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

Abstract

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 pinin-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 µN 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 µm-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 nanoforce tensile testing of on-wafer specimens.

A Uni-Axial Nano-Displacement Micro-Tensile Test of Individual Constituents from Bulk Material

“…In addition, the tensile stage is also designed for in-situ testing inside a SEM. More design details of the micro-tensile stage can be found in [35].…”

“…After mounting the wedge on the micro-tensile stage, accurate specimen alignment with respect to the loading direction is required to avoid complex loading caused by bending [21,35]. Following criteria by Bergers et aI.…”

“…Following criteria by Bergers et aI. [35], for example, for a micro-tensile specimen gauge dimension of 9×3×2.5 μm 3 , the in-plane rotation misalignment between the specimen axis and the loading axis has to be smaller than 11mrad (0.68°), and the out-of-plane tilt misalignment may not exceed 10mrad (0.57°) to limit bending stress to 0.5% of the imposed uni-axial stress.…”

“…The calibrated leaf spring stiffness is designed to yield the desired force range, as explained in Ref. [35]. For such minute forces, the force measurement is highly sensitive to background influences caused by thermal fluctuations, environmental vibrations, and tilt-induced deflections of the leaf spring.…”

“…Therefore, to measure these background influences, a second identical double-leaf spring is designed in a mirrored configuration as part of the same load cell. The corrected force measurement is obtained by subtraction of the specimen force by the background force with a sampling rate of 10 Hz; more details are given in [35]. The effectiveness of this corrected measurement can be seen at the left of Fig.…”