Page 252 - Introduction to Information Optics

4.4. Optical Switching Based on MEMS 237

4.4.1. MEMS FABRICATIONS

MEMS can be fabricated using micromachining techniques. There are three
main micromachining techniques currently in use for making MEMS devices:
bulk micromachining, surface micromachining and LIGA [37, 38, 39].
Bulk silicon micromachining is one of the best-developed micromachining
techniques. Silicon is the primary substrate material used in production
microelectronic circuitry, and so it is the most suitable candidate for the
eventual production of MEMS. Traditional silicon processing techniques,
including photolithography, etching, and packaging, are used in this method.
Bulk micromachining mainly uses isotropic and/or anisotropic etching to make
devices with deep cavities, such as pressure sensors and accelerometers. A
drawback of traditional bulk micromachining is that the geometries of the
structures are restricted by the aspect ratios inherent in the fabrication
methods, as the devices are large.
To get greater complexity out of bulk micromachining, fusion bonding
techniques were developed for virtually seamless integration of multiple wafers.
The process starts with separate fabrication of various elements of a complex
system, and subsequently assembles them. It relies on the creation of atomic
bonds between two wafers to produce a single layer with the same mechanical
and electrical properties.
The second one is surface micromachining. This technique is very simple,
powerful, and versatile. Many optical elements, including moving micromir-
rors, microgratings, and other components used in optical switching, have been
made using this technique. It has a thin-film sacrificial layer selectively added
to and removed from the Si substrate. The mechanical parts of the devices are
made using the thin film, and the Si is often used for interface circuitry. The
process starts with thin films, usually polysilicon, silicon dioxide, and nitride,
as mask and sacrificial layers. Sacrificial etching is the basis of surface
micromachining. A soluble layer is grown or deposited for later removal from
beneath other patterned deposited materials. Since the patterned materials left
behind are separated from the substrate by the thickness of the removed
sacrificial layer, they form freestanding thin-film mechanical structures. Springs
and suspended structures can be constructed with lengths in the plane of wafer
much greater than their widths.
The third technique is called LIGA. The acronym LIGA comes from the
German name for the process (lithography, electroplating, and molding). Tall
structures with submicrometer resolution can be formed with this technique.
Microcomponents can be made from a variety of materials using this method.
The LIGA process begins by generating a photoresist pattern using X-ray
lithography on a conductive substrate. The generated space can be preferen-
tially electroplated right to the brim, yielding a very accurate negative replica
of the original resist pattern. This replica can further be used as a mold for low