Glass frit bonding
Encyclopedia
Glass 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. accelerometer
s or gyroscope
s. This technique utilizes low melting glass ("glass solder") and therefore provides various advantages, i.e. viscosity
of glass decreases with an increase of temperature. The viscous flow of glass has effects to compensate and planarize surface irregularities, convenient for bonding wafers with a high roughness due to plasma etching
or deposition
. A low viscosity promotes hermetically sealed
encapsulation of structures based on a better adaption of the structured shapes. Further, the coefficient of thermal expansion (CTE) of the glass material is adapted to silicon
. This results in low stress in the bonded wafer
pair.
Glass frit bonding can be used for many surface materials, i.e. silicon with hydrophobic and hydrophilic surface, silicon dioxide
, silicon nitride
, aluminium
, titanium
or glass
, as long as the CTE are in the same range. This bonding procedure also allows the realization of metallic feedthrough
s to contact active structures in the hermetically sealed cavity. Glass frit
as a dielectric material does not need additional passivation
for preventing leakage currents at process temperatures up to 125 °C.
Following advantages resulting in using glass frit bonding procedure:
Overview =
The glass frit
bond procedure is used for the encapsulation and mounting of components. The coating of glass frit layers is applied by spin coating
for thickness of 5 to 30 µm or commonly by screen printing for thickness of 10 to 30 µm.
To achieve process temperatures beneath 450 °C leaded glass
or lead silicate glass is used. The glass frit is a paste consisting glass powder, organic binder
, inorganic fillers
and solvent
s. This low melting glass paste is milled into powder (grain size < 15 µm) and mixed with organic binder forming a printable viscous paste. Inorganic fillers, i.e. cordierite
particles (e.g. Mg2Al3 [AlSi5O18]) or barium silicate, are added to the melted glass paste to influence properties, i.e. lowering the mismatch of thermal expansion coefficients between silicon and glass frit. The solvents are used to adjust the viscosity of the organic binder. Several glass frit pastes are commercially available, e.g. FERRO FX-11-0366, and every single one need individual handling after deposition. The choice of the paste depends on various factors, i.e. deposition method, substrate material and process temperatures.
The glass used for MEMS applications consists of particles and lead oxide. Latter lowers the glass transition
temperature below 400 °C. The reduction
of lead oxide by the silicon leads to the formation of lead
precipitations at the silicon-glass interface. Those precipitations decrease the strength of the bond and are reliability risks that have to be considered for the life time predictions of the devices.
The printed glass frit structures are heated to form compact glass. The heating process is necessary to drive out the solvents and binder. This results in a subsequent particle fusion
of the glass powder. Using mechanical pressure the wafers are bonded at elevated temperatures.
The procedural steps of glass frit bonding are divided into the following:
.
Screen printing enables the possibility of selective bonding. So only in areas where bonding is required the glass frit is deposited.
The risk of glass frit flowing into the structures can be prevent by optimization of the screen printing process. Under high positioning precision the sizes of the structures in the range of 190 µm with a minimum spacing of < 100 µm are achievable. The exact positioning of the screen print structures to the cap wafer are required to ensure an accurate bond. The bonded structures are, dependent on the wettability of the printed surface, 10 to 20 % wider than the designed screen.
To ensure a uniform glass thickness, all structures should have the same width. The printed glass frit high is about 30 µm and provides a gap of 5 to 10 µm between the bonded wafers after bonding (compare to cross sectional SEM images). A bond surface activation is not necessary to promote a higher bonding strength.
The initial step comprises drying for 5 to 7 minutes at 100 to 120 °C in order to diffuse solvents out of the interface. This starts the polymerization of the organic binder. The binder molecules are linked to long-chain polymers what solidifies the paste.
The organic binder of the glass paste has to be burned with heating up to a specific temperature (325 to 350 °C) where the glass is not fully melted for 10 to 20 minutes. This so called glazing ensures the outgassing
of the organic additives.
Further, a pre-melting or sealing step heats the material to the process temperature between 410 and 459 °C for 5 to 10 min. The material fully melts and forms a compact glass without any inclusions. The inorganic fillers are melted down and the properties of the bond glass are fixed. The melting of the glass starts at the silicon-glass interface directed to the glass surface. During the melting process the porosity
of the glass eliminates and based on the compression of the intermediate layer the thickness of the glass decreases significantly.
The alignment has to be very precise and stable to prevent shifting. This can be realized using clamps or special pressure plates. Shifting can occur through temporarily staggered pressure, not precise vertical pressure based on misalignment of the bonding tools or the difference of thermal expansion between the bonding tools.
During bonding a supporting tool pressure is applied to improve the thermal input into the bonding glass and equal wafer geometry inadmissibility (i.e. bow and warp) supporting wettability. Based on the sufficiently high viscosity of the glass, bonding can take place nearly without pressure.
The bonding temperature needs to be high enough to reduce the viscosity of the glass material and ensures a good wetting of the bond surface, but also low enough to prevent overspreading of the glass frit material. The heating up over 410 °C enables the wetting of the bond surface. A good wetting
is indicated by a low edge angle. The atomic wafer surface layers are fused into the glass at an atomic level. This forms a thin glass mixture at the interface which forms the strong bond between the glass and the wafer.
s.
