Measurement of excess energies of ion beams extracted from a microwave ion source

Abstract

When an ion beam is used as a tool for analysis based on its energy value, the precision of the measurement depends on how accurately the ion energy is evaluated. However, as far as a plasma type ion source is concerned, the energy value of a very low-energy ion beam, lower than 100 eV specifically, should be carefully treated. Strictly speaking, the ion energy is equal to the ion charge multiplied by the sum of the source bias potential supplied to the chamber and the plasma potential with respect to the chamber. Of course, when the bias potential is sufficiently high, the plasma potential can be neglected. Theoretically, the plasma potential is determined by the electron temperature which is changeable depending on the source operating condition. Furthermore, the fact, that most plasmas of industrially used ion sources have plural electron temperatures and not a single one, makes the problem difficult to resolve. In this paper, we used the magnetic field of a mass separator to directly evaluate the plasma potential in a microwave ion source by utilizing the linear relationship between the squared magnetic flux of the mass separator and the ion acceleration voltage, i.e. the sum of the source bias potential and the plasma potential. The measurement showed that the excess energy of the ion beam due to the plasma potential rises above 100 eV at a microwave power of more than 500 W. Thus, when we handle a low energy ion beam of the order of 100 eV, the real ion energy may be more than twice the value given only by the source bias voltage at the chamber. The result is compared with the theoretical calculation for a plasma model of double electron temperatures.