OPTO - MECHNAICAL RELIABILITY STUDIES OF ALN DRIVEN
CANTILEVERS
A. Andrei, K. Krupa, L. Nieradko, C. Gorecki
Department of Optics LOPMD, Institute FEMTO-ST, University of Franche-Comte
16 Route de Gray 25030 Besancon, France
L. Hirsinger, P. Delobelle
Department of Applied Mechanics LMARC, Institute FEMTO-ST, University of Franche-Comte
24 Chemin de l’Epitaphe 25000 Besancon, France
J. Kacperski, M. Józwik
Institute of Micromechanics and Photonics, Warsaw University of Technology
8 Sw. A. Boboli St., 02-525 Warsaw, Poland
ABSTRACT
The goal of presented study was the developing and investigation of the high quality cantilevers designed as a MEMS/MOEMS
actuators. The cantilevers are composed from silicon beam and a transducer including the aluminium nitride (AlN) layer. It is a
material with the piezoelectric properties, which can be an alternative for PZT films in micromachining technology. After
presenting the fabrication process of the testing devices, the rest of the paper will focus on extraction of AlN physical
parameters combining nanoindentation measurements and deflection data. For non-contact measurements of cantilevers
deflection the interferometric system has been proposed. It gives the possibility to measure static (e.g., initial shape, static outof-plan displacements) and dynamic parameters of samples (e.g., resonance frequencies and amplitude distributions in
vibration modes). Parameters such as residual thin film stresses have been calculated using non approximated equations
capable of taking into account multiple film stacking.
Introduction
MicroElectroMechanical Systems (MEMS) represent an extraordinary technology that promises to transform whole industries
and drive the next technological revolution. These devices can replace bulky actuators and sensors with micrometer scale
equivalents that can be produced in large quantities by silicon micromachining. MEMS improved functionalities and potential
capabilities have brought in range many different application fields, including optical communications, medicine, guidance and
navigation systems, RF devices, weapons systems, biological and chemical agent detection, and data storage. Because the
field of commercial MEMS is still in its infancy, there is nevertheless an important issue for new MEMS which still requires
advanced research, i.e. MEMS reliability.
Aluminium nitride (AlN) films have piezoelectric properties that are already used for acoustic wave propagation in miniature
high frequency bypass filters in wireless communication [1,2]. This is a promising material also for MEMS applications and
sensors using surface acoustic waves have already been proposed [3,4]. For actuation purposes, even if PZT films are most of
the time used for their better piezoelectric properties [5], AlN still represents an alternative that have to be explored.
The objective of the study was the investigation of the high quality cantilevers with AlN layer operating as reliable actuation
elements in more complex MEMS systems. It makes necessary the study of the mechanical and fatigue behaviour of such
components, allowing to understand and analyse their failure mechanisms. In this process the accurate metrology plays a key
role. Material tests were obtained by nanoindentation. For testing under static and dynamic conditions the interferometric
platform has been proposed. It offer the non-contact and precise characterization of microshape and out-of-plane
displacements.
Sample fabrication
The AlN driven microbeams were fabricated on 380-µm thick, 3” (100) oriented Si wafers. The process flow is shown in Figure
1. After realizing a 15 µm thick Si membrane by KOH etch of bottom side of the wafer (step 1), on the top side are
successively deposited three thin layers : two CrNi PVD metal layers (150 nm at step 2 and 350 nm at step 4) that have
between them the AlN film. The Cr50%Ni50% alloy has been sputtered on a heated substrate (200°C) at 230 Watt (0.5 A)
using a 7 mTorr Ar gas. The AlN has been deposited (step 3) in a pulsed reactive DC sputtering machine on a non heated
substrate with an input power of 650 Watt, a Ar/N2 gas ratio of 0.65 sccm / 6 sccm and a pressure of 4.9 mTorr. This
conditions led to a deposition rate around 19 nm/min. Two different wafers have been processed, having a final AlN thickness
of 1 µm and 1.4 µm. The top CrNi electrode patterned at step 4, has been used as a hard mask during AlN and silicon etching
(steps 5 and 7). An intermediary process step allowed the deposition and patterning of 500 nm thick Aluminium pads that were
used for wire bonding. The last RIE step removed the silicon which was not protected by the CrNi hard mask, transforming the
initial Si membranes into an array of free standing cantilevers. They had a constant width of 50 µm and variable lengths from
200 µm up to 900 µm. Figure 2 presents the layout of the cantilever array and a picture of the final device once it has been
diced and connected to the package.