The bonding strength is mainly dependent on the density, the spreading area of the glass frit layer and the surface layer of the bonding interface. It is high enough, around 20 MPa, for most applications and comparable to those achieved with anodic bonding
. The hermeticity ensures the correct function and a sufficient reliability of the bond and therefore the product. Further, the bonding yield of glass frit bonded wafers is very high, normally > 90 %.
Examples =
Glass frit bonding is used to encapsulate surface micro-machined sensors
, i.e. gyroscope
s and accelerometer
s. Other applications are the sealing of absolute pressure sensor
cavities, the mounting of optical window
s and the capping of thermally active devices.
Technical specifications =
See also =
External links =
Glass frit 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 with an intermediate 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...
layer. It is a widely used encapsulation
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...
technology for surface micro-machined structures
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
, i.e. accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
s or gyroscope
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
s. This technique utilizes low melting glass ("glass solder") and therefore provides various advantages, i.e. viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of glass decreases with an increase of temperature. The viscous flow of glass has effects to compensate and planarize surface irregularities, convenient for bonding wafers with a high roughness due to plasma etching
Plasma etching
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge of an appropriate gas mixture being shot at a sample. The plasma source, known as etch species, can be either charged or neutral...
or deposition
Vapor deposition
Vapor deposition can refer to:* Chemical vapor deposition* Physical vapor deposition...
. A low viscosity promotes hermetically sealed
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 :...
encapsulation of structures based on a better adaption of the structured shapes. Further, the coefficient of thermal expansion (CTE) of the glass material is adapted to silicon
Silicon
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...
. This results in low stress in the bonded wafer
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...
pair.
Glass frit bonding can be used for many surface materials, i.e. silicon with hydrophobic and hydrophilic surface, silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
, silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...
, aluminium
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....
, titanium
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 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...
, as long as the CTE are in the same range. This bonding procedure also allows the realization of metallic feedthrough
Feedthrough
A feedthrough is a conductor used to conduct a signal through a printed circuit board. Like any conductor it has a small amount of capacitance. It is used mainly to bypass purposes in ultra-high-frequency circuits. There are many different types of feedthrough but they can be divided into two...
s to contact active structures in the hermetically sealed cavity. Glass frit
Frit
Frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble...
as a dielectric material does not need additional 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...
for preventing leakage currents at process temperatures up to 125 °C.
Following advantages resulting in using glass frit bonding procedure:
- screen printing process applicable on thin, structured wafer
- no electrical potentials during bonding process necessary
- low tension due to low bonding temperature
- selective bonding based on structured intermediate glass layer
- bonding of rough wafer surfaces
- no outgassingOutgassingOutgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
after bonding, better chemical durability, higher strength compared to organic adhesiveAdhesiveAn adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
s - high reliabilityReliability (semiconductor)Reliability of semiconductor devices can be summarized as follows:# Semiconductor devices are very sensitive to impurities and particles. Therefore, to manufacture these devices it is necessary to manage many processes while accurately controlling the level of impurities and particles...
and stable hermetical sealing - easier process compared to metallic or eutectic layer procedures
Overview =
The glass frit
Frit
Frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble...
bond procedure is used for the encapsulation and mounting of components. The coating of glass frit layers is applied by spin coating
Spin coating
Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force...
for thickness of 5 to 30 µm or commonly by screen printing for thickness of 10 to 30 µm.
To achieve process temperatures beneath 450 °C leaded glass
Leaded glass
Leaded glass may refer to:*Lead glass, potassium silicate glass which has been impregnated with a small amount of lead oxide in its fabrication...
or lead silicate glass is used. The glass frit is a paste consisting glass powder, organic binder
Binder (material)
-See also:*Adhesive or Glue*Cement*Paint...
, inorganic fillers
Filler (materials)
Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...
and solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
s. This low melting glass paste is milled into powder (grain size < 15 µm) and mixed with organic binder forming a printable viscous paste. Inorganic fillers, i.e. cordierite
Cordierite
Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...
particles (e.g. Mg2Al3 [AlSi5O18]) or barium silicate, are added to the melted glass paste to influence properties, i.e. lowering the mismatch of thermal expansion coefficients between silicon and glass frit. The solvents are used to adjust the viscosity of the organic binder. Several glass frit pastes are commercially available, e.g. FERRO FX-11-0366, and every single one need individual handling after deposition. The choice of the paste depends on various factors, i.e. deposition method, substrate material and process temperatures.
The glass used for MEMS applications consists of particles and lead oxide. Latter lowers the glass transition
Glass transition
The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass...
temperature below 400 °C. The reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of lead oxide by the silicon leads to the formation of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
precipitations at the silicon-glass interface. Those precipitations decrease the strength of the bond and are reliability risks that have to be considered for the life time predictions of the devices.
