Method and apparatus for controlling the chemical state of an electroless plating bath.

摘要

With the invention the surface chemistry on objects plated in an electroless plating bath, the rate of the plating and the contaminant level in the bath are maintained and optionally controlled. For maintaining and controlling the surface chemistry cyclic voltammetry measurements (31) are made for different pH conditions (29, 30) of the bath. Pourbaix diagrams are determined from these measurements which indicate the transition between metal species being plated by the bath. The open circuit potential (32) of the bath is monitored by a potentiostat and compared with a setpoint open circuit potential (33) which represents a desired metal species on the pourbaix diagram. The monitored open circuit potential and the setpoint are utilized to derive an error voltage (34). The error voltage will control the concentration of a chemical constituent of the plating bath to maintain the desired method species on the plating surface. The plating rate and the contaminant level are preferably monitored by measuring the complex impedance (35) between a reference electrode and a working electrode immersed in the bath. From the real portion (36) of the measured impedance the plating rate and from its imaginary portion (38) the contaminant level are determined. An error signal (37) is produced from the difference between the plating rate derived from the real portion of the impedance and the setpoint plating rate. Controlled by this error signal (40) another constituent of the plating bath is added in order to keep the plating rate constant.