Over the past several decades, quartz crystal resonators (XTALs) and oscillators (XOs) have been used for frequency reference applications in almost all electronic systems due to their stability and superior electrical performance such as low phase noise and frequency stability. To overcome performance limitations and to resolve quartz oscillator reliability issues, several alternative technologies have been trying to replace quartz in recent years, including MEMS oscillators and CMOS oscillators.
These new disruptive technologies provide better robustness against shock and vibration, smaller form-factor, and the potential of monolithic integration. Compared to the quartz counterparts, however, their performance needs to be improved especially for high frequency reference applications. Piezoelectrically transduced pMEMS resonators have emerged as one of the strong candidates for replacing quartz technology for high-frequency, low phase noise reference applications. In this paper, we introduce the design principles and performance characteristics of pMEMS oscillators developed at IDT. Shock and vibration tests, as well as long-term stability measurements, have proven pMEMS oscillators to be reliable and cost-effective replacements for quartz crystal oscillators in high-frequency timing reference applications.
As the name implies, the pMEMS resonator is composed of a piezoelectric material (AlN) with single crystal silicon (SCS), taking advantage of both worlds, i.e., combines the advantages of piezoelectric quartz resonators and the advantages of silicon MEMS resonators. Unlike typical pure silicon capacitive resonators, pMEMS resonators do not require any DC bias voltage for operation. These composite pMEMS resonators offer better motional impedance and linear power handling, and have demonstrated excellent long-term frequency stability. Benefiting from the strong electromechanical coupling of the piezoelectric material and stability and low damping of SCS, pMEMS resonators offer very low motional resistance and an excellent quality factor. Figure 1 presents the schematic illustration of a pMEMS resonator with the piezoelectric layer and electrodes stacked on top of the SCS layer.
Figure 1: Schematic view of a pMEMS resonator
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An electrical stimulus is applied to one of the top electrodes to excite the piezoelectric layer where transverse piezoelectric coefficient e31 is utilized to generate a bulk acoustic wave in the entire device. The resonator then vibrates laterally, and the mechanical motion is transduced through the piezoelectric layer to be sensed by the other top electrode. The resonance frequency of the fundamental mode is determined in Equation 1 by the device’s lateral length L, the effective elastic constant of the combined resonator body Eeff, and the effective mass density.
As the bulk acoustic wave is dispersed in different directions in the resonator body, the energy in the desired length direction can be maximized by designing the resonator’s thickness and width to minimize loss from other directions. Figure 2 shows a typical 107 MHz resonator with measured Q value of 7453 and IL of -12.4dB.
Figure 2: The S21 plot of a typical pMEMS resonator
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The pMEMS resonators are fabricated by a CMOS-compatible process with wafer level packaging (WLP). The fabrication process starts with an SOI wafer deposited with piezoelectric and electrode materials. Once the electrodes are patterned, the resonator body is defined by patterning the piezoelectric layer and SCS stack.
Afterwards a cap wafer is bonded to the device wafer, and a pad metal layer is deposited to form the hermetic seal of the wafer level packaged device. Figure 3 shows the three dimensional view of the 550×450×200µm3 resonator with WLP.
Figure 3: Three-dimensional model of pMEMS resonator with a microshell
A few diced WLP pMEMS resonators on a grain of rice – see figure 4 - put things in perspective.
Figure 4: Singulated wafer level packaged pMEMS resonators on a grain of rice
These pMEMS resonators are wire bonded to an IC die to form an oscillator. Figure 5 shows the teardown of the pMEMS oscillator plastic package.
Figure 5: Teardown of a pMEMS oscillator in a plastic QFN package