The effect of driving RF frequency on H- ion production in TCP H- ion source

Abstract

A hydrogen negative ion source based on Transformer Coupled Plasma (TCP) source with external RF antenna has been developed at SNU, which is able to operate continuously and has an advantage of long lifetime. A driving RF frequency of 13.56MHz has been used to generate plasma in TCP H- ion source. However, it is well known that driving RF frequency affect the performance of H- ion source by varying plasma parameters such as electron temperature and electron density. In volume production, which is one of the most important physical phenomena for H- ion production, the H- ions are generated via dissociative attachment of low energy electrons to ro-vibrationally excited hydrogen molecules. Ro-vibrationally excited hydrogen molecules are generated by collisions with high energy electrons. So, plasma parameters are closely related to H- ion production. To understand the RF frequency dependency of H- ion production, measurements of extracted H- beam current and plasma parameters are performed in the range of driving RF frequency from 12MHz to 27.12MHz. At two RF frequencies, 13.56MHz and 27.12MHz, the experiments of H- beam extraction and plasma diagnostics are carried out to investigate the input RF power dependency of RF frequency effects on H- ion production. Plasma parameters are measured along the center of the ion source chamber by installing the Langmuir probe in the TCP H- ion source through the extraction hole. Bias currents and voltages are also utilized to get the information on plasma parameters near the extraction region. At the same operating pressure and RF power, the more H- beam is extracted with increasing RF frequency. As driving RF frequency increases, electron density also increases while electron temperature decreases. Result of H- beam extraction means that the dissociative attachment of low energy electron to ro-vibrationally hydrogen molecules increases with increasing RF frequency. As driving RF frequency increases, higher power coupling efficiency causes the increase of electron density. Increase of electron density affects H- ion production positively by both processes of exciting hydrogen molecules and inducing associative detachments. On the other hand, as driving RF frequency increase, reduction of high energy electron population with lower electron temperature has positive and negative effects on H- ion production either by increasing dissociative electron attachment or by decreasing collisional excitation of hydrogen molecules. Those effects are believed to determine the change of extracted H- beam currents. Increasing amount of extracted H- beam current with various driving RF frequency has differences in each RF frequency range. At a lower RF frequency range, increasing rate of extracted H- beam current is much larger with varying RF frequency than the rate of electron density, indicating electron temperature also influence on the increase of H- beam current with respect to RF frequency. At discharge region, since electron temperature is almost same at each frequency and high enough to produce ro-vibrationally excited hydrogen molecules, it doesn't affect H- ion production. Reduction of electron temperature at extraction region seems to have more positive effect by increasing dissociative electron attachment than that by decreasing high energy electrons to generate highly excited molecules in this range. Since the population of low energy electron increases, dissociative electron attachment increases, and consequently, much more H-ions are generated. On the other hand, at higher RF frequency range, increasing rate of extracted H- beam current is similar to the rate of electron density increase with varying RF frequency. At this frequency range, positive and negative effects of electron temperature decrease may be canceled out. As electron energy is high, cross section of dissociative attachment is relatively low. As electron energy is higher than 1.5eV, cross section of dissociative attachment is so low. So, the number of H- ion produced by dissociative attachment at 13MHz may be smaller than that of any other RF frequencies. In addition, high energy electron can destroy more H- ions. At higher input RF power, the effect of driving RF frequency on H- ion production is different from that of lower input RF power. As input RF power increases above 1000W, electron temperature doesn't affect the extracted H- beam current anymore because of higher electron temperature at extraction region. As mentioned before, as electron has high energy above 1.5eV, electron temperature doesnt make much difference to the H- ion production, due to a low cross section of associative attachment. In this RF power range, electron density is a major factor to increase population of H-ions, showing similar behavior of H- beam current with respect to RF power. Variation of plasma parameters with varying RF frequency seems to be related to power coupling efficiency at each frequency. So, discharge current and discharge voltage need to be measured for obtaining power coupling efficiency with varying RF frequency. Furthermore, to understand the correlation between driving RF frequency and H- ion production rigorously, H- ion density needs to be measured directly.