Thin-film circuits - MCM and related techniques

Previous article , dedicated to so-called "traditional" thin films, has defined what is meant by a thin film, i.e. a deposit generally less than 5 µm thick, usually obtained in a vacuum. This deposition is global, covering the entire surface of the substrate, and the patterns are obtained by a subtractive method, such as chemical etching.

Thin films have been used for many decades in a wide range of applications. The oldest and still most widespread are probably optical applications. Metallurgy and photography have also used thin films, but the most modern applications now include chemistry, biochemistry and medicine, around sensors of all kinds, from gases to DNA (biochips).

Interconnection, in the form of substrates equipped with deposited conductor tracks, is also one of the preferred fields for thin films.

Although in the 1980s thick films were dethroned in this particular field, the concept of the "multichip module" (MCM), which emerged around 1985, has, thanks to a number of technical innovations, restored a certain interest in thin films, which have regained the opportunity to fully exploit their integration capacities in these applications.

But history only repeats itself. Monolithic integration is constantly evolving and gaining ground on heterolithic integration, pushing it to evolve in its turn. If MCMs are today's (heterolithic) answer to the limitations engendered by (monolithic) ASICs, then (heterolithic) SOPs will be tomorrow's answer to the (monolithic) SOCs that are trying to take MCMs' place today. There's no doubt that thin-film electronics will play a key role in SOPs - superhybrids that combine high-density electrical and optical interconnections, active and passive components, cooling devices and, why not, electromechanical microsystems.