2.4 Alkaline Etchants

1. 2.4 Alkaline Etchants • Alkai hydroxide etchants The hydroxides of alkali metals(KOH, NaOH, etc.)can be used as crystal-oriontation-dependent etchants of silicon. Table of example alkali hydroxide etchant formulations. Note that isopropyl alcohol can be added as a diluent to increase selectivity, as disscussed below. After Peterser(1982)

2. Tetramethyl Ammonium Hydroxide(TMAH) • Thtramethyl ammomium hydroxide(TMAH, (CH3)4, NOH), is one the more useful set etchant chemistries for silicon. It is safer than EDP(discussed below) can be modified with additives so that it does not etch aluminum, begins to slow down for boron-doping levels above approximately 1*1019cm-3, and is relatively low cost. Due to these features, some micromachining operations are showing a trend away from EDP and toward TMAH as a wet silicon etchant where alkali gydroxides are not suitable. In general ,TMAH formulations are reported as a weight percentage of TMAH in water( additives are sometimes used to improve etch properties, as described below). • Selectivity of TMAH Etchants for Various Dielectrics versus (100) Silicon

3. Ethylene Diamine Pyrochatechol(EDP)

4. 2.5 Surface micromachining The Advantage of surface micromachining technology • The ability to produce smaller sensors • b) The fact that this approach shares many common features with current IC technology

5. 2.5.1 Basic Concept of Surface Micromachinning Layer Stacking