Reactive bonding
Encyclopedia
Reactive 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 multilayer system contributes the local heat to bond the solder
films. Based on the limited temperature the substrate material is exposed, temperature-sensitive components and materials with different CTEs
, i.e. metal
s, polymer
s and ceramic
s, can be used without thermal damage.
Overview =
]
The bonding is based on reactive nano scale multilayers providing an internal heat source. These foils are combined with additional solder layers to achieve bonding. The heat that is required for the bonding is created by a self-propagating exothermic reaction of the multilayer system. This reaction is ignited by an energy pulse, i.e. temperature, mechanical pressure, electrical spark or laser pulse. The generated heat is localized to the bonding interface and limited due to a short term heating phase within milliseconds.
This heat is an advantage of this approach, so the used materials are not exposed to high temperatures and allow rapid cooling. A drawback is that this approach is not applicable for bond frame dimensions of few ten micrometres. This is based on the limited handling and structuring abilities of the foils at this small dimensions.
The material used for multilayer systems is a bilayer of alternating elements, commonly Ni
/Al
, Al/Ti
or Ti/a-Si
. The metallic layer is usually 1 to 30 nm thick and can be arranged as horizontal or vertical nano scale material films and are a combination of a reactive and a low melting component. With increased bilayer thickness, the reaction velocity decreases and the reaction heat increases. Therefore a specific balance between high reaction velocity and high reaction heat is necessary.
A commercial example of such material is NanoFoil
. The corresponding bonding process is known as NanoBond.
substrate. The deposition of the multilayer systems on silicon is achieved by magnetron sputtering, electroplating
or etching
. The vertical nano structures are also created directly on the substrate surface.
The substrate surfaces are deposited with a solder
layer, i.e. gold
(Au), using physical vapor deposition
(PVD). The PVD process promotes the wetting
of the solder. The intermixing of the used components during deposition influences the reaction parameters and to prevent this the substrates are cooled.
A commonly used deposition method for multilayer structures is magnetron sputtering. A multilayer system consists of thousands of thin single layers of the component combination that are alternately sputtered on the substrate surface.
For electroplating
or electrochemical deposition (ECD) multilayer deposition two approaches are established. On the one hand a two bath method exist, which means an alternating deposition in two different plating baths. On the other hand a one bath method, with an electrolyte containing both film components in one bath, can be used. The ECD process reduces process time and complexity. In addition, this method enables pattern plating to prevent complex etching process of structures.
Vertical nanostructure
s are created in two steps. At first, needles in the silicon substrate are created by dry etching
. The other used material is deposited using sputtering to cover those needles. This approach reduces the process time and complexity drastically due to the deposition omission of the thousands of single layers. Further, reactive foil patterning can be realizied by applying an electrochemical machining process.
The bonding process is based on the reaction of the nanoscale multilayer to release energy concentrated at the interface. The self-propagating reaction is caused by the reduction of chemical bond
energy in the multilayer system (compare to figure "Schematic self-propagating reaction in a multilayer system after ignition").
The system alloy
, or an intermetallic compound, (AB) is formed from the intermixing elements (A+B) due to atomic diffusion
.
The reactive foil is ignited by an energy pulse resulting in an immediate self-propagating reaction (compare to figure "Schematic reactive bonding process with a reactive multilayer as heat source").
This local intermixing process produces heat that is transmitted to the adjacent element layers. The reaction spreads through the foil in milliseconds. This energy release leads to a high temperature in the bonding interface. Meanwhile the components outside the interface are not exposed to the high temperatures of the reaction. Besides the high interface energy, this reaction is also promoted by the low thickness and therefore the reduced diffusion path of the single metallic layers.
The resulting internal heat melts the solder
layers to form a bond with the multilayer system and the substrate based on diffusion. This exothermic reaction can be ignited in reactive material
s like compacted powders, e.g. Ni/Ti or Ti/Co
, as well as in nanostructured multilayer systems, e.g. Ni/Al. The bonding can take place in various environments, i.e. vacuum
, with a force providing a defined mechanical pressure at room temperature. A high applied mechanical pressure enhances the solder flow and therefore can improve the wetting of the substrate.
