Metal/ceramic interface systems have become of great importance for a variety of scientific research and technological applications. In particular, Aluminum(Al)/ceramic substrate system is widely used in power electronics, which requires device reliability in severe application conditions. Thus, interface strength between Al metal and ceramics is of primal importance. In industries, Al-Silicon(Si) alloy is typically used to lower the bonding temperature. However, the fundamental mechanism of Al/ceramic hetero-interface formation; Si behavior during the bonding processes; interface structures and interfacial strength have not been clarified in detail.

A fundamental issue in characterizing interfacial structures is to identify the crystallographic orientation relationships between adjacent crystals. Orientation relationships (ORs) in metal/ceramic systems have been widely studied because they have direct influence on the hetero interface structures and their properties. Therefore, theoretical calculations are helpful to evaluate the stable ORs for geometrical coherency, which may indicate the minimization of the total interface energy resulting to a stable interface. One method in predicting the crystallographic orientation relationships in hetero systems based on geometrical coherency is coincidence of reciprocal lattice point (CRLP) model. CRLP determines the optimum ORs between two crystals through the calculation of the maximum overlap of the RLPs of two adjoining crystals. The sum of the overlapped volumes represents the degree of parallelism of the sets of planes of the two lattices and their coincidence in interplanar spacings. It is considered that the maximum overlapped volume corresponds to the optimum three-dimensional geometrical coherency.

In this PhD thesis, high-resolution transmission electron microscopy (HRTEM) was used to investigate the stable orientation relationships and interface structures of model metal/ceramic systems formed by liquid-phase bonding and pulsed laser deposition (PLD). To theoretically predict the stable orientation relationships in metal/ceramic systems, coincidence of reciprocal lattice point (CRLP) calculation was performed.

Two common methods of fabricating metal/ceramic interfaces are by liquid phase joining process and thin film deposition. In this research work, a "sandwiched" structure of・-Al2O3(0001)/Al or Al-10weight%Si alloy/・-Al2O3(0001) was fabricated in vacuum using a hot-press process. The sandwiched structures were bonded at a temperature above the melting point of the metal. The same processing conditions were applied using commercially available epitaxial AlN(0001)/・-Al2O3(0001) substrates to study Al/AlN and Al(Si)/AlN hetero interfaces. PLD samples of Al/・-Al2O3 (0001), Al/AlN(0001), Si/・-Al2O3 (0001) and Si/AlN(0001) were prepared at a medium temperature range of 200-700oC. Laser intensity was set at 40 mJ at a vacuum of 10-5Pa in argon environment.

TEM samples were prepared by a standard procedure, using ion thinning method. The cross-sectional interface structure observations and diffraction analyses were performed using TEMs (JEOL JEM-4010, JEM-2010) operated at 400KV and 200KV, respectively. Compositional analyses of the interfaces were performed by a scanning transmission electron microscope (JEOL JEM-2100F) combined with energy dispersive x-ray (EDX) analysis and electron energy loss spectroscopy (EELS) analysis.

The TEM images clearly showed that the step structures of sapphire, ranging from a few nanometers to ~80nm, are formed at the interface during the bonding processes, although the initial substrate surface is expected to be flat. The detailed diffraction analysis showed that there seems to be no strong preferred orientation relationship between Al and ・-Al2O3. The Al/・-Al2O3 interface is atomically sharp in between the steps. The formation of step structures at the hetero interface is also reported elsewhere. Consistent with the earlier studies on aluminum oxidation at the interface, the present results suggest that molten Al and oxygen from the bonding environment and/or Al native oxide layer may react to each other to form epitaxial Al2O3 step structures during the bonding processes.

