On-chip Sensor Research Articles

Theoretical study of microcavity-enhanced absorption spectroscopy for mid-infrared methane detection using a chalcogenide/silica-on-fluoride horizontal slot-waveguide racetrack resonator.

The reported chalcogenide (ChG) rectangular waveguide sensors with a small evanescent field need a large waveguide length to obtain an enhanced light-gas interaction effect. To make such sensors compact and improve the light-gas interaction effect, a microcavity-enhanced absorption spectroscopy technique for methane (CH4) detection was proposed using a mid-infrared chalcogenide/silica-on-fluoride horizontal slot-waveguide racetrack resonator. For the horizontal slot waveguide, an equivalent sensor model (ESM) and related formulations were proposed to simplify the analysis of the racetrack resonator sensor model (RRSM), and the ESM was verified through a comparison between the theoretical result of ESM and the simulation result of RRSM based on the finite element method (FEM). Due to the use of a chalcogenide/silica-on-fluoride horizontal slot-waveguide structure, the waveguide parameters were optimized to obtain a high power confinement factor of 44.63% at the wavelength of 3291 nm, which is at least 5 times higher than other ChG rectangular waveguides. The waveguide length is reduced at least 30 times due to the use of the optimized chalcogenide/silica-on-fluoride horizontal slot-waveguide and racetrack resonator. The limit of detection (LoD) is 3.87 ppm with an intrinsic waveguide loss of 3 dB/cm and an amplitude coupling ratio of 0.1 for the resonator. The response time is less than 5 µs due to the small light-gas interaction area. The influences of environmental pressure and waveguide intrinsic loss on the sensing characteristics were discussed. The compact racetrack resonator sensor structure and equivalent analytical model can also be adopted in the design of an on-chip waveguide sensor for the detection of other gas species.

Automatic Detection of Intestinal Bleeding using an Optical Sensor for Wireless Capsule Endoscopy.

Wireless capsule endoscopy (WCE) has been an effective and safe way to diagnose gastrointestinal (GI) disorders, such as, colon cancers, polyps and bleeding. The detection of bleeding and other anomalies is currently determined through conventional visual inspection of the WCE images by the physicians. An on-chip bleeding sensor is thus required, that can perform an automatic prescreening of the bleeding areas in real-time using blood's optical properties to assist the diagnosis. In this study, a spectrophotometer was initially used to evaluate the chromatic properties of blood. It is found that the reflection ratio pairs of 700 nm to 630 nm and 480 nm to 530 nm provide important statistics to separate blood from non-blood samples. It has been implemented hardware using small LEDs and photodiodes to validate the results. Therefore, the proposed sensor system works as a good candidate to be integrated in a WCE device to detect GI bleeding quickly and in real-time.

Parallelized Wireless Sensing System for Continuous Monitoring of Microtissue Spheroids.

Currently, the use of electrical readout methods for the investigation of microtissue spheroids in combination with lab automation tools is hindered by the cable connections that are required to interrogate the on-chip-integrated electrodes. To overcome this limitation, we developed a wireless sensor scheme, which can detect the size variation of microtissues during long-term culturing and drug exposure assays. The sensor system includes an interrogation board, which is composed of an inductor-capacitor (LC) readout circuit, and the tissue culture platform with integrated split-ring sensors. The magnetic coupling between the LC circuit and the sensors enables the interrogation of the on-chip sensors without any wire connection to the culture platform. By optimizing the sensor dimensions and the LC resonance frequencies, we were able to avoid cross talk between neighboring sensors. We integrated 12 tissue compartments on a standard microscopy slide with a sensor-to-sensor pitch of 9 mm, which is in accordance with standard 96-well plate dimensions. As a proof-of-concept experiment for the developed system, we monitored continuously and during more than four days the growth inhibition of colon cancer microtissue spheroids that had been exposed to different concentrations of doxorubicin, a chemotherapeutic compound. The stability of the measurements during long-term culturing and the compatibility of the sensor scheme with standard lab equipment offer great potential for automated electrical microtissue spheroid characterization.

