The deposition of highly oriented crystalline silicon films at high rates and low temperatures has been of technical and scientific interest, especially in the fabrication of large-area electronic devices where micron-thick epitaxial films are required in order to reduce their cost. Particularly in the photovoltaic industry, the prices of the raw materials such as silicon, ingots/wafers, cells and modules make up more than 55% of the total production costs, and these have continuously risen in the past years. In addition, there is presently a shortage in solar grade silicon raw material which further contributes to the increase in the total price of a finished solar module. This study is thus motivated by the need to develop a new processing route for device-grade epitaxial silicon films.

Several high-rate epitaxial deposition techniques have attained rates of around 10-100 nm/s at temperatures higher than 650 °C. However, high deposition temperatures result in autodoping from and dopant redistribution in the underlying film or substrate, which are otherwise negligible in low-temperature processing. However, epitaxial deposition at low temperatures is in general limited by low deposition rates and the presence of a critical epitaxial thickness, beyond which, the transition from epitaxial to amorphous or polycrystalline film growth occurs. The latter is considered to be mainly due to defect accumulation, kinetic surface roughening or impurity segregation. As a result, the majority of studies on epitaxial deposition at temperatures less than 650 ℃ have reported deposition rates ranging only from 0.02-1.2 nm/s and thicknesses less than a few microns. From a scientific perspective, the development of a new process would require looking beyond the aspects, mechanisms and approaches of conventional processes and techniques. This presents a very interesting and challenging theme both from an experimental and theoretical point of view.

In view of the limitations of the existing silicon deposition techniques, the so-called "mesoplasma" chemical vapor deposition (CVD) technique will be established as a new epitaxial silicon thick film processing route that potentially satisfies both the device performance and economic demands of the industry. This study aims to gain a fundamental understanding of the deposition and growth mechanisms in mesoplasma epitaxy. Particular focus is given to these three aspects which will provide the basic information to obtain a general picture of the mesoplasma process and highlight its unique characteristics from other existing techniques: (a) the simultaneous attainment of high rate and low temperature epitaxy will be demonstrated by investigating key process parameters, namely, source gas (silane) partial pressure and substrate temperature; (b) understanding of precursor/nanocluster formation within the mesoplasma and its role in epitaxy will be elucidated through an in-situ X-ray scattering technique within the gas phase, applied for the first time in such a dynamic environment; and (c) the influence of hydrogen, a key element which has been known to affect silicon epitaxy, in nanocluster-surface and plasma-surface interactions will be investigated.

Mesoplasma CVD

From a processing point of view, the mesoplasma is described as a plasma generated in the 0.1 to 10 Ton range with a high gas flow rate. It is also characterized by its relatively low electron, Te and gas temperatures, Tg, and values of Te<1eV and l<Te/Tg;<10 are anticipated. Under these conditions, effective source gas dissociation and low ion bombardment on the substrate is attained while maintaining relatively moderate to low deposition temperatures compared to thermal plasma processing, thus, making high-rate deposition of thick films at relatively low temperatures possible. In addition, due to the relatively low electron temperature, the dominant plasma chemistry is controlled by atoms and significant ion bombardment on the film leading to degradation of properties is minimized.

In mesoplasma CVD, a thermal boundary layer exists between the plasma and the substrate where vapor condensation and nucleation take place. These processes are expected to contribute to the generation of atomic and nanometer-sized silicon clusters which will subsequently act as the deposition precursors to film growth. The nature of the atoms and clusters, such as cluster size, size distribution and cluster energy, may be controlled through the boundary layer to form a variety of film structures, including epitaxial films, while maintaining the high flux of the precursor and a low substrate temperature.

High-rate and low-temperature silicon epitaxy

Mesoplasma epitaxy is carried out using a radio frequency (rf) inductively-coupled plasma CVD system at a fixed total pressure of 6 Ton and rf power of 22 kW. Argon is used as the main plasma gas while silane, (SiH4) diluted with 20% hydrogen serves as the source gas. In order to simultaneously attain high-rate and low-temperature epitaxy, the amount of silane and substrate temperature were systematically varied. Increasing the amount of silane resulted to a linear increase in deposition rate which is independent of the substrate temperature, indicative of adhesive growth. Interestingly, despite increasing deposition rates and decreasing substrate temperatures, the structural and electrical properties of the epitaxial films are maintained. A high epitaxial deposition rate of around 35 nm/s was achieved even at substrate temperatures as low as 360 ℃, while maintaining the Hall mobility values to around 270 cm2/V-s. Such a combination of deposition rate, temperature and film quality has never been reported for other deposition techniques. With mesoplasma CVD, epitaxial films with thicknesses of several tens of microns can easily be deposited within a few minutes. From a processing perspective, this would translate to significant reductions in cost.

The temperature independence of the deposition rate and electrical properties is also unlike the behavior observed for conventional deposition techniques. Because the power was held constant in the previous set of experiments, the boundary layer thickness did not vary much and precursors having generally similar characteristics were generated. This could suggest the uniqueness of the deposition precursors associated with epitaxy in mesoplasma CVD.

