43 Materials

4.3.1 Materials Characteristics

4.3.1.1 Interconnect Substrate Materials Selection

Cost is a critical factor in material selection. Beyond cost, often the single most important property used in the selection of new dielectric substrate material is that of dielectric constant. The propagation delay of a signal within a computer is inversely proportional to the square root of the substrate material dielectric constant. The lower the dielectric constant of the insulator, the better the performance. Since traditionally aluminum oxide with a dielectric constant of approximately 9.5 to 10 has been used for many years, any new material, if it was going to perform better, should have a dielectric constant well below 10. High-performance silicon chips are frequently joined to the multilayer interconnect substrates through flip-chip or C4 technology, and for large die sizes a close thermal expansion match between the chip and carrier is desirable. Silicon chips can generate many watts of heat in use; so the heating and cooling associated with a computer being turned on and off could be a source of significant fatigue if the thermal expansion mismatch is large, although underfills have much improved fatigue life between die and chip carriers for a variety of module solutions, including both ceramic and especially organic chip carriers where large thermal expansion mismatch is typically large. Another important property of a substrate dielectric material is the processing compatibility between the metal and the ceramic. Can they be processed together to form strong, dense bodies with the desired properties? Molybdenum and tungsten metals have been commonly used with aluminum oxide ceramic because it is necessary to reach temperatures in excess of 1500° C in order to coalesce the Al2O3 particles during firing. At this temperature many metals are molten. While we would like the metal powder particles to sinter into a dense solid conductor, if the metal melts, its properties and the ceramic substrate itself could be altered significantly. Historically, if a highly conductive metal such as copper, gold, or silver are to be used, then the ceramic material's firing temperature must be considerably lower than that of aluminum oxide. Because copper is an attractive conductor material in terms of cost and resistance to elec-tromigration, the firing temperature of the composite should ideally be kept below 1083°C since copper melts at this temperature. Some recent mixtures of a refractory metal such as molybdenum or tungsten mixed with copper have been shown to cofire with alumina-based dielectrics when sintered at reduced temperatures even above the melting point of copper and may provide a means to improve the conductors conductivity but still have a lower propagation speed. One final set of material characteristics that should not be ignored for a chip carrier is the resulting composite mechanical properties. Aluminum oxide is a very robust material. Compared to Al2O3, many materials are much weaker, and if this is not taken into account, many problems can result. Although glass-ceramics generally have a lower modulus compared to Al2O3, they are much stronger than most glass-only compositions. So, even though glass-ceramics are less resistant to cracking or fracture than Al2O3, they are just as good or significantly better than many other possible low dielectric constant ceramic candidates. Listed in Table 4.1 is a summary of the important materials properties for choosing a dielectric substrate material and a comparison between traditional Al2O3 and Mo/W and that of glass-ceramic and copper.

In Table 4.2 a comparison between alumina and Mo/W and glass-ceramics and Cu shows the superiority of glass-ceramics for high-performance applications over alumina ceramic. Many materials were considered before glass-ceramics were selected. When using the properties as shown in Table 4.1 as the selection criteria, some of the candidate materials considered included Si3N4, AlN, BeO, mullite, borosilicate glass, and silica. Each one of these

TABLE 4.1 Materials and Properties Data Comparison

Materials

Glass BoroSilicate

Characteristic Al2O3 -Ceramic Mullite BeO Si3N4 SiO2 AlN Glass

TABLE 4.1 Materials and Properties Data Comparison

Materials

Glass BoroSilicate

Characteristic Al2O3 -Ceramic Mullite BeO Si3N4 SiO2 AlN Glass

Coefficient of

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