• Micromachining is defined as the process of shaping Si or other material to realize 3 dimensional mechanical structure in miniature form.
• The oldest micromachining technology is bulk micromachining.
• This technique involves the selective removal of the substrate material in order to realize miniaturized mechanical components.
• Bulk micromachining can be accomplished using chemical or physical means.
• A widely used bulk micromachining technique is chemical wet etching, which involves the immersion of a substrate into a solution of reactive chemical that will etch exposed regions of the substrate at measurable rates.
• Chemical wet can provide a very high etch rate and selectivity.
• There are two general types of chemical wet etching in bulk micromachining: isotropic wet etching and anisotropic wet etching.

Isotropic wet etching:

• The etch rate is not dependent on the crystallographic orientation of the substrate
• The etching proceeds in all directions at equal rates.
• Common masking materials for isotropic wet silicon etching include silicon dioxide and silicon nitride.

Anisotropic wet etching:

• The etch rate depends upon the crystallographic orientation of the substrate.
• Anisotropic etching etches more slowly along the <111> planes than all the other.
• It provides high resolution etch capability with reasonably tight dimensions control.
• Anisotropic etching techniques are commonly used in manufacturing of Si pressure sensors as well as bulk micro-machined accelerometers.

Unit processes required in Bulk Micromachining

• Deposition of thin films - Physical or Chemical vapor deposition (for metals, polySi, SiO2, Si3N4). Spin coating (for resists, SU-8, other polymeric materials)
• Doping- To change conductivity of PolySi and to change chemical (etch rate) characteristics
• Pattern transfer & Etching- Lithography, Wet etching of Silicon, Dry Etching of Silicon

Bulk micromachining for a cantilever