1. Introduction

Wafer bonding offers flexible and inexpensive ways for manufacturing silicon-on-insulator (SOI), micro/nano electromechancal systems (MEMS/NEMS), three-dimensional (3D) integration, and 3D-nanostrutures with unprecedented functions. High-precision wafer bonding with submicron or even nanometer scale is necessary to achieve well-aligned bonding structure enabling fabrication of many advanced devices and realizing accurate interconnections to be made between layers. The required wafer diameters are increased from 300 mm to 450 mm in near future, whereas the minimum device dimensions of wafers are continuously reduced to provide increasing circuit density. Thus, to achieve high-precision wafer bonding, perfect alignment method as well as low temperature bonding process is indispensable.

Nowadays, a variety of direct alignment and indirect alignment methods have been developed over the last decades. For most of typical alignment methods, the alignment process is generally performed by means of "cross-and-box" or "cross-on-cross" alignment marks, alternatively known as "alignment keys". In either direct or indirect alignment, optical or infrared (IR) microscope is used to detect the positions of alignment marks. Thus, the alignment accuracy is limited by diffraction effects. Current available wafer-to-wafer alignment accuracies are in the range of 0.5~1 μm in laboratory-environments and in the range of 1~ 3 μm for commercial industry tools. Vernier patterns are extensively used to calibrate wafer-to-wafer alignment due to its higher accuracy more than just optics. But that the verniers line-up is still difficult to be confirmed using optical microscope directly especially in nanometer scale. Moire pattern is a promising candidate to resolve this problem as well as convenient for observation. Moire fringes created by two similar linear gratings or concentric circular gratings have been commonly used to measure linear displacements, since they enable the manifestation of very small relative displacements in large moire fringe movements. Several types of moire fringes superimposed by two radial gratings, skewed-radial gratings, Fresnel zone plates, and elongated circular gratings are known for the metrology of planar displacements. However, the differences between perfectly aligned moire fringes and very small misaligned ones, e.g. sub-pitch misalignments, are still difficult to be identified by microscope without external reference. Sensitive, no relying on any external reference, two-dimensional (2D) moire gratings are preferred to assist high-precision alignment.

On the other hand, conventional wafer bonding is usually performed in ambient air at room temperature, and annealing at temperatures higher than 1000°C is necessary to achieve high bonding strength. To realize a low-temperature annealing, process, many efforts have been made in recent years. Among them, O2 (or Ar, N2) plasma activated bonding is a low-cost solution since it does not requiring a high-vacuum system. But the bonding strength of bonded wafer pairs, e.g. silicon direct bonding, is weak at room temperature and low-temperature annealing (200-400°C) is still necessary. It means even if the wafer-to-wafer alignment has been carried out perfectly, misalignments resulting from temperature gradient and thermal expansion mismatch between integrated materials are undesirable in annealing process. Moreover, a large number of voids are generated during the annealing process. A room temperature bonding with no requiring annealing is an optimum method to avoid temperature-related problems on high-precision wafer bonding and device fabrication. Although many works have concentrated on plasma activated bonding , to our knowledge, the room temperature silicon direct bonding by plasma activation without wet chemical cleaning step has not been reported yet. It is a challenge but strongly needed to realize high-precision wafer bonding.

2. Purpose

To realize high precision wafer bonding, the alignment with high accuracy and low temperature bonding are two separate and vital steps. This research work focuses on development of two novel processes that are able to achieve perfect alignment and room temperature wafer bonding, respectively. The purpose of this research includes (1) development of 2D moire pattern for realization of wafer-to-wafer alignment with nanometer-scale accuracy; (2) development of a novel room temperature wafer bonding method without requiring annealing and wet chemical cleaning processes; and (3) by means of moire fringe assisted alignment method and fluorine containing plasma activated bonding process, strategies of high-precision wafer bonding are proposed.

3. Summary of Chapters

Chapter 1 reviews wafer bonding and its applications briefly. Wafer alignment and direct bonding methods, two key techniques for precise wafer bonding, are mainly overviewed in this chapter. We address the limitation of typical alignment methods and their improvements. On the other hand, the problems of current wafer bonding methods are described.

In Chapter 2, the motivation and purpose of this dissertation are presented.

In Chapter 3, a novel 2D-centrosymmetric square moire pattern is developed for non-destructive measurement of misalignment between the bonded wafer pairs. Simulation results show the mismatched moire fringes produced by the two centrosymmetric square patterns are highly sensitive with the misalignments and misaligned directions without requiring any external reference. In our experiment, the misalignments in the order of ±0.2 μm in X-Y axis can be resolved by IR microscopy images, as an example. This value can be further improved to sub-100 nm range if the parameters of the moire gratings are optimized. Using two pairs of these moire square gratings, misalignments of the bonded wafers are determined in X-Y-O axis simultaneously on wafer-scale. Moreover, the measurement of misalignments using the moire gratings does not depend on the gap between the two wafers changing from tens of micrometers to zero during alignment process. Thus, these moire gratings are suitable for being the secondary alignment mark to achieve high-precision alignment. The moire fringe assisted alignment method is proposed for realization of nanometer-range precision alignment. By optimizing parameters of the square moire pattern, the alignment accuracy assisted by the moire fringes is expected to be tens of nanometers.

In Chapter 4, room temperature wafer bonding method without wet chemical cleaning as well as no requiring annealing was developed by fluorine containing plasma activated bonding process.

