Electroless Plating on Aluminium Based Semiconductors

Cleaning the pads off any organic or silicon based contaminates which may occur due to wafer handling, storage, or variations in the manufacturing process.

Removing any native oxide that may have built up on the aluminum pad surface. This is typically performed using caustic based etching chemicals.

Activation of the surface of the aluminum. The most commonly used wet chemical system for this is “zincation”, where a zinc oxide solution is used to replace some of the pad aluminum with zinc in an electrochemical reaction. Empirical research has shown that by stripping this zinc layer off and then reforming it in a second zincation step, a higher quality layer of zinc is created (so called “double zincation”). This zinc layer changes the electric potential of the aluminum pad, and when immersed in a nickel sulfate solution, nickel replaces this zinc and an autocatalytic nickel reaction continues. By adjusting the time, temperature, pH, and chemical concentrations of the nickel plating bath, nickel structures between 1 and 25 microns tall can be created.

For most applications the deposition of solder does not immediately follow the nickel deposition process. As the nickel surface will oxidize fairly quickly a thin layer of a noble metal is typically deposited on top of the nickel to protect the surface from oxidation. There are two common metals (Pd and Au) which are compatible with the electroless nickel process and can be deposited sequentially within the same plating line using either immersion or electroless based processes.