Ww Winfred Stoffels

Study of initial dust formation in an Ar‐SiH4 discharge by laser induced particle explosive evaporation

The initial step of particulate growth in a dust forming low pressure radio‐frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE). With respect to the conventional light scattering, this method has been found much more efficient to observe small nanometer size particles, especially in the case of UV excimer laser radiation. Experimental results interpreted by a simple model of laser‐particle interaction show that the intensity of LIPEE continuum emission depends on the particle radius roughly as r4. This interaction is essentially different from Rayleigh scattering, as the latter varies as r6. A study of time evolution of powder formation by LIPEE emission reveals the initial formation of nanometer size crystallites and the coalescence process leading to larger scale particles. It could be demonstrated that the critical step of dust formation is the initial clustering process leading to nanometer scale crystallites.

Plasma chemistry and surface processes of negative ions

This work reviews formation processes of negative ions in low-pressure laboratory plasmas. There are many topics of discussion in the chemistry of negative ions. In most studies only volume production by dissociative electron attachment is considered. However, a typical problem is that experiments reveal higher negative ion densities than one would expect based on attachment rates to ground-state molecules. Apparently, there exist other, more efficient ion production channels. Excitation and chemical conversion of the parent gas under plasma conditions can significantly increase the effective attachment rates. This is due to extremely high attachment cross sections of rovibrationally and electronically excited molecules, as well as radicals and large polymeric species. Alternatively, negative ions can be efficiently generated in the plasma sheath, due to interactions of high-energy positive ions with neutrals or with the surface. Sheath chemistry can have a large impact on the bulk plasma, so it has to be studied in more detail to obtain a complete understanding of electronegative plasmas. Both chemical reactions in the volume and sheath collisions must be included in plasma models.

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 r...

Energy influx from an rf plasma to a substrate during plasma processing

The energy influx delivered by an rf plasma to a metal substrate has been studied by a calorimetric method with a thermal probe. By changing the substrate voltage, the influence of the kinetic energy of the charge carriers to the thermal power could be determined. The measured energy influx for an argon plasma can be explained mainly by ions, electrons, and their recombination. In the case of an oxygen plasma, where the energy influx is under comparable conditions about 50% higher, also other transfer mechanisms such as surface-aided atom association and relaxation of rovibrational states have to be taken into consideration.

Instantaneous and delayed responses of line intensities to interruption of the RF power in an argon inductively coupled plasma

Abstract Instantaneous and delayed responses of line intensities to the sudden interruption of the RF power have been studied in an argon inductively coupled plasma (ICP). The instantaneous responses are caused by equilibrium shifts in the balances of elementary processes that control the populations of the excited states. It has been found that excited levels of Ar and H are predominantly populated by recombination of free electrons with ionic species, while most levels of metals such as Mg, Cd, Na, Fe, Al and Cu are populated by excitation from the ground state atom. Also charge transfer between Mg 1 and Ar 1 has been observed in the temporal behaviour of line intensities of two Mg + states, quasi-resonant for charge transfer. Furthermore, we observed that in the inner part of the plasma the temperature remains constant during the recombination decay time, after an initial cooling of the electrons to the heavy particle temperature. When the power is switched on again, the electron temperature seems to increase temporarily to a value that is higher than the steady state value. The delayed responses are caused by disturbances created in the expansion zone of the plasma during and after the interruption. It was found that these disturbances travel through the plasma with a velocity of 12 m s −1 .

Measurements of radical densities in radio‐frequency fluorocarbon plasmas using infrared absorption spectroscopy

Densities of CF2 radicals, rotational temperatures, and the degree of dissociation in radio‐frequency fluorocarbon plasmas have been measured using Fourier transform infrared absorption spectroscopy and tunable diode laser infrared absorption spectroscopy. The CF2 densities obtained in CF4, CHF3, C2F6, and CF2Cl2 plasmas indicate that the partial pressure of CF2 is around 1%–5% of the total pressure. From the spatial dependence of the CF2 density it was established that at high pressure, CF2 is produced either on the rf electrode or close to the rf electrode. Furthermore, a comparison between measured absorption spectra and a simulation of the rotational distributions has revealed that the rotational temperatures of CF4, CF2, and HF are all close to room temperature. FTIR spectra indicate that in plasmas of gases with a low F/C ratio (due to the presence of H or Cl) the source gas is converted for a significant part into other species.

Micro‐Disperse Particles in Plasmas: From Disturbing Side Effects to New Applications

The research effort in the area of dusty plasmas initially aimed at avoiding particle formation and controlling the contamination level in industrial reactors. Nowadays, dusty plasmas have grown into a vast field and new applications of plasma-processed dust particles are emerging. There is demand for particles with special properties, and for particle-seeded composite materials. Low-pressure plasmas offer a unique possibility of confinement, control and fine tailoring of particle properties. The role of plasma technology in treatment and surface modification of powder grains is reviewed and illustrated with examples. The interaction between plasma and injected micro-disperse powder particles can also be used as a diagnostic tool for the study of plasma surface processes.

Speed of streamers in argon over a flat surface of a dielectric

A pin–pin electrode geometry was used to study the velocities of streamers propagating over a flat dielectric surface and in gas close to the dielectric. The experiments were done in an argon atmosphere, at pressures from 0.1 to 1 bar, with repetitive voltage pulses. The dielectric surface played a noticeable role in discharge ignition and propagation. The average speed of the discharge decreased with higher pressure and lower voltage pulse rise rate. It was higher when the conductive channel between the electrodes was formed over the dielectric, rather than through the gas. Space resolved measurements revealed an increase in velocity of the discharge as it travelled towards the grounded electrode.

Dust formation and charging in an Ar/SiH4 radio-frequency discharge

The formation and charging of submicrometer dust particles in a low pressure argon/silane radio‐frequency (rf) discharge was studied using laser‐induced photodetachment in combination with a microwave resonance technique. This method allows a measurement of the spatially averaged electron density, the spatially resolved negative ion density, and/or the charge on small clusters in the plasma as a function of time during particle formation. The loss frequency of photodetached electrons yields information about the recharging of small clusters. During the first second after plasma ignition dust particles are formed. Simultaneously, the electron density decreases from about 2×1015 m−3 to about 4×1014 m−3. In the first 10 ms after discharge ignition, charged particles are not present in the plasma and the photodetachment experiment gives a negative ion density of 4×1015 m−3. During the first 50 ms after plasma ignition, nanocrystallites are formed, which is reflected by a strong increase of the loss frequency ...

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 dis...