The printed glass frit structures are heated to form compact glass. The heating process is necessary to drive out the solvents and binder. This results in a subsequent particle fusion
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
of the glass powder. Using mechanical pressure the wafers are bonded at elevated temperatures.
The procedural steps of glass frit bonding are divided into the following:
- Deposition of glass paste
- Burn out and firing (in order to form glass layer)
- Bonding process
- Cooling down
Glass frit deposition
Screen printing, as a commonly used deposition method, provides a technique of structuring for the glass frit material. This method has the advantage of material deposition on structured cap wafers without any additional processes, i.e. photolithographyPhotolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
.
Screen printing enables the possibility of selective bonding. So only in areas where bonding is required the glass frit is deposited.
The risk of glass frit flowing into the structures can be prevent by optimization of the screen printing process. Under high positioning precision the sizes of the structures in the range of 190 µm with a minimum spacing of < 100 µm are achievable. The exact positioning of the screen print structures to the cap wafer are required to ensure an accurate bond. The bonded structures are, dependent on the wettability of the printed surface, 10 to 20 % wider than the designed screen.
To ensure a uniform glass thickness, all structures should have the same width. The printed glass frit high is about 30 µm and provides a gap of 5 to 10 µm between the bonded wafers after bonding (compare to cross sectional SEM images). A bond surface activation is not necessary to promote a higher bonding strength.
Thermal conditioning
Thermal conditioning transforms the glass paste into glass layer and is important to prevent voids inside the glass frit layer. The conditioning process consists of:- Glazing of organic binder and solvents
- Melting of glass particle to compact glass
- Formation of solid connection between glass and wafer surface
The initial step comprises drying for 5 to 7 minutes at 100 to 120 °C in order to diffuse solvents out of the interface. This starts the polymerization of the organic binder. The binder molecules are linked to long-chain polymers what solidifies the paste.
The organic binder of the glass paste has to be burned with heating up to a specific temperature (325 to 350 °C) where the glass is not fully melted for 10 to 20 minutes. This so called glazing ensures the outgassing
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
of the organic additives.
Further, a pre-melting or sealing step heats the material to the process temperature between 410 and 459 °C for 5 to 10 min. The material fully melts and forms a compact glass without any inclusions. The inorganic fillers are melted down and the properties of the bond glass are fixed. The melting of the glass starts at the silicon-glass interface directed to the glass surface. During the melting process the porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of the glass eliminates and based on the compression of the intermediate layer the thickness of the glass decreases significantly.
Bonding process
The glass frit bonding, starting with alignment of the wafers, is a thermo-compressive process that takes place in the bonding chamber at specific pressure. Under bonding pressure wafers are heated up to the process temperature around 430 °C for a few minutes. On the one hand a short bonding time causes the glass frit to spread insufficiently, on the other hand a longer bonding time causes the glass frit to be overflown subsequently leaving voids.The alignment has to be very precise and stable to prevent shifting. This can be realized using clamps or special pressure plates. Shifting can occur through temporarily staggered pressure, not precise vertical pressure based on misalignment of the bonding tools or the difference of thermal expansion between the bonding tools.
During bonding a supporting tool pressure is applied to improve the thermal input into the bonding glass and equal wafer geometry inadmissibility (i.e. bow and warp) supporting wettability. Based on the sufficiently high viscosity of the glass, bonding can take place nearly without pressure.
The bonding temperature needs to be high enough to reduce the viscosity of the glass material and ensures a good wetting of the bond surface, but also low enough to prevent overspreading of the glass frit material. The heating up over 410 °C enables the wetting of the bond surface. A good 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...
is indicated by a low edge angle. The atomic wafer surface layers are fused into the glass at an atomic level. This forms a thin glass mixture at the interface which forms the strong bond between the glass and the wafer.
Cooling down
During cooling down under pressure a mechanically strong and hermetically sealed wafer bond is formed. The cooling process leads especially at higher temperatures to thermal stress in the glass frit layer that has to be considered in the lifetime analysis of the bond frame. The wafer pair is removed from the bond chamber at lower temperatures to prevent thermal cracking of the wafers or the bond interface by thermal shockThermal shock
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...
s.
The bonding strength is mainly dependent on the density, the spreading area of the glass frit layer and the surface layer of the bonding interface. It is high enough, around 20 MPa, for most applications and comparable to those achieved with anodic bonding
Anodic bonding
Anodic 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...
. The hermeticity ensures the correct function and a sufficient reliability of the bond and therefore the product. Further, the bonding yield of glass frit bonded wafers is very high, normally > 90 %.
Examples =
Glass frit bonding is used to encapsulate surface micro-machined sensors
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
, i.e. gyroscope
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
s and accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
s. Other applications are the sealing of absolute pressure sensor
Pressure sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
cavities, the mounting of optical window
Optical window
The meaning of this term depends on the context:* In astronomy, the optical window is the optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground...
s and the capping of thermally active devices.
Technical specifications =
| Materials | Substrate:
Intermediate layer:
|
| 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...
- 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...
- 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...
- 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 =
Glass frit bonding at Fraunhofer ENAS