Thin film formation methods for organic materials, biomaterials and polymers are becoming important for organic light emitting diode (OLED) display, organic semiconductors, bio-chips and biosensors. Concerning those thin film formation technology, not only pattern resolution, but also thin film quality such as thickness uniformity, controllability, costs and speed are the key issues.

In case of inorganic materials, using established process based on the photolithography combined with thermal deposition, sputtering and CVD (Chemical Vapor Deposition) and so on, it is possible to make a high resolution thin film pattern with high quality. However, these processes require high temperatures, high vacuum process and/or chemical treatment such as etching and development, which are not compatible with organic materials, biomaterials nor polymers.

For these reasons, direct patterning methods such as ink-jet printing, spin coating, micro spotting and micro contact printing have been proposed. In these methods, samples are patterned or deposited directly on the substrate without thermal and chemical process. These methods are low damage process, however these are wet process, which may cause non-uniform film thickness by ring stain effect upon drying.

To cope with these problems, dry-direct patterning methods combined with electrostatic collection such as electrospray deposition (ESD) and surface acoustic wave atomizer and electrostatic deposition (SAW-ED) have been proposed in 1999 and 2005 respectively. Concepts of these methods are as follows: (1) sample is atomized in tiny droplets and charged by high voltage, (2) tiny droplets are quickly dried up to be nano-sized particles (3) charged nanoparticles are deposited or patterned on the substrate by electrostatic force.

Advantages of dry patterning/deposition methods are that it is performed under atmospheric pressure and room temperature, and that it does not require any thermal or chemical processes that may damage target material. Moreover, size of dried particles can be controlled by concentration of liquid samples. Thus, not only high uniformity, but also high resolution patterning is possible at the same time. Furthermore, thick film or multi-layered structure can be also fabricated. Dry-direct pattering methods have a lot of merits, compared with other wet patterning methods. They will be important techniques for thin film fabrication of micro/nano patterns of bioactive/organic materials.

The purpose of this research is to increase the deposition speed keeping the quality of the deposition and to expand the application fields of dry patterning method.

In order to increase deposition quality, smaller nanoparticles are required. Particle size is mainly determined by the size of atomized drop and concentration of sample. One of the simple methods to obtain small nanoparticles is to reduce concentration of sample, however sample quantity and deposition time are drastically increased with lowered concentration. For this reason, balanced condition between particle size and deposition time must be compromised. To increase deposition speed keeping particle size, small drop size and high atomization speed should be simultaneously achieved.

Quality of the deposition can be evaluated by particle size as well as formation of particles making a thin film. As mentioned in the previous paragraph, it is possible to reduce particle size by control of drop size or sample concentration. In the case of particle formation, it is not easy to fabricate film with regular thickness using dry direct pattering. Since charged nanoparticles are deposited by the electrostatic force, their flight paths as well as deposition characteristics strongly depend on the electrostatic fields. Electrostatic fields are fluctuated during deposition process since charged nanoparticles lead to variation of boundary conditions on target electrodes. For this reason, pin hole or non-regular thickness film is easily generated. For the purpose of electronic functional layers, generation of pin hole is critical demerits, which may cause break down and malfunction of devices.

Finally, to expand the application fields of dry patterning method, some of drawback needs to be solved. The most distinctive demerit is that it is hard to deposit on the non-conductive substrate. Conductive substrate is required for attracting charged nanoparticles by electrostatic force. Deposition on the non-conductive substrate is necessary for variety of applications. For example, it may require to pattern on the non-conductive substrate, such as insulator layer of glass/plastics for micro total analysis system (microTAS) and electronic devices. In addition, many applications like OLED, electrode arrays are composed of conductive and non-conductive substrate. Thus, advanced deposition method is required for deposition on the non-conductive substrate.

To verify above mentioned purpose, performances of the dry direct pattering according to the atomizer are compared since performance of atomizer is the most important parameter which determines size of nanoparticles. Three types of atomizers, electrospray, surface acoustic wave (SAW) atomizer, and mesh type nebulizer, are compared in terms of particle size, collection efficiency and atomizing speed. As the results, average diameters of deposited particles are 0.1 (ESD), 0.11 (SAW-ED) and 0.25 (Nebulizer-ED) micrometers by using 0.5 mg/ml protein solution. Size of initial atomized droplet can be estimated from deposited particles. Mean diameters of initial atomized droplets are 1.3 (ESD), 1.4 (SAW-ED) and 3.2 (Nebulizer-ED) micrometer, respectively. Collection efficiency is 26 % (ESD), 2.2 % (SAW-ED), and 0.8 % (Nebulizer-ED). Atomization speed is 0.01 (ESD), 0.3 (SAW-ED) and, 7 (Nebulizer-ED) microliter per second, respectively. In case of ESD method, it is possible to atomize tiny and uniform size droplets compared with other methods, collecting efficiency is much higher, but atomizing speed was slower than others. In case of SAW-ED method, mean diameter of deposited particle is a little bit larger than ESD. However, atomizing speed is higher approximately 30 times than ESD. And, collection efficiency is lower than ESD. In case of nebulizer-ED, it showed higher atomizing speed than ESD and SAW-ED, but, particle size is largest and collecting efficiency is lowest.

