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◆ In this paper, nano wires have been studied from the viewpoint of a mechanical sensor application. Polycrystalline silicon (poly-Si) nano wire piezoresistors were fabricated by the electron beam (EB) direct writing and RIE processes. Electrical and electro-mechanical (piezoresistive effect) characteristics of the poly-Si nano wire piezoresistors were investigated in order to verify abilities as sensing elements of mechanical sensors.
◆ Figure 1 shows the cross section of the poly-Si piezoresistor. The poly-Si nano wire piezoresistor, whose minimum cross section area is 100nm x 100nm, was realized by the combination of the electron beam (EB) direct writing and RIE processes. Electrodes were fabricated by Al vacuum evaporation and photolithography. Table 1 shows the dimensions and resistance of the fabricated piezoresistors. Figure 2 shows an SEM image of the fabricated nano wire piezoresistor. Grain size of the nano wire piezoresistor is approximately 150 nm.
◆ Figure 3 shows the relation between longitudinal (Pi_l ) and transverse (Pi_t ) piezoresistive coefficients, and the cross section area of the piezoresistors. The stress was applied to the piezoresistors by using a simple cantilever system. The cantilevers were prepared by dicing a Si (100) substrate into rectangular strips, on which the piezoresistors were fabricated and whose longitudinal directions were along [011]. The Pi_l of the nano wire piezoresistor increased with a decrease in the cross section area, while the Pi_t was approximately zero and invariant despite variation of the cross section area. The increase in the longitudinal piezoresistive coefficient Pi_l of the nano wire piezoresistor with a decrease in the cross section area might be due to an increase in mobility change ratio against applied stress in a low dimensional band structure. FEM analysis reveals that transverse normal stresses of nano wire piezoresistors in Fig.3 are less than 1/10 of stress on substrate. This is reason why the apparent transverse piezoresistive coefficient Pi_t was relatively small.
◆ The maximum value of the Pi_l of the nano wire piezoresistor is 22 x 10 ^-5 (1/MPa) at N = 5 x 10 ¹⁹ (cm ^-3 ) and it has enough sensitivity for mechanical sensor application.

Fig.1 Cross section of the piezoresistor

Table １ Dimensions and resistance of piezoresistors.

Fig.2 SEM image of the fabricated nano wire
piezoresistor (width = 100 nm, thickness=100 nm).

Fig.3 Relation between piezoresistive coefficients Pi_l and Pi_t ,
and cross section area of piezoresistors.