them about two orders of magnitude smaller than the atomic flux.   

Extreme ultraviolet spectroscopy of low pressure helium microwave driven discharges

Surface wave driven discharges are reliable plasma sources that can produce high levels of vacuum and extreme ultraviolet radiation (VUV and EUV). The richness of the emission spectrum makes this type of discharge a possible alternative source in EUV/VUV radiation assisted applications. However, due to challenging experimental requirements, publications concerning EUV radiation emitted by microwave plasmas are scarce and a deeper understanding of the main mechanisms governing the emission of radiation in this spectral range is required. To this end, the EUV radiation emitted by helium microwave driven plasmas operating at 2.45 GHz has been studied for low pressure conditions. Spectral lines from excited helium atoms and ions were detected via emission spectroscopy in the EUV/VUV regions. Novel data concerning the spectral lines observed in the 23 - 33 nm wavelength range and their intensity behaviour with variation of the discharge operational conditions are presented. The intensity of all the spectral emissions strongly increases with the microwave power delivered to the plasma up to 400 W. Furthermore, the intensity of all the ion spectral emissions in the EUV range decreases by nearly one order of magnitude as the pressure was raised from 0.2 to 0.5 mbar.   

A computationally assisted spectroscopic technique to measure secondary electron emission coefficients in technological rf plasmas

A Computationally Assisted Spectroscopic Technique to measure secondary electron emission coefficients (y-CAST) in capacitive rf plasmas is proposed. This non-intrusive, sensitive diagnostic is based on a combination of Phase Resolved Optical Emission Spectroscopy and PIC simulations. Under most conditions in electropositive plasmas the spatio-temporally resolved electron-impact excitation rate features two distinct maxima adjacent to each electrode at different times within one rf period. One maximum is the consequence of an energy gain of the electrons due to sheath expansion. The second maximum is produced by electrons accelerated towards the plasma bulk by the sheath electric field at the time of maximum voltage drop across the sheath. Due to the different excitation mechanisms the ratio of the intensities of these maxima is very sensitive to y, which allows for its determination via comparing the experimentally measured excitation profiles with corresponding simulation data obtained with various y-coefficients. This diagnostic is tested here in a geometrically symmetric reactor, for stainless steel electrodes and argon gas. An effective secondary electron emission coefficient of y=0.067+-0.010 is obtained, which is in excellent agreement with previous experimental results.   

Actinometry of O, N and F atoms

Actinometry is optical non-invasive diagnostics which is often applied for measurements of atom concentrations in various plasmas. However, it’s accuracy relies on knowledge of both apparent excitation cross sections and electron energy distribution function in plasma. Applicability and accuracy of this method for measuring the absolute concentrations of O, N and F atoms in discharge plasma was studied. For this purpose, concentrations of the atoms produced in ICP discharge were measured by two methods: actinometry and appearance potential mass-spectrometry (APMS). Comparison of the results showed good agreement between both methods in a range of experimental errors for oxygen. Since the excitation cross sections for $O(3p ^3P)$ and $O(3p ^5P)$ states (most often used in actinometry of atomic oxygen) are well known and experimentally validated, the influence of the EEDF shape was accurately evaluated. Following this approach, the theoretical excitation cross sections of $N(3p ^4P^o)$, $F(3p ^2P^o)$ and $F(3p ^4D^o)$ states (used for actinometry of N and F atoms) were absolutely calibrated at first time by fitting the actinometry results to that of APMS.   

Challenges in Optical Emission Spectroscopy

Collisional-radiative models (CRMs) are widely used to investigate plasma properties such as electron density, electron temperature and the form of the electron energy distribution function. In this work an extensive CRM for argon is presented, which models 30 excited states and various kinds of processes including electron impact excitation/de-excitation, radiation and radiation trapping. The CRM is evaluated in several test cases, i.e. inductively and capacitively coupled plasmas at various pressures, powers/voltages and gas admixtures. Deviations are found between modelled and measured spectra. The escape factor as a means of describing radiation trapping is discussed as well as the cross section data for electron impact processes.   

Plasma monitoring of nanoparticles formation in SiH4/H2 discharges

Radio-frequency SiH4/H2 discharges is the most common technique for the growth of silicon thin films. Nanoparticles formation and uncontrollable agglomeration to dust is common drawback of such type of discharges due to the extensive reactivity of the species produced in the gas phase. In this work, we deposited silicon films in different plasma conditions while monitoring at the same time nanoparticles formation. The experiments were performed under Continuous Wave (CW) and Pulsed Plasma generation in order to control particles formation. Different time-resolved plasma diagnostics, such as Optical ${\rm E}$mission Imaging, Laser Light Scattering and self-bias voltage (Vdc) measurements were used for the detection of particles. Mass spectrometry was also used to record higher silanes formation during the deposition. The deposited films were characterized in terms of crystallinity, hydrogen content and optical properties by Laser Raman, Fourier Transformed Infrared and UV/Vis spectroscopy. Finally, Atomic Force Microscopy (AFM) was applied to monitor the morphology and roughness of the films. The properties and the morphology of the deposited films are compared in order to determine the effect of the particles formation on the material's quality.