Lijc Lambert Bergers

Measuring time-dependent deformations in metallic MEMS

The reliability of metallic microelectromechanical systems (MEMS) depends on time-dependent deformation such as creep. Key to this process is the interaction between microstructural length scales and dimensional length scales, so-called size-effects. As a first critical step towards studying these size-effects in time-dependent deformation, a purely mechanical experimental methodology has been developed, which is presented here. The methodology entails the application of a constant deflection to a lm-sized free-standing aluminum-alloy cantilever beam for a prolonged period of time. After this load is removed, the deformation evolution is immediately recorded by acquiring surface topographies through confocal optical profilometry. Image correlation and an algorithm based on elastic beam theory are applied to the full-field beam profiles to correct drift and improve limited optical profilometry precision, yielding the tip deflection as function of time with a precision of 7% of the surface roughness. A proof-of-principle measurement reveals a remarkable time-dependent deflection recovery. Assumptions and errors of the methodology are analyzed. Finally, it is concluded that the methodology is most suitable for the investigation of creep due to the simplicity of specimen handling, preparation and setup design, while maximizing long term stability and deformation precision.

On-wafer time-dependent high reproducibility nano-force tensile testing

Time-dependent mechanical investigations of on-wafer specimens are of interest for improving the reliability of thin metal film microdevices. This paper presents a novel methodology, addressing key challenges in creep and anelasticity investigations through on-wafer tensile tests, achieving highly reproducible force and specimen deformation measurements and loading states. The methodology consists of a novel approach for precise loading using a pin-in-hole gripper and a high-precision specimen alignment system based on three-dimensional image tracking and optical profilometry resulting in angular alignment of?<0.1?mrad and near-perfect co-linearity. A compact test system enables in situ tensile tests of on-wafer specimens under light and electron microscopy. Precision force measurement over a range of 0.07??N to 250?mN is realized based on a simple drift-compensated elastically-hinged load cell with high-precision deflection measurement. The specimen deformation measurement, compensated for drift through image tracking, yields displacement reproducibility of?<6?nm. Proof of principle tensile experiments are performed on 5??m-thick aluminum-alloy thin film specimens, demonstrating reproducible Young?s modulus measurement of 72.6???3.7?GPa. Room temperature creep experiments show excellent stability of the force measurement and underline the methodology?s high reproducibility and suitability for time-dependent nano-force tensile testing of on-wafer specimens.

Characterization of time-dependent anelastic microbeam bending mechanics

This paper presents an accurate yet straightforward methodology for characterizing time-dependent anelastic mechanics of thin metal films employed in metalic microelectromechanical systems (MEMS). The deflection of microbeams is controlled with a mechanical micro-clamp, measured with digital holographic microscopy and processed with global digital image correlation (GDIC). The GDIC processing directly incorporates kinematics into the three-dimensional correlation problem, describing drift-induced rigid body motion and the beam deflection. This yields beam curvature measurements with a resolution of <1.5???10?6??m?1, or for films thinner than 5??m, a strain resolution of <4???. Using a simple experimental sequence, these curvature measurements are then combined with a linear multi-mode time-dependent anelastic model and a priori knowledge of the Youngs modulus. This allows the characterization of the material behaviour in the absence of an additional explicit force measurement, which simplifies the experimental setup. Using this methodology we characterize the anelasticity of 5??m-thick Al(1?wt%)-Cu microbeams of varying microstructures over relevant timescales of 1 to 1???105?s and adequately predict the time and amplitude response of experiments performed for various loading conditions. This demonstrates the validity of the methodology and the suitability for thin film mechanics research for MEMS development.

Enhanced Global Digital Image Correlation for Accurate Measurement of Microbeam Bending

Microbeams are simple on-chip test structures used for thin film and MEMS materials characterization. Profilometry can be combined with Euler-Bernoulli (EB) beam theory to extract material parameters, like the E-modulus. Characterization of time-dependent microbeam bending is required, though non-trivial, as it involves long term sub-microscale measurements. Here we propose an enhanced global digital image correlation (GDIC) procedure to analyze time-dependent microbeam bending. Using GDIC we extract the full-field curvature profile from optical profilometry data of thin metal microbeam bending experiments, whilst simultaneously correcting for rigid body motion resulting from drift. This work focusses on the implementation of this GDIC procedure and evaluation of its accuracy through a numerical assessment of the proposed methodology.

