Thermocompression bonding
Encyclopedia
Thermocompression 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 (Au)
-gold (Au)
, are brought into atomic contact applying force and heat simultaneously. The diffusion
requires atomic contact between the surface
s due to the atomic motion
. The atoms migrate from one crystal lattice to the other one based on crystal lattice vibration. This atomic interaction sticks the interface
together.
The diffusion process is described by the following three processes:
This method enables internal structure protecting device packages and direct electrical interconnect structures without additional steps beside the surface mounting process.
Overview =
The most established materials for thermocompression bonding are copper (Cu)
, gold (Au)
and aluminium (Al)
because of their high diffusion rates. In addition, aluminium and copper as relatively soft metals have good ductile
properties.
The bonding with Al or Cu requires temperatures ≥ 400 °C to ensure sufficient hermetical sealing
. Furthermore, aluminium needs extensive deposition and requires a high applied force to crack the surface oxide
, as it is not able to penetrate through the oxide.
Using gold for diffusion, a temperature around 300 °C is needed to achieve a successful bond. Compared to Al or Cu, it does not form an oxide. This allows to skip a surface cleaning procedure before bonding.
Copper has the disadvantage that the damascene process is very extensive. Also it forms immediately a surface oxide that can be removed by formic acid
vapor
cleaning. The oxide removal conduces also as surface passivation.
The metal diffusion requires a good control of the CTE differences between the two wafers to prevent resulting stress. Therefore, the temperature of both heaters needs to be matched and center-to-edge uniform. This results in a synchronized wafer expansion.
in the metal films affect the diffusion reactions by reducing the diffusion rates. Therefore, clean deposition practices and bonding with oxide removal and re-oxidation prevention steps are applied. The oxide layer removal can be realized by various oxide etch chemistry methods
. Dry etching
processes, i.e. formic acid vapor cleaning, are preferred based on the minimization of the immersion in fluids and the resulting etching of the passivation
or the adhesion layer. Using the CMP
process, which is especially for Cu and Al required, creates a planarized surface with micro roughness around several nanometer and enables the achievement of void
free diffusion bonds. Further, a surface treatment for organic removal, e.g. UV-ozone exposure, is possible.
Methods, i.e. plasma
surface pretreatment, provide an accelerated diffusion rate based on an increased surface contact. Also the use of an ultra planarization step is considered to improve the bonding due to a reduction of material transport required for the diffusion. This improvement is based on a defined height Cu
, Au
and Sn
.
, sputtering
or electroplating
. Evaporation and sputtering, producing high quality films with limited impurities, are slow and hence used for micrometre and sub-micrometre layer thicknesses. The electroplating is commonly used for thicker films and needs careful monitoring and control of the film roughness and the layer purity.
The gold film can also be deposited on a diffusion barrier
film, i.e. oxide
or nitride
. Also, an additional nano crystalline metal film, e.g. Ta, Cr, W, or Ti, can enhance the adhesion strength of the diffusion bond at decreased applied pressure and bonding temperature.
Surface diffusion, also referred to as atomic diffusion, describes the process along the surface interface, when atoms move from surface to surface to free energy.
The grain boundary diffusion terms the free migration of atoms in free atomic lattice spaces. This is based on polycrystalline layers and its boundaries of incomplete matching of the atomic lattice and grains.
The diffusion through bulk crystal is the exchange of atoms or vacancies within the lattice that enables the mixing. The bulk diffusion starts at 30 to 50 % of the materials melting point increasing exponentially with the temperature.
To enable the diffusion process, a high force is applied to plastically deform the surface asperities in the film, i.e. reducing bow and warp of the metal. Further, the applied force and its uniformity is important and depends on the wafer
diameter and the metal density
features. The high degree of force uniformity diminish the total force needed and alleviate the stress gradients and sensitivity to fragility. The bonding temperature can be lowered using a higher applied pressure and vice versa, considering that high pressure increases the chances of damage to the structural material or the films.
