Invited paper

Nano Fabrication and Metrology for Nano ElectroMechanical Sensing Devices
Sunao Ishihara, Shin’ichi Warisawa and Reo Kometani
The University of Tokyo, Tokyo, Japan
Abstract – Nano electromechanical systems (NEMS) work in conjunction with nanomechanical resonators.
Owing to their ultra-small size, nanomechanical resonators have extremely high fundamental resonancefrequencies and quality (Q) factors. When operating in the resonant mode, they show outstanding
performance in sensing and signal processing, realizing ultrafast and ultrasensitive sensors.
In this review, the technological challenges in nanostructure fabrication, metrology, and application tosensing devices are described. After the fundamental properties of nanomechanical resonators are reviewed,
two key technological studies are described: 1) nano-vibration measurement and resonance characterization
using an electron beam (EB) and an atomic force microscope (AFM) probe, and 2) nanofabrication
technologies to make three-dimensional (3D) nanomechanical structures using an EB and a focused ion beam(FIB). Technologies to increase the Q-factor are considered the key to successful fabrication. Finally, thefabrication and demonstration of several sensing devices is described.
Keywords: Nanomechanical resonator, 3D nanofabrication, resonance property characterization, sensing device

1. Introduction
In the modern world, almost every system, piece ofequipment, device, and material has had its structure
size decreased in a bid to improve performance. A
typical example is the Si semiconductor LSI. Driven by
microfabrication technology, circuit patterns have been
continuously miniaturized to the size of several tens ofnanometers. In a simultaneous evolution, microelectromechanical systems (MEMS) composed ofmechanical 3D structures fabricated using suchmicrofabrication technologies have been widely
developed and used for high performance sensors andactuators.
From the late 1990s, several scientific works on
extreme resolution sensing have been reported,
including nano-scale mechanical resonators with quite
high resonant frequency and Q-factor [1], ultra-small
charge detectors with nanomechanical structures [2],and femtometer resolution displacement measurements[3]. These studies demonstrated that the excellent
mechanical properties of small-size resonators could bequite useful for ultra-high sensitivity sensing devices.
This has resulted in the development of nanoelectromechanical systems (NEMS), and the general
micro/nano-systems called MEMS/NEMS.
This paper reviews recent NEMS research activities
focusing on nanostructure fabrication, nano-vibration
measurement, and mechanical property characterizationand how they can be applied to sensing devices.
2. Dynamic property of nanoresonators [4]NEMS have excellent potential to realize ultimate
sensing or actuation performances. As shown in Fig. 1,
nanomechanical resonators made of semiconductors,ceramics, or nano-carbon materials have a size of tens
to hundreds of nanometers [5–9]. They are mostly
operated in resonant modes and perform extremely highfundamental resonance frequencies due to their small
mass and significantly high Q-factors in the rangeof 103–105. These attributes are technically suitable forapplications to high-sensitivity sensors, fast actuators,
signal processing, and so on.

Figure 2 shows the basic configuration of ananomechanical resonator sensing device. Whena disturbances affects the resonating vibrator, theresonant characteristics (such as peak frequency) willchange. In this case, a “disturbance” is considered an“input signal”, and the device works as a sensor todetect displacements, masses, forces, flowing charges,
pressure, temperature, and so on by monitoring changesof the resonance characteristics. Typical characteristics
useful for sensing are resonance frequency f0, vibration
amplitude A0, and Q-factor f0 /ԥ f. The Q-factor is a
performance index indicating resonance sharpness
determined by various energy losses in a resonancesystem. It is the dominating factor for achieving highsensitivity.

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

9Invited paper

The resonance features of nanomechanical resonators
described
above
show
that
theessential
research tasks to realize fast, highly sensitive sensors
using nanomechanical resonators are 1) to measurenano-scale vibrations and characterize their features, 2)
to develop 3D nanostructure fabrication technology, and3) to design and fabricate highly sensitive sensingdevices.
3. Nano-scale vibration measurement
Laser interferometry is typically the most widelyused technique for vibration measurement. However, inthe case of measuring the behavior of NEMS
components, optical methods have difficulty probing
very small samples, mainly because of the opticaldiffraction limit. When the dimension of the vibrating
elements is much smaller than the size of the laser spot,
an interferometer can barely catch or distinguishbetween the reflected lights from the samples. Several
nano-scale vibration measurement techniques using
electron beams or atomic force microscopy havetherefore been investigated.
An electron beam (EB) is suitable for probing verysmall structures due to its own small spot size, which isjust a few nanometers. The schematic of a vibration
measurement system using EB is shown in Fig. 3 [10].
When an EB is irradiated to a vibrating structure acrossthe beam spot, secondary electrons (SEs) emitted fromthe resonator surface are modulated to an on-off signalby the vibration, and detected by the SE detector. The
network analyzer then obtains the resonance10characteristics in the frequency domain by sweeping theactuation frequency and analyzing the modulated
signals [11].
If a different irradiation direction is adopted, themeasurement performance can be considerably
improved. For example, when the EB is irradiatedparallel to the vibrating plane with the appropriate
inclination, the detection signal is also modulated by theSE emission dependency on the tilting angle of theresonator. These EB measurement methods can detect
sub-nanometer amplitude at frequencies of up to several
MHz. Moreover, in the latter case, very accurate
resonance frequency identification and vibration
amplitude evaluation can also be performed [10, 12].
In order to evaluate the dynamic characteristics of anano-resonator, a vibration measuring technique using
an atomic force microscope (AFM) was developed. As
shown in Fig. 4, a GaAs membrane resonator isactuated with Coulomb force generated by AC voltage
applied between the resonator and the probe.
Simultaneously, the vibration amplitude is detected asheight data of the AFM operating in the dynamic mode,
resulting in the detection of the envelope of resonance.
By sweeping the actuation frequency, the AFM output
draws the frequency characteristics, as shown in thefigure [13]. An added advantage of this method is thatthe resonance profile can be measured by using AFM
scanning.