A 137-μW 1.78-mm2 30-Frames/s Real-Time Gesture Recognition SoC for Smart Devices

Gesture recognition has increasingly become one of the most popular human-machine interaction techniques for smart devices. Existing gesture recognition systems suffer from either excessive power consumption or large size, limiting their applications for ultralow-power wearable devices. This article presents an accurate area-efficient and low-power real-time gesture recognition system for smart wearable devices. The proposed work utilizes an accurate peak-based gesture classification engine with less memory and a low-resolution and low-power on-chip image sensor for achieving high area efficiency and low power. In addition, the feature extraction architecture removes fixed-pattern noises from the low-power on-chip image sensor for accuracy improvement and employs parallelism for recognition speed enhancement. Thus, the proposed system accomplishes accurate real-time gesture recognition for eight motion hand gestures with an average recognition accuracy of 90.6% and latency of 4.228 ms. Measurement results of a test chip fabricated in 65-nm CMOS demonstrate that the proposed system consumes 137.0 μW at 30 frames/s while occupying only 1.78 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which achieves the lowest power and smallest area among the recently reported gesture recognition systems.

A High Dynamic Range Closed-Loop Stiffness-Adjustable MEMS Force Sensor

We present a microelectromechanical system (MEMS)-based closed-loop force sensor, capable of measuring bidirectional forces along one axis of motion. The device comprises an on-chip electrothermal displacement sensor and electrostatic actuators. It features a voltage-controlled stiffness-adjustment mechanism, providing the user with an additional means of tuning the sensor’s characteristics. This silicon-on-insulator device is an improvement on our previous design, offering a better dynamic range and a less complex calibration process. The sensor is fabricated based on a standard SOI MEMS process. It is then fully characterized and instrumented with a feedback controller. The closed-loop characterization reveals a ${1 {\sigma }}$ -resolution of 4.74 nN and a force sensing range of $- {211.7\, {\mu }\text {N}}$ to $\mathrm {211.5\, {\mu }N}$ , leading to a dynamic range of 92.9 dB. [2019-0248]

Soft-Switching Techniques for Single-Inductor Multiple-Output LED Drivers

Although a plethora of prior-art single-inductor multiple-output (SIMO) light-emitting diode (LED) drivers have been reported, they remain inadequate for resource-constraint drone applications due to compromised power efficiency and weight. This article proposes a monolithic SIMO red–green–blue–white LED driver that features high power efficiency and light weight. The former is obtained by employing our proposed soft-switched cross-current-eliminated controller and proposed on-chip bidirectional current sensor. The latter is achieved by means of employing a tiny inductor attributed to boundary conduction mode and high switching frequency. To the best of authors’ knowledge, the proposed SIMO LED driver is the first-ever design that features soft switching for all power switches and innate elimination of current cross-regulation. In comparison with the state-of-the-art designs listed in the benchmark table, the proposed design features the highest power efficiency of 96.2%, smallest output inductor of 820 nH, shortest LED current settling time of 900 ns, and the highest switching frequency of 2 MHz.

Comparative study of Er3+-doped Ga-Ge-Sb-S thin films fabricated by sputtering and pulsed laser deposition

Despite the renewed interest in rare earth-doped chalcogenide glasses lying mainly in mid-infrared applications, a few comprehensive studies so far have presented the photoluminescence of amorphous chalcogenide films from visible to mid-infrared. This work reports the fabrication of luminescent quaternary sulfide thin films using radio-frequency sputtering and pulsed laser deposition, and the characterization of their chemical composition, morphology, structure, refractive index and Er3+ photoluminescence. The study of Er3+ 4I13/2 level lifetimes enables developing suitable deposition parameters; the dependency of composition, structural and spectroscopic properties on deposition parameters provides a way to tailor the RE-doped thin film properties. The surface roughness is very low for both deposition methods, ensuring reasonable propagation optical losses. The effects of annealing on the sulfide films spectroscopy and lifetimes were assessed. PLD appears consistent composition-wise, and largely independent of the deposition conditions, but radiofrequency magnetron sputtering seems to be more versatile, as one may tailor the film properties through deposition parameters manipulation. The luminescence via rare earth-doped chalcogenide waveguiding micro-structures might find easy-to-use applications concerning telecommunications or on-chip optical sensors for which luminescent sources or amplifiers operating at different wavelengths are required.

