Actinometric Study of RF Hydrogen Discharge at 13.56 MHz and 27 MHz

Hydrogen radio frequency (RF) discharge was studied by optical emission spectroscopy and actinometry. The main aim of this work was the comparison of RF discharges at 2 frequencies – 13.56 MHz and 27 MHz as sources of atomic hydrogen. The power delivered to discharge was 30 W. The concentration of atomic hydrogen was determined by actinometric method in the 200 – 600 Pa pressure range with 5% argon as the actinometer. The hydrogen Hα line at 656.3 nm and an argon line at 811.5nm line were used. From the Fulcher-α band (d Πu – a Σg) the temperature of the gas was determined. The electron temperature was determined by Boltzmann plot of the hydrogen Balmer series (Hα, Ηβ, Ηγ, Ηδ). Introduction Atomic hydrogen and sources of hydrogen atoms play an important role in chemical vapour deposition of diamond and other functional materials or as source of VUV radiation for absorption measurements due to Lyman-α emission line at 121 nm. High-density hydrogen atoms are often generated from hydrogen molecules by different kinds of electrical discharges. In all kinds of hydrogen atom-involved applications, it is essential to determine the concentration of hydrogen atoms as functions of the operating parameters. There are several techniques for monitoring atomic density as titration [Duan et al., 2003], using catalytic probes [Mozetic et al., 1996], two-photon absorption laser induced fluorescence (TALIF), [Jolly et al., 2005], vacuum ultraviolet absorption spectroscopy (VUVAS) [Takashima et al., 2001] or actinometry [Thomaz et al., 1999]. Advantage of actinometry is its simplicity. Principle of actinometry lies in adding of small amount of probing gas (several per cent) to investigated gas, usually argon. Subsequently the intensities of emission lines of actinometer and investigated gas are compared. Atomic concentration is derived from this ratio. In some cases, adding of probing gas is not necessary and it can be replaced by emission system radiating in investigated gas. It was used in nitrogen with second positive system as actinometric system or Fulcher-α band in hydrogen discharge [Kitajima et al., 2008; Lavrov et al., 2003]. The aim of our investigation is comparison of effectiveness of dissociation capacitively excited discharge in hydrogen at two frequencies (13.56 and 27 MHz) at pressure range (200–600) Pa. Experimental setup The experimental setup is shown schematically in Figure 1. RF discharge was powered by high voltage power supply with external rings (width 0.5 cm, distance between rings was 2 cm and diameter 1 cm). The frequency of power supply was 13.6 MHz and 27 MHz. We used wattmeter Bird to determine the power of discharge and set it to value of 30 W. Discharge was maintained in quartz tube 55 cm long and with inner diameter 0.65 cm. Pressure was measured by capacitive gauge. Measurements were realised at gas pressures (200, 300, 400, 500 and 600) Pa. The gas flow was monitored by mass MKS flow controller. The abundance of H2 was 95% and 5% of argon. It represents gas flow from 20.0 sccm to 63.3 sccm. The gas flow of Ar varies from 1.0 sccm to 3.2 sccm. Light emitted from discharge axis passed through MgF2 window and was focused by MgF2 lens to optical fiber connected to Andor Mechelle 5000 spectrometer coupled with Andor IStar intensified camera and processed with PC. Influence of actinometer was checked by comparing discharge with and without actinometer. 105 WDS'12 Proceedings of Contributed Papers, Part II, 105–110, 2012. ISBN 978-80-7378-225-2 © MATFYZPRESS