The ideal ceramic substrate for high reliability microelectronic interconnection would match the expansion coefficient of the semiconductor (usually silicon) as semiconductor chips have grown in size and, being rigidly attached to the substrate, could easily become stressed and fail. Ideally the ceramic substrate would also have a low dielectric loss and high thermal conductivity, as high-density circuits and high power devices dissipate a lot of heat in a small area. The reason why alumina has been the most widely used substrate for thick film microelectronics is that it has good thermal expansion, mechanical and dielectric properties. It is easy to make to a high tolerance and finish, as its small grain size is important for fine definitions, and it has good adhesion with the glazes/pastes used for metallisation etc. It has a relatively high thermal conductivity, but beryllia (thermal conductivity half that of copper) is used in some high performance thick and thin film electronics. Aluminium nitride and silicon carbide (doped with beryllia) have comparable high thermal conductivities and are sometimes used, particularly as aluminium nitride (thermal conductivity 9 times that of alumina) has a thermal expansion coefficient closer to that of silicon.

Ceramics with tailored dielectric constants are used depending on the application. Speed is vital in modern microelectronics, and delay in the signal propagating across the substrate is proportional to the square root of the dielectric constant (k). For silicon itself k=4 (square root=2) so the delay is twice that of a vacuum (k=1). For alumina k=10 so the delay is just over 3 times that in a vacuum. However, circuit features such as resonators require a high k to reduce their size, so compromises have to be made.

17.1.2 Manufacturing Process

Ceramic thick film technology has its origins in pottery decoration methods used by Chinese potters over 2000 years ago using printed metal oxide colours. They used silk as a mesh, blocking out areas of the silk with pitch, then pressed dye through this pattern, hence the term silk-screen printing. The process would be repeated several times for a multilayer pattern. Its use for microelectronics dates from the 1920s with the development of modern silk-screen printing. World War II and the development of the transistor and integrated circuit in the 40s and 50s led to its rapid growth.

Stainless steel screen printing mesh
- source Wikipedia via Alibaba

So the thick film screen-printing process for circuitry is very similar to that described previously for pottery. The ceramic material is formed as a thixotropic “ink” or paste that can be deposited onto a substrate by forcing it through a patterned polymer or metal screen or mesh using a sponge or “squeegee”. Thixotropic materials are viscous when still but fluid when worked. Usually the paste would contain a binder and solvent as well as the main constituent. The non-volatile binder holds the material in suspension and is removed during sintering at typically 350 ºC, whereas the solvent used to control the viscosity is removed at around 100 ºC. Often the required circuit pattern is formed photographically on a stainless steel mesh. This mesh is coated in a UV sensitive emulsion that is exposed to the required pattern of lines and features, which when developed leaves spaces where the ink is required. A typical mesh has 200-400 wires per inch or 80-160 per cm, each wire is 15-50 microns diameter, and around 200-micron wide lines can be resolved on the substrate.

Cross section diagram illustrating multi-layer
printed circuit - source emerald insight

The initial process of making ceramic thick film circuitry is the sequential screen printing and firing of dielectric, conductors and resistors onto a suitable substrate. The ability to build up several layers of circuitry using insulating dielectric layers is a key advantage of this technology. Conductive metallisation is formulated with glasses and oxides to aid densification of the metal when fired at 600 to 950 ºC. Ceramic dielectrics are chosen based partly on their dielectric constant, since a low value is needed for use as insulation, to avoid capacitive “crosstalk” or signal leakage, while a high value is needed in forming capacitors themselves.

Capacitor dielectrics are usually formed from glasses to achieve the necessary high density and low porosity at processing temperatures, but include a ceramic filler such as doped barium titanate to increase the dielectric constant. Thick-film capacitors typically have a dielectric thickness of 25 microns.