Page 137 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

5

Silicon Micromaching: Bulk

5.1 INTRODUCTION

The emergence of silicon micromachining has enabled the rapid progress in the field of
microelectromechanical systems (MEMS), as discussed previously in Chapter 1. Silicon
micromachining is the process of fashioning microscopic mechanical parts out of a silicon
substrate or, indeed, on top of a silicon substrate. It is used to fabricate a variety of
mechanical microstructures including beams, diaphragms, grooves, orifices, springs, gears,
suspensions, and a great diversity of other complex mechanical structures. These mechan-
ical structures have been used successfully to realise a wide range of microsensors 1
and microactuators. Silicon micromachining comprises two technologies: bulk micro-
machining and surface micromachining. The topic of surface micromachining is covered in
the next chapter. Further details can be found in the two-volume Handbook of Microlithog-
raphy, Micromachining, and Microfabrication (Rai-Choudhury 1997).
Bulk micromachining is the most used of the two principal silicon micromachining
technologies. It emerged in the early 1960s and has been used since then in the fabrication
of many different microstructures. Bulk micromachining is utilised in the manufacture of
the majority of commercial devices - almost all pressure sensors and silicon valves and 90
percent of silicon acceleration sensors. The term bulk micromachining expresses the fact
that this type of micromachining is used to realise micromechanical structures within the
bulk of a single-crystal silicon (SCS) wafer by selectively removing the wafer material.
The microstructures fabricated using bulk micromachining may cover the thickness range
from submicrons to the thickness of the full wafer (200 to 500 um) and the lateral size
ranges from microns to the full diameter of a wafer (75 to 200 mm).
Etching is the key technological step for bulk micromachining. The etch process
employed in bulk micromachining comprises one or several of the following techniques:

1. Wet isotropic etching
2. Wet anisotropic etching

3. Plasma isotropic etching
4. Reactive ion etching (RIE)
5. Etch-stop techniques

1
Chapter 8 is devoted to this topic.