Mechanical and microstructural characterization of high aspect ratio through-wafer electroplated copper interconnects

• Published in: Journal of micromechanics and microengineering Vol. 17; no. 9; pp. 1749 - 1757
• Main Authors: Dixit, Pradeep, Xu, Luhua, Miao, Jianmin, Pang, John H L, Preisser, Robert
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.09.2007; Institute of Physics
• Subjects: Applied sciences; Electronics; Exact sciences and technology; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Mechanical engineering. Machine design; Mechanical instruments, equipment and techniques; Microelectronic fabrication (materials and surfaces technology); Micromechanical devices and systems; Physics; Precision engineering, watch making; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Atomic force microscopy; Great depth; Nanoindentation; Mechanical properties; Thermomechanical properties; Nanostructure; Metal; Hardness; X ray diffraction; Indentation; Microhardness; Reactive ion etching; Size effect; Electrodeposition; Silicon; Copper; Microelectromechanical device; High aspect ratio microstructure HARM
• ISSN: 0960-1317; 1361-6439
• DOI: 10.1088/0960-1317/17/9/001

In this paper we present the mechanical and microstructural characterization results of through-wafer electroplated copper interconnects. Copper was deposited in very high aspect ratio (~15), narrow (15 mum) through-vias in silicon, which were earlier created by deep reactive ion etching. The two critical mechanical properties, i.e. hardness and modulus of elasticity, and the microstructure of the electroplated copper interconnect were determined by nanoindentation, atomic force microscope and x-ray diffraction techniques. A location-dependent hardness characteristic was shown along the length of electroplated copper interconnects. The modulus and the hardness of copper interconnects at the bottom segment (124 GPa and 1.8 GPa) were found to be higher than those at the top segment (116 GPa and 1.1 GPa). The reason behind these variable hardness values in the copper interconnect was due to the different grain sizes and the microstructure in the electroplated copper. These varying grain sizes were caused by the incremental current densities used during electrodeposition. The thermal strain, generated due to the coefficient of thermal expansion mismatch, was measured by the digital image speckle correlation technique. From the results, the thermal strain in the Y-direction was found to be more dominant than that in the X-direction. The grain sizes and the preferred texture orientation in the electroplated copper were characterized by the x-ray diffraction technique.