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Hamrock Fundamentals of Machine Elements

Chapter 20: Elements of

Microelectromechanical Systems (MEMS)

There is plenty of room at the

bottom.

Richard Feynman

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Figure 20.1 The Texas Instruments digital pixel technology

(DPT) device.

DPT Device

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Figure 20.2 Pattern transfer by lithography. Note that the mask

in step 3 can be a positive or negative image of the pattern.

Lithography

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4/20Hamrock Fundamentals of Machine Elements

Figure 20.3 Etching directionality. (a) Isotropic etching: etch proceeds

vertically and horizontally at approximately the same rate, with significant

mask undercut. (b) Orientation-dependant etching (ODE): etch proceeds

vertically, terminating on {111} crystal planes with little mask undercut.

(c) Vertical etching: etch proceeds vertically with little mask undercut.

Etching Directionality

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Figure 20.4 Schematic illustration of

bulk micromachining. (a) Diffuse

dopant in desired pattern. (b) Depositand pattern masking film. (c)

Orientation-dependant etch (ODE),

leaving behind a freestanding structure.

Bulk Micromachining

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Figure 20.5 Schematic illustration of the steps in surface micromachining.

(a) deposition of a phosphosilicate glass (PSG) spacer layer; (b) etching of

spacer layer; (c) deposition of polysilicon; (d) etching of polysilicon; (e)

selective wet etching of PSG, leaving the silicon substrate and deposited

polysilicon unaffected.

Surface Micromachining

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Figure 20.6 (a) SEM image of a deployed micromirror. (b) Detail of the

micromirror hinge. (Source: Sandia National Laboratories.)

Micromirror

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Figure 20.7 Schematic illustration of the steps required to manufacture a

hinge. (a) Deposition of a phosphosilicate glass (PSG) spacer layer and

polysilicon layer. (b) deposition of a second spacer layer; (c) Selective

etching of the PSG; (d) depostion of polysilicon to form a staple for the

hinge; (e) After selective wet etching of the PSG, the hinge can rotate.

Micro-Hinge Manufacture

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LIGA

Figure 20.8 The LIGA (lithography, electrodeposition and molding)

technique. (a) Primary production of a metal final product or mold insert. (b)

Use of the primary part for secondary operations, or replication

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Table 20.1 Summary of important beam situations for MEMS devices.

Beams in MEMS

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Atomic Force Microscope Probe

Figure 20.9 Scanning electron microscope images of a diamond-tipped

cantilever probe used in atomic force microscopy. (a) Side view with detail

of diamond; (b) bottom view of entire cantilever.

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Table 20.2 Coefficients and for analysis of rectangular plate pressure

sensor.

Rectangular Plate

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Figure 20.10 Illustration of electrostatic actuatuation. (a) Attractive forces

between charged plates; (b) forces resulting from eccentric charged plate

between two other plates; (c) schematic illustration of a comb drive.

Electrostatic Actuation

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Figure 20.11 A comb

drive. Note the springs in

the center provide a

restoring force to returnthe electrostatic comb

teeth to their original

position. From Sandia

National Laboratories.

Comb Drive

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Figure 20.12 (a) Schematic illustration of a rotary electrostatic

motor, sometimes called a slide motor; (b) scanning electron

microscope image of a rotary micromotor.

Rotary Electrostatic Motor

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Figure 20.13 Capilary tube for microflow. (a) Schamitic illustration

of tube construction; (b) induced traveing wave and fluid flow.

Capilary Tube for Microflow

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Thermal Inkjet Printer

Figure 20.14 (a) Sequence of operation of

a thermal inkjet printer. (a) Resistiveheating element is turned on, rapidly

vaporizing ink and forming a bubble. (b)

Within five microseconds, the bubble has

expanded and displaced liquid ink from

the nozzle. (c) Surface tension breaks theink stream into a bubble, which is

discharged at high velocity. The heating

element is turned off at this time, so that

the bubble collapses as heat is transferred

to the surrounding ink. (d) Within 24microseconds, an ink droplet (and

undesirable satellite droplets) are ejected,

and surface tension of the ink draws more

liquid from the reservoir.

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Piezoelectric Inkjet Mechanism

Figure 20.15 Schematic illustration of a piezoelectric driven inkjet

printer head.

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Metal Oxide Sensors

Table 20.4 Common metal oxide sensors.

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Accellerometer

Figure 20.16 (a) Schematic illustration of accellerometer; (b) photograph of

Analog Devices ADXL-50 accelerometer with a surface micromachined

capacitive sensor (center), on-chip excitation, self-test and signal conditioning

circuitry. The entire chip measures 0.500 by 0.625 mm.

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