Abstract
In this dissertation, the effects of high frequency pulse plating on copper deposition in acid copper system were theoretically and experimentally studied. In the part of theoretical analysis, numerical simulations were employed to predict the influence of additive with various inclusion rates and capacities on potential response and mass transfer. The analyses of potential responses at various pulse periods were employed to account for the variation of current efficiency by pulse current. In the experimental analysis, copper electrodeposition in plated-through-hole (PTH) process and onto copper seed layer over silicon wafer by high frequency pulse current (PC) and pulse-reverse current (PR) were characterized by linear sweep voltammetry (LSV), cyclic voltammogram (CV), impedance, scanning electron microscopy (SEM) and x-ray diffraction (XRD). 80 ppm chloride ions (Cl-), 100 ppm polyethylene glycol (PEG) with 4000 average molecular weight and 40 ppm 3-mercapto-1-propanesulfonate (MPS) were used as additives and compared with the baths without additives. In the absence of additives, capacitive current density can be neglected owing to the smaller overpotential. However, the double layer effect can not be neglected in the millisecond range pulse plating in the presence of additives. Overpotential response is strongly affected by the presence of additives. Capacitive current density not only increases in the presence of additives due to increase in overpotential but also increases with increasing capacity of double layer. Then, the variation of surface concentrations of additives and metal ions diminishes as a result of the fast charging and discharging of the double layer. This prediction is consistent with the experimental overpotential response on a copper rotating disk electrode in a bath with and without 100 ppm PEG. The current efficiency of copper deposition decreases with shortening pulses in the millisecond range but increases with shortening pulses in the microsecond range. Shortening the pulse period could change the rate-determining step from the first-step charge transfer and surface diffusion to the first-step charge transfer. Only in the millisecond range, the current efficiency decreases with shortening pulse period due to the disproportionation of cuprous ions and the dissolution of copper adatom. However, in the microsecond range, the current efficiency was found to increase with decreasing pulse period because the adatoms are directly incorporated into steps and kink sites, and the disproportionation of cuprous ions or the dissolution of copper adatoms has less chance to occur. High frequency PC and PR can improve the deposition uniformity in PTH. In an additive-free electrolyte, the deposition uniformity in PTH with high frequency PC and PR at 20 mA/cm2 average current density is better than that with low frequency PC and PR. Based on impedance analysis, plating at low frequency is controlled by the diffusion of cuprous or cupric ion while plating at high frequency is controlled by charge transfer. Changing the rate-determining step from diffusion control to charge transfer control results in an improved metal distribution. In the presence of additives (PEG+MPS+Cl), the uniformity improves with DC at 20 or 10mA/cm2. The trend in the baths with additives using PC and PR at high and low frequency is similar to that in the bath without additives. However, the improvement by high frequency PC and PR is not significant since the plating is under adsorption control. The resistivity of copper deposit decreases with increasing PC and PR frequency in the baths with and without additives. The deposit becomes smoother with larger grain size as the frequency increases. Since overpotential decreases with increasing pulse frequency, it results in lower resistivity. In addition, the increase in the fraction of (200) at high frequency also reduces the resistivity. In the presence of additives, the copper resistivity of a bath with additives is generally lower than that without additives. A larger grain size can be obtained in the presence of additives because the additives tend to decrease overpotential, which naturally lead to lower resistivity.