A miniaturized ECR plasma flood gun for wafer charge neutralization

In modern ion implanters, a plasma flood gun (PFG) is used to neutralize wafer charge during the doping process, preventing the breakdown of floating wafers caused by the space charge accumulation. Typically, there are two kinds of PFGs, namely, dc arc discharge with filament and RF discharge. As a PFG, the filament one has limited lifetime and cannot avoid metallic contamination because of the thermal emitting filament. RF discharge PFG has been developed to solve these problems, including prolonging the source lifetime and avoiding metal pollution. Recently, a 2.45 GHz electron cyclotron resonance (ECR) ion source is also regarded as a potential choice for PFG. However, the dimension of the 2.45 GHz ECR source system including the size of the source itself and its meter's length RF subsidiary limits its application within an ion implanter. At Peking University, a miniaturized 2.45 GHz permanent magnet electron cyclotron resonance plasma flood gun with a coaxial RF transmission line has been built and tested. The dimensions of the ECR source body are Φ60 mm × Φ88 mm with a Φ30 mm × Φ40 mm plasma chamber. Its RF transmission line consists of a 200 W microwave generator, a 30 cm coaxial line, a 7 cm coaxial-to-waveguide transducer, and a microwave window that also serves as a vacuum seal. In continuous wave experiments, the electron extraction currents can be as high as 8.8 mA at an input RF power of 22 W with argon gas. The gas flow is less than 1.0 SCCM for this test.