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Micromachined array-type Mirau interferometer for
parallel inspection of MEMS
J. Albero, S. Bargiel, N. Passilly, P. Dannberg, M. Stumpf, U.D. Zeitner, C.
Rousselot, K. Gastinger, C. Gorecki
To cite this version:
J. Albero, S. Bargiel, N. Passilly, P. Dannberg, M. Stumpf, et al.. Micromachined array-type Mirau
interferometer for parallel inspection of MEMS. Journal of Micromechanics and Microengineering,
IOP Publishing, 2011, 21 (6), pp.065005. �10.1088/0960-1317/21/6/065005�. �hal-00655057�
Micromachined array-type Mirau interferometer
for parallel inspection of MEMS
JAlbero
1
, S Bargiel
1
, N Passilly
1
, P Dannberg
2
, M Stumpf
2
,UDZeitner
2
,
C Rousselot
1
,KGastinger
3
and C Gorecki
1
1
D
´
epartement MN2S, FEMTO-ST (UMR CNRS 6174), 16 Route de Gray, 25030 Besan¸con Cedex,
France
2
Fraunhofer IOF, A-Einstein-Str. 7, 07745-Jena, Germany
3
SINTEF IKT Optical Measurement Systems and Data Analysis, N-7465 Trondheim, Norway
E-mail:
christophe.gorecki@univ-fcomte.fr.
Abstract
We present the development of an array type of micromachined Mirau interferometers, operating in the regime of low
coherence interferometry (LCI) and adapted for massively parallel inspection of MEMS. The system is a combination of
free-space micro-optical technologies and silicon micromachining, based on the vertical assembly of two glass wafers. The
probing wafer carries an array of refractive microlenses, diffractive gratings to correct chromatic and spherical aberrations
and reference micro-mirrors. The semitransparent beam splitter plate is based on the deposition of a dielectric multilayer,
sandwiched between two glass wafers. The interferometer matrix is the key element of a novel inspection system aimed to
perform parallel inspection of MEMS. The fabricated demonstrator, including 5 × 5 channels, allows consequently
decreasing the measurement time by a factor of 25. In the following, the details of fabrication processes of the micro-optical
components and their assembly are described. The feasibility of the LCI is demonstrated for the measurement of a wafer of
MEMS sensors.
1. Introduction
The potential of non-destructive optical techniques was
demonstrated in the determination of local material properties
and for the dynamic characteristics study of MEMS. In
particular, optical interferometry may be carried out at wafer
level without sample preparation, offering high sensitivity,
non-contact measurements and automatic analysis of the
results for the reconstruction of the exact 3D shape of
MEMS structures. Various techniques were developed
including conventional or phase-shifting interferometry [
1, 2],
holographic interferometry and digital holography [
3], white
light stroboscopic interferometry [
4], speckle interferometry
[
5] and electronic speckle pattern interferometry (ESPI) [6].
According to the measuring principle and detection
system, each optical technique is mostly specialized in the
characterization of static and/or dynamic behavior of micro-
components, measuring the out-of-plane deformation, in-
plane deformation or both. Thus, two-beam interferometry
requires mirror-like surfaces, while ESPI is able to operate
with rough surfaces. Commercially available interference
microscopes [
7] often use two types of interferometric
objectives, which are modifications of the Michelson
interferometer: the Mirau and Linnik configurations. In
a Michelson interferometer objective, suitable for small
magnifications, a beamsplitter cube and a reference mirror
are inserted between the objective and the sample. At
magnifications up to 50×, the working distance becomes
too short to squeeze in a beam splitter cube. A Mirau
interferometric objective is therefore used instead. The
advantage of such a configuration is to place a reference
mirror at the center of the objective lens, and interposing
1
Figure 1. Schematic view of a single-channel LCI. The dashed line encloses the system scope of this paper.
