Large-Scale Integration of Nanoelectromechanical Systems for
Gas Sensing Applications
I. Bargatin
†,‡,§
,
E. B. Myers
†
,
J. S. Aldridge
†,
∥
,
C. Marcoux
‡
,
P. Brianceau
‡
,
L. Duraffourg
‡
,
E. Colinet
‡
,
S. Hentz
‡
,
P. Andreucci
‡
, and
M. L. Roukes
†
†
Kavli Nanoscience Institute and Department of Physics, Caltech, Pasadena, CA
‡
CEA/LETI - MINATEC, Grenoble, France
Abstract
We have developed arrays of nanomechanical systems (NEMS) by large-scale integration,
comprising thousands of individual nanoresonators with densities of up to 6 million NEMS per
square centimeter. The individual NEMS devices are electrically coupled using a combined series-
parallel configuration that is extremely robust with respect to lithographical defects and
mechanical or electrostatic-discharge damage. Given the large number of connected
nanoresonators, the arrays are able to handle extremely high input powers (>1 W per array,
corresponding to <1 mW per nanoresonator) without excessive heating or deterioration of
resonance response. We demonstrate the utility of integrated NEMS arrays as high-performance
chemical vapor sensors, detecting a part-per-billion concentration of a chemical warfare simulant
within only a 2 s exposure period.
Keywords
NEMS arrays; gas chromatography; gas detectors; mass sensing
In the last several years, individual nanoelectromechanical resonators have been used to
establish record sensitivities in force,
1
position,
2
mass,
3,4
and gas concentration.
5
The
© 2012 American Chemical Society
§
Present Addresse
: Department of Electrical Engineering, Stanford University, Stanford, CA.
Present Addresse
: Integrated Photonics Technology, Inc., Carlsbad, CA.
Supporting Information
A description of the fabrication procedure and measurement circuit, as well as a more detailed analysis of series-parallel piezoresistive
NEMS detection, is provided. This material is available free of charge via the Internet at
http://pubs.acs.org
.
NIH Public Access
Author Manuscript
Nano Lett
. Author manuscript; available in PMC 2013 November 25.
Published in final edited form as:
Nano Lett
. 2012 March 14; 12(3): . doi:10.1021/nl2037479.
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
miniscule size of nanomechanical systems (NEMS) sensors clearly gives them
unprecedented sensitivity to external perturbations, but this sometimes comes at a cost. For
example, the power these devices can use and the magnitude of signal they can produce both
decrease at smaller sizes. Moreover for gas sensors, the interaction cross-section with
particular analytes in a gas or liquid environment can rapidly decrease as the active
mechanical element becomes smaller, whether due to increased analyte diffusion time,
interaction with nonactive sensor regions, or noisy, stochastic absorption/desorption of trace
analyte levels.
6
In this limit of “needle in a haystack” detection, individual NEMS may have
difficulty capturing even a single molecule of the analyte. Such challenges can make it
difficult to exploit the full potential of individual NEMS sensors in the next generation of
real-world microanalytical tools.
It is therefore critical to scale up the interaction cross-section of NEMS sensors while still
maintaining, or even enhancing, their extraordinary sensitivities and useful attributes. A
straightforward and commonly used approach to this task is simply to combine individual
devices into arrays. For chemical sensors, different devices within the array can serve as
sensors of different chemical compounds. Such arrays have previously been fabricated from
microscale cantilever resonators,
7
microscale membrane resonators,
8
nanoscale cantilevers,
9
nanoscale doubly clamped beam resonators,
10
and nanowire resonators.
11
Alternatively, one
can use the collective response of multiple elements of the array to enhance the signal-to-
noise ratio or other properties. For example, by engineering the mechanical coupling
between individual resonators, one can produce a collective mode of oscillation that inherits
the positive characteristics of individual resonators, such as high frequency and quality
factor, but is able to handle more power.
