When mirror-polished, flat, and clean wafers of almost any material are brought into contact at room temperature, they are locally attracted to each other by van der Waals forces and adhere or bond. This phenomenon is referred to as wafer bonding. The most prominent applications of wafer bonding are silicon-on-insulator (SOI) devices, silicon-based sensors and actuators, as well as optical devices. The basics of wafer-bonding technology are described, including microcleanroom approaches, prevention of interface bubbles, bonding of III-V compounds, low-temperature bonding, ultra-high vacuum bonding, thinning methods such as smart-cut procedures, and twist wafer bonding for compliant substrates. Wafer bonding allows a new degree of freedom in design and fabrication of material combinations that previously would have been excluded because these material combinations cannot be realized by the conventional approach of epitaxial growth.

Semiconductor wafer bonding

A multistep pulsed-laser deposition (PLD) process is presented for epitaxial, nominally undoped ZnO thin films of total thickness of 1 to 2 μm on c-plane sapphire substrates. We obtain reproducibly high electron mobilities from 115 up to 155 cm2/V s at 300 K in a narrow carrier concentration range from 2 to 5×1016 cm−3. The key issue of the multistep PLD process is the insertion of 30-nm-thin ZnO relaxation layers deposited at reduced substrate temperature. The high-mobility samples show atomically flat surface structure with grain size of about 0.5–1 μm, whereas the surfaces of low-mobility films consist of clearly resolved hexagonally faceted columnar grains of only 200-nm size, as shown by atomic force microscopy. Structurally optimized PLD ZnO thin films show narrow high-resolution x-ray diffraction peak widths of the ZnO(0002) ω- and 2Θ-scans as low as 151 and 43 arcsec, respectively, and narrow photoluminescence linewidths of donor-bound excitons of 1.7 meV at 2 K.

High electron mobility of epitaxial ZnO thin films on c-plane sapphire grown by multistep pulsed-laser deposition

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

Infrared dielectric functions and phonon modes of high-quality ZnO films

An intermediate substrate includes a handle substrate bonded to a thin layer suitable for epitaxial growth of a compound semiconductor layer, such as a III-nitride semiconductor layer. The handle substrate may be a metal or metal alloy substrate, such as a molybdenum or molybdenum alloy substrate, while the thin layer may be a sapphire layer. A method of making the intermediate substrate includes forming a weak interface in the source substrate, bonding the source substrate to the handle substrate, and exfoliating the thin layer from the source substrate such that the thin layer remains bonded to the handle substrate.

Bonded intermediate substrate and method of making same

Elastic effects on surface physics

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” approaches) are also considered. Finally, recent developments in the area of so-called “compliant universal substrates” based on twist wafer bonding are discussed.

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Fundamental issues in wafer bonding

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 PLD-YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (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 fulfil 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 1,000 double-sided 3-inch diam. films a high degree of homogeneity and reproducibility of j/sub c/ and R/sub s/ is reached. The extension up to 8-inch substrate diameter will increase the productivity of the flexible PLD technique considerably.

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High-quality Y-Ba-Cu-O thin films by PLD-ready for market applications

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.



Mikrorisse von epitaktisch c-orientierten YBa2Cu3O7–δ- und GdBa2Cu3O7–δ-Dunnschichten zeigen sich bevorzugt auf mit Pufferschicht versehenen, R-orientierten Saphirsubstraten von 3 in Durchmesser im Vergleich zu kleineren Substraten und anderem Substratmaterial wie MgO, SrTiO3 und LaAlO3. Die Flachendichte und das kristallographische Bild der Risse kann betrachtlich variieren, und zwar langs einer gegebenen Oberflache, von Probe zu Probe sowie in Abhangigkeit von der Temperaturbehandlung. Diese Abhangigkeiten deuten auf einen starken Einflus der Mikrostruktur. Letztere wird mittels optischer, Elektronen- und Rastersondenmikroskopie untersucht. Die TEM-Querschnittsabbildung zeigt Risse, welche die HTSL-Schicht lokal von der Pufferschicht trennen. Die Lichtmikroskopie im polarisierten Durchlicht erweist sich als eine rationelle, empfindliche Ubersichtsmethode zur Beobachtung des Risverhaltens.

Microcracks observed in epitaxial thin films of YBa2 Cu3O7–δ and GdBa2Cu3O7–δ†

We present a technique for the fabrication of materials integration of (100) silicon and (100) gallium arsenide by direct wafer bonding GaAs wafers 3 in in diameter were hydrophobically bonded to commercially available 3 in silicon-on-sapphire wafers at room temperature After successive annealings in hydrogen and arsenic atmospheres at temperatures up to 850 °C the Si/GaAs interfacial energy was increased by the formation of strong covalent bonds Due to the difference in the lattice constants of about 41%, extra Si lattice planes were observed at the interface No threading dislocations were introduced into the GaAs

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Materials integration of gallium arsenide and silicon by wafer bonding

A novel fabrication process of ferroelectric-semiconductor heterostructures based on direct wafer bonding has been demonstrated. Polycrystalline Bi4Ti3O12 ferroelectric thin films were deposited on 3 in. silicon wafers using chemical solution deposition. The films were polished and then directly bonded to silicon wafers in a micro-cleanroom. After thermal annealing in air at 500 °C for 12 h, the bonding energy increases up to 1.5 J/m2. High resolution transmission electron microscopy shows the difference between the bonded and reacted interfaces. Obtaining a metal-ferroelectric-silicon (MFS) structure containing the ferroelectric-Si bonded interface was achieved by polishing down and etching the handling wafer. The Bi4Ti3O12 film kept its ferroelectric properties as shown by C–V measurement.

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G. Kästner

Papers

Fundamental issues in wafer bonding

High-quality Y-Ba-Cu-O thin films by PLD-ready for market applications

Microcracks observed in epitaxial thin films of YBa2 Cu3O7–δ and GdBa2Cu3O7–δ†

Materials integration of gallium arsenide and silicon by wafer bonding

Ferroelectric-semiconductor heterostructures obtained by direct wafer bonding