Dielectrics, thick-film compositions

PASTES. Conductor, resistor, dielectric, seal glass, polymer and soldering compositions are available in paste or ink form. They are used to produce hybrid circuits, networks and ceramic capacitors. The materials are often called thick film compositions. 

Ellipsometry measurements can provide infomiation about the thickness, microroughness and dielectric ftinction of thin films. It can also provide infomiation on the depth profile of multilayer stmctiires non-destmctively, including the thickness, the composition and the degree of crystallinity of each layer [39]. The measurement of the various components of a complex multilayered film is illustrated m figure Bl.26.17 [40]. 

Polymer thick films also perform conductor, resistor, and dielectric functions, but here the polymeric resias remain an iategral part after cuting. Owiag to the relatively low (120—165°C) processiag temperatures, both plastic and ceramic substrates can be used, lea ding to overall low costs ia materials and fabrication. A common conductive composition for flexible membrane switches ia touch keyboards uses fine silver particles ia a thermoplastic or thermoset polymeric biader. 

The copyrolysis of 1 wt% dibromotetrafluoro-p-xylylene with commercially available hexafluoro-p-xylene (Aldrich) with metals was examined and it was found that it was indeed possible to prepare films that were spectroscopically indistinguishable from those deposited from dimer. The PA-F films obtained are of excellent quality, having dielectric constants of2.2-2.3 at 1 MHz and dissociation temperatures up to 530°C in N2. A uniformity of better than 10% can be routinely achieved with a 0.5-gm-thick film on a 5-in. silicon wafer with no measurable impurities as determined by XPS. During a typical deposition, the precursor was maintained at 50°C, the reaction zone (a ceramic tube packed with Cu or Ni) was kept at 375-550°C, and the substrate was cooled to -10 to -20°C. The deposited film had an atomic composition, C F 0 = 66 33 1 3 as determined by XPS. Except for 0, no impurities were detected. Within instrumental error, the film is stoichiometric. Poly(tetrafluoro-p-xylylene) has a theoretical composition ofC F = 2 1. Figure 18.2 illustrates the XPS ofthe binding energy... 

The temperature dependence of the dielectric of the multil er thick film deposited from titanate /mixed solvent (alcohol/acetylacetone) system is shown in Figure 3. This multilayer (5 layers) FGM sample was made using the suspension of composition 0, 30,46, 63 and 100... 

F i g u r e 7 is a plot of normalized capacitance of 85, 95 and 100 v/o BT. All these samples show sharp transition. Figure 8 is a plot of Curie temperature as a fiinction of composition. 75 v/o sample shows a Curie point 16°C. Figure 9 is a dielectric response of a multilayer thick film on a Pt substrate made from titanate/ethanol suspensions of composition 100,75, 50 and 25 v/o BT. This sample shows broad transition temperature (80°-120"C). Although pure BT has a transition temperature 120 C and next nearest transition temperature by 75 v/o BT( 16"C). This indicates inter-layer diffiision of the cation resulted the broadening as well as shifting of the peak towards lower temperature. Dielectric constant at transition temperature is -5,000 in IkHz. This preliminary results indicate that by chosing appropiate suspension composition, individual layer thickness and sintering time and... 

Efforts at loading titanium oxide nanoparticles in PVA (commercially available from Nanophase) have been reported [65], In this report, titanium nanoparticles are dispersed in an aqueous solution of PVA with poly(melamine-co-formaldehyde). The solution is spun onto substrate and heated to generate a cross-Unked polymer-nanoparticle dielectric. A modest enhancement of dielectric constant is achieved for 600-nm thick films. Thin-film transistors using this composite show excellent pen-tacene mobility (> 0.2 cm V s ) and reasonable on/off ratios 10. Vj- —TV is reasonably high, suggesting static charge at the dielectric-semiconductor interface. 

For thick-film capacitor applications the dielectric composition will contain a material with a high dielectric... 

A description of the application of ceramic and photopolymer technologies to achieve high-resolution electronic patterns follows. The first section discusses ceramic dielectric vias, and the second, conductive circuitry. Improved photosensitive ceramic coating compositions and more particularly, compositions that function as precursors to fired dielectric ceramics, are mainly useful in preparation of multilayer thick-film substrates. 

As previously mentioned, the glass composition is also critical to compatibUity of thick-fihn conductors with other thick film materials such as resistors and dielectrics. This requires additional considerations when the formulation is designed. Figure 8.11 shows conduc-tor/resistor incompatibilities at the interface resulting in cosmetic as well as performance-related issues. Glass chemistry selection is also very dependent on the firing atmosphere, as discussed earlier in the section on Cu conductors. 

Thick-film formulations exhibit different types of dominant sintering mechanisms, depending on their composition. For example, thick-film conductors can sinter like sohd-state metal particles or spheres, while sintering of resistors and dielectrics is more complex and largely dependent on glass sintering behavior. 

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. 

M. K. Rao et al., Effects of Infra-Red Firing on the Properties of Low AT Thick Film Dielectrics Compositions, Proc. Inti. Symp. Microelec., Atlanta, pp. 119-123,1986. 

A coating composition was prepared by dissolving the step 1 product (3.00 g), and 1,3,5,7-adamantanetetracarboxylic acid (0.552 g) in /V,/V-dimcthylacctamidc (20.13 g) and then filtering through 0.2 pm membrane. The coating solution was then spin coated onto an 8-inch silicon wafer and heated to 300°C for 30 minutes and then further heated to 400°C for an additional 30 minutes. The film that formed had a thickness of 298 nm, a density of 1.05 g/cm3, and a dielectric constant of 2.3. 

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