Dielectrics, thick-film

Dielectric compositions for insulator fabrication include insulator dielectrics or glass filled with ceramic dielectrics (alumina and silica added to the glass), ceramic-filled glasses (glass and refractory oxides), and crystallizing dielectrics (crystallizable glass and crystallization agents). 

---

Properties of Thick-Film Dielectric Materials Multilayer Dielectric Materials... 

Thick-film dielectric materials are used primarily as insulators between conductors, either as simple crossovers or in complex multilayer structures. Small openings, or vias, may be left in the dielectric layers so that adjacent conductor layers may interconnect. In complex structures, as many as several hundred vias per layer may be required. In this manner, complex interconnection structures may be created. Although the majority of thick-fihn circuits can be fabricated with only three layers of metallization, others may require several more. If more than three layers are required the yield begins dropping dramatically with a corresponding increase in cost. 

DuPont Electronic Materials Division and Hughes Aircraft introduced the first LTCC materials system at the 1983 ISHM Conference in Philadelphia. DuPont then marketed a complete line of tapes and conductor systems with which to build complete systems. Solderable conductors and screened resistor inks soon followed. The properties of these DuPont materials are shown in Table 1.13. It can be seen that their properties are similar to those of alumina and thick film dielectrics. 

Base Metal Conductors. Copper conductors have gained some acceptance over the past decade because of their relatively low metal cost, low resistivity, good adhesion on AljOj substrates, excellent solder leach resistance, and low migration tendency. Advances in compatible thick film dielectric formulations have resulted in significant use of Cu in multilayer interconnect boards, primarily for military applications. Also, uses of Cu conductor materials have included power hybrid and microwave-related applications. Their applications in more complex systems and networks have been limited by the availability of state-of-the-art nitrogen-firable resistor systems. However, there are additional factors that complicate the widespread usage of this versatile material. 

Thick-film dielectrics are used in a variety of hybrid microelectronics apphcations. The most common functions of thick-film dielectrics are to ... 

Dielectrics used to satisfy the first and the second requirements are electric insulators with relatively low dielectric constants (less than 15). Capacitor dielectrics with dielectric constants greater than 20 are used as charge storage devices. Thus, thick-film dielectric formulations can be broadly categorized as either insulator or capacitor dielectrics. Insulator dielectric materials can be further categorized as sealing glasses, crossover dielectrics, and multilayer dielectrics. 

FIGURE 8.25 DTA trace of glass used in a glass-filled ceramic thick film dielectric. 

Fused silica has the lowest K among ceramic materials. The dielectric constants of polymeric materials are in the range of 2.7-3.7. Thick-film dielectric materials based on fnsed sihca filled with quartz, cordierite, sol-gel sihca, and other proprietary glass ceramic formnlations... 

Thick-Film Dielectrics for Other Applications. So far the information presented in this section has dealt with thick-fihn dielectric materials for AljOj substrates. Other substrate materials used in the hybrid industry include beryUia (BeO), AIN, and porcelainized steel substrates. The display industry uses a variety of glass substrates. These other substrate materials aU require the design and development of specific formulations because of their chemical and mechanical differences. " ... 

Aluminum nitride is a nonoxide ceramic material. The GTE of AIN (4.5 ppm/°C) is substantially lower than that of alumina. Early attempts to use thick-film dielectric materials designed for alumina substrates on AIN were unsuccessful. Many of the glasses used in thick-film formulations designed for alumina substrates are chemically incompatible with AIN. Furthermore, the CTE differences between dielectrics designed for alumina and AIN substrates lead to cracking. Special thick-fihn dielectric formulations have been developed for use on AIN substrates. Table 8.12 shows properties of a typical thick-fihn dielectric on AIN substrates. " ... 

TABLE 8.13 Typical Properties of High-A" Thick-Film Dielectrics... 

Microstructural features of dielectric materials include the chemical composition and structure of the various phases as well as porosity and its distribution. These microstructural features, coupled with macroscopic defects such as pinholes, mesh marks, foreign matter, and interaction with conductor materials, determine dielectric properties. Very little information is published about composition of thick-film dielectrics, which are complex materials systems. The following section is a general discussion of dielectric microstructure development. 

Most often, the AT of a material is obtained from capacitance measurements of a parallel plate capacitor made from thick-film dielectric and conductor materials. The electrical capacitance C of a parahel plate capacitor with electrode area A and dielectric thickness d may be expressed as follows ... 

Insulator Dielectrics. Insulator thick film dielectrics are multiphase materials. The electronic, ionic, and interfacial polarization mechanisms all contribute to the dielectric constant of glass-ceramic materials. Electronic polarization is directly proportional to the density of electrons in the glass-ceramic. Thus, dielectrics based on glasses containing oxides of high atomic number elements (e.g., lead) or high density exhibit high dielectric constants. 

The fired micro structure of a thick film dielectric contains ferroelectric phases and other phases produced by sintering additives or modifiers. The dielectric constant of this complex mixture of phases can be estimated by using various mixture rules. For example, one of the mixture rules used in estimating the dielectric constant of a mixture is... 

J. Barrett and P. Moran, Laser Drilling of Vias in Unfired Thick Film Dielectrics, Proc. Inti. 

Gary P. Shorthouse et al., A Hermetic Very Low AT High Resolution Thick Film Dielectric for High Speed High Density Interconnections, Proc. Inti. Symp. Microelec., pp. 528-532,1991. 

---