Gokhan Hatipoglu

High-Speed Ultrasmooth Etching of Fused Silica Substrates in SF 6 , NF 3 , and H 2 O-Based Inductively Coupled Plasma Process

This paper presents a new paradigm for high aspect ratio etching of fused silica substrates using a modified inductively coupled plasma (ICP) etch chamber. In particular, we have incorporated a stainless steel gas diffuser ring on the mechanical substrate clamping plate of the etcher to introduce NF₃ and H₂O gases right above the wafer, whereas SF₆ is introduced through the ICP source. This configuration of plasma etching allows for incomplete breakdown of NF₃ + H₂O gas mixture, thereby creating a high local density of F, NF_x, and HF (Hydrogen Fluoride) while achieving large flux of SF_x⁺ ion bombardment. Using this configuration, source power of 2500 W, substrate power of 400 W, and SF₆/NF₃/H₂O flow rates of 60/100/50 sccm, we were able to achieve a surface roughness of ~5 Å at an etch rate of ~1 μm/min. In situ residual gas analysis of the plasma conditions show high concentrations of F, HF, and SF_x, along with a large concentration of NF_x species. The highest etch rate was also found to be a function of the ion flux. The anisotropy of the etch was enhanced by the formation of an inert nickel fluoride/oxide skin layer on the sidewalls of the etched features.

A highly aromatic and sulfonated ionomer for high elastic modulus ionic polymer membrane micro-actuators

A high modulus, sulfonated ionomer synthesized from 4,6-bis(4-hydroxyphenyl)-N,N-diphenyl-1,3,5-triazin-2-amine and 4,4?-biphenol with bis(4-fluorophenyl)sulfone (DPA-PS:BP) is investigated for ionic polymer actuators. The uniqueness of DPA-PS:BP is that it can have a high ionic liquid (IL) uptake and consequently generates a high intrinsic strain response, which is >1.1% under 1.6?V while maintaining a high elastic modulus (i.e. 600?MPa for 65?vol% IL uptake). Moreover, such a high modulus of the active ionomer, originating from the highly aromatic backbone and side-chain-free structure, allows for the fabrication of free-standing thin film micro-actuators (down to 5??m thickness) via the solution cast method and focused-ion-beam milling, which exhibits a much higher bending actuation, i.e. 43??m tip displacement and 180?kPa blocking stress for a 200??m long and 5??m thick cantilever actuator, compared with the ionic actuators based on traditional ionomers such as Nafion, which has a much lower elastic modulus (50?MPa) and actuation strain.

Micromachined magnetoflexoelastic resonator based magnetometer

In this paper, we demonstrate the performance of a magnetoflexoelastic magnetometer consisting of a micromachined ultra-thin (7.5 μm) quartz bulk acoustic resonator on which 500 nm thick magnetostrictive Metglas® (Fe85B5Si10) film is deposited. The resonance frequency of the unimorph resonator structure is sensitively affected by the magnetostrictively induced flexoelastic effect in quartz and is exploited to detect low frequency (<100 Hz) and nanoTesla magnetic fields. The resonance frequency shift is measured by tracking the at-resonance admittance of the resonator as a function of the applied magnetic field. The frequency shifts are linearly correlated to the magnetic field strength. A minimum detectable magnetic flux density of ∼79 nT has been measured for 10 Hz modulated magnetic field input signals which corresponds to a frequency sensitivity of 0.883 Hz/μT.

Experimental studies in magnetically induced transverse force-frequency effect in thin quartz microresonators

In this work, the transverse force-frequency sensitivity of magnetostrictive Metglas® (Fe85B5Si10) thin film coated AT-cut thickness shear mode quartz thin plate microresonator (500 μm × 500 μm × 19 μm) is experimentally measured and modeled in Lagrangian formulation by coupling magnetostrictive deformation equations with the basic plate equations from the theory of small deformation. The quartz plate resonator is fabricated by micromachining techniques and released into fixed-free structure using focused ion beam milling. Application of a magnetic field results in the out-of-plane bending of the structure due to elastic coupling between the magnetostrictive Metglas® and quartz resonator layers. As a result of the transverse loading and out-of-plane bending, the admittance characteristics of the resonator shifts, and these shifts are recorded in real time utilizing a network analyzer. The sensitivity is experimentally measured to be 162.3 mdeg/Oe for phase, corresponding to a frequency sensitivity of Δf/H = 11 Hz/Oe. The equivalent force-frequency sensitivity can then be calculated as 2.36 μN/Hz using the developed model. The coupled domain analysis fits well with the experimental data. Further reduction of quartz thickness and optimization of the thickness ratio of the magnetostrictive to quartz layers offers the possibility of exploiting the stress sensitivity of plate microresonators as sensitive magnetic field sensors capable of low nanoTesla to picoTesla level magnetic flux densities.

