Experimental investigation of a low pressure capacitively-coupled discharge

In this thesis, a low-pressure, capacitively-coupled plasma (CCP) was investigated using the well established techniques of actinometry, two-photon laser-induced fluorescence (TALIF), appearance potential mass spectrometry (APMS), Langmuir and hairpin probes. The behaviour of atomic oxygen density in mixtures of O2/SF6 was investigated using TALIF and a Langmuir probe. A significant five-fold increase of [O] was observed when O2 plasma was diluted with SF6 (5 - 10%). This was attributed to a combination of a change in surface conditions and also due to a shift in the effective electron temperature, Tef f . It was found that Tef f dramatically increased from ≈ 1 to 8 eV as the SF6 content varied from 0 - 60% which consequently resulted in a threefold increase in the atomic oxygen production rate. [O] increased by over a factor of three after fluorinating the walls with SF6 plasma. Electron energy distribution functions, EEDFs, required for calculating excitation rate constants needed for actinometry were measured with a Langmuir probe. When compared to TALIF measurements; actinometry showed excellent quantitative agreement in pure O2 plasma but did not predict the rapid rise of [O] with SF6 addition and grossly overestimated [O] in O2 plasma containing SF6. However, reasonable qualitative agreement was shown for mixtures in the range of 10 to 60% SF6. A Langmuir probe was used to investigate various electron heating phenomena in O2 and Ar plasmas. The so called ‘α − γ’ mode transition in O2 plasma (100 mTorr) was identified from the power evolution (3 - 600 W) of the electron energy probability function (EEPF) electron density (ne) and Tef f . The EEPF evolved from Druyvesteyn to bi-Maxwellian with increasing power which resulted in a rapid decrease and abrupt increase in Tef f and ne respectively. Comparisons were made to the same mode transition for simi-17 ar conditions in Ar plasma. Low energy electrons were effectively heated in Ar plasma when compared to O2 plasma. This was unusual since Ar atoms have an abnormally low momentum transfer cross-section (Ramsauer effect) which does not favour collisional heating of low energy electrons. The pressure evolution of the EEPF, ne, and Tef f was also investigated in O2 plasma operated at 30 and 200 W. The number of high energy electrons decreased with increasing pressure (10 - 70 mTorr), however, high energy electrons were enhanced for pressures above 70 mTorr at 30 W and greater than 400 m Torrat 200 W. This behaviour is contrary to the inverse relationship between the ionization rate and gas density (particle balance). The mechanisms behind such unusual behaviour are still unknown and require further investigation. Although, structure was observed on the EEPFs which could indicate the presence of super-elastic collisions; a process which is known to enhance the tail of the EEPF. Atomic fluorine density was measured in both SF6/Ar (95/5%) and SF6/O2/Ar (85/10/5%) discharges using appearance potential mass spectrometry (APMS). The dissociation fraction, D, increased from 0.3×10−3 - 2.4×10−3 with increasing power (50 - 300 W) in SF6/O2/Ar plasma at 40 mTorr. D in SF6/Ar plasma was over 22 times lower than in SF6/O2/Ar plasma. The presence of O2 in the feedstock significantly enhanced [F] in the discharge. The F+ signal increased by a factor of 60 when 10 % O2 was added to SF6 plasma, whereas, actinometry predicted only a threefold increase of [F]. Similarly, absolute [F] measured with APMS increased by a factor of 10 with increasing power in SF6/O2/Ar plasma whereas actinometry only showed a twofold increase. The comparison of these techniques showed that actinometry for measuring [F] was unreliable for the condition explored in this work

Application of two-photon absorption laser induced fluorescence to validate actinometry measurements of absolute atomic oxygen number density based on improved EEDFs obtained from PIC simulations.

Actinometry is a non-invasive optical technique that allows absolute atomic oxygen density determination within a plasma provided certain conditions are met. However, the technique is sensitive to to the accuracy of the Electron Energy Distribution Function (EEDF). A Maxwellian distribution is often used for actinometry calculations, but this is typically just an approximation. A Particle in Cell (PIC) code is used to try and generate a more accurate EEDF to improve the actinometry results. To do this the electron density in the plasma is measured using a hairpin probe and compared to the electron density predicted by the PIC code. The code is adjusted to get a reasonable agreement with the hairpin probe electron densities. The corresponding EEDF from the PIC code is then used in the actinoometry model to calculate the atomic oxygen density in the plasma. The actinometry results are compared to oxygen density measurements made using Two-photon Absorption Laser Induced Fluorescence to validate the actinometry results

