Direct bonding

Infrared photograph of initiation and propagation of bonding wave in wafer bonding of silicon wafers. (l) wafers are separated by an air layer and the bond process starts by pressure on top wafer. (m) bond wave moves to the edge. (r) a perfectly bonded wafer pair, not reflecting IR light.[8]

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.[2] 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.[9] 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.[10]

Diagram of the surface energy of hydrophilic and hydrophobic bonded wafers [2]

Before contacting the wafers, those have to be aligned.[1] 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.[8]

${\displaystyle {\ce {{Si-OH}+ OH-Si ->[{\ce {polymerization}}] {Si-O-Si}+ H2O}}}$

${\displaystyle {\ce {{Si-OH}+OH-Si->[{\ce {slow~fracture}}]{Si-O-Si}+H2O}}}$

After the pre-bonding in air, in a special gaseous atmosphere or vacuum, the wafers have to pass an annealing process for increasing the bonding strength. The annealing therefore provides a certain amount of thermal energy which forces more silanol groups to react among each other and new, highly stable chemical bindings are formed. The kind of binding which forms directly depends on the amount of energy which has been delivered or the applied temperature respectively. In consequence the bonding strength rises with increasing annealing temperatures.[2]

Between room temperature and 110 °C the interface energy remains low, water molecules diffuse at the bond interface, leading to a rearrangement, causing more hydrogen-bonds. At temperatures from 110 °C to 150 °C silanol groups polymerize to siloxane and water, but also a slow fracture takes place. This reaction equates a thermo dynamical equilibrium and a higher density of silanol groups results in a higher number of siloxane and an increasing bond strength.

${\displaystyle {\ce {{Si-OH}+ Si-OH <=> {Si-O-Si}+ HOH}}}$

No further processes are observed at the interface between 150 °C and 800 °C until all OH-groups are polymerized and the composite strength remains constant.

Above 800 °C native oxide gets viscous and starts to flow at the interface, which increases the area of contacted surfaces. So, the diffusion of trapped hydrogen molecules along the interface is enhanced and interface voids may reduce in size or disappear at all. The annealing process is finished by the cooling of the wafer stack.[8]

The interface energy increases to more than 2 J⁄m² at 800 °C with a native oxide layer or at 1000 °C if the wafers are covered by thermal oxide (compare diagram of surface energy). In case one wafer contains a layer of thermal oxide and the other wafer is covered by a native oxide, the surface energy development is similar to a wafer pair both covered with a native oxide layer.[2]

A hydrophobic surface is generated if the native oxide layer is removed by either plasma treatment or by fluoride containing etching solutions, e.g. hydrogen fluoride (HF) or ammonium fluoride (NH₄F). This process enhances the formation of Si-F bonds of the exposed silicon atoms. For hydrophobic bonding it is important to avoid re-hydrophilization, e.g. by rinsing and spin-drying, since Si-F bonds contacted with water result in Si-OH.[1]

Prior to bonding the surface is covered with hydrogen and fluorine atoms. The bonding at room temperature is mostly based on van-der-Waals forces between those hydrogen and fluorine atoms. Compared to bonding with hydrophilic surfaces, the interface energy is lower directly after contacting. This fact builds up the need for a higher surface quality and cleanliness to prevent unbonded areas and thereby to achieve a full-surface contact between the wafers (compare infrared photograph of a bond wave).[1] Similar to bonding of hydrophilic surfaces, the pre-bond is followed by an annealing process.

From room temperature to 150 °C no important interface reactions occur and the surface energy is stable. Between 150 °C and 300 °C more Si-F-H-Si bonds are formed. Above 300 °C the desorption of hydrogen and fluoride from the wafer surface leads to redundant hydrogen atoms that diffuse in the silicon crystal lattice or along interface. As a result, covalent Si-Si bonds start to establish between opposing surfaces. At 700 °C the transition to Si-Si bonds is completed.[11] The bonding energy reaches cohesive strengths of bulk silicon (compare diagram of surface energy).[2]