(Invited) Graphene-Based Micro Biosensors

On the surface of graphene, energy transfer occurs when molecules are located close to the surface. The energy transfer yield depends on the degree of molecular interaction between the adsorbed molecules and the graphene surface. For example, when a fluorescent molecule such as a dye is located very close to the graphene surface, the dye does not exhibit fluorescence because of the efficient fluorescence resonance energy transfer (FRET). The degree of energy transfer varies when the molecules on the surface changes their molecular states and structures. This allows us to visualize biological/chemical reactions by converting those invisible molecular interactions into the measurable physical quantities such as light and electricity. This makes graphene a promising material for a novel biosensor.We have created a unique type of biosensor, which works on a graphene surface by modifying it with a specific DNA called an aptamer, for the detection of biologically important proteins such as cancer markers. One end of the aptamer is labeled with a fluorescent dye and the other end is connected to a pyrene linker molecule, which shows a strong affinity to the graphene. Thus, the aptamer is firmly fixed to the graphene surface. The graphene aptasensor detection mechanism is as follows. In the initial stage, the dye-conjugated aptamer is adsorbed on the graphene surface via physical adsorption (π-π interactions), and thus the dye is located close to the graphene surface. Here, the fluorescence of the dye is well quenched by graphene via FRET and is barely observable (Fig. 1(a)). If the target of the aptamer is present in the system, the aptamer forms a complex with the target and leaves the graphene surface. At the same time, the dye molecule also leaves the graphene surface and the dye recovers its fluorescence (Fig. 1(b)). We can detect the target molecule by observing the fluorescence [1]. The system allows us to perform molecular detection on a solid surface, which is a powerful tool to realize a two-dimensional (2D) on-chip sensor. By using the on-chip sensor, detection of the target protein is possible simply by adding a sample smaller than 1 μL to a sensor chip and is completed in about a minute. The system also allows us to perform real-time molecular detection, the simultaneous detection of multiple target molecules on a single chip [2], and the molecular design of a probe for enhancing the sensitivity by using single-stranded DNA spacer (Fig. 2)[3].We then extend the biosensing platform from a 2D plane to a hollow three-dimensional (3D) space by building the protein detection system on the inner surfaces of flexible layered polymer films. The different strain gradients of the polymeric bilayer are used as the driving force behind the 3D transformation [4]. Detection of human serum albumin (HAS) in a 3D aptasensor has been successfully demonstrated (Fig. 1(e-f)). Since the formation of the micro-roll avoids any cytotoxic processes, they can be used for encapsulating cells in vitro [4, 5]. To demonstrate the detection of proteins from cells by the proposed sensor, human hepatoma cells (HepG2) were seeded inside the rolls which were functionalized with an aptamer for cytochrome C. After overnight incubation, staurosporine (apoptosis inducer) was added to increase cytochrome C concentration in cytoplasma. Fluorescence intensity increased only around the cells after 3 h incubation (Fig. 3(a)). Moreover, increased intensity was observed after cell rupturing (Fig. 3(b)). These results show that the 3D aptasensor can detect the cytochrome C in the cytoplasma after necrosis. Our sensor is expected to be applied to spatiotemporal detection of several secreted proteins such as growth factors in near future.

Balanced Detection Method Using Optical Affinity Sensors for Quick Measurement of Biomolecule Concentrations.

