Dielectrics, thick-film applications

A laser-induced ToF mass spectrometer (LIMA-2A) was manufactured by Cambridge Mass Spectrometry Ltd., Cambridge, UK, for micro local analysis and was used to analyze thin sections of biological samples in the transmission mode or bulk material in the reflection mode.150,151 Typical LIMA applications in microelectronics include identification of impurities in dielectrics, microlocal analysis, depth profiling, thick film analysis and investigations on hybrid circuits. 

Plasma oxide has found utility in high-frequency applications for dual-layer isolation,8 because of its low dielectric constant and high breakdown voltage. Also, it is in compression when deposited, so that it can be used as the dielectric when thick films (2 to 5 microns) are needed. Such thick films when deposited by thermal CVD (which is deposited in tension) tend to crack. One final advantage to the use of plasma oxide rather than plasma nitride is that... 

The effective transmission or reflection may be measured, for example, in the production of semi-transparent films. In most cases, however, the optical film thickness of transparent dielectric films is determined, exploiting the fact that the reflection and transmission change periodically with increasing film thickness (provided that the refractive indices of the film and the substrate are different). The following connection between geometrical and optical film thickness is applicable ... 

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. 

A variation on the above photoetching process uses thick-film inks that are photosensitive. This eliminates the need for the photoresist application and removal steps. Although photosensitive conductors and dielectrics for use with gold and silver are available, similar materials for use with copper are not. 

It must be noted at this point that transparent PbTiO glass-ceramics are of interest for electro-optic and electro-luminescent applications (Kokubo and Tashiro, 1976). Non-transparent, opaque PbTi03 glass-ceramics, however, can be used for thick-film capacitors with a dielectric constant of 94 and tan 6 of 0.0130 (at 10 c/s) on alumina substrates that have been heated to temperatures up to approximately 600°C. The dielectric constant of the capacitor as a function of temperature is almost linear from room temperature to 270 C, with a temperature coefficient of 0.00083 K Therefore, it was found that glass powders were suitable for preparing capacitors with a high dielectric constant (Kokubo and Tashiro, 1976). 

The properties of inorganic materials with dielectric constants less than 12 are described in this section under the headings of Ceramics, Glasses, Micas, Substrates, Glass-Ceramics, and Thick Films. Since micas and glasses are also used as dielectrics, the properties associated with capacitor applications are discussed in the section on 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. 

Test Pattern. The basic conductor properties can be measured using a single test pattern, as illustrated in Fig. 8.15. These include resistivity, print definition and film thickness, film density, solder leach resistance, wettability, adhesion, and wire bondability. Each property will be discussed individually with reference to Fig. 8.15. Many applications require functional use tests which usually require specific test patterns and even multilayer construction processes. Similarly, numerous applications require standard conductor tests on thick-fihn dielectrics instead of the bare substrate. 

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. " ... 

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

Early work in ellipsometry focused on improving the technique, whereas attention now emphasizes applications to materials analysis. New uses continue to be found however, ellipsometry traditionally has been used to determine film thicknesses (in the rang 1-1000 nm), as well as optical constants. " Common systems are oxide and nitride films on silicon v ers, dielectric films deposited on optical sur ces, and multilayer semiconductor strucmres. 

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