Effects of C4F8 plasma polymerization film on etching profiles in the Bosch process

The Bosch process is a deep etching method for silicon that uses C4F8 plasma-deposited polymerized films as passivation films to protect the silicon sidewalls. This study measured the deposition rate of the passivation films and the etch rate with F-radical exposure and analyzed the chemical composition of the films. Additionally, we observed the deformation of the passivation films during the Bosch process and assessed its influence on the etch profiles. As the C4F8 flow rates increased, the deposition rates attained a local maximum, subsequently decreased to a local minimum and then increased again. The deposition rates were extremely low when the pressure exceeded 10 Pa. With the increasing C4F8 flow rates, inductively coupled plasma power, and pressure, the respective bond content varied up to 10%, and C—CFX and C—C bond contents were replaced with CF2 and CF contents, respectively. The results indicated that the chemical composition of the films did not affect the etch rates of the films, and upon exposure to F radicals, the chemical composition of all films transformed into an identical chemical composition with a higher CF2 bond content. Polymerized films with low CF2-bond content deformed with F-radical exposure, enabled the passage of F radicals, and did not serve as passivation films. In addition to high deposition rates and high F-radical resistance, the Bosch process requires passivation films with high CF2 bond content. The present findings will aid in tuning the parameters of the Bosch process and increase the productivity of silicon deep reactive-ion etching.