BITS Faculty Publications

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    Heterogeneous CMOS-MEMS based Boost Converter for 2.4 GHz RF energy Harvester
    (IEEE, 2024) Rao, V. Ramgopal
    Internet of Things (IoT) has experienced a significant growth in last few years. Billions of battery-powered wireless sensors are expected to be employed as the IoT becomes an integral part of our daily lives. Therefore, ambient energy resources such as light, RF source, EM radiation, thermal energy can be utilized to prolong the lifetime of batteries for sensors. In this work, ambient RF energy source is used for energy harvesting to power up the wireless sensors and low power electronic devices. For the first time, we experimentally demonstrated RF energy harvester to scavenge 2.45 GHz from Wi-Fi sources using commercially available CMOS-MEMS (micro electromechanical switch) hybrid switches. The use of MEMS switches in the boost converter instead of conventional NMOS switches reduces the leakage current, stabilize the ON-state resistance, and improves the overall efficiency. Our experimental result indicates that the use of MEMS switches increases the efficiency of the energy harvester more than 15% as compared to its NMOS counterpart.
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    Influence of Slotting and Boss Radius on the Response of MEMS Based Intracranial Pressure Sensor
    (UPI Journals, 2018) Rao, Venkatesh K.P.
    In the present paper, we design a Microelectro- Mechanical System (MEMS) piezoresistive pressure sensor for intracranial pressure monitoring. The pressure sensor design presented in this paper consists of a square diaphragm. The slots were introduced to square diaphragm increases the stresses developed thus enhancing the sensitivity of the sensor In addition to slots, a central boss was introduced to enhance the sensitivity of the sensor. We carried out numerical simulation to evaluate the sensitivity of the sensor. Parametric studies were done to optimize the central boss radius to enhance the sensitivity of the sensor.
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    Effect of external electric potential on the mechanical resonance of MEMS cantilever resonator
    (IOP, 2022-08) Rao, Venkatesh K.P.
    This work presents an analytical model to predict the natural frequency of electrostatically actuated micromachined cantilever beam under the application of DC voltage. The analytical modelling uses an energy based method with a sinusoidal vibration assumption. The electric field between the cantilever electrodes is assumed to be vertical and fringing field effects are neglected. The behaviour of natural frequency and a closed-form expression for pull-in voltage are evaluated. The electrostatic spring softening effect of DC bias on the resonant natural frequency is studied in particular. Results are compared with simulation in ANSYS.
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    Design and Analysis of Single Drive Tri-Axis MEMS Gyroscope
    (Springer, 2022-11) Rao, Venkatesh K.P.
    MEMS based single and dual-axis gyroscopes have been widely explored for potential application in automotive, space, defense, and consumer electronics sectors. Tri-axis gyroscopes based on MEMS, however, have been sparsely studied. This work presents a novel design for tri-axis MEMS gyroscope and an analytical model to obtain the natural frequencies in drive and sense modes. These frequency values have been compared with the numerically obtained frequencies using Finite Element Analysis (FEA). The analytical results lie within 10% of their numerically obtained values. The frequency matching process involves many iterations of geometric dimensions if the end application requires minor design changes. The proposed analytical model will make the design customization easy as the frequencies of each mode will be expressed as a function of critical geometrical parameters saving multiple numerical runs required for design optimization.
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    System integration design in MEMS — A case study of micromachined load cell
    (Springer, 2009-10) Rao, Venkatesh K.P.
    One of the critical issues in large scale commercial exploitation of MEMS technology is its system integration. In MEMS, a system design approach requires integration of varied and disparate subsystems with one of a kind interface. The physical scales as well as the magnitude of signals of various subsystems vary widely. Known and proven integration techniques often lead to considerable loss in advantages the tiny MEMS sensors have to offer. Therefore, it becomes imperative to think of the entire system at the outset, at least in terms of the concept design. Such design entails various aspects of the system ranging from selection of material, transduction mechanism, structural configuration, interface electronics, and packaging. One way of handling this problem is the system-in-package approach that uses optimized technology for each function using the concurrent hybrid engineering approach. The main strength of this design approach is the fast time to prototype development. In the present work, we pursue this approach for a MEMS load cell to complete the process of system integration for high capacity load sensing. The system includes; a micromachined sensing gauge, interface electronics and a packaging module representing a system-in-package ready for end characterization. The various subsystems are presented in a modular stacked form using hybrid technologies. The micromachined sensing subsystem works on principles of piezo-resistive sensing and is fabricated using CMOS compatible processes. The structural configuration of the sensing layer is designed to reduce the offset, temperature drift, and residual stress effects of the piezo-resistive sensor. ANSYS simulations are carried out to study the effect of substrate coupling on sensor structure and its sensitivity. The load cell system has built-in electronics for signal conditioning, processing, and communication, taking into consideration the issues associated with resolution of minimum detectable signal. The packaged system represents a compact and low cost solution for high capacity load sensing in the category of compressive type load sensor.
