P erformance-enhanced Fabry–Perot microcavity

Microelectronic Engineering 70 (2003) 102–108
www.elsevier.com / locate / mee
Performance-enhanced Fabry–Perot microcavity structure with a
novel non-planar diaphragm
a,
a
a
a
b
W.J. Wang *, R.M. Lin , T.T. Sun , D.G. Guo , Y. Ren
a
Centre for Mechanics of Micro-Systems, School of Mechanical & Production Engineering, Nanyang Technological University,
50 Nanyang Avenue, Singapore, 639798, Singapore
b
Tissue Engineering Lab, School of Mechanical & Production Engineering, Nanyang Technological University, 50 Nanyang Avenue,
Singapore, 639798, Singapore
Received 31 March 2003; received in revised form 7 May 2003; accepted 27 May 2003
Abstract
A new Fabry–Perot (FP) microcavity structure with a novel, single and deeply corrugated diaphragm (SDCD) is presented
for use as a pressure-sensing element. Both the zero-pressure offset due to the non-planar diaphragm structure and the signal
averaging effect caused by the non-planar deflection of the diaphragm in response to an external pressure load are studied.
The FP microcavity was fabricated on a single-chip using both surface and bulk micromachining techniques. The theoretical
analyses as well as the measurements have shown that the signal-averaging effect can substantially be reduced for the
proposed FP microcavity pressure sensor by using the flatness-enhancing diaphragm structure.
 2003 Elsevier B.V. All rights reserved.
Keywords: Fabry–Perot; Pressure sensor; Corrugated diaphragm
1. Introduction
Optical metrology offers particular advantages in
remote sensing and has been suggested as a valuable
alternative to commercially available piezoresistive
and capacitive sensing techniques for devices operable in harsh environments where electronics cannot
operate. Of the various optical sensing techniques
available, Fabry–Perot (FP) interferometery shows
considerable promise in terms of high sensitivity,
versatility and immunity to environmental noise. An
*Corresponding author. Tel.: 165-790-4051; fax: 165-7911975.
E-mail address: mwjwang@ntu.edu.sg (W.J. Wang).
FP interferometer is an optical element consisting of
two partially reflecting, low-loss, parallel mirrors
separated by a gap. Since the reflectance or transmittance of the FP cavity is a function of both the
gap spacing and the wavelength, the FP cavity can
be realized as a sensor of either parameter.
Microelectromechanical systems (MEMS) technology is effective for manufacturing of these sensors since considerable flexibility in choosing
measurand response ranges, bandwidth and sensitivity can be achieved with the small and precise size of
sensing elements. FP microcavity based sensors,
such as pressure sensors, temperature sensors and
chemical sensors [1–4] have been fabricated using
micromachining techniques. Recent advances in
0167-9317 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved.
doi:10.1016 / S0167-9317(03)00399-X
W. J. Wang et al. / Microelectronic Engineering 70 (2003) 102–108
silicon micromachining techniques have made optical sensing devices more feasible for commercialization by reducing sizes and improving performance
and manufacturability. One of the major obstacles to
commercialization of the micromachined FP cavity
pressure sensors is the degradation of device performance arising from non-planar deflection of an
edge-clamped diaphragm / mirror when subjected to
an external pressure load. Two alternative techniques
have been proposed to date to address the issue [4].
One technique towards minimizing the non-planarity
of the deflecting mirror is to maximize the area ratio
of the moving mirror to the stationary one. Another
alternative flatness-enhancing technique is to form
corrugations in the deflecting diaphragm. Unfortunately, due to the restrictions stemming from either the
configurations of the FP microcavity or the process
technology, the effect on enhancing the flatness of
the diaphragm is significantly limited to date.
It has been well established from our previous
studies that compared to the conventional flat diaphragm and shallowly corrugated diaphragm of
comparable size and thickness, significantly higher
sensitivity can be achieved with the single deeply
corrugated diaphragm (SDCD) by both partly releasing the initial stress and reducing the mechanical
stiffness of the diaphragm simultaneously [5]. In this
study, the SDCD structure will be extended into the
development of a novel FP microcavity. Both analysis and finite element model (FEM) simulation have
shown that the flatness-enhancing effect can be
achieved with the SDCD structure, and therefore the
signal averaging effect can substantially be reduced
for the SDCD FP microcavity pressure sensor.
