Tuneable RF MEMS components using SU-8

With the rapid progress in the wireless communication field, radio frequency microelectro- mechanical systems (RF MEMS) are seen as one of the promising technologies to replace the existing high power communication systems. MEMS based tuneable devices such as varactors and phase shifters offer many advantages over their conventional diode-based counterparts including low loss, low power consumption and high linearity. MEMS varactors in particular can be integrated into many reconfigurable modules such as switching and reconfigurable matching networks. Moreover, distributed MEMS transmission line (DMTL) phase shifters with their linear phase characteristic can be applied to wideband phased array antennas for microwave medical imaging which requires beam steering and high gain antenna systems. This thesis focuses on the design and development of two RF MEMS devices which are a high tuning ratio digital MEMS varactor and a low frequency DMTL phase shifter using SU-8 polymer. The design and simulation of a 4-bit and a 5-bit digital MEMS varactors have been carried out in the first phase of this study. One of the limitations of the digital MEMS varactors fabricated on silicon substrates is the high fringing field capacitance that reduces the overall capacitance ratios of the devices. To reduce the effect of the fringing fields, two methods have been proposed to elevate the varactors from the silicon substrate. In the first method, a 26.35 μm deep trench is etched in the silicon substrate under the 4-bit digital MEMS varactor which is able to achieve a high capacitance ratio of 35.7. In the 5-bit digital MEMS varactor design, SU-8 material is used to form a 20 μm thick separation layer between the varactor and the silicon substrate instead of the deep trench method applied in the 4-bit MEMS varactor. The simulated capacitance ratio of the 5-bit digital MEMS varactor is 34.8. Additionally, the SU-8 also serves as a sacrificial layer to release the MEMS bridges on the devices hence reducing the fabrication process compared to the conventional MEMS release process that uses oxide as the sacrificial material. To verify the performance of using the thick SU-8 dielectric layer in reducing the fringing field capacitance in the varactor design, single-bridge varactors with different lengths and widths have been fabricated and analysed. A novel truss bridge structure has been proposed in order to reduce the pull-in voltage of the varactors. It is found that by using the truss structure, the measured pull-in voltage of the bridge can be reduced by 12.5% compared to the conventional solid fixed-fixed bridge structure. However, due to the high residual stress from the fabrication process which causes the bridge to warp over its width, the achievable average down-state capacitance of the fabricated single-bridge varactor is limited to 211 fF compared to the simulated value of 1.28 pF. Nevertheless, the capacitance ratio of the device fabricated on the SU-8 layer increases by 56.75% over a similar device fabricated without the polymer which proves that the fringing field capacitance has been reduced. Furthermore, fabrication of the single-bridge MEMS varactors on low-resistivity silicon has been carried out with the use of SU-8 as the passivation layer without affecting the performances of the varactors. This finding can lead to the realisation of low-cost MEMS varactors in the future. The second part of this thesis investigates the development of distributed MEMS transmission line (DMTL) phase shifters for operation in the frequency range of 2 GHz to 4 GHz (S-band). The proposed phase shifters are a 2-bit and 3-bit digital DMTL phase shifters. One of the potential applications of the proposed phase shifters is for phased array antenna systems for microwave head imaging that requires wideband performance. The 2-bit and 3-bit DMTL phase shifters have been designed and simulated with 41 MEMS bridges and 105 MEMS bridges respectively. The simulated phase shifts of the 2-bit phase shifter design are 00, 900, 1800 and 2700 whereas for the 3-bit phase shifter, 8 phase shifts have been achieved namely 00, 450, 900, 1350, 1800, 2250, 2700 and 3150. To validate the performance of the proposed low frequency DMTL phase shifter, the 2-bit phase shifter design has been fabricated and analysed. The measured impedance matching of the phase shifter shows good performance with reflection coefficients of less than -10 dB across the operating frequency range for all the states of the phase shifter. The measured differential phase shifts of the device are 00, 17.890, 34.510 and 52.390. The lower measured differential phase shifts compared to the simulated values can be attributed to the warping of the bridges over their width which causes a formation of an air gap between the bridge and dielectric layer hence reducing the down-state capacitance of the varactors in the phase shifter. Nevertheless, this is the first DMTL phase shifter to achieve a maximum differential phase shift of 52.390 at 2.45 GHz. Based on the measured differential phase shifts, the phase shifter can provide a maximum steering angle of ±5.730 for a 4-element phased array antenna at 2.45 GHz. The maximum measured transmission loss of the phase shifter is -10.51 dB at 2.45 GHz. The high loss of the phase shifter is due to the skin depth effect since the co-planar waveguide (CPW) transmission line of the phase shifter is fabricated using 300 nm thick aluminium. Therefore, further investigation has been carried out to provide better estimation of the transmission loss of the phase shifter by fabricating a CPW transmission line with the same configuration to that of the transmission line in the fabricated phase shifter by using 2 μm thick aluminium. The measured loss of the transmission line is -2.39 dB which shows significant improvement over the loss obtained from the phase shifter. Moreover, several CPW transmission lines with different centre conductor’s widths have been fabricated and analysed to further reduce the losses of the transmission lines. An attenuation loss of only 0.122 dB/cm has been achieved using a 500 μm-width centre conductor in the fabricated CPW transmission line which can lead to the realisation of a low-loss DMTL phase shifter for low microwave frequency range. The characterisation and optimisation of the varactors and phase shifters using SU-8 provide the initial step towards the development of tuneable RF MEMS devices for wide range of applications including wireless communications and radar systems. Moreover, the proposed DMTL phase shifters for operation at the lower end of microwave spectrum particularly in the frequency range of 2 GHz to 4 GHz are vital for the realisation of wideband phased array antennas for microwave medical imaging applications

