Andreas Dr. Plößl

Wafer direct bonding: tailoring adhesion between brittle materials

Abstract It is a well-known phenomenon that two solids with sufficiently flat surfaces can stick to each other when brought into intimate contact in ambient air at room temperature. The attraction between the two bodies is primarily mediated through van der Waals forces or hydrogen bonding. Without a subsequent heating step, that type of bonding is reversible. Annealing may increase the energy of adhesion up to the cohesive strength of the materials concerned. The wafer bonding phenomena in brittle materials systems, especially in silicon, is reviewed in the experiment. The focus is on low temperature bonding techniques. The pivotal influence chemical species on the surfaces have on the subsequent type of bonding (van der Waals, hydrogen, covalent bonding, mechanical interlocking) is discussed. Methods of modifying the surface chemistry for tailoring bonding properties are addressed. The paper is aimed at providing an overview of the current understanding of the factors determining the bondability and strength of the bonding obtainable. The authors assess the present state of the experimental methods for determining basic parameters governing the adhesion. A number of examples illustrate the applicability of fusion bonding for as diverse fields as opto-electronics, microsystems technology, and fabrication of advanced substrates like silicon-on-insulator wafers.

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.

GaAs wafer bonding by atomic hydrogen surface cleaning

A method of large-area wafer bonding of GaAs is proposed. The bonding procedure was carried out in an ultrahigh vacuum. The wafer surfaces were cleaned at 400 and 500 °C by application of atomic hydrogen produced by thermal cracking. The wafers were brought into contact either immediately after the cleaning, or at temperatures as low as 150 °C, without application of a load, and successfully bonded over the whole area. High-resolution transmission electron microscopy revealed that the wafers could be directly bonded without any crystalline damage or intermediate layer. From a mechanical test, the fracture surface energy was estimated to be 0.7–1.0 J/m2, which is comparable to that of the bulk fracture. Furthermore, this bonding method needs no wet chemical treatment and has no limits to wafer diameter. Moreover, it is suitable for low temperature bonding.

Silicon-on-insulator: materials aspects and applications

Abstract The purpose of this contribution is to give an overview of silicon-on-insulator (SOI) technology with emphasis on the fabrication of SOI substrates and their material properties. Although the concept of SOI has been around for several decades, only recent material science advances made the fabrication of thin-film substrates possible whose material quality is comparable to bulk wafers. SIMOX wafers benefitted from lowering the oxygen dose needed for ion-beam synthesis of buried oxide layers and optimisation of the thermal annealing cycles. Through improved thinning technologies, the wafer-direct-bonding approach for the burial of thermal oxide layers became competetive for thin-film SOI, especially when complemented with the salvaging of the “sacrificial” wafer.