2. Design and simulation of the proposed FP
microcavity
The proposed FP microcavity sensing element, as
shown in Fig. 1, consists of three major components:
a SDCD structure consists of flexible suspending
sidewalls and a flat bottom-region that serves as the
moving mirror; a bottom diaphragm that serves as
the stationary mirror; and an air gap between the
moving and stationary mirrors of the FP microcavity.
The FP microcavity pressure-sensing element is
103
Fig. 1. Three-dimensional schematic with cross-section (a); and
bottom-side view (b) of the proposed SDCD FP microcavity
pressure-sensing element.
expected to operate at wavelengths greater than 1.15
mm, where the silicon substrate is highly transparent.
The unique feature of this design is that it employs
the flexible suspending sidewalls serving as the stress
concentrator and buffer for the moving mirror to
enhance the flatness of the diaphragm in response to
external pressure load, where its whole sensing area
is at the bottom of the corrugation. While similar
optical characteristics may be achieved for the FP
microcavity with conventional flat diaphragm provided that the diaphragm is large enough compared
with optical beam size, the proposed SDCD FP
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W. J. Wang et al. / Microelectronic Engineering 70 (2003) 102–108
microcavity offers significant advantage in terms of
small size and dense arrays.
The SDCD structure consists of a 0.5-mm-thick
low-stress silicon nitride (Si x N y ) layer sandwiched
between two 0.5-mm-thick polysilicon cladding
layers. The thickness of each layer in the composite
diaphragm was chosen mainly to satisfy the mechanical constraints such as low tensile residual
stress and suitable mechanical compliance with the
size of the flat bottom-region of the SDCD structure
being chosen as 200 mm.
The stationary mirror comprises also three layers
of polysilicon, Si x N y and silicon substrate. The 200nm-thick Si x N y and 350-nm-thick polysilicon layers
are used to fine-tune the total reflectance of the FP
microcavity. It should be mentioned that finesse is a
measure of the sharpness of the transmission peaks.
From the sensing elements perspective, the use of
mirrors possessing moderately low reflectance, r, i.e.,
a low finesse FP cavity, is more suitable since the
total reflectance of the FP cavity would become
broad and almost sinusoidal in shape for moderately
small r, values while larger r values will cause the
reflectance to have a sharp peak and little variation
over the air gap variation of interest, and too small
an r value will cause the reflectance vary in a small
range over the air gap variation.
The initial air gap (i.e., a gap at an applied
pressure load of zero) of the FP microcavity was
selected as 1.6 mm to obtain a linear response over
the desired pressure range and to minimize the
measurement tolerance arising from the layer thickness variation induced by the fabrication process. FP
microcavity with a large gap (for example, .5 mm)
has a multi-cycle operation and thus relies on fringe
counting techniques for the determination of the
measurand [6]. Short-gap FP microcavity has a
spectral width comparable to a single fringe so that
any shifts of the resonance peak can directly be
related to the magnitude of the parameter of interest.
Short-gap type FP microcavity is, therefore, used in
this study because of its compatibility with a simpler
detection scheme as opposed to the cumbersome and
ambiguous fringe counting circuits associated with a
large-gap FP microcavity.
The signal averaging effect is referred to as a
pressure-dependent degradation of optical response
which arises because the negative intensity values
can cancel out with the positive values due to the
sinusoidal nature of the optical response reflected
from or transmitted through a FP microcavity, leading to an averaged optical response from different
positions of the non-uniformly deflected diaphragm.
For this reason, the actually detected optical response
may be lower than the expected value from the FP
microcativity with an idealized piston-like diaphragm, and the degradation increases with increasing the degree of the curvature of a deflecting
diaphragm in a FP microcavity. To simulate the
actual optical response, the shape of the deflected
diaphragm should first be determined. FEM was
employed to simulate the shape of the proposed
SDCD structure. For comparison, the conventional
flat diaphragm was also involved in the simulation.
We simulated both the square flat diaphragm of
2003200 mm 2 in area and 1.5 mm in thickness, as
well as the SDCD of identical bottom area and
thickness with a corrugation depth of 40 mm. The
pre-stressed diaphragms were modeled using a threedimensional shell element (element Shell 63 in
ANSYS 5.6). The initial stress was induced in the
diaphragms by using the equivalent cooling temperature method as proposed by Zhang and Wise [7]. Fig.
