Compact Model for Flexible Ion-Sensitive Field-Effect Transistor

This paper presents the theoretical modelling, and simulation of bending effects on an ion-sensitive field-effect transistor (ISFET), towards futuristic bendable integrated circuits and microsystems for biomedical applications. Based on variations of threshold voltage and drain current under different bending conditions and orientations of the channel of the device, the bendable ISFET macro-model has been implemented in Verilog-A, and compiled into the Cadence environment. The effects of bending on the behaviour of the device have been simulated over a user-defined range of pH, and sensitivities in a standard 0.18-μm CMOS technology. It has been found that the transfer curves (Id-Vg) of ISFET vary up to 4.46% for tensile and up to 5.15% for compressive bending stress at pH 2, and up to 4.99% for tensile and 5.61% for compressive bending stress at pH 12 with respect to its planar counterpart, while the sensitivity of the device has been found to remain the same irrespectively of the bending stress. The proposed model has been validated by comparing the results with those obtained by other macro-models and experimental results in literature

Modeling of CMOS devices and circuits on flexible ultrathin chips

The field of flexible electronics is rapidly evolving. The ultrathin chips are being used to address the high-performance requirements of many applications. However, simulation and prediction of changes in response of device/circuit due to bending induced stress remains a challenge as of lack of suitable compact models. This makes circuit designing for bendable electronics a difficult task. This paper presents advances in this direction, through compressive and tensile stress studies on transistors and simple circuits such as inverters with different channel lengths and orientations of transistors on ultrathin chips. Different designs of devices and circuits in a standard CMOS 0.18-μm technology were fabricated in two separated chips. The two fabricated chips were thinned down to 20 μm using standard dicing-before-grinding technique steps followed by post-CMOS processing to obtain sufficient bendability (20-mm bending radius, or 0.05% nominal strain). Electrical characterization was performed by packaging the thinned chip on a flexible substrate. Experimental results show change of carrier mobilities in respective transistors, and switching threshold voltage of the inverters during different bending conditions (maximum percentage change of 2% for compressive and 4% for tensile stress). To simulate these changes, a compact model, which is a combination of mathematical equations and extracted parameters from BSIM4, has been developed in Verilog-A and compiled into Cadence Virtuoso environment. The proposed model predicts the mobility variations and threshold voltage in compressive and tensile bending stress conditions and orientations, and shows an agreement with the experimental measurements (1% for compressive and 0.6% for tensile stress mismatch)

Device Modelling of Silicon Based High-Performance Flexible Electronics

The area of flexible electronics is rapidly expanding and evolving. With applications requiring high speed and performance, ultra-thin silicon-based electronics has shown its prominence. However, the change in device response upon bending is a major concern. In absence of suitable analytical and design tool friendly model, the behavior under bent condition is hard to predict. This poses challenges to circuit designer working in the bendable electronics field, in laying out a design that can give a precise response in a stressed condition. This paper presents advances in this direction and investigates the effect of compressive and tensile stress on the performance of NMOS and PMOS transistor and a touch sensor comprising a transistor and piezoelectric capacitor

Ultra-thin and flexible CMOS technology: ISFET-based microsystem for biomedical applications

