Thick film processing

Equally important as tape casting in the fabrication of multilayer ceramics is thick film processing. Thick film technology is widely used in microelectronics for resistor networks, hybrid integrated circuitry, and discrete components, such as capacitors and inductors along with metallization of MLC capacitors and packages as mentioned above. 

One of the attractive features of the thick film process is that the capital investment for thick film printing is relatively low. Basically, it involves a thick film printer, an oven, and a furnace. One of the notable commercial successes of using thick film techniques is the manufacture of glucose strips for diabetic patient management. The strip involves thick film printing and is cost-effective, making the disposable strip a reality. 

Pollution legislation continues driving further reductions in engine emissions. The dominant exhaust-gas sensors today and in the near future are oxygen partial pressure sensors - also called lambda sensors. Due to the high temperature of exhaust gas, these sensors are made by ceramics technology in combination with thick-film processing. 

A thin-film electrode is relatively dense, as the metallic film does not have the electrocatalytic properties that a porous electrode has. Therefore, in many instances, the surface of the thin film is chemically or electrochemically modified to enhance its electrocatalytic activity. For instance, thin platinum film electrodes can be platinized electrochemically forming a porous platinum black layer. This platinum black layer is electrocatalytically more active than the thin platinum film. Thin-film processes are more capital and labor intensive and the process is more complicated than thick-film processes. Thin-film deposition is also a batch process which may produce sensors of limited numbers of silicon substrates. This is very desirable in prototype development, for it allows modification on prototypes with minimum cost. 

In assembling hybrid microcircuits or multichip modules, ceramic interconnect substrates fabricated using thin-film or thick-film processes are attached to the inside base of a ceramic or metal package. Generally, film adhesives that have been cut to size are used to attach large substrates (greater than 1-inch square) while either paste or film adhesives may be used for smaller substrates. Substrates may be alumina, beryllia, aluminum nitride, or silicon. 

MCM-C interconnect substrates are produced from either low-temperature cofired ceramic (LTCC) or high-temperature cofired ceramic (HTCC). Either process can produce multilayer substrates having high numbers of conductor layers (up to 100), although for most applications 2-20 layers are sufficient. The fabrication of MCM-C involves thick-film processes that have wider lines and spacings (5-20 mils) than MCM-D, but are lower in cost. 

Thick-film resistors are made by diluting a conductive oxide with an electrical insulator such as glass. (The practical aspects of thick-film processing were described in... 

Production scale. Although it can be a mass production technique, it is also adaptable to small batch production. The thick film process is therefore extremely useful in research where varying parameters may be tested on a small number of samples. Also, the eventual commercial market for such devices is probably not sufficient to warrant silicon IC fabrication on a massive scale. 

The term thick film (T-F) technology is accepted to mean that field of microelectronics in which specially-formed pastes are applied and fired on to a ceramic substrate in a defined pattern and sequence to produce a set of individual components such as resistors and capacitors, or a complete functional circuit (19). Figure 11.3 shows a flow diagram of a standard thick film process. 

The lines and spaces are much smaller than can be attained by the thick-film process. In a production environment, the thin-film process can produce lines and spaces 0.001 in. in width, whereas the thick film process is limited to 0.01 in. 

The line definition available by the thin-film process is considerably better than that of the thick-film process. Consequently, the thin-film process operates much better at high frequencies. 

To obtain finer lines and smaller vias, one can use photoimageable thick-film process for dielectric and conductors and diffusion patterning. The photoimageable thick-film process involves the use of a photoactive paste printed on a substrate and exposed through artwork or a mask to define circuif characteristics, lines, and vias. The materials are developed in an aqueous process and then fired using the conventional thick-film fechnique. Copper, silver, and gold metallizations are used, and layer coxmts of up to 10 circuit layers are possible. 

After cooling, further thick-film processes, such as resistor printing and firing and laser trim, can be performed. A comparison of HTCC and LTCC technologies is presented in Table 4.13. The reader can refer to Chapter 6 for a more detailed discussion of the HTCC and LTCC processes. 

In the thick-film process, the ceramic can be metallized with a circuit pattern in one of two ways screen printing the pattern or etching the pattern. 

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