Cristian Cassella

Anchor Losses in AlN Contour Mode Resonators

In this paper, we analyze possible sources of dissipation in aluminium nitride (AlN) contour mode resonators for three different resonance frequency devices (fr) (220 MHz, 370 MHz, and 1.05 GHz). For this purpose, anchors of different widths (Wa) and lengths (La) proportional to the acoustic wavelength (λ) are designed as supports for resonators in which the dimensions of the vibrating body are kept fixed. The Q extracted experimentally confirms that anchor losses are the dominant source of damping for most anchor designs when fr is equal to 220 and 370 MHz. For specific anchor dimensions (Wa/λ is in the range of 1/4-1/2) that mitigate energy leakage through the supports, a temperature-dependent dissipation mechanism dominates as seen in higher fr resonators operating close to 1.05 GHz. To describe the Q due to anchor losses, we use a finite-element method with absorbing boundary conditions. We also propose a simple analytical formulation for describing the dependence of the temperature-dependent damping mechanism on frequency. In this way, we are able to quantitatively predict Q due to anchor losses and qualitatively describe the trends observed experimentally.

Aluminum Nitride Cross-Sectional Lamé Mode Resonators

This paper demonstrates a new class of AlN-based piezoelectric resonators for operation in the microwave frequency range. These novel devices are identified as cross-sectional-Lamé-mode resonators (CLMRs) as they rely on the piezoelectric transduction of a Lamé mode, in the cross section of an AlN plate. Such a 2-D mechanical mode of vibration, characterized by motion along both the lateral and the thickness directions, is actuated and sensed piezoelectrically through the coherent combination of the e31 and e33 piezoelectric coefficients of AlN. This special feature enables the implementation of CLMRs with high values of electromechanical coupling coefficient. In particular, we experimentally demonstrated kt2 values in excess of 4.6% and 2.5% in CLMRs using, respectively, two or one metallic interdigitated metallic electrodes, and operating around 1 and 2.8 GHz. Furthermore, despite the dependence of the cross-sectional Lamé mode on both the thickness and the width of the AlN plate, lithographic tunability of the resonance frequency, by changing only the in-plane dimensions of the device, can be achieved without a substantial degradation of kt2. The capability of achieving high kt2 and multiple operating frequencies on the same chip, without additional fabrication costs (lithographic tunability of the resonance frequency), makes this technology one of the best candidates for the implementation of multifrequency and low insertion loss filter banks for reconfigurable radiofrequency front ends.

Reduction of anchor losses by etched slots in aluminum nitride contour mode resonators

This paper presents a new technique to increase the quality factor, Q, of AlN Contour Mode Resonators (CMRs). The technique uses etched slots in the body of AlN CMRs to reduce energy dissipation through the anchors. The reduction of the energy lost through the supporting anchors improves the device Q without altering its electromechanical coupling, kt2. An almost 50% improvement in the Figure of Merit, FoM, defined as the product between Q and kt2, has been measured in 220 MHz AlN CMRs.

Quality Factor Dependence on the Inactive Regions in AlN Contour-Mode Resonators

This paper investigates the dependence of the quality factor (Q) of AlN contour mode resonators (CMRs) on the characteristics of the inactive regions located between the main resonator body (active region) and the stress-free surfaces placed beside the anchors. This paper shows that it is possible to reduce the energy leakage through the anchors by optimally sizing the width of such regions. To validate this concept, we built 16 different configurations of 225-MHz AlN CMRs, differing just by the size of the resonator inactive regions.

Cross-Sectional Lamé Mode Ladder Filters for UHF Wideband Applications

This letter reports on the first demonstration of ladder filters based on the recently demonstrated cross-sectional Lamé mode resonator (CLMR) technology. These filter prototypes show a fractional bandwidth (BW3dB) as high as 3.3% and an insertion loss as low as 0.4 dB. As the resonance frequency of CLMRs can be lithographically controlled without significantly degrading their electromechanical coupling coefficient (k2t), multiple contiguous frequency bands can be covered by this filter technology without adding fabrication complexity. This unique feature addresses one of the most crucial challenges associated with the development of miniaturized mobile platforms adopting carrier-aggregation. Furthermore, the capability of achieving large BW3dB, around lithographically defined center frequencies, enables the fabrication of transmit and receive modules of the next-generation radio-frequency front ends on the same chip without adding fabrication steps.