Nanocluster role in epitaxy

As stated previously, the condensation of atomic silicon within the thermal boundary layer results. to the generation of nanoclusters. Detection and characterization of the silicon nanoclusters, as they are formed in the vapor phase, is an essential key to gain fundamental understanding about the deposition mechanism and to establish better control in the mesoplasma CVD process. Utilizing a laboratory-scale X-ray generator and a two-dimensional position-sensitive proportional counter, a small-angle X-ray scattering system was constructed and incorporated to the existing mesoplasma CVD chamber. Through the SAXS system, free silicon nanoclusters generated in the gas phase have been successfully detected in situ during silicon film deposition. Spherical nanoclusters acting as deposition precursors around 2 nm in size were identified and associated with epitaxial film growth. These nanoclusters were characterized to have a diffuse electron density distribution in the vicinity of the nanocluster surface, which is indicative of a loosely-bound structure, the so-called "hot cluster". Due to the thermal energy provided by the plasma, the atoms comprising the nanoclusters responsible for epitaxy are loosely-bound and in a non-rigid and thermally energized state. The high internal energy thus facilitates cluster deformation upon impingement even on a relatively low temperature substrate, allowing the individual atoms to migrate on the surface for lateral growth, leading to high quality films. On the other hand, the emergence of a small amount of larger clusters with a relatively rigid form was associated with the transition from an epitaxial to agglomerated film structure.

By simple control of the deposition parameters, the nanocluster characteristics can be effectively controlled in mesoplasma CVD. This nanocluster-based approach for film deposition by mesoplasma CVD has great potential for depositing device quality films simultaneously at high rates and low temperatures, which are otherwise not attainable with other deposition techniques. The potential of SAXS as an in situ plasma diagnostic technique has also been demonstrated. Further improvements could eventually establish SAXS as an effective plasma diagnostic tool for real-time and in situ process monitoring and control.

Plasma-nanocluster-surface interactions-influence of hydrogen

Hydrogen, which is present as the diluent gas and generated from the inevitable decomposition of silane, is known to affect silicon epitaxy, especially at low temperatures. Thus, in mesoplasma epitaxy where the gases are highly dissociated, atomic hydrogen is expected to influence the deposition process. Numerous investigations describing the effects of hydrogen have been reported, some even with totally contradicting claims. At low temperatures, some assert that atomic hydrogen enhances epitaxy by promoting the surface diffusion of precursors, or by abstracting adsorbed surface hydrogen to expose available sites for silicon bonding. Still, other models report that hydrogen has a disruptive role in epitaxy by segregating on the surface leading to defect formation, or by causing a buildup of surface roughness due to limited adatom mobility. Regardless of whether hydrogen is favorable or detrimental to epitaxy, it is generally known that the effect is chemical in nature. In the investigation of the effect of hydrogen in mesoplasma CVD, the approach involves looking into two critical aspects of the deposition process, namely, on the generation of in-flight deposition precursors and on the evolution of film morphology and structure. Identifying which aspect is critically affected by hydrogen could enable more effective control of the deposition process and further improvement of the electrical properties.

Through SAXS, the effect of varying amounts of hydrogen on the characteristics of the nanoclusters was studied. It was found that at fixed values of rf power, substrate temperature and silane corresponding to epitaxy, varying the hydrogen partial pressure from 150 to 460 mTorr does not critically alter the characteristics of the nanoclusters (~2 nm in size) as growth precursors. The nanoclusters exhibit similar characteristics as the nanoclusters previously associated with epitaxy, that is, having a loosely-bound, non-rigid structure. However, examination of the deposited films revealed that at hydrogen partial pressures less than 220 mTorr, the film exhibited a polycrystalline structure, whereas at higher hydrogen partial pressures, the films are epitaxial with Hall mobilities exceeding 200 cm2N-s. Another set of experiments involving exposure only to an Ar-H2 plasma under varying hydrogen partial pressures showed that the silicon substrate surface is smoother with increasing hydrogen introduction. At high hydrogen amounts, the chemical effect of hydrogen is likely promoted and provides favorable conditions for the atoms of the nanoclusters to migrate on the substrate surface and incorporate at step edges or kinks. Thus, in order to effectively utilize the unique nanocluster characteristics to attain epitaxy, a minimum amount of hydrogen is needed. It is concluded that the interplay of the nanoclusters characteristics and the hydrogen-surface interaction is a key to further improve the deposition process.

The aforementioned results have demonstrated the unique features and advantages offered by mesoplasma CVD. The advancements gained in the study, both from scientific and technological perspectives could pave the way for the eventual establishment of the mesoplasma CVD as a new processing route for the fabrication of high quality epitaxial silicon films at high rates and low temperatures.

