Onur Tigli

Zinc oxide nanostructures: from growth to application

Zinc oxide’s (ZnO) physical and chemical properties make it a viable and extremely attractive compound to use in a variety of nanotechnology applications. Some of these applications include biomedical, energy, sensors, and optics. As the research in ZnO nanostructures continue to grow, it has inspired a whole host of new innovative applications. Complementing its unique chemical qualities, it also has a simple crystal-growth technology and offers significantly lower fabrication costs when compared to other semiconductors used in nanotechnology. Several processes have been developed in order to synthesize high quality ZnO nanostructures—specifically in the case of nanowires. Here we offer a comprehensive review on the growth methods currently employed in research, industry, and academia to understand what protocols are available to meet specific needs in nanotechnology. Methods examined include: the vapor–liquid–solid, physical vapor deposition, chemical vapor deposition, metal–organic chemical vapor deposition, and the hydrothermal-based chemical approach. Each of these methods is discussed and their strengths and weaknesses are analyzed with objective comparison metrics. In addition, we study the current state-of-the-art applications employing ZnO nanostructures at their core. A historical perspective on the evolution of the field and the accompanying literature are also presented.

Piezoelectric energy harvesting: State-of-the-art and challenges

Piezoelectric energy harvesting has attracted wide attention from researchers especially in the last decade due to its advantages such as high power density, architectural simplicity, and scalability. As a result, the number of studies on piezoelectric energy harvesting published in the last 5 years is more than twice the sum of publications on its electromagnetic and electrostatic counterparts. This paper presents a comprehensive review on the history and current state-of-the art of piezoelectric energy harvesting. A brief theory section presents the basic principles of piezoelectric energy conversion and introduces the most commonly used mechanical architectures. The theory section is followed by a literature survey on piezoelectric energy harvesters, which are classified into three groups: (i) macro- and mesoscale, (ii) MEMS scale, and (iii) nanoscale. The size of a piezoelectric energy harvester affects a variety of parameters such as its weight, fabrication method, achievable power output level, and potential application areas. Consequently, size-based classification provides a reliable and effective basis to study various piezoelectric energy harvesters. The literature survey on each scale group is concluded with a summary, potential application areas, and future directions. In a separate section, the most prominent challenges in piezoelectric energy harvesting and the studies focusing on these challenges are discussed. The conclusion part summarizes the current standing of piezoelectric energy harvesters as possible candidates for various applications and discusses the issues that need to be addressed for realization of practical piezoelectric energy harvesting devices.

Fabrication and Characterization of a Surface-Acoustic-Wave Biosensor in CMOS Technology for Cancer Biomarker Detection

Design, fabrication, and characterization of a novel surface acoustic wave (SAW) biosensor in complementary metal-oxide semiconductor (CMOS) technology are introduced. The biosensor employs a streptavidin/biotin-based five-layer immunoassay for detecting a prominent breast cancer biomarker, mammoglobin (hMAM). There is a growing demand to develop a sensitive and specific assay to detect biomarkers in serum that could be used in the early detection of breast cancer, determining prognosis and monitoring therapy. CMOS-SAW devices present a viable alternative to the existing biosensor technologies by providing higher sensitivity levels and better performance at low costs. Two architectures (circular and rectangular) were developed and respective tests were presented for performance comparison. The sensitivities of the devices were analyzed primarily based on center frequency shifts. A frequency sensitivity of 8.704 pg/Hz and a mass sensitivity of 2810.25 m2 /kg were obtained. Selectivity tests were carried out against bovine serum albumin. Experimental results indicate that it is possible to attach cancer biomarkers to functionalized CMOS-SAW sensor surfaces and selectively detect hMAM antigens with improved sensitivities, lowered costs, and increased repeatability of fabrication.

A Novel Saw Device in CMOS: Design, Modeling, and Fabrication

The design, finite element modeling, fabrication, and characterization of a novel surface acoustic wave (SAW) delay line for bio/chemical and telecommunication applications in CMOS technology are introduced. A full modeling was carried out. The devices are designed in a standard semiconductor foundry 1.5-mum two-metal two-poly process. A unique maskless postprocessing sequence is designed and completed. The three postprocessing steps are fully compatible with any standard integrated circuit technology such as CMOS. This allows any signal control/processing circuitry to be easily integrated on the same chip. ZnO is used as the piezoelectric material for SAW generation. A thorough characterization and patterning optimization of the sputtered ZnO was carried out. The major novelties that are introduced in the SAW delay line features are the embedded heater elements for temperature control, compensation, and acoustic absorbers that are designed to eliminate edge reflections and minimize triple transit interference that is amplified by edge reflections. Both of these attributes are designed by using CMOS materials without disturbing SAW performance

Experimental study of PDMS bonding to various substrates for monolithic microfluidic applications

This paper presents a comprehensive experimental study and characterization of material and bonding of PDMS based structures to various substrates. A previously published method [1] of bonding is further improved with the inclusion of more substrate material and additional characteristics. Uncured PDMS is used as an adhesive to bond PDMS devices reversibly to various substrates including a number of commonly used substrate materials that are not supported by the widely used plasma treatment method. We have optimized parameters such as PDMS base to curing agent ratio, curing temperature, and PDMS device age to obtain better bond strengths and quality. Bond strengths are presented for semiconductor substrates (silicon, zinc oxide, and silicon dioxide), metals (gold, aluminum), photoresists (SU-8, AZxx) and glass. Silicon based substrates experienced minor amounts of surface residue, but the method is fully reversible for other tested substrates. Bond strengths were measured as maximum endurable pressure between PDMS and substrates. Maximum average bond strengths of more than 0.4 MPa were achieved for substrates with Si-O groups. Other substrates exhibited maximum average bond strengths in the range 0.2–0.3 MPa. Also presented is a method that avoids alignment step for PDMS microfluidic device bonding, named the non-aligned method. This method provides bond strengths of more than 0.1 MPa. Presented methods do not need special equipment or processes such as plasma generators or temperature increases. Biocompatibility tests are performed for materials used in fabrications to ensure applicability in bio-sensing related devices.