2 shows the comparison of the FEM simulated
deflections along the moving mirror between the
conventional flat diaphragm and the SDCD structure
under a l-kPa pressure load, in which the point at 0
mm corresponds to the center of the mirror having
the maxmal deflection, and the deflection at each
point is normalized with respect to the maximal
Fig. 2. The deflection along movable mirror for an applied
pressure of 1 kPa, in which the point at 0 mm corresponds to the
center of the moving mirror, and the deflection at each point is
normalized with respect to the maximal deflection at the center of
the mirror.
W. J. Wang et al. / Microelectronic Engineering 70 (2003) 102–108
deflection at the center of the mirror. It is evident
from Fig. 2 that the degree of flatness of the moving
mirror subjected to external pressure load has substantially been enhanced by the SDCD when compared to the conventional flat diaphragm. Since the
variation in the amount of deflection over the moving
mirror can be reduced by the suspending sidewalls,
we propose that the pressure-dependent degradation
of the optical response (i.e., the signal averaging
effect), can be significantly reduced for the FP
microcavity based pressure sensor.
Following the determination of the shape of the
deflecting diaphragm, the weighted average response
was calculated from [8]
O
f R( g) ? A( g) g
R avg 5 ]]]]]
A( g)
O
(1)
where R( g) is the reflectance of the FP microcavity
at gap g and A( g) is the area with gap g. The
reflectance of the FP microcavity of a given gap
distance was calculated using the characteristic matrix method [9]:
S
h0 B 2 C
R 5 ]]]
h0 B 2 C
DS
D
h0 B 2 C *
]]]
h0 B 2 C
(2)
where * denotes complex conjugate, and
B
q
3 4 1P 3
3 4
C
5
i 51
cos(ko n i z i )
j
] sin(ko n i z i )
hi
jhi sin(ko n i z i )
cos(ko n i z i )
42
1
hm
(3)
where hm and h0 are the admittance of the environment outside the FP microcavity and of the medium
from which the light comes, respectively; k 0 5 2p /l;
n i and z i are the refractive index and thickness of the
ith layer, respectively.
Fig. 3 shows the simulated reflectance as a function of mechanical displacement at the center of the
diaphragm for FP microcavity with the conventional
flat diaphragm and the proposed SDCD structure
with a corrugation depth of 40 mm in comparison
with an idealized piston-like diaphragm. It can be
seen from Fig. 3 that compared with the idealized
piston-like diaphragm, degradation is present in the
105
Fig. 3. Evaluation of signal averaging effect of the FP microcavity with a conventional flat diaphragm and the proposed SDCD
structure in comparison with an idealized piston-like diaphragm.
reflectance of the FP microcavity with the actual
bowed flat diaphragm. It is evident that the reflectance of the FP microcavity with the flat diaphragm
does not have a period of one half the illumination
wavelength of 1310 nm, demonstrating the signal
averaging effect caused by the non-uniform deflection of the deflected diaphragm. However, it can
clearly be seen that using the SDCD structure as the
moving mirror of the FP microcavity can substantially reduce the signal averaging effect. The substantial
reduction in the signal averaging effect is believed to
be attributed to the flexible suspending sidewalls that
act as a stress concentrator and buffer for the moving
mirror (i.e., the flat-bottomed region of the SDCD
structure), partly change their own shapes rather than
the one of the entire diaphragm when absorbing the
external pressure load, thereby enhancing the degree
of flatness of the moving mirror in response to an
applied pressure load.
On the other hand, simulations performed using
FEM have shown that the SDCD structures having
internal stress exhibit a central deflection under zero
applied pressure (i.e., a zero-pressure offset). This
phenomenon is not observed in conventional flat
diaphragm structures having internal stress, which
remain flat under zero applied pressure. It should be
pointed out that the zero-pressure offset is different
from buckling, which occurs only in case of compressive stress and only if the compressive stress has
exceeded a critical value [10]. The direction and
shape of a buckled diaphragm cannot be exactly
predicted. However, the zero-pressure offset direc-
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W. J. Wang et al. / Microelectronic Engineering 70 (2003) 102–108
Fig. 4. FEM simulated zero-pressure offset versus corrugation
depth for different initial stress.
tion and magnitude of the non-planar diaphragm can,
in principle, be predicted if the dimensions and the
stress are known [10]. It has been established that the
offset direction of the SDCD structure is dependent
on whether the initial stress is compressive or tensile,
while the offset magnitude is a function of diaphragm stress level and diaphragm geometry [5].