To determine the concentration of biomolecules using a label-free optical biosensor, it is necessary to measure the serial signal from the reaction starting point, which is inconvenient for practical applications. Here, we propose an alternative detection method for determining the concentration of a biomolecule. The method, which is derived from the fraction bound equation of the Langmuir adsorption model, determines the concentration relative to a reference sample with required accuracy, with a single measurement at any point in time. We also experimentally demonstrated the method and its accuracy by detecting streptavidin-biotin complexes using on-chip optical sensors based on active disk resonators integrated with microfluidic circuits. By performing the proposed method in a simultaneous parallel measurement scheme, signal fluctuations evenly induced in the detectors by external perturbations could be automatically suppressed, similar to the balanced detection method. We expect our approach to be applicable to practical applications where fast and accurate detection responses are needed.

On-chip biochemical sensor using wide Gaussian beams in silicon waveguide-integrated plasmonic crystal.

An on-chip biochemical sensor based on two-dimensional waveguide-integrated plasmonic crystal formed by a nanogap tile (NGT) array is realized. By using on-chip optical lenses, an ultra-wide collimated Gaussian beam is launched, coupled with surface plasmonic crystals and collected with relatively low additional insertion loss, allowing a large sensing area. The optical field enhancement and stop-band shift of the NGT device for biochemical sensing are numerically and experimentally demonstrated with sensitivity reaching up to ${\sim}{260}\;{\rm nm/RIU}$∼260nm/RIU. Our sensor is demonstrated with monolayer thiol molecules illustrating that it can be functionalized with this class of molecule which is commonly used with bulk surface plasmon resonance sensors.

Suspended slotted photonic crystal cavities for high-sensitivity refractive index sensing.

We present the design, fabrication, and characterization of a suspended slotted photonic crystal (SSPhC) cavity sensor based on the silicon-on-insulator platform. The sensing performance can be dramatically enhanced by the optimized SSPhC cavity as most of the light energy can be distributed in the low index region (∼57%). By measuring the spectrum response of the cavity sensor immersed in NaCl solutions with different mass concentrations, an ultra-high sensitivity around 656 nm/RIU has been experimentally demonstrated. Furthermore, the whole size of the cavity sensor (including the grating couplers) is 320 × 40 µm2, making the high-sensitivity device attractive for the realization of large-scale multi-channel on-chip sensors.

Enhanced Electrode Deposition for On-Chip Integrated Micro-Supercapacitors by Controlled Surface Roughening.

On-chip micro-supercapacitors (MSCs), integrated with energy harvesters, hold substantial promise for developing self-powered wireless sensor systems. However, MSCs have conventionally been manufactured through techniques incompatible with semiconductor fabrication technology, the most significant bottleneck being the electrode deposition technique. Utilization of spin-coating for electrode deposition has shown potential to deliver several complementary metal–oxide–semiconductor (CMOS)-compatible MSCs on a silicon substrate. Yet, their limited electrochemical performance and yield over the substrate have remained challenges obstructing their subsequent integration. We report a facile surface roughening technique for improving the wafer yield and the electrochemical performance of CMOS-compatible MSCs, specifically for reduced graphene oxide as an electrode material. A 4 nm iron layer is deposited and annealed on the wafer substrate to increase the roughness of the surface. In comparison to standard nonroughened MSCs, the increase in surface roughness leads to a 78% increased electrode thickness, 21% improvement in mass retention, 57% improvement in the uniformity of the spin-coated electrodes, and a high yield of 87% working devices on a 2″ silicon substrate. Furthermore, these improvements directly translate to higher capacitive performance with enhanced rate capability, energy, and power density. This technique brings us one step closer to fully integrable CMOS-compatible MSCs in self-powered systems for on-chip wireless sensor electronics.

