Plasma activated bonding
Encyclopedia
Plasma 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 expansion (CTE).
Surface activation
prior bonding has the typical advantage that no intermediate layer is needed and sufficiently high bonding energy is achieved after annealing
at temperatures below 400 °C.
Overview =
The decrease of temperature is based on the increase of bonding strength using plasma activation
on clean wafer
surfaces. Further, the increase is caused by elevation in amount of Si-OH groups, removal of contaminants
on the wafer surface, the enhancement of viscous flow of the surface layer and the enhanced diffusivity
of water and gas trapped at the interface. Based on ambient pressure
, two main surface activation fields using plasma
treatment are established for wafer preprocessing to lower the temperatures during annealing. To establish maximum surface energy at low temperatures (< 100 °C) numerous parameters for plasma activation and annealing need to be optimized according to the bond material.
Plasma activated bonding is based on process pressure divided into:
Atmospheric Pressure-Plasma Activated Bonding (AP-PAB) =
This method is to ignite plasma without using a low pressure environment, so no expensive equipment for vacuum
generation is needed.
Atmospheric Pressure-Plasma Activated Bonding enables the possibility to ignite plasma at specific local areas or the whole surface of the substrate. Between the two electrodes plasma gas is ignited via alternating voltage.
The wafer pairs pass the following process flow:
The optimal gas mixture for the plasma treatment is depending on the annealing temperature. Furthermore, treatment with plasma is suitable to prevent bond defects during the annealing procedure.
If using glass
, based on the high surface roughness, a chemical-mechanical planarization (CMP)
step after rinsing is necessary to improve the bonding quality. The bond strength
is characterized by fracture toughness
determined by micro chevron tests
. Plasma activated wafer bonds can achieve fracture toughnesses that are comparable to bulk material.
enables a stable plasma at atmospheric pressure
. To avoid sparks
, a dielectric
has to be fixed on one or both electrodes
. The shape of the electrode is similar to the substrate geometry used to cover the entire surface. The principle of an AP-activation with one dielectric barrier is shown in figure "Scheme of dielectric barrier discharge".
The activation equipment consists of the grounded
chuck
acting as wafer carrier and an indium tin oxide (ITO)
coated glass electrode. Further, the glass substrate is used as dielectric barrier and the discharge is powered by a corona generator.
Low Pressure-Plasma Activated Bonding (LP-PAB) =
The Low Pressure-Plasma Activated Bonding operates in fine vacuum (0.1 - 100 Pa) with a continuous gas flow. This procedure requires:
The plasma exposed surface is activated by ion bombardment and chemical reactions through radicals
. Electrons of the atmosphere
move towards the HF electrode during its positive voltage
. The most established frequency of the HF electrode is 13.56 MHz.
Further, the electrons are not able to leave the electrode within the positive half wave of applied voltage, so the negative electrode is charged up to 1000 V (bias voltage). The gap between the electrode and the chuck is filled with plasma gas. The moving electrons of the atmosphere are banging into the plasma gas atoms and hit out electrons. Due to its positive orientation the massive ions, that are not able to follow the HF field, move to the negatively charged electrode, where the wafer is placed. Within those environment the surface activation is based on the striking ions and radicals interacting with the surface of the wafer (compare to figure "Scheme of a plasma reactor for low pressure plasma activated bonding").
The surface activation with plasma at low pressure is processed in the following steps :
The electrode attached to the HF-Generator is used as carrier of the wafer. Following, the surfaces of the wafers charge up negatively caused by the electrons and attract the positive ions of the plasma. The plasma ignites in the RIE-reactor (shown in figure "Scheme of a plasma reactor for low pressure plasma activated bonding").
The maximal bond strength is achieved with nitrogen
and oxygen
as process gases and is sufficiently high with a homogeneous dispersion over the wafers after annealing at 250 °C. The bond energy is characterized > 200 % of non-activated reference wafer annealed at the same temperature. The surface activated wafer pair has 15 % less bond energy compared to a high temperature bonded wafer pair. Annealing at 350 °C results in bonding strengths similar to high-temperature bonding.
Remote plasma is using chemical components where mainly neutral radicals are reacting with the surface. The advantage of this process is less damaged surface through missing ion bombardment. Further, the plasma exposure times could be arranged longer than with, e.g. RIE method.
source and an ion trapping metal plate are used for the generation of radicals. The effect of plasma on the surface changes from chemical/physical to chemical plasma treatment. This is based on the reactions between radicals and atoms on the surface.
