de Fj Frits Hoog

Measurements of negative ion densities in 13.56‐MHz rf plasmas of CF4, C2F6, CHF3, and C3F8 using microwave resonance and the photodetachment effect

The high‐power density of a frequency quadrupled pulsed Nd‐YAG laser has been used to photodetach electrons from negative ions in rf plasmas generated within a microwave cavity. Negative ion densities have been determined by measuring the frequency shift of the resonance transmission, the shift being caused by the photoelectrons created by irradiating the plasma with the laser pulse. By measurement of the shape of the resonance curve as a function of time and of microwave frequency, and consecutive fitting of a parabola to the top of the resonance curve, the negative ion density has been determined as a function of gas pressure, rf power, and position in the plasma. Measurements were performed in plasmas of CF4, C2F6, CHF3, and C3F8. The results indicate that the negative ion densities are about one order of magnitude larger than the electron density, which is in good agreement with a fluid model calculation. The pressure and power dependence of the electron density and of the negative ion density gives i...

Temporal behavior of the electron and negative ion densities in a pulsed radio‐frequency CF4 plasma

Electron and negative ion densities in the afterglow and in the plasma initiation phase of a 13.56‐MHz rf discharge in CF4 were measured by using a microwave cavity method and a laser photodetachment technique. Measurements were carried out at low rf powers (≲10 W) and in the pressure range from 100 to 300 mTorr. The electron density in the afterglow showed an enhanced decay rate due to the presence of negative ions. Electrons originating from negative ions through associative collisional detachment with neutral radicals were also detected in the afterglow. Decay curve analysis of the negative ion density gave an ion–ion (presumably CF+3−F−) recombination rate constant of (5±2)×10−13 m3 s−1, and showed that, in the active plasma, the negative ion loss rates by associative detachment and ion–ion recombination are of the same order of magnitude. The behavior of the electron and negative ion densities in the plasma initiation phase indicates that molecules and radicals that slowly accumulate in the plasma do...

Transport of argon ions in an inductively coupled high-density plasma reactor

The first direct observation of the velocity distribution of the metastable Ar+*(2G9/2) ions in the presheath of an inductively coupled plasma has been achieved by using the Doppler shifted laser induced fluorescence technique. Drift of the ions along the electric field in the presheath is observed and distribution functions of the velocity in both parallel and perpendicular directions, relative to the E field, are deduced at 5 and 40 mTorr. Present results show that in high density plasmas the velocity distribution of the metastable ions is directly related to that of the ground state argon ions. Neutral gas temperature of around 600 K is also measured from the absorption profile of a diode laser beam, set on one of the 772.4 nm argon lines.

Thomson scattering using an atomic notch filter

One of the biggest problems in performing Thomson scattering experiments in low-density plasmas is the very high stray light intensity in comparison with the Thomson scattering intensity. This problem is especially present in fluorescent lamps because of the proximity of the glass tube. We propose an atomic notch filter in combination with a dye laser and an amplified spontaneous emission (ASE) filter as a way of reducing this stray light level. The dye laser produces 589 nm radiation which is guided through the ASE filter that increases the spectral purity. The beam is then guided in the fluorescent lamp, where the Thomson scattering process takes place. The scattered light is collected and guided through a sodium vapor absorption cell, where the stray light is absorbed because it is resonant to the D2 transition of sodium. The spectral width of the Thomson scattering light is large enough to be transmitted through the absorption cell. In this way we only measure the Thomson scattering light.

Mass-resolved ion energy measurements at the grounded electrode of an argon rf plasma

The mass‐resolved ion energy distribution (IED) at the grounded electrode has been determined in a 13.56‐MHz parallel‐plate plasma in argon. The IED is determined of Ar+, Ar2+, Ar2+, ArH+, H3O+, and H3+ for several plasma conditions. At pressures higher than 10 mTorr, collisions in the sheath become important. The IED of Ar+ is particularly defined by charge exchange collisions in the sheath while the IED of the other ions shows only features generated by elastic scattering. This is confirmed by Monte Carlo simulations. The measurements clearly show the necessity of simultaneous mass and energy separation, rather than the nonmass‐resolved IED reported in the literature.

