st popular micromachining material. Limiting our analysis to optical switching applications, there are essentially two tech- niques that can be used to realize optical switches with silicon-based MEMs: bulk micro- machining and surface micromachining [44,45]. Bulk micromachining is the most mature and simple micromachining technology; it involves the removal of silicon from the bulk silicon substrate by etchants. Depending on the type of etchants that are used, etching can be isotropic, that is, oriented along one of the silicon crystal planes, or anisotropic, that is, oriented along a plane different from the crystal main planes. Anisotropic etching is generally used to expose the silicon crystal planes in order to build mirrors or similar other structures. Using bulk micromachining the need of complex planar processes is reduced to a mini- mum, but only the simplest structures can be created. Surface micromachining is a more advanced fabrication technique. It is based on the use, near the layer of structural materials that are destined to compose the final structure, of layers of sacrificial materials, which are destined to be etched out SO as to leave empty spaces between structural layers. An example is shown in Figure 5.32, which demonstrates how a sacrificial material layer is removed in order to create a suspended mirror. In addition, from the architecture point of view there are essentially two possible ways of constructing an MEMS optical switch: 2D MEMS and 3D MEMS. Telecommunication Networks Structural material Sacrifical material Sacrifical material Silicon bulk Silicon bulk Step 1 Step 3 Structural material Sacrifical material Silicon bulk Silicon bulk Step 2 Step 4 FIGURE 5.32 Schematic process flow for the creation of a suspended mirror in a planar MEMS. Step 1: sacrifical material deposition; step 2: sacrifical material shaping etchant 1; step 3: structural material deposition; step 4: sacrifical material elimination etchant 2. In a 2D architecture the movable mirrors are positioned at the interse