Method for ion implantation

A method for ion implanting a species into a workpiece, said method comprising placing said workpiece (121) in a processing zone of a chamber (105) having a pair of ports (155,160) near generally opposite sides of said processing zone and connected together by a conduit (150) external of said chamber, introducing into said chamber a process gas (125) comprising a first species to be implanted, generating from said process gas a plasma current and causing said plasma current to oscillate in a circulatory reentrant path comprising said conduit and said processing zone, and coupling a bias to said workpiece, and setting said bias to a level corresponding to a desired depth distribution below a surface of the workpiece at which said species is to be implanted.