Ting-Ta Yen

Temperature-compensated aluminum nitride lamb wave resonators

In this paper, the temperature compensation of AlN Lamb wave resonators using edge-type reflectors is theoretically studied and experimentally demonstrated. By adding a compensating layer of SiO2 with an appropriate thickness, a Lamb wave resonator based on a stack of AlN and SiO2 layers can achieve a zero first-order temperature coefficient of frequency (TCF). Using a composite membrane consisting of 1 ¿m AlN and 0.83 ¿m SiO2, a Lamb wave resonator operating at 711 MHz exhibits a first-order TCF of -0.31 ppm/°C and a second-order TCF of -22.3 ppb/°C2 at room temperature. The temperature-dependent fractional frequency variation is less than 250 ppm over a wide temperature range from -55°C to 125°C. This temperature-compensated AlN Lamb wave resonator is promising for future applications including thermally stable oscillators, filters, and sensors.

Thermally compensated aluminum nitride Lamb wave resonators for high temperature applications

In this letter, temperature compensation for aluminum nitride (AlN) Lamb wave resonators operating at high temperature is presented. By adding a compensating layer of silicon dioxide (SiO2), the turnover temperature can be designed for high temperature operation by varying the normalized AlN film thickness (hAlN/λ) and the normalized SiO2 film thickness (hSiO2/λ). With different designs of hAlN/λ and hSiO2/λ, the Lamb wave resonators were well temperature-compensated at 214 °C, 430 °C, and 542 °C, respectively. The experimental results demonstrate that the thermally compensated AlN Lamb wave resonators are promising for frequency control and sensing applications at high temperature.

Corrugated aluminum nitride energy harvesters for high energy conversion effectiveness

Aluminum nitride energy harvesters based on corrugated cantilever structures have been proposed, designed and demonstrated by means of micromachining processes with high energy conversion effectiveness. Corrugated cantilever design with a single piezoelectric layer prevents the common problem of an energy cancellation issue in a piezoelectric cantilever, by using a simple fabrication process similar to those in making the unimorph energy harvesters. Furthermore, corrugated structure can have an energy conversion effectiveness comparable to a conventional bimorph design. Experimentally, a prototype energy harvester with measured resonance frequency of 2.56 kHz has been fabricated. Under an input acceleration of 0.25 G, the amplitude of output voltage from the energy harvester has been recorded as 92 mV at a load resistance of 0.86 MΩ and the calculated output power is 4.9 nW. Furthermore, a multifold device resonating at 853 Hz with output power of 0.17 µW under acceleration of 1 G has been recorded.

Characterization of aluminum nitride lamb wave resonators operating at 600°C for harsh environment RF applications

In this paper, aluminum nitride (AlN) Lamb wave resonators (LWR) operating in air from room temperature up to 600°C are demonstrated for the first time. To date, no AlN RF devices have been tested and characterized at extreme temperatures. This paper describes the design, fabrication and characterization of temperature compensated AlN Lamb wave resonators at 600°C. Temperature coefficients of frequency (TCF) of both uncompensated and compensated devices were measured. Quality factors against temperature were also recorded. This supports the use of piezoelectric AlN as the material platform for radio-frequency (RF) components and sensing applications in harsh environments.

High-Q capacitive-piezoelectric AlN Lamb wave resonators

The use of capacitive-piezoelectric transducers, formed by separating a piezoelectric structure from its electrodes by sub-micron gaps, has raised the measured quality factor of aluminum nitride (AlN) Lamb wave resonators (LWR) from the ~1,000 of typical square-edged conventional devices (with contacting electrodes) to over 5,000 at 940 MHz, posting the highest reported Q for non-overmoded pure AlN resonators using d31 (e31) transduction at this frequency range. The Q · f product achieved here is significantly higher than that of a previous 1.2-GHz capacitive-piezoelectric contour-mode ring, mainly due to the use of Lamb wave modes that allow better support isolation to prevent energy loss to the substrate. In addition, the use of interdigital transducer (IDT) electrodes successfully decouples the resonance frequency from overall device dimensions, offering a CAD-definable design parameter for fine-frequency control. The effective coupling coefficient of keff2 = 0.3% achieved by this device is lower than the 1.6% typically observed for conventional AlN Lamb wave resonators, but still sufficient to avoid pass-band distortion in the 0.1% bandwidth filters needed for next-generation RF channel-selecting communication front-ends.

