Piezoelectric suspended microchannel resonators

The objective of this PhD thesis is the development of a microfluidic platform for the real-time measurements of the mechanical properties of biological analytes at the single entity level.The systems developed for these purposes consist of piezoelectrically-transduced suspended microchannel resonators (SMRs). Their operation is based on the tracking of their resonance frequency when samples are flowing through the SMR, because the resonance frequency depends on the mechanical properties of the system (e.g. stiffness or mass).The structure of the SMRs is made of low-stress silicon nitride (ls-SiNx), a material offering multiple advantages (good mechanical properties, high temperature and chemical resistance, transparency, amongst others). The cross-sectional area of the fluidic channel is designed to accommodate the detection of red blood cells and circulating tumor cells, biological entities that are known to have mechanical properties correlating with various pathologies, including cancer. The length of the devices ranges from 50 to 1000 ÎŒm, covering multiple orders of magnitudes in resonance frequencies. Each chip consists of a microfluidic network comprising 2 to 4 SMRs of different lengths to enable measurements of the same analyte with different devices.The fabrication relies on a 7-mask process flow. The manufacture of the channels requires two electron-beam lithographies, multiple depositions of ls-SiNx, as well as a polysilicon sacrificial layer. The piezoelectric (PZE) stack is fabricated on top of the flat channel surface and consists of platinum electrodes and an aluminum nitride active layer. Fluidic accesses are then etched from the backside of the wafer before the resonators are released on the front.The chips fabricated can be assembled in a dedicated experimental setup. A fluidic connector along with the implementation of o-rings ensure leak-free delivery of fluidic samples from a pressured-controlled pump to the SMR chip. The electrical transduction is achieved with a PCB that is connected to a lock-in amplifier. A vacuum chamber sealed with o-rings is pumped and guarantees operation of SMRs at low pressure, while temperature control is achieved with a Peltier module and a thermistor.Multiple characterization steps are performed to assess the performance of the devices from a piezoelectric standpoint both in static and dynamic mode. The frequency stability of empty and filled SMRs is also studied through measurements of the Allan deviation, an established metrics for resonators. Measurements with a 200-ÎŒm-long SMR demonstrate a theoretical buoyant mass resolution of 150 ag (at an integration time of 400 ms), a performance close to the state of the art for devices of similar dimensions.The SMRs are also evaluated as density sensors and show that a theoretical density resolution close to 0.5 g/m3 is achievable, which is an order of magnitude better than commercial devices. It is also demonstrated that the piezoelectric nature of the transduction allows to estimate how the resonators are affected by the heat absorption inherent to the operation of an optical detection system such as a Laser Doppler Vibrometer. Finally, SMRs enable the measurement of the buoyant mass of a population of bacteria isolated from lake water at the single analyte level. To the best of our knowledge, this is the first characterization of the mechanical properties of single entities with piezoelectric suspended microchannel resonators.

