In contrast to the bulk micromachining described above, where three-dimensional structures are etched into the substrate wafer, surface micromachining consists of building them by layering thin films of new material onto the surface of the substrate. Usually, sacrificial spacer layers are used to create freestanding structures like air-bridges; after depositing such a sacrificial layer and patterning it using microlithographic steps described above, the material for the final structure is deposited. Afterwards, the spacer layer is removed by an appropriate etchant, freeing the desired structure.
Surface micromachining was invented in the late 1960s, when a cantilever beam was produced by underetching the applied material on top of a sacrificial layer. The techniques used in this area emerged in the early 1980s, using polysilicon as structural material. Many different structures created using surface micromachining have been demonstrated, e. g., springs, gears, sliders and sealed cavities. However, the first commercial application based on this process was announced 1991 by Analog Devices (an accelerometer for the automobile industries).
Figure 4 shows the basic surface micromachining process, applied to the production of an air-bridge structure.
Figure 4: Basic surface micromachining process. (a) Spacer layer deposition.
(b) Pattering of the spacer layer. (c) Deposition of the microstructure layer.
(d) Patterning of desired structure. (e) Stripping of the spacer layer resolves
The steps for producing the air-bridge are clearly visible. Different materials can be used for the spacer layer, with photoresist being the simplest choice, reducing the steps necessary for patterning. Photoresist can be exposed with an appropriate mask and simply developed, revealing the structure needed for the following deposition of the microstructure layer. If a metallic material is used for this purpose, often a seed layer is deposited in order to enable the final structure to be applied via electrodeposition. This seed layer can be evaporated to a thickness of ~100 Å; afterwards, the sample is electroplated for an appropriate time in order to achieve the desired thickness of the final structure.
A variation of the standard surface micromachining process is called lift-off. Its aim is to apply a (metallic in most cases) layer to the substrate only in specific areas. Therefore, a sacrificial layer (usually photoresist) is applied and patterned, opening the regions that are to be covered with the metal film. After deposition of the metal, it contacts the substrate only in those regions. By removing the photoresist with a solvent that does not attack the metal layer, the material on top of the sacrificial layer is "lifted off", leaving the metal only at the desired areas (see Figure 5).
Figure 5: Principle of the lift-off process
Important for the success of the lift-off procedure is the use of relatively thick photoresist in order to provide a very thin metallic layer on the sidewalls of the opening. This allows the lift-off to be completed without breaking the metallic film too easily. This is also the main difference between lift-off and normal surface micromachining, where thicker sidewalls are required in order to provide stability of the free structure.
The advantage of the lift-off procedure is the ability to work with metallic layers such as platinum or gold that are difficult to pattern by etching directly.