Substrate bonding

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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.

Technologies

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	Low Melting Point metals and materials	1-10 Å/sec	10-100ºC	Highly directional - no sidewall coverage	Poor	High	Poor	10-100nm	Liftoff metal deposition especially e-beam resists
Anodic	Silicon and ionic glass	1-15 Å/sec	10-100ºC	Highly directional - no sidewall coverage	Poor	Low	Poor	10-100nm	Liftoff or thicker metal deposition
Eutectic	Metals and dielectrics	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)	Mainly Dielectrics	5-200Å/sec	200-400ºC	Some sidewall coverage	Good	Very Low	Good	10-100nm	Lower temp oxide/nitride deposition
Glass frit	Mainly Dielectrics	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	Conductive Materials	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

Adhesive bonding

Main article: Adhesive bonding

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

Anodic bonding

Main article: Anodic bonding

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.

Bond is hermetically sealed and displays good resistance to thermal and chemical effects Very reliable bond, probably the most forgiving with substrate flatness

Eutectic bonding

Main article: Eutectic bonding

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

Main article: Fusion 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

Main article: 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

Technology 6

Main article: 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?

FOM 1

Subheadings are optional.

Applications

How is this technology used in nanofabrication and what types of devices/research areas is it useful in?

Equipment

Specific equipment for each technology can be found on its page above. Additionally, below is a list of all metrology equipment in the LNF:

See also

Other related wiki pages

Further reading

• Other stuff, e.g. technology workshop slides
• External links (can be in another section below, if appropriate)