12
Such collective modes can then be further
optimized to produce the desired overall response, for example, that of a bandpass filter.
13
In this paper, we report the first application of large-scale-integrated (LSI) fabrication
techniques to NEMS array fabrication, which has allowed us to utilize the collective
response of thousands of NEMS resonators to enable new paradigms in NEMS-based
sensing. For our first realization of a LSI-NEMS sensor, we take an approach that requires a
highly uniform array of nearly identical submicrometer-scale elements over a much larger
(millimeter-scale) area. Our design relies on the essentially coherent response of thousands
of independent resonators, connected electrically in a manner that provides natural noise
averaging, increased collective power handling capability, and fault-tolerant robustness.
LSI-NEMS arrays, while dramatically increasing the interaction cross-section of individual
NEMS resonators, furthermore provide a potential route to orders-of-magnitude sensitivity
improvements over individual resonator elements. As a proof-of-design, we demonstrate
parts-per-billion sensing of the chemical warfare agent simulant diisomethylphosphonate
(DIMP) within a 2 s exposure window using a functionalized LSI-NEMS.
Our NEMS arrays were fabricated from CMOS-compatible materials using state-of-the-art
microelectronic lithography and etching techniques. The devices were fabricated on 200 mm
SOI wafers with a 160 nm-thick silicon layer and a 400 nm thick buried oxide layer. A 70
nm-thick film of aluminum silicide
14
was sputtered on top of the SOI wafer and patterned
using 248 nm deep-UV lithography. Anisotropic etching of the thin metal film was then
used to define the self-sensing piezoresistive elements of the NEMS array itself as well as
the lead wires and the wirebonding pads.
5
This patterned metallization layer also served as a
mask for the anisotropic etching of the structural silicon layer down to the buried oxide.
Finally, the NEMS cantilevers were released using a carefully timed vapor HF etch of the
buried oxide. The details of the fabrication procedure are described in the Supporting
Information.
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Figure 1 shows a completed 200 mm wafer of LSI-NEMS arrays, as well as a representative
array and an individual cantilever element. Arrays were fabricated with different individual
NEMS element dimensions across the wafer with the lengths varying between 1.6 and 5
m,
and the widths varying between 800 nm and 1.2
m. A typical array contained 20 rows and
140 columns with a 6.5
m linear pitch for a total of 2800 array elements. The largest arrays
employed in the present work contained 6800 NEMS with an integration density of nearly 6
million NEMS per square cm.
A straightforward approach to harnessing the power of many individual NEMS in large-area
arrays is to connect them electrically in a combined series-parallel configuration, shown in
the inset of Figure 1c. A single-port (signal and ground) connection to the array can then be
made through electrodes on opposite edges of the array. The individual NEMS were excited
into vibration using electrothermally generated strain,
15
which allowed us to actuate each
array element with essentially identical driving phases. This phase coherence is critical for
harnessing the collective benefits of the array geometry (see Supporting Information) and
cannot be easily achieved with off-chip actuation methods. For example, piezoshaker
actuation suffered from large phase lags due to propagation and interference of acoustic
waves across the full extent of the millimeter-scale array. Since electric signals propagate at
close to speed of light, rather than speed of sound, purely electric actuation, such as the
thermoelastic mechanism, provides much more coherent driving signal to all array elements.
The resulting motion was detected electrically through the metal surface layer’s
piezoresistive response.
16
Details of the actuation and detection circuitry are in the
Supporting Information. The most immediately apparent benefits of this configuration are
the simplicity of electrical connection and the high degree of robustness with respect to
lithographic defects and mechanical or electrical damage. Arrays of this type can be used as
sensors even if most individual cantilevers prove defective, as long as there remains a
conductive path through the array. In addition, the arrays of the series-parallel connected
cantilevers are highly resistant to electrostatic discharge (ESD) because of their much larger
collective power handling ability.