Fig. 4 shows the magnitude of zero-pressure offset of
the diaphragm versus the initial stress. It can be seen
from Fig. 4 that the higher the internal stress level,
the larger the zero-pressure offset. An interesting
phenomenon is that, for a specific initial stress level,
a larger zero-pressure offset is observed at the
extremes of the corrugation depth range with a
minimum value in mid range. This is because the
four sidewalls at a smaller corrugation depth are
more rigid as compared to those of a larger corrugation depth. Thus, the relaxation of the diaphragm
structure is more difficult when it is being released.
For a large corrugation depth, however, the SDCD
structure has become more flexible, which makes it
more vulnerable at high initial stress. As for the
offset direction, the observed zero-pressure deflections were in the direction of bending down from the
substrate for the low tensile stress composite diaphragms in this study, which is consistent with the
FEM simulation results.
3. Fabrication and characterizations
As shown in Fig. 5, the proposed FP sensing
Fig. 5. Process flow of the proposed FP microcavity.
element is fabricated using both surface and bulk
micromachining techniques. The process starts with
a double-sided polished n-type silicon (100) wafer
upon which a masking layer of silicon nitride, as
shown in Fig. 5a, was formed by low-pressure
chemical vapor deposition (LPCVD). After the
masking layer was patterned on the front-side of the
wafer, a deep cavity was etched with 40% aqueous
KOH at 60 8C (50-mm thick silicon was remained)
(Fig. 5b). The masking layer of silicon nitride was
then removed using hot H 3 PO 4 . Low-stress silicon
nitride and polysilicon were then formed by LPCVD
successively, followed by deposition of phosphosilicate glass (PSG) of 1.6 mm serving as the sacrificial
layer. A reflow of the PSG was carried out at a
temperature of 850 8C for 1 h to eliminate the effect
W. J. Wang et al. / Microelectronic Engineering 70 (2003) 102–108
of stress concentration at sharp corners. A trilayer of
0.5 mm of polysilicon, 0.5 mm of low-stress silicon
nitride, and again 0.5 mm of polysilicon, were
deposited successively by LPCVD. The resultant
tensile residual stresses of the stacked layers of the
diaphragms were measured as 70 MPa. The stress of
the film at every process stage was obtained by
measuring a dummy wafer using a curvature-based
film-stress meter.
The polysilicon / Si x N y / polysilicon sandwich, the
sacrificial PSG layer, as well as the polysilicon and
the low-stress silicon nitride layers at the back-side
of the wafer were then removed, followed by the
creation of access holes using deep reactive ion
etching with photoresist as its masking layer (Fig.
5c). The diaphragm was then released in the 40% HF
solution (Fig. 5d). Sequential replacement of acetone
and methanol was carefully processed to protect the
diaphragm from sticking. Fig. 6 shows the SEM
micrograph of the fabricated SDCD FP microcavity.
The wavelength-dependent transmittance of the FP
microcavity was measured without any external
pressure load. The measurements combined with the
simulated results are shown in Fig. 7. Included in the
simulation is the variation of the refractive index of
each film as a function of wavelength, using the data
from [11]. The zero-pressure offset of approximately
40 nm responsible for the phase shift of the optical
response is considered, as well. It can be seen from
Fig. 7 that reasonable agreement was obtained
Fig. 6. SEM micrograph of the fabricated FP microcavity; (a) the
top view and (b) the bottom-corner region.
107
Fig. 7. Simulated and measured transmittance of the FP microcavity versus wavelength; solid line: simulated response; dotted
line: measured response.
between the measured and simulated transmittance,
indicating the layer thickness and indices of refraction used in the calculation were correct, and the
structure proposed in this study is feasible to be
realized as a pressure sensor.
An applied pressure deflects the diaphragms and
changes the optical path length of the cavity, which
is sensed by the change in the intensity of the light
reflected from the FP microcavity. Since there are
access holes in the bottom diaphragm, when a
pressure differential exists, it is actually the top
diaphragm that is deflected. The prototype pressure
sensor has been tested under static pressure. The
experimental setup is sketched in Fig. 8. As shown in
Fig. 8, a laser was routed to the one end of the input
ports of the 232 coupler. The reflected light was
measured from the other end of input ports. One end
of output ports is connected to the FP microcavity.
