Validity of actinometry to monitor oxygen atom concentration in microwave discharges created by surface wave in O2-N2 mixtures

Abstract

The validity of actinometry to monitor oxygen atom concentration in O2-N2 microwave discharges created by surface wave is investigated. The plasma is created with a gas flow in a quartz tube of inner diameter 16 mm at pressures in the Torr range. First, it is shown that the reliability of actinometry can be deduced from the longitudinal profile of the actinometry signal. Second, absolute concentrations of oxygen atoms are estimated from the experimental actinometry signal and agree satisfactorily with concentrations simultaneously measured by vacuum ultraviolet (VUV) absorption downstream from the plasma. Moreover, upon varying the nitrogen percentage (0%–100%), it is evidenced that the actinometry signal is proportional to the concentration measured by VUV absorption. Furthermore, it is evidenced that the oxygen dissociation rate is only 2% in pure oxygen plasmas, while it reaches 15% (433 MHz) or 30% (2450 MHz) for mixtures containing more than 20% of nitrogen. This drastic increase in [O] upon the addition of N2 is extensively discussed and, finally, attributed to a decrease in the recombination frequency of oxygen atoms on the quartz wall, in the presence of nitrogen.

Use of particle-in-cell simulations to improve the actinometry technique for determination of absolute atomic oxygen density

“…They also found that inclusion of a correction factor for dissociative excitation at lower pressures and currents where the dissociation fraction was found to be < 1% allowed reasonable quantitative results to be obtained for their system. Granier et al [19] applied the method to determine [O] in an O 2 /N 2 microwave surface wave discharge. They monitored atomic oxygen emission at 777 nm and 844 nm in the usual manner.…”

“…Work done by previous authors has shown that application of the actinometry method does not always yield results that reflect the true atomic oxygen number density in the plasma. The validity of actinometry for atomic oxygen has been investigated for various plasma sources: RF capacitive plasma [14,15,16], ECR source [17], DC glow discharge [18], microwave source [19,20], helicon source [21], inductive source [22], and micro-scale atmospheric pressure jet source [23]. Walkrup et al [14] investigated the validity of actinometry for atomic oxygen in RF capacitive plasmas containing both pure O 2 and O 2 /CF 2 mixtures by examining line emission intensity ratios.…”

Role of admixtures on dissociation of molecular gases

“…Many authors [1,2,3,4,5] demonstrated that even a small admixture can dramatically change the dissociation of the gas. This phenomenon is generally supposed to be caused by one or more of the following effects: change of reduced electric field E/N in the discharge, various homogeneous reactions or by decrease of the wall recombination coefficient due to heterogeneous reactions.…”

Real-time plasma control in a dual-frequency, confined plasma etcher

“…In Ref. 17, it is shown that actinometry can be used to monitor the atomic concentration in O 2 and O 2 −N 2 low-pressure ͑67− 266 Pa͒ microwave plasmas.…”