Atomization speed of SAW atomizer is faster than ESD, however particle size is larger than ESD as well as experimental formula proposed by Lang. Main reason of differences between theoretical estimation and performance was that second peak on the larger diameter in the size distribution was generated. To investigate the reason of second peak generation, a new standing wave type SAW atomizer was proposed. Driving circuits was also improved, which is possible to apply not only intermittent burst driving, but also continuous driving. Then, the earlier progressive wave and the proposed new type are compared by means of vibration mode, atomization speed, and electrostatic deposition tests. In case of vibration mode and atomization speed test, standing wave type showed higher Standing Wave Ratio (SWR) and slower atomization speed than progressive wave type. In the electrostatic deposition test, liquid sample is atomized by two type of SAW atomizers combined with two different driving modes, which are continuous and intermittent drive. And then, deposited dry particles are measured by field-emission type scanning electron microscopy (FE-SEM) and its sizes are calculated by image processing software. Among them, only standing wave with continuous drive showed no second peak within size distribution.

Proposed standing wave type SAW atomizer showed high performance on the regular size distribution. However, it is still larger than mean drop size by ESD. For this reason, high frequency type of SAW atomizer is proposed. Based on the theory of ultrasonic atomization, drop size is reduced according to the frequency rise. SAW atomizers which excitation frequency ranging from 50 to 95 MHz type were constructed and evaluated by atomization speed and SAW-ED methods. On the atomization speed test, the minimum power required for atomization was approximately 4 W (50 MHz), 11 W (75 MHz) and 24 W (95 MHz). In this power, atomization speed was 0.06 (50 MHz), 0.04 (75 MHz) and 0.01 (95 MHz) microliter per second. Finally, the estimated mean diameter of atomized droplets was 5.7 (50 MHz), 4.4 (75 MHz) and 2.7 (95 MHz) micrometer, respectively. Drop size and atomization speed were reduced by the increasing frequency, however the required power for atomization is increased.

Not only drop size and atomization speed, applicability is also important. In the dry direct patterning, conductive substrate is requited for the generation of high intensity electrostatic fields. For the deposition on the non-conductive substrate, a new deposition method combined with corona discharge and SAW-ED was proposed. Process is composed of two steps: The first is patterning of the electric charges on the substrate using a corona discharge; The second is deposition of the dried nanoparticles on the pre-charged area using SAW-ED method. To generate charge pattern by corona discharge, a high voltage of 4.5 kV was applied between thin metal wire and conductive base plate in a distance of 3-4 cm. Then, 5 microliter of 5 mg/ml Bovine Serum Albumin (BSA) solution was deposited on the charge-patterned substrate by SAW-ED process. A high voltage of 5 kV is applied between conductive base plate and thin conductive wires placed just above the atomizer. The distance between atomizer and conductive base plate is approximately 15 cm. Corona discharge is applied for 10 minutes and atomization time was 20 seconds. As a result, approximately 200 micrometer width lines are deposited successfully.

To expand the application fields of dry patterning method, film quality need to be improved. For this reason, a new fabrication method for thin film for OLED was proposed. The basic concept of the proposed method is that nanoparticles are deposited on the target substrate just before they become completely dry. This is done by mixing in an additional solvent which has an evaporation speed that is relatively lower than that of the original solution. To investigate the morphology of the deposited layer, different concentrations of poly(2-methoxy-5-(2-ethylhexoxy)-1, 4-phenylenevinylene (MEH-PPV) solutions were sprayed on and then measured using field-emission scanning electron microscopy (FE-SEM) and a white-light interferometer. In the morphology test, some of sample shows good average surface roughness under the 1 nanometer. Based on these results, a small OLED pixel was fabricated, and current-voltage curve and luminescence characteristics were measured. Distinctive merit of proposed method is that it is possible to fabricate thin and regular-thickness films without pin hole.

