1 Structural devices: a static role
The superb manufacturing techniques of microelectronics enable designers of integrated circuits to exercise complete control over the electrical characteristics of each component, such as a transistor, by specifying the shapes and sizes of their active regions. Using photolithographic mask-drawing software on their workstations, they can copy and paste blocks of identical devices all over the chip, knowing that when the design is finally realised in silicon, this extreme uniformity will be faithfully reproduced. Figure 1 shows an example.
It is perhaps surprising that it took more than twenty years of microelectronics manufacturing before any serious efforts were made to apply these methods to the making of large numbers of tiny, precisely similar mechanical structures. The first part of this text is devoted to a case study of the manufacture of one such mechanical structure – one that would be impossible to make so reliably by any other means – using processes adapted from microelectronics, augmented by a few novel ones. The new technology, combining mechanical and electrical function on this small scale, is given the generic name of microelectromechanical systems (or MEMS for short), and the device you'll be looking at is the microcantilever and probe tip assembly used in atomic force microscopes. But before that, a description of the manufacturing steps that go into the making of a very simple micromachined sensor – essentially a hot wire – will serve to illustrate some of the challenges that have to be overcome when making precisely defined devices on a tiny scale.
Photolithography is the universal tool for microelectronic chip making. The term encompasses all of the process steps that go into the removal of parts of a thin layer of material to create a pattern (such as, for example, a network of metal tracks connecting transistors to one another on the chip).