Reduction of Electron Shading Damage Using Synchronous Bias in Pulsed Plasma

A novel method is proposed for reducing charging damage due to the “electron shading” effect. The concept is to selectively utilize the coolest electrons in a pulsed plasma at the end of its off period by synchronizing rf bias; thereby one should be able to reduce the negative charge build-up responsible for the damage. This concept has been examined using an inductively coupled plasma (ICP) apparatus. Exposure to a cw Ar ICP damaged most MOS capacitors with a 6-nm-thick gate oxide and connected to 105 shaded antennas. This damage was reduced only slightly even with 5-µ s- on/10-µ s- off pulse modulation when the rf bias was asynchronous (60 kHz). When the rf bias (66.7 kHz) synchronized at the expected optimal phase, the most significant reduction of the damage was observed. This effect is discussed based on the results of time-resolved probe and optical emission measurements.