RF Passive Components Based on Aluminum Nitride Cross-Sectional Lamé-Mode MEMS Resonators

This paper presents a new class of monolithic integrated RF passive components based on the recently developed aluminum nitride (AlN) MEMS cross-sectional Lamé-mode resonator (CLMR) technology. First, we experimentally demonstrate a 920-MHz CLMR showing the values of electromechanical coupling coefficient (k_t²) and quality factor (Q_load) in excess of 6.2% and 1750, respectively. To the best our knowledge, the resulting figure of merit (= Q·k_t²), in excess of 108, is the highest ever reported for AlN-based piezoelectric resonators using interdigitated metallic electrodes (IDTs) and operating in the same frequency range. Second, we report the measured performance of an 870-MHz ladder filter, synthesized using three degenerate CLMRs. This device shows the values of fractional bandwidth (BW_3dB) in excess of 3.8% and an insertion loss of ~1.5 dB. Finally, we report the performance of the first piezoelectric transformer (PT) based on the CLMR technology. This device, dubbed “cross-sectional Lamé-mode transformer,” exploits the high-k_t² of the CLMR technology to achieve high values of open-circuit voltage-gains (G_v) in excess of 39. To the best of our knowledge, such a high G_v-value is the highest ever reported for MEMS-based PTs operating in the microwave frequency range.

Analysis of the impact of release area on the quality factor of contour-mode resonators by laser Doppler vibrometry

Energy dissipation through the anchors of an aluminum nitride (AlN) MEMS contour-mode resonator (CMR) plays an important role in setting the device quality factor at frequencies under 500MHz [1]. The acoustic energy leaving the resonator is effected, not only by the anchor, but also the portion of the surrounding device layer that has been released from the substrate during fabrication. Typical device simulations used for design do not take into account the motion of this additional area. Using laser Doppler vibrometery and COMSOL simulations we show a variation in device Q by 28% as a result of the motion of this released region.

AlN Two-Dimensional-Mode Resonators for Ultra-High Frequency Applications

In this letter, we present the first prototype of aluminum nitride two-dimensional-mode resonators (2DMRs) for operation in the ultra-high-frequency range. The 2DMRs in this letter are made of thick AlN films (5.9μm) and rely on both the d₃₁ and the d₃₃ coefficients of AlN to attain high electromechanical coupling (k_t² ), low motional resistance, and a limited lithographic control of the resonance frequency. k_t² > 3.4%, a mechanical quality factor larger than 2400, and >10% lithographic variation of the center frequency were demonstrated.

Dynamics of microscale thin film AlN piezoelectric resonators enables low phase noise UHF frequency sources

Miniaturized, multi-band and high frequency oscillators that are compatible with CMOS processes are highly desirable for the synthesis of compact, stable, and low power frequency sources for reconfigurable radio frequency communication systems and cognitive radios. Aluminum nitride (AlN) contour mode MEMS resonators (CMR) are emerging devices capable of high Q, low impedance, and multi-frequency operation on a single chip. The frequency stability of these AlN MEMS devices is of primary importance in delivering oscillators that exhibit low phase noise, and low sensitivity to temperature and acceleration. In this article we describe how the resonator dynamics impacts oscillator performance and present some preliminary demonstrations of ultra-high-frequency (UHF) oscillators. An example of an oscillator prototype we synthesized with a 586 MHz AlN CMR exhibited phase noise <; - 91 dBc/Hz and - 160 dBc/Hz at 1 kHz and 10 MHz offsets, temperature stability of 2 ppm from - 20 to + 85°C, and acceleration sensitivity <; 30 ppb/G.

Piezoelectric nonlinear nanomechanical temperature and acceleration insensitive clocks

This work presents the development of high frequency mechanical oscillators based on non-linear laterally vibrating aluminum nitride (AlN) piezoelectric resonators. Our efforts are focused on harnessing non-linear dynamics in resonant mechanical devices to devise frequency sources operating around 1 GHz and capable of outperforming state-of-the-art oscillators in terms of phase noise and size. To this extent, we have identified the thermal and mechanical origin of non-linearities in micro and nanomechanical AlN resonators and developed a theory that describes the optimal operating point for non-linear oscillators. Based on these considerations, we have devised 1 GHz oscillators that exhibit phase noise of < -90 dBc/Hz at 1 kHz offset and < -160 dBc/Hz at 10 MHz offset. In order to attain thermally stable oscillators showing few ppm shifts from - 40 to + 85 °C, we have implemented an embedded ovenization technique that consumes only few mW of power. By means of simple microfabrication techniques, we have included a serpentine heater in the body of the resonator and exploited it to heat it and monitor its temperature without degrading its electromechanical performance. The ovenized devices have resulted in high frequency stability with just few ppm of shift over the temperature range of interest. Finally, few of these oscillators were tested according to military standards for acceleration sensitivity and exhibited a frequency sensitivity lower than 30 ppb/G. These ultra stable oscillators with low jitter and phase noise will ultimately benefit military as well as commercial communication systems.