Another potential benefit of LSI piezoresistive NEMS sensor arrays emerges when we
consider how the measured response of the entire array, that is, the change in overall
resistance, relates to the changes in resistance of the individual array elements. Assuming for
simplicity that all cantilevers have the same at-rest resistance, r, and that the fractional
variations in the resistance of individual cantilevers are small, it is easy to derive that the
relative change in the resistance of the array is the average of the relative change in the
resistance of individual array elements (see Supporting Information for details)
(1)
where
r
ij
is the change of the resistance of the cantilever situated in the ith row and jth
column of the array and N = lm is total number of array elements. The resistance
contribution from each resonator
r
ij
will be a combination of mechanical signal and
aggregate noise (e.g., Johnson noise, phase noise, thermal fluctuation noise).
17
However, if
every element of the NEMS array is identical, that is, if all element signals are at the same
frequency and phase, then signal and noise do not add similarly in eq 1. In essence, the
mechanical signals will sum coherently, while many of the individual resonator noise
contributions add together incoherently. A full analysis (see Supporting Information) shows
that, in the ideal case of identical resonators driven at the same frequency and phase, the
signal-to-noise ratio scales as the square root of the number of array elements. For arrays
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comprised of thousands of elements, this can potentially result in orders of magnitude
improvement in SNR for NEMS arrays vs single NEMS.
In practice, it is of course impossible to realize the ideal LSI NEMS array, since it would
require zero process variations over the large area of the array. However, as we detail in the
Supporting Information, achieving reasonably high (albeit imperfect) frequency uniformity
can still yield enhanced SNR over that of single resonators. The frequency response of such
an array can then be approximately described by a Lorentzian, just like in the case a single
driven harmonic oscillator. However, the effective quality factor, Q
eff
, of the array response
is set by both the individual element quality factor,
Q
, and the width of the resonance
frequency distribution,
f
, of the array. To wit, Q
eff
≈
1/(
Q
−1
+
Q
distr
−1
), where
Q
distr
≈
f
/
f
describes the relative magnitude of resonance frequency dispersion across the array. In our
arrays, the dispersion of resonance frequencies across the array was of the order of 1%,
corresponding to
Q
distr
≈
100.
Since the absolute size of typical lithographic and etching imperfections does not depend on
the dimensions of the cantilever, the relative frequency dispersion decreases (
Q
distr
increases) with increasing cantilever length. However, the quality factor of individual
cantilevers in air,
Q
air
, generally increases with resonance frequency and therefore decreases
with increasing cantilever length.
18
As a result, there is some optimal cantilever length, for
which the
Q
distr
≈
Q
air
, and Q
eff
≈
Q
distr
/2
≈
Q
air
/2. While we have studied arrays of
various dimensions, we obtained optimal results from arrays made out of 2
m long, 800 nm
wide cantilevers, whose individual resonance frequencies were approximately 24 MHz. The
quality factors of such individual cantilevers in vacuum and air were on the orders of 1000
and 100, respectively. In vacuum, the overall observed array response was thus largely
determined by the frequency dispersion,
Q
eff,vac
≈
Q
distr
≈
100; whereas in air the trade-off
was close to optimal:
Q
air
≈
Q
distr
≈
100 and
Q
eff,air
≈
(1/
Q
air
+1/
Q
distr
)
−1
≈
50.
The motion of the array cantilevers was actuated thermoelastically and detected
piezoresistively using the two-port downmixing measurement scheme described in the
Supporting Information. In the two-port scheme, a single metal loop on a resonator is used
for both thermoelastic actuation and piezoresistive detection. This measurement scheme
worked in the same way for arrays as for individual cantilevers, the only difference being
the need to supply more RF power. To further maximize the resonance signal visibility, we
measured two arrays at the same time using the balanced differential scheme.
