Understanding Smart Sensors - Nomads.usp

20 Understanding Smart Sensors Surface orientation(a)Thin diaphragm54.74°SiO mask2CantileverstructureSiO mask2(b) Surface orientation(c)Silicon nitridemaskFigure 2.1 Bulk micromachined structures: (a) anisotropic, (b) isotropic with agitation, and(c) alternative crystal orientation and mask material.control than anisotropic etching, and the level of agitation influences theresults. Undercutting and suspended structures are achieved with anisotropicetching with mask patterns and extended etch times. To expand the applicationsof micromachining, other chemicals, such as sodium hydroxide (NaOH),are used for etching the and plane. Also, other materials such assilicon nitride can be used for the masking layer, as shown in Figure 2.1(c).Table 2.2 lists the etch rates of common materials, and possible etchants aredescribed in Section 2.5.2 [2]. The etch rates for 317 combinations of 16 materials(single-crystal silicon, doped and undoped polysilicon, several types of silicondioxide, stoichiometric and silicon-rich silicon nitride, aluminum,tungsten, titanium, titanium-tungsten alloy, and two brands of photoresist)used in the fabrication of MEMS devices and ICs in 28 wet, plasma, andplasmaless-gas-phase etches have been reported in one study [7].Etch stop techniques enhance the accuracy of wet chemical etching. Themost common techniques for etch-depth control in bulk micromachining areshown in Figure 2.2 [6]: Precisely controlled diffusions (a), epitaxially grownlayers in the silicon crystal (b), or field enhanced depletion layers (c) slow downthe etching process at the interface, allowing accurate structures to be obtained.Boron etch stops using EDP or TMAH can be used to produce layers as thick