This paper reports a novel generation of CMOS stress mapping chips comprising 32 square field effect transistors (FET) with four source/drain contacts (piezoFETs) exploiting the shear piezoresistive effect in n-type (NMOS) or p-type (PMOS) inversion layers. The sensor chips with a total die area of 2.5 x 2 mm2 are integrated with analog circuitry and digital logic. When exposed to homogenous shear or normal stress, all 32 integrated stress sensors show a linear response in excellent agreement with theoretical predictions and exhibit identical stress sensitivities. Piezo-FETs fabricated as separate devices are characterized with respect to stress sensitivity, intrinsic offset, and noise behavior. Stress sensitivities are enhanced by incorporating a central hole into the piezo-FETs. Sensitivities of -448 iV/(V MPa) and 477 iV/(V MPa) were measured for NMOS and PMOS devices, respectively.
The piezoresistive response of n-and p-type hydrogenated nanocrystalline silicon thin films, deposited by hot-wire (HW) and plasma-enhanced chemical vapor deposition (PECVD) on thermally oxidized silicon wafers, has been studied using four-point bending tests. The piezoresistive gauge factor (GF) was measured on patterned thin-film micro-resistors rotated by an angle θ with respect to the principal strain axis. Both longitudinal (GF L) and transverse (GF T) GFs, corresponding to θ = 0º and 90º, respectively, are negative for n-type and positive for p-type films. For other values of θ (30º, 45º, 120º and 135º) GFs have the same signal as GF L and GF T and their value is proportional to the normal strain associated with planes rotated by θ relative to the principal strain axis. It is concluded that the films are isotropic in the growth plane since the GF-values follow a Mohr's circle with the principal axes 2 coinciding with those of the strain tensor. The strongest p-type pirezoresistive response (GF L = 41.0, GF T = 2.84) was found in a film deposited by PECVD at a substrate temperature of 250ºC and working pressure of 0.250 Torr, with dark conductivity 1.6 Ω-1 cm-1. The strongest n-type response (GF L =-28.1, GF T =-5.60) was found in a film deposited by PECVD at 150ºC and working pressure of 3 Torr, with dark conductivity 9.7 Ω-1 cm-1. A model for the piezoresistivity of nc-Si is proposed, based on a mean-field approximation for the conductivity of an ensemble of randomly oriented crystallites and neglecting grain boundary effects. The model is able to reproduce the measured GF L values for both n-and p-type films. It fails however to explain the transversal GF T data. Both experimental and theoretical data show that nanocrystalline silicon can have an isotropic piezoresistive effect of the order of 40% of the maximum response of crystalline silicon.
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