NEMS switch with 30 nm thick beam and 20 nm high air gap for high density non-volatile memory applications

Abstract

As design rule is scaled down in complementary metal-oxide-semiconductor (CMOS) device, the several disadvantages based on electric field effect in CMOS device were emerged such as short channel effect, junction leakage and gate oxide leakage current [1]. Recently, non-CMOS based device using micro/nanoelectromechanical systems (MEMS/NEMS) switch have been proposed as one of the alternatives [2][3][4]. Devices based on MEMS/NEMS switch show excellent on-off current characteristics due to an almost zero off current and abrupt on-off current transition. Also, they have robustness under harsh environments such as X-ray, radiation, and low/high temperature. In this work, two types of two terminal NEMS switch with the smallest dimensions ever made were proposed and fabricated. Moreover, their electrical characteristics were provided. Fig.1 shows that two terminal NEMS cantilever switch (NCLS) and NEMS clamp switch (NCS) were fabricated utilizing conventional CMOS fabrication process. A poly-Si layer was used as a sacrificial layer to form an ultra thin and uniform air gap on shallow trench isolation (STI) because the air gap affects the pull-in voltage. Also, we chose a TiN beam with a high stiffness and low electrical resistivity as the structure layer to get reliable operation of NEMS switches. Finally, an exposed poly-Si mask and sacrificial layer were removed by wet etchant having highly etching selectivity of the TiN beam and field oxide. Fig. 2 shows that two terminal NCLS and NCS with 30 nm thick TiN beam and 20 nm high air gap was successfully manufactured by thin poly Si deposition technology and high selectivity poly-Si etchant without any inner gap residue and stiction. As shown in Fig. 2(a-1), when releasing the sacrificial layer, the curling up problem of the TiN beam which cause rising the pull-in voltage of NCLS was occurred by a residual stress gradient of the TiN film. This curl-up problem can be solved by applying TiN annealing process before removing the poly-Si sacrificial layer ( Fig. 2(a-2)). The I Beam -V WL hysteresis curves of NCLS and NCS with 30 nm thick TiN beam and 20 nm high air gap show a switching operation behavior in Fig. 3(a) and Fig. 3(b), respectively. The NCLS shows an abrupt on/off current characteristics, which have the pull-in voltage of 13.6 V and pull-out voltage of 8.5 V. The same size NCLS shows a good endurance under multiple DC bias sweeps in air ambient and the measured pull-in voltages are well converged nearby 13 V as shown in Fig. 4. Under the AC bias sweep condition (f=5 Hz, t= several tens of second), the NCLS demonstrates a good pulse response according to an AC signal as shown in Fig. 5. In case of the NCS with the 1000 nm long and 200 nm wide TiN beam, even if the pull-in voltage is measured at 11.2 V, an abrupt pull-out operation is not clearly shown due to the degradation or fracture of the long NCS (Fig. 3).As an application of two terminal NCLS which has more efficiency on area, a suspended beam memory (SBM) cell array structure was suggested for high density...

Circuit designs for increasing reliability and reducing energy

“…Previous studies have looked into using NEMs for memory applications [112], [93], [113], [110], [114], [115], [116]. Some of them address the use of NEM switches to replace normal memory arrays, such as SRAM.…”

“…Some of these studies also discuss non-volatile memory arrays [113], [115], [110], [114]. The memory array structure disclosed in [114] is of particular interest to our proposal as we explain in Section 5.3.…”