1,362 research outputs found
Exploring the temporally resolved electron density evolution in EUV induced plasmas
We measured for the first time the electron density in an Extreme
Ultra-Violet induced plasma. This is achieved in a low-pressure argon plasma by
using a method called microwave cavity resonance spectroscopy. The measured
electron density just after the EUV pulse is m. This
is in good agreement with a theoretical prediction from photo ionization, which
yields a density of m. After the EUV pulse the density
slightly increase due to electron impact ionization. The plasma (i.e. electron
density) decays in tens of microseconds.Comment: 3 pages, 4 figure
Facilitating Retention and Transfer of Physics Concepts with Challenging Assignments in Design-Based Learning Projects
Revealing latent trajectories of (intended) train travel during and after COVID-19
This study investigates whether the decline in public transit ridership is a temporary phenomenon or indicative of a structural shift in travel patterns and attitudes. We estimate a latent class trajectory model using data from a comprehensive and large-scale survey administered by the Dutch national train operator conducted at eight different points in time after the onset of the pandemic. Six latent trajectories in train use and stated future intentions to use the train are revealed, showing different ‘recovery’ pathways. Whereas low-educated frequent commuters travel almost as much as before, highly educated frequent commuters and mixed-purpose travellers still travel much less, even in the last wave when all restrictions are lifted. The results indicate that travellers belonging to these classes have structurally changed their behaviour. The shift to working from home is more pronounced than the shift to private car use
Measurement of the gas temperature in fluorocarbon radio frequency discharges using infrared absorption spectroscopy
The translational gas temperature was measured in 13.56 MHz radio-frequency (rf) discharges in CF4 and CHF3. Infrared absorption spectra of CF4 and CF2 were recorded using a tunable diode laser and the gas temperature was deduced from the linewidths of the absorption lines of these molecules. It is shown that linewidth measurements yield a simple and direct method to determine the gas temperature, with an accuracy up to ~ 10 K. The results obtained in CF4 and CHF3 plasmas indicate that the translational temperatures of all particles investigated in these plasmas are, at most, 50 K above the room temperature. The temperature increases with increasing gas pressure and rf power, but it is independent of the flow rate. This is attributed to an increased heating rate of the gas. Moreover, it was found that the temperature rise is significantly smaller in CHF3 than in CF4, under the same plasma conditions. This can be attributed to a higher power dissipation by chemical conversion of the parent gas in a CHF3 discharge, as compared with a CF4 plasm
Commentary: The Impact of the Time Interval Between Radiation and Hyperthermia on Clinical Outcome in Patients With Locally Advanced Cervical Cancer
The chemistry of a CCl2F2 radio frequency discharge
A systematic study of the chemistry of stable molecules and radicals in a low pressure CCl2F2 radio frequency discharge for dry Si etching has been performed. Various particle densities have been measured and modeled. The electron density, needed as an input parameter to model the CCl2F2 dissociation, is measured by a microwave cavity method. The densities of stable molecules, like CClF3, CF4, 1,2-C2Cl2F4 and the etch product SiF4, are measured by Fourier transform absorption spectroscopy. The density of the CF2 radical is measured by means of absorption spectroscopy with a tunable diode laser. Its density is in the order of 1019 m-3. All density measurements are presented as a function of various plasma parameters. Moreover, optical emission intensities of Cl and F have been recorded as a function of plasma parameters. It appears that the feed gas (CCl2F2) is substantially dissociated (about 70%–90%) in the discharge. Based on the obtained data the dissociation rates of several molecules have been estimated. The measured total dissociation rate of CCl2F2 is 8×10-15 m3¿s-1. For this molecule the dissociation rate is substantially higher than the dissociative attachment rate (10-15 m3¿s-1). The dissociation rate for CClF3 is 2×10-15 m3¿s-1 and for CF4 about 3×10-16 m3¿s-1. The total dissociation rate of C2Cl2F4 is higher than 5 ×10-15 m3¿s-1, of which 2.5±0.5 × 10-15 m3¿s-1 is due to dissociative attachment. Furthermore it has been found that the presence of a silicon wafer strongly affects the plasma chemistry. Optical emission measurements show that the densities of halogen radicals are significantly depleted in presence of Si. Moreover, the densities of several halocarbon molecules display a negative correlation with the density of the etch product SiF4. © 1995 American Vacuum Societ
Production and destruction of CFx radicals in radio-frequency fluorocarbon plasmas
Spacially resolved densities of CF, CF2, and CF3¿ radicals in capacitively coupled 13.56 MHz radio-frequency (rf) discharges in CF4¿and CHF3 were determined by means of infrared absorption spectroscopy employing a tunable diode laser spectrometer. It was established that the stationary CF2 density and density profile in a CF4 plasma depend strongly on the electrode material. This is attributed to different sticking coefficients of CF2 on different surfaces. Furthermore, it was found that the densities of all CFx radicals increase near the electrodes at high gas pressures and rf powers in a CHF3 plasma. This leads to the conclusion that production of CFx radicals takes place in the sheath region close to the electrodes. It is proposed that collisions between ions and source gas molecules are responsible for this production of CFx radicals. In the presence of a destruction process in the plasma glow (e.g., by three-body recombination with other radicals) and the absence of a fast surface loss process this results in the observed increase of CFx densities near the electrodes. In order to study the radical kinetics time dependent measurements were performed during power modulation of the plasma. It was found that the decay time of the CF2 density in the afterglow of a CF4 plasma is much shorter than the corresponding decay time in a CHF3¿discharge. This suggests that the surface loss is relatively less important in the latter case, in agreement with measurements of spatial density distributions. This is explained by the presence of a (CFx)n layer, which is readily deposited on the electrodes in a CHF3 discharge, and by low sticking probabilities of CF and CF2 radicals on such a layer. © 1996 American Vacuum Societ
In situ ellipsometry during plasma etching of silica films on Si
The etching of SiO2 films on a Si substrate in an rf plasma in CF4 has been studied with i n s i t uellipsometry. The etch rate was measured as a function of flow, rf power and pressure. An accurate analysis of the experimental data using numerical simulations based on multilayermodels has yielded information both on the refractive index of the etched SiO2 film and on the existence of a top layer. It could be established that a layer is present on top of the SiO2 during etching, which is probably caused by roughening of the SiO2 layer. Furthermore at high pressures (>8 Pa) it was demonstrated that after the complete removal of the SiO2 film a polymer layer starts growing on the Si substrate
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