Developments in Hybrid Electro-Electroless Deposition (HEED)
Authors: Petro Robert 
Published: February 2020

Hybrid electro-electroless deposition (HEED) is a novel thin film metal deposition technique that combines electroplating and electroless deposition within a single electrolyte. The hybridization of HEED is the result of each deposition process targeting a distinct metal ion, or combination of ions, for reduction. The nature of the deposit is determined by the composition, pH, and temperature of the electrolyte as well as the electroplating potential applied; all of which affect the relative deposition rate of each process. Composite, alloyed, or compositionally modulated layered deposits can all readily be formed using HEED. This work details advancements in controlling the composition and production of both HEED alloys and multi-layers.
HEED alloys are produced by using electroplating to either preferentially deposit one component of the electroless alloy, or including a new metal into the electroless deposit. The deposition of Zn enhanced Ni-Zn-P HEED alloys, representing the former case, is shown in connection to the production of corrosion resistant coatings. The electroless process produces coatings over the entire surface while electroplating provides increased Zn content in electroplated regions. The formation of concentration gradients within the coating provides control over where corrosion is most likely to occur. Energy-dispersive X-ray spectroscopy (EDS) was used to determine an increase in Zn content from 26% using electroless deposition to 50% using HEED [1]. The process was carried out on both Cu substrate and Mg alloy substrates.
Compositionally modulated HEED, or multi-layer HEED, deposits are produced in a similar fashion to traditionally electroplated multi-layer deposits by modulating the applied deposition potential [1, 2]. Provided appropriate conditions and ion choice, slowing the electroless deposition rate minimizes inclusion of the electroless alloy or metal. With HEED, metals of similar nobility, such as Ni and Co, can be deposited in a multi-layered arrangement [2]. In the multi-layered configuration Ni-P is electrolessly deposited while Co is reduced using electroplating. The atomic composition of the electroless Ni-P layers, tested on Sn/Pd treated glass, was consistently 76% Ni, 22% P, with no more than 2% Co [2], consistent with Ni3P. Within an acidic electrolyte the electroless Ni-P layers are effectively free of Co while the purity of electroplated Co layers is dependent on the electroless deposition rate and relative Co2+ ion availability in solution. Adjusting the pH to the alkaline regime results in alloying of each layer as Co is co-deposited in the electroless deposition phase.
Figure 1 shows the competition between Co electroplating and electroless Ni-P deposition at low current density 0.95 A∙dm-2 on a Cu substrate [2]. The low current density slows electroplating while the temperature promotes hastened electroless deposition. The resulting deposit is a composite of each distinct process with the electroless deposition material, EDS Spot 1, and electroplated material, EDS Spot 2, shown to have distinct structures and composition. Increasing the current density to 3.8 A∙dm-2 produces homogeneous alloys as electroplating becomes the dominant process. Lowering the temperature of the electrolyte further increases the dominance of electroplating.
[1] Robert Petro and Mordechay Schlesinger, “Development of Hybrid Electro-Electroless Deposit (HEED) Coatings and Applications”, Journal of The Electrochemical Society, 161(10), (2014), p.D1-D6
[2] Robert Petro and Mordechay Schlesinger, “Deposition of Cobalt-Nickel Hybrid Electro-Electroless Deposited (HEED) Modulated Multilayers”, Journal of The Electrochemical Society, 162(4), (2015), p.D154-D158

Figure 1