MEMS sensors are currently undergoing a phase of exciting technological development,
not only enabling advancements in traditional applications such as accelerometers
and gyroscopes, but also in emerging applications such as microfluidics, thermoelectromechanical,
and harsh environment sensors. While traditional MEMS sensors have found wide applications
in motion sensing, navigation, and robotics, emerging MEMS sensors are likely to open
up applications in the rapidly expanding fields of wearables, internet-of-things,
point-of-care detection, and harsh environment monitoring. Novel applications, enabled
by advancements in system miniaturization, design innovation and cutting edge fabrication
techniques promise an exciting era for MEMS-based sensors and systems development.
However, to fully realize their potential several challenges still need to be overcome.
Among these challenges, long-term sensor reliability and performance parameter modeling
for expedient and robust designs are significant. Additionally, there are issues of
cost and power consumption, especially for mass applications requiring small size
and weight.
There are 17 papers published in this Special Issue focusing on a wide range of MEMS
sensor applications and fabrication methodologies. Almost a third of the papers, [1,2,3,4,5],
and [6], present various accelerometer and gyroscope designs and their performance
evaluation. Three of the papers, [7,8], and [9], explore novel fabrication methodologies
for MEMS devices. The remaining papers cover various novel MEMS sensors and actuators
focusing on inertial micro-switch [10], micro hot plates [11], near IR spectrometry
[12], magnetic microactuator [13], resonant microfluidic chip [14], high temperature
pressure sensors [15], thermoelectric power sensors [16], and a review of the photonic
crystal nanobeams for sensing [17].
In particular, Yang et al. [1] proposed a z-axis magnetoresistive accelerometer with
electrostatic force feedback in a three-layer design, taking advantage of the change
in a magnetic field caused by input acceleration, which is measured by a pair of magnetoresistive
sensors at the top layer. They achieved a good sensitivity of 8.85 mV/g for a plate
gap of 1 mm. Qin et al. investigated the effect of anisotropy in single-crystal silicon
vibrating ring gyroscope, and found out that the frequency split is much more for
the [100] direction compared to [111] direction for n = 2 mode, concluding that fabrication
in the latter direction is preferable [2]. Liu et al. presented an ASIC-based design
process for a monolithic CMOS MEMS accelerometer [3]. They also presented a low-noise
and low zero-g offset design of MEMS accelerometer using a low noise chopper circuit
and telescopic architecture, which significantly reduced noise and zero-g offset,
but increased the power requirement [4]. Jia et al. addressed an important problem
of frequency mismatch in MEMS gyroscopes, and presented an approach for reducing it
by designing a dual-mass gyroscope that utilizes a quadrature modulation signal [5].
A maximum frequency mismatch of less than 0.3 Hz was demonstrated using their design.
Fang et al. proposed a novel adaptive control algorithm incorporating a back-stepping
technique to compensate for model uncertainties, disturbances, and unknown parameters
in micro-gyroscopes, which are very pertinent issues in their performance optimization
[6].
On the fabrication techniques for the MEMS sensors, Smiljanić et al. reported on the
deep wet etching of Si substrate in various crystallographic directions and performed
theoretical modeling of the etch profiles, which agreed well with the experimental
results [7]. Wu et al. presented an innovative fabrication method for a catalytic
gas sensor based on a Pt coil addressing the non-uniformity of pellistor material
at the inside surface of the coil [8]. Using a droplet-based coating methodology they
demonstrated uniformly coated and reliable pellistor sensors. Kim et al. presented
a femtosecond laser-based micro-welding technique for bonding glass and fabricate
reliable microfluidic channels. They compared the microfluidic channels fabricated
using this method with those fabricated using a glue-based technique, highlighting
their relative ease of fabrication and reliability [9].
On the new device applications side, Peng et al. presented an inertial microswitch
with a very low threshold of 5 g and high threshold accuracy, leveraging squeeze film
damping [10]. Liu et al. presented novel designs of micro hot-plates with significantly
improved temperature non-uniformity [11]. Huang et al. reported on a novel MEMS-based
infrared spectrometer operating in the range of 800–1800 nm with a wavelength resolution
of 10 nm, which compared favorably with similar commercial systems [12]. Feng et al.
designed, simulated and fabricated a linear magnetic microactuator with bistable behavior
with less than 1 ms response time [13]. An LC resonant circuit-based sensor for detecting
metallic debris in hydraulic fuel is proposed by Yu et al., where they were able to
successfully demonstrate selective detection of iron and copper particles with diameters
down to tens of microns [14]. Gajula et al. designed a GaN circular membrane-based
pressure sensor capable of operating at high temperatures. The pressure sensors exhibited
high sensitivity at temperatures in excess of 200 °C, which is a significant improvement
over their Si counterparts [15]. Zhang et al. presented a MEMS-based thermoelectric
power sensor for measuring microwave power using a floating slug design to minimize
microwave power loss. The sensor was implemented with GaAs MMIC technology and exhibited
very good sensitivity up to 25 GHz [16]. Finally, Qiao et al. presented a comprehensive
review of photonic crystal nanobeam-based sensors providing a ready reference for
researchers interested in this area. They specifically focused on the sensing of refractive
index changes, nanoparticle sensing, optomechanical sensing, and temperature sensing
[17].
I would like to take this opportunity to thank all the authors for submitting their
papers to this Special Issue. I would also like to thank all the reviewers for dedicating
their time and helping to improve the quality of the submitted papers.