P. Kopperschmidt

Wafer bonding for microsystems technologies

In microsystems technologies, frequently complex structures consisting of structured or plain silicon or other wafers have to be joined to one mechanically stable configuration. In many cases, wafer bonding, also termed fusion bonding, allows to achieve this objective. The present overview will introduce the different requirements surfaces have to fulfill for successful bonding especially in the case of silicon wafers. Special emphasis is put on understanding the atomistic reactions at the bonding interface. This understanding has allowed the development of a simple low temperature bonding approach which allows to reach high bonding energies at temperatures as low as 150°C. Implications for pressure sensors will be discussed as well as various thinning approaches and bonding of dissimilar materials.

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.”

MATERIALS INTEGRATION OF GALLIUM ARSENIDE AND SILICON BY WAFER BONDING

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 4.1%, extra Si lattice planes were observed at the interface. No threading dislocations were introduced into the GaAs.

High bond energy and thermomechanical stress in silicon on sapphire wafer bonding

Silicon on sapphire wafer pairs are formed by direct wafer bonding of 3-in. silicon and sapphire wafers. Subsequent annealing commonly used to increase the bond energy imposes serious thermomechanical strain. The corresponding bending, recorded in situ as a function of temperature, reveals relaxations by de- and rebonding until the silicon wafer cracks into small fragments that mostly remain bonded. After further annealing up to 800 °C and cooling to room temperature, a strong curvature of the fragments indicates a frozen-in high temperature bond state with elastic energies around 100 J/m2. Cross-sectional transmission electron microscopy of the interface reveals an amorphous intermediate layer the thickness of which considerably increases with increasing the oxygen partial pressure during annealing.

Optimisation of the wire-shadow TEM cross-section preparation technique

Abstract The wire-shadow technique is a simple and easy-to-use preparation method of cross-section specimens for TEM investigations. The method has been optimised for thin films on silicon and oxide (sapphire, MgO) substrates. A wire is glued onto the film. During thinning in a modified commercial ion-milling sample holder the shadow of the small wire protects part of the film. Non-shadowed areas of film and substrate are removed by ion sputtering. During ion thinning a sharp edge evolves in the wire shadow. As a consequence, the interface between film and substrate becomes transparent to electrons just at the point at which the wire is eroded away. Different wire materials (tungsten, Al 2 O 3 , carbon, nicalon (SiC) and nylon) and thinning geometries were tested. The best results were achieved using an amorphous carbon fibre with a diameter of 7 μm.

Interface defects in integrated hybrid semiconductors by wafer bonding

The integration of materials by wafer bonding offers novel device fabrication for applications in micromechanics, microelectronics, and optoelectronics. Two mirror-polished surfaces are brought into intimate contact by adhesive forces regardless of their crystallography, crystalline orientation and lattice mismatch. Followed by a thermal treatment at several hundred degrees centigrade, the interface energy of the material combination is increased to energies of covalent interatomic bonds. Attempts to break the bond lead to fracturing of the materials. In particular, thermomechanic stress in dissimilar material combinations may result in bending, gliding and cracking of the bonded wafers during annealing. The bonding interface of various hybrid semiconductor materials was studied by transmission electron microscopy. Occasionally, microscopic imperfections at the bonding interface were found in Si/Si, Si/GaAs, GaAs/GaAs, GaAs/Al2O3, GaAs/InP and moreover Al2O3/Al2O3 bonded wafer pairs. The imperfections were identified as voids, negative crystals, and oxide-containing precipitates ranging from 5 to 20 nm in diameter. Microscopic defects at the bonding interface in integrated bulk materials do not affect the mechanical and electrical properties of the device very much. However, in bonding of thin films the defects or precipitates may thread through the thin film, if the diameter of the precipitate surpasses the thickness of the film. These pinholes-containing thin films have a high leakage current, low electrical breakthrough and crystallographic disorder. Epitaxy of material on a pinholes containing, disordered surface results on deposition of bicystalline grains. In between the grains tilt grain boundaries were observed raising from the bonding interface. Bonding related defects at the interface can be avoided by alternative bonding techniques like UHV wafer bonding and low temperature wafer bonding. r 2001 Elsevier Science B.V. All rights reserved.

Phase identification of micro and macro bubbles at the interface of directly bonded GaAs on sapphire

Direct wafer bonding (DWB) of 3″ GaAs and R-cut sapphire was performed in a microcleanroom using ultra pure water as cleaning agent. The initial bonding is mediated by Van der Waals forces and hydrogen bridges. The bond energy is released by subsequent heating up to temperatures of 500°C. During heating the formation of macroscopic bubbles at the interface was observed. Details of the interface structure were investigated by cross-sectional as well as plan-view transmission electron microscope (TEM) micrographs. The chemical composition of the elements at the interface was measured by energy dispersive X-ray analysis (EDX) and electron energy loss spectroscopy (EELS). A high density of micro bubbles in bonded areas, a network of micro channels in the transition region and macro bubbles in debonded areas could be distinguished. The macro bubbles are filled with a porous oxide. X-ray diffraction (XRD) and selected area electron diffraction (SAED) revealed the growth of textured γ-Ga2O3 and elemental arsenic.