Flexible milimeter-wave microstrip patch antenna array for wearable RF energy harvesting applications

In this paper, a series-fed milimeter-wave microstrip patch antenna array operating at 28 GHz is presented for wearable radio-frequency (RF) energy harvesting applications. The antenna array is made of 4×4 rectangular microstrip elements on a polyethylene terephthalate (PET) substrate to provide conformability when directly attached on human body parts. A 4-way Wilkinson power divider is connected to the array for RF power combining. The overall size of the antenna is 47×28×0.25 mm. The half-power beamwidth (HPBW) of the antenna array can be increased up to 151.9⁰ via structural deformation making it suitable for energy harvesting applications. The performance of the antenna array is investigated in terms of impedance matching, gain and radiation pattern. The average simulated specific absorption rate (SAR) of the antenna is 0.52 W/kg which is well below the safety limit of 1.6 W/kg averaged over 1 g of tissue for 100 mW of input power

Review on strain sensors for detection of human facial expressions recognition systems

Facial expression plays an important factor in human communication which helps us to understand the intentions and emotions of others. Generally, people infer the emotional states of other people such as fear, sadness, joy and anger just by looking at the facial expression and vocal tone. Moreover, facial expression can also be used to deliver messages especially for those who are paralyzed which their only means of communication is through facial expression. Therefore, by exploiting the facial expression of a paralyzed patient, a sensory system could be developed which would allow the patient to communicate with others and to assist them to actuate robotic limbs in order to improve their mobility. Conventional methods such as vision sensors that use cameras to detect facial expression have suffered from low mobility, high complexity, high cost and difficulty to adapt as wearable. Stretchable electronic devices have been developed for various applications including heaters, energy converters, transistors and sensors. Wearability, conformability to the skin, less complicated design and low cost promotes the use of strain sensor as part of a system for facial expression detection. This review paper presents the development of stretchable strain sensors for human facial expression detection focusing mainly on the materials and fabrication strategies. In addition, this paper also provides fundamental structural design as well as challenges and opportunities in realizing stretchable strain sensor and their various potential applications

Design and modeling of MEMS SAW resonator on Lithium Niobate

Surface Acoustic Wave (SAW) resonators are essential components for modern communication systems. They can function as filters and frequency synthesizers. SAW resonators operate based on the principle of acoustic waves propagating along the surface of a solid piezoelectric material. The waves are generated by injecting electrical energy using interdigitated transducers (IDTs) into the piezoelectric material which transforms it into propagating mechanical waves. This project intends to study the key design parameters that affect the performance of SAW resonator such as optimum spacing between IDT and reflector, optimum spacing of IDTs and the numbers of reflector in order to get the highest mechanical displacement. Key requirements of a SAW resonator include having precise resonant frequency (fr), low insertion losses, and high quality factors (Q). To meet these requirements, it is necessary to investigate the key design parameters; number of reflectors, number of IDTs, periodic distance of transducer fingers ( #x03BB;), spacing between IDT and reflector. Finite element simulations to determine the optimum SAW resonator design was performed using COMSOL Multiphysics #x2122;

Development of low cost screen-printed piezoresistive strain sensor for facial expressions recognition systems

The ability to automatically detect human facial expressions using sensor technology can potentially improve the quality of human life. The facial expressions can indicate the intentions and physiological conditions of a person which is valuable to healthcare sector. This paper presents the development of a low-cost stretchable strain sensor for human facial expression detection by utilizing stretchable silver ink. Screen printing technique was used to fabricate the sensor on commercially available medical dressing tape, Tegaderm. Mechanical characterization was carried out for the medical tape to evaluate its ultimate tensile strength and Young’s modulus. Moreover, a new theoretical model for the hyper elastic Tegaderm film was developed based on Rivlin-Mooney two parameter model. The strain sensor exhibits several benefits such as high sensitivity, easy fabrication and conformable to human skin deformation. The sensor has the maximum gauge factor (GF) of around 84 with the highest stretchability of up to 20% strain. Experiments were conducted to evaluate the feasibility of applying the strain sensors for facial expressions detection system. Three facial expressions which are happy, sad and disgust were successfully distinguished. Due to the distinctive features of the sensor, it has wide potential applications in the field of facial expression detection, human body monitoring and robotics