19
In the balanced measurement technique, the two arrays must have different center
frequencies but similar overall resistances. In that case, the coherent backgrounds generated
in each of the arrays cancel each other, but the resonance responses do not because the
resonances occur at different frequencies for the two arrays. Figure 2a shows a typical
resonance response measured in vacuum using two arrays with cantilevers of nominal
lengths of 2.0 and 2.1
m. The graph features two resonance peaks with the lower-frequency
one corresponding to the array with longer cantilevers and vice versa.
Note that the vacuum response curves featured many sharp features that are caused by the
individual resonances of cantilevers whose frequency lied outside the majority of resonance
peaks of the array. These features were reproducible and larger than the amplitude of
measurement noise by approximately 2 orders of magnitude. According to theory, the
individual resonances should be smaller than the overall peak by a factor of
N
× Q
eff
/
Q
≈
2800(100/1000) = 280. However, we found the sharp features to be approximately 100 times
smaller than the overall peak, suggesting that they are not individual resonances but rather
superpositions of several resonances. In air, these variations were smoothed out, as shown in
Figure 2b.
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To further study the dispersion of frequencies in the arrays, we have also done
measurements using thermoelastic actuation and optical detection in vacuum. The optical
detection setup was a simple reflection interferometer with a spot size of approximately 10
m, previously used in ref 20. The results are shown in Figure 3, where we plot the
interferometer signal of an array of 2.8
m long, 1.2
m wide cantilevers versus excitation
frequency and the position of the beam spot. The position of the beam spot was stepped
every 5
m across the width of an array (see Figure 3b).
The small size of the optical spot allowed us to detect only about a dozen cantilevers within
the beam spot instead of the entire array of 2800 cantilevers. The majority of individual
resonances were situated near the central frequency of 18.6 MHz, and these resonances
formed the main peak of the array response. Similar to electrical data on other arrays,
however, there were also a number of “outliers”, especially at frequencies above the central
peak. Some of these resonances were sufficiently well resolved to be fitted individually. The
quality factor of such individual resonances in vacuum was approximately 1100.
One of the main advantages of NEMS arrays over individual devices is the much improved
power matching and collective power handling capability. Since the total resistance of the
array consisting of
l
rows and
m
columns is given by (
mr
)/
l
, it is possible to produce
impedance-matched arrays from a wide range of individual resonators simply by changing
the row and column count. The individual cantilevers that we used to build arrays had
typical resistances of approximately 7 Ohms, so that 20 × 140 arrays we typically used had
total resistances of 7
× 140/20
≈
50
, which were optimally matched to commercial
power electronics.
Apart from easy power matching, NEMS arrays have dramatically increased collective
power handling capabilities compared to individual devices. As a rule of thumb, arrays can
handle powers that are larger by a factor of
N
, the total number of array elements. While
individual devices of the dimensions that we used typically had a maximum power handling
ability of less than one milliwatt, our arrays could easily handle maximum powers of a few
watts. For example, the collective RF power applied to the two arrays in the balanced
measurement scheme was 900 mW per array for the data shown in Figure 2b. Applying even
higher RF power of approximately 2 W per array produced significant frequency drift but no
permanent damage to the arrays. This suggests that even higher power levels can be used
with improved heat sinking at the die level.
The primary goal of developing the cantilever arrays described in this work was to create
sensors that are more robust, easier to use, and potentially more sensitive than individual
devices. One sensor application where arrays can offer large improvements is gas sensing
and, in particular, NEMS-based gas chromatography.
9
In this application, the sensor arrays
need to be integrated with the gas delivery system. In prior work from our group, this was
accomplished by encapsulating the nanomechanical sensor in a microfluidic flow cell that
could then be directly connected to an external gas delivery system, such as a commercial
gas chromatography (GC) system.
9
This way, the nanoscale resonator is only exposed to the
analytes and carrier gas and remains isolated from the air of the environment. In addition,
the volume of the cell and, therefore, the diffusion time, which often limits the speed of gas
chromatography analysis, can be minimized with proper design.
