Thickness dependence of Al0.88Sc0.12N thin films grown on silicon

In this study, aluminum nitride (AlN) thin films were deposited on Si (100) and investigated on the minimum film residual stress with varying two critical deposition conditions in N2 gas flow and power. The pulsed DC reactive sputtering of aluminum targets was carried out in gas ratio of nitrogen (N2) and argon (Ar) plasma with N2:Ar ratios from 15:15 to 60:15 and power from 400 to 1000 W. According to the large-scale data of in situ optical emission spectroscopy (OES) study in connection with film microstructure information by analyzing X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) micrographs, transmission electron microscope (TEM) analyses, Fourier-transform infrared spectroscopy (FTIR) spectra, and alpha-step profilers, the deposited film stress states can be highly affected by the critical processing parameters of N2 flow rate and power. In addition, the proposed value of residual stress (VRS) can be calculated by PC1-DEV (the standard deviation in the first principal component direction) method for accurate prediction of the stress states of deposited films, i.e., compressive stress or tensile stress, which will provide valuable information on residual stress characterization as the in situ monitoring tool for the AlN thin film deposition process. In summary, a methodology based on large data of OES by which principal component analysis (PCA) was utilized to reduce its dimension can be used to determine residual stress characterization from a simple measurement in situ plasma monitoring tool is suggested.

Minimizing film residual stress with in situ OES big data using principal component analysis of deposited AlN films by pulsed DC reactive sputtering

In this study, we submit a complex set of in-situ data collected by optical emission spectroscopy (OES) during the process of aluminum nitride (AlN) thin film. Changing the sputtering power and nitrogen(N2) flow rate, AlN film was deposited on Si substrate using a superior sputtering with a pulsed direct current (DC) method. The correlation between OES data and deposited film residual stress (tensile vs. compressive) associated with crystalline status by X-ray diffraction spectroscopy (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) measurements were investigated and established throughout the machine learning exercise. An important answer to know is whether the stress of the processing film is compressive or tensile. To answer this question, we can access as many optical spectra data as we need, record the data to generate a library, and exploit principal component analysis (PCA) to reduce complexity from complex data. After preprocessing through PCA, we demonstrated that we could apply standard artificial neural networks (ANNs), and we could obtain a machine learning classification method to distinguish the stress types of the AlN thin films obtained by analyzing XRD results and correlating with TEM microstructures. Combining PCA with ANNs, an accurate method for in-situ stress prediction and classification was created to solve the semiconductor process problems related to film property on deposited films more efficiently. Therefore, methods for machine learning-assisted classification can be further extended and applied to other semiconductors or related research of interest in the future.

Machine Learning Assisted Classification of Aluminum Nitride Thin Film Stress via In-Situ Optical Emission Spectroscopy Data.

Thin film through-thickness stress gradients produce out-of-plane bending in released microelectromechanical systems (MEMS) structures. We study the stress and stress gradient of Al0.68Sc0.32N thin films deposited directly on Si. We show that Al0.68Sc0.32N cantilever structures realized in films with low average film stress have significant out-of-plane bending when the Al1−xScxN material is deposited under constant sputtering conditions. We demonstrate a method where the total process gas flow is varied during the deposition to compensate for the native through-thickness stress gradient in sputtered Al1−xScxN thin films. This method is utilized to reduce the out-of-plane bending of 200 µm long, 500 nm thick Al0.68Sc0.32N MEMS cantilevers from greater than 128 µm to less than 3 µm.

/pdf/compensation-of-the-stress-gradient-in-physical-vapor-31amr8vh.pdf

Compensation of the Stress Gradient in Physical Vapor Deposited Al1−xScxN Films for Microelectromechanical Systems with Low Out-of-Plane Bending

For an optimized performance of micro electromechanical systems (MEMS) double-clamped bridge-type resonators for mechanical strain sensing, a modified sputter process was developed which exploits the influence of varying sputter pressure during deposition on the intrinsic stress component of functional thin films. In detail, four different, polycrystalline aluminum nitride layers, synthesized with different sputter parameter sets were characterized related to their microstructure with techniques, such as X-ray diffraction, scanning electron microscopy and atomic force microscopy, respectively. Furthermore, the intrinsic thin film stress and the longitudinal piezoelectric coefficient were evaluated. The best performing layers were integrated in the fabrication process of two MEMS resonant strain sensor devices to study the stress-related impact on the resonant device performance. The initial static buckling of the sensor devices was studied by white light interferometric measurements, whereas the frequency response as a function of externally applied as well as intrinsic strain was analyzed by laser Doppler vibrometry. The behavior of the sensor devices was compared to theoretical predications and the influence of intrinsic thin film stress on the resonance frequency as a function of strain was studied. With a precise tailoring of the intrinsic film stress, a responsivity of ∼17000 is measured, representing an improvement by a factor of ∼5 compared to state-of-the-art resonant strain sensors. 

Stress engineering of polycrystalline aluminum nitride thin films for strain sensing with resonant piezoelectric microbridges

https://iopscience.iop.org/article/10.1088/1361-6439/aad91a/pdf

Method for controlling stress gradients in PVD aluminum nitride

http://www.heartrhythmjournal.com/article/S1547527123007154/pdf

En-452411-3 are drivers recurring or ephemeral? observations from serial mapping of persistent atrial fibrillation in paced canines

Past studies have examined the differences between <tex>$R$</tex> and <tex>$S$</tex> waves of unipolar atrial signals in patients with atrial fibrillation <tex>$(AF)$</tex> and have shown a difference in the <tex>$R$</tex> to <tex>$S$</tex> ratio <tex>$(R:S)$</tex> in certain regions of the atria compared to a healthy population. This work indicates a potential use of <tex>$R:S$</tex> as a marker for <tex>$AF$</tex>. In this study, we further examine these claims and investigate temporal changes in <tex>$R:S$</tex> over <tex>${A}F$</tex> development in animals. Four canines underwent <tex>$AF$</tex> development protocols and endocardial sinus rhythm maps were recorded as <tex>$AF$</tex> progressed. Unipolar signals gathered from mapping were used to calculate <tex>$R:S$</tex> within the left atrium of each animal. Calculations were performed at time points: before <tex>${A}F$</tex> initiation, 3–4 months of chronic <tex>${A}F$</tex> and 6 months of chronic <tex>${A}F$</tex>. From our analysis, we observed an increase in R-dominant signals within the left atrium once <tex>${A}F$</tex> is induced. Temporal results show that <tex>$R$</tex> dominance may be an indicator for chronic <tex>${A}F$</tex> patients and may be associated with the presence of arrhythmogenic substrate. With the addition of regional information, this unipolar signal analysis could guide therapeutic strategies.

Bram Hunt

Papers

Method for controlling stress gradients in PVD aluminum nitride

En-452411-3 are drivers recurring or ephemeral? observations from serial mapping of persistent atrial fibrillation in paced canines

Unipolar R:S Development in Chronic Atrial Fibrillation