IMPROVED WAFER CHARGE NEUTRALIZATION SYSTEM IN VARIAN HIGH-CURRENT IMPLANTERS

Control of wafer charging during high current implantation plays a key role in achieving higher device yields. As certain device dimensions shrink with a corresponding reduction in gate oxide thicknesses, their sensitivity to charging increases. This characteristic of devices, compounded with the increasing beam currents available in commercial high current implanters (up to 25 mA or more), poses critical challenges to techniques used for wafer charge control during implantation. In this paper, we will present the key features of an auto emission flood control system which holds the disc current constant at a desired setting, and minimizes any charge fluctuations on the wafer in real time. The features of this system were compared to those of "no electron flood" and "constant emission flood system" using a variety of wafers (e.g., bare, poly-Si, poly-Si-on-oxide and photoresists, etc.). In all cases, the charging characteristics of each wafer were monitored in real time using a capacitive charge collecting device. Correlations between disc current, flood emission current and the charge collectors response were used to demonstrate the merits of the auto emission flood control system. In addition, the flood gun options available on the XP series of high current implanters, including the features of the charge collecting device, will be presented.