Piezoelectric transducers are widely used in the microelectro-mechanical system (MEMS) transducers such as inertial sensors, actuators, and energy harvesters. Lead zirconate titanate (PZT) thin films prepared with sol-gel or sputtering methods could offer high piezoelectric constants (d(33)~360 pC/N), whereas the resonant frequencies of the MEMS piezoelectric devices are often very high due to their large Young's moduli. Soft piezoelectric polymers, such as polyvinylidene fluoride (PVDF), have been widely investigated, but their piezoelectric responses are somewhat limited to low values (d(33)<35 pC/N). Recently, cellular polymer electrets, which contain a large number of micro-scale voids with implanted charges, have attracted much attention due to their low effective Young's moduli, tolerances for large deflection, and high electromechanical sensitivities

In previous studies, porous structures are formed in solid films by thermal expansion or physical foaming methods, which result in random distribution of ellipsoidal voids with different dimensions. Although inflation and stretching processes are often employed to optimize the void heights and thus the porosity of the film, somewhat broad distribution of the void dimensions would result in higher stiffness of the structure and/or nonuniform polarization of the polymer film through dielectric barrier discharges in the cavities with different heights. In addition, previous cellular polymers exhibit relatively poor structural stabilities. Under external forces, high-pressure gases inside the voids could leak to the outside, which leads to a decrease of the film thickness and deterioration of the piezoelectric response. Cellular structures with only a few layers, such as cellular polydimethylsiloxane (PDMS), can be made by thermal molding or bonding polymer substrates with cavities. However, it is not straightforward process to bond many layers firmly without crushing voids at an elevated temperature around the glass transition temperature.

Since most investigations focus on one-face charged single electret, few reports on heterocharges on double-faces charged electret and feasibility of charging multi-layer electrets. For one-face charged single film, either positive or negative charges could be successfully implanted into the electret film, and for some sort of polymer electrets, the negative charge implanted into the electret film is more stable, than the positive one. This fact has already reached a consensus for homocharges property, but still few works have been made to investigate the stability of positive and negative charges (heterocharges) in the dipole unit. The fundamental experiments and theoretical analyze should be carried out to well investigate electret properties for cellular electret.

Based on the above consideration, the objectives of present study are

・To confirm the fundamental properties of heterocharges in single/multi-later electrets and feasibility of soft X-ray uniformly charging multi-layer electrets in series.

・To propose a novel microfabricated high-aspect-ratio and high-density cellular polymer electret based on trench-filled parylene technology.

・To design and establish an in-plane MEMS piezoelectret transducer based on parylene piezoelectret spring to sense low-frequency vibration.

・To propose an improved high-performance piezoelectret with embedded PEDOT electrodes for vibration-driven energy harvester.

Firstly, the theoretical model of homocharges and heterocharges are compared. The relative low external electric field in heterocharges electret might own better stability than homocharges electret, which is confirmed by the experimental data. The nearly equal positive and negative surface potentials in double-faces charged single electret film are observed, which demonstrates the feasibility of soft X-ray charging single electret with heterocharges. On the other hand, two parallel electret films can be charged in series and show the similar surface potentials, which should become the strong evidence to confirm the feasibility of the soft X-ray charging multi-layer electrets.

A novel microfabricated high-aspect-ratio cellular polymer electret based on trench-filled parylene process is proposed, which could solve the long-term stability or reproducibility and charging issues introduced by thermal expansion or physical foaming preparation methods. In order to realize good mechanical property and electret performance, we employ parylene-C as main structural material and also a thin Dix-F layer to improve the electret performance. Both of parylene-C and Dix-F would offer a conformal coating even in deep Si trenches. After releasing the structure, the free-standing cellular parylene structure is obtained.

Based on trench-filled parylene process, the MEMS-based piezoelectret transducer is developed, which enable in-plane oscillation with a low resonant frequency. To realize the macroscopic dipoles inside the high-aspect-ratio cellular parylene structures, charges should be implanted onto the vertical walls of the cellular structures to form electret. Since corona ions cannot be used for charging through the narrow gap due to charge built-up near the opening, we employ the soft X-ray charging, in which in-situ photoionization of air molecules inside the gap using soft X-rays. When the bias voltage is applied across the gap, the abundant positive and negative charges are accelerated and implanted onto the opposite vertical parylene. The uniform artificial dipoles moments could vary along with parylene structural deformation driven by the inertia of a seismic mass.

With respect to the in-plane resonant oscillation, the prototype could provide the acceleration sensitivity from 152 to 197 mV/g at the low frequency of 30 Hz, and the piezoelectric charge and voltage sensitivities of 1522 pC/N and 152 V/N could be obtained at 1 g acceleration. At the resonant frequency of 149 Hz, the peak to peak voltage of 4.8 V was obtained, corresponding to the sensitivities of as large as 9600 pC/N and 960 V/N, respectively.

