We have already seen, in connection with our discussion of the Casimir force, in Chapter 5, a structure in which a flat plate was freely suspended above a silicon surface by two torsion fibers. This structure (see Figures 5.5 and 5.6) is an example of one in which a "sacrificial layer" (in this case silicon dioxide) was initially grown.
A deposited polysilicon layer, suitably patterned, was grown above the sacrificial layer, also providing supports to the underlying crystal. Then the SiO2 layer was etched away, leaving the suspended "paddle" free to rotate in response to electric and Casimir forces. The method is seen in Figure 7.1 .
Referring to Figure 7.1, panel (1) shows e-beam exposure of a PMMA (polymethylmethacrylate) resist, which has been uniformly applied above the three layers comprized by the base silicon, the sacrificial oxide layer, and a polysilicon deposited layer. Panel (2) shows a patterned metal etch mask on the polysilicon layer. Panel (3) shows liftoff of the unexposed resist layer, followed in panel (4) by dry etch to delineate the suspended bridge. In panel (5) a wet etch, such as HF, undercuts the oxide beneath the suspended layer, leaving it freely suspended. Finally, in panel (6), metal electrodes are applied.
This scheme is used in the class of MEMS (NEMS) devices Micro (Nano)Electro-MechanicalSystems, which include the accelerometers used in automobile collision
1) E-beam exposure of PMMA
Development of PMMA 2) and metal deposition metal mask
Wet-etch undercut through oxide contact metal
6) Evaporation of contact metal
Figure 7.1 Steps in formation of a suspended silicon plate above a silicon chip [after 2].
sensors. It is also used in the recent "Millipede" prototype data storage device, where more than a thousand individual atomic force microscope sensors are produced, using the silicon photolithographic process illustrated in Figure 7.1, on a single chip
Another example of this elegant version of the silicon process is illustrated in Figure 7.2, in which a single-electron-transistor (SET) is used to sense the 0.1 GHz vibrations of a suspended nanometer-scale beam [4,5].
The structure in Figure 7.2 panel (b) is patterned and etched from a GaAs-GaAlAs single crystal heterostructure. The vibrating beam is a doubly clamped single crystal of GaAs with dimensions 3 mmx250nm wide x 200 nm thick, with a resonant frequency of 116 MHz. The beam (the gate electrode of the SET) was patterned  using electron beam lithography and a combination of reactive ion etching (to make the vertical cuts) and dilute HF etching (to remove the sacrificial GaAlAs layer originally under the beam). The beam, which serves as the gate electrode, is capacitively coupled to the Al island, the closest approach being 250 nm, with a gate-island capacitance of 0.13 fF.
Figure 7.2 Single-electron-transistor used to sense vibration of freely suspended crystal beam [4,5]. In panel (b) a "crystal beam" is sus> pended and free to vibrate at 116 MHz. It is metallized and acts as gate electrode in the FET transistor, denoted "island". The "island" is connected to source and drain electrodes (right and left) by Al-AlOx-Al tunnel barrier
Figure 7.2 Single-electron-transistor used to sense vibration of freely suspended crystal beam [4,5]. In panel (b) a "crystal beam" is sus> pended and free to vibrate at 116 MHz. It is metallized and acts as gate electrode in the FET transistor, denoted "island". The "island" is connected to source and drain electrodes (right and left) by Al-AlOx-Al tunnel barrier b b
contacts. At fixed gate bias and fixed source-drain voltage, motion ofthe gate electrode induces charge on the island, and sensitively controlsthe source-drain current. Panel (a): Schematic diagram of electrical operation of the device. A motion of the bar by 2 fm (about the radius of an atomic nucleus) can be detected with 1 Hz bandwidth at 30 mK.
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