Substrate bonding refers to attaching two or more substrates of material such as glass or silicon, to each other by means of various chemical and physical effects. Substrate bonding is mainly used in MEMS, where sensor components are encapsulated in the application. The most common types of bonding are adhesive, anodic, eutectic, fusion, glass frit, direct wafer or fusion bonding, and metallic diffusion.
There are 7 primary methods which define substrate bonding technology. These have been mentioned above and are described in detail below.
|Bonding method||Substrate types||materials used||common temperature ranges||pressure ranges||flatness requirements||hermetic seal||relative bond strength||best tool to use||Primarily Used for:|
|Adhesive bonding||almost any||variety of materials including polymers, adhesives, epoxies, dry films, BCB, polyimides, and UV curable compounds||10-100ºC||Highly directional - no sidewall coverage||Poor||High||Poor||10-100nm||Liftoff metal deposition especially e-beam resists|
|Anodic||Si, ionic glass||Si, thin SiO2, ionic glass (pyrex)||300-450ºC||Highly directional - no sidewall coverage||Poor||Low||Poor||10-100nm||Liftoff or thicker metal deposition|
|Eutectic||Si, glass(with adhesion layer)||0.1-10 Å/sec||50-300ºC||Some sidewall coverage||Good||Low||Good||~10nm||More conformal metal and dielectric thin film deposition. Compounds that do not evaporate while keeping stoichiomettry|
|Fusion (direct wafer)||Si, SiO2||5-200Å/sec||200-400ºC||Some sidewall coverage||Good||Very Low||Good||10-100nm||Lower temp oxide/nitride deposition|
|Glass frit||Si, SiO2, Si2N4, Al, Ti, Glass||10-100 Å/sec||600-1200ºC||Isotropic - good sidewall coverage||Very Good||Very Low||Very Good||1-10nm||Better quality oxide/nitride dep where substrate can handle higher temp|
|Metallic diffusion||materials with similar CTE's||Depends on process||0-100ºC||Isotropic - good sidewall coverage||Good||Depends on process||Depends on process||Depends on Process||Thicker films deposition with good conformality|
Uses a layer of polymer or adhesive, including epoxies, dry films, BCB, polyimides, and UV curable compounds between the substrates to bond the substrates together. This introduces a foreign material between your substrates, but there is a wide variety of materials that can make this method desirable for it's extremely wide range of temperature and control.
Relatively low temperatures help protect sensitive components
This type of bonding involves encapsulating components on a silicon wafer by means of ionic glass. When ionic glass is heated, it becomes increasingly less insulating. When a high voltage is applied, current is able to flow which causes the migration of the ions in the glass. The resistance of the ionic glass dramatically decreases with increased temperatures, thus successful bonding can occur through a range of temperatures and voltages depending on your device tolerances. Altering the surface energy of the substrate materials through plasma activation can also provide some process lattitude depending on your stress, temperature, and voltage tolerances.
Bond is hermetically sealed and displays good resistance to thermal and chemical effects Very reliable bond, probably the most forgiving with substrate flatness
Eutectic wafer bonding takes advantage of the special properties of eutectic metals. Similar to soldering alloys, such metals melt already at low temperatures. This property allows planar surfaces to be achieved.
In order to control reflow of the eutectic material, eutectic bonding requires precise dosing of the bonding force and even temperature distribution.
High mechanical strength High resistance to thermal and chemical effects
Fusion (direct wafer) bonding
Fusion bonding refers to spontaneous adhesion of two planar substrates. The process involves rinsing the polished discs and rendering them largely hydrophilic, then placing them in contact and tempering them at high temperatures. Plasma pretreatment allows the substrates to be bonded at room temperature.
Less process time required Low temperatures protect sensitive components
Glass frit bonding
This process involves screen-printing glass frits onto the bonding surfaces. This results in structures that are subsequently heated and fused when the two substrate surfaces are placed in contact. On cooling, a mechanically stable bond results.
Low production costs
Metallic Diffusion bonding
Metal diffusion bonding is based on Cu-Cu, Al-Al, Au-Au and other metallic bonds. In addition, the use of metal diffusion allows two wafers to be bonded both mechanically and electrically in a single step. The technique is required for bonding in 3D applications such as 3D stacking.
Highest level of impermeability among all common bonding processes High alignment precision following bonding
Figures of Merit
What is important to look for in the technology? Etch/dep rate? Resolution?
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How is this technology used in nanofabrication and what types of devices/research areas is it useful in?
Specific equipment for each technology can be found on its page above. Additionally, below is a list of all metrology equipment in the LNF:
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