In the TEM observation of Al-Si alloy/・-Al2O3 interface, the step height is significantly reduced compared with the pure Al case. Presence of Si at the interface was detected by TEM-EDS. Precipitates were also observed at the interface by TEM. These precipitates were confirmed to be Si by electron diffraction and STEM-EELS analyses. HRTEM images of the interfaces between silicon precipitates and sapphire substrate showed two different orientation relationships with sapphire for the bonding temperatures of 645°C and 610°C. At the higher temperature bonding condition (645°C), a stable OR between epitaxial Si and・-Al2O3 substrate was determined to be [110(_)]Si//[110(_)0]・-Al2O3, (111)Si//(0001)・-Al2O3. While at the lower bond temperature (610°C), a different OR between the Si precipitate and the・-Al2O3 substrate was observed. In both temperatures, silicon precipitates formed atomically sharp interfaces with sapphire (0001) surfaces. Experimentally found strong preference of silicon at the interface suggests that aluminum oxidation at the interface may be strongly influenced by the Si segregation. First principles calculations also support the tendency for Si to segregate at the interface. The formation of silicon precipitates with stable interface structures may also be one of the reasons to inhibit step growth reaction at the interface.

Pure Al and Al-10weight%Si alloy were also bonded to AlN(0001) ceramic substrate. AlN is considered to be a very good thermal conductor for power electronics applications. The detailed diffraction analysis showed that there seems to be no epitaxial relationship between Al and AlN in the liquid state bonded interfaces. Unlike in the Al/・-Al2O3 interfaces, Al/AlN interface is relatively flat without any step structures. In the Al-Si alloy/AlN case, Si segregation at the interface was also confirmed by TEM-EDS. Upon extending the bonding time to a few hours, formation of a textured layer of Al-Si was observed by TEM. In specific areas at the interface, Al (with some dissolved Si) formed an orientation relationship with AlN substrate with about 4o tilt of Al(111) on AlN(0001) interface plane. A stable unique OR was discovered for the observed tilt on Al(111) and was identified as [11(_)0]Al//[112(_)0]AlN , (001)Al//(22(_)03)AlN by CRLP calculations. The preferred parallelism of Al(001) and AlN(22(_)03) suggests a strong interaction between the two planes.

To determine the stable interface structures for Al and Si on ceramic substrates using a different bonding process, a simple pulsed laser deposition technique was used. Al and Si islands were formed on the・-Al2O3 and AlN substrates using PLD method. The OR of epitaxial Al/・-Al2O3 was determined to be [110(_)]Al//[110(_)0]・-Al2O3, (111)Al//(0001)・-Al2O3. In contrast, the detailed diffraction analysis showed that there seems to be no epitaxial orientation relationship between Al and AlN. Only a textured growth of Al(111) plane parallel to the basal plane of AlN was observed. CRLP calculation showed that preferred Al/AlN OR has a lower geometrical coherency and with the associated higher misfit parameter compared with the most stable OR of Al/・-Al2O3. CRLP calculation then supports the textured Al(111) plane on AlN(0001) which can orient itself in multiple directions without experiencing a large change in misfit at the interface. Similarly, Si shows a texture of (111) plane to be parallel to・-Al2O3 (0001) and AlN(0001). Locally oriented Si/・-Al2O3 was observed in specific areas at the interface. The orientation relationship was determined as (111)[11(_)0]Si//(0001)[11(_)00]・-Al2O3, which is the same as the most stable orientation relationship of Si precipitates in liquid-phase bonded Al-Si/・-Al2O3 interface. PLD Si/AlN also shows a textured layer with local epitaxial relationship of (111)[11(_)0]Si//(0001)[112(_)0]AlN.

CRLP can predict the most stable primary OR as well as other preferentially stable orientations, which hereafter will be referred as secondary ORs. Lattice misfits of the perpendicular planes to the interface werer also calculated for each respective OR. Lattice misfit with respect to the substrate was estimated as:・ = (dsapphire - dmetal)/dsapphire x 100%. The minimum misfit of the perpendicular planes at the interface was considered in this thesis.

