Inception and propagation of positive streamers in high-purity nitrogen: effects of the voltage rise-rate

Controlling streamer morphology is important for numerous applications. Up to now, the effect of the voltage rise rate was only studied across a wide range. Here we show that even slight variations in the voltage rise can have significant effects. We have studied positive streamer discharges in a 16 cm point-plane gap in high-purity nitrogen 6.0, created by 25 kV pulses with a duration of 130 ns. The voltage rise varies by a rise rate from 1.9 kV/ns to 2.7 kV/ns and by the first peak voltage of 22 to 28 kV. A structural link is found between smaller discharges with a larger inception cloud caused by a faster rising voltage. This relation is explained by the greater stability of the inception cloud due to a faster voltage rise, causing a delay in the destabilisation. Time-resolved measurements show that the inception cloud propagates slower than an earlier destabilised, more filamentary discharge. This explains that the discharge with a faster rising voltage pulse ends up to be shorter. Furthermore, the effect of remaining background ionisation in a pulse sequence has been studied, showing that channel thickness and branching rate are locally affected, depending on the covered volume of the previous discharge.Comment: 16 pages, 9 figure

Access Point Security Service for wireless ad-hoc communication

This paper describes the design and implementation of a security solution for ad-hoc peer-to-peer communication. The security solution is based on a scenario where two wireless devices require secure communication, but share no security relationship a priori. The necessary requirements for the security solution described here comprise topics such as energy efficiency, security standards and ad-hoc networks. The devised solution is called Access Point Security Service (APSS). APSS is able to provide security by delivering a symmetric key to two wireless devices that require ad-hoc peer-to-peer communication. The main principle of APSS is that it makes use of an existing security relationship between a network provider and its customers. The existing security relationship enables the network provider to deliver security to two or more communicating parties in the form of a shared key. An implementation of APSS is provided making use of the existing Wi-Fi security standards

Etching of silicon in alkaline solutions: a critical look at the {111} minimum

Anisotropic wet-chemical etching of silicon in alkaline solutions is a key technology in the fabrication of sensors and actuators. In this technology, etching through masks is used for fast and reproducible shaping of micromechanical structures. The etch rates Image depend mainly on composition and temperature of the etchant. In a plot of etch rate versus orientation, there is always a deep, cusped minimum for the {1 1 1} orientations. We have investigated the height of the {1 1 1} etch-rate minimum, as well as the etching mechanisms that determine it. We found that in situations where masks are involved, the height of the {1 1 1} minimum can be influenced by nucleation at a silicon/mask-junction. A junction which influences etch or growth rates in this way can be recognized as a velocity source, a mathematical concept developed by us that is also applicable to dislocations and grain boundaries. The activity of a velocity source depends on the angle between the relevant {1 1 1} plane and the mask, and can thus have different values at opposite {1 1 1} sides of a thin wall etched in a micromechanical structure. This observation explains the little understood spread in published data on etch rate of {1 1 1} and the anisotropy factor (often defined as Imag

Streamers in air splitting into three branches

We investigate the branching of positive streamers in air and present the first systematic investigation of splitting into more than two branches. We study discharges in 100 mbar artificial air that is exposed to voltage pulses of 10 kV applied to a needle electrode 160 mm above a grounded plate. By imaging the discharge with two cameras from three angles, we establish that about every 200th branching event is a branching into three. Branching into three occurs more frequently for the relatively thicker streamers. In fact, we find that the surface of the total streamer cross-sections before and after a branching event is roughly the same.Comment: 6 pages, 7 figure

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 $2.6\cdot10^{16}$ m$^{-3}$. This is in good agreement with a theoretical prediction from photo ionization, which yields a density of $4.5\cdot10^{16}$ m$^{-3}$. 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

Streamer inception and imaging in various atmospheres

Streamers are the first stage of many discharges involving high voltages.They consist of a propagating ionization front leaving behind atrail of conductive, quasi-neutral plasma. In this contribution we willshow experiments on streamers revealing some of their most importantproperties: their inception and their propagation and branchingbehaviour.We study streamer inception by applying repetitive high voltagepulses and studying the statistics of inception delay. By means of smallbias pulses between the high voltage pulses, we are able to manipulatethese statistics, which reveals a lot on the processes governing theinception.Secondly, we study the propagation and branching of streamers bya combination of stereoscopic and stroboscopic measurements of ’lowcomplexity’ streamer discharges. We have developed automated routineswhich can determine propagation velocities, branching angles andmuch more from these and can directly compare these against numericalresults, thereby also giving unprecedented insights into the fundamentalsof such discharges

Simulation of anisotropic wet-chemical etching using a physical model

We present a method to describe the orientation dependence of the etch rate of silicon, or any other single crystalline material, in anisotropic etching solutions by analytical functions. The parameters in these functions have a simple physical meaning. Crystals have a small number of atomically smooth faces, which etch (and grow) slowly as a consequence of the removal (or addition) of atoms by rows and layers. However, smooth faces have a roughening transition (well known in statistical physics); at increasing temperature they become rougher, and accordingly the etch and growth rates increase. Consequently, the basic physical parameters of our functions are the roughness of the smooth faces and the velocity of steps on these faces. This small set of parameters describes the etch rate in the two-dimensional space of orientations (on the unit sphere). We have applied our method to the practical case of etch rate functions for silicon crystals in KOH solutions. The maximum deviation between experimental data and simulation using only nine physically meaningful parameters is less than 5% of the maximum etch rate. This method, which in this study is used to describe anisotropic etching of silicon, can easily be adjusted to describe the growth or etching process of any crysta

Plasma accumulation effects in Extreme Ultra-Violet generated plasmas

In order to meet the demand of increasing computer speed and memory capacity, industries are striving to reduce the size of computer chips. This miniaturization can be achieved by reducing the wavelength in lithography machines to Extreme Ultra-Violet (EUV, 92 eV). The low-pressure (around 1 Pa) transparent background gas (e.g. H2 and He) in the lithography machine is partially ionized by the absorption of EUV photons. The study of these low-density 1E15 m^-3 pulsed plasmas is interesting and experimentally challenging. The electron density is measured with microwave cavity resonance spectroscopy (MCRS). In MCRS measurements the resonance frequency in a cavity is determined, this frequency depends on the electron density in the cavity. In this research the plasma accumulation is studied in an EUV-generated plasma in argon. The EUV source generates EUV pulses with a repetition frequency between 500 Hz and 10 kHz. The accumulation of plasma is clearly observed at frequencies above 1 kHz