Wafer bonding
Encyclopedia
Wafer bonding is a packaging technology on wafer
-level for the fabrication of microelectromechanical systems
(MEMS), nanoelectromechanical systems
(NEMS), microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation. The wafers' diameter range from 100 mm to 200 mm (4 inch to 8 inch) for MEMS/NEMS and up to 300 mm (12 inch) for the production of microelectronic devices.
Overview =
In microelectromechanical systems (MEMS) and nanoelectromechanical systems
(NEMS), the package is an important part of the device. It enhances a proper function and protects the sensitive internal structures from environmental influences such as temperature, moisture, high pressure and oxidizing species. So, the long-term stability and reliability of the functional elements directly depend on the encapsulation process as well as a significant fraction on the overall device costs. In conclusion, the package has to fulfill the following requirements:
Techniques =
The commonly used and developed bonding methods are as follows:
Requirements =
The bonding of wafers requires specific environmental conditions which can generally be defined as follows:
The actual bond is an interaction of all those conditions and requirements. Hence, the applied technology needs to be chosen in respect to the present substrate and defined specification like max. bearable temperature, mechanical pressure or desired gaseous atmosphere.
Evaluation =
The bonded wafers are characterized in order to evaluate a technology's yield, bonding strength and level of hermeticity either for fabricated devices or for the purpose of process development. Therefore, several different approaches for the bond characterization
have emerged. On the one hand non-destructive optical methods to find cracks or interfacial voids are used beside destructive techniques for the bond strength evaluation, like tensile or shear testing. On the other hand the unique properties of carefully chosen gases or the pressure depending vibration behavior of micro resonators are exploited for hermeticity testing.
External links =
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
-level for the fabrication of microelectromechanical systems
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
(MEMS), nanoelectromechanical systems
Nanoelectromechanical systems
Nanoelectromechanical systems are devices integrating electrical and mechanical functionality on the nanoscale. NEMS form the logical next miniaturization step from so-called microelectromechanical systems, or MEMS devices...
(NEMS), microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation. The wafers' diameter range from 100 mm to 200 mm (4 inch to 8 inch) for MEMS/NEMS and up to 300 mm (12 inch) for the production of microelectronic devices.
Overview =
In microelectromechanical systems (MEMS) and nanoelectromechanical systems
Nanoelectromechanical systems
Nanoelectromechanical systems are devices integrating electrical and mechanical functionality on the nanoscale. NEMS form the logical next miniaturization step from so-called microelectromechanical systems, or MEMS devices...
(NEMS), the package is an important part of the device. It enhances a proper function and protects the sensitive internal structures from environmental influences such as temperature, moisture, high pressure and oxidizing species. So, the long-term stability and reliability of the functional elements directly depend on the encapsulation process as well as a significant fraction on the overall device costs. In conclusion, the package has to fulfill the following requirements:
- protection against environmental influences
- heat dissipation
- integration of elements with different technologies
- compatibility to the surrounding periphery
- maintenance of energy and information flow
Techniques =
The commonly used and developed bonding methods are as follows:
- Direct bondingDirect bondingDirect 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....
- Plasma activated bondingPlasma activated bondingPlasma activated bonding is a derivative, directed to lower processing temperatures for direct bonding with hydrophilic surfaces. The main requirements for lowering temperatures of direct bonding are the use of materials melting at low temperatures and with different coefficients of thermal...
- Anodic bondingAnodic bondingAnodic bonding is a wafer bonding procedure without any intermediate layer. This bonding technique, also known as field assisted bonding or electrostatic sealing, is mostly used for connecting silicon/glass and metal/glass through electric fields...
- Eutectic bondingEutectic bondingEutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer. Those eutectic metals are alloys that transform directly from solid to liquid state at a specific composition and temperature without passing a two phase equilibrium, i.e...
- Glass frit bondingGlass frit bondingGlass frit bonding, also referred to as glass soldering or seal glass bonding, describes a wafer bonding technique with an intermediate glass layer. It is a widely used encapsulation technology for surface micro-machined structures, i.e. accelerometers or gyroscopes. This technique utilizes low...
- Adhesive bondingAdhesive bondingAdhesive bonding describes a wafer bonding technique with applying an intermediate layer to connect substrates of different materials. These produced connections can be soluble or insoluble. The commercially available adhesive can be organic or inorganic and is deposited on one or both substrate...
- Thermocompression bondingThermocompression bondingThermocompression bonding describes a wafer bonding technique and is also referred to as diffusion bonding, pressure joining, thermocompression welding or solid-state welding. Two metals, e.g. gold -gold , are brought into atomic contact applying force and heat simultaneously. The diffusion...
- Reactive bondingReactive bondingReactive bonding describes a wafer bonding procedure using highly reactive nanoscale multilayer systems as an intermediate layer between the bonding substrates. The multilayer system consists of two alternating different thin metallic films. The self-propagating exothermic reaction within the...
Requirements =
The bonding of wafers requires specific environmental conditions which can generally be defined as follows:
- substrate surface
- flatness
- smoothness
- cleanliness
- bonding environment
- bond temperature
- ambient pressure
- applied force
- materials
- substrate materials
- intermediate layer materials
The actual bond is an interaction of all those conditions and requirements. Hence, the applied technology needs to be chosen in respect to the present substrate and defined specification like max. bearable temperature, mechanical pressure or desired gaseous atmosphere.
Evaluation =
The bonded wafers are characterized in order to evaluate a technology's yield, bonding strength and level of hermeticity either for fabricated devices or for the purpose of process development. Therefore, several different approaches for the bond characterization
Bond characterization
The wafer bond characterization is based on different methods and tests. Considered a high importance of the wafer are the successful bonded wafers without flaws. Those flaws can be caused by void formation in the interface due to unevenness or impurities...
have emerged. On the one hand non-destructive optical methods to find cracks or interfacial voids are used beside destructive techniques for the bond strength evaluation, like tensile or shear testing. On the other hand the unique properties of carefully chosen gases or the pressure depending vibration behavior of micro resonators are exploited for hermeticity testing.
External links =