F J de Hoog

Analysis of a Cu-Ne hollow cathode glow discharge at intermediate currents

In a copper-neon hollow cathode glow discharge the authors have measured several parameters. At pressures between 250 and 1000 Pa and cathode current densities between 0.01 and 0.1 A cm-2 the authors obtained gas temperatures between 900 and 1400K with use of Doppler broadened line profiles. The electron transport temperature was obtained from the time dependent optogalvanic effect to be 0.2 to 1.2 eV. Electron densities were derived from Stark broadening of line profiles recorded with saturated absorption spectroscopy; values were between 1 and 9*1019 m-3. Copper ground state densities have been found with the absorption of resonance radiation and were between 0.2 and 6*1019 m-3. Densities of several excited states, including the neon metastables, were measured with the absorption of dye laser radiation; metastable density was of the order of 1018 m-3. They are compared with values obtained from a collisional radiative model. It turns out that ionisation and excitation of neon is caused by beam electrons, ionisation of copper occurs by charge transfer and loss of ions is caused by ambipolar diffusion only. These processes are incorporated in a transport model which has been solved using a sputtering condition at the cathode boundary. In this sputtering condition thermal diffusion of copper atoms and the sticking coefficients of copper atoms and ions are included. The results of the model are in good agreement with the measurements.

Electron density fluctuations in a dusty Ar/SiH4 rf discharge

Many investigators have been working on the problems related to dusty low pressure discharges’V2 and significant progress has been made in understanding particle formation,3 trapping,4’5 charging6 and tran~port.~~~ Also the interaction of particles with the plasma has been studied.9V*0 In this paper we will discuss the behavior of the electron density in a dusty Ar/SiH4 rf discharge. It has been shown that particle formation in this chemical system (under typical conditions: 5% SiH4 in argon at 120 mTorr) is a three-step process.3*11 Within several milliseconds after plasma ignition nanometer size particles are formed. In this process nearly all silicon in the gas phase is incorporated into these particles. In the next formation step these particles coalesce into structures of approximately 20 nm. At ambient temperature the coalescence phase lasts for about 50 ms. As the duration of this process is shorter than a typical residence time of species in the plasma (100 ms and longer), no new active gas is supplied and consequently the total particle mass remains constant during the coalescence. This implies that the particle density decreases. When the grain size reaches about 20 nm, the particles acquire a permanent negative charge and the repulsive Coulomb force prevents further coalescence. The negative charge traps the particles in the positive plasma glow, where they continue to grow by deposition of plasma species. This third formation phase lasts typically for several seconds. Afterwards the particles become too large, so they are (partly) expelled from the discharge and the cycle begins again. Such a formation mechanism results in homogeneously distributed particles with a well defined, monodisperse size distribution. However, several generations of monodisperse particles can be simultaneously present in the discharge.3 Below we will show that the charging of particles results in a decrease of the electron density in the plasma during the particle forma

Laser-particulate interactions in a dusty RF plasma

The interaction of particulates formed in an argon RF discharge containing 1-5% CCl/sub 2/F/sub 2/ admixture with a pulsed infrared laser (Nd:YAG, intensity /spl sim/10/sup 9/ W m/sup /spl minus/2/, pulse duration /spl sim/10/sup /spl minus/4/ s) has been studied in situ. The white light emitted during this process has been monitored as a function of time and wavelength using a fast photo diode and an optical multichannel analyser. The spectra have been fitted with blackbody curves with a standard deviation of 5%. A spectral temperature of about 3500 K has been obtained for various plasma conditions and attributed to the decomposition temperature of the particulate material. A model based on laser heating, internal heat conduction and chemical decomposition is compared with the experimental results. This model predicts the time constants for heating and decomposition of the particulates and explains the dependence of the measured emission intensity on the laser intensity. >

Sheath properties of RF plasmas in a parallel-plate etch reactor; the low-frequency regime (ω<ωi)

The time-dependent Poisson equation has been solved in the space charge sheaths of a parallel-plate RF plasma reactor. The frequency of the applied electric field is assumed to be smaller than the ion plasma frequency. The self-bias voltage has been calculated as a function of the electrode area ratio. The calculated time-dependent sheath voltages, the sheath thicknesses and the resulting current waveforms are non-sinusoidal. The sheath voltages agree with reported measurements.