Fig. 8. Schematic view of the optical measurement set-up.
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W. J. Wang et al. / Microelectronic Engineering 70 (2003) 102–108
a novel SDCD structure as a moving mirror has been
designed, fabricated and characterized. The device
performances were evaluated by both theoretical
analysis and measurements. Results show that the
signal averaging effect was substantially reduced for
the SDCD FP pressure sensor by the flatness-enhancing diaphragm structure. The initial testing of the FP
microcavity has shown encouraging results. Since
the proposed FP microcavity is significantly
miniaturized, they will find applications to advantage
small size and dense arrays, which play an important
role in medical applications.
Fig. 9. Simulated and measured reflectance as a function of
differential pressure.
Fig. 9 shows the simulated and measured optical
reflectance of the SDCD FP pressure sensor. In the
simulation of the proposed SDCD FP pressure
sensor, the residual stress of 70 Mpa is assumed to
be present in the SDCD with corrugation depth of 40
mm, according to the previous measurements. Both
zero-pressure offset and signal averaging effect were
included in the simulation. It can be seen from Fig. 9
that the experiments show good consistence with the
calculations. The maximum sensitivity is obtained
over the pressures ranging from about 10 to 25 psi at
the wavelength l 5 1310nm, which can be tailored
to a particular range of pressures according to the
specific requirements of applications by tuning the
laser wavelength. It is noticed from Fig. 9 that at
wavelength l 5 1320nm, the response curve shifts
along the horizontal axis of Fig. 9, resulting in the
maximum sensitivity over the pressures ranging from
12 to 27 psi. The validity of the extracted deflection
distance from the reflectance plot in Fig. 9 is
evaluated by comparing with the measured deflection
distance. The measured deflection distance of the
SDCD mirror, 0.63 mm, is comparable with the one
peak-to-peak period of the reflectance, 0.655 mm,
demonstrating the SDCD FP microcavity is promising in realization as a pressure sensor.
4. Conclusions
In this work, a new FP microcavity structure with
References
[1] Y. Kim, D.P. Neikirk, Micromachined Fabry–Perot cavity
pressure transducer, IEEE Photonics Technol. Lett. 7 (12)
(1995) 1471–1473.
[2] D.C. Abeysinghe, S. Dasgupta, J.T. Boyd, H.E. Jackson, A
novel MEMS pressure sensor fabricated on an optical fiber,
IEEE Photonics Technol. Lett. 13 (9) (2001) 993–995.
[3] J. Han, Fabry–Perot cavity chemical sensors by silicon
micromachining techniques, Appl. Phys. Lett. 74 (3) (1999)
445–447.
[4] J. Han, J.Y. Kim, T.S. Kim, J.S. Kim, Performance of
Fabry–Perot microcavity structures with corrugated diaphragms, Sensor. Actuat. A79 (2000) 162–172.
[5] W.J. Wang, R.M. Lin, Study of single deeply corrugated
diaphragm for high sensitivity microphones, J. Micromech.
Microeng. 13 (2003) 184–189.
[6] G.L. Mitchell, A review of Fabry–Perot interferometer
sensors, in: Optical fiber sensors, Proceedings in Physics,
Vol. 44, Springer Verlag, 1989, pp. 450–457.
[7] Y. Zhang, K.D. Wise, Performance of non-planar silicon
diaphragms under large deflections, J. Microelectromech.
Syst. 3 (1994) 59–68.
[8] Y. Kim, D.P. Neikirk, Micromachined Fabry–Perot cavity
pressure transducer with optical fiber interconnects, SPIE
2624 (1995) 242–249.
[9] H.A. Macleod, Thin-film Optical Filters, 3rd Edition, Adam
Hilger Ltd, 2001.
[10] P.R. Scheeper, W. Olthuis, P. Bergveld, The design, fabrication and testing of corrugated silicon nitride diaphragms, J.
Microelectromech. Syst. 3 (1) (1994) 36–42.
[11] E.D. Palik (Ed.), Handbook of Optical Constants of Solids,
Academic, Orlando, FL, 1985.