Dry direct patterning using electrically charged nanoparticles generated by atomizer has proved to have many merits. First, performances of ordinary atomizers as a dry direct patterning were compared. In this test, SAW atomizer showed high atomization speed, but particle size was larger than ESD. To reduce particle size, standing wave type SAW atomizer which is possible to atomize tiny and regular drops was proposed. High frequency SAW atomizers were also utilized and tested. Then, new patterning method for deposition on the non-conductive substrate was proposed by combination of SAW-ED and corona discharge. Finally, OLED film was successfully fabricated by controlling drying speed using additional solvent. In conclusion, this study has proved that dry direct patterning using electrically charged nanoparticles generated by atomizers has many advantages over conventional methods and that it can be applied to various thin/thick film deposition including biomaterials, polymers and organic semiconductors/ electroluminescent materials.

本論文は「A Study on Electrostatic Deposition Method using Nanoparticles Generated by Atomizer」、(霧化器により形成されたナノ粒子を用いた静電成膜法の研究)と題し、有機ディスプレイ、半導体デバイスなど様々な分野で応用するための高品質な薄膜をナノ粒子と静電気力を用いたパターニング方法を利用し達成することを目的として、新たな霧化器の開発やデポジション法の開発を取り組んだ研究成果を纏めたものである。

本論文は、次の8章から構成されている。

第1章は「序論」であり、本研究の背景と目的を記述し、本論文の構成について述べている。有機・生体高分子の薄膜形成のためにはスピンコーティング、インクジェット、スクリーンプリント などのウェットプロセスとエレクトロスプレー・デポジション(ESD)や弾性表面波霧化器と静電気力を利用した方法(SAW-ED)などのドライプロセスがあるが、従来のドライプロセスはピンホールの発生や膜形成スピードが遅という問題のため、ディスプレイなどに応用するのは困難であった。そこで、この問題を解決するために薄膜形成時間を短縮しながら品質を上げてドライプロセスの応用分野を広げることを本博士論文の目的とすることを述べている。

第2章「霧化器の性能比較」では、液滴サイズ、霧化スピード、捕集効率などの薄膜形成に関係するパラメーターを三種類の霧化器(エレクトロスプレー、弾性表面波霧化器、メッシュ型ネブライザー)について、それぞれの霧化の原理に基づき、理論的考察により比較している。その結果、10nm程度の粒子を利用して100nmの膜厚を形成する時、従来の霧化器ではパターニングに要する時間が1000時間以上かかることが推定できることを明らかにしている。

第3章「静電場でのデポジション特性に関する検討」では、静電成膜法で形成された膜の微細構造を研究し、デポジションされたナノ粒子の間に微細な空間あるいはピンホールが発生することを記述している。

第4章「定在波型弾性表面波霧化器」では、従来の進行波型のSAW振動子で形成されたパーティクルの粒径が理論値と大きく離れている問題を解決するために、新たに、定在波型SAW振動子を提案している。この新しい定在波型振動子を用いた霧化器は、従来の進行波型SAW振動子を用いたものに比べて、高能率に、より微細なパーティクルが形成できることを示している。

第5章「高周波波型弾性表面波霧化器」では、従来のSAW霧化器での駆動周波数は10MHz程度であったが、周波数を大きくすると粒径の微細化が期待できることから、50MHz、75MHz、95MHzを駆動周波数とする高周波型弾性表面波霧化器を製作した。 その結果、予想に反して、10MHzに比べて、霧化特性を改善できることを示す実験結果は得られなかったことを述べ、過度に高い駆動周波数を用いる必要のないことを明らかにしている。

第6章「コロナ放電とSAW-EDを利用した薄膜形成法」では、コロナ放電を利用し非導電性表面にチャージをパターン化する手法を利用し、導電性の基板が必要だった既存の静電成膜法の問題を解決した新たな薄膜形成方法を提案して、その有効性を実証している。

第7章「エレクトロスプレー・デポジション法を利用した有機薄膜形成」では、静電成膜法には根本的にピンホールが発生しやすいということと膜形成スピードが遅い問題点があり、有機半導体材料の薄膜形成の実現が困難されていたが、2 種類の異なる蒸発速度を持つ溶媒を適切な割合で混合しながらESD 法を実施する手法を考案し、ピンホールの生成を抑え、高品質な有機EL薄膜を早いスピードで形成することに成功している。

第8章「結論」では本研究で得られた成果を纏めるとともに、開発した新しい技術の今後の研究課題と将来展望を述べている。

このように、本論文では新しいドライパターニング技術として注目されているSAW-EDと ESDの諸性能の向上に取り組み、コロナ放電を利用するSAW-ED法を考案するとともに、2種の蒸発速度の異なる溶媒を混合するESDの新手法を開発している。そして、 ピンホールのない有機薄膜を形成し、有機EL膜の発光に成功している。

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。