Source for doubly charged ions

Abstract

High energy ions are required for implantation of p- and n-wells in C-MOS and also for bipolar ICs, e.g. direct implantation of buried layers, in particular for modern shallow structures. For this purpose ion energies above 1 MeV are required. It is of course more economical to make use of doubly or even higher-charged ions, than of expensive high voltage apparatus. The presence of a relative large component of the high current singly charged ions is a major problem when operating the ion implantation apparatus with its source producing multiply charged ions. The operation of the ion source, similar to that by Bethge [1], with radiofrequency voltage instead of d.c. voltage, offers several advantages. The probability of ionization at r.f. voltages (of typical frequencies of 13.65 MHz) is higher and therefore the ion source operates at a lower gas pressure, which in turn insures cleaner operating conditions. Furthermore, the yield of, e.g., doubly charged ions to singly charged ions is larger for r.f. voltages, which reduces the above-mentioned contamination problem. The operation of the ion source with an r.f. power source, which consists of a generator, power amplifier and in particular a matching network between the power amplifier and the source at high potential, requires special attention. The impedance of the ion source as seen by the power amplifier is a function of the plasma condition, i.e. temperature and intensity. In particular the initial condition differs from the steady state condition. Results of a comparitive study of the d.c. and r.f. powered ion source with regard to the yields of singly, doubly and trebly charged ions for various species of ions as required by the semiconductor technology will be discussed.