a semi-transparent plate between the objective lens and the
specimen. These components are arranged in such a manner
that an interference pattern will appear if the system is
focused upon the sample. For magnifications of 100× and
more, which require even shorter working distances, a Linnik
interferometric objective is used. In the Linnik configuration,
a beam splitter cube directs the beam onto two objectives; one
beam illuminates the reference mirror and the other is directed
to the test surface. However, the vertical and lateral resolutions
of a Mirau interferometer are well suited for the analysis of
most MEMS devices and its range of working distances is well
adapted to the requirements of wafer-level measurements. In
this field, most of the metrology architectures are based on
bulk microscopes, performing serial measurements. Here,
there is a need for miniature and low cost instruments, able
to read the data in parallel—this is the proposal of the present
contribution.
We will demonstrate that, when fabricated as a
combination of free-space micro-optical technology and
standard micromachining, an array implementation of
miniature Mirau interferometers is well suited to facilitate
the in-line metrology of wafer-scale MEMS objects with
high throughput. The proof-of-concept of such an
approach to MEMS metrology has been demonstrated
by the authors [
8] and a multifunctional inspection
platform with two exchangeable wafer-level sensing heads
(called probing wafers) has been developed for MEMS
metrology. Each probing wafer contains a network of
5 × 5 interferometers where the architecture and the pitch
between elementary channels are adapted and aligned on
the MEMS wafer to be measured. In addition, the
illumination, detection and sample excitation modules can be
exchanged in accordance to the category of measurements
(topography, dynamic characterization, partially coherent or
monochromatic illumination, etc), thanks to their modularity.
Two versions of probing wafer configurations are available,
each of them being developed over a 4 inch wafer: (i) a
network of miniature Mirau interferometers operating in the
regime of low coherence interferometry (LCI); and (ii) a
network of Twyman–Green laser interferometers (LI). The LCI
is dedicated to static measurements of MEMS wafers, whereas
the LI is used to characterize dynamic parameters of MEMS.
In the final version, the size of each matrix is aimed to reach
10 × 5 elements and therefore be adapted when assembled
into an 8 inch basis wafer. Each array of interferometers is
made to be mounted on commercially available positioning
systems, such as the PA 200 prober from Karl Suss [
9]. Light
sources are arranged in an array and positioned on each side
of each interferometric unit. The light is guided by a beam
splitter towards the probing wafer.
In this paper, we focus on the implementation of an array-
type micromachined miniature Mirau interferometer, detailing
the fabrication process of micro-optical components and their
assembly. In section 2 we present the architecture of the
LCI and its main parameters. In section
3, we focus on
the fabrication of the probing wafer itself, with a discussion
on the design and fabrication process of each micro-optical
building block as well as on the complete assembly. To check
out the imaging quality of the LCI, an infrared sensor test
wafer is measured in section
4, demonstrating the experimental
validation of the working principles. Finally, conclusions and
perspectives are given in section
5.
2. LCI architecture
The architecture of the proposed interferometric system
consists of a 5 × 5 matrix of single-channel low-coherence
interferometers based on a Mirau configuration. In this paper,
we will focus, at some points, on the fabrication of a single-
channel interferometer for clarity although the fabrication
processes involve the 25 channels systematically. To match
several concrete specifications of MEMS/MOEMS metrology
2
(a)
(b)
(c)
(d)
Figure 2. Flowchart of the fabrication process of the interferometer: (a) Al reference mirrors and the a-Si absorbent layer are created by
lift-off, (b) the DOE compensation is fabricated by polymer UV-molding, (c) the microlens is formed by polymer UV-molding and (d)the
beam-splitter plate is fabricated by dielectric materials thin-film deposition and assembled by using of a spacer substrate.
in an industrial environment, the matrix is fabricated on
a 4-inch basis within a square of nearly 70 × 70 mm
2
,
where the pitch between the channels is 13.76 mm [
10].