Magnetoviscoelastic Ferrofluid-Based Magnetometer

In this paper, we present a novel concept for magnetic sensing that is based upon the sensitive monitoring of the magnetoviscoelastic effects in magnetic nanoparticle containing ferrofluids using micromachined shear mode bulk acoustic wave quartz crystal resonators (μQCR). The unique sensor concept is based on the application of, hitherto unexplored, magnetic field induced viscoelastic response of a thin interfacial ferrofluid layer placed atop a high-frequency shear wave quartz resonator, which can be sensitively monitored through the at-resonance impedance characteristics of the resonator. The high magnetic susceptibility of ferrofluid suspensions results in the modulation of the viscoelasticity due to applied magnetic fields. A bias magnetic field perpendicular to the resonator surface was applied to realize a dense agglomeration of the ferrofluid particles at the immediate interface of the resonator surface. Viscoelastic changes due to in-plane incident magnetic field shifts the at-resonance conductance characteristics of μQCR, and is tracked in real time to achieve a novel magnetic sensing mechanism to detect and quantify the low-frequency low strength magnetic fields. For improved sensitivity, the in-plane sensed magnetic flux density is concentrated using a high relative permeability (μr = 7000) thin film of Metglas (Fe85B5Si10) deposited on the resonator electrode. Furthermore, by patterning the Metglas film in a bow-tie shape and aligned at the center of the μQCR electrode, both 2-D vector sensing and improvement in the sensitivity were achieved. Using these improvements, a minimum detectable field of 1.5 nT/√ Hz at 1 Hz has been experimentally demonstrated.

Application of ionic liquid doped ionomers for organic vapor sensing

In this work we explore the use of room temperature ionic liquid doped ionomers as the sensing material for volatile organic compounds (VOCs). Pixels of a monolithic, micromachined quartz crystal resonator (μQCR) array were functionalized using a sulfonated, aromatic, high thermal stability polymer as a template which is able to contain high amount of ionic liquids (up to 200 wt %). Ionic liquid: [C4mim][BF4] (1-butyl-3-methylimidazolium tetrafluoroburate) was used as the detection material. Exposure to VOCs results in mass loading and changes in the viscoelastic properties of the ionomer and manifests as changes in the frequency and phase of the quartz resonators.

Ion distribution in ionic electroactive polymer actuators

Ionic electroactive polymer (i-EAP) actuators with large strain and low operation voltage are extremely attractive for applications such as MEMS and smart materials and systems. In-depth understanding of the ion transport and storage under electrical stimulus is crucial for optimizing the actuator performance. In this study, we show the dominances of ion diffusion charge and we perform direct measurements of the steady state ion distribution in charged and frozen actuators by using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). High temperature actuators that consist Aquivion ionomer membrane and high melting temperature ionic liquid 1-butyl-2,3-dimethylimidazolium chloride (BMMI-Cl]) served in this study. Electrical impedance, I-V characteristics, and potential step charging of the actuator are characterized at 25°C and 100°C. The conductivity of the actuator is 0.3mS/cm at 100°C and 2.9μS/cm at 25°C, respectively. The electrochemical window of the device is 3V and a 2mm tip displacement is observed under 2.5V 0.2Hz at 100°C. A semi-quantitative depth profile of the relative ion concentration in charged and frozen actuators is measured by ToF-SIMS. The result shows that, unlike semiconductors, ions do not deplete from the electrodes with same signs. Due to a strong cluster effect between the ions, Cl- and BMMI+ accumulate near both cathode and anode. Furthermore, the profile indicates that the ion size difference causes the BMMI+ space charge layers (~6um) much thicker than those of Cl- (~0.5um).

Passively-powered wireless micromachined quartz magnetoflexoelastic magnetometer

Highly sensitive, wirelessly powered, and maintenance-free sensors are of great interest to the biomedical, geological, hazardous environment, and traffic control communities. This work demonstrates the passively-powered wireless operation of a magnetoflexoelastic magnetometer. The wireless coupling is achieved using a coupled near-field resonant loop antennas, which excite the high Q-factor (∼6000) micromachined quartz resonator. Magnetostrictive curves are acquired both wired and wirelessly at distances up to 45 mm to confirm the phenomenon is magnetoflexoelastic in nature. A 49.1 Hz/Oe sensitivity was achieved in wireless operation and the ultimate detectable limit was 7 μΤ at 0.5 Hz.

A modified inductively coupled plasma for high-speed, ultra-smooth reactive phase etching of silica glass

We report on the etching of borosilicate glass substrates in a conventional and modified inductively coupled plasma - reactive ion etch (ICP-RIE) tool. We present the etch rates and surface roughness of borosilicate glass in various fluorine based plasmas using C₄F₈, SF₆, Ar, NF₃, and H₂O gases. In the conventional ICP-RIE etching mode an etch rate of 0.55 μm/min at a rms surface roughness of 25 nm was obtained at C₄F₈, SF₆ flow rates of 5 sccm, O₂ flow rate of 50 sccm, 2000 W of ICP power, 475 W of substrate power. A maximum etch rate of 0.67μm/min was obtained at a high rms surface roughness of 450 nm by increasing flow rate of C₄F₈ to 50 sccm. Using the modified ICP-RIE system consisting of a gas diffuser ring clamped to the substrate holder, the physical component of the etching was considerably reduced and we have been able to achieve etch rates ~0.72 μm/min with surface smoothness of ~1 nm for borosilicate glass and fused silica respectively after 5 minutes etches.

Modeling perturbations induced in plate resonator characteristics due to flexural bending

In this work, the changes in frequency occurring in thickness-shear mode quartz resonators due to bending are discussed and modeled. First order strain gradients and second and third order elastic coefficients for micromachined thin plate quartz resonators are important in the calculation of the frequency shifts. These gradients and coefficients are summarized and derived using Lees theory. This theory is later combined with quartz/magnetostrictive layer unimorphs, where the tip deflection formulations are given. As a case study, the theoretical frequency shifts in a micromachined quartz resonator laminated with magnetostrictive thin film is calculated and later compared with experimental data. The combined model fits well with the experiments.