Date of Award
Master of Science (MS)
Committee Member 1
Committee Member 2
Wire bonding is the process of forming electrical connection between the integrated circuit (IC) and its structural package. ICs made of material with low dielectric constant (low-k) and ultra low-k are porous in nature, and are prone to fracture induced failure during packaging process. In recent years, there is increasing interest in copper wire bond technology as an alternative to gold wire bond in microelectronic devices due to its superior electrical performance and low cost. Copper wires are also approximately 25% more conductive than Au wires aiding in better heat dissipation. At present, validated constitutive models for the strain rate and temperature dependent behavior of Cu free-air ball (FAB) appear to be largely missing in the literature. The lack of reliable constitutive models for the Cu FAB has hampered the modeling of the wire bonding process and the ability to assess risk of fracture in ultra low-k dielectric stacks. The challenge to FAB characterization is primarily due to the difficulty in performing mechanical tests on spherical FAB of micrometers in size. To address this challenge, compression tests are performed on FAB using custom-built microscale tester in the current study. Specifically, the tester has three closed-loop controlled linear stages with submicron resolution, a manual tilt stage, a six-axis load cell with sub-Newton load resolution for eliminating misalignment, a milliNewton resolution load cell for compression load measurement, a capacitance sensor to estimate sample deformation and to control the vertical stage in closed loop, a high working depth camera for viewing the sample deformation, and controllers for the stages implemented in the LabVIEW environment. FAB is compressed between tungsten carbide punches and a constitutive model is developed for Cu FAB through an inverse modeling procedure. In the inverse procedure, appropriate constitutive model parameter values are iterated through an automated optimization workflow, until the load-displacement response matches the experimentally observed response. Using the material properties obtained from the experiment, a "macroscale" finite element model for the impact and ulatrasonic vibration stages of wire bonding process is constructed to simulate (a) Plastic deformation of the Cu FAB at different time steps (b) Evolution of contact pressure (c) Phenomenon such as pad splash and lift-off. The deformations from the macroscale model are provided as input to a microscale model of the dielectric with copper vias as well as line-type heterogeneities. The microscale model is used to identify potential crack nucleation sites as well as the crack path within the ILD stack during wire bonding. The modeling provides insight into the relative amounts of damage accumulated during the impact and the ultrasonic excitation stages.
In general, Bonding over Active Circuit (BOAC) has made wire bonding a considerable challenge due to the brittleness of the dielectric. Identifying and locating microscale fractures beneath the bond pads during wire bonding require extensive sample preparation and investigation for microscopic characterization. While simulations of fracture are an attractive alternative to trial and error microscopic characterization, the length scale of components involved in wire bonding varies from millimeters to nanometers. Therefore, constructing a finite element mesh across the model is computationally costly. Also, a multi-scale simulation framework is necessary. Such a modeling framework is also developed in this work to predict crack nucleation and propagation in wire bond induced failure.
Paranjothy, Sai Sudharsanan, "Experimental Characterization Of Cu Free-Air Ball And Simulations Of Dielectric Fracture During Wire Bonding" (2014). Open Access Theses. 362.