Thermocompression 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
Microelectronics is a subfield of electronics. As the name suggests, ...
technique and is also referred to as
diffusion bonding, pressure joining, thermocompression welding or solid-state welding. Two metals, e.g.
gold
Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
-
gold (Au), are brought into atomic contact applying force and heat simultaneously.
The
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 ...
requires atomic contact between the
surface
A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...
s due to the atomic
motion
In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...
. The atoms migrate from one
crystal lattice
In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystal, crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that ...
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:
*
surface diffusion
Surface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters ( adparticles) at solid material surfaces.Oura, Lifshits, Saranin, Zotov, and Katayama 2003, p. 325 The process can generally be thought of in ...
*
grain boundary
In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and thermal c ...
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
Vapor-compression evaporation is the evaporation method by which a Centrifugal fan, blower, gas compressor, compressor or jet ejector is used to Gas compression, compress, and thus, increase the pressure of the vapor produced. Since the pressure ...
bonding are
copper (Cu),
gold (Au) and
aluminium (Al) because of their high diffusion rates.
In addition, aluminium and copper are relatively soft metals with good
ductility
Ductility refers to the ability of a material to sustain significant plastic Deformation (engineering), deformation before fracture. Plastic deformation is the permanent distortion of a material under applied stress, as opposed to elastic def ...
.
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 penetrate the surface
oxide
An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
, as it is not able to penetrate through the oxide.
When 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 Damascene may refer to:
* Topics directly associated with the city of Damascus in Syria:
** A native or inhabitant of Damascus
** Damascus Arabic, the local dialect of Damascus
** Damascus steel, developed for swordmaking
** "Damascene moment", the ...
process is very extensive.
It also immediately forms a surface oxide which can, however, be removed by
formic acid
Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid. It has the chemical formula HCOOH and structure . This acid is an important intermediate in chemical synthesis and occurs naturally, most notably in some an ...
vapor
In physics, a vapor (American English) or vapour (Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R ...
cleaning. Oxide removal doubles as surface passivation.
The diffusion of these metals requires good knowledge 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 for synchronized wafer expansion.
Procedural steps
Pre-conditioning
Oxidation
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
and
impurities
In chemistry and materials science, impurities are chemical substances inside a confined amount of liquid, gas, or solid. They differ from the chemical composition of the material or compound. Firstly, a pure chemical should appear in at least on ...
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
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 trichloride; ...
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 nanometres 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.
Deposition
The metal films can be deposited by
evaporation
Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...
,
sputtering
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and c ...
or
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 ...
. 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
A diffusion barrier is a thin layer (usually micrometres thick) 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.
Adhesion of a plated metal layer to i ...
film, i.e.
oxide
An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
or
nitride
In chemistry, a nitride is a chemical compound of nitrogen. Nitrides can be inorganic or organic, ionic or covalent. The nitride anion, N3−, is very elusive but compounds of nitride are numerous, although rarely naturally occurring. Some nitr ...
.
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
Migration, migratory, or migrate may refer to: Human migration
* Human migration, physical movement by humans from one region to another
** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
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
Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
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
A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
or
forming gas
Forming gas is a mixture of hydrogen (mole fraction varies) and nitrogen. It is sometimes called a "dissociated ammonia atmosphere" due to the reaction which generates it:
:2 NH3 → 3 H2 + N2
It can also be manufactured by thermal cra ...
environment, e.g. N
2.
The pressure atmosphere supports the
heat conduction
Thermal conduction is the diffusion of thermal energy (heat) within one material or between materials in contact. The higher temperature object has molecules with more kinetic energy; collisions between molecules distributes this kinetic energy u ...
and prevents thermal gradients vertically across the wafer and re-oxidation.
Based on the difficult control of
thermal expansion
Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions).
Substances usually contract with decreasing temp ...
differences between the two wafers, precision alignment and high quality
fixtures are used.
The bonding settings for the most established metals are following (for 200 mm wafers):
; 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 and 80 kN for 20 to 60 min
Examples
1. Thermocompression bonding is well established in the
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss
", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...
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
Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...
,
accelerometers
An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change of velocity) of the object relative to an observer who is in free fall (that is, relative to an inertia ...
,
gyroscopes
A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining Orientation (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...
and RF MEMS.
2. Typically, thermocompression bonds are made with delivering heat and pressure to the mating surface by a hard faced bonding tool.
Compliant bonding[A.Coucoulas, “Compliant Bonding” Proceedings 1970 IEEE 20th Electronic Components Conference, pp. 380-89, 1970. http://commons.wikimedia.org/wiki/File:CompliantBondingPublic_1-10.pdf https://www.researchgate.net/publication/225284187_Compliant_Bonding_Alexander_Coucoulas_1970_Proceeding_Electronic_Components_Conference_Awarded_Best_Paper] is a unique method of forming this type of solid state bond between a gold lead and a gold surface since heat and pressure is transmitted through a compliant or deformable media. The use of the compliant medium ensures the physical integrity of the lead by controlling the extent of wire deformation. The process also allows one to bond a multiple number of gold wires of various dimensions simultaneously since the compliant media ensures contacting and deforming all the lead wires.
Technical specifications
See also
*
Compliant bonding
*
Thermosonic bonding
Thermosonic bonding is widely used to wire bond silicon integrated circuits into computers. Alexander Coucoulas was named "Father of Thermosonic Bonding" by George Harman, the world's foremost authority on wire bonding, where he referenced Coucoul ...
References
{{Wafer bonding
Electronics manufacturing
Packaging (microfabrication)
Semiconductor technology
Wafer bonding