Oleg Kononchuk

Internal Dissolution of Buried Oxide in SOI Wafers

High temperature anneal of SOI wafers in oxygen-free atmosphere results in internal buried oxide dissolution and top Si layer etching. Dissolution rate is determined by interstitial oxygen diffusion through the top Si layer and evaporation from the top Si surface in the form of SiO. It has been observed that kinetics of the process follows linear-parabolic law. Simple thermodynamic model is proposed, which explains observed dependences on temperature and top Si layer thickness.

Development of microcracks in hydrogen-implanted silicon substrates

The development of microcracks in hydrogen-implanted silicon has been studied up to the final split using optical microscopy and mass spectroscopy. It is shown that the amount of gas released when splitting the material is proportional to the surface area of microcracks. This observation is interpreted as a signature of a vertical collection of the available gas. The development of microcracks is modeled taking into account both diffusion and mechanical crack propagation. The model reproduces many experimental observations such as the dependence of split time upon temperature and implanted dose.

Electrical levels of dislocation networks in p- and n-type Si

The results of deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) investigations on directly bonded n- and p-type silicon wafers with small twist misorientation angles ranging from 1 to 5 degrees are presented and discussed. Both shallow and deep levels in the upper half of a band gap are found and a good correspondence between the DLTS and MCTS data on n- and p-type samples was established. The dependence of DLTS-peak magnitude on twist and tilt misorientation angles (density of dislocations) was investigated and the origin of different levels is suggested.

Surface self-diffusion of silicon during high temperature annealing

The atomic-scale mechanisms driving thermally activated self-diffusion on silicon surfaces are investigated by atomic force microscopy. The evolution of surface topography is quantified over a large spatial bandwidth by means of the Power Spectral Density functions. We propose a parametric model, based on the Mullins-Herring (M-H) diffusion equation, to describe the evolution of the surface topography of silicon during thermal annealing. Usually, a stochastic term is introduced into the M-H model in order to describe intrinsic random fluctuations of the system. In this work, we add two stochastic terms describing the surface thermal fluctuations and the oxidation-evaporation phenomenon. Using this extended model, surface evolution during thermal annealing in reducing atmosphere can be predicted for temperatures above the roughening transition. A very good agreement between experimental and theoretical data describing roughness evolution and self-diffusion phenomenon is obtained. The physical origin and time-evolution of these stochastic terms are discussed. Finally, using this model, we explore the limitations of the smoothening of the silicon surfaces by rapid thermal annealing.

Electronic States of Oxygen-Free Dislocation Networks Produced by Direct Bonding of Silicon Wafers

The results of experimental investigations of the dislocation-related DLTS-peaks originated from the dislocation networks (DN) are presented. Samples with DNs were produced by direct bonding of p-type silicon wafers and no enhancement of oxygen concentration was detected near the DN plane. Origins of the DLTS peaks were proposed and a correlation with the dislocation-related photoluminescence data was established based on known dislocation structure of the samples. Two types of shallow DLTS peaks exhibited Pool-Frenkel effect, which could be linked to the dislocation deformation potential. One of the shallow DLTS peaks was related to straight parts of screw dislocations and another - to the intersections of the dislocations.

Determination of the Origin of Dislocation Related Luminescence from Silicon Using Regular Dislocation Networks

The investigation of regular dislocation networks (DN) formed by direct wafer bonding suggests that the D1 and D2 peaks of dislocation-related luminescence (DRL) in silicon is linked to screw dislocations, whereas edge dislocations are responsible for D3 and D4 DRL peaks. Non-radiative recombination activity in DN could be attributed to edge dislocations and could be related to enhanced ability of these dislocations to getter impurity atoms. Obtained relation of DRL intensity with the density of screw dislocations suggests existence of the optimum twist angle for the wafer-bonding geometry for which the DRL intensity has a maximum. The dependence of DRL intensity on the spacing between screw dislocations has the maximum at about 7 nm. Reported radiative and non-radiative recombination properties of DN present substantial interest not only for possible LED applications in all-Si photonics but also for photovoltaics, since DNs represent a model system for grain boundaries controlling carrier lifetime in microcrystalline-Si material.

Dislocation Structure, Electrical and Luminescent Properties of Hydrophilically Bonded Silicon Wafer Interface

The dislocation-related luminescence (DRL) in the vicinity of D1 band (0.8 eV) in hydrophilically bonded n- and p-type silicon wafers is investigated by means of recently developed pulsed trap refilling enhanced luminescence technique (Pulsed-TREL). The shallow and deep dislocation related electronic states in both upper and lower part of the band gap are determined and characterized by means of DLTS. Among those traps we have established ones which directly participate in D1 DRL. We have shown that D1 luminescence goes via shallow dislocation related states (SDRS) located close to the conduction and valence bands with thermal activation energy of about 0.1 eV whereas deep levels do not participate in D1 DRL. The model explaining the fact how the 0.8 eV luminescence may go through levels which interlevel energy is at least 0.97 eV in terms of Coulomb interaction between ionized SDRS is suggested.

Mechanisms of Dislocation Network Formation in Si(001) Hydrophilic Bonded Wafers

Structures of Si(001) hydrofillic bonded wafers have been studied by transmission electron microscopy. Model of three-fold nods generation during interaction of intersecting mixed and screw dislocations has been suggested and applied to analyze geometrical features of dislocation networks. Possible mechanisms of dislocation generation at the interface between Si bonded wafers are discussed.

Interface water diffusion in silicon direct bonding

The kinetics of water diffusion through the gap formed by the direct bonding of two silicon wafers is studied using two different techniques. X-ray reflectivity is able to monitor the interface density changes associated with the water front progression. The water intake is also revealed through the defect creation upon annealing, creating a rim-like pattern whose extent also gives the water diffusion law. At room temperature, the kinetics observed by either technique are consistent with the Lucas-Washburn law for diffusion through a gap width smaller than 1 nm, excluding any significant no-slip layer thickness.

Novel Trends in SOI Technology for CMOS Applications

High temperature annealing of SOI wafers in non-oxidized ambient leads to internal Buried Oxide (BOX) dissolution. The underlying mechanisms and kinetics of this effect are discussed. High quality SOI wafers with very thin BOX down to 2nm are demonstrated utilizing optimized annealing conditions. Hybrid SOI/bulk wafers are obtained by the new process applying silicon nitride mask on the wafer surface. Stability of SOI and Si3N4/SOI systems at high temperatures is discussed and optimized process window is determined.