Kazuhiko Kano

Enhancement of Piezoelectric Response in Scandium Aluminum Nitride Alloy Thin Films Prepared by Dual Reactive Cosputtering

Adv. Mater. 2009, 21, 593–596 2009 WILEY-VCH Verlag Gm The industrial demand for higher-temperature piezoelectric sensors is drastically increasing, for the control of automobile, aircraft, and turbine engines and the monitoring of furnace and reactor systems, because environmental problems, such as carbon dioxide (CO2) and nitrogen oxide (NOx) reduction, are becoming more globally serious. The sensors are also desirable for health monitoring coal-fired electric-generation plants and nuclear plants. It is generally known that piezoelectric materials with a higher Curie temperature possess a lower piezoelectric coefficient. Furthermore, the results of a study (Fig. 1) of the relationship between maximum use temperature and piezoelectric coefficient d33 shows that the piezoelectric materials with a higher maximum use temperature possess a lower piezoelectric coefficient d33. [3–9] For example, the Curie temperature and piezoelectric coefficient d33 of lead zirconium titanate (PZT), which is widely used in many electronic devices, are 250 8C and 410 pCN , respectively. The maximum use temperature and d33 of aluminum nitride (AlN), which is a typical hightemperature piezoelectric material, are 1150 8C and 5.5 pCN . It is difficult to achieve a good balance between high maximum use temperature and large piezoelectricity in a material, and no effective piezoelectric materials with these characteristics have yet been found. In this communication, we report a hightemperature piezoelectric material that exhibits a good balance between high maximum use temperature and large piezoelectricity. This was achieved by the combination of the discovery of a phase transition in scandium aluminum nitride (ScxAl1 xN) alloy thin films and the use of dual co-sputtering, which leads to nonequilibrium alloy thin films. Sc0.43Al0.57N alloys exhibit a large piezoelectric coefficient d33 of 27.6 pCN , which is at least 500% larger than AlN. The large piezoelectric coefficient d33 is the highest piezoelectric response among the tetrahedrally bonded semiconductors, despite the fact that the crystal structure of scandium nitride (ScN) is rock-salt (nonpolar). Moreover, the large piezoelectricity is not changed by annealing at 500 8C for 56 h under vacuum. This work demonstrates the new route to design of this high-temperature piezoelectric material. ScN has a rock-salt structure (nonpolar). However, Takeuchi reported the existence of a (meta)stable wurtzite structure in ScN, and the possible fabrication of Sc-IIIA-N nitrides by firstprinciples calculations. Farrer et al. predicted that the wurtzite structure is unstable in ScN, and that the hexagonal structure is (meta)stable in ScN, unlike the wurtzite structure. The piezoelectric responses of hexagonal ScxGa1 xN and ScxIn1 xN alloys can be enhanced by an isostructural phase transition (from wurtzite to layered hexagonal). However, the piezoelectric responses and Curie temperatures of the nitride alloys have not yet been confirmed by experiments. AlN, GaN, and InN are IIIA nitrides and have a wurtzite structure (polar). In particular, the thermal stability and piezoelectricity of AlN are the highest among the IIIA nitrides. AlN is a piezoelectric material compatible with the Complementary metal–oxide– semiconductor (CMOS) manufacturing process, and is a promising material for integrated sensors/actuators on silicon substrates. Wurtzite and rocksalt structures have rather different lattice forms and unit sizes. The formation of

Influence of growth temperature and scandium concentration on piezoelectric response of scandium aluminum nitride alloy thin films

The authors have investigated the influence of growth temperature and scandium concentration on the piezoelectric response of scandium aluminum nitride (ScxAl1−xN) films prepared by dual reactive cosputtering. The piezoelectric response strongly depends on the growth temperature and scandium concentration. The piezoelectric response of the films prepared at 400 °C gradually increases with increasing scandium concentration. On the other hand, the piezoelectric response of the films prepared at 580 °C drastically decreases and increases in the scandium concentration from 30% to 40%. We think that the drastic change of the piezoelectric response is due to the disordered grain growth.

High-performance surface acoustic wave resonators in the 1 to 3 GHz range using a ScAlN/6H-SiC structure

This paper describes application of Sc-doped AlN (ScAlN) to wideband SAW devices in the 1 to 3 GHz range. First, it is shown theoretically that large SAW velocity and electromechanical coupling factor are simultaneously achievable when the ScAlN film is combined with a base substrate with extremely high acoustic wave velocities, such as diamond and SiC. Next, SAW delay lines are fabricated on the ScAlN/6H-SiC structure, and reasonable agreement between the theory and experiment is obtained. Finally, one-port SAW resonators are fabricated on the structure, and it is shown that high-performance is achievable in the 1 to 3 GHz range by use of the structure.