The bonding process itself takes place in a vacuum
or forming gas
environment, e.g. N2. The pressure atmosphere supports the heat conduction
and prevents thermal gradients vertically across the wafer and re-oxidation. Based on the difficult control of thermal expansion
differences between the two wafers, precision alignment and high quality fixtures are used.
The bonding settings for the most established metals are following:
Aluminium (Al): bonding temperature can be from 400 to 450 °C with an applied force above 70 kN for 20 to 45 min
Gold (Au): bonding temperature is between 260 and 450 °C with an applied force above 40 kN for 20 to 45 min
Copper (Cu):bonding temperature lies around 380 to 450 °C with an applied force between 20 to 80 kN for 20 to 60 min
Examples =
Thermocompression bonding is well established in the CMOS
industry and realizes vertical integrated devices and production of wafer level packages with smaller form factors. This bonding procedure is used to produce pressure sensors, accelerometers, gyroscopes and RF MEMS.
Technical Specifications =
See also =
External links =
Thermocompression 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...
technique and is also referred to as diffusion bonding, pressure joining, thermocompression welding or solid-state welding. Two metals, e.g. gold (Au)
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...
-gold (Au)
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...
, are brought into atomic contact applying force and heat simultaneously. The 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...
requires atomic contact between the surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
s due to the atomic motion
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...
. The atoms migrate from one crystal lattice to the other one based on crystal lattice vibration. This atomic interaction sticks the interface
Interface (chemistry)
An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...
together.
The diffusion process is described by the following three processes:
- surface diffusionSurface diffusionSurface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters at solid material surfaces. The process can generally be thought of in terms of particles jumping between adjacent adsorption sites on a surface, as in figure 1...
- grain boundaryGrain boundaryA grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material...
diffusion - bulk diffusion
This method enables internal structure protecting device packages and direct electrical interconnect structures without additional steps beside the surface mounting process.
Overview =
The most established materials for thermocompression bonding are copper (Cu)
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, gold (Au)
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...
and aluminium (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....
because of their high diffusion rates. In addition, aluminium and copper as relatively soft metals have good ductile
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
properties.
The bonding with Al or Cu requires temperatures ≥ 400 °C to ensure sufficient hermetical 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 :...
. Furthermore, aluminium needs extensive deposition and requires a high applied force to crack the surface oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
, as it is not able to penetrate through the oxide.
Using gold for diffusion, a temperature around 300 °C is needed to achieve a successful bond. Compared to Al or Cu, it does not form an oxide. This allows to skip a surface cleaning procedure before bonding.
Copper has the disadvantage that the damascene process is very extensive. Also it forms immediately a surface oxide that can be removed by formic acid
Formic acid
Formic acid is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early...
vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
cleaning. The oxide removal conduces also as surface passivation.
The metal diffusion requires a good control of the CTE differences between the two wafers to prevent resulting stress. Therefore, the temperature of both heaters needs to be matched and center-to-edge uniform. This results in a synchronized wafer expansion.
Pre-conditioning
Oxidation and impuritiesImpurity
Impurities are substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound.Impurities are either naturally occurring or added during synthesis of a chemical or commercial product...
in the metal films affect the diffusion reactions by reducing the diffusion rates. Therefore, clean deposition practices and bonding with oxide removal and re-oxidation prevention steps are applied. The oxide layer removal can be realized by various oxide etch chemistry methods
Etching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
. 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...
processes, i.e. formic acid vapor cleaning, are preferred based on the minimization of the immersion in fluids and the resulting etching of the passivation
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
or the adhesion layer. Using the CMP
Chemical-mechanical planarization
Chemical Mechanical Polishing/Planarization is a process of smoothing surfaces with the combination of chemical and mechanical forces. It can be thought of as a hybrid of chemical etching and free abrasive polishing.-Description:...
process, which is especially for Cu and Al required, creates a planarized surface with micro roughness around several nanometer and enables the achievement of void
Void
-In science and engineering:*Void , the empty spaces between galaxy filaments*Lack of matter, or vacuum*Void, in boiling heat transfer, formed where there is a departure from nucleate boiling, causing a critical heat flux...
free diffusion bonds. Further, a surface treatment for organic removal, e.g. UV-ozone exposure, is possible.
