The thermal conductivity reduction due to grain boundary scattering is widely interpreted using a scattering length assumed equal to the grain size and independent of the phonon frequency (gray). T...

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Thermal Conductivity of Nanocrystalline Silicon: Importance of Grain Size and Frequency-Dependent Mean Free Paths

This review discusses recent advances in materials engineering to control thermal conductivity. We begin by presenting theories of heat conduction for general material classes, focusing on common approximations and trends. Next, we discuss characterization techniques for measuring thermal conductivity and the underlying transport properties. Advanced materials at the frontiers of thermal transport, such as rattlers, complex unit cells, nanowires, and nanocomposites, are treated in depth using experimental data and theoretical predictions. The review closes by highlighting several promising areas for further development.

Advances in Thermal Conductivity

Failure of metals III: Fracture and fatigue of nanostructured metallic materials

The measurement of the mechanical properties of materials with submicrometer dimensions is extremely challenging, from the preparation and manipulation of specimens to the application of small loads and extraction of accurate stresses and strains. A new on-chip nanomechanical testing concept has been developed in order to measure the mechanical properties of submicrometer freestanding thin films allowing various loading configurations and specimen geometries. The basic idea is to use internal stress present in one film to provide the actuation for deforming another film attached to the first film on one side and to the substrate on the other side. The measurement of the displacement resulting from the release of both films gives access to the stress and the strain applied to the test specimen provided the Young's modulus and mismatch strain of the actuator film are known. Classical microelectromechanical-systems-based microfabrication procedures are used to pattern the test structures and release the films from the substrate. The design procedures, data reduction scheme, and a generic fabrication strategy are described in details and implemented in order to build a suite of test structures with various combinations of dimensions. These structures allow the characterization of different materials and mechanical properties and enable high throughputs of data while avoiding any electrical signal or external actuation. Results obtained on ductile aluminum and brittle polysilicon films demonstrate the potential of the method to determine the Young's modulus, yield stress or fracture stress, fracture strain, and strain hardening in ductile materials.

New On-Chip Nanomechanical Testing Laboratory - Applications to Aluminum and Polysilicon Thin Films

Silicon devices are ubiquitous in many micro- and nano-scale technological applications, most notably microelectronics and microelectromechanical systems (MEMS). Despite their widespread usage, however, issues related to uncertain mechanical reliability remain a major factor inhibiting the further advancement of device commercialization. In particular, reliability issues related to the fracture of MEMS components have become increasingly important given continued reductions in critical feature sizes coupled with recent escalations in both MEMS device actuation forces and harsh usage conditions. In this review, the fracture strength of micro- and nano-scale silicon components in the context of MEMS is considered. An overview of the crystal structure and elastic and fracture properties of both single-crystal silicon (SCS) and polycrystalline silicon (polysilicon) is presented. Experimental methods for the deposition of SCS and polysilicon films, fabrication of fracture-strength test components, and analysis of strength data are also summarized. SCS and polysilicon fracture strength results as a function of processing conditions, component size and geometry, and test temperature, environment, and loading rate are then surveyed and analyzed to form overarching processing-structure-property-performance relationships. Future studies are suggested to advance our current view of these relationships and their impacts on the manufacturing yield, device performance, and operational reliability of micro- and nano-scale silicon devices.

Fracture strength of micro- and nano-scale silicon components

On-chip laboratory suite for testing of free-standing metal film mechanical properties, Part II -Experiments

Stronger silicon for microsystems

Failure Analysis of Electrothermal Actuators Subjected to Electrical Overstress (EOS) and Electrostatic Discharge (ESD)

We report on the development of single-frequency VCSELs (vertical-cavity surface-emitting lasers) for sensing the position of a moving MEMS (micro-electro-mechanical system) object with resolution much less than 1nm. Position measurement is the basis of many different types of MEMS sensors, including accelerometers, gyroscopes, and pressure sensors. Typically, by switching from a traditional capacitive electronic readout to an interferometric optical readout, the resolution can be improved by an order of magnitude with a corresponding improvement in MEMS sensor performance. Because the VCSEL wavelength determines the scale of the position measurement, laser wavelength (frequency) stability is desirable. This paper discusses the impact of VCSEL amplitude and frequency noise on the position measurement.

Michael Shaw

Papers

On-chip laboratory suite for testing of free-standing metal film mechanical properties, Part II -Experiments

Stronger silicon for microsystems

Failure Analysis of Electrothermal Actuators Subjected to Electrical Overstress (EOS) and Electrostatic Discharge (ESD)

VCSELs for interferometric readout of MEMS sensors