近年著しい展開を示す大面積デバイス分野では、高品質デバイスの基幹となる薄膜シリコン製造プロセスが全コストの50%以上を占め、シリコン原料不足も顕在化し始めており、従来技術に代わる、高効率原料利用、高品質薄膜の低温・高速堆積の相反する要求を両立する革新的な結晶系シリコン薄膜化技術が切望されている。これを背景に、本論文は、半ば等閑された数Torr前後の中間的圧力領域のメゾプラズマに着目し、その中核を成すナノクラスターのその場計測に基づくメゾプラズマエピタキシー技術の学術基盤展開と、低温高速エピタキシャル薄膜堆積技術確立を目指した研究をまとめている。

本論文は以下の五章から成る。

第一章は序論であり、大面積シリコンデバイスの中でも特に太陽電池応用に関わる技術動向と技術要求、多様なエピタキシャル薄膜堆積技術の原理並びに現状と将来展開に向けた課題について詳述し、学術的、技術的観点より本研究の位置付け、目的を明確化している。

第二章では、メゾプラズマCVDに関連する先行研究をまとめ、従来のシリコン薄膜堆積技術と対比しながら、メゾプラズマの特徴、基本的堆積機構の相違点について詳述されている。具体的には、低ガス温度(Tg)でありながら、電子温度(Te)が低いためイオン衝撃が抑制され(Te<1eV, 1< Te/Te<10)、加えてプラズマフローによる高原料ガス流束を特徴とすることから、従来の低圧プラズマや熱プラズマとは異なるプラズマ特性を有することを示唆している。またプラズマ分光により原料シランガスの完全分解に伴うプラズマ内部での原子状シリコンを確認し、基本堆積機構は基板直上でのプラズマガスの凝縮に伴い成膜前駆体が形成されること、境界層が前駆体形成制御に重要であることを示している。

第三章はメゾプラズマCVDで実現する低温高速エピタキシーの特徴をまとめている。具体的には、圧力を6Torrに固定して、成膜過程に大きく影響を与えるシラン原料ガス流量、基板温度に注目し、薄膜組織と電気特性との相関について詳細に調査している。特筆すべき結果として、高周波入力22kWにて、350℃程度の低温でありながら、40nm/secを超える堆積速度にてエピタキシャル薄膜を高々2sccmのSiH4ガス流量で実現したこと、更にホール移動度で代表される電気特性も基板温度に依存せず、300cm2/Vs程度の高い値を維持できることを挙げている。また、エピタキシャル成長速度が基板温度に影響を受けず原料ガス濃度に比例して増加することから、従来のエピタキシャル成長とは異なる成長モードである事を明らかにしている。

第四章では、プラズマ/基板境界層内(-数100μm)で形成されるナノクラスターが低温高速エピタキシーを実現する決定因子であることを提唱している。具体的には、計測領域(散乱ベクトル)0.2-1.8nm-1を0.004nm-1の精度でその場計測しうる実験室系X線小角散乱装置を組み上げ、メゾプラズマエピタキシャル成長時に、平均2nm程度の粒径を有するナノクラスターその場計測に世界に先駆けて成功した。特筆すべきは、球状クラスター表面の拡散的な電子構造を反映したスペクトルにより、シリコン原子が粗に結合し熱的に活性な状態にある所謂ホットクラスター様の基本構造を見いだしたこと、また多結晶成長時には5nm程度の大型でrigidなクラスターが形成され、これが多結晶化の一因となりうる事を見いだしたことである。本計測結果に基づいて、当該ナノクラスターが基板表面衝突に伴い原子へ分解され、これら熱的に活性な構成原子が低温でありながら十分に表面拡散しうることが低温高速沿面成長を実現させる基本メカニズムであると提案している。

第五章ではクラスター支援成長の特徴をシリコン薄膜表面でのプラズマ/基板間の相互作用により考察している。特に水素原子の効果について検討し、その成膜機構への影響、メゾプラズマ環境下での役割、電気特性との相関を示している。具体的には、AFMによる成長表面形態を精査すると共にCorrelation functionを用いた確率論的手法を併用して、エピタキシャル成長時のステップ成長を確証すると共に多結晶成長時には局所的な表面拡散が支配する成長モードである事、水素導入によりナノクラスター性状には顕著な変化は見られないが220mTorr以上の添加でステップ成長が促進される事を示している。またプラズマ照射に伴うシリコン表面の特異な形態を確認し、1x1018/cm2sec程度と推算される極めて大きな原子状水素フラックスがメゾプラズマ環境では容易に得られ、本プロセスを特徴づける要因であることを示している。

第六章は総括であり本研究で得られた成果を総括している。

以上を要するに、本研究は、メゾプラズマ及び特異なナノクラスターについて焦点を絞り、既往の薄膜堆積機構とは異なるアプローチにより、新規低温高速エピタキシャル薄膜化技術を展開し大面積デバイス薄膜化技術、並びにプラズマプロセス分野に新たな視点を導入する成果として、材料工学に対する貢献は大きい。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。