Ion beam implantation by expanding instead of scanning

Abstract

The beam expansion is an alternative concept to the conventional beam scan applied to ion implantation for semiconductors without exception. By beam expansion the wafer and the photoresist mask on the wafer are treated very gently since continuously and simultaneously everywhere on the wafer one implants only with the average current density. By beam scanning however discontinuously and sequentially the positions on the wafer are implanted with 100 to 1000 times larger current density. The beam expansion facility consists of an electrical quadrupole for beam broadening, a rather unique ion reflector as a nonlinear beam transport element, a rotating target, and fourteen Faraday cups at the target for measuring the radial distributions of current density and implantation dose. By varying the quadrupole voltage qualitatively different current density distributions are generated. For implantation experiments such distributions are sequentially applied for times which are precalculated with a non-negative linear least squares method for getting a dose distribution with minimum non-uniformity. For this purpose the facility is computer-controlled. Most experiments were performed with Ar + ion beams of 100 keV energy and 0.5 mA current. For implantation in silicon wafers a 0.1 mA B + ion beam was used. The relative accuracy of the Faraday cups and current integrators is at least 0.5%, ultimately confirmed by sheet resistance measurements at simultaneously implanted wafers. By superposing different beam current distributions the implantation dose has a non-uniformity of 1.6% on average and 0.5% best value. Such results are obtained from 40 implantation experiments. For short time < 20 s implantation experiments one applies the most uniform current density distribution. On average one can then achieve homogeneous dose distributions between 1 and 2% non-uniformity regularly and 0.7% best value according to experiments over months.