4. 3-D nanostructure fabrication
Key materials in a nanomechanical resonator aresemiconductors (Si, SiN, SiC, GaAs, or GaN) andcarbons (DLC, CNT, or grapheme). When fabricating
nanomechanical resonators, conventional micro/nanofabrication technologies for making semiconductor
LSIs/MEMS that improve their resolution capability are
typically used. Recently, bottom-up technologies basedon self-assembly, crystal growth, and moleculardeposition are also frequently being used. One of thevery useful technologies for fabricating 3D
nanostructures that we use in our own NEMS research
is focused ion beam chemical vapor deposition(FIB-CVD).
As described above, the most attractive properties ofnanomechanical resonators are their high resonance

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

Invited paper

frequency and high Q-factor. From the fabrication
viewpoint, however, the Q-factor tends to decrease as
the mass of the resonator decreases, as shown in Fig. 5[14]. A key issue is therefore maintaining or evenincreasing the Q-factor when fabricating small size
resonators.
One effective method of increasing the Q-factor isapplying tensile stress/strain along the resonating
structure. A typical example is shown in Fig. 6. The
doubly clumped beam is made of a tensile
strain-induced heterostructure fabricated with latticemismatched heteroepitaxy. In this case, the Q-factor
was increased by more than one degree of magnitudewith 0.35% strain induce [15].

We examined a new lithography technique for 3D
nanostructure fabrication to clarify its excellent
lithography performance. As shown in Fig. 7, anultra-thin strained carbon beam resonator can be
fabricated using modified lithography and annealing
processes. This new technique, which is called FIB/EB
dual-beam lithography, utilizes the different penetration
depths of the resist materials between ion and electron
beams. In this process, a small penetration Ga+ ion
exposes the vibration beam and a deep penetration EB
exposes the supporting base. A beam resonator is theninstantly developed, followed by curing and annealing
to achieve a carbonization and hardening of the resist
material. Tensile stress is simultaneously induced into
the beam owing to the shrinkage of the supporting base[16, 17].

Recently, carbon nanotubes (CNTs) and graphene
have been widely applied to nanoresonators due to theiroutstanding mechanical properties [8, 9, 18]. We studied
a graphene resonator to determine how to best make useof its extremely stiff and ultra-thin structure. In thisstudy, the shrinkage property of the resist material isused to fabricate strained graphene resonators with ahigh Q-factor. As shown in Fig. 8, graphene resonatorswith several layers were fabricated on the trench of an
SU-8 resist. The resonators were then clamped withdiamond-like carbon (DLC) deposited by FIB-CVD andtrimmed by FIB etching. Annealing was used to apply
tensile strain to the graphene resonator by SU-8 drastic
shrinkage. As shown in the figure, the Q-factor
increased after annealing to over 7,000 at roomtemperature [19].

5. Sensing device application
We made full use of the technologies for measuring
and fabricating nanostructures mentioned in theprevious section to fabricate sensing devices withmicro/nanomechanical resonators. Here, two device
making trials are described.
Figure 9 shows a high-sensitivity charge detection
device using antisymmetric vibration in two coupled
GaAs resonators. In this coupled beam resonator, theantisymmetric mode under in-phase simultaneous
driving of the two beams disappears with perfectfrequency tuning. The piezoelectric stress induced by asmall gate-voltage modulation breaks the balance of thetwo resonators, leading to the re-emergence of theantisymmetric mode. Measurement of the amplitude

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

11

Invited paper

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06GJ01[1]change enables detection of the applied voltage or,
equivalently, added charges. In contrast to thefrequency-shift detection using a single resonator, thismethod enables a large readout up to the strongly driven
nonlinear response regime, providing a high RT
sensitivity of 147 e/Hz0.5 [20].

Another trial is a highly sensitive displacement
detection device using a GaAs-based electromechanicalresonator integrated with high electron mobility
transistor (HEMT). As shown in Fig. 12, piezoelectric
voltage generated by the vibration of the resonator isapplied to the gate of the HEMT, resulting in the onchip amplification of the signal voltage. This device
achieved a displacement sensitivity of 9pm/Hz0.5, which
is one of the highest displacement detection schemes atroom temperature [21].
6. Summary
The remarkable dynamic features of nanomechanicalresonators make them promising candidates for the
fabrication of extremely sensitive NEMS sensing
devices. Extensive nanotechnology research on
fabrication and metrology technologies for highQ-factor nanoresonators are currently being performed
to enable the design and creation of such devices.
Moreover,severalsensingdevicesusing
micro/nanomechanical resonators have been made anddemonstrate
outstanding
sensitivity
atroomtemperature.
We expect there to be extensive further research
conducted in this area.
㻌12ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.