Design and Validationof a Testing
to Measure
Experimental
designfor Setup
uniaxial
tensileTensile Properties of Materials at the Microscalemeasurements
atthemicroscale
Daniele Casari , Laszlo Pethö , Patrik Schürch , Laetitia Philippe , Johann Michler ,aaaaa

Philippe Zyssetb, Jakob Schwiedrzika
aEmpa, Swiss Federal Laboratories for Materials Science and Technology, Thun, Switzerlandb
Institute for Surgical Technology and Biomechanics, University of Bern, Bern, Switzerland

Abstract: Focused ion beam (FIB) technology was employed to to produce microtensile specimens through a time-saving manufacturing protocol while minimizing FIB artefacts. Thesample geometry was optimized via finite element (FE) to mimic ASTM 638 tensile specimens stress conditions. A combination of reactive ion etching and FIB milling was used tomanufacture self-aligning grippers which are able to ensure less than 5% error for in-plane misalignments up to 2° and up to 1/3 of the sample width. Single crystal Si and GaAs
specimens were tested to validate the tensile setup.

1. Sample Geometry

2. FIB−SEM Sample Fabrication Protocol

Self-standing sample geometry with stresses
profile comparable to the one found in theASTM 638 standard geometries: the FE
optimized sample geometry ensures uniaxial
and homogeneous stress in the gage section
while minimizing stress concentrations.

FIB milling allow the fabrication of microsized
specimens of any material stable in vacuum
and under the influence of electron beam.
Nevertheless this method produces artefacts
that could affect the mechanical test if notcontrolled.

Geometry
Factor:

Minimizing FIB Induced Artefacts:• Curtaining is reduced by depositing a Pt
layer cap (1-2 µm) on the sample head prior
FIB milling.
• Attached to its original substrate
to facilitate handling and avoid
pre-test damage.

• Damage layer is reduced by decreasing
voltage and beam current. Estimateddamage layer of less than 25 nm.

• Stress deviations <5% with ratio
junction radius/width sample ≥4.

• Re-deposition minimized by parallel millingof the specimens.

• Size is in the order of few µm.

• Taper to tolerances of <0.5° by overtitling
the sample between 1 to 4° in the last step,
depending on the material milled and thebeam settings.

• Sample head designed to allow
gripper guiding during selfalignment.

20 um
1 µm

3. Microtensile Setup

4. Silicon Grippers

Stress strain data can be obtained with a correction for the system compliance. The compliance ofthe tested specimen is found by knowing the compliance of the system (which is fixed and can befound with calibration samples). With the help of simulations two geometry factor describing thevariation of stiffness in function of specimen width and thickness are defined:+"#"$

ΔA

1"#"$ (ΔA, ΔC) = E= 1++ "#"$AE1+ΔC
CE+34$567$8

ΔA
134$567$8 (ΔA, ΔC) = E= 1+AE+ 34$567$8

ΔC E.GHI1+CEHence, even when FIB milling does not produce the exact specimen shape defined above, one canstill extrapolate both modulus and displacement in the gage section:2=

134$567$8

1

0.398

+&'&

−

1=>

!"#"$ = ! &'& − )+3<3&$7

1+&'&−

1+"#"$= ! &'& − )

1+&'&

−

10.746 1"#"$ 2Error Propagation (setup accuracy):
Assuming an error of 100 nm for all the geometric dimensions necessary to calculate the stress strain
data, a peak-peak noise of ΔF =10 µN for the load cell and a misalignment 0.5 µm, this study
revealed an uncertainty in the measured elastic modulus of only 7.2%.

5. Setup Validation

Laterally compliant silicon (Si) grippers are fabricated by deep reactive ion etching (Bosch etch). This
process allows the manufacturing of 96 gripper of 200 µm thickness in one single Si wafer. The grippers
have high reproducibility and the process allow resolution up to 0.5 µm. To ease the manoeuvring of
the tensile gripper on the sample, a part of the gripper end can be removed by Xe plasma-FIB milling.
FE analysis showed that by using laterally compliant grippers instead of a rigid gripper is advantageous
in terms of measurement accuracy of both stress and strain.

6. Conclusions

SiGaAsVideo

Stress strain data

Accuracy setup:
Measured modulus: 81.4 ± 4.6 GPa
Theoretical modulus: 85.5 GPa

Ø The versatility of FIB manufacturing techniques is highlighted as it allows
applications such as the study of micromechanical properties of materials.

Sample Geometry:Ø The testing setup was design to be insensitive to in plane misalignments and it
might be used to test all class of material that can be milled by FIB methods.
Ø Sample geometry was optimized through FE analysis to avoid stress concentrations
while a self-aligning tensile setup was developed using a compliant gripper.
Ø The methodology was demonstrated experimentally on single crystal GaAs and Si
specimens using an Alemnis nanoindenter.

(Self-alignment)

Gage section failure in morethan 90% of the samples.

Grants: # 165510 # 174192

Corresponding author:
Daniele Casari
Swiss Federal Laboratories forMaterials Science and Technology
Feuerwerkerstrasse 39
3602 Thun
Switzerlanddaniele.casari@empa.ch+41 58 765 62 31