Experimental study of temperature-compensated aluminum nitride Lamb wave resonators

In this paper, the temperature compensation of aluminum nitride (AlN) Lamb wave resonators using edge-type reflectors is experimentally studied and demonstrated. By adding one compensating silicon dioxide (SiO2) layer with an appropriate thickness, the Lamb wave resonator can achieve a zero linear temperature coefficient of frequency (TCF). With the composite membranes including 1 µm AlN and 0.83 µm SiO2, a Lamb wave resonator operating at 711 MHz exhibits a secondorder TCF of −21.5 ppb/°C2. The temperature-dependent frequency variation is less than 250 parts per million (ppm) over a wide temperature range from −55 °C to 125 °C. This temperature compensated AlN Lamb wave resonator is promising for future applications to thermally stable oscillators, filters, and sensors.

Fine frequency selection techniques for aluminum nitride Lamb wave resonators

This paper reports fine frequency selection techniques for aluminum nitride (AlN) Lamb wave mode resonators (LWR) to control the relative frequency of resonators in an array to 0.1%. The technique that works the best is by adjusting the so-called AlN “overhang” dimension, OH, measured from the center of the outermost electrode to the edge of AlN plate independently of the interdigital transducer (IDT) pitch. Experimental results suggest the center frequency can be linearly adjusted by up to ±2.5% with no significant effect on other resonator parameters including Q, Rm, C0, and k2t. Preliminary results of filter banks at 735 MHz utilizing this technique demonstrate that the relative center frequency of each channel can be evenly spaced by 0.05% without post-process trimming.

Thermal compensation for aluminum nitride Lamb wave resonators operating at high temperature

Thermal compensation for aluminum nitride (AlN) Lamb wave resonators operating at high temperature is experimentally demonstrated in this study. By adding a compensating layer of silicon dioxide (SiO<inf>2</inf>), the turnover temperature can be designed for high temperature operation by varying the normalized AlN thickness (h<inf>AlN</inf>/λ) and the normalized SiO2 thickness (h<inf>SiO2</inf>/λ) in the AlN/SiO<inf>2</inf> composite stack. With different designs of h<inf>AlN</inf>/λ and h<inf>SiO2</inf>/λ, the Lamb wave resonators were well temperature-compensated at 214°C, 430°C, and 542°C, respectively. Furthermore, several testing cycles in the full temperature range from 25°C to 700°C were taken to demonstrate the repeatability of the frequency characteristics. This thermal compensation technology is promising for future applications to piezoelectric resonators, filters, and sensors at high temperature.

Synthesis of narrowband AlN Lamb wave ladder-type filters based on overhang adjustment

By adjusting the aluminum nitride (AlN) “overhang” dimension, OH, measured from the center of the outermost electrode to the edge of AlN plate, independently of the interdigital transducer (IDT) pitch, AlN Lamb wave mode resonators (LWRs) with 0.25% relative frequency control were demonstrated. Unlike adjusting electrode number, NE, device length, L, and electrode coverage, η, all of which affect device parameters such as Q, Rm, C0, and kt2, overhang frequency selection technique can linearly shift the center frequency up to 2.5% with no influence on other resonator parameters. Preliminary results of narrowband ladder filters using these finely spaced AlN LWRs as building blocks were fabricated and measured. Fine tuning of bandwidth and center frequency were studied.

Intrinsic characteristics of thermally stable AlN Lamb wave resonators at high temperatures

The thermal compensation at high temperatures for aluminum nitride (AlN) Lamb wave resonators utilizing the lowest symmetric (S₀) mode is theoretically and experimentally demonstrated in this work. The turnover temperature can be designed at high temperatures by changing the normalized AlN film thickness (h_AlN/λ) and the normalized silicon oxide (SiO₂) layer thickness (h_SiO2/λ) in the AlN/SiO₂ composite layer. The AlN Lamb wave resonators were well temperature-compensated at 214°C and 430°C, respectively, by using different ratios of h_AlN/λ to h_SiO2/λ. Even though the intrinsic quality factor (Q) degrades and the intrinsic motional impedance (R_m) increases at high temperatures, a Lamb wave resonator shows a Q of 760 at its turnover temperature, 430°C. These results demonstrate that thermally stable AlN Lamb wave resonators have the great potential for harsh environment applications.