The latter is a multiple of the widespread period in batch
fabrication of MEMS systems which allows scanning an
entire wafer by small displacements of the wafer within this
13.76 mm square zone. A schematic view of a single-
channel interferometer is shown in figure
1. The system
includes an illumination/detection block made of a matrix
of LED sources operating at a wavelength of 470 nm and
having a 30 nm spectral width. It is adapted for the needs
of a smart-pixel camera, which detects and demodulates the
interference signal. The light sources, the beam-splitter cube
and the detectors are assembled by mechanical means on the
complete architecture of the measurement system. Thus, in
the following, we only focus on the fabrication and working
demonstration of the probing wafer of micro-optical Mirau
interferometers (surrounded by the dashed line in figure
1),
without detailing the principles of illumination and detection
blocks.
In a single-channel Mirau interferometer, the incident
light beam from a LED source, reflected by a cube beam-
splitter, is collected by a microlens and directed towards the
sample. A diffractive optical element (DOE) is used in order to
compensate chromatic and spherical aberrations and therefore
to improve the lateral resolution. The collected light passes
through a semi-transparent plate, i.e. the beam-splitter plate
that reflects half of it back to a reference mirror while the rest
of light is transmitted towards the surface to be measured. The
beams reflected by the sample and by the reference mirror
interfere and the generated interference pattern is directed by
the microlens towards the smart-pixel camera module, which
detects and demodulates the interference signal [
11].
3. Fabrication of micro-optical building blocks
The probing system includes four different micro-optical
components. The refractive microlens has a spherical shape,
a diameter of 2.5 mm and a sagitta (sag) of 162 μm in order
to achieve a numerical aperture (NA) of 0.135 (focal length
≈ 9.3 mm). The correction of the aberration induced by
the microlens is achieved by a DOE placed on the backside
of the microlens and presenting a phase function of radial
symmetry. The 700 × 700 μm
2
reference square micromirror
is located at the center of the DOE. Finally, the beam-splitter
plate, made of a dielectric multi-layer stack, is located at nearly
half of the focal length; thanks to spacers whose function is
both to adjust the optical path length and to reduce the cross-
talk between channels. This system forms the core of the
LCI. It can be noted that this assembled multi-wafer stack,
where defects such as thickness variations of substrates or
possible deformations of wafers will affect the experimental
performances, must be as free of stress as possible in order to
match the specifications.
Figure
2 shows a schematic flowchart of the complete
fabrication process of the probing wafer. First, the aluminum
(Al) reference mirrors, whose upper face is absorbent thanks
to an amorphous silicon (a-Si) layer, are created by a lift-off
process over a glass substrate (figure
2(a)). In the fabrication
flow, the replication of the DOE is done prior to the microlens
replication due to the much lower polymer thickness, the
nearly planar appearance and the lower bowing of the wafer
after polymer curing. However, a test replication of the
microlenses is performed prior to the DOE fabrication in
order to be characterized and accordingly adjust the DOE
design. Once the DOE is replicated around the micromirror
(figure
2(b)), the microlens is formed by polymer UV-molding
(figure
2(c)). Finally, the beam-splitter plate is fabricated
by thin-film deposition of dielectric materials and assembled
by using a spacer substrate that equalizes the optical path
lengths of the reference and the measurement arms of the
interferometer (figure
2(d)). All the glass substrates used for
the fabrication are Borofloat 33 from Schott [12].
3
(b)(a)
Figure 3. Reflectivity measurements of a-Si/Al micromirrors: (a) setup, (b) comparison between the measured reflectivity of Al (at point
P1) and a-Si
/Al (at point P2) through the Borofloat 33 substrate as a function of the incident angle of the light beam.
3.1. Micromirrors
The function of the micromirrors in the Mirau architecture is
to create the reference beam of the interferometer (figure
2(a)).
They are made of a thin film of aluminum, deposited on the
surface of a 500 μm thick glass substrate that eventually
receives the microlenses on its opposite side. However, for
this architecture, the Al micromirrors also reflect back the
incoming light (through the lens) towards the camera module.
