Eutectic bonding

Such excellent or at least adequate capillary behaviour is also typical of the process variant known as eutectic bonding in which the transient creation of a liquid phase is caused by the interdiffusion of two chemically different metal alloy component materials. In the laboratory variant process known as partial transient liquid phase bonding, (Shalz et al. 1992), a coated interlayer is used for ceramic-ceramic or ceramic-metal joints. In this process the interlayer is a ductile metal or alloy whose surface is coated with a thin layer of a lower melting temperature metal or alloy, for example Ni-20Cr coated with 2 microns of Au. The bonding temperature is chosen so that only the coating melts and the ductility of the interlayer helps to accommodate mismatches in the coefficient of thermal expansion of the component materials. 

Development of metal/intermetallic compound functionally graded material produced by eutectic bonding method. 

On production of FGM by the eutectic bonding method, a metal and an intermetallic compound are combined using a eutectic reaction which exists between both the substances. An importcint advantage in this combination of the substances is that relieving thermal stress is easy. Because the difference in thermal expansion coefficients between metals and intermetallic compounds is much smaller than that between metals and ceramics. 

The eutectic bonding method can be applied to a combination of shaped metal parts and intermetallic compound powder, and results in coating of the shaped metal part by intermetallic compound. We intend to call this technique eutectic coating method . 

From above results, it is concluded that the eutectic bonding method is effective for production of FGM. 

Fig.3 Heat treatment patterns for fabricating Fig.4 Heat treatment patterns for fabricating Ti/TisSn FGMs by eutectic bonding method. Ti/TisSis FGMs by eutectic bonding method.

Figure 5. Micrographs of eutectic bonding parts in Ti/TisSn FGMs.

Eutectic bonding is a process adopted from IC packaging and uses an eutectic gold-based alloy preform, such as 80% Au/20% Sn [43]. Slow creeping of the eutectic alloy is the main drawback of this technique. 

When using isotropically conductive adhesives, placement of components is performed by the same equipment as used for nonadhesive attachment techniques. Die bonders are similar to those used for eutectic bonding except for the type of adhesive dispenser. Surface-mounted placement machines developed for solder paste assembly can also be... 

Eutectic bonding is a bonding method based on the melting of a metallic intermediate layer of eutectic alloy composition which is used for chip bonding and wafer bonding. 

Eutectic Bonding, Fig.1 Au-Si phase diagram (According to Okamoto and Massalski 1990)... 

Eutectic bonding of wafers consists mainly of the three following steps ... 

However, for the bonding procedure, a higher temperature than the eutectic temperature is usually preferred, so that the bonding strength of the eutectic bond is enhanced. In spite of the texturization appearance under a higher temperature, the bonding process is normally implemented in practice at 390 °C, so that the heat between the wafers and alloy is successfully transferred. Only in this way, a reliable bonding process will be accomplished. 

Figure 2 shows a cross section of bonded wafers after an eutectic bonding process. An intermediate layer of gold (Au) is utilized for bonding of two silicon wafers (Si), one covered with oxide (SiOa) and a diffusion barrier (Ti), the other one after oxide removal prior to bonding. During the bonding process, the eutectic alloy forms and local voids occur within the AuSi layer. 

Eutectic bonding has been mainly applied in microelectronics technology. It is used to stack silicon dies, to bond silicon wafers, and it has been adopted in the packaging of silicon... 

Eutectic Bonding, Fig. 2 Cross section SEM image of wafer interface after oxide removal at bottom wafer and subsequent eutectic bonding Si-Si02-Ti-AuSi-Si (Fraunhofer ENAS)... 

Eutectic Bonding, Fig. 3 High speed dual head die... 

Low-temperature aluminum-germanium (Al-Ge) eutectic bonding at a low temperature of 435 °C has been investigated for monolithic three-dimensional integrated circuit (3DIQ applications (Cmogorac et al. 2009). 

Cmogorac F, Btrringer R, Dauskardt R, Pease F (2009) Aluminum-germanium eutectic bonding for 3D integration. IEEE international conference on 3D system integration 3DIC, San Francisco... 

Wang Q, Choa SH, Kim WB, Hwang IS, Ham SJ, Moon CY (2006) Application of Au-Sn eutectic bonding in hermetic radio-frequency microelectromechanical system wafer level packaging. J Electron Mater 35(3) 425 32... 

Cheng YT, Lin L, Najafi K (2000) Localized silicon fusion and eutectic bonding for MEMS fabrication and packaging. J Microelectromech Syst 9(l) 3-8... 

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