MEMS Scale PVDF-TrFE-Based Piezoelectric Energy Harvesters

This paper presents the design, the fabrication, and the performance characterization of microelectromechanical systems (MEMS) scale cantilever-type piezoelectric energy harvesters (PEHs) that utilize the piezoelectric polymer polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). Ranges are determined for device dimensions according to the calculations based on mathematical models. Designed devices were fabricated using standard MEMS fabrication techniques. Electrodes were formed with sputtered Al and Ti/Al thin films, and a 1.3-μm-thick PVDF-TrFE film was deposited using spin coating. Cantilevers were suspended using a two-step process: backside DRIE to perform the bulk etch, followed by XeF2 gaseous etch for the final release. Remnant polarization and coercive field of the fabricated devices were measured as 6.1 μC/cm2 and 74.9 V/μm, respectively. Piezoelectric performances were evaluated by a press-and-release type of measurement. For these measurements, custom-made probe tips attached to micropositioners were used. Based on the experimental results, maximum power output was calculated as 35.1 pW for a peak tip displacement of 500 μm from a 1200 μm × 300 μm cantilever, which corresponds to a power output density of 97.5 pW/mm2. The proposed method has the potential to create the PEHs that are monolithically integrated with complementary metal-oxide-semiconductor circuits and lead to self-sustained low power electronics.

Integrated CMOS surface acoustic wave gas sensor: design and characteristics

The development of inexpensive and miniaturized Surface Acoustics Wave (SAW) gas sensors that are highly selective and sensitive is described. These sensors are implemented with micro-electro-mechanical systems (MEMS) in CMOS technology. IDT equivalent circuit and model for SAW delay line is introduced. Simulation results are included for characterization and design of the sensor. In this paper we will describe the design and post-processing steps to implement SAW device in CMOS technology. Design parameters of this device were obtained using modeling equivalent circuit to characterize sensor. Two approaches were used in the design of the SAW gas sensor that will be discussed. A CMOS chip was fabricated using MOSIS. Thin film ZnO was grown on Silicon based die and its characterizations is presented.

A Novel Circular SAW (Surface Acoustic Wave) Device in CMOS

The design, modeling and fabrication of a novel circular SAW device in CMOS (complementary metal oxide semiconductor) are introduced. The results obtained by Tigli and Zaghloul (2007) demonstrate that it is possible to design and fabricate SAW based sensors in CMOS with comparable performance to conventional devices. It is of great interest to improve the transfer characteristics and to reduce the losses of conventional rectangular SAW architectures for obtaining highly selective sensor platforms. Performance deficiencies of regular SAW devices in CMOS were addressed with this new architecture for improved performance. A 3D model for the novel architecture was constructed. A detailed finite element analysis was carried out to examine the transient, harmonic and modal behavior of the new architecture under excitation. The devices were fabricated in 0.5 mum AMI technology and the post processing was carried out using the methods developed by Tigli and Zaghloul (2005). The results demonstrate that it is possible to obtain highly oriented surface acoustic waves by using the novel circular architecture. A 17.4 dB insertion loss and a 10.8 MHz of 3dB bandwidth improvement were achieved when compared to a conventional rectangular device.

Design and fabrication of a novel SAW bio/chemical sensor in CMOS

The design, fabrication, post-processing and characterization of a novel SAW (Surface Acoustic Wave) based bio/chemical sensor in CMOS technology is introduced. The sensors are designed in AMI 1.5 mum 2 metal, 2 poly process. A unique maskless post processing sequence is designed and completed. The three post-processing steps are fully compatible with any CMOS technology. This allows any signal control/processing circuitry to be easily integrated on the same chip. ZnO is used as the piezoelectric material for the SAW generation. A thorough characterization and patterning optimization of the sputtered ZnO was carried out. The major novelties that are introduced in the SAW delay line features are: The embedded heater elements for temperature control, compensation and acoustic absorbers that are designed to eliminate edge reflections and minimize triple transit interference. Both of these attributes are designed by using the CMOS layers without disturbing the SAW performance

Finite element method analysis of surface acoustic wave devices with microcavities for detection of liquids

This paper introduces the use of finite element method analysis tools to investigate the use of a Rayleigh type surface acoustic wave (SAW) sensor to interrogate minute amounts of liquids trapped in microcavities placed on the delay line. Launched surface waves in the ST-X quartz substrate couple to the liquid and emit compressional waves. These waves form a resonant cavity condition and interfere with the surface waves in the substrate. Simulations show that the platform operates in a different mechanism than the conventional mass loading of SAW devices. Based on the proposed detection mechanism, it is able to distinguish between variations of 40% and 90% glycerin based on phase relations while using liquid volumes smaller than 10 pl. Results from shallow microcavities show high correlation with sound velocity parameter of the liquid whereas deeper microcavities display high sensitivities with respect to glycerin concentration. Simulated devices yield a maximum sensitivity of −0.77°/(% glycerin) for 16 μm wavelength operation with 8 μm deep, 24 μm wide, and 24 μm long microcavities.