Figure 1. Technological process flow chart.
Figure 2. Layout of the single chip and photography of packaged sample
The analytical model for multilayer cantilevers
The technological process of cantilevers fabrication has an influence on the initial shape of microstructures. The deposition
and etch of multiple thin layer on an initially flat silicon substrate introduces internal stress and generates bending. For
estimation of stress magnitude we introduce analytical model of simplified cantilever structure. The use of the same metal
(CrNi) for the top and bottom electrodes, allows us to consider that the silicon cantilever has on its surface only two films : the
AlN film and a metal electrode film with a thickness equal to the sum of the top and bottom electrodes thicknesses. The stress
in the AlN and metal films are causing a mechanical deflection δm of the cantilever that can be written as :
δm =
with the global stress σ0 :
2
3(1 − υ ) E s hs − (E f h f + E e he )(he + h f ) 2
L σ0
2
E s hs + E f h f + E e he
E eq heq
(1)
σ0 =
where
σ f h f + σ e he
(2)
heq
E = Young Modulus
h = thickness
σ = stress
The indexes s, f, e corresponds to the parameter for silicon substrate, the AlN film and the metal electrodes. L represents the
cantilever’s length and υ the Poisson coefficient considered the same for all the materials. The equivalent thickness heq is
equal to the sum of hs, hf, and he, while Eeq represents the equivalent modulus that can be written as :
Eeq =
K1 + K 2 + K 3 + K 4 + K 5
3
f h f + E s h f + E e he ) heq
(3a)
(E
with
K1 = E 4f h 4f + E s4 hs4 + Ee4 he4
(
(3b)
K 2 = 2 E s E e hs he 2h + 2h + 3hs he
2
s
(
E h h (2h
2
e
)
K 3 = 2 E f Ee h f he 2h 2f + 2he2 + 3h f he
K4 = 2 E f
s
f
s
2
s
+ 2h + 3hs h f
2
f
)
)
K 5 = 12 E s E f hs h f he heq
(3c)
(3d)
(3e)
(3f)
Under an applied voltage V, the piezoelectric properties of the AlN film will cause an additional deflection δp that can be related
to the AlN piezoelectric coefficient d31 by :
δp = −
E f (E s hs + Ee he )
3
d 31 L2 V
2
E eq heq E s hs + E f h f + Ee he
(4)
Putting in the above equations he=0 leads to the classic equations for a piezoelectric film deposited on a silicon cantilever [6].
Furthermore, a first order development of the Eq. (1) gives a generalized writing of the Stoney equation :
σf
E s hs2 δ m
=
3(1 − υ )h f L2




E f h f  3h f h 2f 

1 +
+
1 + 4
2hs hs2 
E s hs 
 1
424
3  1424
3

A
B


(5)
For our particular study, using the Stoney equation would had introduce a significant under-estimation of the stress in the film,
since the values of the terms A and B are 0.97 and 0.11 respectively.
Interferometric system
To investigate the behaviour of the cantilevers the interferometry method has been proposed. The measurement system was a
multifunctional interferometric platform dedicated MEMS/MOEMS testing [7]. It gives the possibility to combine the capabilities
of interferometry methods to find static (e.g., initial shape, static out-of-plan displacements) and dynamic parameters of
samples (e.g., resonance frequencies and amplitude distributions in vibration modes). The scheme of measurement platform is
presented in Fig. 3. The monochromatic light comes from the collimated laser diode (λ = 665 nm). The main optical part
consists of Twyman-Green interferometer and long working distance microscope (LDM). The interferometer is formed with a
beam-splitter cube and mirror as reference reflective surface. Both the plane reference wavefront and the measuring
wavefront, exiting collinearly from the beamsplitter. Obtained interference pattern is magnified by microscope optics and
acquired by the CCD camera coupled with a frame grabber. To introduce phase shift in interferograms, needed by temporal
phase shift method [8], the mirror is shifted by piezoelectric transducer. An object under test may be loaded by voltage signal
(optionally amplified) supplied from generator. After calibration procedure of PZT transducer measurement is performed
automatically and results from acquired interferograms are calculated in software.