PZE-transduced Suspended Microchannel Resonators for sensing applications

Annalisa De Pastina

This PhD thesis aims at developing a system which can measure the mechanical properties of fluidic samples in the picoliter range. The ultimate goal is the characterization of cancer cells and viscoelastic fluids (i.e. biological fluids), in order to study their mass and stiffness for diagnostic applications. The core of this project is the design, fabrication and characterization of hollow micromechanical resonators containing embedded microfluidic channels and provided with integrated piezoelectric (PZE) transduction. The measurement strategy consists in monitoring the device resonance frequency over time, while the analyte flows through the microchannel. Arrays of singly-clamped and doubly-clamped resonators are made of low stress Silicon Nitride (ls-SiNx) and vibrate in a frequency range from tens of kHz to few MHz. Microfluidic channels span 25%, 60% and 100% of the cantilever length in order to compare the resonance frequency shift induced by the same particle at different positions, along identically-designed beams. The goal is to disentangle analyte mass and stiffness contribution from the suspended microchannel resonator (SMR) mechanical response. The cross section of the suspended fluidic channel, 6 um x 10 um, is designed to measure particles in the same size range (i.e. red blood cells or circulating tumor cells). SMR energy dissipation is studied via FEM simulation, as a function of fluidic properties and geometrical dimensions. A coupled fluid-structure interaction (FSI) problem, based on linear elasticity and linearized Navier Stokes equations, is studied. The model is found well in agreement with theory and experiments presented in literature, considering the viscosity range of interest in case of biosensing application. The fabrication of transparent and PZE-transduced suspended microchannel resonators is achieved via a 6-mask process flow. 250, 500, 750 and 1000 um long singly- and doubly-clamped SMRs are fabricated. Microfluidic channels are defined via the etching of high aspect ratio-trenches through a sacrificial layer of polysilicon, and filled with ls-SiNx. Discontinuous etch apertures are defined on top of the buried fluidic network and enable channel emptying in 25 minutes via KOH etching. PZE electrodes in platinum and aluminum nitride are deposited via sputter deposition and provide independent actuation and readout of each resonator, for the first time in SMR arrays. The development of an experimental platform enables fluidic and electrical connections, as well as temperature control during devices characterization. The mechanical response of fabricated sensors is measured in air and vacuum environment, while channels are filled with air, water and Isopropyl Alcohol. PZE electrodes demonstrate efficient actuation, in the order of 1.5 nm/V for 250 um- long singly-clamped SMRs. However, PZE detected signals result very low in amplitude, thus optical detection via laser Doppler vibrometer is preferred. Frequency stability of fabricated sensors is studied in order to estimate their sensing performances and characterize their physical limitations. 250 um-long SMRs filled with DI water exhibit a frequency stability of 30 ppb at 400 ms integration time, in air. This result is in agreement with values found in literature for SMRs sensors and translates in an estimated mass sensitivity of few femtograms.

