Direct bonding, or fusion bonding, is a wafer bonding process without any additional intermediate layers. It is based on chemical bonds between two surfaces of any material possible meeting numerous requirements. These requirements are specified for the wafer surface as sufficiently clean, flat and smooth. Otherwise unbonded areas so called voids, i.e. interface bubbles, can occur.

The procedural steps of the direct bonding process of wafers any surface is divided into

Even though direct bonding as a wafer bonding technique is able to process nearly all materials, silicon is the most established material up to now. Therefore, the bonding process is also referred to as silicon direct bonding or silicon fusion bonding. The fields of application for silicon direct bonding are, e.g. manufacturing of Silicon on insulator (SOI) wafers, sensors and actuators.

The silicon direct bonding is based on intermolecular interactions including van der Waals forces, hydrogen bonds and strong covalent bonds. The initial procedure of direct bonding was characterized by a high process temperature. There is demand to lower the process temperature due to several factors, one is for instance the increasing number of utilized materials with different coefficients of thermal expansion. Hence, the aim is to achieve a stable and hermetic direct bond at a temperature below 450 °C. Therefore, processes for wafer surface activation i.e. plasma treatment or chemical-mechanical polishing (CMP), are being considered and are actively being researched. The upper limit of 450 °C bases on the limitations of back-end CMOS processing and the beginning of interactions between the applied materials.

The adhering effect of smooth and polished solid surfaces is first mentioned by Desaguliers (1734). His discovery was based on the friction between two surfaces of solids. The better the surfaces are polished the lower the friction is between those solids. This statement he described is only valid until a specific point. From this point on the friction starts to rise and the surfaces of the solids start to adhere together. First reports of successful silicon direct bonding were published 1986 among others by J. B. Lasky.

Direct bonding is mostly referred to as bonding with silicon. Therefore, process techniques are divided in accordance with the chemical structure of the surface in hydrophilic (compare to scheme of a hydrophilic silicon surface) or hydrophobic (compare to scheme of a hydrophobic silicon surface).

The surface state of a silicon wafer can be measured by the contact angle a drop of water forms. In the case of a hydrophilic surface the angle is small (< 5 °) based on the excellent wettability whereas a hydrophobic surface shows a contact angle larger than 90 °.

Before bonding two wafers, those two solids need to be free of impurities that can base on particle, organic and/or ionic contamination. To achieve the cleanliness without degrading the surface quality, the wafer passes a dry cleaning, e.g. plasma treatments or UV/ozone cleaning, or a wet chemical cleaning procedure. The utilization of chemical solutions combines sequential steps. An established industrial standard procedure is SC (Standard Clean) purification by RCA. It consists of two solutions