Figure 3. Scheme of the interferometric platform
Results of AlN films study
Prior the fabrication of the devices, a series of test structures has been realized. They allowed the measurement of the stress
in the films and the influence of the bottom electrode nature over the stress in the AlN film. Nano-indentation tests were
performed on the AlN films of the test structures using a Nanoindenter IIS (Nano-Instruments) with a Berkovich tip. The study
was conducted following the continuous contact stiffness measurement procedure [9] with a frequency of 45 Hz and an
indenter vibration amplitude of 1 nm during the penetration on the tip into the sample. It allowed the determination of the
M<hkl> modulus for AlN deposited on Pt and CrNi electrodes. The indentation modulus M<hkl> for AlN was found to be strongly
dependent of the metal electrode on which the AlN film has been deposited : M<hkl> = 340 GPa for CrNi electrode for only
M<hkl> = 290 GPa for the Pt elecrtode. Using the elastic constants Cij for AlN films given in the literature [10,11] it can be shown
that the value of M<001> is equal to 335 GPa (maximum value for M<hkl>). Therefore, between the two metal electrodes that
have been tested, the CrNi electrode proved to be the best choice since it led to a well <001> orientated AlN film. Furthermore,
the bending of the test structures cantilevers before and after the AlN deposition on their CrNi and respectively Pt electrodes,
gave valuable information about the stress levels in the films. The measurements shown in Figure 4 indicated that the stress in
the CrNi film is tensile, while the Pt film was under compression.
Figure 4. Measured test structures cantilever bending before and after AlN deposition as a function of their square length
The change of slope after AlN deposition indicates a compressive stress in this film. Numerical calculations using equation 1
led to values of +1110 MPa tension stress for CrNi and -700 MPa compressive stress for Pt. After its deposition on the two
different metal electrodes, the stress value in the AlN film deposited on CrNi was of -220 MPa, versus -564 MPa for the one
deposited on Pt. CrNi proved again to be a better choice as metal electrode since its allows the deposition of less stressed
AlN. Moreover, if the thicknesses of the films are well chosen, it is possible to obtain a structure with a global stress equal to
zero (at a given constant temperature).
The compressive stress in the AlN film has been related in the literature to the high DC power input [12]. W.J. Meng and al.
[13] have also shown that the stress was a function of the AlN film thickness : it had a minimum for thicknesses between 500
nm and 1000 nm and it is increasing for grater thicknesses. These observations have been confirmed in our study. Figure 5
shows the calculated AlN stress in the test structures but also in the final devices that had AlN thicknesses of 1 µm (wafer1)
and 1.4 µm (wafer 2). The AlN compressive stress is significantly increasing with its thickness. Therefore, if the objective was
to realize an AlN driven structure with low global stress, the CrNi electrodes thicknesses should be adjusted not only to the AlN
thickness value, but also to its thickness depended stress value.
Figure 5. Calculated stress in the AlN film as a function of its thickness
Results of cantilevers study
In order to evaluate the cantilevers behaviour, the static and parameters of the cantilevers have been determined by using the
interferometric system described in previous section. The technological process of cantilever fabrication has an influence on
the flatness and initial shape of microstructures. The deposition and etch of multiple thin layer on an initially straight Si
generates bending. The initial static deflection h0 of 800 µm long cantilevers with both : 1 µm and 1,4 µm of AlN film thickness
has been performed at the beginning. After the 1st resonance frequency of vibration was measured. The obtained results for
10 cantilevers of each type are collected in Table 1.
No.
tAlN =1 µm
f [kHz]
h0 [µm]
tAlN =1.4 µm
h0 [µm]
f [kHz]
1
2
3
4
5
6
7
8
9
10
16.76
16.43
16.11
19.84
16.28
16.59
16.91
14.77
14.95
14.89
36.54
36.64
36.78
35.05
36.98
36.81
36.60
38.14
38.04
38.14
31.27
31.45
31.28
32.21
31.55
33.98
33.96
28.75
28.96
28.01
37.56
37.49
37.48
38.00
37.79
36.86
36.89
37.59
37.45
36.58
Average
16.71
36.69
31.81
37.46
Table 1. Data of interferometric measurement
Conclusions
The presented process allowed the fabrication of AlN driven cantilevers that have good piezoelectrical behavior. CrNi proved
to be a better choice as bottom electrode, since the AlN deposited on it had better <001> orientation and lower stress. The
influence of AlN thickness on its stress level has also been investigated, the obtained results being a valuable information in
designing AlN driven structures with a low global stress. All the physical parameters have been calculated using non
approximated analytical equations. The parameters entering in these relations were determined by the experimental
techniques: nanoindentation and interferometry. The future works will focus on reliability procedure determining long term
stability of parameters and life time of cantilevers.
Acknowledgments
This research was supported by European Network of Excellence in Microoptics (NEMO) and grant no N505 004 31/0670 of
the Polish Ministry of Science and Higher Education.
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