9
In this work, we did not use a microfluidic cell but instead operated the arrays in ambient air
and used a micropositioner to move the end of the 90 cm long column of the gas
chromatography system approximately 100–200
m above the sensor array (Figure 4a).
While not appropriate for industrial applications, this simple configuration allowed rapid
device testing and did not result in increased diffusion time compared to the microfluidic
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cell experiments.
11
To demonstrate the possibility of using arrays for detection of specific
analytes, we coated the arrays with the polymer DKAP, a silicone copolymer developed at
Sandia National Laboratory for detection of phosphonate gas molecules—precursors and
simulants of nerve gas agents.
21,22
A droplet of DKAP solution was put on the array chip
surface and left to dry in air, leaving a thin (10–20 nm) film of polymer on the array
cantilevers.
Functionalization of the array did not have a measurable effect on the frequency or the
effective quality factor of the array response. However, the quality factor was noticeably
affected by the flow of the hydrogen carrier gas through the column. When the GC system
was in operation, the hydrogen carrier gas was forced out of the bottom end of the column at
the rate of 1–2 sccm (standard cubic centimeters per minute) and largely displaced the air in
the immediate vicinity of the array (see inset of Figure 4a). Because the viscosity of
hydrogen is lower than that of air, this increased the quality factor of individual resonances,
Q, and therefore the effective quality factor of the entire array in accordance with the
formula Q
eff
= 1/(
Q
−1
+
Q
distr
−1
). In practice, the effective quality factor typically increased
from approximately 50 to approximately 60.
In order to test the gas sensing functionality of the arrays, we have performed open-loop
measurements of frequency shift of the array sensor after injecting solutions of diisopropyl
methylphosphonate (DIMP), a nerve gas simulant, in CS
2
solvent through a GC column as
described above. The open-loop frequency measurements of frequency shift were carried out
by monitoring the dispersive quadrature of the Lorentzian response (see, for example, curve
Y in Figure 2b). If the drive and bias frequencies remain constant and the central peak
frequency of the arrays
R
changes by
R
due to an absorbed mass
m, the dispersive
components of the resonance voltage signal will change by
(2)
where
A
arr
is the voltage amplitude of the array resonance peak, and
m
c
is the mass of the
cantilever. Therefore, as long as the total frequency shift is smaller than the resonance width,
we can easily infer the changes in the resonance frequency from the measured changes in the
dispersive quadrature of the signal.
Figure 4b shows the resulting chromatograms for a wide range of DIMP mass injections.
The downward shift in the frequency response of the array was caused by the uptake of
DIMP molecules by the DKAP polymer that covered the cantilevers as the chemical was
eluted from the open end of the GC column. The total observed frequency shift did not
exceed 0.4% << 1/Q
eff
≈
2%, ensuring a linear relationship between the measured
dispersive component and the loading mass.
The resulting average concentrations c in the eluted peaks are calculated as
9
c
= (
c
l
V
l
V
m
S
R
)/
(MW
F
t
), where
c
l
is the mass density of DIMP in the liquid sample,
V
l
is the liquid
volume of sample injected into the column,
V
m
= 22.4 L/mol is the molar volume of an ideal
gas at ambient temperature and pressure,
S
R
is the injection split ratio, MW is the molecular
weight of the analyte,
t
is the peak width in time, and
F
is the column flow rate.
At large concentrations (above 1 ppm), the frequency shift peak area did not follow a linear
relationship with the concentration of DIMP due to saturation of the polymer film. The
response was more linear at smaller concentrations, with both the shape and the delay of the
peak independent of the DIMP concentration (see inset to Figure 4b). The data demonstrate
a minimum detectable concentration of approximately 1.2 ppb in a 1 Hz bandwidth, which is
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roughly optimal for detecting short-column pulses a few seconds in length. This limit does
not surpass the minimum equilibrium sensitivity that was demonstrated in our group for
individual NEMS cantilevers using long columns and averaging times,
9
but it improves
upon the short-term sensitivity of individual devices in that work, which were obtained
using similar high-speed GC measurements with pulse lengths of only a few seconds. We
are currently verifying whether this sensitivity improvement is fully explained by the array’s
improved signal-to-noise ratio and active surface area, or is also due to the different
measurement geometries.