Photodetachment effect in a radio frequency plasma in CF4

Experiments to study negative ion densities have been carried out using the photodetachment effect in a rf plasma in CF4. Electrons are detached from the negative ions under the influence of the pulse of a Nd:YAG laser. The induced increase of the electron density is measured as a function of time using the shift of the resonance frequency of a microwave cavity containing the plasma. The negative ion density [F−] is found to be about (4±1)×1015 m−3, a factor 4±1 higher than the electron density.

Optogalvanic effects in a hollow cathode glow discharge plasma

The hollow cathode glow discharge is a very suitable source to measure the optogalvanic effect (OGE). A detailed study of this effect in neon has been made using a tunable, single‐frequency dye laser. It was found that the lower level of the irradiated transition together with the plasma conditions determine the sign of the effect. Three situations occur: the lower level can either be metastable, resonant, or highly excited. With a five‐level model taking into account electronic collisions and radiative decay a qualitative description of all the cases found in the experiment can be given. Further, it is shown that from a frequency scan of the OGE, line profiles can be obtained which, under certain conditions, contain the gas temperature in their Doppler part. Temperatures ranging from 700 to 1300 K determined this way are consistent with other results. This method makes it possible to measure the temperature as a function of the position, showing that inside the glow the gas temperature is constant. Final...

Mass-resolved ion energy measurements at both electrodes of a 13.56 MHz plasma in CF4

The ion energy distributions (IEDs) at the electrodes in a capacitively coupled 13.56 MHz plasma in CF4 have been measured mass resolved with a Balzers quadrupole in combination with a home‐built energy analyzer. Mass‐resolved determination offers the possibility to compare the IED of different ions achieved in the same sheath. The IEDs have been determined at both the largest and the smallest electrode. Apart from the IEDs of the CF4 species, the IEDs of ionic species in plasmas in argon and nitrogen also were determined. Apart from the CF4 ionic species CF+3, CF+2, CF+, and F+, CHF+2 ions also are present in the CF4 plasma due to residual water in the reactor. Because the CHF+2 ions are not produced in the sheath and because we do not detect elastically scattered ions, the IEDs of these ions show the typical bimodal distribution for rf plasmas which corresponds to an IED of ions which have not collided in the sheath. From these IEDs we can obtain the sheath characteristics, such as the averaged sheath p...

Measurement of the complex refractive index of liquids in the infrared using spectroscopic attenuated total reflection ellipsometry: correction for depolarization by scattering

With spectroscopic ellipsometry one can measure the real and imaginary parts of the refractive index of a medium simultaneously. To determine this index in the infrared for a number of technical liquids, use was made of attenuated total internal reflection at the glass-liquid interface of a specially designed prism. This attenuated total reflection approach warrants minimal signal loss and is, for strongly absorbing liquids, the only way to measure the complex refractive index. A surprising phenomenon, observed when BK-7 prism glass was used, is scattering in the vicinity of the absorption wavelengths of the glass. A simple model that can be used to describe the relations among absorption, scattering, and depolarization was successfully used to correct the measurements. Refractive indices for demineralized water, Freon 113, heptane, benzene, gas oil, and crude oil in the wave number range from 5000 to 10,000 cm(-1) (1-2 µm) are presented.

Negative ions in a radio-frequency plasma in CF4

Experiments to study negative ion densities in a radio-frequency CF4 plasma have been carried out using a photodetachment technique. Electrons are photodetached from the negative ions using the pulse of a Nd-YAG laser at the tripled (355 nm) or the quadrupled (266 nm) frequency. The photodetached electrons are detected by a microwave method as a sudden increase of the electron density in the plasma. The negative ion density, which consists mainly of F- is found to be typically four times higher than the stationary electron density at a pressure of 13 Pa, an RF power of 15 W and a CF4 flow of 15 SCCM. The measured decay of the detached electrons after the laser pulse has been interpreted in terms of electron attachment and ambipolar diffusion. The results demonstrate the possibilities for use of this technique to evaluate attachment coefficients in active plasmas. The attachment rate constant for CF4 is found to be (7+or-1)*10-17 m3 S-1 at RF powers of 15 W. The electron diffusion coefficient is 0.13+or-0.12 m2 s-1 at standard conditions of 1 Torr and 300 K.