Pressureless Bonding Using Sputtered Ag Thin Films

Abstract

To improve the performance and reliability of power electronic devices, particularly those built around next-generation wide-bandgap semiconductors such as SiC and GaN, the bonding method used for packaging must change from soldering to solderless technology. Because traditional solders are problematic in the harsh operating conditions expected for emerging hightemperature power devices, we propose a new bonding method in this paper, namely a pressureless, low-temperature bonding process in air, using abnormal grain growth on sputtered Ag thin films to realize extremely high temperature resistance. To investigate the mechanisms of this bonding process, we characterized the microstructural changes in the Ag films over various bonding temperatures and times. We measured the bonding properties of the specimens by a die-shear strength test, as well as by x-ray diffraction measurements of the residual stress in the Ag films to show how the microstructural developments were essential to the bonding technology. Sound bonds with high die strength can be achieved only with abnormal grain growth at optimum bonding temperature and time. Pressureless bonding allows for production of reliable high-temperature power devices for a wide variety of industrial, energy, and environmental applications.

Nano-volcanic Eruption of Silver

“…Pressure-less, low-temperature Ag-to-Ag bonding has recently been developed for die-attachment in advanced wide bandgap (WBG) electronics 6 11 12 . Compressive stress has been proposed to be the fundamental driving force of direct metal bonding in various systems, such as Ag-to-Ag, gold (Au)-to-Au, and copper (Cu)-to-Cu interconnections 6 11 12 13 14 15 .…”

“…Pressure-less, low-temperature Ag-to-Ag bonding has recently been developed for die-attachment in advanced wide bandgap (WBG) electronics 6 11 12 . Compressive stress has been proposed to be the fundamental driving force of direct metal bonding in various systems, such as Ag-to-Ag, gold (Au)-to-Au, and copper (Cu)-to-Cu interconnections 6 11 12 13 14 15 . Unlike Cu-to-Cu bonding, which typically involves rather complex processes and strict processing environments 13 , Ag-to-Ag bonding, which exhibits extraordinary mechanical properties, can be established simply by employing sputtered Ag contacts 6 11 12 .…”

“…Compressive stress has been proposed to be the fundamental driving force of direct metal bonding in various systems, such as Ag-to-Ag, gold (Au)-to-Au, and copper (Cu)-to-Cu interconnections 6 11 12 13 14 15 . Unlike Cu-to-Cu bonding, which typically involves rather complex processes and strict processing environments 13 , Ag-to-Ag bonding, which exhibits extraordinary mechanical properties, can be established simply by employing sputtered Ag contacts 6 11 12 . The formation of strong Ag/Ag joints has also been considered to be a stress-driven phenomenon, which is associated with the aforementioned formation of abnormal abundant Ag hillocks 3 .…”

“…The formation of abundant Ag hillocks and the pressure-less Ag-to-Ag bonding have both been attributed to the stress-migration mechanism that is driven by thermal stress and the residual stresses in sputtered Ag films 3 6 11 12 . The very large stress gradient has been estimated by X-ray diffractometry to be as high as 200 MPa 6 11 , and the mass transfer is associated with the columnar grain boundaries that are perpendicular to the substrate 3 . However, experiments have shown that the primary driving force for diffusion, which is the very large thermal-stress gradient, diminished instantly in the very early stage of annealing 11 .…”

Contact Resistance and Metallurgical Connections Between Silver Coated Polymer Particles in Isotropic Conductive Adhesives

“…Oh et al have done extensive research on the hillock or abnormal grain growth dynamics in sputtered Ag thin films due to stress migration. [19][20][21][22] When compressive stresses are induced in silver thin films, stress relaxation will lead to increased diffusion of silver atoms along the grain boundaries, which can cause grain growth on the film surfaces. A similar process may occur in the AgPS-based ICAs.…”