Yaoyu Pang

University of Texas at Austin, Austin, Texas, United States

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Publications (9)9.16 Total impact

  • Yaoyu Pang, Rui Huang
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    ABSTRACT: A stressed epitaxial film can undergo surface instability. The stress field and the interface interaction have profound effects on the dynamics of surface evolution that leads to self-assembled quantum dots. In this paper, by using a nonlinear evolution equation, we investigate pattern evolution of self-assembled quantum dots under general biaxial stresses. It is found that the shape of quantum dots and their spatial ordering are strongly influenced by the relative magnitudes of the biaxial stresses. Linear perturbation analysis and nonlinear numerical simulations are conducted to elucidate the effect of stress anisotropy on the process of self-assembly that selects different patterns.
    MRS Online Proceeding Library 01/2011; 921.
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    Yaoyu Pang, Rui Huang
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    ABSTRACT: Stress-induced surface instability and evolution of epitaxial thin films leads to formation of a variety of self-assembled surface patterns with feature sizes at micro- and nanoscales. The anisotropy in both the surface and bulk properties of the film and substrate has profound effects on the nonlinear dynamics of surface evolution and pattern formation. Experimentally it has been demonstrated that the effect of anisotropy strongly depends on the crystal orientation of the substrate surface on which the film grows epitaxially. In this paper we develop a nonlinear model for surface evolution of epitaxial thin films on generally anisotropic crystal substrates. Specifically, the effect of bulk elastic anisotropy of the substrate on epitaxial surface pattern evolution is investigated for cubic germanium (Ge) and SiGe films on silicon (Si) substrates with four different surface orientations: Si(001), Si(111), Si(110), and Si(113). Both linear analysis and nonlinear numerical simulations suggest that, with surface anisotropy neglected, ordered surface patterns form under the influence of the elastic anisotropy, and these surface patterns clearly reflect the symmetry of the underlying crystal structures of the substrate. It is concluded that consideration of anisotropic elasticity reveals a much richer dynamics of surface pattern evolution as compared to isotropic models.
    International Journal of Solids and Structures 01/2009; 46(14):2822-2833. · 2.04 Impact Factor
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    ABSTRACT: A series of studies have been conducted for mechanical behavior of elastic thin films on compliant substrates. Under tension, the film may fracture by growing channel cracks. The driving force for channel cracking (i.e., the energy release rate) increases significantly for compliant substrates. Moreover, channel cracking may be accompanied by interfacial delamination. For a film on a relatively compliant substrate, a critical interface toughness is predicted, which separates stable and unstable delamination. For a film on a relatively stiff substrate, however, a channel crack grows with no delamination when the interface toughness is greater than a critical value. An effective energy release rate for the steady-state growth of a channel crack is defined to account for the influence of interfacial delamination on both the fracture driving force and the resistance, which can be significantly higher than the energy release rate assuming no delamination. Alternatively, when the film is under compression, it tends to buckle. Two buckling modes have been observed, one with interfacial delamination (i.e., buckle-delamination) and the other without delamination (i.e., wrinkling). By comparing the critical stresses for the onset of buckling, we give a criterion for the selection of the buckling modes, which depends on the stiffness ratio between the film and the substrate as well as the interface defects. A general conclusion from these studies is that, whether tension or compression, the interfacial properties are critical in controlling the morphology and failure of elastic thin films on compliant substrates.
    Thermal and Thermomechanical Phenomena in Electronic Systems, 2008. ITHERM 2008. 11th Intersociety Conference on; 06/2008
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    Yaoyu Pang, Rui Huang
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    ABSTRACT: Surface instability of epitaxial thin films leads to a variety of surface patterns. Anisotropy in surface and bulk properties has profound effects on the dynamics of pattern formation. In this paper, we theoretically predict that under anisotropic mismatch stresses, a bifurcation of surface pattern occurs in addition to generic symmetry breaking from isotropic systems. Numerical simulations based on a nonlinear evolution equation demonstrate pattern selection at an early stage and nontrivial patterns for long-time evolution. © 2007 American Institute of Physics.
    Journal of Applied Physics 01/2007; 101. · 2.21 Impact Factor
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    Haixia Mei, Yaoyu Pang, Rui Huang
