Preparation of Surfaces of Germanium and Kovar Substrates by Electroplating Prior to Soldering

Germanium is used as a window material in packaging infrared detector structures, due to its high refractive index and transmittance to infrared radiation. It must be joined to the dewar body made of Kovar^® (registered trademark of Carpenter Technology Corporation) material by soldering during packaging. A surface coating (metallization) well adhered to both substrates was developed as a surface preparation step prior to soldering to overcome the poor wetting characteristics of germanium. Successive electroplating of 3 mm wide nickel and gold frames to the outer edge of a circular germanium window and kovar body was applied to attain successful joining of germanium to kovar.

Surface cleaning of parts to be joined or plated is very essential to remove contaminants from the surface and reduce oxides to the highest possible extent. Onsia and Conard [1], reported that wet chemical processes need to be developed to prepare germanium surface for subsequent growth processes. The wet chemical treatment for germanium surface preparation used in this study followed the reciepe originally proposed by Shiyu Sun et. Al. [2]. The samples were first dipped into deionized water having a resistivity of 18.6 MΩ cm, for 30 seconds to dissolve the native oxide. Then, they were placed into 10% H₂O₂ solution for 30 seconds to eliminate the carbon contaminations on the surface. Finally, the samples were immersed into a hydrofluoric acid solution for 10 minutes to remove the oxides. X-Ray Photoelectron Spectroscopy (XPS) results showed that more than 70% of oxides on the surface could be eliminated by the surface preparation treatment proposed above. A chemical polishing method was applied for kovar surface cleaning. Solution bath was composed of 750 mL CH₃COOH (Sigma Aldrich, 1005706), 250 mL HNO₃ (Sigma Aldrich, 438073) and 3 mL HCl (Sigma Aldrich, 438073). The temperature was kept constant at 60°C and the solution was not stirred during cleaning. 

The nickel was deposited at 55^oC from the Watts solution, containing 240 g/L NiSO₄.6H₂O, 45 g/L NiCl₂.6H₂O and 30 g/L boric acid (Merck, 100165), because it is easy to control the bath and deposits have lower internal stresses. The wave characteristic of pulse current and saccharin addition have important effects in nickel plating [3]  in terms of porosity, grain size and densty of coatings Addition of saccharin to the bath and the use of pulse plating decreased the grain size of the nickel and changed the morphology of the grains from pyramidal structure to colony like in coating.

A thin layer of gold, as pore free as possible, was required to protect the nickel surface from oxidation. Gold was electrodeposited on the nickel coated kovar substrate from the acid cyanide bath at 50^oC by pulse and direct current plating in this study. The bath was composed of 10 g/L potassium gold cyanide and 180 g/L potassium citrate. The gold films obtained by pulse plating exhibited stronger adhesion to nickel surface and contained less porosity compared to DC plating. The porosity of gold coating was determined by the IPC-TM-650 test method. The solution used in porosity test was composed of 980 mL ethanol, 7 g dimethylglyoxim (Sigma Aldrich, 162574), 20 mL de-ionized water and 1 g ammonium chloride (Merck, 168320). The increase in gold deposit thickness in 0.1 to 0.4 µm range decreased porosity. 

References

[1] B. Onsia, T. Conard, Solid State Phenomena Vols. 103-104 pp 27-30, 2005

[2] Brenn Shiyu Sun, ProQuest Dissertations and Theses7, 2007

[3] M.S. Chandrasekar, M. Pushpavanam, Electrochimica Acta 53 3313–3322, 2008