Gold electroplating for high aspect ratio X-ray optics

Schematic representation of the electroplating 3-electrode configuration. The sample to be plated is designated as the working electrode, Pt wire used as a counter electrode and a mercury sulfate (sat. K2SO4) reference electrode.

We recently modified the existing electroplating set up to a considerably faster, cleaner and, safer method by moving from a cyanide based Au electrolyte (commonly used) to a sulfite based Au electrolyte. Strict control of the deposition parameters is possible by using a 3-electrode configuration in conjunction with a potentiostat. The potentiostat applies a variable potential difference between the sample to be plated (working electrode) and the Pt or Au wire (the counter electrode), using a mercury sulfate electrode as the potential reference (reference electrode). In addition to rotating the sample in the plating solution, this process enables faster deposition rates and controllable growth modes in the presence of different p-block metal additives.

One such example is bottom-up Au filling of high aspect ratio silicon for use in grating-based X-ray technologies. Typically, in high aspect ratio/recessed features, sidewall growth can lead to early closing of the features to be filled resulting in voids – typically detrimental with respect to X-ray optics applications. The same is true for conformal growth in very deep structures, with the added disadvantage that conformal growth above the recessed features would lead to overfilling/waste and may require additional processing steps such as etching. With strict control of the deposition parameters as previously explained, bottom-up accelerated growth (Au deposition) can be realised. Resulting in, fully-filled, dense structures, free of any voids and with minimal deposition on the top of the field controllable by potential and the rotation rate.

Apart from bottom-up Au electroplating, the TOMCAT group is also involved in Au pulsed electrodeposition and Pd electroless deposition. Non electrochemical techniques have been also developed such as Ir atomic layer deposition and metal casting of Au-Sn and Pb based alloys.