Plasma Technology for Advanced Devices
Charging effects during plasma etching of high aspect ratio structures can cause gate oxide degradation during gate etching and profile deformation issues such as notching or bowing. Charging effects become important for aspect ratios higher than 2:1. The origin of this phenomenon is due to the difference in directionality between ions and electrons when they cross the plasma sheath and interact with three dimensional structures (slide 1).
At low and medium frequencies (< 10 MHz), the ions enter the plasma sheath at different phases of the RF cycle resulting in a bi-modal ion energy distribution. For higher frequencies, the energy distribution exhibits only one peak since the period of the RF signal is much smaller than the time it takes for the ion to travel through the sheath resulting in the ion experiencing just an average field. Because of the acceleration in the sheath, the angular distribution is very directional for ions.
Electrons can respond to the instantaneous electric field, they enter the sheath with a Maxwellian energy distribution in speed but very isotropic directionality. The difference in directionality between ions and electrons leads to charging effects which may strongly impact plasma processes (slide 2).
The following discussion is adapted from Hwang and Giapis, JVST B15, 70, (1997). For featureless surfaces, ion and electron fluxes onto large open areas are equal. Any vertical surface on the wafer (feature sidewall) screens a part of the electron flux: the net flux of electrons arriving on the surface decreases. In contrast, the ion flux is not impacted. If the surface at the bottom is an insulator, it charges positively, possibly leading to a partial deviation of the ion flux. If the mask is an insulator, it will charge negatively. For two adjacent sidewalls are present (space between two lines or trenches), the shadowing effect becomes even more pronounced. The insulator between the lines will be charged even more (slide 3).
Simulations by Hwang and Giapis show that it takes about 1000 RF cycles for the ion and electron fluxes to reach a steady state. This leads to a stongly assymetric potenial distribution. A very strong peak potential develops inside the structure close to the last polysilicon line. The dramatic re-distribution of potential occurs leads to the conditions necessary for an ion trajectory deflexion and ion acceleration towards the silicon sidewalls (slide 4).
In the steady state etching regime, 15 eV ions can get repelled away from the outer sidewall. The large positive potential at the trench bottom can slow down energetic ions so that they can be deflected and accelerated towards the lower part of the poly silicon sidewall. Increasing the ion energy to 30 eV reduces these effects (slide 5).
The very strong peak potential that develops close to the edge of the structure induces a ion trajectory distortion. Close to the edge of polysilicon, ions never reach SiO2 at the bottom. They get deflected by the high potential at the bottom that develops on the SiO2 surface and reach the polysilicon sidewalls. There, the ions can generate some etching if their energy is high enough to punch through the passivation layer (slide 6).