Redistribution layer bonding

Another approach to 3D integration is to use wafer bonding to stack die before singulation this approach is referred to as wafer-level 3D. There have been a variety of approaches to wafer-level 3D that have been demonstrated, which can be categorized by the wafer-bonding approach used oxide-to-oxide, copper-to-copper, polymer-to-polymer (or adhesive bonding), and mixtures of these approaches (such as redistribution layer bonding). Each of these four approaches will be introduced in this section, with emphasis placed on their application to 3D. The bond unit processes are described further in Section 15.4.2, and their associated CMP issues are discussed in detail in Section 15.5. 

FIGURE 15.8 Schematic representation of the redistribution layer bonding approach to 3D integration demonstrated by McMahon et al. (from Ref. 51). 

McMahon JJ, Lu J-Q, Gutmann RJ. Wafer bonding of damascene-patterned metal/ adhesive redistribution layers for via-lirst 3D interconnect. In Proceedings of the IEEE Electronic Components and Technology Conference 2005. p 331-336. 

Gutmann R, McMahon J, Rao S, Niklaus F, Lu J-Q. Wafer-level via-first 3D integration with hybrid-bonding of Cu/BCB redistribution layers. Proceedings of the International Wafer Level Packaging Congress 2005. 

The iCSP is assembled at the wafer level. In a typical construction shown in Fig. 23, a sheet of glass the same dimension as the silicon wafer is patterned with a dam of epoxy that mirrors the outline of each of the die on the wafer. This is placed over, and bonded to the wafer. Vias are formed through the back of the image sensor substrate to reveal the underside of the specially designed connection pads. An isolation layer is placed onto the back of the die to isolate it prior to patterning an interconnect or redistribution layer (RDL), which is used to route connections from the silicon bond pads to the solder balls. 

For chips that are flip-chip bonded, additional conductor layers may be required to redistribute the I/Os from underneath the chip to the engi-... 

The most advanced implementation of cofired-ceramic-packaging technology is the thermal conduction module (TCM) used in large-scale computers (IBM) (4, 72, 74). This package can accommodate over 100 flip-chip-bonded ICs on a 90 by 90 mm cofired ceramic substrate. The multilayer ceramic substrate contains 33 metal layers for chip pad redistribution, signal interconnection, and power distribution (Figure 14). Each chip contains 120 bonding pads, and 1800 pins are brazed to the bottom of the substrate for connection to a PWB. 

Mineral grinding leads to distorsion of chemical and ionic bonds between atoms and ions. In the fracture areas binding and coordination states get asymmetric, and new electron and electric valences occur. Spontaneous reactions in the crystalline structure and with contact phases are the consequence of the distorsion. Surface distorsion of the crystalline structure may be diminished or completely abolished. At the same time, the free surface energy decreases due to polarization of surface ions. These ions are redistributed in the inner or outer layer of the crystalline surface and/or due to chemisorption of molecules and ions1. All these changes occur side by side, but one of them can suppress the effect of the others in a decisive manner. 

The application of absolute reaction rate theory to a chemical change at an interface is only useful if the calculations refer to an identified, or at least reliably inferred, model of the controlling bond redistribution step. This is a problem, because it is particularly difficult to characterize the structures of the immediate precursors to reaction in many solid state rate processes of interest. The activated species are inaccessible to direct characterization because they are usually located between reactant and product phases. The total amount of reacting material present within this layer, often of molecular dimensions, is small and irreversible chemical and textural changes may accompany opening of such specialized structures for examination or analysis. Moreover, the presence of metallic and/or opaque, ill-crystalUzed product phases may prevent or impede the experimental recognition of participating intermediates or essential textural features. 

The diagram in Fig. 6 on the left shows another interesting effect. Perpendicular to the fibre direction side maxima can be observed. In an ideal CPRP material, all fibres are parallel and isolated each from another. In reality, the manufacturing process leads to a certain fibre deformation and redistribution. Both the fibres within a layer and the fibres of neighbouring layers can contact each other thus causing variations in the eddy current field. This effect can be used for evaluating some matrix and bonding properties. 

In summary, whereas extensive chemical interactions did not occur and observable interfacial reaction layers did not form at the C-C/CuAgTi interfaces, some redistribution of alloying elements occurred. Large Ti concentrations occurred at the C-C/braze interface, indicating favorable surface modification due to a carbide-forming reaction that promoted bonding. 

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