Low energy ion implantation using non-equilibrium glow discharge

The cathode sheath in the pulsed glow discharge plays a key role in defining the energy of ions reaching the wafer. Understanding the structure, dynamics and collisional properties of the sheath is critical for successful application of these discharges to low energy plasma implantation. Measurements of the time resolved ion energy distribution in the cathode sheath of the dc pulsed discharge through boron trifluoride were performed both for collisional and collision-less sheath. In the case of a collision-less sheath, ion energy distribution is dominated by the high energy peak. For the collisional sheath, ion energy distribution is influenced by the complexity of elastic and inelastic collisions. The measured BF2+ ion energy distributions for different number of collisions in the sheath are consistent with a charge transfer model. Computations using Davis and Vanderslice model confirmed that charge transfer is a major inelastic collision process in the sheath. However, the experiments indicate that other inelastic ion-atom collisions resulting in BF2+ dissociation inside the sheath cause a breakdown of the charge transfer model. Experimental ion energy distributions were also used to compute the dopant profile distribution in the silicon. A remarkable agreement between the calculated and experimental depth profiles is achieved.