Parasitic Capacitances, Inductive Coupling, and High-Frequency Behavior of AMR Sensors

Anisotropic Magneto-Resistive (AMR) sensors are popular in sensitive applications, especially in planar on-chip integrated form which implies the presence of several on-chip capacitive and inductive parasitic elements. However, there is a lack of equivalent circuit models capturing these parasitics and allowing the modeling of sensors' behavior at higher frequencies or in stability-sensitive closed-loop operation. This work fills this gap in the literature by introducing a high-frequency equivalent circuit model, for planar on-chip AMR sensors with integrated offset and Set/Reset coils, capturing the main capacitive and inductive parasitic elements. A general methodology for estimating the values of the parasitic elements via a set of measurements is presented and it is applied to a particular commercial AMR sensor. Using the introduced circuit model, the transfer function from the offset coil voltage to the output voltage of the sensor, via the parasitic capacitive coupling path, is derived. The transfer function from the S/R coil voltage to the offset coil voltage is also derived. Both are used to create a system-level model of the AMR sensor which can be used in system-level analysis to help the designer predict and optimize the open or the closed-loop sensor circuit system behavior. The introduced high-frequency equivalent circuit model and the two related transfer functions have been verified experimentally using the component values derived from the examined commercial AMR sensor.

Assessment of On-Chip Current Sensor for Detection of Thermal-Neutron-Induced Transients

This article assesses, for the first time, a body/bulk built-in current sensor (BBICS) in a CMOS 65-nm test chip under thermal neutron, high-energy neutron, and laser radiation. Experimental results suggest that the on-chip current sensor is effective to detect transient faults in different case-study subcircuits of the chip exposed to the accelerated radiation effects, opening prospects for embedding this type of sensor in reliable, secure, and low-power integrated circuit applications.

Vernier effect-based tunable mid-infrared sensor using silicon-on-insulator cascaded rings.

Vernier effect has been captivated as a promising approach to achieve high-performance photonic sensors. However, experimental demonstration of such sensors in mid-infrared (MIR) range, which covers abundant absorption fingerprints of molecules, is still lacking. Here, we report Vernier effect-based thermally tunable photonic sensors using cascaded ring resonators fabricated on the silicon-on-insulator (SOI) platform. The radii and the coupling gaps in two rings are investigated as key design parameters. By applying organic liquids on our device, we observe an envelope shift of 48 nm with a sensitivity of 3000 nm/RIU and an intensity drop of 6.7 dB. Besides, our device can be thermally tuned with a sensitivity of 0.091 nm/mW. Leveraging the characteristic molecular absorption in the MIR, our work offers new possibilities for complex index sensing, which has wide applications in on-chip photonic sensors.

Mid Infrared Optical Gas Sensor Using Plasmonic Mach-Zehnder Interferometer

In this work, we propose an optimized design for on-chip gas sensor using metal-insulator (MI) plasmonic waveguide in the mid infrared range and utilizing a Mach-Zehnder Inetrferometer (MZI). The MI waveguide utilizes a high index dielectric layer on top of the metal to enhance the sensitivity of the sensor. The thickness and the refractive index of this layer are optimized to achieve high sensitivity. Using this layer, a design that exhibits high performance for both wavelength and intensity interrogation schemes is achieved. In addition, another one that furtherly enhances the sensor performance for intensity interrogation is also proposed. This design also minimizes the sensor sensitivity to wavelength variations. Intensity interrogation scheme has the advantage of eliminating the size and cost needed by wide wavelength band measurements including either spectrometer or tunable laser in wavelength interrogation. The first design sensitivity has reached 10000 nm/RIU with wavelength interrogation figure of merit (FOMλ) of 133RIU−1 and intensity interrogation FOMI of 239RIU−1. While the second one exhibit FOMI of 363RIU−1, both with length of 250 µm around 4.6 µm wavelength. Finally, these structures are cheap, compact, and easy to fabricate.

Hybrid plasmonic grating slot waveguide with high field enhancement for an on-chip surface-enhanced Raman scattering sensor.

