Micro and Nanosystems - Volume 3, Issue 2, 2011

Film bulk acoustic resonator (FBAR) usually consists of a sputtered piezoelectric thin film (ZnO) sandwiched by two metal layers (top and bottom electrodes). A resonance condition occurs if the thickness of the piezoelectric thin film is equal to an integer multiple of a half of the acoustic wavelength. The fundamental resonant frequency is then inversely proportional to the thickness of the piezoelectric material used. The first generation of FBAR was reported in the early 1980s. Since then, it has gained mainstream attention both as filters and as high sensitivity sensors. Due to the small size, low fabrication cost and high sensitivity, FBAR has great potential to be employed in a variety of sensing applications. On one hand, it can be used as a mass sensor, because the resonant frequency of FBAR decreases linearly with the mass accumulated on top of the resonator. Different coatings can be applied on FBAR for the absorption of various sensing targets, especially in biological applications. On the other hand, environmental factors, such as ultraviolet radiation, relative humidity and some harmful gases can alter the density of the ZnO film in FBAR. In this way, it is feasible to utilize FBAR as environment monitors. The aim of this special issue is to give a brief overview of the latest research progress on FBAR sensors. It will contain a collection of works from the most active research groups in this field. The first paper discussed the design of a FBAR oscillator to improve the sensitivity of the device, especially aimed at biological sensing applications. The second paper explored the response of FBAR to infrared radiation. In the third paper, the ZnO based FBAR sensor was applied to monitor ultraviolet radiation, relative humidity and different gases, such as acetone (reducing gas) and ozone (oxidizing gas). The last paper investigated different structures of FBAR (thickness field excitation FBAR and later field excitation FBAR) and compared their distinct response behaviors to DC voltage. These works can serve as a useful reference for researchers to have a comprehensive understanding of the current research status of FBAR sensors and the trends for future investigation.

We present a design of FBAR (film bulk acoustic resonator) oscillator to achieve low phase noise. Between the series and parallel resonances, FBAR serves as an inductor in our design. We adopt the Colpitts topology for the oscillator due to its good phase noise performance. FBAR was modeled by Modified Butterworth-Van Dyke circuit in our design. Parasitic inductances of interconnects were well extracted and included in our simulation. By following the proper tuning guidelines, we demonstrated a FBAR oscillator at 1.6GHz with a phase noise of -95dBc/Hz at 10kHz offset and - 115dBc/Hz at 100kHz offset.

This paper described an infrared (IR) radiation sensor based on Film Bulk Acoustic Resonator (FBAR). The resonant frequency of the FBAR decreased linearly when there was IR (peak wavelength at 780 nm) illumination on the device. The sensing mechanism was attributed to the temperature-dependent Young's modulus of the resonator material (ZnO), which subsequently shifted the resonant frequency. The noise equivalent temperature difference (NETD) and the detection limit for 780 nm IR were 16 mK at 25 °C and 2.9 μW/mm2, respectively. This study has proven the feasibility of detecting IR using ZnO film based FBAR.

Different environmental factors, such as ultraviolet radiation (UV), relative humidity (RH) and the presence of reducing (acetone) or oxidizing (ozone) gases, play an important role in the daily life of human beings. Traditionally, semiconductor metal oxide sensors are the major candidates employed to monitor these factors. However, they suffer from low sensitivity, poor selectivity and huge power consumption. In this paper, Zinc Oxide (ZnO) based Film Bulk Acoustic Resonator (FBAR) was developed to monitor UV, RH, acetone and ozone in the environment. FBAR generally consists of a sputtered piezoelectric thin film sandwiched between two electrodes. It has been well developed both as filters and as high sensitivity mass sensors in recent years. FBAR offers high sensitivity and excellent selectivity for various environment monitoring applications. As the sensing signal is in the frequency domain, FABR has the potential to be incorporated in a wireless sensor network for remote sensing. This study extended our current knowledge of FBAR and pointed out feasible directions for future exploration.

This paper describes different DC response in Thickness Field Excitation (TFE) and Lateral Field Excitation (LFE) Film Bulk Acoustic Resonators (FBAR). As DC voltage was applied to TFE FBAR, the resonant frequency shifted linearly with a sensitivity of -4.5 ppm/V. This DC response was independent of temperature (from room temperature to 80 °C). On the other hand, DC voltage did not have an apparent impact on the quality factor (Q). In the case of LFE FBAR, DC voltage cannot alter the resonant frequency. However, an enhanced Q was observed with increasing voltage. This was a result of the acoustoelectric amplification effect due to the lower acoustic velocity in LFE FBAR.

Some interesting practical uses of nonconducting poly(o-aminophenol) (POAP) films in the fields of bioelectrochemistry and electrocatalysis are discussed in this paper. Particular emphasis is given to the effects of applied potential, solution pH and interferents on the response current of biosensors based on POAP. Also, the behavior of POAP to improve the electroactivity of some electrocatalysts and its own electrocatalytic activity in coenzyme detection are described.

Droplet coalescence is one of the most attractive manipulation schemes in droplet-based microfluidic systems, which enables droplet-based functions such as mixing and microreactor to be achieved in lab-on-a-chip applications. This paper systematically presents an overview on techniques used for droplet coalescence in microfluidic systems. In this paper, techniques employed for droplet coalescence are categorized as passive and active types. The basic theory and mechanism behind these techniques are also presented.

The use of microfluidic bioreactor platforms for cell culturing holds considerable promise for a range of fields which include drug discovery, tissue engineering, bioprocessing optimisation and cell based screening studies. Microfluidic bioreactor systems have length scales that are well matched to the physical dimensions of most cells and microorganisms. In view of this, microfluidic bioreactors have attractive features which make them ideal to study the behaviour of cells and their internal organisation in their native microenvironment. Due to their small footprint microbioreactor platforms offer a number of advantages over conventional macroscale systems including improved biological function, higher quality cell-based data, reduced volume of reagents, ease of integration and lower cost. This review highlights the basic concepts, designs and operational requirements of microbioreactors for cell based studies. An illustrative outline of different applications of microbioreactors and some indication of new trends and progress in recent years are provided. Specific examples of applications of microbioreactors are drawn for cytotoxicity assays, tissue engineering, stem cells, microbial fermentations, single cell analysis and in vitro fertilisation.

Calcium silicate nanosheets have been synthesized by a hydrothermal deposition process using silicon dioxide and calcium oxide as the raw materials. X-ray diffraction and electron microscopy observations show that the width and thickness of the calcium silicate nanosheets with triclinic calcium silicate structure are about 10 μm and 100-200 nm, respectively. Hydrothermal deposition temperature and time have important roles in the formation and size of the calcium silicate nanosheets. The nanosheets exhibit strong blue light emission centered at 421 nm and 472 nm suggesting that the nanosheets have good optical properties.

This paper presents simulations of 3D nanoscale flow inside non-smooth pipe with molecular dynamics simulation method. For nanoscale flow problems the influence of non-smooth surface is very important. This work constructs a nanoscale pipe with non-smooth inside surface and analyzes the steady liquid flow in the non-smooth pipe. The distribution of density, temperature and flow velocity is studied. Lennard-Jones potential is used in the simulations. Differences are obvious between smooth and non-smooth solutions. The numerical results show that the non-smooth surface should be considered for many nanoscale flow problems. The results of this paper are helpful to analyze wetting and non-wetting phenomenon at a solid-fluid interface.