Properties of an Internal U-Shaped Inductively Coupled Plasma Source Operated by Superimposed Dual Frequency

Plasma properties of a U-shaped internal inductively coupled plasma (ICP) source (2.3 m long) operated by superimposed dual frequency was investigated as a function of power ratio of dual frequency composed of 2/13.56 MHz and 6/13.56 MHz. For the 2.3-m long linear internal-type ICP source, the use of 13.56 MHz as the operating frequency could result in nonuniform plasma due to the standing wave effect along the antenna line. By using the superimposed dual frequency composed of low frequency (2 or 6 MHz)/high frequency (13.56 MHz) at a fixed total power, higher plasma uniformity could be observed possibly due to the decreased standing wave effect compared to that obtained by 13.56 MHz without changing the plasma density. The use of 2/13.56 MHz instead of 6/13.56 MHz exhibited higher plasma uniformity. In addition, by decreasing the capacitive coupling component by the operation at the lower RF frequency, the ion bombardment energy to the substrate was lower for the operation with the superimposed dual frequency (2 or 6/13.56 MHz) compared to that with 13.56 MHz only.