Fluid Nozzles

Nozzles are among the simplest microstructures to fabricate using anisotropic etching of silicon, electroforming, or laser drilling of a metal sheet. A series of U.S. patents issued in the 1970s to IBM Corp. [7] describes the fabrication of silicon nozzles and their application for inkjet printing. The Ford Motor Company experimented in the 1980s with micromachined nozzles for engine fuel injection. With the expiration of most key patents on nozzle formation, micromachined nozzles are becoming common features in the design of atomizers, medical inhalers, and fluid spray systems. Nozzles need not necessarily be of silicon. MicroParts GmbH of Dortmund, Germany, manufactures a drug-inhaling device for asthma patients that incorporates a precise plastic nozzle fabricated using the LIGA electroplating and molding process described in the previous chapter.

A simple square silicon nozzle can be readily fabricated by depositing silicon nitride on both sides of a (100) wafer and patterning a square in the silicon nitride layer on the back side. Anisotropic etching in potassium hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH) forms a port through the wafer with walls defined by the {111} planes of silicon. The dimensions of the backside opening in the silicon nitride must be larger than 71% of the wafer thickness in order to etch through the wafer (see Figure 4.3).

Figure 4.3: Schematic illustrations of square and circular nozzles on the wafer surface with their corresponding fabrication steps.

Forming nozzles of circular or arbitrary shape in silicon involves additional fabrication...