A plasma diagnostic technique using a floating probe for the dielectric deposition process

Low-temperature RF discharges have been widely used in processing applications. Plasma diagnostics provide useful information about the plasma state which is important for processing results. In the deposition process, as the dielectric material is coated onto the probe surface, electrical diagnostic techniques using a dc current cannot be applied. Instead, an ac voltage is applicable for measuring plasma parameters. In this paper, electron temperatures and plasma densities were measured with an anodized aluminum probe using the floating-type harmonic method and the self-bias method. The Al2O3 layer on the probe surface and the sheath were modeled as a series connection of a capacitor and a resistor, respectively. The applied ac voltage was divided into the two parts depending on their impedances, and the voltage across the sheath was determined by the phase between the voltage and the current. According to experimental results, the conventional harmonic method, which uses the first and second harmonic current, was not valid to measure the electron temperature when the dielectric layer was thick. In contrast, the electron temperature measured by the self-bias method, which uses only the first harmonic current, was reliable regardless of the thickness of the dielectric layer.