Microwave interferometry at 160.28 GHz with Gaussian beam propagation (beam waist: 5 mm) and laser photodetachment were combined for the analysis of negative atomic oxygen ions in the bulk plasma of an asymmetric capacitively coupled 13.56 MHz discharge (cc-rf). The line-integrated negative oxygen ion density amounts to between 2.5 × 10 14 and 10 15 m −2 depending on the oxygen pressure and rf power. Furthermore, the measured decay of the detachment signal reveals two modes of rf oxygen plasma characterized by different electronegativities. High electronegativity, α > 2, is associated with a low decay time constant of only a few microseconds, whereas in oxygen plasmas with low electronegativity, α < 1, the relaxation of electron density needs much longer with typical decay time constants of up to about 100 µs. The transition between the two modes shows a step-like characteristic and was observed at a specific rf power depending on the oxygen pressure. In the case of high electronegativity the electron density relaxation can be described by a simple 0D-attachment-detachment model, taking into consideration a constant density for positive ions and neutral oxygen species. Using the appropriate rate coefficients from the literature and the experimentally determined effective rate coefficients of first order kinetics, the evaluation of the attachment and detachment rates indicates the significant role of O 2 (a 1 g ) in the formation and loss of negative atomic oxygen ions.
In this series of two papers, the E-H transition in a planar inductively coupled radio frequency discharge (13.56 MHz) in pure oxygen is studied using comprehensive plasma diagnostic methods. The electron density serves as the main plasma parameter to distinguish between the operation modes. The (effective) electron temperature, which is calculated from the electron energy distribution function and the difference between the floating and plasma potential, halves during the E-H transition. Furthermore, the pressure dependency of the RF sheath extension in the E-mode implies a collisional RF sheath for the considered total gas pressures. The gas temperature increases with the electron density during the E-H transition and doubles in the H-mode compared to the E-mode, whereas the molecular ground state density halves at the given total gas pressure. Moreover, the singlet molecular metastable density reaches 2% in the E-mode and 4% in the H-mode of the molecular ground state density. These measured plasma parameters can be used as input parameters for global rate equation calculations to analyze several elementary processes. Here, the ionization rate for the molecular oxygen ions is exemplarily determined and reveals, together with the optical excitation rate patterns, a change in electronegativity during the mode transition.
Key words Low pressure rf discharge, oxygen plasma, electron and negative ion density, plasma instability.The capacitively coupled radio frequency plasma at 13.56 MHz in oxygen was systematically studied by 160 GHz Gaussian beam microwave interferometry at high temporal resolution (200 ns) and simultaneous laser photodetachment for electron and negative ion density analysis. Additionally, spatio-temporally resolved electric probe measurements were performed for comparison with microwave interferometry. A high and low electronegative operation mode was found in the asymmetric rf discharge. In the high electronegative mode it was shown the significant role of the metastable excited oxygen molecules in electron attachment and detachment processes. In particular, a temporary electron density increase is observed in the early afterglow of a pulsed rf plasma. The transition between both modes is driven by the rf power and the self-bias voltage, respectively. In connection with the phase resolved optical emission spectroscopy and the study of the electron heating mechanisms the transition into the low electronegative mode at higher rf power shows a relation to the alpha to gamma mode transition. Furthermore, electron density fluctuations are measured over a wide field of processing parameters, e.g. due to the attachment-induced ionization instability. PIC-MCC simulation and fluid model calculation of a symmetric oxygen rf discharge confirm the different electron heating mechanisms and the dominance of negative atomic oxygen ions.
Periodic fluctuations in the frequency range from 0.3 to 3 kHz were experimentally investigated in capacitively coupled radio frequency (13.56 MHz) oxygen plasma. The Gaussian beam microwave interferometry directly provides the line integrated electron density fluctuations. A system of two Langmuir probes measured the floating potential spatially (axial, radial) and temporally resolved. Hence, the floating potential fluctuation development is mapped within the discharge volume and provides a kind of discharge breathing and no wave propagation. Finally, it was measured the optical emission pattern of atomic oxygen during the fluctuation as well as the RF phase resolved optical emission intensity at selected phase position of the fluctuation by an intensified charge-coupled device camera. The deduced excitation rate pattern reveals the RF sheath dynamics and electron heating mechanisms, which is changing between low and high electronegativity during a fluctuation cycle. A perturbation calculation was taken into account using a global model with 15 elementary collision processes in the balance equations for the charged plasma species (O2+, e, O−, O2−) and a harmonic perturbation. The calculated frequencies agree with the experimentally observed frequencies. Whereby, the electron attachment/detachment processes are important for the generation of this instability.
160 GHz Gaussian beam microwave interferometry is realized for electron density analysis in low pressure rf plasmas. Measurement of electron densities lower than 10 16 m −3 with corresponding phase shift less than 0.3 • demands high stability of the interferometer frequency and minimum disturbance due to external interfering voltages and mechanical vibrations of the optical components. The interferometer consists of a frequency stabilized (phase lock loop) heterodyne system operating at a frequency of f MWI = 160.28 GHz and wavelength of λ MWI = 1.87 mm, respectively. A quasi-optical setup is used, considering specially designed horn antennas and elliptical mirrors as well as components which have to comply with the aperture limit in relation to the Gaussian microwave beam and its optimal coupling and focusing into the plasma center. A spatial and temporal resolution of about 10 mm (beam waist 5 mm) and 0.2 µs is achieved, respectively. In cc-rf plasma the lowest measurable phase shift is in the order of 0.01 • , which corresponds to a line-integrated electron density of about 5 × 10 13 m −2 or an electron density of 5 × 10 14 m −3 averaged over the electrode diameter. Results are presented and discussed concerning line-integrated electron density in an asymmetric argon cc-rf plasma in dependence on rf power and total pressure.
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