Methods, i.e. plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
surface pretreatment, provide an accelerated diffusion rate based on an increased surface contact. Also the use of an ultra planarization step is considered to improve the bonding due to a reduction of material transport required for the diffusion. This improvement is based on a defined height Cu
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, Au
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...
and Sn
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
.
Deposition
The metal films can be deposited by evaporationEvaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
, sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
or 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...
. Evaporation and sputtering, producing high quality films with limited impurities, are slow and hence used for micrometre and sub-micrometre layer thicknesses. The electroplating is commonly used for thicker films and needs careful monitoring and control of the film roughness and the layer purity.
The gold film can also be deposited on a diffusion barrier
Diffusion barrier
A diffusion barrier is a thin layer of metal usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other....
film, i.e. oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
or nitride
Nitride
In chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications....
. Also, an additional nano crystalline metal film, e.g. Ta, Cr, W, or Ti, can enhance the adhesion strength of the diffusion bond at decreased applied pressure and bonding temperature.
Bonding
The factors of the chosen temperature and applied pressure depend on the diffusion rate. The diffusion occurs between the crystal lattices by lattice vibration. Atoms can not leap over free space, i.e. contamination or vacancies. Beside the most rapid diffusion process (surface diffusion), the grain boundary and the bulk diffusion exist.Surface diffusion, also referred to as atomic diffusion, describes the process along the surface interface, when atoms move from surface to surface to free energy.
The grain boundary diffusion terms the free migration of atoms in free atomic lattice spaces. This is based on polycrystalline layers and its boundaries of incomplete matching of the atomic lattice and grains.
The diffusion through bulk crystal is the exchange of atoms or vacancies within the lattice that enables the mixing. The bulk diffusion starts at 30 to 50 % of the materials melting point increasing exponentially with the temperature.
To enable the diffusion process, a high force is applied to plastically deform the surface asperities in the film, i.e. reducing bow and warp of the metal. Further, the applied force and its uniformity is important and depends on the wafer
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...
diameter and the metal density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
features. The high degree of force uniformity diminish the total force needed and alleviate the stress gradients and sensitivity to fragility. The bonding temperature can be lowered using a higher applied pressure and vice versa, considering that high pressure increases the chances of damage to the structural material or the films.
The bonding process itself takes place in a 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...
or forming gas
Forming gas
Forming gas is a mixture of hydrogen and nitrogen. It is sometimes called a "dissociated ammonia atmosphere" due to the reaction which generates it:...
environment, e.g. N2. The pressure atmosphere supports the heat conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
and prevents thermal gradients vertically across the wafer and re-oxidation. Based on the difficult control of thermal expansion
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...
differences between the two wafers, precision alignment and high quality fixtures are used.
The bonding settings for the most established metals are following:
Aluminium (Al): bonding temperature can be from 400 to 450 °C with an applied force above 70 kN for 20 to 45 min
Gold (Au): bonding temperature is between 260 and 450 °C with an applied force above 40 kN for 20 to 45 min
Copper (Cu):bonding temperature lies around 380 to 450 °C with an applied force between 20 to 80 kN for 20 to 60 min
Examples =
Thermocompression bonding is well established in the CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
industry and realizes vertical integrated devices and production of wafer level packages with smaller form factors. This bonding procedure is used to produce pressure sensors, accelerometers, gyroscopes and RF MEMS.
Technical Specifications =
| Materials |
|
| Temperature |
|
| Advantages |
|
| Drawbacks |
|
| Researches |
|
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...
- 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...
- 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...
External links =
Thermocompression bonding at Fraunhofer ENAS