Electrochemical Deposition of Sn-Cu Alloys for Applications in 3D Stacking in Microelectronics Industry

Abstract

The use of eutectic Sn-Cu alloys in packaging applications in microelectronics industry is not something entirely new. However, this alloy was mostly used in large features and fabricated using metal powders and metallurgical deposition techniques. The diameter of an individual grain of the eutectic Sn-Cu powder is typically similar in size to the diameter of pillars or bumps currently used in 3D stacking, and if we were to check its usefulness as a solder on this scale, an alternative manufacturing technique had to be used. We have electrochemically deposited Sn-Cu alloys having up to 10 wt.% Cu and explored its possible benefits when used as a solder in combination with different Under bump metallization (UBM) materials. Sn-Cu alloys were deposited on blanket and patterned coupons, and promising experiments then transferred to a wafer-scale, i.e. Sn-Cu alloys were deposited on 300 mm wafers in an industrial-type plating tool. A typical sample/wafer had a Cu seed with a diffusion barrier (e.g. TiW) underneath and patterned features defined with a photoresist mask. Characteristic dimensions of the patterned features were on the order of mm and cm (lines), (diameter, d)50µm × (height, H)60µm (pillars), and (d)8µm × (H)15µm (bumps). Cu, Ni, or Co were used as UBM layers. Chemical composition of deposited alloys has been examined by using X-ray fluorescence (XRF), Micro X-ray fluorescence (Micro-XRF), Inductively coupled plasma mass spectrometry (ICP-MS), and Electron probe microanalysis (EPMA). Surface morphology and the shape of deposited features have been examined using Scanning electron microscopy (SEM) and Laser scanning microscopy (LSM), while thermodynamic properties have been studied using Differential scanning calorimetry (DSC). Within-wafer (WIW) and within-die (WID) height uniformity of the pillars has been analyzed using Falcon 630 Plus tool (Camtek Ltd), capable of capturing heights of all the structures on the wafer simultaneously. The kinetics of Intermetallic compound (IMC) formation have been studied in the number of selected samples by combining ex-situ Focused ion beam (FIB) and in-situ measurements of resistance change of the features during anneal/thermal ageing [1]. We will discuss possible benefits and drawbacks for use of electrochemically deposited Sn-Cu alloy solders in combination with different UBM layers based on these results. References: [1] Lin Hou, Jaber Derakhshandeh, Eric Beyne, and Ingrid De Wolf, “A Novel Resistance Measurement Methodology for in-situ UBM/Solder Interfacial Reaction Monitoring”, DOI 10.1109/TCPMT.2019.2950448, IEEE Transactions on Components, Packaging and Manufacturing Technology. Figure 1