Fast photon counting in negative corona discharges in the Trichel regime

Using a fast pulse counting system with a time resolution of 0.7 ns, the intensity has been measured of the light originating from a negative corona discharge in the Trichel regime in air, resolved in time, in position in the gap and in wavelength. The dominant spectral lines originate from the second positive system of N2. Both the current and the emission from the plasma carrying the current show a fast increase on a nanosecond timescale and a successive slower decrease. Repetition frequencies are in the MHz range. The maximum of the light pulse intensity corresponds with the maximum of the Trichel pulse. The continuum emission measured originates from the attachment processes during the maximum in the current pulse. The electron density estimated from the continuum measurements is 1018 m-3 at maximum for air at a pressure of 100 kPa. From observed Cul line emission the sputter rate for positive ions impinging on the cathode tip has been determined to be 4*10-5 atoms ion-1.

Infrared spectroscopy of a dusty RF plasma

In situ Fourier transform infrared spectroscopy has been used to study particulate formation in a CCl2F2/Ar RF discharge. Strong absorption bands at 1000-1100 cm-1 have been found and attributed to C-F and Si-F absorption. Furthermore continuous extinction due to Rayleigh and Mie scattering has been observed. The relative intensities of C-F, Si-F and scattering signals vary with plasma conditions. There are several experimental indications that the clusters are formed on the surface and ejected into the plasma. An SEM study of the substrate surface has allowed us to establish the mechanism for the particulate production in this discharge.

Detection of particulates in a rf plasma by laser evaporation and subsequent discharge formation

Nd:YAG‐laser‐induced evaporation of particulates formed in an Ar‐CCl2F2 rf plasma and the subsequent discharge in the vapor have been investigated in situ by means of optical emission spectroscopy. The estimated threshold for discharge formation is 5×106 W/cm2. The maximum laser‐induced emission intensity is observed when the laser is operated in the long‐pulse mode (about 200 μs pulse duration) at the fundamental frequency. The wavelength integrated intensity of this continuum emission has been compared with light scattering intensity at the same laser energy. It has been found that the laser‐induced emission intensity can be more than ten times higher than the scattering intensity, especially for particulates with a diameter much smaller than the wavelength of the laser. Therefore, this effect provides a sensitive particulate detection method.

High-resolution infrared spectroscopy of etching plasmas

Infrared absorption spectroscopy has been used to measure the absolute densities of neutral particles in various fluorocarbon RF plasmas. The densities of CF2 radicals have been measured using a tunable diode laser. Moreover, broadband absorption spectra obtained using a Fourier transform spectrometer have been used to determine the densities of stable reaction products and to assess the degree of dissociation in the plasma. The results indicate that the rotational temperatures of all species involved are slightly above room temperature. The density of CF2 at high gas pressures increases close to the electrodes, indicating production near or on the electrodes and loss in the gas phase. The dissociation degree in plasmas of gases with a high C/F ratio can be as high as 90%. From an analysis of the flow dependence of the degree of dissociation the total dissociation rate coefficients of CF4, CF2Cl2, CF3Cl and C2F4Cl2 have been calculated.

Temperature of neutral copper and neon atoms in a hollow cathode laser discharge

Abstract From measurements with a Fabry-Perot interferometer it was found that neon and copper neutrals in the active part of a hollow cathode charge transfer laser discharge have kinetic temperatures of 1000 K and 2100 K, respectively.

Negative ions and particle formation in low-pressure halocarbon discharges

The growth of particles in a radiofrequency (RF) (13.56 MHz) plasma at pressures from 25 to 200 mTorr in mixtures of CF4, CF2Cl2 and argon has been studied experimentally. A planar configuration was used, with a silicon wafer on the powered electrode. The electron density has been measured with microwave resonance spectroscopy using a cylindrical cavity surrounding the plasma. The same geometry has been used to measure the density of various species of negative ions by detecting the extra electrons created by laser-induced photo detachment. It appears that the negative ion density is much larger in the case of CF2Cl2 than in the case of CF4. There seems to be hardly any dependence of the negative ion concentration on the CF2Cl2/Ar partial pressure ratio in the range where powder growth occurs. However, the attachment rate to chlorine is found to be much higher than to fluorine. Furthermore the gas phase discharge chemistry has been studied using infrared absorption spectroscopy. Both a tunable diode laser system, and a Fourier transform spectrometer have been applied. The CF2 concentration appears to decrease strongly when powder growth occurs. The SiF4 concentration then has a maximum. The results indicate that the presence of chlorine in the plasma feed gas is essential. In CF4 no particle formation is detected. The wafer surface is blackened during powder formation. SEM inspection indicates that this is caused by micromasking. Considering all the information, we arrive at the conclusion that particle growth is initiated by micromasking at the Si surface combined with a highly directional etching process. Due to residual isotropic etching the particles are released from the surface and enter the plasma, where they start coalescing and growing under the influence of CF2 polymerization.