St. Senz

FUNDAMENTAL ISSUES IN WAFER BONDING

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.

“Compliant” twist-bonded GaAs substrates: The potential role of pinholes

By twist wafer bonding, thin (100) GaAs layers were transferred onto (100) GaAs handling wafers in order to fabricate structures like those suggested in the literature as “compliant universal substrates.” Heteroepitaxial InP and InGaAs films were grown on the GaAs twist-bonded layers. Twisted and untwisted grains of the epitaxial film with diameters from 0.1 to several μm without threading dislocations were observed by transmission electron microscopy. Twisted grains grew on the twist-bonded layer, while the untwisted grains grew directly on the GaAs handling wafer and were caused by pinholes in the twist-bonded GaAs layer. It is suggested that the lateral limitation of the epitaxial growth of grains on the thin twisted GaAs layer caused by the presence of pinholes reduces the density of threading dislocations in the strain-relaxed film and might be a mechanism for the observed low density of threading dislocations in lattice-mismatched epitaxial films grown on twist-bonded “compliant universal substrates.”

Macroporous-silicon-based three-dimensional photonic crystal with a large complete band gap

A photonic crystal (PC) structure is described revealing a complete three-dimensional (3D) photonic band gap of about 25% if realized as a silicon/air structure. It is based on two systems of parallel circular pores being orthogonal to each other. The gap size depends on the degree of mutual penetration of the pore systems. A possible fabrication route is based on macroporous silicon (lattice constant a=0.5 μm), into which orthogonal pores are drilled, e.g., by focused-ion-beam etching. This yields a 3D photonic crystal with a complete band gap in the near infrared. The dispersion behavior of the PC is theoretically analyzed (band structure, density of states), varying the pore radii. We discuss the influence of pore shape variations and topological modifications on the size of the gap.

Crystallographic orientation and morphology of epitaxial In2O3 thin films grown on MgO(001) single crystal substrates

Thin epitaxial In2O3 films were grown on MgO(001) single crystal substrates by electron beam evaporation in a high-vacuum chamber. During the evaporation, the substrate was heated to temperatures between 600 and 850°C, and different oxygen background pressures were applied. The films were first characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). Structure, morphology and crystallographic orientation of the films and the MgO/In2O3 interfaces were then investigated by transmission electron microscopy/selected area electron diffraction (TEM/SAED). In all samples, the major part of the film area grows with the In2O3(111) plane parallel to the MgO(001) surface, and shows an in-plane orientation with four domains occurring, characterized by the In2O3(110) plane being parallel to one of the four different MgO(110) planes. These parts of the In2O3 films consist of grains with a columnar structure, with a typical grain diameter of 50 to 200 nm for a film thickness ranging from 120 to 150nm, and an extreme small rocking curve FWHM up to 0.1°. A minor part of the film area was found in a “cube-to-cube” orientation: In2O3(001)∥MgO(001);In2O3[100]∥MgO[100].The amount of the minor part film area decreases with decresing substrate temperature down to lower than 0.3%

Formation of an amorphous product phase during the solid state reaction between a vitreous SiO2 thin film and a (001) BaTiO3 substrate

The solid state chemical reaction of a thin SiO2 film with a single-crystal (001) BaTiO3 substrate has been investigated as a model system for processes occurring during the sintering of BaTiO3 ceramics with the sintering additive SiO2. Thin amorphous SiO2 films on (001) BaTiO3 substrates react with the substrate during annealing at 1000C for 5 to 15 min. Transmission electron microscopy (TEM) has revealed the formation of a glassy reaction product after 5 min of heating. The composition of this glass has been analyzed by energy dispersive X-ray spectrometry (EDX), showing mainly SiO2 and about 16 at.% of BaTiO3. The Ba:Ti ratio of the glass is 1:1. During prolonged heating, or if the initial BaTiO3 surface is decorated with Pt markers, the reaction product crystallizes, with the fresnoite Ba2TiSi2O8 forming as the dominant crystalline phase. # 2000 Elsevier Science Ltd. All rights reserved.