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    ABSTRACT: Channeling cracks in brittle thin films have been observed to be a key reliability issue for advanced interconnects and other integrated structures. Most theoretical studies to date have assumed no delamination at the interface, while experiments have observed channel cracks both with and without interfacial delamination. This paper analyzes the effect of interfacial delamination on the fracture condition of brittle thin films on elastic substrates. It is found that, depending on the elastic mismatch and interface toughness, a channel crack may grow with no delamination, with a stable delamination, or with unstable delamination. For a film on a relatively compliant substrate, a critical interface toughness is predicted, which separates stable and unstable delamination. For a film on a relatively stiff substrate, however, a channel crack grows with no delamination when the interface toughness is greater than a critical value, while stable delamination along with the channel crack is possible only in a small range of interface toughness for a specific elastic mismatch. An effective energy release rate for the steady-state growth of a channel crack is defined to account for the influence of interfacial delamination on both the fracture driving force and the resistance, which can be significantly higher than the energy release rate assuming no delamination.
    International Journal of Fracture 01/2007; 148(4):331-342. · 1.25 Impact Factor
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    Yaoyu Pang, Rui Huang
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    ABSTRACT: Channeling cracks in low-k dielectrics have been observed to be a key reliability issue for advanced interconnects. The constraint effect of surrounding materials including stacked buffer layers has been studied. This paper analyzes the effect of interfacial delamination on the fracture condition of brittle thin films on elastic substrates. It is found that stable delamination along with the growth of a channel crack is possible only for a specific range of elastic mismatch and interface toughness. An effective energy release rate is defined to account for the influence of interfacial delamination on both the driving force and the fracture resistance, which can be significantly higher than the case assuming no delamination.
    MRS Proceedings. 12/2006; 990.
  • Yaoyu Pang, Rui Huang
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    ABSTRACT: An epitaxial thin film can undergo surface instability and break up into discrete islands. The stress field and the interface interaction have profound effects on the dynamics of surface evolution. In this work, we develop a nonlinear evolution equation with a second-order approximation for the stress field and a nonlinear wetting potential for the interface. The equation is solved numerically in both two-dimensional (2D) and three-dimensional configurations using a spectral method. The effects of stress and wetting are examined. It is found that the nonlinear stress field alone induces “blow-up” instability, leading to cracklike grooving in 2D and circular pitlike morphology in 3D. For thin films, the blow-up is suppressed by the wetting effect, leading to a thin wetting layer and an array of discrete islands. The dynamics of island formation and coarsening over a large area is well captured by the interplay of the nonlinear stress field and the wetting effect.
    Physical review. B, Condensed matter 01/2006; 74(7). · 3.66 Impact Factor
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    ABSTRACT: The present study investigated the effect of passivation on the kinetics of interfacial mass transport by measuring stress relaxation in electroplated Cu films with four different cap layers: SiN, SiC, SiCN, and a Co metal cap. Stress curves measured under thermal cycling showed different behaviors for the unpassivated and passivated Cu films, but were essentially indifferent for the films passivated with different cap layers. On the other hand, stress relaxation measured under an isothermal condition revealed clearly the effect of passivation, indicating that interface diffusion controls the kinetics of stress relaxation. The relaxation rates in the passivated Cu films were found to decrease in the order of SiC, SiCN, SiN, and metal caps. This correlates well with previous studies on the relationship between interfacial adhesion and electromigration. A kinetic model based on coupling of interface and grain-boundary diffusion was used to deduce the interface diffusivities and the corresponding activation energies.
    Journal of Materials Research - J MATER RES. 01/2006; 21(6).
  • Rui Huang, Se Hyuk Im, Yaoyu Pang
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    ABSTRACT: Thin films of nanoscale thickness are common in integrated systems and devices. Subjected to interactions of diverse physical origins, the nanoscale thin films often undergo structural and/or morphological instability and develop a variety of surface patterns. Two examples are discussed in this paper. An epitaxial thin film undergoes surface roughening and form self-assembled quantum dots [1]. A thin metal film bonded to a polymer substrate develops various wrinkle patterns [2]. We develop nonlinear models and numerical methods to simulate the evolution processes. The results reveal very rich dynamics of surface pattern formation and suggest potential means for the control and making of ordered patterns.
    ASME 4th Integrated Nanosystems Conference; 01/2005