EPFL2018

Piezoelectric thin films for bulk acoustic wave resonator applications

Roman Lanz

Bandpass filters for microwave frequencies realized with thin film bulk acoustic wave resonators (FBAR) are a promising alternative to current dielectric or surface acoustic wave filters for use in mobile telecommunication applications. With equivalent performance, FBAR filters are significantly smaller than dielectric filters and allow for a larger power operation than SAW filters. In addition, FBARs offer the possibility of on-chip integration, which will result in substantial volume and cost reduction. The first passive FBAR devices are now appearing on the market. They mainly cover needs in miniaturized RF-filters for the new bands around 2 GHz. A FBAR is essentially a thin piezoelectric plate sandwiched between two electrodes and acoustically isolated from the environment for energy trapping purposes. If the isolation is effectuated by an acoustic Bragg reflector, one speaks of solidly mounted resonators (SMR). Piezoelectric aluminum nitride (AlN) thin films are predominantly used in the emerging FBAR technology because AlN exhibits a sufficient electromechanical coupling coefficient kt2 , low acoustic losses at microwave frequencies, a low temperature coefficient of frequency, and its chemical composition is compatible with CMOS requirements. This thesis has two research directions. In the first part, FBAR structures based on AlN thin films were investigated for applications at X-band frequencies (7.2-8.5 GHz), i.e. operating at much higher frequencies than the ones used for present products. The goal was to identify property limitations related to such high frequencies, and to demonstrate to industry high performing SMR filters at 8 GHz. In the second part, a new material for FBAR devices was studied. The motivation is that AlN allows for a restricted filter bandwidth only, limited by its coupling factor of maximal 7%. Monocrystalline KNbO3 appears as an ideal alternative with its high coupling factor kt2 of 47%, and relatively large sound velocity of 8125m/s for longitudinal waves along the [101] direction. Piezoelectricity of KNbO3 films grown on electrodes has never been characterized. Single crystal results indicate that the optimal film texture would be (101). In this thesis, the growth of KNbO3 films on Pt electrodes was studied with the goal to achieve this texture uniformly, and to characterize piezoelectric properties. X-band FBAR's were first studied with numerical simulations based on a one-dimensional theory of the thickness-extensional bulk acoustic wave (BAW). Thickness, acoustic properties and electrical conductivity of the electrodes have a large impact on the resonator characteristics. There are conflicting requirements with respect to optimum acoustic and electrical properties of the electrode materials. An optimum thickness was calculated for 8GHz FBARs that use Pt bottom and Al top electrodes. The characteristics of ladder filters have been calculated based on the impedances of single resonators. The adjustable filter parameters, i.e. the areas of series and shunt resonators, frequency de-tuning between series and parallel resonators, and number of π-sections were screened for a process window offering maximum filter bandwidth with lowest ripple and low insertion loss for a given out-of-band rejection. An important result of the numerical simulations was that the bandwidth of ladder filters can be doubled by de-tuning the series and parallel resonators by more (1.3 times) than the difference of resonance and anti-resonance frequency. This also leads to a flatter passband while keeping the ripples below ±0.2dB. Solidly mounted resonators and filters were fabricated using an acoustic multilayer reflector consisting of AlN and SiO2 λ/4 layers. All films were sputter deposited in a high vacuum sputter cluster system with 4 process chambers. The films were patterned using standard photolithography and dry etching processes. The SMR exhibited a strong and spurious-free resonance at 8GHz with a high quality factor of 360 and electromechanical coupling coefficient of 6.0%. The temperature coefficient of frequency was -18ppm/K, and the voltage coefficient of frequency was -72ppm/V. Passband ladder filters with T- and π-topology consisting of 3 to 14 SMR were successfully demonstrated with a center frequency of 8GHz. These filters were optimized for maximum bandwidth and exhibited an insertion loss of 5.5dB, a rejection of 32dB, a 0.2dB bandwidth of 99MHz (1.3%), and a 3dB bandwidth of 224MHz (2.9%). There was good correspondence between measured and simulated filter and resonator characteristics. For perfect agreement, parasitic elements needed to be taken into account. These were a series resistance of 5Ω and a parallel conductance of 2mS in case of single resonators. The series resistance can be explained with resistive losses in the electrodes, whereas the parallel conduction was due to conduction along the surface. For π-filters, an additional series inductance of 100pH was needed to obtain a satisfactory fit. This inductance increased the out-of band rejection and insertion loss. Besides the group delay variation, all industrial specifications were met. KNbO3 was in-situ sputter deposited at 500 to 600°C using a rf magnetron source. A dedicated sputter chamber with load-lock and oxygen resistant substrate heater was built for this purpose. The high volatility of potassium oxide requires a potassium enrichment of the target. Targets with several excess concentrations (in the form of K2CO3) were studied. Stoichiometric KNbO3 films were obtained with targets containing 25 and 40% excess K. Zero and 10% excess yielded K deficient films, whereas 100% and 200% excess K led to highly unstable targets with K accumulation on the target surface, resulting in K rich second phases. The potassium-to-niobium ratio in the films depends strongly on sputter pressure and substrate temperature. Dense films, nucleated with cubic {100} texture, were obtained on platinized silicon substrates with a 10nm thick IrO2 seed layer at substrate temperatures of 520°C. At lower temperatures the films were amorphous, and at higher temperatures the films were composed of individual and facetted KNbO3 grains. The cubic high-temperature {100} texture results in a mixed (101)/(010) texture in the orthorhombic room temperature phase. The measured relative permitivity of 420 indicates that both orientations are equally present. Micro-Raman confirms the orthorhombic line splitting. Piezoelectrical and ferroelectrical activity were verified by means of a piezoelectric sensitive atomic force microscope. A very large piezoelectric activity was observed on some of the grains, and the polarization could be switched on most of the grains. However, the average d33,f = e33/c33, as measured by means of laser interferometry, showed a modest value of 24pm/V. The effective coupling factor is derived as kt2=2.8%, which is small relative to the theoretical value of 47%. The high dielectric constant and the absence of piezoelectric activity along the [010] direction are responsible for the reduction of the kt2 factor. Film roughness, complexity of deposition process and open poling issue make KNbO3 integration into BAW devices a difficult task.

EPFL2004

Micromachined tunable devices based on silicon integrated BaxSr1-xTiO3 thin films