Design and fabrication of Surface Acoustic Wave resonators on Lithium Niobate

Surface Acoustic Wave (SAW) resonators are essential components in communication devices and are used mainly as oscillators, frequency synthesizers and transceivers. Common piezoelectric substrates are quartz, Lithium Tantalate (LiTaO3) and Lithium Niobate (LiNbO3). In this paper we describe the design and fabrication of SAW resonators on LiNbO3. The design of the SAW resonators was simulated using COMSOL MultiphysicsTM. Two SAW resonators with resonance frequency of 218 MHz with varying number of reflectors were fabricated and measured. Measurements conducted using an RF Probe station and network analyzer yielded losses of -48.3 dB and -49.32 dB for Resonator 1 and Resonator 2 respectively

Mechanical and Thermal Evaluation of Carrageenan/Hydroxypropyl Methyl Cellulose Biocomposite Incorporated with Modified Starch Corroborated by Molecular Interaction Recognition

Vegetarian hard capsule has attracted surging demand as an alternative to gelatin; however, only few have been commercialized. Carrageenan extracted from seaweed has the potential to be utilized as a hard capsule material. Improving the mechanical and thermal properties of carrageenan biocomposite is therefore of great importance for future use in the drug delivery system. Hence, carboxymethyl sago starch (CMSS) was incorporated to strengthen the carrageenan biocomposite in a concentration range from 0 to 1.0% w/v. The intermolecular hydrogen bonding formed between carrageenan and CMSS was revealed via density functional theory (DFT) calculations and substantiated by 1H NMR and FTIR spectra. The result showed that the hydrogen bond is established between hydroxyl (carrageenan)–carbonyl (CMSS) groups at a distance of 1.87 Å. The bond formation subsequently increased the tensile strength of the biocomposite film and the loop strength of the hard capsule by 20.6 and 7.7%, respectively. The glass transition temperature of the film was increased from 37.8 to 47.8 °C, increasing the thermal stability. The activation energy upon decomposition of the film is 74.4 kJ·mol–1, representing a 26.2% increase over the control carrageenan. These findings demonstrate that incorporation of CMSS increases the properties of carrageenan biocomposite and provides a promising alternative to animal-based hard capsules

Mechanical and Thermal Evaluation of Carrageenan/Hydroxypropyl Methyl Cellulose Biocomposite Incorporated with Modified Starch Corroborated by Molecular Interaction Recognition

Vegetarian hard capsule has attracted surging demand as an alternative to gelatin; however, only few have been commercialized. Carrageenan extracted from seaweed has the potential to be utilized as a hard capsule material. Improving the mechanical and thermal properties of carrageenan biocomposite is therefore of great importance for future use in the drug delivery system. Hence, carboxymethyl sago starch (CMSS) was incorporated to strengthen the carrageenan biocomposite in a concentration range from 0 to 1.0% w/v. The intermolecular hydrogen bonding formed between carrageenan and CMSS was revealed via density functional theory (DFT) calculations and substantiated by 1H NMR and FTIR spectra. The result showed that the hydrogen bond is established between hydroxyl (carrageenan)–carbonyl (CMSS) groups at a distance of 1.87 Å. The bond formation subsequently increased the tensile strength of the biocomposite film and the loop strength of the hard capsule by 20.6 and 7.7%, respectively. The glass transition temperature of the film was increased from 37.8 to 47.8 °C, increasing the thermal stability. The activation energy upon decomposition of the film is 74.4 kJ·mol–1, representing a 26.2% increase over the control carrageenan. These findings demonstrate that incorporation of CMSS increases the properties of carrageenan biocomposite and provides a promising alternative to animal-based hard capsules

RF Remote Blood Glucose Sensor and a Microfluidic Vascular Phantom for Sensor Validation

Diabetes has become a major health problem in society. Invasive glucometers, although precise, only provide discrete measurements at specific times and are unsuitable for long-term monitoring due to the injuries caused on skin and the prohibitive cost of disposables. Remote, continuous, self-monitoring of blood sugar levels allows for active and better management of diabetics. In this work, we present a radio frequency (RF) sensor based on a stepped impedance resonator for remote blood glucose monitoring. When placed on top of a human hand, this RF interdigital sensor allows detection of variation in blood sugar levels by monitoring the changes in the dielectric constant of the material underneath. The designed stepped impedance resonator operates at 3.528 GHz with a Q factor of 1455. A microfluidic device structure that imitates the blood veins in the human hand was fabricated in PDMS to validate that the sensor can measure changes in glucose concentrations. To test the RF sensor, glucose solutions with concentrations ranging from 0 to 240 mg/dL were injected into the fluidic channels and placed underneath the RF sensor. The shifts in the resonance frequencies of the RF sensor were measured using a network analyzer via its S-11 parameters. Based on the change in resonance frequencies, the sensitivity of the biosensor was found to be 264.2 kHz/mg center dot dL(-1) and its LOD was calculated to be 29.89 mg/dL