A creep tensile tester for thin metallic films
Citation for published version (APA):Bergers, L. I. J. C., Hoefnagels, J. P. M., & Geers, M. G. D. (2010). A creep tensile tester for thin metallic films. Poster session presented at Mate Poster Award 2010 : 15th Annual Poster Contest.
Document status and date: Published: 01/01/2010 Document Version:
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Section Mechanics of Materials
A creep tensile tester for thin
metallic films
L. I. J. C. Bergers, J. P. M. Hoefnagels and M. G. D. Geers
l.i.j.c.bergers@tue.nl
/department of mechanical engineering
Introduction
Prolonged deformation of metallic thin film radio-frequency micro-electromechanical switches effect the long term device characteristic. Here µ-scale time-dependent mechanical prop-erties differ from the macroscale and need characterization to improve reliability.
Figure 1:Prolonged deformation of metallic RF-MEMS.
Goal:
• design anin-situSEM hi-res (nm,nN) tensile creep tester.
Challenges:
• fabrication and manipulation of µm-sized samples! • nN - mN load application, control and measurement over
long periods!
Design of experiment
Careful evaluation of reported methods [1] leads to the follow-ing optimized design.
Versatile sample fabrication
t = 0.5 - 50 μm F = 0.1 N - 50 mN dx=<200 m μ μ w = 1 - 25 μm l = ~500 μm free pad: ~400 x ~400 μm μm Tensile specimen (top view)
MEMS foundry processing:
Anchor fixed to chip Displacementmarkers Pin-in-holeloading point
High aspect ratio t/l => bending stress!!
10mm
10 mm
Specimen chip
(top view)
Fapplied
Figure 2: On-chip tensile specimens: easy specimen handling, load-ing, variation and reproduction.
Precise loading Gripper SOI-substrate Au-coating on bottom gripper pin isolation 2p rotation alignment θy θz z y x Specimen chip x-axis of gripper x-axis of specimen
θyalign: electrical contacting
2-mask Silicon micromachining
Figure 3: Load transfer using a ’gripper’: pin-in-hole contact and align-ment of axes minimizes moalign-ments and bending stresses.
Sensitive load measurement
Capacitive displacement sensors: - u < 5 nm - u = 250 res range μm Environmental effects measured by unloaded mechanism Fapplied -- F ~10 nN F = kleafspringsuapplied res,min ‘Gripper’ on ‘weak’ leafsprings: 10 mm
Figure 4: An exchangeable load cell for various force ranges.
Compact in-situ electron/light microscope setup
Specimen chip Chip holder +heater Chip rotation Coarse xy-positioning Fine xyz-positioner (piezo) Loadcell Loadcell to actuator alignment Coarse z-positioning
Figure 5: The creep tensile tester. SEM/light microscopy combined with image correlation allows 2D-deformation measurements down to the nm.
Summary
• Easy dimensional variations and sample handling through MEMS foundry processing.
• Pin-in-hole ’gripper’ and precise specimen alignment for pure tensile loading.
• Load cell: simple yet sensitive leafspring mechanism. • System forin-situ SEM and optical microscopy
deforma-tion measurements.
Outlook
• Characterize size-effects in creep of metallic MEMS.
References:
[1] HEMKER, K. J. , SHARPE, W. N. JR., MICROSCALE CHARACTERIZATION OF ME