In a wide survey of metal/・-Al2O3 systems, four common ORs were experimentally identified which belong to either (111)Metal//(0001)・-Al2O3 or (110)Metal//(0001)・-Al2O3. OR A was identified as: (111)[01(_)1]Metal//(0001)[01(_)10]・-Al2O3. OR B can be obtained by a 30o rotation of the metal around the [111] axis with respect to OR A, and described as: (111)[11(_)0]Metal//(0001)[211(__)0]・-Al2O3. On the other hand, in precipitation and some textured film processes, stable ORs sometimes become very different from the above ORs. These ORs are Pitsch-Schrader (PS) described as (110)[001]Metal//(0001)[101(_)0]・-Al2O3, and Burgers described as (110)[1(_)11]Metal//(0001)[11(_)00]・-Al2O3.

CRLP predicted ORs are in good agreement with the experimentally observed ORs in the whole range of metal lattice parameters. While (111)Metal//(0001)・-Al2O3 is widely observed in epitaxial thin film deposition processes for both bcc and fcc metals, (110)Metal//(0001)・-Al2O3 iss commonly found in precipitation methods where parallelism of close-packed planes is a key factor for the growth mechanism of the precipitates. CRLP predicted these orientations as secondary preferentially stable ORs. These results suggest that CRLP is most powerful in the predication of stable ORs of metals on・-Al2O3, which are grown by film deposition methods.

In order to demonstrate the applicability of CRLP to predict not only for primary ORs but also for secondary ORs, Al and Si/・-Al2O3 systems were considered as representative systems from the list of surveyed metal/・-Al2O3 systems. OR A matched the experimentally observed most stable OR for the epitaxial Al/・-Al2O (PLD) and for the high temperature Si precipitate in the liquid-phase bonded Al-Si/・-Al2O3 interface. In addition, the three-dimensional profiles suggest the tendency of Al and Si to form its secondary ORs around two orthogonal axes (・・・・・. The second and third peaks were found for Al (OR B and OR C) around the [111] rotational axis (・・. OR C was identified as: (111)[51(_)4(_)]Metal//(0001)[110(_)0]・-Al2O3 at ・=11o, which was experimentally observed similarly for Pt and Cu. However, Si showed a significant secondary peak at・=35o around the [101(_)] rotational axis, which matched with the PS relationship as also observed experimentally in this present study on liquid-phase bonded Al-Si interface. In these model systems, CRLP clearly showed the most stable OR as well as the tendency for forming secondary ORs, which well matched with the previous experimental reports.

In summary, this PhD thesis has shown the stable interface structures in metal/ceramic systems using two common bonding methods, which are liquid phase joining and pulsed laser deposition. The liquid state hot-press process conditions, did not show any epitaxial relationship of aluminum on sapphire. Silicon was observed to significantly decrease the sapphire step structures at the interface. It was found that the bonding temperature of 645・C is enough for silicon precipitate to form an epitaxial relationship with sapphire. The same low energy OR can be found in most FCC metals epitaxial with the basal plane of sapphire. The present results indicate that Si is likely to segregate to the ・-Al2O3 surface and affect the interfacial reaction between Al and sapphire. On the other hand, Al on AlN showed a 4otilt of Al(111) with respect to AlN(0001), which corresponds to a unique OR. In pulsed laser deposition method, epitaxial thin film of Al on・・-Al2O3(0001) was successfully fabricated. In contrast only a textured Al(111) on AlN(0001) was observed. While Si on ・-Al2O3(0001) and AlN(0001) showed texturing with localized epitaxy. CRLP method was used to predict the stable ORs, which were found in this present study as well as in a wide range of other metal/・-Al2O3 hetero interface systems. CRLP calculations could successfully estimate these preferential ORs and their relative stabilities. CRLP is thus considered to be a powerful tool to predict the most stable OR and other preferentially stable secondary ORs for most metal/・-Al2O3 systems.