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    Design and characterization of in-plane MEMS yaw rate sensor
    (Springer, 2009-10) Rao, Venkatesh K.P.
    In this paper, we present the design and characterization of a vibratory yaw rate MEMS sensor that uses in-plane motion for both actuation and sensing. The design criterion for the rate sensor is based on a high sensitivity and low bandwidth. The required sensitivity of the yawrate sensor is attained by using the inplane motion in which the dominant damping mechanism is the fluid loss due to slide film damping i.e. two–three orders of magnitude less than the squeeze-film damping in other rate sensors with out-of-plane motion. The low bandwidth is achieved by matching the drive and the sense mode frequencies. Based on these factors, the yaw rate sensor is designed and finally realized using surface micromachining. The inplane motion of the sensor is experimentally characterized to determine the sense and the drive mode frequencies, and corresponding damping ratios. It is found that the experimental results match well with the numerical and the analytical models with less than 5% error in frequencies measurements. The measured quality factor of the sensor is approximately 467, which is two orders of magnitude higher than that for a similar rate sensor with out-of-plane sense direction.
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    High Yield Polymer MEMS Process for CMOS/MEMS Integration
    (Springer, 2011-02) Rao, V. Ramgopal
    MEMS community is increasingly using SU-8 as a structural material because it is self-patternable, compliant and needs a low thermal budget. While the exposed layers act as the structural layers, the unexposed SU-8 layers can act as the sacrificial layers, thus making it similar to a surface micromachining process. A sequence of exposed and unexposed SU-8 layers should lead to the development of a SU-8 based MEMS chip integrated with a pre-processed CMOS wafer. A process consisting of optical lithography to obtain SU-8 structures on a CMOS wafer is described in this paper.
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    ZnO Nanorod Based Ultra Sensitive and Selective Explosive Sensor
    (IEEE, 2013-02) Rao, V. Ramgopal
    A small scale (20 μm), ultra sensitive (50 ppb) and highly selective sensor based on ZnO nanostructures using Micro-electro-mechanical system (MEMS) platform has been reported here for the detection of explosive and Volatile Organic Compound (VOC) vapors. Flower and rod like architectures of nanorods were used as a sensing layer. The nanorods prepared via chemical synthesis were uniform with diameters of 50-80 nm and lengths about 3-4 μm. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) reveal that the nanostructures are well oriented with the c-axis, perpendicular to the substrate. A relatively higher selectivity for 2, 4, 6-Trinitrotoluene (TNT) vapors compared to other VOCs at room temperature were observed. The intensity of deep level green emission peak associated with point defects decreases after exposure as revealed from Photoluminescence (PL) spectra.
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    Nano-electro-mechanical transduction and packaging solutions for polymer MEMS devices
    (IEEE, 2015) Rao, V. Ramgopal
    Polymeric micro/nano electromechanical sensors and devices have recently gained much attention over their Si counterparts because of their lower cost and higher sensitivity. In this paper, we have reviewed various reported schemes for nano-electro- mechanical transduction for polymeric sensors particularly for physical, chemical and bio-sensing applications. We have also reviewed the packaging aspects and challenges for these polymeric sensors for various applications.
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    A lab-on-a-chip system for detection of multiple macronutrients in the soil
    (IEEE, 2016) Rao, V. Ramgopal
    In this work, we have demonstrated for the first time the sensing of soil macronutrients nitrate and potassium with the use of MTDAN ionophore, nitrate ionophore VI and 18crown6 ether in a PVC/DOS matrix using a highly sensitive piezoresitive silicon oxide cantilevers. The complete portable electronics system along with a liquid cell for on the field experimentation has been demonstrated.