Technical Specifications =
See also =
External links =
Direct bonding at Fraunhofer ENAS
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
for direct bonding
Direct bonding
Direct 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....
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 expansion (CTE).
Surface activation
Activation
Activation in chemical sciences generally refers to the process whereby something is prepared or excited for a subsequent reaction.- Chemistry :...
prior bonding has the typical advantage that no intermediate layer is needed and sufficiently high bonding energy is achieved after annealing
Annealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
at temperatures below 400 °C.
Overview =
The decrease of temperature is based on the increase of bonding strength using plasma activation
Plasma activation
Plasma activation . It is done with the intent to alter or improve adhesion properties of surfaces prior to coating, painting, etc...
on clean 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...
surfaces. Further, the increase is caused by elevation in amount of Si-OH groups, removal of contaminants
Contamination
Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...
on the wafer surface, the enhancement of viscous flow of the surface layer and the enhanced diffusivity
Diffusivity
Diffusivity can refer to:*Diffusivity of heat*Diffusivity of mass:** Molecular diffusivity ** Eddy diffusivity*Momentum diffusivity...
of water and gas trapped at the interface. Based on ambient pressure
Ambient pressure
The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, which comes into contact with the object....
, two main surface activation fields using 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...
treatment are established for wafer preprocessing to lower the temperatures during annealing. To establish maximum surface energy at low temperatures (< 100 °C) numerous parameters for plasma activation and annealing need to be optimized according to the bond material.
Plasma activated bonding is based on process pressure divided into:
- Atmospheric Pressure-Plasma Activated Bonding (AP-PAB)
- Dielectric barrier dischargeDielectric barrier dischargeDielectric-barrier discharge is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Originally called silent discharge and also known as ozone production discharge or partial discharge, it was first reported by Ernst Werner von Siemens in 1857.- Process...
- Corona dischargeCorona dischargeIn electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...
- Plasma torchPlasma torchA plasma torch is a device for generating a directed flow of plasma. The plasma jet can be used for applications including plasma cutting, plasma spraying, and plasma arc waste disposal....
(Jet)
- Dielectric barrier discharge
- Low Pressure-Plasma Activated Bonding (LP-PAB)
- RIE (Reactive Ion Etching)
- ICP RIE (Inductively Coupled PlasmaInductively coupled plasmaAn inductively coupled plasma is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.-Operation:...
Reactive Ion Etching) - Sequential Plasma (SPAB)
- Remote Plasma
Atmospheric Pressure-Plasma Activated Bonding (AP-PAB) =
This method is to ignite plasma without using a low pressure environment, so no expensive equipment for 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...
generation is needed.
Atmospheric Pressure-Plasma Activated Bonding enables the possibility to ignite plasma at specific local areas or the whole surface of the substrate. Between the two electrodes plasma gas is ignited via alternating voltage.
The wafer pairs pass the following process flow:
- RCA cleaning
- Surface activation at atmospheric pressure
- Treatment duration ~ 40 s
- Process gases used for silicon
- Synthetic air (80 vol.-% N2 + 20 vol.-% O2)
- Oxygen (O2)
- Process gases used for glass or LiTaO3
- Ar/H2 (90 vol.-% Ar + 10 vol.-% H2)
- Humid oxygen (O2dH2O)
- Rinsing in de-ionized water
- Treatment duration 10 minutes
- Reduction of particle concentration
- Pre-bonding at room temperature
- Annealing (room temperature to 400 °C)
The optimal gas mixture for the plasma treatment is depending on the annealing temperature. Furthermore, treatment with plasma is suitable to prevent bond defects during the annealing procedure.
If using glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, based on the high surface roughness, a chemical-mechanical planarization (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:...
step after rinsing is necessary to improve the bonding quality. The bond strength
Bond strength
In chemistry, bond strength is measured between two atoms joined in a chemical bond. It is the degree to which each atom linked to another atom contributes to the valency of this other atom...
is characterized by fracture toughness
Fracture toughness
In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications. The fracture toughness of a material is determined from the...
determined by micro chevron tests
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...
. Plasma activated wafer bonds can achieve fracture toughnesses that are comparable to bulk material.