Fluorine containing O2 RIE plasma treatment is successfully used for surface activation. The activated wafers without wet chemical cleaning are brought into contact in ambient air. After storage at room temperature (~25°C) for 24 h, high bonding strength of Si/Si (~2.4 J/m2) is achieved without requiring any annealing step. This value is close to the bulk-fracture strength of silicon. No void is observed at Si/Si interfaces. Furthermore, this fluorine containing oxygen plasma activated process is also effective for combination of Si/SiO2, SiO2/SiO2, Si/Si3N4 wafer pairs. The bonding mechanism can be explained by the reaction between reactive fluorinated silicon oxide layers resulting in covalent bonding at room temperature. In addition, adding fluorine to oxygen plasma can mitigate void formation in subsequent annealing process. Especially for a short fluorine containing plasma treatment, it is nearly void-free even if the bonded wafer pairs are heated from 200 to 800°C.

In Chapter 5, based on nanoprecision alignment and room temperature bonding, the process flow of high-precision with ex-situ or in-situ strategy is proposed to provide potential for high-precision wafer bonding equipment.

4. Conclusions

The conclusions of this dissertation is summarized as below

(1) A novel 2D moire pattern is developed to assist realization of high-precision wafer-to-wafer alignment and non-destructive measurement of misalignments for wafer bonding. Its accuracy can be improved into tens of nanometers.

(2) Room temperature wafer bonding method without wet chemical cleaning as well as no requiring annealing is developed by simple fluorine containing plasma activated bonding process. It is effective for combination of silicon and silicon-based materials.

(3) By means of the moire fringe assisted alignment and room temperature fluorine containing plasma activated bonding, high-precision wafer bonding process is expected to be realized with ex-situ or in-situ strategy. It can be applied for not only the future 3D integration of wafer-scale, but also the fabrication of 3D nanostructures and wafer-level NEMS packaging.

本論文の目的は、モアレ縞による高精度位置合わせマーク計測およびフッ素添加プラズマ活性化手法による低温接合を用いることにより、シリコンウエハの高精度位置決め接合を可能にする手法を開発したものである。

ウエハ接合は、半導体の3次元実装や高密度実装のため,近い将来,100nmのオーダの位置決め精度での高精度接合が必要とされている。しかし、現状では、通常のアライメントマークのIRカメラでの読み取りでは、0.5~1μmの位置合わせが限界とされている。

本論文では、これに対し、従来ウエハ接合のアライメントには使われていなかったモアレパターンを利用して高精度の位置決めを実現することを提案している。接合するウエハの対向する場所に、ピッチのわずかに異なる縞状のパターンを形成し、これが接合時に重なるようにすると、いわゆるモアレ縞が観察される。ウエハの位置がわずかにずれると、このモアレ縞が数10~100倍の倍率で移動する。これにより、分解能の低いIRカメラによる透過像であっても、高精度の位置合わせが可能になる。モアレ縞による位置決めについては、これまで、2つのモアレパターンを組み合わせ、1次元の位置決めに利用した例が知られている。しかし、これの従来のモアレパターンをそのまま2次元に利用しても、1)対応するウエハの位置に対称なパターンを形成する必要があり、それぞれのウエハに別のモアレパターン形成用のマスクが必要である、また、2)高精度位置決めの前に粗位置決めを行うため、従来の位置決めマークを併用する必要があり、その分、位置決めマークが占める面積が大きくなる、という欠点が予想される。

本研究の独創的な点は、これに対し、正方形を8分割した領域にピッチの異なる縞状パターンを交互に配置した中心対称パターンをモアレパターンをして使うことを提案した点にある。これにより、一つのマークで粗位置決めと高精度位置決めの両方を行うことが可能となり、かつ、同じマークを接合するウエハの対応する位置に形成すればいいので、パターン形成用のマスクについても1枚で済むことになる。さらに、この正方形の角を落とした菱形マークで同じ位置決めが可能なことから、マークが占める面積を最小限にすることができる。

審査の過程では、この提案された手法について、精度の限界がどの程度になるのかが議論となった。これについて、本論文では、モアレパターンを利用した位置決め精度の限界について詳細な検討が行われ、最終的に、この手法により、10 nmオーダの位置決めが可能であることが示された。

また、高精度のウエハ接合の必要条件としての低温接合についても、本研究では、新しい手法が提案された。すなわち、従来の酸素プラズマ活性化接合に対し、フッ素添加ガスを用いたプラズマ活性化が、接合強度の向上ならびに、アニーリング時のボイド発生の低減に劇的な効果があることを示したものである。本研究では、フッ素添加ガスの種類、流量、プラズマ励起源、電力、照射時間、接合雰囲気、加圧条件、保持時間などのプロセス条件を最適化し、従来のプラズマ処理では0.6J/m2程度であった接合強度をシリコンのバルク強度に近い2.5J/m2に近い接合強度を実現した。また、プラズマ処理面の濡れ性の検討やXPSによる表面電子状態の検討、FTIRによる接合界面の観察、ボイド発生の条件の検討を通して、フッ素添加プラズマによる接合強度の上昇のメカニズムを提案、具体的に検証した。

審査の過程では、この低温接合の新プロセスの提案と前述のモアレ縞を利用した高精度位置決めの関係について、その組み合わせの必然性が明確でないとの指摘があった。これに対し、最終論文では、この2つの新しい提案を組み合わせた、高精度低温ウエハ接合の新しい接合装置とプロセスが提案された。すなわち、2つのex-situ手法と1つのin-situ手法であり、それぞれのプロセスについて、容積、スループット、コストなどの詳細検討比較が行われ、提案手法の従来手法に対する優位性が極めて明快に示されることになった。

以上の結果から、本研究では、従来の手法では困難であった高精度のウエハ接合を高精度の位置決め手法の提案と、低温接合の新しいプロセスの提案、さらにその組み合わせにより可能であることを明確に示したものであり、その独創性が高く評価された。以上のように、本研究で得られた工学的知見は極めて大きく、また、工学の発展に寄与するところは多大である。

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。