Examples =
Reactive bonding approach is used to assemble MEMS components including die attachment
and the hermetic sealing
of micro-system packages
. The process is used to join temperature sensitive biological activated substrates for diagnostics or medical device
s. In addition disposable microfluidic devices with sensing function and immobilized cells can be fabricated.
Technical Specifications =
See also =
External links=
Reactive bonding at Fraunhofer ENAS
Wafer bonding
Wafer bonding is a packaging technology on wafer-level for the fabrication of microelectromechanical systems , nanoelectromechanical systems , microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation...
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
Exothermic reaction
An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...
within the multilayer system contributes the local heat to bond the solder
Solder
Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...
films. Based on the limited temperature the substrate material is exposed, temperature-sensitive components and materials with different CTEs
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
, i.e. metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s, polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s and ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s, can be used without thermal damage.
Overview =
]
The bonding is based on reactive nano scale multilayers providing an internal heat source. These foils are combined with additional solder layers to achieve bonding. The heat that is required for the bonding is created by a self-propagating exothermic reaction of the multilayer system. This reaction is ignited by an energy pulse, i.e. temperature, mechanical pressure, electrical spark or laser pulse. The generated heat is localized to the bonding interface and limited due to a short term heating phase within milliseconds.
This heat is an advantage of this approach, so the used materials are not exposed to high temperatures and allow rapid cooling. A drawback is that this approach is not applicable for bond frame dimensions of few ten micrometres. This is based on the limited handling and structuring abilities of the foils at this small dimensions.
The material used for multilayer systems is a bilayer of alternating elements, commonly Ni
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
/Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, Al/Ti
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
or Ti/a-Si
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
. The metallic layer is usually 1 to 30 nm thick and can be arranged as horizontal or vertical nano scale material films and are a combination of a reactive and a low melting component. With increased bilayer thickness, the reaction velocity decreases and the reaction heat increases. Therefore a specific balance between high reaction velocity and high reaction heat is necessary.
A commercial example of such material is NanoFoil
NanoFoil
NanoFoil is the trademarked name for a reactive multi-layer foil material, sometimes referred to as a pyrotechnic initiator of two mutually reactive metals, aluminium and nickel, sputtered to form thin layers to create a laminated foil...
. The corresponding bonding process is known as NanoBond.
Preprocessing
Two different reactive structures are established, conventional lateral layer-by-layer (multilayer) and vertical arranged structures. Based on difficulties, that occur during handling, patterning and positioning of the freestanding foils, the multilayer films are directly deposited onto the siliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
substrate. The deposition of the multilayer systems on silicon is achieved by magnetron sputtering, electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
or etching
Etching
Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal...
. The vertical nano structures are also created directly on the substrate surface.
The substrate surfaces are deposited with a solder
Solder
Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...
layer, i.e. gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
(Au), using physical vapor deposition
Physical vapor deposition
Physical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...
(PVD). The PVD process promotes the wetting
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...
of the solder. The intermixing of the used components during deposition influences the reaction parameters and to prevent this the substrates are cooled.
A commonly used deposition method for multilayer structures is magnetron sputtering. A multilayer system consists of thousands of thin single layers of the component combination that are alternately sputtered on the substrate surface.
For electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
or electrochemical deposition (ECD) multilayer deposition two approaches are established. On the one hand a two bath method exist, which means an alternating deposition in two different plating baths. On the other hand a one bath method, with an electrolyte containing both film components in one bath, can be used. The ECD process reduces process time and complexity. In addition, this method enables pattern plating to prevent complex etching process of structures.