Dielectric barrier discharge (DBD)
The usage of dielectric barrier dischargeDielectric barrier discharge
Dielectric-barrier discharge is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Originally called silent discharge and also known as ozone production discharge or partial discharge, it was first reported by Ernst Werner von Siemens in 1857.- Process...
enables a stable plasma at atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
. To avoid sparks
Spark
-Physical sparks:* Spark , a small glowing particle or ember* An electric spark, usually with a flash and a sharp noise-Computer science:* SPARK...
, a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
has to be fixed on one or both electrodes
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
. The shape of the electrode is similar to the substrate geometry used to cover the entire surface. The principle of an AP-activation with one dielectric barrier is shown in figure "Scheme of dielectric barrier discharge".
The activation equipment consists of the grounded
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
chuck
Chuck (engineering)
A chuck is a specialized type of clamp used to hold an object, usually an object with radial symmetry, especially a cylindrical object. It is most commonly used to hold a rotating tool or a rotating workpiece...
acting as wafer carrier and an indium tin oxide (ITO)
Indium tin oxide
Indium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...
coated glass electrode. Further, the glass substrate is used as dielectric barrier and the discharge is powered by a corona generator.
Low Pressure-Plasma Activated Bonding (LP-PAB) =
The Low Pressure-Plasma Activated Bonding operates in fine vacuum (0.1 - 100 Pa) with a continuous gas flow. This procedure requires:
- Vacuum
- Process gases
- High frequency (HF) electrical field between two electrodes
The plasma exposed surface is activated by ion bombardment and chemical reactions through radicals
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
. Electrons of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
move towards the HF electrode during its positive voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
. The most established frequency of the HF electrode is 13.56 MHz.
Further, the electrons are not able to leave the electrode within the positive half wave of applied voltage, so the negative electrode is charged up to 1000 V (bias voltage). The gap between the electrode and the chuck is filled with plasma gas. The moving electrons of the atmosphere are banging into the plasma gas atoms and hit out electrons. Due to its positive orientation the massive ions, that are not able to follow the HF field, move to the negatively charged electrode, where the wafer is placed. Within those environment the surface activation is based on the striking ions and radicals interacting with the surface of the wafer (compare to figure "Scheme of a plasma reactor for low pressure plasma activated bonding").
The surface activation with plasma at low pressure is processed in the following steps :
- RCA cleaning
- Surface activation at low pressure
- Treatment duration ~ 30 - 60 s
- Process gases (N2, O2)
- Rinsing in de-ionized water
- Treatment duration 10 min
- Reduction of particle concentration
- Pre-bonding at room temperature
- Annealing (room temperature to 400 °C)
Reactive Ion Etching (RIE)
The RIE mode is used in dry etching processes and through reduction of parameters, i.e. HF power, this method is usable for surface activation.The electrode attached to the HF-Generator is used as carrier of the wafer. Following, the surfaces of the wafers charge up negatively caused by the electrons and attract the positive ions of the plasma. The plasma ignites in the RIE-reactor (shown in figure "Scheme of a plasma reactor for low pressure plasma activated bonding").
The maximal bond strength is achieved with nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
as process gases and is sufficiently high with a homogeneous dispersion over the wafers after annealing at 250 °C. The bond energy is characterized > 200 % of non-activated reference wafer annealed at the same temperature. The surface activated wafer pair has 15 % less bond energy compared to a high temperature bonded wafer pair. Annealing at 350 °C results in bonding strengths similar to high-temperature bonding.
Remote plasma
The procedure of remote plasma is based on creating plasma in a separate side chamber. The input gases enter the remote plasma source and are transported to the main process chamber to react. A scheme of the system is shown in figure "Remote plasma system".Remote plasma is using chemical components where mainly neutral radicals are reacting with the surface. The advantage of this process is less damaged surface through missing ion bombardment. Further, the plasma exposure times could be arranged longer than with, e.g. RIE method.
Sequential Plasma (SPAB)
The wafers are activated with short RIE plasma followed by a radical treatment in one reactor chamber. An additional microwaveMicrowave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
source and an ion trapping metal plate are used for the generation of radicals. The effect of plasma on the surface changes from chemical/physical to chemical plasma treatment. This is based on the reactions between radicals and atoms on the surface.
Technical Specifications =
| Materials |
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| Temperature |
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| Advantages |
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| Drawbacks |
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| Researches |
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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....
- 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...
- 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 =
Direct bonding at Fraunhofer ENAS