As an early power generator, the output power of 78 nW could be obtained at the resonant frequency of 149 Hz with 1.5 g acceleration. These results demonstrate the advantages of the cellular electret based on the trench-filled parylene for in-plane low-frequency transducers.

We also propose an improved cellular structure with embedded PEDOT electrodes for higher surface charge density. PEDOT conductive polymer is infused into the parylene spring by the capillary force. In order to electronically isolate positive/negative electrodes yet to mechanically connect parylene springs, short parylene beams with narrow isolation structure are devised to avoid the surface-tension driven PEDOT flow in between the positive and the negative sides. The good robustness of the PEDOT electrodes is confirmed by the oscillation experiment.

Thus, the bias voltage up to 150 V could be directly applied on the each parylene spring, which is expected to increase the power output to several μW.

環境発電とは,環境の中に薄く広く存在するエネルギーから微小電力を取り出す技術であり,長期間持続可能な無線センサネットワークのためのメンテナンスフリーの持続型電源などとして,大きな期待が寄せられている.特に,環境振動発電は,航空機,橋脚や構造物の健全性監視,居住空間における空調のきめ細かい制御,農畜産業での個体管理などへの応用が期待されているが,環境の振動は周波数帯が100Hz以下と低く,電磁誘導や圧電素子の性能を十分活かすことができない. 本論文では,小容積において最も効率良く電気的エネルギーを取り出す方法と考えられるエレクトレットを用い,MEMS技術を用いたハニカム樹脂ばね構造とそれに荷電をすることによって得られる圧電ポリマー構造を持つ新しいマイクロ振動発電器を提案したものである.

第1章では,まず,従来のエレクトレット,PVDFなどの圧電ポリマー材料を概観した後,多孔質のポア内壁に荷電をすることによって圧電性を持たせる多孔質圧電ポリマーについて述べ,延伸法などを用いたこれまでの製作方法ではポアの寸法,分布が均一でないために圧電係数が低い値に留まっていたことを説明している.また,近年の微細加工技術を用いた試みも散見されるが,原理的に重ね合わせる層の数を増やすことができないという問題を指摘し,本研究の目的である,トレンチ埋込み技術を用いた新しいハニカム樹脂ばね構造を持つ圧電ポリマーの提案について述べている.

第2章では,パリレン埋込み技術を用いた,圧電性を示すハニカム樹脂ばね構造の設計とその試作手法について述べている.まず,ポリマー層空気層とポリマー層を重ねた1次元モデルによって,ポリマー材料の比誘電率によって,最適な空気層とポリマー層の厚みの比が決定されること,圧電係数が表面電荷密度の2乗に比例することを示している.また,本論文で用いる軟X荷電の場合,荷電終了時にポリマー層の両側に絶対値の等しい異符号の電荷が打ち込まれることを解析的に示し,実際に荷電実験によってそれを確認している.また,パリレン樹脂がばね材料としてもエレクトレット材料としても適していることを示している.さらに,振動方向のばね定数が小さく垂直方向の剛性が高い,高アスペクト比ハニカム構造を提案し,シリコン基板の深掘りエッチング,パリレン樹脂によるトレンチ埋込み,XeF2ガスによるモールドSiの除去による,パリレン樹脂のハニカム構造の試作を行っている.

第3章では,実際にMEMS技術を用いて,ハニカム樹脂ばね構造を持つ圧電デバイスを試作し,140 Hz 程度の低い共振周波数を実現している.また,軟X線による荷電後,外部振動によって150-200 mV/gの大きな電圧出力が得られることを示し,MEMS技術に基づく本論文の圧電ポリマー構造が,従来の圧電ポリマーに比べて高い性能を示すことを明らかにしている.一方,圧電係数に大きな影響のある表面電位は,軟X線荷電の際の電界の強さによって決まるが,初期プロトタイプでは多数のハニカム構造全体にバイアス電圧を印加していたため,14Vと低い値に留まることを指摘している.

第4章では,表面電位を向上させるため,パリレンばね内部の空隙に導電性ポリマーを表面張力によって流し込み,ハニカム構造内部に電極を埋め込むプロセスについて検討をしている.これによれば,荷電時の電場を増大させ,出力を最大2桁向上させることができると見積もっている.そして,試作サンプルを用いて,50V程度の表面電位を実現し,本手法による圧電性能向上の実現可能性を示した.

第5章では,本研究で得られた結論をまとめている.

以上要するに,本論文は,トレンチ埋込みプロセスによるハニカム樹脂ばね構造を用いた圧電ポリマーの提案と試作,軟X線荷電による圧電ポリマーの形成とその評価,ばね空隙内への導電性ポリマーの導入,振動発電実験などにより,低共振周波数,大振幅を実現しうる,環境振動発電のための新たなMEMSデバイスの提案を行ったもので,環境発電技術,センサ工学,マイクロ・ナノ工学などの進展に寄与するところが大きい.

よって本論文は博士(工学)の学位請求論文として合格と認められる。