Direct bonding, or fusion bonding, describes a wafer bonding process without any additional intermediate layers. The bonding process 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 wafer preprocessing, pre-bonding at room temperature and annealing at elevated temperatures. 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.
Overview
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.
History
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.
Conventional direct bonding
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 °.
Bonding of hydrophilic silicon wafers
Wafer preprocessing
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 SC1 (NH4 OH (29%) + H2O2 (30%) + Deionized H2O [1:1:5]) and SC2 (HCl (37%) + H2O2 (30%) + Deionized H2O [1:1:6]). SC1 is used for removing organic contaminations and particles at a temperature of 70 °C to 80 °C for 5 to 10 min and SC2 is used for removing metal ions at 80 °C for 10 min. Subsequently, the wafers are rinsed with or stored in deionized water. The actual procedure needs to be adapted to every application and device because of usually existing interconnects and metallization systems on the wafer.
Pre-bonding at room temperature
Before contacting the wafers, those have to be aligned. If the surfaces are sufficiently smooth, the wafers start to bond as soon as they get in atomic contact as shown in infrared photograph of a bond wave.
The wafers are covered with water molecules so the bonding happens between chemisorbed water molecules on the opposing wafer surfaces. In consequence a significant fraction of Si-OH (silanol) groups start to polymerize at room temperature forming Si-O-Si and water and a sufficient bonding strength for handling the wafer stack is assured. The formed water molecules will migrate or diffuse along the interface during annealing.