Alex Hsing M.S. Student Department of Materials Science and Engineering Stanford University Phone: 650-725-2643 Fax: 650-725-4034 Office: Durand Bldg., Rm. 157 Email: hsing@stanford.edu

Educational Background

Research Interests

Novel Micro-Mechanical Testing

Current materials selection is made with limited understanding of key interfacial adhesion properties and the mechanics related to packaging of fragile die-level interconnect structures. The detailed mechanisms of subcritical debonding related to complex residual stresses, fatigue and vibrational loading, and the effects of processing and in-service environment and temperature are poorly understood. There is a need for new testing metrologies that will assess the strength and compliance of BEOL structures and provide quantitative data that can be used to determine allowable chip/package interactions. There is a need for an improved fundamental understanding of the mechanisms that control resulting adhesion, fracture and thermomechanical properties of new materials and interfaces in complex package structures.

The objective of this project is to develop a novel micro-mechanical test system to impose tensile, compressive, and cyclic stresses on thin-film stacked structures. We are particularly interested in measuring the strength and compliance of fragile BEOL structures (Figure 1) and determining properties such as stiffness, yielding, fracture and fatigue crack growth. This will further the understanding of complex chip-package interactions.

Figure 1: Cross section of a silicon die showing BEOL interconnect layers and the chip package

We will study cracking and debonding that occur from complex loading, thermomechanical fatigue cycling, environment (gaseous and aqueous) and temperature.  By introducing weak regions we can determine where in the package/BEOL structure we initiate and propagate cracking.  A novel test system will be used to impose tractions on the BEOL structures to study the effects of back end strength and compliance on damage mechanisms and crack growth processes resulting from packaging contact stresses. 

The metrology system consists of a piezoelectric actuator, a small form-factor load cell, and a micron-sized probe. Precise control of the probe position allows for mapping of properties like stiffness, which can be correlated to material density within the complex structure.

Figure 2 shows a feature density map of a 10 mm x 10 mm die. Figure 3 shows a stiffness map obtained by applying compressive stresses on the die using the micro-mechanical test system. The probe can also be soldered to die bumps, allowing tensile fracture and fatigue testing.

 Figure 2: Map of on-chip feature densities

Figure 3: Stiffness map across the die obtained from compressive micro-mechanical testing