A new paradigm of silicon technology is the ultra-thin chip (UTC) technology and the emerging applications. Very thin integrated circuits (ICs) with through-silicon vias (TSVs) will allow the stacking and interconnection of multiple dies in a compact format allowing a migration towards three-dimensional ICs (3D-ICs). Also, extremely thin and therefore mechanically bendable silicon chips in conjunction with the emerging thin-film and organic semiconductor technologies will enhance the performance and functionality of large-area flexible electronic systems. However, UTC technology requires special attention related to the circuit design, fabrication, dicing and handling of ultra-thin chips as they have different physical properties compared to their bulky counterparts. Also, transistors and other active devices on UTCs experiencing variable bending stresses will suffer from the piezoresistive effect of silicon substrate which results in a shift of their operating point and therefore, an additional aspect should be considered during circuit design. This thesis tries to address some of these challenges related to UTC technology by focusing initially on modelling of transistors on mechanically bendable Si-UTCs. The developed behavioural models are a combination of mathematical equations and extracted parameters from BSIM4 and BSIM6 modified by a set of equations describing the bending-induced stresses on silicon. The transistor models are written in Verilog-A and compiled in Cadence Virtuoso environment where they were simulated at different bending conditions. To complement this, the verification of these models through experimental results is also presented. Two chips were designed using a 180 nm CMOS technology. The first chip includes nMOS and pMOS transistors with fixed channel width and two different channel lengths and two different channel orientations (0° and 90°) with respect to the wafer crystal orientation. The second chip includes inverter logic gates with different transistor sizes and orientations, as in the previous chip. Both chips were thinned down to ∼20m using dicing-before-grinding (DBG) prior to electrical characterisation at different bending conditions. Furthermore, this thesis presents the first reported fully integrated CMOS-based ISFET microsystem on UTC technology. The design of the integrated CMOS-based ISFET chip with 512 integrated on-chip ISFET sensors along with their read-out and digitisation scheme is presented. The integrated circuits (ICs) are thinned down to ∼30m and the bulky, as well as thinned ICs, are electrically and electrochemically characterised. Also, the thesis presents the first reported mechanically bendable CMOS-based ISFET device demonstrating that mechanical deformation of the die can result in drift compensation through the exploitation of the piezoresistive nature of silicon. Finally, this thesis presents the studies towards the development of on-chip reference electrodes and biodegradable and ultra-thin biosensors for the detection of neurotransmitters such as dopamine and serotonin

Microdroplet Based Organic Vapour Sensor on a Disposable GO-Chitosan Flexible Substrate

With rising hazardous organic vapours in the environment, the detection of volatile organic vapour compounds (VOCs) is important for human safety. To this end, this paper presents a conductive droplet-based disposable sensor. Unlike conventional sensors, the droplet system is easily replaceable and is capable of detecting multiple vapours based on surface tension gradient. The response time for the presented sensing arrangement was found to be 3-4 seconds which is better than the solid-state counterparts. The chemiresistive sensor used in this work, is fabricated on 2.5 μm thick ultra-flexible graphene oxide-chitosan (GOC) bioresorbable substrate with Pt electrodes which are 60 μm apart. The presence of GO in the GOC substrate provides optimum hydrophobicity to the droplet for efficient operation. The electrostatic interaction and strong hydrogen bonds between GO and polysaccharide groups in chitosan provides tunable hydrophobicity and stability to the droplet. Moreover, biocompatibility, low-toxicity and bioresorbability of GOC substrate are highly desirable in the disposable sensing applications. With a conductive droplet of ~10 μL of aq. NaCl as an active sensing material, dispensed in-between the Pt electrodes, it was observed that the droplet shows 14-21% change in resistance in presence of VOCs

Device modelling for bendable piezoelectric FET-based touch sensing system

Flexible electronics is rapidly evolving towards devices and circuits to enable numerous new applications. The high-performance, in terms of response speed, uniformity and reliability, remains a sticking point. The potential solutions for high-performance related challenges bring us back to the timetested silicon based electronics. However, the changes in the response of silicon based devices due to bending related stresses is a concern, especially because there are no suitable models to predict this behavior. This also makes the circuit design a difficult task. This paper reports advances in this direction, through our research on bendable Piezoelectric Oxide Semiconductor Field Effect Transistor (POSFET) based touch sensors. The analytical model of POSFET, complimented with Verilog-A model, is presented to describe the device behavior under normal force in planar and stressed conditions. Further, dynamic readout circuit compensation of POSFET devices have been analyzed and compared with similar arrangement to reduce the piezoresistive effect under tensile and compressive stresses. This approach introduces a first step towards the systematic modeling of stress induced changes in device response. This systematic study will help realize high-performance bendable microsystems with integrated sensors and readout circuitry on ultra-thin chips (UTCs) needed in various applications, in particular, the electronic skin (e-skin)

