Plasma and Electrode Potentials

An RF plasma system can be represented by an equivalent circuit in the capacitive sheath approximation (slide 1). The cathode sheath and the anode (wall) sheath are represented by parallel ohmic and capacitive resistors as well as diode elements. The bulk plasma is an ohmic resistance. The RF signal generates a dc potential on the electrode which can be calculated knowing the RF voltage and the capacitances of the cathode and anode sheaths. For a symmetric reactor (areas of cathode and anode are approximately equal), the average plasma voltage is half of the sum of the RF and dc voltages.

Slide 2 illustrates the plasma and excitation electrode potentials for dc and capacitively coupled plasmas for different cathode to anode area ratios. Most plasma etch reactors are capacitively coupled and the anode (wall and reactor lid) have a much larger area compared to the cathode. For this type of reactor, the dc voltage is negative (which attracts positively charged ions to enhance the etch process) and the RF signal is positive with respect to ground only for a very short period of time. At this point in time, the plasma and RF voltages are equal. The plasma voltage is zero when the RF voltage reaches it's minimum.

Slide 3 shows the ion and electron currents to the powered electrode and the reactor walls for the capacitively coupled asymmetric reactor. A constant ion current is interrupted by a short burst of electron current when the peak voltage of the RF signal becomes positive with respect to ground. The time integrated ion and electron currents are equal maintaining the overall charge balance of the plasma.

The change of the electrode and wall potentials during the RF duty cycle is illustrated in slide 4 (compare to the lower right figure on slide 2).

More plasma etch fundamentals …