Relaxation of an epitaxial InGaAs film on a thin twist-bonded (100) GaAs substrate

A 30 nm (100) GaAs layer was transferred by twist wafer bonding to a (100) GaAs handling wafer. A similar structure was proposed in the literature as a “compliant substrate.” Transmission electron microscopy and x-ray diffraction of 40 and 300 nm epitaxial InGaAs films (0.5% misfit) showed no evidence of a relaxation mechanism specifically attributed to a compliant substrate. The 40 nm film was nearly pseudomorphic without any evidence of a relaxation mechanism, like grain-boundary sliding. The possibility of grain-boundary slip along the twist-bonded interface is discussed.

Epitaxial Bi-layered perovskite ferroelectric thin film heterostructures by large area pulsed laser deposition

Abstract Epitaxial thin films of Bi4Ti3O12, SrBi2Ta2O9 and BaBi4Ti4O15 have been epitaxially deposited onto 3-inch substrates by large area pulsed laser deposition. The out-of-plane orientation of the layers is characterized by a FWHM of the rocking curve Δωin the range of 0.9° to 2.1° and their in-plane orientation by a FWHM of the phi-scan ω π ranging from 3° to 4.5°. The composition is uniform across the whole 3-inch wafer and a thickness uniformity in the range of 5 to 10% of the mean thickness has been achieved. The ferroelectric properties of the Bi-layered perovskite layers depend strongly on their microstructure and crystallographic orientation. The remnant polarization of SrBi2Ta2O9 films is ranging from Pr = 0.2 μC/cm2 for films having only c-oriented crystallites to Pr = 1 μC/cm2 for films containing a substantial fraction of crystallites with their c-axis in the plane of the film.

Characterisation of interfacial properties in sputtered Co/Cu multilayers: X-ray reflectometry compared with TEM and AFM

Combining localised and global structural information we suggest that differences in the magnetoresistive effect in the first maximum of antiferromagnetic coupling in Co/Cu multilayers can be attributed to the number of magnetic shortcuts localised at grain boundaries. In particular evidence is given that the increased rms-roughness in samples without buffer as compared to samples grown on a Fe buffer can be attributed to a break off of the multilayer structure in adjacent grains.

Structural and electrical properties of metal-ferroelectric-silicon heterostructure fabricated by a direct wafer bonding and layer transfer process

Abstract Structural and electrical investigations revealed that a direct wafer bonding and layer transfer process yields good quality ferroelectric/Si interfaces for SBT, PZT and BiT ferroelectric thin films. C-V characteristics and interface trap measurements show a large difference for Au-Ferroelectric-Si structures depending on whether the interface is fabricated by bonding or by direct deposition. For reacted interfaces the trap densities are ranging from 2×1012 cm−2 eV−1 for SBT/Si and BiT/Si to 2×1013 cm−2eV−1 for PZT/Si. For bonded interfaces, independent of the top ferroelectric layer, the trap density is about 4×1011 cm−2eV−1.

Epitaxial Ferroelectric Aurivillius-Type Phases on Metallic Oxides by Pulsed Laser Deposition

Bi-based layered perovskites, also called Aurivillius-type phases, are superior to simple perovskite materials with regard to their ferroelectric long-term stability. Another way to alleviate fatigue and aging problems in metal-ferroelectric-metal (MFM) heterostructures is to replace the bottom metallic electrode with a conductive oxide electrode. An attempt to combine the two approaches has been made to investigate whether a further improvement in stability can be achieved. To promote an oriented growth of the ferroelectric films, epitaxial buffer layers (YSZ, Ce0 2 ) and epitaxial electrodes of (La 0.5 Sr 0.5 )Co0 3 (LSC) have been consecutively deposited onto Si (100). Finally a ferroelectric thin film of the layered perovskite Bi 4 Ti 3 0 12 (BiT) has been grown. Rocking curve measurements demonstrate good epitaxial growth of both the buffer and the electrode layers. The ferroelectric thin films show a preferred c-axis orientation. Cross-section TEM images reveal a twinned superstructure in the LSC layer with a tripling of the lattice parameter. EDX line-scans show that a Co-enriched and Bi-depleted layer had formed at the BiT/LSC interface. After deposition of Au electrodes on both the BiT and the LSC layer, a hysteretic behavior could be detected and the ferroelectric properties of the c-oriented BiT film be confirmed.