Yifang Cao

Mechanical Properties of Au Films on Silicon Substrates

This paper presents the results of recent studies of the effects of film thickness on the mechanical properties of electron-beam (e-beam) deposited Au films on silicon substrates. Following a brief description of film microstructure and surface topography, film mechanical properties (Youngs modulus and hardness) are determined using nanoindentation techniques. The effects of stiff silicon substrates on the Youngs modulus are analyzed within the framework established by King [6 8]. The effects of indentation size on film hardness are also explained within the context of strain gradient plasticity theories and substrate effects. The plasticity length scale parameters are shown to scale with film thickness. The amount of material pile up is also shown to increase with decreasing film thickness, for a given ratio of indentation depth to film thickness. The implications of the work are discussed for applications of Au films on silicon substrates.

Patterning of active organic materials by direct transfer for organic electronic devices

We demonstrate the localized patterning of organic thin films based on direct material transfer from a stamp to a substrate. This process, based on van der Waals bonding between contacting organic films, extends the range of application of patterning via stamping to devices where the active organic materials must be locally deposited on the substrate. A mechanical analysis combined with finite element numerical simulations successfully describes the details of the material transfer process over substrate irregularities and step edges. To demonstrate local patterning, we fabricate an array of organic light-emitting devices, showing that the organic-organic interface formed by the transfer process does not significantly affect device performance. This technique expands the application of a unique attribute of organic materials—the ability to compositionally pattern materials to locally modify, or optimize a device function or property within the substrate plane.

Effects of dust particles and layer properties on organic electronic devices fabricated by stamping

The mechanical properties of organic semiconductor thin films are determined using nanoindentation. The measured mechanical properties are incorporated into finite element simulations of deformation that arise during cathode patterning of organic electronic devices by pressure stamping methods. Simulations show that dust particles interposed between the stamp and film surface affect the evolution of contact areas when silicon or compliant polydimethyl-siloxane stamp dies are employed. We also examine the effects of the transferred metal layer thickness and stamp bulk modulus. Experimental and modeling results are found to be in good agreement. The implications of the results are discussed for the fabrication of a range of organic electronic devices.

Adhesion and the cold welding of gold-silver thin films

This paper presents the results of a combined experimental and theoretical study of adhesion in cold-welded Au–Ag interfaces that are relevant to the fabrication of organic electronic structures. Focused ion beam /transmission electron microscopy and electron energy loss spectroscopy techniques are used to reveal interfacial impurities associated with the cold welding of nanoscale Au and Ag thin films. A theoretical model is also developed and used to predict the contact profiles around impurities between cold-welded thin films. The model is shown to provide new insights into how adhesion affects the surface contacts that occur during cold welding. The implications of the results are then discussed for the design of cold-welding processes.

Nano- and microscale adhesion energy measurement for Au-Au contacts in microswitch structures

This paper presents a study of adhesion energies that are relevant to Au–Au microswitch contacts at the nano- and micronscales. Adhesion measurements are obtained from cantilevered Au microelectromechanical system (MEMS) microswitch structures with varying lengths. Scanning electron microscopy measurements of the microbeam profiles are combined with fracture mechanics model for the estimation of the adhesion energy. Adhesion contact and pull-off experiments are combined with theoretical models for the extraction of adhesion energies associated with Au-coated atomic force microscopy tips and Au microswitch substrates. Finite element method simulation was also performed to account for crack-tip shielding contributions from asperities in contact between Au–Au microscale cantilevered MEMS structure. The estimates of adhesion energies obtained for micron-scale contacts are shown to be in good agreement with those obtained for nanoscale contacts when appropriate “crack-tip shielding” corrections are used to acc...

Nanoindentation Measurements of the Mechanical Properties of Ni Thin Films: Effects of Film Microstructure and Substrate Modulus

This paper presents the results of nanoindentation measurements of the hardness and moduli of normally and obliquely deposited nanocrystalline Ni films on substrates of SiO2, Si, and bulk Ni. Following an initial characterization of film microstructure and surface topography with atomic force microscopy (AFM), the paper examines the effects of film microstructure, film thickness, and substrate modulus on the measured film mechanical properties. Obliquely deposited films are shown to have lower hardness values than normally deposited films. The measured hardness values and material pile-up are also shown to depend significantly on the mismatch between the film modulus and substrate modulus. A framework is presented for quantifying the effects of substrate modulus mismatch on basic film mechanical properties.

Investigation of Spreading and Traction of Human Osteosarcoma Cells on Microgrooved Polydimethylsilaxone Surfaces

The paper presents the recent results of the spreading and traction of human osteosarcoma cells on soft polydimethylsiloxane (PDMS) microgrooved surfaces. The results show that the cell tractions resulted in significant deformation of the microgrooves. The tractions were calculated, and found to be in good agreement with the results from other studies. The results suggest that the cell spreading-induced soft substrate deformation needs to be considered in the design of implantable bioMEMS structures.

Fabrication of organic light-emitting devices by direct transfer of active organic materials using organic-organic adhesion

We demonstrate a patterning method for organic electronics based on direct transfer of active organic materials by stamping, and apply it to the fabrication of organic light-emitting devices.