金属/セラミックス界面は工業的に様々な分野に応用されており、材料特性を決定づける重要な因子である。特に、金属アルミニウム(Al)/セラミックス界面はパワーエレクトロニクスの分野で盛んに用いられており、過酷な環境下で安定して用いることのできる信頼性、耐久性が求められている。界面の信頼性、耐久性の向上には、界面構造を制御する指針を構築し、的確な材料設計を行うことが不可欠であるが、Al/セラミックス界面の界面構造に関しては未だ統一的な理解に乏しく、試行錯誤による界面形成が行われているのが現状である。そこで、本研究ではAl/セラミックス界面の界面構造形成メカニズムの解明と界面構造を決定づける因子の抽出を目指して、実験・理論両面からのアプローチを駆使して研究を行った。特に実験では、高分解能透過型電子顕微鏡法(HRTEM)を用いた原子スケールからの界面構造解析、理論計算においては第一原理計算及び逆格子一致計算(CRLP)を行った。本論文は5章から構成されている。

第1章は緒言であり、これまでのAl/セラミックス界面に関する研究及び実応用例について概説し、界面形成における微構造制御及びその解析の必要性と重要性について述べている。また、その中で、本研究の位置づけ、必要性、新規性、独創性などについて記述し、本研究の主目的について述べている。

第2章では、液相接合法によって作製したAl/サファイア界面、Al-Si/サファイア界面、Al/AlN界面、Al-Si/AlN界面に関してHRTEMによる高分解能構造解析とTEM-EDSによる組成分析、電子回折による界面方位解析を行った結果について述べている。その結果、今回用いた液相接合条件においてはAlとセラミック基板に優先的な結晶方位関係は存在しないことが明らかとなった。また、液相法では界面部にステップ構造が形成されやすいことが明らかとなり、この構造は添加されたSiが界面偏析することで大幅に抑制できることが明らかとなった。このようなSi偏析の傾向は第一原理計算による理論解析によっても示されており、本結果は微量な添加元素により界面構造の制御が可能であること示唆している。またSiはナノメーターサイズで析出する場合も存在し、その際にはSiとセラミック基板には優先的な結晶方位関係があることを見出した。

第3章では、第2章とは異なる界面形成プロセスであるパルスレーザー堆積法により作製したAl/サファイア、Al/AlN、Si/サファイア、Si/AlN界面に関してHRTEMによる系統的な観察を行った結果を述べている。その結果、液相法とは異なり、本手法により作製された界面には優先的な結晶方位関係が存在することが明らかとなった。この結果は、界面形成プロセスに依存して原子レベルの界面構造は大きく変化し、真空堆積法により形成されたAl/セラミック界面はエピタキシャルな方位関係を取りやすいことが明らかとなった。このように、異なるプロセスによる同一界面を系統的に調査することにより、界面構造のプロセス依存性を明確に抽出することに成功した。

第4章では、今回観察されたAl/セラミック界面における優先結晶方位関係を理解するために、逆格子一致法(CRLP)による系統的な解析を行った。CRLP法は、Alとセラミック結晶の逆格子をコンピュータ上で再現し、各逆格子点に適当な体積を与えたのち、3次元的に可能なすべての方位関係における逆格子球の重なり体積を計算し、重なり体積のトータル値によって界面の整合性を評価する手法である。本手法を用いてAlとセラミック結晶の整合性を評価した結果、本研究で観察された優先方位関係はいずれも結晶学的に極めて整合性の高い方位関係であることが明らかとなった。このように整合性が高い界面は構造的に安定な界面であることが予測されることから、プロセス制御により安定界面を形成できる可能性が示唆された。また、一方向からの観察では方位関係的に整合性が低いと予測された界面が、3次元的に解析することによって高い整合性を有することが初めて明らかとなり、界面方位を3次元的に考えるアプローチの重要性が示された。

第5章は総括である。

このように本論文は、工業的に重要なAl/セラミック界面に対して実験・理論両面を用いて系統的な解析を行っており、プロセスの重要性、添加物効果の重要性、結晶方位関係の重要性に関して明確に説明することに成功している。また、これらの因子を制御する指針に関しても新たな知見を提案しており、科学的に得られた知見のみに留まることなく実際のデバイス開発への展開も見据えた研究内容として評価できる。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。