Bulk composition of r.f. magnetron sputter deposited calcium phosphate coatings on different substrates (polyethylene, polytetrafluoroethylene, silicon)

Calcium phosphate (CaP) ceramic coatings are often applied to improve the biological response of an implant. CaP ceramic was deposited using r.f. magnetron sputter deposition on different substrates (polyethylene (PE), polytetrafluoroethylene (PTFE), and Si), with varying Ar gas pressures, and on different positions in the deposition chamber. With increasing Ar gas pressure, the coating thickness decreases. Surprisingly, the coating composition varied among the different substrates. This was found to be the result of two phenomena. Firstly, negative charging of the insulating substrates during the deposition of the coating influences the severity of a bombardment by negative oxygen ions, which escape from the sputtering target. The varying degree of (preferential) resputtering of P from the coatings gives rise to different coating compositions. Secondly, in the case of PTFE, the escape of fluorine from the PTFE (which we found to be induced by UV light from the plasma) influences the coating composition, probably because the F reacts with P in the coating, to form a gas molecule. Finally, we found that the coating composition can be influenced, during coating deposition, by a 500 eV ion bombardment.