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Semiconductor wafer bonding has increasingly become a technology of choice for materials integration in microelectronics, optoelectronics, and microelectromechanical systems. The present overview concentrates on some basic issues associated with wafer bonding such as the reactions at the bonding interface during hydrophobic and hydrophilic wafer bonding, as well as during ultrahigh vacuum bonding. Mechanisms of hydrogen-implantation induced layer splitting ͑''smart-cut'' and ''smarter-cut'' approaches͒ are also considered. Finally, recent developments in the area of so-called ''compliant universal substrates'' based on twist wafer bonding are discussed.

Large-area pulsed laser deposition (PLD) has reached a state in terms of film quality and reproducibility which makes possible now real market applications of PLDYBazCu3074 (YBCO) thin films on both sides of R-plane sapphire substrates as HTSC devices in satellite and mobile communication systems. Bandpass filters optimized from PLD-YBCO thin films presently fulfill the requirements of the main national companies which are active in future communication techniques. A relatively simple PLD arrangement with fixed laser plume and rotating substrate, with an offset between the laser plume and the center of the substrate is employed to deposit laterally homogeneous 3-inch diameter Ag-doped YBCO thin films. With the experience of more than 1, OOO doublesided 3-inch diam. films a high degree of homogeneity and reproducibility of j, and R is reached. The extension up to &inch substrate diameter will increase the productivity of the flexible PLD technique considerably.Index Terms-High-temperature superconducting thin films, microwave devices, pulsed laser deposition, YBazCu3074.

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Epitaxial c-oriented YBCO films laser deposited onto 3 in diameter CeO 2-buffered sapphire wafers and LaAlO 3 cylinders as well as sputter deposited onto 2 in diameter LaAlO 3 wafers were characterized by integral and spatially resolved measurements of the critical current density j c and the microwave surface resistance R s , by microstructure investigations using optical and electron microscopy and by x-ray diffraction. Epitaxial misorientations of in-plane-rotated as well as of a-axis-oriented grains were found in amounts up to 10%. The in-plane rotation seriously degraded R s while the a orientation mainly lowered j c. Moreover, a degradation of R s and of the microwave power handling could be clearly correlated with the density of microcracks occasionally found in YBCO films on sapphire. Inhomogeneities like a-axis-oriented grains were observed to 'disperse' microcracks, probably in favour of the electrical properties. The impact of further microstructure imperfections on R s , in particular of the typical twin lamellae and their domains, is discussed in view of findings from transmission electron microscopy.

Microcracking of epitaxially c‐oriented YBa2Cu3O7–δ and GdBa2Cu3O7–δ thin films is observed to preferentially occur on buffered, R‐cut sapphire substrates of 3 in diameter in comparison to smaller substrates and other substrate materials such as MgO, SrTiO3, and LaAlO3. The area density and the crystallographic appearance of the crack pattern can vary considerably across a given surface, from sample to sample, and in dependence on the temperature treatment. These dependencies indicate the crack behaviour to be seriously affected by the microstructure. The latter is studied by optical, electron, and atomic force microscopy. Cross‐sectioning TEM reveals cracks to locally separate the HTSC film from the buffer layer. Optical microscopy using transmitted polarized light is found to be a straightforward, sensitive survey method to observe the microcracking behaviour.