Influence of a direct current bias on the energy of ions from an electron cyclotron resonance plasma

The energy distributions of ions (IED) on the substrate in an electron cyclotron resonance discharge have been measured for methane at pressures of 5.4 × 10−2 and 1.1 × 10−1 Pa, for substrate holders with diameters ranging from 2.2 to 9.5 cm and dc-bias voltages between 0 and −120 V. The applied dc bias is fully converted into an increase of ion energy only with the smallest substrate plate, and has no effect on the ion energy for the largest diameter. For intermediate diameters, the ion energy increases with increasing pressure at constant bias. This is explained by a model taking into account the sheath and anisotropic plasma resistivity. The current density does not increase with bias. From the merely small changes in full width at half-maximum and the percentage in low energy ions with increasing dc-bias voltage and sheath thickness it can be concluded that the sheath is collisionless.