AlN-based BAW resonators with CNT electrodes for gravimetric biosensing

Solidly mounted resonators (SMRs) with a top carbon nanotubes (CNTs) surface coating that doubles as an electrode and as a sensing layer have been fabricated. The influence of the CNTs on the frequency response of the resonators was studied by direct comparison to identical devices with a top metallic electrode. It was found that the CNTs introduced significantly less mass load on the resonators and these devices exhibited a greater quality factor, Q (>2000, compared to ∼1000 for devices with metal electrodes), which increases the gravimetric sensitivity of the devices by allowing the tracking of smaller frequency shifts. Protein solutions with different concentrations were loaded on the top of the resonators and their responses to mass-load from physically adsorbed coatings were investigated. Results show that resonators using CNTs as the top electrode exhibited a higher frequency change for a given load (∼0.25 MHz cm2 ng−1) compared to that of a metal thin film electrode (∼0.14 MHz cm2 ng−1), due to the lower mass of the CNTelectrodes and their higher active surface area compared to that of a thin film metal electrode. It is therefore concluded that the use of CNTelectrodes on resonators for their use as gravimetric biosensors is a significant improvement over metallic electrodes that are normally employed

AlN ja Sc0.2Al0.8N ohutkalvojen märkäkemiallinen etsaus

Aluminium nitride is a piezoelectric material commonly used in piezoelectric microelectromechanical systems (MEMS) in the form of thin films deposited by sputtering. AlN-based devices are found in wireless electronics in the form of acoustic filters, but they also have prospective applications in a wide variety of sensor systems. To enhance the piezoelectric properties of AlN, some of the Al can be replaced with scandium, which is required for next-generation devices. However, addition of Sc makes both the deposition and patterning of the film more difficult. This work focuses on patterning of AlN and Sc0.2Al0.8N thin films with wet etching. Both materials are etched anisotropically, which in theory enables etching the materials with little deviation from the mask dimensions. However, in practise, undercutting at the mask edges occurs easily making the structures narrower compared to the etch mask. This work investigates and compares the mechanisms and etch rates of AlN and Sc0.2Al0.8N. Tetramethyl ammonium hydroxide was mostly used for etching, but also H3PO4 and H2SO4 were tested. Addition of 20 atom-% Sc lowered the etch rate of the material and resulted in more undercutting. The causes behind mask undercutting were examined by using 11 differently deposited etch masks, and the undercutting was minimized by optimizing the mask deposition, using thermal annealing, and optimizing the etching temperature. Finally, the work identifies and discusses the relevant factors in depositing and patterning the AlN, ScxAl1-xN and mask films

Development of surface micromachined Aluminum Nitride air-bridges for piezoelectric MEMS/NEMS applications by Metal Organic Vapor Phase Epitaxy techniques

Group III-nitrides have attracted considerable attention for piezoelectric Micro/Nano electromechanical (MEMS/NEMS) applications due to their excellent bio compatibility, well developed growth techniques for high quality thin films and structural stability at high temperatures when compared to the commonly used piezoelectric metal oxides. Among the group III-nitrides Aluminum Nitride (AlN) possess superior material properties such as highest piezoelectric coefficient and good mechanical properties. Growth techniques for fabricating group III-nitride MEMS/NEMS by metal organic vapor phase epitaxy (MOVPE) techniques have involved sacrificial layers such as epitaxial group III-nitrides/ alloys, nanocrystalline films and porous interlayers. However, the material properties of the MOVPE grown films on the amorphous sacrificial layers such as silicon oxide have not been adequately investigated to evaluate potential MEMS/NEMS devices such as piezoelectric micro/ nanofluidic channels.;This work demonstrates a process for the fabrication of Aluminum nitride (AlN) thin film air-bridges using MOVPE techniques on silicon templates. Micro-FTIR techniques were used to study the crystallographic orientation of the AlN thin film air-bridges with lateral dimensions as low as 100 mum. FTIR results also show that the wet etching process to remove the underlying sacrificial layer also improves the material properties of the AlN films on SiOx. The study indicates that AlN air-bridges are polycrystalline in nature and are preferentially c-axis oriented after wet etching. Lateral field excitation of the piezoelectric films and laser Doppler vibrometer techniques were combined to investigate the piezoelectric response of the AlN films on the sacrificial layer. Lateral field excitation of the AlN films grown on the amorphous sacrificial layer shows that the AlN films exhibit piezoelectric properties. The displacement of the AlN air-bridges obtained by lateral field actuation is around 1 nm over an air-gap of 130 nm after the removal of the sacrificial layer. However, the mismatch in the coefficient of thermal expansion between the substrate and thin films induces significant residual stress in the heterostructure. The AlN air-bridges on silicon substrate exhibit fracture due to the tensile residual stress exceeding the fracture limit

Piezoelectric aluminum nitride thin films for microelectromechanical systems

This article reports on the state-of-the-art of the development of aluminum nitride (AlN) thin-film microelectromechanical systems (MEMS) with particular emphasis on acoustic devices for radio frequency (RF) signal processing. Examples of resonant devices are reviewed to highlight the capabilities of AlN as an integrated circuit compatible material for the implementation of RF filters and oscillators. The commercial success of thin-film bulk acoustic resonators is presented to show how AlN has de facto become an industrial standard for the synthesis of high performance duplexers. The article also reports on the development of a new class of AlN acoustic resonators that are directly integrated with circuits and enable a new generation of reconfigurable narrowband filters and oscillators. Research efforts related to the deposition of doped AlN films and the scaling of sputtered AlN films into the nano realm are also provided as examples of possible future material developments that could expand the range of applicability of AlN MEM

Design and Implementation of Silicon-Based MEMS Resonators for Application in Ultra Stable High Frequency Oscillators

The focus of this work is to design and implement resonators for ultra-stable high-frequency ( \u3e 100MHz) silicon-based MEMS oscillators. Specifically, two novel types of resonators are introduced that push the performance of silicon-based MEMS resonators to new limits. Thin film Piezoelectric-on-Silicon (TPoS) resonators have been shown to be suitable for oscillator applications due to their combined high quality factor, coupling efficiency, power handling and doping-dependent temperature-frequency behavior. This thesis is an attempt to utilize the TPoS platform and optimize it for extremely stable high-frequency oscillator applications. To achieve the said objective, two main research venues are explored. Firstly, quality factor is systematically studied and anisotropy of single crystalline silicon (SCS) is exploited to enable high-quality factor side-supported radial-mode (aka breathing mode) TPoS disc resonators through minimization of anchor-loss. It is then experimentally demonstrated that in TPoS disc resonators with tethers aligned to [100], unloaded quality factor improves from ~450 for the second harmonic mode at 43 MHz to ~11,500 for the eighth harmonic mode at 196 MHz. Secondly, thickness quasi-Lamé modes are studied and demonstrated in TPoS resonators for the first time. It is shown that thickness quasi-Lamé modes (TQLM) could be efficiently excited in silicon with very high quality factor (Q). A quality factor of 23.2 k is measured in vacuum at 185 MHz for a fundamental TQLM-TPoS resonators designed within a circular acoustic isolation frame. Quality factor of 12.6 k and 6 k are also measured for the second- and third- harmonic TQLM TPoS resonators at 366 MHz and 555 MHz respectively. Turn-over temperatures between 40 °C to 125 °C are also designed and measured for TQLM TPoS resonators fabricated on degenerately N-doped silicon substrates. The reported extremely high quality factor, very low motional resistance, and tunable turn-over temperatures \u3e 80 °C make these resonators a great candidate for ultra-stable oven-controlled high-frequency MEMS oscillators