Tolerance analysis and optimization of a lateral deformable NEMS zeroth-order gratings

This paper discusses the error analysis and optimization of a lateral deformable NEMS zeroth-order grating transducer based on an anomalous diffraction phenomenon, namely Wood's type anomaly, in which tiny changes in the displacement of the nanostructured grating elements lead to a dramatic increase or decrease of the optical reflection amplitude. With this special feature, this structure is ideal to measure very small displacement. However, parameters such as wavelength, period, duty ratio play very important roles in the performance of the device. In this paper, the performance of the structures with different parameter settings is analyzed and the original structure is optimized through 3-D Finite Difference Time Domain method (FDTD). Simulation demonstrates the new structure is of higher sensitivity, namely 0.705%/nm. Moreover, in order to enlarge the tiny size of the original structure which is closely related to the incident wavelength, the wavelength of incident light is changed into longer ones, e.g. 1053 nm, 1310 nm, and 1530 nm. The calculation predicts the optimized structure corresponding to 1530 nm not only has extinction ratio higher than the original one, namely 96% but also twice larger size, namely twice less strict demand for processing precision, which makes them possible to fabricate with current surface micromachining processing similar to that used for the fabrication of polysilicon MEMS. Besides, structures of several visible incident wavelengths, such as 532 nm, 632.8 nm, 670 nm, and 753 nm, are also analyzed and optimized, which gives great convenience to the installation and calibration of such device as well as observation of its performance. All the calculated data enables us to apply the structure into fields required for different sensitivities with different grating designs and thus broadens the further usage of such novel structure, as structures of different parameters are of different sensitivities and signal strengths. In addition, a tolerance analysis of the device in terms of fabrication, illumination, alignment is discussed in detail to get a guidance of successful realization of an actual device. Especially, during the analysis an unexpected performance of the device is discovered, in which the signal of the devices corresponding to 1530 nm changes into pulse signal with slop of 2.292%/nm, i.e. 0.36 dB/nm when the air gap is reduced to 300 nm, indicating sensitivity over four times higher than that of the original one, namely 0.5%/nm, i.e. 0.04 dB/nm.