A Wearable Fabric-Based RFID Skin Temperature Monitoring Patch

This paper presents a novel design of wearable radio frequency identification (RFID) sensor patch make of conductive fabric and integrated on clothes. The wearable RFID with similar design is also implemented on a Polyimide (PI) substrate to show the effectiveness of the system. We also demonstrate the wearable and washable RFID patch by using conductive fabric coil antenna as well as non-conductive fabric substrate. The conductive fabric offers great flexibility and comfortability as it can be sewed into clothes and connect the components of the patch. As a proof of concept, we developed the conductive fabric based RFID for temperature sensing and demonstrate its use by measuring variations in the skin temperature. We observed that the proposed antenna is strain independent during bending. Further, it has the advantage of simplicity and is relatively free from issues such as degradation of performance

Polydimethylsiloxane as polymeric protective coating for fabrication of ultra-thin chips

The bendable silicon-based ultra-thin chips (UTCs), with thickness below 50 μm are needed to provide high-performance flexible electronics for several emerging applications ranging from flexible displays to robotic e-skin. The UTCs from standard silicon wafer are obtained by etching the bulk material from the backside of the wafer using a wet chemical etchant. During the etching process, it is imperative to protect the front processed side from the etchant as in most cases, the etchant is incompatible with the metals and other materials used in the fabrication of devices. This paper reports a new method using polydimethylsiloxane (PDMS) as the protective coating during wet etching of silicon. The silicon sample is thinned to sub-25 μm thickness using Tetramethylammonium hydroxide (TMAH), while PDMS acting as a protective coating, which is removed after thinning by using a chemical composition involving a nucleophilic attack on siloxane bond. As a bulk material with low- temperature processing requirements, PDMS offers an interesting alternative to other commercially available materials. The presented approach offers a range of advantages compared to other polymeric materials that are being used for the above-mentioned purpose

Screen Printed Thick Film Reference Electrodes for Electrochemical Sensing

This paper presents printed thick film Ag|AgCl|KCl reference electrodes (RE) for electrochemical sensors. The screenprinted REs with 10-μm thick glass-KCl salt matrix layer exhibit a stable potential of 5 mV. Cyclic voltammetric analysis shows that the anodic and cathodic peak current of the RE increases with the scan rate in the range of 25-150 mVs -1 . The analytical performance of the REs shows a stable open circuit potential for the NaCl concentrations in the range of 30-100 mM. Testing the presented REs for electrochemical pH sensor application (with RuO 2 -based sensitive electrode) the sensitivity of 55 mV/pH was noted in the pH range of 4.5-9. Evaluating the effect of temperature on the performance of REs, a potential variation of -3.8 mV/ °C was observed. Finally, a LabVIEW interface was developed to store, analyze, and calculate the sensitivity of the sensor under different temperature conditions. The LabVIEW interface can also be used to calculate the pH-value/temperature of unknown solutions under known temperature/pH conditions

Ultra-thin chips with ISFET array for continuous monitoring of body fluids pH

This paper presents ISFET array based pH-sensing system-on-ultra-thin-chip (SoUTC) designed and fabricated in 350 nm CMOS technology. The SoUTC with the proposed current-mode active-pixel ISFET circuit array is desined to operate at 2V and consumes 6.28 W per-pixel. The presented SoUTC exhibits low sensitivity to process, voltage, temperature and strain-induced (PVTS) variations. The silicon area occupancy of each active-pixel is 44.9x33.5 m2 with an ion-sensing area of 576 m2. The design of presented ISFET device is analysed with finite element modeling in COMSOL Multiphysics using compact model parameters of MOSFET in 350 nm CMOS technology. Owing to thin (~30m) Si-substrate the presented SoUTC can conform to curvilinear surfaces, allowing intimate contact necessary for reliable data for monitoring of analytes in body fluids such as sweat. Further, it can operate either in a rolling shutter fashion or in a pseudo-random pixel selection mode allowing the simultaneous detection of pH from different skin regions. Finally, the circuits have been tested in aqueous Dulbeccos Modified Eagle Medium (DMEM) culture media with 5-9 pH values, which mimics cellular environments, to demonstrate their potential use for continuous monitoring of body-fluids pH