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SPG Mitteilungen Nr. 40
Structural MEMS Testing
Alex Dommann and Antonia Neels*
EMPA, Lerchenfeldstrasse 5, 9014 St. Gallen, alex.dommann@empa.ch
*CSEM, Microsystems Technology Division, 2002 Neuchâtel
In single crystal silicon (SCSi) based devices, stress and
loading in operation introduces defects during the Micro-
ElectroMechanical Systems (MEMS) life time and increases
the risk of failure. Reliability studies on potential
failure sources have an impact on MEMS design and are
essential to assure the long term functioning of the device.
Defects introduced by Deep Reactive-Ion Etching (DRIE),
thermal annealing, dicing and bonding and also the device
environment (radiations, temperature) influence the crystalline
perfection and have a direct impact on the mechanical
properties of MEMS and their aging behavior. Defects and
deformations are analyzed using High Resolution X-ray Diffraction
Methods (HRXRD) such as Reciprocal Space Maps
(RSM). Micro systems technology can be highly reliable,
but can be different from those of solid-state electronics.
Therefore testing techniques must be developed to accelerate
MEMS-specific failures [1 - 4].
which is detected via the broadening of the X-ray peak
in a "rocking-curve" (RC) measurement. Analysis is performed
with high resolution X-ray diffraction. The set-up is
composed by curved multilayer x-ray mirrors named after
Herbert Göbel (Göbel mirror) in front of the X-ray tube, followed
by a monochromator for monochromatization and
collimation of X-ray beams by using a 4-Crystal monochromator
with two channel-cut Ge [220] called Bartels monochromator
(Fig. 2). Two configurations are possible for the
diffracted beam side, depending on the methods applied:
Rocking curve (RC) or Reciprocal space map (RSM). Both
methods allow measuring strain and defects concentration
in a crystal. The instrument used for HRXRD is shown (Figure
1).
New MEMS fabrication processes and packaging concepts
find applications in areas where a high reliability is needed
such as in aerospace, automotive or watch industry. This
creates a strong demand in quality control and failure analysis
and also brings new challenges, particularly in the fields
of testing and qualification. Non-destructive HRXRD methods
are applied to monitor the mobility of defects and strain
Figure 1: Diffractometer used for HRXRD applications.
HRXRD allows measuring the strain of a crystal with high
resolution (Fig. 1). We use HRXRD to assess the strain in
DRIE etched processed silicon beams. Strain deforms the
silicon beam leading to an appreciable sample curvature
A rocking curve (RC) is obtained as angular distribution
of the reflected X-ray beam, when the detector is
set at a specific Bragg angle and when the sample is
rotated about small angles normal to the Bragg plane
axis. The rocking curve is broadened by disruptions of
the plane parallelity and by crystal defects like those
introduced by mechanical stress. Reciprocal space
mapping (RSM) adds, by the restriction of the angular
acceptance of the detector, another dimension to
the information available from the HRXRD experiment.
Strain and tilt elements being present in a sample are
identified separately. An excellent introduction to both
analytical methods can be found at http://prism.mit.
edu/xray/tutorials.htm. See 'Basics of High Resolution
X-Ray Diffraction for Studying Epitaxial Thin Films'.
Fig 2: Diffractometer setup for RSM’s (detector position 1) and RC’s (detector position 2)
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