Experimental investigation of electron heating modes in capacitively coupled radio-frequency oxygen discharge

A Langmuir probe has been used to investigate electron heating mechanisms in a capacitively coupled oxygen discharge over a wide pressure range (50 – 800 mTorr) at a fixed applied power (200 W). Evidence presented here from experimentally obtained electron energy distribution functions (EEDFs) illustrates discharge transition from collisionless (stochastic) to collisional (ohmic) dominant regime with increasing oxygen pressure. The discharge exhibited a bi-Maxwellian EEDF in the collisionless regime, dominated by stochastic heating whereas Druyvesteyn-like EEDFs in the collisional dominant regime. Moreover, in the transition between these two regimes, parameters such as electron density, effective electron temperature and electron-neutral collision frequency exhibited significant variations

Experimental investigation of a low pressure capacitively-coupled discharge

In this thesis, a low-pressure, capacitively-coupled plasma (CCP) was investigated using the well established techniques of actinometry, two-photon laser-induced fluorescence (TALIF), appearance potential mass spectrometry (APMS), Langmuir and hairpin probes. The behaviour of atomic oxygen density in mixtures of O2/SF6 was investigated using TALIF and a Langmuir probe. A significant five-fold increase of [O] was observed when O2 plasma was diluted with SF6 (5 - 10%). This was attributed to a combination of a change in surface conditions and also due to a shift in the effective electron temperature, Tef f . It was found that Tef f dramatically increased from ≈ 1 to 8 eV as the SF6 content varied from 0 - 60% which consequently resulted in a threefold increase in the atomic oxygen production rate. [O] increased by over a factor of three after fluorinating the walls with SF6 plasma. Electron energy distribution functions, EEDFs, required for calculating excitation rate constants needed for actinometry were measured with a Langmuir probe. When compared to TALIF measurements; actinometry showed excellent quantitative agreement in pure O2 plasma but did not predict the rapid rise of [O] with SF6 addition and grossly overestimated [O] in O2 plasma containing SF6. However, reasonable qualitative agreement was shown for mixtures in the range of 10 to 60% SF6. A Langmuir probe was used to investigate various electron heating phenomena in O2 and Ar plasmas. The so called ‘α − γ’ mode transition in O2 plasma (100 mTorr) was identified from the power evolution (3 - 600 W) of the electron energy probability function (EEPF) electron density (ne) and Tef f . The EEPF evolved from Druyvesteyn to bi-Maxwellian with increasing power which resulted in a rapid decrease and abrupt increase in Tef f and ne respectively. Comparisons were made to the same mode transition for simi-17 ar conditions in Ar plasma. Low energy electrons were effectively heated in Ar plasma when compared to O2 plasma. This was unusual since Ar atoms have an abnormally low momentum transfer cross-section (Ramsauer effect) which does not favour collisional heating of low energy electrons. The pressure evolution of the EEPF, ne, and Tef f was also investigated in O2 plasma operated at 30 and 200 W. The number of high energy electrons decreased with increasing pressure (10 - 70 mTorr), however, high energy electrons were enhanced for pressures above 70 mTorr at 30 W and greater than 400 m Torrat 200 W. This behaviour is contrary to the inverse relationship between the ionization rate and gas density (particle balance). The mechanisms behind such unusual behaviour are still unknown and require further investigation. Although, structure was observed on the EEPFs which could indicate the presence of super-elastic collisions; a process which is known to enhance the tail of the EEPF. Atomic fluorine density was measured in both SF6/Ar (95/5%) and SF6/O2/Ar (85/10/5%) discharges using appearance potential mass spectrometry (APMS). The dissociation fraction, D, increased from 0.3×10−3 - 2.4×10−3 with increasing power (50 - 300 W) in SF6/O2/Ar plasma at 40 mTorr. D in SF6/Ar plasma was over 22 times lower than in SF6/O2/Ar plasma. The presence of O2 in the feedstock significantly enhanced [F] in the discharge. The F+ signal increased by a factor of 60 when 10 % O2 was added to SF6 plasma, whereas, actinometry predicted only a threefold increase of [F]. Similarly, absolute [F] measured with APMS increased by a factor of 10 with increasing power in SF6/O2/Ar plasma whereas actinometry only showed a twofold increase. The comparison of these techniques showed that actinometry for measuring [F] was unreliable for the condition explored in this work