A potential rise acting as a local retarding field for drifting ions has been observed in front of a positively biased electron-absorbing substrate in a downstream, electron cyclotron resonance plasma. Increasing positive substrate bias shifted the plasma potential upward and decreased the sheath potential to a certain minimum (5–13 V) depending upon pressure (0.7–5 mTorr), to satisfy a current balance between the substrate holder and the surrounding wall. For the minimal sheath potential, the incident ion flux to the substrate was shown to decrease with increasing bias due to prevention of ion arrival by the retarding field. Soft ion bombardment at the minimal sheath potential in a hydrogen plasma was then used to etch silicon wafer surfaces as in vacuo pretreatment for nanocrystalline diamond deposition. The highest diamond particle density of the order of 107 cm-2 was finally obtained on a clean and smooth surface with minimal damage formed by low energy (≃10 eV), high flux ion irradiation.