Understanding Smart Sensors - Nomads.usp

Micromachining 19The relative ease of accomplishing both bulk and surface micromachininghas led to many researchers investigating a variety of applications. Some of theareas being investigated will lead to smarter sensors through higher levels ofintegration. A key process associated with micromachining is the bonding ofsilicon to a silicon or substrate material. The processes used for micromachining,associated processes, and application of the technology to sensors are coveredin this chapter.2.2 Bulk MicromachiningBulk micromachining is a process for making three-dimensional microstructuresin which a masked silicon wafer is etched in an orientation-dependentetching solution [3]. Using micromachining technology, several wafers can befabricated simultaneously and lot-to-lot consistency is maintained by controllinga minimal number of parameters. Key parameters in bulk micromachininginclude crystallographic orientation, etchant, etchant concentration, semiconductorstarting material, temperature, and time. Photolithography techniquescommon in IC technology precisely define patterns for etching both sides ofsilicon wafers. The crystallographic orientation, etchant, and semiconductorstarting material are chosen by design, leaving etchant concentration, temperature,and time as lot-to-lot control items.Silicon ICs are typically fabricated (manufactured) using or silicon. In bulk micromachining, an anisotropic (unidirectional) etchant, suchas ethylene-diamine-pyrocatechol (EDP), hydrazine (N 2 H 4 ), tetramethylammoniumhydroxide (TMAH), or potassium hydroxide (KOH), attacks the plane of silicon. The plane is etched at a much faster rate than the plane, typically 35 times faster. N-type silicon is etched at a much fasterrate (>50 times faster) than p+-type, so n-type material is often used as the startingmaterial. P+-type material can be epitaxially grown on the wafer or diffusedinto the wafer to add a further control element in defining the dimensions. Agitationmaintains uniform concentration during anisotropic etching. The characteristicshape (preferential etching) of anisotropic etching of silicon isshown in the cross-section of Figure 2.1(a), which produces a 54.7-degree anglefor the silicon [4, 5]. The top view of etching into the surface of the siliconappears as a pyramid-shaped pit.Etch rates of 1.0–1.5 mm/minute occur in the plane of silicon withetch temperatures of 85–115°C for common etchants such as EDP and KOH[6]. Isotropic etching, shown in Figure 2.1(b), has etch rates independent of thecrystallographic orientation. Isotropic etching allows undercut and cantileverstructures to be produced. In bulk silicon, however, it is more difficult to