These reflections must be reduced as much as possible in order
to avoid stray light and thus a contrast reduction of the desired
interference image. We have chosen to deposit an amorphous
silicon (a-Si) layer between the glass substrate and the Al
layer, because of its absorptive behavior at the wavelength of
λ = 470 nm and its good compatibility with the fabrication
flowchart.
The Al and a-Si layers are patterned in a lift-off process.
First, a glass substrate is spin-coated with a layer of LOR10B
resin (Microchem), specially conceived as the under-layer
for positive lift-off processes [
13]. Since the LOR is not
photosensitive, it is covered with a layer of a positive
photoresist S1813 (Shipley). In the standard photolithography
process, squares of 700 × 700 μm
2
are created in S1813,
whereas LOR provides the necessary undercut for lift-off.
After patterning the resists, a 130 nm layer of a-Si is produced
by plasma enhanced chemical vapor deposition (PECVD) at
a temperature lower than the typical values (80
◦
C instead of
360
◦
C) to preserve the resist layers. Afterwards, a 150 nm
thick layer of Al is evaporated in a highly energetic process
(deposition rate 4–5 nm s
−1
) specially developed to achieve
very low values of surface roughness and, at the same time,
to compensate the increased surface roughness caused by the
a-Si deposition. Note that the values of average roughness
R
a
, obtained using a contact profilometer AlphaStep (Tencor)
on top of the Al micromirror are in the order of 0.6 nm, and
thus below typical roughness specifications for optical quality
components [
14]. The glass substrate is then immersed in 1165
remover (Microchem) in order to eliminate the unnecessary
metalized zones surrounding the micromirrors. After rinsing
and cleaning the surface with an O
2
plasma process by reactive
ion etching (RIE), the substrate is ready to receive the other
micro-optical components.
Table 1. Essential parameters of the micromirrors.
Parameter Measured value
Dimensions 700 × 700 μm
2
Materials PECVD a-Si/evaporated Al
Layer thickness 130 nm
/150 nm
Surface roughness 0.457 nm
Reflectivity of a-Si
/Al stack
(direct illumination)
84% (angle 10
◦
, λ = 470 nm)
Reflectivity of Al (illumination
through glass)
79% (angle 10
◦
, λ = 470 nm)
Reflectivity of a-Si
/Al stack
(illumination through glass)
15% (angle 10
◦
, λ = 470 nm)
The characterization of the reflectivity, on the one hand of
the Al layer alone, and on the other hand of the a-Si/Al layer
stack, when the light (commercial LED source Newport, λ =
470 nm) is incident through the Borofloat 33 glass substrate,
demonstrates the efficiency of the absorbent layer. Indeed,
reflectivity measured through the glass substrate is below 17%
for the stack whereas it reaches 85% without the absorbant
layer, corresponding to a reduction of the undesired reflections
by a factor 5 (see figure
3). The main fabrication parameters
and measurement results are listed in table
1.
3.2. Microlenses
The polymer microlenses are replicated on top of the
glass wafer that contains the micromirrors and the DOEs.
As mentioned before, the fabrication of the DOE, used
to compensate the aberrations introduced by the imaging
microlenses [15], is prior to the replication of the latter.
However, since the DOE structure can be adjusted in order
to compensate possible deviations of the real geometry of
the fabricated microlenses compared to the designed one, the
microlens master is fabricated first.
Photolithography and resist reflow are basic technologies
in our development for lens mastering, as shown in figure
4
[16, 17]. First, a clean glass substrate is spin-coated with a
layer of Clariant AZ4562 photoresist of submicron thickness
homogeneity (figure
4(a)). It can be noted that a thickness
of 100 μm is sufficient to achieve the needed sag at the end
of the process (≈162 μm). The photoresist is patterned by
photolithography and 2.5 mm diameter cylinders are obtained
4
