Investigation of O-atom kinetics in O₂, CO₂, H₂O and O₂/HMDSO low pressure radiofrequency pulsed plasmas by time-resolved optical emission spectroscopy

Abstract

In the present paper, O-atom kinetics is investigated in low pressure (around 0.7 Pa) O 2 /hexamethyldisiloxane pulsed plasmas by time-resolved optical emission spectroscopy. In order to distinguish direct O-atom excitation by electron impact from dissociative excitation of O 2 , CO, CO 2 and H 2 O molecules formed in O 2 /HMDSO plasmas, measurements were first carried out in pure O 2 , CO 2 and H 2 O plasmas. From these measurements it was concluded that direct O-atom excitation by electron impact was the dominant mechanism and that actinometry could be used to monitor O-atom kinetics provided molecular oxygen dissociative excitation is taken into account. Second, the surface O-atom loss coefficient measured in oxygen pulsed plasmas is in agreement with the literature (10 −1 ) and is five times higher than the one measured in CO 2 pulsed plasma (2 × 10 −2 ). This latter result is attributed to competition effect at surface sites between CO and O. Finally, we point out the influence of water molecule wall adsorption on O-atom kinetics in H 2 O and O 2 /HMDSO pulsed plasmas. During the post-discharge, H 2 O and/or OH molecules adsorbed at the reactor walls strongly reduce the O-atom loss coefficient. During the discharge, ion bombardment enhances desorption of molecules leading to an increase in the O-atom loss coefficient.

Etching and Post‐Treatment Surface Stability of Track‐Etched Polycarbonate Membranes by Plasma Processing Using Various Related Oxidizing Plasma Systems

“…Although the dissociative excitation of ground state O 2 likely makes a minor contribution to measured in the pure O 2 plasma, this pathway can be ignored in the CO 2 and H 2 O plasma systems. Bousquet et al investigated the atomic oxygen excitation kinetics in O 2 , CO 2 , and H 2 O plasmas. They found that production of O* via dissociative excitation of O 2 , CO 2 , CO, H 2 O, or OH in CO 2 and H 2 O plasma systems was negligible.…”

Oxygen (3P) atom recombination on a Pyrex surface in an O2 plasma

“…This is preferable to using the O atom density since in this case the effect of gas temperature variations on the total gas density, N, are eliminated. The [O]/N variations were followed by time-resolved actinometry, using Ar (added as 5% of the gas flow) as the actinometer gas [1,3,4,6,[9][10][11]13,[35][36][37][38][39][40][41][42]. The effect of this small addition of Ar on the EEDF and other discharge parameters is negligible, since the EEDF is largely determined by electron collisions with the majority gas, O 2 [43].…”

“…Oxygen gas is widely used in many plasma processing applications, and oxygen atoms are one of the key reactive species present in them, leading to recurrent interest in O atom kinetics over a range of different discharge conditions. In low-pressure pure oxygen plasmas these atoms are principally created by electron impact dissociation of O 2 , and predominantly lost by surface recombination; therefore these two processes define the atom density in the plasma volume [1][2][3][4][5][6][7][8][9][10][11][12]. The surface recombination of O atoms has been previously studied on a range of different materials, including dielectrics, semiconductors and metals [1][2][3][4][5][6][7][8][9][10][11][12].…”

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

“…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.…”

“…They concluded that actinometry using the O * (844)/Ar * (750) emission ratio was valid for modest Ar/O 2 flow ratios (smaller than 2) when the total pressure in their system exceeded 1 Torr while the O * (777)/Ar * (750) ratio gave misleading results. Bousquet et al [21] used actinometry in a low pressure (5 mTorr) pulsed O 2 /Hexamethyldisiloxane helicon source to monitor atomic oxygen density. They determined that direct excitation of atomic oxygen was the main source of emission at 844 nm with only a small contribution due to dissociative excitation of O 2 and that the actinometry technique using the O * (844)/Ar * (750) emission ratio was valid under these experimental conditions provided dissociative excitation of O 2 was also taken into account.…”