J Job Beckers

Pulse, dc and ac breakdown in high pressure gas discharge lamps

An optical study of pulse, dc, and ac (50?400?kHz) ignition of metal halide lamps has been performed by investigating intensified CCD camera images of the discharges. The ceramic lamp burners were filled with xenon gas at pressures of 300 and 700?mbar. In comparison with dc and pulse ignition, igniting with an ac voltage decreases the ignition voltage by up to 56% and the breakdown time scales get much longer (~10?3?s compared with ~10?7?s for pulse ignition). Increasing the ac frequency decreases the ignition voltages and changes the ionization channel shapes. External irradiation of UV light can have either an increasing or a decreasing effect on ignition voltages.

Temperature dependence of nucleation and growth of nanoparticles in low pressure Ar/CH4 RF discharges

The gas temperature (Tg) dependence of nucleation and growth processes of hydrocarbon nanoparticles in low pressure Ar/CH4 RF discharges has been investigated. Measuring the electron density with the microwave cavity resonance technique allowed us to monitor nucleation processes on small (µs) time scales. On larger time scales, coagulation times and growth rates are determined by means of measuring the phase angle between the RF voltage and current in correlation with laser light scattering. The experimental results show a significant gas temperature dependence of both powder nucleation and growth processes. Within the measured gas temperature range (20-130 °C) the particle growth rate decreases by a factor of ~3.7, while the coagulation time increases by a factor of ~6.5 with increasing Tg. Moreover, in this paper we present a simplified model which uses the experimentally determined growth rates and coagulation times to predict the value of the critical density of nanometre sized neutral particles, necessary to initiate coagulation. This model estimates a critical density of 3.5 × 1015 m-3 at room temperature which decreases with increasing temperature.

Fast and interrupted expansion in cyclic void growth in dusty plasma

Low-pressure acetylene plasmas are able to spontaneously form dust particles. This will result in a dense cloud of solid particles that is levitated in the plasma. The formed particles can grow up to micrometers. We observed a spontaneous interruption in the expansion of the so-called dust void. A dust void is a macroscopic region in the plasma that is free of nanoparticles. The phenomenon is periodical and reproducible. We refer to the expansion interruption as ‘hiccup’. The expanding void is an environment in which a new cycle of dust particle formation can start. At a certain moment in time, this cycle reaches the (sudden) coagulation phase and as a result the void will temporarily shrink. To substantiate this reasoning, the electron density is determined non-intrusively using microwave cavity resonance spectroscopy. Moreover, video imaging of laser light scattering of the dust particles provides their spatial distribution. The emission intensity of a single argon transition is measured similarly. Our results support the aforementioned hypothesis for what happens during the void hiccup. The void dynamics preceding the hiccup are modeled using a simple analytical model for the two dominant forces (ion drag and electric) working on a nanoparticle in a plasma. The model results qualitatively reproduce the measurements.

Anion dynamics in the first 10 milliseconds of an argon–acetylene radio-frequency plasma

The time evolution of the smallest anions (C2H− and H2CC−), just after plasma ignition, is studied by means of microwave cavity resonance spectroscopy (MCRS) in concert with laser-induced photodetachment under varying gas pressure and temperature in an argon–acetylene radio-frequency (13.56 MHz) plasma. These anions act as an initiator for spontaneous dust particle formation in these plasmas. With an intense 355 nm Nd : YAG laser pulse directed through the discharge, electrons are detached only from these anions present in the laser path. This results in a sudden increase in the electron density in the plasma, which can accurately and with sub-microsecond time resolution be measured with MCRS. By adjusting the time after plasma ignition at which the laser is fired through the discharge, the time evolution of the anion density can be studied. We have operated in the linear regime: the photodetachment signal is proportional to the laser intensity. This allowed us to study the trends of the photodetachment signal as a function of the operational parameters of the plasma. The density of the smallest anions steadily increases in the first few milliseconds after plasma ignition, after which it reaches a steady state. While keeping the gas density constant, increasing the gas temperature in the range 30–120 °C limits the number of smallest anions and saturates at a temperature of about 90 °C. A reaction pathway is proposed to explain the observed trends.

