Yukihiro Takeuchi

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

Capacitive accelerometer with high aspect ratio single crystalline silicon microstructure using the SOI structure with polysilicon-based interconnect technique

We have developed a new processing technique for a capacitive mechanical sensor with a single crystalline silicon microstructure using the SOI structure which enables electrical isolation and interconnected wiring. This technique can make the sensor surface completely flat, allowing the formation of a cap for resin molding and a vacuum package for an angular rate sensor.

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.

Improvement of Si/SiO/sub 2/ mask etching selectivity in the new D-RIE process

This paper describes an improvement of Si/SiO/sub 2/ mask etching selectivity in the new D-RIE process that we presented in MEMS 2000. This process, which repeats the conventional D-RIE (ASE process) and O/sub 2/ plasma irradiation processes alternately, can improve the aspect ratio due to the prevention of lateral etching. However, the SiO/sub 2/ mask erosion of this process was 2.7 times as high as that of the conventional D-RIE process because the SiO/sub 2/ mask is sputtered by oxygen ion in the O/sub 2/ plasma irradiation process. Therefore the highest aspect ratio:46 was restricted by mask consumption. In this study, we suppressed the SiO/sub 2/ mask consumption. This suppression improves etching selectivity and increases the highest aspect ratio up to 60. Furthermore, the required process time is reduced to 2/3 of the prior result.

Improvement of high aspect ratio Si etching by optimized oxygen plasma irradiation inserted DRIE

This paper describes an advanced Si-deep etching process achieving a high aspect ratio with excellent verticality by the improvement of the O2 plasma source condition in the oxygen plasma irradiation inserted deep reactive ion etching (OP-DRIE) process that we have developed. The conventional DRIE process which we call the Bosch process has a trade-off relation between the high aspect ratio and verticality in the trench profile. Our developed process technique, repeating the conventional DRIE and the O2 plasma irradiation process alternately, can achieve the vertical trench profile with a higher aspect ratio than that of the conventional DRIE process. In order to maximize an advantage of the developed process, a thickness of the SiO2 layer formed by irradiation of O2 plasma should be large enough as a protection layer. However, because of insufficient SiO2 thickness formed by O2 plasma, the aspect ratio has been limited in previous work. Furthermore, mask material (SiO2) erosion which is another limitation factor of the aspect ratio is increased by the insertion of O2 plasma irradiation. In this paper, we have investigated optimum O2 plasma source conditions that allow an increase in SiO2 thickness with a high oxidation rate, and at the same time, with less mask erosion on the top of the wafer. We have clarified the effects of frequency and pulsed/CW modes of the plasma source on the effectiveness in SiO2 formation. From the obtained oxygen plasma source condition, we achieved the etched Si trench having an aspect ratio of over 70 with excellent verticality (uniform trench width).

High-Power Vertical-Cavity Surface-Emitting Laser under a Short Pulsed Operation

We report on the lasing characteristics of a single vertical-cavity surface-emitting laser (VCSEL) with InGaAs/GaAs multiple-quantum-wells under high-power pulsed operations. External quantum efficiency was improved by decreasing carrier overflow in the active region, and it was found that increasing the number of quantum wells suppressed carrier overflow. The highest peak pulsed power of over 12.5 W at an injection current of 20 A was achieved by a single VCSEL with five InGaAs quantum wells (QWs) in the active region.

High-Pulsed-Power (49W) Vertical-Cavity Surface-Emitting Laser with Five Quantum Wells by Uniform Current Injection into Large Emitting Area

We report on the high-power lasing characteristics of a large area bottom-emitting vertical-cavity surface-emitting laser (VCSEL). There have been difficulties in uniform current injection for large area VCSELs, which are caused by the band discontinuity at the interface between AlAs and GaAs of the n-type Distributed Bragg Reflector (DBR). We have reported the n-type DBR using a graded composition interface of 20nm thick suppresses the crowding of current to the edge of emitting area and improves external efficiency [1]. The highest peak pulsed power of over 49W was achieved by a five-quantum-well VCSEL with a current aperture diameter of 200µm.

Improvement of the optical transmittance of a micro prism made from a Si substrate by DRIE, oxidation and SiO2 film refilling

In this paper, we report on an improved fabrication process of micro-optical elements (e.g. lens and prism) with a height of 100 µm monolithically fabricated on a silicon substrate. The previously reported fabrication process is composed of (1) fine-pitched trench etching using DRIE followed by (2) thermal oxidization converting Si trench walls to transparent SiO2. Though 100% of Si walls had been converted to SiO2, it was difficult to completely squeeze out the remaining voids between SiO2 layers. Here, SiO2 deposition by CVD was added to the previous process as the third step to fill up the remaining voids. The first step (i.e. DRIE) was also modified to form trenches with a larger tapered angle at the top region than the other regions. The improved process made it possible to refill the trenches without voids with an only exception at the top region. The transmittance of the optical devices has improved from 67 to 91% by our new process.

SOI-based Si/SiO2 high-mesa waveguides for a compact infrared sensing system

High-mesa waveguides have been fabricated for a compact infrared sensing system, as they have a benefit of having optical evanescent field outside of their solid waveguides and thus this contributes to the sensing of gas or liquid in a compact area. Fabricated semiconductor on insulator (SOI)-based Si/SiO2 high-mesa waveguides, by using neutral loop discharge (NLD) plasma etching technique, showed extremely low propagation loss compared to those fabricated by using conventional reactive ion etching (RIE) technique. Moreover, we also demonstrate actual sensing for liquid methanol. By using 6 mm SOI-based Si/SiO2 high-mesa waveguide with waveguide width of 0.7 micro-meters, we could successfully obtain sufficient infrared absorption for the first time, therefore, this proved that the proposed waveguide had an optical field outside of the waveguide.

A new fabrication process for micro optical elements using drie and oxidation

We have developed a new fabrication process of micro optical elements by applying DRIE (Deep Reactive Ion Etching) process and thermal oxidation, which enables us to make micro lenses and prisms on a silicon substrate without assembling. This process can also form other optical elements, such as light wave-guides by changing mask pattern.