Development of the gel plating process for selective gold metallization
Author(s)Svedberg, Lynne Miriam, 1972-
Michael J. Cima.
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A Gel Plating Process for selective gold metallization was developed in this thesis. The Gel Plating Process selectively plates metal on catalytic features without exposing sensitive area, of the substrate surface to a corrosive plating bath. This process utilizes an electroless plating bath and polymeric thickening agent to formulate a gel. The gel is selectively printed onto the areas of the substrate that require plating. Plating occurs autocatalytically at an elevated temperature by the simultaneous anodic oxidation of the reducing agent and catalytic reduction of the metal complex onto the catalytic features under the gel print. The substrate is cleaned, leaving a selectively metallized substrate surface. Aluminum nitride (AIN) is a potential packaging material for high power electronic applications; however, it is severely corroded in the alkaline gold electroless plating solutions often used to selectively plate packaging substrates with electrically isolated lines. An ammonium ion selective electrode was used to study the in situ corrosion of AlN as a function of pH. It was shown that the aluminum trihydroxide corrosion product created on the surface of AlN during exposure to aqueous solutions acts as a barrier layer that decreases the corrosion of AlN in the pH range where this aluminum trihydroxide product is insoluble. An aluminum oxynitride layer of 200 angstroms was found to act as an insoluble barrier layer to corrosion of AIN at a pH of 9.5. A plating gel was formulated for use in the Gel Plating Process by adding a polymeric thickening agent to an electroless gold plating bath chemistry. The plating chemistry incorporated sodium gold thiosulfate as the gold complex and sodium ascorbate as the reducing agent . The gold concentration in the plating gel was increased to 40 g/L gold from the 4 g/L gold found in typical commercial electroless plating baths. This resulted in a high ion concentration plating solution that was thickened with a hydroxyethyl celJulose (HEC) thickening agent. A stabilizer, 2-mercaptobenzimidazole, was found to increase the room temperature and plating temperature stability of the plating gel. Addition of excess sodium ascorbate caused a significant improvement in plate color. Addition of a surfactant to the plating gel formulation enhanced the printability of the plating gel and reduced extraneous plating of gold onto the noncatalytic areas of the substrate. The rest viscosity of the plating gel was found to be 70,000 cP at 25 °C and 38,000 cP at 60 °C, the highest plating temperature used. The plating mechanism of the Gel Plating Process was found to be reaction rate controlled at short times and diffusion controlled at longer times. A reaction rate model and a diffusion model with a heterogeneous reaction were fit to plating rate data at 50 °C and 60 °C. The reaction rate model was used to determine the rate constant, k, for the cathodic reaction at the plating surf ace at short times. A diffusion model with a heterogeneous chemical reaction was used to identify the diffusion coefficient, D, of the gold complex in the plating gel at long times. At 60 °C, it was found that k = 1.5 x 10-6 emfs and D = 4 x 10·7 cm2/s. At 50 °C, it was found that k = 3 x 10·6 emfs and D = 2 x 10·7 cm2/s. The thickening agent was found to decrease both the reaction rate constant for the cathodic reaction at the plating surface and the diffusivity of the gold complex. The gold plate thickness and microstructure was found to be influenced by the gold complex and reducing agent chemistry, the stabilizer concentration, and thickening agent additions. Plating through a gel resulted in a fine grained microstructure. Plating through a solution resulted in a large grained microstructure.- SEM cross sections indicated that a dense gold plate resulted from gel plating. The gold film obtained by the Gel Plating Process is free of trapped organics.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999.Includes bibliographical references (leaves 194-196).
DepartmentMassachusetts Institute of Technology. Dept. of Materials Science and Engineering
Massachusetts Institute of Technology
Materials Science and Engineering