G. Kästner

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.

Microstructure defects in YBCO thin films: A TEM study to discuss their influence on device properties

Abstract the microstructure of epitaxial c oriented YBa 2 Cu 3 O 7−δ films deposited and patterned by different routine methods typical of device layouts is studied by TEM. Film systems up to three layers were either deposited or sputtered onto SrTiO 3 , LaAlO 3 , MgO, and R-cut sapphire buffered by CeO 2 or YSZ. Specific grain boundaries of accidental grains either a -axis up oriented or in-plane rotated are discussed in terms of their growth, flux pinnig, and weak-link behavior. These properties should be affected by adhering Y 2 O 3 inclusions which otherwise occur highly dispersed in the bulk, preferably of the sputtered films. Dislocations, probably attributed to twin boundaries and to faulty c -stacking, are considered in view of the film growth, mechanical relaxation, and flux pinning. Within a device pattern of YBCO/SrTiO 3 /YBCO, thoroughly good epitaxial orientation is also found on top of crossing layer edges in agreement with the measured high j c at the crossovers. Particular reference is given to the literature on the origin and the effects of microstructure defects.

Ferroelectric-semiconductor heterostructures obtained by direct 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.

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.

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.

Surface quality and laser-damage behaviour of chemo-mechanically polished CaF2 single crystals characterized by scanning electron microscopy

A significant increase in the ultraviolet laser-damage threshold of CaF2 (1 1 1) single-crystal surfaces after surface finishing by chemomechanical polishing (CMP) with colloidal silica has been demonstrated as compared to conventional mechanical-abrasive polishing (MAP). It was shown that CMP yields an up to 12-fold increase of the damage threshold fluence up to Fth = 30 J cm−2 for 1-on-1 nanosecond pulses of 248 and 193 nm excimer laser irradiation. Even after 5-on-1 irradiations, the damage threshold remains as high as Fth = 15 J cm−2 in the case of CMP. For both polishing procedures, the change in dielectric surface properties has been characterized by means of scanning electron microscopy (SEM) using electron beam-induced charge-up phenomena. These were mainly detected by the variation of emitted secondary electron (SE) yield δSE depending on the primary electron (PE) energy. Two kinds f charge-up phenomena were employed: (i) the onset or vanishing of statistically fluctuating SE yield bursts during slow-scan imaging (“stripe pattern” method), and (ii) the temporal decay of the electron beam-induced charge-up inside an electrically conducting mask (charge decay method). Both these phenomena disappeared after CMP. It is concluded that this disappearance results from removing the subsurface damage layer which is typical of MAP.

Compound semiconductor interfaces obtained by direct wafer bonding in hydrogen or forming gas

A technique of direct bonding wafers up to 6-inch diameter without mechanical load after being heated to elevated temperatures in H2 or in non-flammable forming gas (5% H2/95% N2) was applied to GaAs and GaAs/GaP. Electron microscopy revealed crystallographic bond interfaces containing the typical dislocation network. The density of dislocations can be easily chosen below a limit adverse to electrical properties. Current–voltage characteristics of doped GaAs wafer pairs showed ohmic behavior of p–p and n–n junctions, whereas p–n diodes showed 2 to 3 V breakdown voltage as well as an ideality factor between 1.0 and 1.8.

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.

Microstructure of device‐related YBCO thin films

A preliminary brief review is given on various kinds of microstructure defects in view of their possible effects on flux pinning, junction behaviour, and film growth. Then, transmission electron microscope investigations are reported concerning thin films of YBCO on LaAlO3 (laser‐deposited), on SrTiO3 and MgO (hollow‐cathode sputtered), and on a Mg‐Ti‐spinel buffer layer on MgO. The usual ‘‘c‐epitaxy’’ with the typical twin lamellae has been thoroughly observed, also in case of the buffer layer and within the edge region of photolithographically patterned films where the twins provide conditions of superconductivity. Grains (islands) in the mm size region of a 45° rotated c‐epitaxy were observed in YBCO films on MgO (001). Furthermore, much smaller inhomogeneities of some 1 nm size—possibly nuclei of the ‘‘a‐epitaxy’’—were found which have to be considered with respect to flux poinning.