Vertical nanostructure
Nanostructure
A nanostructure is an object of intermediate size between molecular and microscopic structures.In describing nanostructures it is necessary to differentiate between the number of dimensions on the nanoscale. Nanotextured surfaces have one dimension on the nanoscale, i.e., only the thickness of the...
s are created in two steps. At first, needles in the silicon substrate are created by dry etching
Dry etching
Dry etching refers to the removal of material, typically a masked pattern of semiconductor material, by exposing the material to a bombardment of ions that dislodge portions of the material from the exposed surface...
. The other used material is deposited using sputtering to cover those needles. This approach reduces the process time and complexity drastically due to the deposition omission of the thousands of single layers. Further, reactive foil patterning can be realizied by applying an electrochemical machining process.
Bonding
The bonding process is based on the reaction of the nanoscale multilayer to release energy concentrated at the interface. The self-propagating reaction is caused by the reduction of chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
energy in the multilayer system (compare to figure "Schematic self-propagating reaction in a multilayer system after ignition").
The system alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
, or an intermetallic compound, (AB) is formed from the intermixing elements (A+B) due to atomic diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
.
The reactive foil is ignited by an energy pulse resulting in an immediate self-propagating reaction (compare to figure "Schematic reactive bonding process with a reactive multilayer as heat source").
This local intermixing process produces heat that is transmitted to the adjacent element layers. The reaction spreads through the foil in milliseconds. This energy release leads to a high temperature in the bonding interface. Meanwhile the components outside the interface are not exposed to the high temperatures of the reaction. Besides the high interface energy, this reaction is also promoted by the low thickness and therefore the reduced diffusion path of the single metallic layers.
The resulting internal heat melts the solder
Solder
Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...
layers to form a bond with the multilayer system and the substrate based on diffusion. This exothermic reaction can be ignited in reactive material
Reactive material
In the US military, reactive materials are a new class of materials currently being investigated by the Office of Naval Research and others as a means to increase the lethality of direct-hit or fragmentation warheads...
s like compacted powders, e.g. Ni/Ti or Ti/Co
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, as well as in nanostructured multilayer systems, e.g. Ni/Al. The bonding can take place in various environments, i.e. vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, with a force providing a defined mechanical pressure at room temperature. A high applied mechanical pressure enhances the solder flow and therefore can improve the wetting of the substrate.
Examples =
Reactive bonding approach is used to assemble MEMS components including die attachment
Die attachment
Die attachment is the step during the integrated circuit packaging phase of semiconductor device fabrication during which a die is mounted and fixed to the package or support structure....
and the hermetic sealing
Hermetic seal
A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method and conditions of usage.-Etymology :...
of micro-system packages
Electronic packaging
Electronic packaging is a major discipline within the field of electronic engineering, and includes a wide variety of technologies. It refers to enclosures and protective features built into the product itself, and not to shipping containers...
. The process is used to join temperature sensitive biological activated substrates for diagnostics or medical device
Medical device
A medical device is a product which is used for medical purposes in patients, in diagnosis, therapy or surgery . Whereas medicinal products achieve their principal action by pharmacological, metabolic or immunological means. Medical devices act by other means like physical, mechanical, thermal,...
s. In addition disposable microfluidic devices with sensing function and immobilized cells can be fabricated.
Technical Specifications =
| Materials | Substrate:
Solder:
Reactive component:
|
| Temperature |
|
| Advantages |
|
| Drawbacks |
|
See also =
- Wafer bondingWafer bondingWafer bonding is a packaging technology on wafer-level for the fabrication of microelectromechanical systems , nanoelectromechanical systems , microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation...
- 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...
- Measurement and characterization for wafer level packaging technologiesBond characterizationThe 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...
- Reactive materials
- NanoFoilNanoFoilNanoFoil is the trademarked name for a reactive multi-layer foil material, sometimes referred to as a pyrotechnic initiator of two mutually reactive metals, aluminium and nickel, sputtered to form thin layers to create a laminated foil...
External links=
Reactive bonding at Fraunhofer ENAS