We design and theoretically investigate a surface-enhanced Raman scattering (SERS) sensor based on the hybrid plasmonic grating slot waveguide. The sensor is formed by combining a dielectric deep slot waveguide and a metallic grating slot waveguide. The proposed sensor exhibits a high field enhancement with a maximum enhancement factor of 7580.9 at the wavelength of 785 nm, revealing that the electric field in such hybrid plasmonic grating slot waveguide can be extremely strengthened. To better characterize the performance of the sensor in the SERS application, the total normalized volumetric enhancement factor (TNVEF) is proposed, which is determined by both the |E|4-approximation-based volumetric field enhancement and Raman scattered light collection efficiency. The TNVEF is utilized to characterize the influences of the structural parameters on the sensor and further optimize the sensing structure. Such on-chip SERS sensor can be integrated with a micro-laser and a micro-multiplexer on a photonic platform to realize an all-integrated on-chip SERS detection system.

Secure Assay Execution on MEDA Biochips to Thwart Attacks Using Real-Time Sensing

Digital microfluidic biochips (DMFBs) have emerged as a promising platform for DNA sequencing, clinical chemistry, and point-of-care diagnostics. Recent research has shown that DMFBs are susceptible to various types of malicious attacks. Defenses proposed thus far only offer probabilistic guarantees of security due to the limitation of on-chip sensor resources. A micro-electrode-dot-array (MEDA) biochip is a next-generation DMFB that enables the real-time sensing of on-chip droplet locations, which are captured in the form of a droplet-location map. We propose a security mechanism that validates assay execution by reconstructing the sequencing graph (i.e., the assay specification) from the droplet-location maps and comparing it against the golden sequencing graph. We prove that there is a unique (one-to-one) mapping from the set of droplet-location maps (over the duration of the assay) to the set of possible sequencing graphs. Any deviation in the droplet-location maps due to an attack is detected by this countermeasure because the resulting derived sequencing graph is not isomorphic to the original sequencing graph. We highlight the strength of the security mechanism by simulating attacks on real-life bioassays. We also address the concern that the proposed mechanism may raise false alarms when some fluidic operations are executed on MEDA biochips. To avoid such false alarms, we propose an enhanced sensing technique that provides fine-grained sensing for the security mechanism.

Design and analysis of slow-light Bloch slot waveguides for on-chip gas sensing

The performance of on-chip gas sensors based on light absorption is mainly determined by the light–gas interaction. In this paper, slow-light Bloch slot waveguides (BSW) are proposed to improve sensing performance. The sensing performance is enhanced in two mechanisms. On the one hand, light is confined in the slot to increase the overlap of the mode field and the gas; on the other hand, the slow-light effect is achieved by adjusting the subwavelength grating period to increase the group index. By joint engineering the evanescent fields and group index, for a low pump power of 10 mW and a propagation loss of 3 dB/cm, the detection limit of 0.034 ppm in the near-infrared and the detection limit of 0.29 ppm in the mid-infrared at the optimum propagation length of 1.45 cm are obtained, respectively. The proposed BSW provides a promising platform for high-performance gas sensing.

Nanowire Ring Embedded in a Flexible Substrate for Local Strain Detection.

Optical sensing has attracted more and more attention in recent years with the advance in planar waveguide fabrication processes. The photon, as a carrier of information in sensing areas, could have a better performance than electrons. We propose a novel end-to-end ring cavity to fabricate sensitive units of a strain sensor. We then propose a method of combining a flexible substrate with an end-to-end semiconductor nanowire ring cavity to fabricate novel strain sensors. We used a tuning resonant wavelength detected by a homebuilt excitation and detection system to measure applied strain. The resonant wavelength of the strain gauge was red-shift and linear tuned with increasing strain. The gauge factor was about 50, calculated through experiments and theory, and Q was 1938, with structural parameters L = 70 µm and d = 1 µm. The high sensitivity makes it possible to measure micro deformation more accurately. End-to-end coupling active nanowire waveguides eliminate the shortcomings of side by side coupling structures, which have the phasing shift with no minor optical density loss. This resonator in flexible substrates could be used not only as on-chip strain sensors for micro or nano deformation detecting but also as tunable light sources for photonic integrated circuits.