Andreas Nöth

Thin Film Bulk Acoustic Wave Resonators (TFBARs) had been developed a decade ago and since then were implemented extensively in mobile communications devices. The "heart" of a TFBAR consists of a piezoelectric film that operates as an acousto-electric transducer, stabilizing the transmission at a given predetermined frequency. For reasons such as space economy in hand-held devices, it is of interest to make these TFBARs tunable, so that a single TFBAR is multi-band responsive. This thesis demonstrates for the first time electrically tunable, single-component TFBARs. A theory describing the tuning behavior of dc bias induced acoustic resonances was developed. Then we made the hypothesis that dc bias induced piezoelectric BaxSr1-xTiO3 (BST) thin films – namely, paraelectric, non-piezoelectric films operating under dc bias – can be used to make electrically tunable TFBARs and that the devices can be switched on or off depending on the dc bias state. The devices were then fabricated: We integrated BST lms onto silicon substrates, micromachined the substrate to create the TFBARs and a new type of suspended planar capacitor which were then characterized, analyzed, and modeled, demonstrating successfully the new concept: We developed a theory describing the electrical tuning behavior of the dc bias induced acoustic resonances in paraelectric thin lms in terms of material parameters. The field dependent constitutive piezoelectric equations were derived from the Landau free energy P-expansion by taking the linear and nonlinear electrostrictive terms as well as the background permittivity into account. We considered two modes of excitation for the tuning of the acoustic resonances, namely the thickness excitation (TE) mode and the lateral field excitation (LFE) mode. The tuning behavior of the two types of resonators based on BST thin films was modeled and discussed. For the modeling we calculated the relevant tensor components controlling the tuning of the BST resonators from the available literature data. The fabrication of the membrane-type TFBARs was realized by integrating BST thin films onto silicon substrates and using micromaching technologies. We showed that the developed TFBARs can be switched on or off with a dc bias. At a dc electric field of 615 kV/cm we observed a tuning of -2.4% (-66 MHz) and -0.6% (-16 MHz) for the resonance and antiresonance frequencies of the device, while the resonance frequency at a dc electric field extrapolated to 0 kV/cm was 2.85 GHz. The effective electromechanical coupling factor k2eff of the device increased up to 4.4%. The tuning was non-hysteretic. The Quality-factor (Q-factor) of the device was about 200. The developed micromaching processes for the TFBARs were used to fabricate coplanar BST capacitors on silicon. Micromaching was used to remove the Si substrate under the active area of the device. Comparing this new micromachined coplanar capacitor with conventional non-micromachined capacitors, we demonstrated that the removal of the substrate from the active device area resulted in a reduction of parasitic effects. The micromachined coplanar capacitor showed an increased tunability and a reduced loss tangent in comparison to the non-micromachined capacitor. The micromachined capacitor showed a relative tunability of 37% at a dc electric field of 1100 kV/cm. The loss tangent was 0.08 and 0.06 at zero and maximum dc bias, respectively, at a measurement frequency of 20 GHz. The integration of epitaxial BST thin films on silicon was studied as well because of its potential improvement of the device performance in comparison to devices based on polycrystalline films. Two different electrode/buffer layer systems, YBa2Cu3O7-x (YBCO) / CeO2 / YxZr1-xO2-x/2 (YSZ) and TiN were used for the integration. Epitaxial BST thin films were grown with both structures. For the BST / YBCO / CeO2 / YSZ / Si structure, the BST unit cell was rotated by 45° in the in-plane dimension with respect to the substrate. The BST layer exhibited good structural quality as indicated by a Full Width at Half Maximum (FWHM) of 0.5° of the rocking curve of the BST(002) diffraction peak. For the BST / TiN / Si structure, the BST layer was grown with a cube-on-cube epitaxial relationship on the silicon substrate, thus demonstrating epitaxially grown BST on silicon using conventional bottom electrode that is easily acceptable by the microelectronic industry. However, in this case, the structural quality of the BST layer was reduced in comparison to the BST / YBCO / CeO2 / YSZ / Si structure. The FWHM of the rocking curve of the BST(002) diffraction peak was 2.2°. We established the temperature (T=550 to 600 °C) and pressure (p ≈ 10-7 to 5 × 10-4 Torr) conditions for the growth of epitaxial BST thin films on TiN-buffered Si. At too high temperatures and/or oxygen pressures epitaxial BST thin film growth was impeded due to the oxidation of the TiN layer. By introducing experimental results from the electrical characterization of the micromachined devices with the polycrystalline BST into our developed theory, we could successfully model the tuning behavior of our fabricated TFBARs. The modeling allowed us to de-embed the intrinsic electromechanical properties of a freestanding BST layer. The effect of increasing mechanical load on the tuning performance of the device was modeled and studied experimentally. Under strong mechanical load, the tuning of both resonance and antiresonance frequency was reduced. The effect was attributed to a reduction in the tuning of k2eff of the device and of the sound velocity of the BST layer.

EPFL2009