Influence of oxygen concentration in sputtering gas on piezoelectric response of aluminum nitride thin films

The authors have investigated the influence of oxygen concentration in sputtering gas on the piezoelectric response of aluminum nitride (AlN) thin films prepared on silicon substrates. The piezoelectric response strongly depends on the oxygen concentration, and changes from +6.8to−7.0pC∕N with increasing oxygen concentration from 0% to 1.2%. The polar direction drastically inverts from the Al polarity to N polarity. When the oxygen concentration in sputtering gas was 1.2%, the oxygen concentration in the AlN films was 7at.%. Furthermore, the growth rate of the AlN films gradually decreases with increasing oxygen concentration in sputtering gas.

Influence of sputtering pressure on polarity distribution of aluminum nitride thin films

The authors have investigated the influence of sputtering pressure on the polarity distribution of aluminum nitride (AlN) films. They have found that sputtering pressure strongly influences the polarity distribution of AlN films prepared on molybdenum electrodes. The polarity distribution of the AlN films was observed by piezoresponse force microscopy. The polarity orientation is decided with respect to each fine grain constituting the AlN films, and polarity conversion from Al polarity to N polarity is observed with increasing sputtering pressure. The piezoelectric response of the films changes from +3.7to−4.4pC∕N with increasing sputtering pressure from 0.36to4.0Pa.

Polarity inversion in aluminum nitride thin films under high sputtering power

The authors have investigated the influence of sputtering power on the piezoelectric response of aluminum nitride (AlN) thin films prepared on titanium nitride bottom electrodes. The piezoelectric response strongly depends on the sputtering power. The polar inversion was found by piezoresponse force microscopy. The polarity gradually changes from the N polarity to Al polarity with increasing sputtering power. The piezoelectric response of the films changes from −2.7to+4.3pC∕N with increasing sputtering power from 100to500W. Furthermore, the polarity inversion from the N polarity to Al polarity is observed by increasing sputtering power during growth.

ScAlN Lamb wave resonator in GHz range released by XeF 2 etching

Recently, it was reported that the piezoelectric characteristic of AlN was enhanced by doping Sc, and that 40% Sc-doped AlN (Sc_0.4Al_0.6N) had approximately 5 times higher piezoelectricity than pure AlN. In this study, we designed, fabricated and evaluated Lamb wave resonators using a Sc_0.4Al_0.6N thin film. First, we examined elastic constants c₄₄^E and c₄₄^D of Sc_0.4Al_0.6N using a thickness-shear wave resonator excited by parallel electric field, and then designed Lamb wave resonators using revised material constants. The Lamb wave resonators were fabricated by electron beam lithography, reactive ion etching, XeF₂ release etching etc. The maximum and average electromechanical coupling coefficients were 8.3% and 6.8%, respectively, for 2.6 GHz devices working in S₀ mode.

Parametric evaluation of mid-range wireless power transmission

Quality factor is the most essential parameter in the concept of the mid-range wireless power transmission. The theoretical formula of the transmission efficiency was redefined as a function of coupling coefficient and quality factor. This redefinition enables simple comparison between the conventional close-range electromagnetic induction transmission and the mid-range transmission. The realizable quality factor was precisely analyzed by the additional calculations of the wire cover loss and the capacitor loss. And the actual transmission system was developed. It verified the redefined efficiency formula and the new calculation method of quality factor.

A new deep reactive ion etching process by dual sidewall protection layer

This paper describes a new deep reactive ion etching (D-RIE) process which drastically improves the aspect ratio of the etched trench. The conventional D-RIE process obtains the high aspect ratio trench etching with the protection layer, such as a polymeric layer. The etching anisotropy is limited in this process because this protection layer prevents not only lateral etching, but also vertical etching. In contrast, the new process we developed intensively prevents lateral etching with a dual protection layer consists of a polymeric layer and a SiO/sub 2/ layer on the trench sidewall. Therefore the etching anisotropy and the aspect ratio can be improved. Furthermore, this process can only be performed by switching the introducing gas into the etching chamber.

Compact and Tunable Transmitter and Receiver for Magnetic Resonance Power Transmission to Mobile Objects

As electronic devices are becoming more mobile and ubiquitous, power cables are turning to the bottlenecks in the full-fledged utilization of electronics. While battery capacities are reaching their limits, wireless power transmission with magnetic resonance is expected to provide a breakthrough for this situation by enabling power feeding available anywhere and anytime. This chapter studies the feasibility of magnetic resonance power transmission to mobile objects mainly focusing on the resonator quality factor and impedance matching control systems. Transmission efficiency reaches a reasonably high level when the transmitting and receiving resonators satisfy two conditions. The first is to have high quality factors. The second is to tune and match the impedance to the transmission distances. The second section explains the theoretical grounds for these conditions. The third section describes a developed wireless power transmission system prototype which was made compact and tunable to be applied to mobile objects. The later sections evaluate the quality factor and the impedance matching of the prototype.