Reactive bonding describes a

wafer bonding 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 ...

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 In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...

within the multilayer system contributes the local heat to bond the

solder Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...

films. Based on the limited temperature the substrate material is exposed, temperature-sensitive components and materials with different CTEs, i.e.

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

s and

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

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 nickel, 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.

Procedural steps

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

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

substrate. The deposition of the multilayer systems on silicon is achieved by

magnetron sputtering Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re ...

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

etching Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other type ...

. The vertical nano structures are also created directly on the substrate surface. The substrate surfaces are deposited with a

layer, i.e.

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...

(Au), using

physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and poly ...

(PVD). The PVD process promotes the

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. This happens in presence of a gaseous phase or another liquid phase not miscible with ...

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

. A multilayer system consists of thousands of thin single layers of the component combination that are alternately sputtered on the substrate surface. For

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

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 A nanostructure is a structure of intermediate size between microscopic and molecular structures. Nanostructural detail is microstructure at nanoscale. In describing nanostructures, it is necessary to differentiate between the number of di ...

s are created in two steps. At first, needles in the silicon substrate are created by

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 (usually a plasma of reactive gases such as fluorocarbons, oxygen, chlorine, boron trichlorid ...

. 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 realized by applying an

electrochemical machining Electrochemical machining (ECM) is a method of removing metal by an electrochemical process. It is normally used for mass production and is used for working extremely hard materials or materials that are difficult to machine using conventional me ...

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 A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing o ...

energy in the multilayer system (compare to figure "Schematic self-propagating reaction in a multilayer system after ignition"). The system

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...

, or an

intermetallic compound An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic ele ...

, (AB) is formed from the intermixing elements (A+B) due to atomic

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

. 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

layers to form a bond with the multilayer system and the substrate based on diffusion. This exothermic reaction can be ignited in reactive materials 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 A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

, 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 Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

components including

die attachment In electronics manufacturing, integrated circuit packaging is the final stage of semiconductor device fabrication, in which the block of semiconductor material is encapsulated in a supporting case that prevents physical damage and corrosion. ...

and the

hermetic seal A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger categor ...

ing of micro-system

packages Package may refer to: Containers or Enclosures * Packaging and labeling, enclosing or protecting products * Mail, items larger than a letter * Chip package or chip carrier * Electronic packaging, in electrical engineering * Automotive package, i ...

. The process is used to join temperature sensitive biological activated substrates for diagnostics or

medical device A medical device is any device intended to be used for medical purposes. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assur ...

s. In addition disposable

microfluidic Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field tha ...

devices with sensing function and immobilized cells can be fabricated.

Technical specifications

References

{{cite journal, author1=Qiu, X. , author2=Wang, J. , name-list-style=amp , title = Bonding silicon wafers with reactive multilayer foils, journal = Sensors and Actuators A: Physical, year = 2008, volume = 141, pages = 476–481, number = 2, doi = 10.1016/j.sna.2007.10.039 Electronics manufacturing Packaging (microfabrication) Semiconductor technology Wafer bonding Brazing and soldering