Measuring time-dependent mechanics in metallic MEMS

The reliability of metallic microelectromechanical systems (MEMS) depends on time-dependent deformation such as creep. The interaction between microstructural length scales and dimensional length scales, so-called ‘size-effects’, play a prominent role in this. As a first critical step towards studying these size effects in time-dependent deformation, a purely mechanical experimental methodology has been developed, which is discussed here. It is found most suitable for the investigation of creep due to the simplicity of sample handling and preparation and setup design, whilst maximizing long term stability and displacement resolution. The methodology entails the application of a constant deflection to a µm-sized free-standing aluminum cantilever beam for a prolonged period of time. After this load is removed, the deformation evolution is immediately recorded by acquiring surface height profiles through confocal optical profilometry. Image correlation and an algorithm based on elastic beam theory are applied to the full-field beam profiles to yield the tip deflection as function of time. From a discussion on the sources of experimental error, it is concluded that the methodology yields the tip deflection as function of time with ∼3 nm precision.

A nano-tensile tester for creep studies

Free-standing metallic thin films are increasingly used as structural components in MEMS. In commercial devices, long-term reliability is essential, which requires determining time-dependent mechanical properties of these films. The uniaxial tensile test is a preferred method due to uncomplicated determination of the stress and strain state. However, at the MEMS-scale this method is not straightforward: specimen handling and loading, force and deformation measurement need careful consideration. Here we discuss the challenges of the application and measurement of nano-Newton forces, nanometer deformations and micro-radians rotation alignment ensuring negligible bending in on-chip tensile test structures during long periods. We then present a novel tensile-testing instrument with in-situ capabilities in SEM and Optical Profilometry. The design solutions to measure these small forces and deformations whilst ensuring a uniaxial stress state will be presented.

Anelasticity in Al-alloy thin films: a micro-mechanical analysis

Micro-electromechanical systems enable many novel high-tech applications. Aluminum alloy thin films would be electrically favorable, but mechanical reliability forms fundamental challenges. Notably, miniaturization reveals detrimental time-dependent anelasticity in free-standing Al-alloy thin films. Yet, systematic experimental studies are lacking, perhaps due to challenges in microscale testing.

Mechanically probing time-dependent mechanics in metallic MEMS

The reliability of metallic micro-electromechanical systems (MEMS) depends on time-dependent deformation such as creep. To this end, a purely mechanical experimental methodology for studying the time-dependent deformation of free-standing microbeams has been developed. It is found most suitable for the investigation of creep due to the simplicity of sample handling and preparation and setup design, whilst maximizing long term stability and displacement resolution. The methodology entails the application of a constant deflection to a µm-sized free-standing aluminum cantilever beam for a prolonged period of time. After this load is removed, the deformation evolution is immediately recorded by acquiring surface height profiles through confocal optical profilometry. Image correlation and an algorithm based on elastic beam theory are applied to the full-field beam profiles to yield the tip deflection as a function of time. The methodology yields the tip deflection as function of time with ~3 nm precision.

Creep measurements in free-standing thin metal film micro-cantilever bending

Creep is a time-dependent deformation mechanism that affects the reliability of metallic MEMS. Examples of metallic MEMS are RF-MEMS capacitors/switches, found in wireless/RF applications. Proper modeling of this mechanism is yet to be achieved, because size-effects that play a role in MEMS are not well understood. To understand this better, a methodology is setup to study creep in Al-Cu alloy thin film micro-cantilevers micro-fabricated in the same MEMS fabrication process as actual RF-MEMS devices. The methodology entails the measurement of time-dependent deflection recovery after maintaining cantilevers at a constant deflection for a prolonged period. Confocal profilometry and a simple mechanical setup with minimal sample handling are applied to control and measure the deformation. Digital image correlation, leveling and kinematics-based averaging algorithms are applied to the measured surface profiles to correct for various errors and improve the precision to yield a precision < 7% of the surface roughness. A set of measurements is presented in which alloy microstructure length scales at the micrometer-level are varied to probe the nature of this creep behavior.