Page 106 - An Introduction to Microelectromechanical Systems Engineering

Passive Micromachined Mechanical Structures 85

magnetized thin films. Electrical current in a conductive element that is located
within a magnetic field gives rise to an electromagnetic force—the Lorentz force—in
a direction perpendicular to the current and magnetic field. This force is propor-
tional to the current, magnetic flux density, and length of the element. A conductor
1 mm in length carrying 10 mA in a 1-T magnetic field is subject to a force of 10 µN.
Lorentz forces are useful for closed-loop feedback in systems employing electro-
magnetic sensing. Two yaw-rate sensors and a beam steering micromirror described
later make use of this method.

Actuation Using Shape-Memory Alloys
Finally, of all five schemes, shape-memory alloys undoubtedly offer the highest
energy density available for actuation. The effect, introduced in Chapter 2, can
provide very large forces when the temperature of the material rises above the
critical temperature, typically around 100ºC. The challenge with shape-memory
alloys lies in the difficulty of integrating their fabrication with conventional silicon
manufacturing processes.

Passive Micromachined Mechanical Structures

Fluid Nozzles
Nozzles are among the simplest microstructures to fabricate using anisotropic etch-
ing of silicon, electroforming, or laser drilling of a metal sheet. A series of U.S. pat-
ents 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 expira-
tion of most key patents on nozzle formation, micromachined nozzles are becoming
common features in the design of atomizers, medical inhalers, and fluid spray sys-
tems. Nozzles need not necessarily be of silicon. MicroParts GmbH of Dortmund,
Germany, manufactures a drug-inhaling device for asthma patients that incorpo-
rates 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).
Forming nozzles of circular or arbitrary shape in silicon involves additional fab-
rication steps. The most common approach is to grow on a (100) wafer a p-type epi-
−3
19
taxial layer of silicon with a high boron concentration ( >1 × 10 cm ). The shape
of the nozzle is patterned and etched into the p-type silicon layer using standard
lithography and plasma etching (or RIE). A protective layer of silicon nitride is
deposited on both sides of the wafer and patterned in the shape of a square on the
back side. Double-sided lithography provides accurate alignment between the noz-
zle opening and the square on the back side. The fabrication is complete with the