Electron density measurement of inductively coupled plasmas by terahertz time-domain spectroscopy (TH z -TDS)

The electron densities of argon inductively coupled plasmas were measured by terahertz time-domain spectroscopy (TH z -TDS). At a low pressure, the electron densities were also measured with a Langmuir-type double probe and the validity of TH z -TDS electron-density measurement in a plasma has been corroborated. As the input radio-frequency (RF) power increases, the plasma density and gas temperature increase, which makes the probe measurement less reliable or even impossible, due to the large heat load to the probe surface. On the contrary, the TH z -TDS measurement is unaffected by the gas temperature and becomes more reliable due to the higher electron density at higher input power for plasma generation.