50 nm thick AlN Films for Actuation and Detection of Nanoscale Resonators

We report on the optimization of a multilayer platinum/aluminum nitride (AlN) stack with an AlN active layer thickness of 50 nm for piezoelectric actuation in nanoelectromechanical systems (NEMS). Experiments in reactive sputtering of AlN were used to find optimal parameters for good crystallinity and low residual stress of a four layer film stack, shown in Figure 1. A direct relationship was measured between the AlN seed layer thickness and the x-ray diffraction (XRD) rocking curve’s full width at half maximum (FWHM) of the AlN active layer. However, no influence of seed layer thickness was found in double beam laser interferometric measurements of the clamped longitudinal piezoelectric coefficient d33,f, relating thickness strain from an applied electric field, of the XRD measured four layer film stacks.

Effect of AlN seed layer on crystallographic characterization of piezoelectric AlN

Waqas Bashir, Annalisa De Pastina, Kaitlin Marie Howell, Ramin Matloub Aghdam, Paul Muralt, Luis Guillermo Villanueva Torrijo

Ultrathin aluminum nitride (AlN) films are of great interest for integration into nanoelectromechanical systems for actuation and sensing. Given the direct relationship between crystallographic texture and piezoelectric (PZE) response, x-ray diffraction has become an important metrology step. However, signals from layers deposited below the PZE AlN thin film may skew the crystallographic analysis and give misleading results. In this work, the authors compare the use of a Ti or AlN seed layer on the crystallographic quality of PZE AlN. The authors also analyze the influence of several AlN seed layer thicknesses on the rocking curve FWHM of PZE AlN and demonstrate a larger effect of the AlN seed layer on the theta-2 theta AlN crystallographic peak for increasing AlN seed layer thickness. Published by the AVS.

2019

Dielectric Actuation Techniques at the Nanoscale: Piezoelectricity and Flexoelectricity

Kaitlin Marie Howell

Micro- and nanoelectromechanical systems (MEMS/NEMS) have long shown their potential to disrupt the established technologies. Over the past 15 years, MEMS have become fundamental components in filters, accelerometers, gyroscopes and gas sensors. MEMS are now an essential part of most of the electronic devices we own. However, their smaller counterpart, NEMS, are still far from industrialization due to two challenges: their fabrication and the transduction of their motion. One transduction technique to control the motion of NEMS is piezoelectricity, which can be directly integrated, has low power consumption and exhibits linear transduction. Piezoelectric aluminum nitride (AlN) deposited by reactive sputtering has been applied in several MEMS applications and recently in NEMS. There is a good understanding of the sputtering parameters and substrate conditions needed to fabricate high quality micrometer thick AlN films, but less is known for nanometer thick films. One part of the thesis was focused on the impact of reactive sputtering parameters and substrate conditions on the c-axis texture and piezoelectric coefficients of 50-100 nm thick AlN films. We found that substrate temperature, sputtering power and type as well as quality of substrate have a strong impact on AlN c-axis texture. By optimizing these conditions, we were able to fabricate 50 nm thick AlN films with crystalline and piezoelectric properties similar to micrometer thick films. Recently, AlN doped with scandium (AlScN) has been developed due to its larger piezoelectric coefficients compared to undoped AlN. We found a strong influence of two sputtering parameters, argon gas concentration and substrate bias, on the c-axis texture and density of abnormal grains in 1 micrometer thick Al0.6Sc0.4N films. A fundamental issue in piezoelectric NEMS is that an asymmetric thickness cross section is necessary to create a flexural curvature and the asymmetry will reduce with decreasing system thickness. To avoid this issue, an alternative transduction technique based on flexoelectricity can be implemented, which has three advantages over piezoelectricity: it is theorized to exist in all dielectrics, it is predicted that its coefficients do not decrease with decreasing film thickness and any thickness cross section can be used to fabricate flexural NEMS. Most experimental work in flexoelectricity has been on ferroelectric or paraelectric materials; little research has been conducted on materials like CMOS-compatible, high-k dielectrics which could replace piezoelectric thin films in NEMS. Therefore, another part of this thesis focused on the fabrication and characterization of flexoelectric actuators based on amorphous hafnium oxide (HfO2). The fabricated 40 nm thick HfO2 films were non-ferroelectric and an effective flexoelectric coefficient of 37 pC/m was measured. The final part of the thesis analyzed the potential curvature generation of piezoelectric and flexoelectric films with nanoscale thicknesses, as well as provided a better understanding of the relationship between experimental flexoelectric coefficients and relative permittivity. Based on these analyses, we envision that flexoelectric thin films based on high-k dielectric materials with relative permittivities between 50-100 are a promising alternative to piezoelectric thin films in nanoscale actuation.

EPFL2019

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.