Question

4. Explain why the potential of a glow discharge (plasma region) used in dry etching is positive, relative to ground.

Answer 1

Vdc is an extremely important parameter for understanding the dry etching mechanism. In the parallel-plate dry etching equipment [reactive-ion etching (RIE)] a lower electrode, on which a wafer is placed, is connected to an RF power supply through a blocking capacitor, and the opposite electrode (upper electrode) is connected to the ground. A frequently used RF frequency is 13.56 MHz; in other words, the direction of the electric field changes 13.56 × 106 times every second. Because electrons have a small mass, they are able to follow the oscillation of the electric field. On the other hand, ions are much heavier, with masses that are approximately 100,000 times larger, and they are unable to follow the RF oscillation and do not move much from where they are.
As a result, only the electrons accelerated by the electric field jump into the electrodes. Because the lower electrode is connected to a blocking capacitor, it is gradually biased to a negative potential. A direct current (DC) bias that is thus generated is referred to as a self-bias and is represented by Vdc. While the Vdc value is dependent
on the RF power, it would be in the tens of volts to several hundreds volts for the etching of conductive materials such as Si and Al. The electrode voltage turns positive only for a small period of time during each cycle. Electron currents flow onto the electrode during this period. On the other hand, ion currents flow almost continuously.
The sum of the amounts of injected charges in each cycle is zero in a steady state.