Exploring the electron density in plasmas induced by extreme ultraviolet radiation in argon

The new generation of lithography tools use high energy EUV radiation which ionizes the present background gas due to photoionization. To predict and understand the long term impact on the highly delicate mirrors It is essential to characterize these kinds of EUV-induced plasmas. We measured the electron density evolution in argon gas during and just after irradiation by a short pulse of EUV light at 13.5 nm by applying microwave cavity resonance spectroscopy. Dependencies on EUV pulse energy and gas pressure have been explored over a range relevant for industrial applications. Our experimental results show that the maximum reached electron density depends linearly on pulse energy. A quadratic dependence - caused by photoionization and subsequent electron impact ionization by free electrons - is found from experiments where the gas pressure is varied. This is demonstrated by our theoretical estimates presented in this manuscript as well.

Exploring the temporally resolved electron density evolution in extreme ultra-violet induced plasmas

We measured the electron density in an extreme ultra-violet (EUV) 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 2.6 × 1016 m−3. This is in good agreement with a theoretical prediction from photo-ionization, which yields a density of 4.5 × 1016 m−3. After the EUV pulse the density slightly increases due to electron impact ionization. The plasma (i.e. electron density) decays in tens of microseconds.

Exploring the electron density in plasma induced by EUV radiation:II. Numerical studies in argon and hydrogen

We used numerical modeling to study the evolution of EUV-induced plasmas in argon and hydrogen. The results of simulations were compared to the electron densities measured by microwave cavity resonance spectroscopy. It was found that the measured electron densities can be used to derive the integral amount of plasma in the cavity. However, in some regimes, the impact of the setup geometry, EUV spectrum, and EUV induced secondary emission should be taken into account. The influence of these parameters on the generated plasma and the measured electron density is discussed.

Light emission of metal halide lamps under micro- and hypergravity conditions

The wavelength-integrated light output from a metal halide discharge lamp is measured for gravity conditions varying from 0to1.8g during parabolic flights. The results show that the changing gravity affects the convection flow in the lamp, which in turn changes the total light output. For vertically burning lamps, the sign and magnitude of the effect can be predicted using the demixing parameter: the ratio of typical diffusion to convection times. In horizontally burning lamps at 0g, the absence of convective mixing results in a reduced light emission.

Exploring the electron density in plasma induced by EUV radiation: I. Experimental study in hydrogen

Plasmas induced by EUV radiation are unique since they are created without the need of any discharge. Moreover, it is essential to characterize these plasmas to understand and predict their long term impact on highly delicate optics in EUV lithography tools. In this paper we study plasmas induced by 13.5 nm EUV radiation in hydrogen gas. The electron density is measured temporally resolved using a non-invasive technique known as microwave cavity resonance spectroscopy. The influence of the EUV pulse energy and gas pressure on the temporal evolution of the electron density has been explored over a parameter range relevant for industry. Our experimental results show that the maximum electron density is in the order of 1014 m−3 and depends linearly on the EUV pulse energy. Furthermore, the maximum electron density depends quadratically on the pressure; the linear term is caused by photoionization and the quadratic term by subsequent electron impact ionization. The decay of the plasma is governed by ambipolar diffusion and, hence, becomes slower at elevated pressures. Similarities and differences of the same processes in argon are highlighted in this paper.

Surprising temperature dependence of the dust particle growth rate in low pressure Ar/C2H2 plasmas

We have experimentally monitored the growth rate of dust particles in a low pressure Ar/C2H2 radiofrequency discharge as a function of the gas temperature Tg and independent of the C2H radical density and the gas density. Used diagnostics are laser light scattering and measurements of the phase angle between the RF voltage and current. In contrast to most literature, we demonstrate that the growth rate is not a monotonically decreasing function of Tg but shows a maximum around Tg = 65 °C. In addition, we demonstrate that the phase angle is an accurate measure to monitor the particle growth rate.