Seunghee Han

Measurement of expanding plasma sheath from a target biased by a negative pulse with a fast rise time

The pulse rise time effect on sheath size is investigated. Experiments are carried out on a planar target biased by a negative pulse with a fast rise time in a plasma source ion implantation (PSII) system. The fast rise time of the pulse provides that the speed of sheath expansion is larger than the Bohm speed during the pulse rise time. Results show that the sheath size increases with increasing pulse voltage and pulse rise rate (the ratio of the pulse voltage and the rise time), which is a very important factor for characterizing the expanding sheath in PSII. During the rise time, sheath size is proportional to the square root of the pulse rise rate over plasma density but it is insensitive to ion mass. Experimental results are in good agreement with the developed model based on the expanding ion matrix sheath model. After the full pulse has been attained, the sheath increases linearly with a constant speed as the Bohm speed.