CSEM Scientific and Technical Report 2008

Massively Parallel Optical Tomography for Quality Inspection
S. Beer, B. Schaffer, R. Cook, L. Bonjour, Y. Li, A. Perkins, D. Beyeler, S. Neukom, T. Baechler
Interferometry is a versatile tool for the test and characterization of MOEMS. As part of the EU project SMARTIEHS such a massively parallelized
system is under development. CSEM is in charge of the interferometry imager yielding a resolution of 150x150 pixels.
Interferometry can be applied for the passive characterization
of the 3-dimensional shape, as well as for the active
characterization of deformation and resonance frequencies of
MOEMS (Micro- Opto- Electro Mechanical Systems)
structures [1] . Long measurement times, due to present serial
inspection procedures available wafer-based MOEMS test
systems, are not suitable for mass production tests and
characterizations.
A new inspection approach has been developed in the EU
project SMARTIEHS [ 2] . With the introduction of a wafer-towafer
inspection concept parallel testing and characterization
of tens of MOEMS structures within one measurement cycle
has been made possible. Exchangeable micro-optical probing
wafers are developed and aligned with the MOEMS-waferunder-
test. Two different probing wafer configurations, a 5x5
array of low-coherence interferometers (LCI) and a 5x5 array
of laser interferometers (LI), use an array of 5x5 smart-pixel
cameras featuring optical lock-in detection at the pixel level [3] .
Vibration measurement plays an important role in the
characterization of micromechanical objects, delivering key
information on reliability, functionality and material properties
as well as enabling the detection of defects. The rapid
development of micromechanical technologies – principally in
view of addressing increased expectations in terms of product
quality and complexity – drives research and development of
suitable measurement methods with high accuracy and high
speed. Laser Doppler Vibrometry (LDV) is considered as the
most accurate one. As a point method, however, it requires
additional scanning to obtain an amplitude distribution over
the object-under-test, considerably increasing measurement
time. To achieve short measurement times and increase in the
amplitude ranges to be measured, new vibration
measurement techniques – similar to the time-averaging
interferometry – are being implemented using sinusoidal light
modulation and a smart-pixel camera.
This smart-pixel image sensor dedicated to optical
characterization of MOEMS, called SMOC, offers processing
of the detected intensity signal on each pixel. In an
interferometer, this functionality enables to pre-process the
interference signal in order to increase measurement speed
and accuracy. The SMOC imager, with its 150x150 pixels,
features background suppression and direct I-Q demodulation.
Global shutter allows integrate-while-read operation. The
sensor has column-parallel 10-bit ADCs and 5 output pads
operating at 41.5 MHz each. Maximum frame rate of more
than 400 fps at full resolution is supported. Higher frame rates
can be achieved by region-of-interest (ROI) sub-frame viewing.
On the digital side, a fully programmable sequencer has been
implemented. The camera composed of the 5x5
synchronously operating smart-pixel imagers has a
CameraLink-interface running at 3 Gbps.
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The principle of the camera module allowing individual
mechanical adjustment of each imager module is shown in
Figure 1.
Figure 1: Schematical representation of the SMARTIEHS camera
module (from the pixel to the high-speed data interface to the PC)
A prototype of the smart-pixel imager is under development
and will be taped out at the beginning of 2009. Further
versions will have quadrupled number of pixels (300x300) and
a significantly increased frame rate (more than 9 kfps).
This project is mainly funded under the Grant Agreement
223935 of the 7th Framework Program Objective 2007-3.6.
[1] K. Gastinger, et al., “Multi-technique platform for dynamic and
static MEMS-characterization”, SPIE Vol. 6616 (2007)
[2] K. Gastinger, et al., “SMARTIEHS – Smart inspection system for
high-speed and multifunctional testing of MEMS and MOEMS”,
14th Micro Optics Conference 2008 (MOC’08), Brussels (BE)
[3] S. Beer, et al., “Smart-pixels for real-time optical coherence
tomography”, SPIE Vol.5302 (2004), 21-32