We conclude by noting some of the further possibilities enabled by developing NEMS
technology compatible with LSI fabrication techniques. For example, our process for
fabricating NEMS arrays is completely amenable to integration with the CMOS electronics
needed to drive and detect the resonances of NEMS devices. In the future, such integration
will allow us to fabricate arrays of independently operating nanomechanical oscillators (as
opposed to resonators). Such oscillators will be needed in extremely low analyte
concentration regimes (parts-per-quadrillion and below), where the number of interacting
molecules per individual NEMS approach one and collective averaging no longer improves
the SNR. Such oscillators should also prove useful for NEMS-based mass spectroscopy,
3
as
well as a new test bed for studying nonlinearly interacting oscillators. Furthermore, it may
be possible to incorporate low-power actuation and detection techniques into arrays. In
particular, piezoelectric detection and actuation should allow more efficient transduction of
RF power into mechanical motion of NEMS arrays. Piezoelectric detection on the array
level would also avoid the problem of signal shorting by parasitic capacitances, which
generally prevents the use of piezoelectric detection in individual NEMS devices. The use of
piezoelectric actuation would also dramatically reduce the total power dissipation of NEMS
arrays without reducing their power handling capability, thereby improving the overall
power efficiency of NEMS array sensors.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
The authors thank R. Karabalin for help with optical measurements. We gratefully acknowledge support from the
Defense Advanced Research Projects Agency via DARPA/MTO-MGA Grant NBCH1050001, as well as support
from the Institut Carnot via the Carnot-NEMS project.
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Figure 1.
(a) Photograph of a full 200 mm wafer with patterned NEMS arrays. (b) Zoomed-in
photograph of one 20 mm wafer die containing a variety of nanofabricated resonator array
structures. (c) Scanning electron micrograph of a section of a cantilever array. Inset:
Schematic of a combined series-parallel electrical connection of array elements. (d)
Scanning electron micrograph (oblique view) of an individual array component.
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Figure 2.
(a) Resonant response of the two arrays in vacuum. Both quadratures of the lock-in
response, X and Y, are shown. The inset shows a zoomed-in version of a part of the
measured response curve. Note that the noise level in these measurements was more than an
order of magnitude smaller than the amplitude of the rapid variations shown in the inset. (b)
Same for measurements in air. Note that the amplitudes of the resonance peaks were much
larger than in (a) because much more RF power was applied to drive and detect the
resonances in air.
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Figure 3.
Optically detected spectrum of cantilevers in a representative array for different positions of
the laser spot (spot size approximately 10
m). (b) Top-view schematic of a 140 × 20 array
of cantilevers. Individual cantilevers are not visible in this image. The dotted red line
schematically shows the positions of the laser spot used to acquire the spectra.
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Figure 4.
(a) Integration of the array sensors into a commercial Hewlett-Packard 5890 gas
chromatography system. The photograph shows the inner chamber of the GC system with
the injector, column, and column heating wires as well as the printed circuit board with the
sensor arrays and a micropositioner. The inset shows schematically how the lower end of the
column is positioned above the array and how the effluent flows from the column. (b) Gas
chromatogram of DIMP obtained with an array sensor in the bridge configuration with 10 W
resistive heating of the 90 cm long column. The inset shows zoomed-in versions of the
chromatograms for the lowest DIMP concentrations. The averaging time was 150 ms in
these measurements, corresponding to a bandwidth of 1/(2
× 0.15 s)
≈
1 Hz. The rms
amplitude of noise corresponds to a frequency shift of 3 ppm and therefore a concentration
sensitivity of approximately 1.2 ppb. The total RF power used in these measurements was
approximately 360 mW per array.
Bargatin et al.
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