Ionized physical vapor deposition of Cu on 300 mm wafers: A modeling study

Abstract

A two-dimensional model has been used to understand the physics and process engineering issues associated with a conceptual 300 mm Cu internal-coil ionized physical vapor deposition reactor. It has been found that inductive coupling from the coil is the primary source of plasma production. Since the coil is in direct contact with the plasma, a significant fraction of the coil power is deposited in the gas capacitively as well. This results in sputtering of the Cu coil, which tends to improve Cu flux uniformity at the outer edges of the wafer. Since the Cu ionization threshold is much lower than Ar, Cu+ density is comparable to Ar+ density even though ground state Cu density is much smaller than Ar. Significant fraction of the neutral Cu flux to the wafer is in the metastable or athermal state. The effects of several actuators, reactor dimensions, and buffer gas on important plasma and process quantities have also been investigated. Electron density in the reactor and Cu ionization fraction increases with increasing total coil power because of enhanced ionization. Total coil power however does not affect the Cu density appreciably, except near the coil where enhanced coil sputtering increases the Cu density. Decrease in dc target voltage with increasing coil power decreases Cu+ loss to the target and results in an increase in total Cu flux to the wafer. Electron and Cu density in the reactor increase with increasing dc target power. This is due to enhancement in target sputtering and consequent ionization of the sputtered Cu. While this increases the total Cu flux to the wafer, ionization fraction is not affected much. It is demonstrated that uniformity of Cu flux to the wafer and ionization fraction can be controlled by means of the terminating capacitor at the coil. Decreasing the terminating capacitance increases the coil voltage, enhances coil sputtering and enhances Cu flux toward the outer edges of the wafer. This, however, decreases the amount of power that is transferred to the plasma inductively, reducing the ionization efficiency. Increasing the coil–wafer distance results in fewer sputtered Cu atoms being ionized as the target–coil distance becomes smaller than the mean free path for thermalization of hot sputtered Cu atoms. Also, one can control the ionization fraction of Cu flux to the wafer by replacing Ar by Ne or Xe, without significantly impacting the total Cu flux.