Nanometer gap detection method using GMR grating

Abstract

ABSTRACT In this paper the displacement of the reflection resonant peak resulting from the change of the gap between the guided-mode resonance (GMR) grating and substrate is used to measure the nanometer gap. The paper calculates double layer model and metal substrate model using rigorous coupled-wave analysis (RCWA). It is revealed that nano-gap detection using GMR grating is feasible for both dielectric and metal substrate. The detection range of resonant wavelength and gap is tunable. The detect sensitivity is investigated by varying the parameters of grating (thickness, period and refractive index), the thickness of films, and polarization. Tolerance of grating implies an advantage for manufacture. An optimized result presents an 18nm resonant shift for 100nm gap with the max sensitivity achieving 0.85. Keyword: guided-mode resonance (GMR); Nano-gap; rigorous coupled-wave analysis (RCWA) 1. INTRODUCTION Nanometer gap detection has become the urgent research topic in industrial and science. It has been widely developed in high accuracy manufacture of semiconductor, the control of flying height of magnetic read-write heads, calibration of high accuracy se nor, inspection of super-smooth surface and measurement of weak vibration. Moon et al.[1]used “Chirped-Talbot effect” to measure the gap between two glass substrate. Though its sensitivity achieved 1nm below, the detecting range is 1 to 30 m. Suzuki et al. [2] presented “Two lasers optical heterodyne interference” method. They obtained accuracy gap at 1.3um. While multiple interference fringes in the small scale complicated computation. Wu et al.[3] demonstrated that surface plasmons are applied to detect the gap from 100nm to 300nm with th e sensitivity about 10nm. However the gap belo w 100nm is still a to ugh task to detect. GMR is a peculiar phenomenon in sub-wavelength grating structures under certain conditions. Such structures have very narrow bandwidth, high diffraction efficiency and extreme sensitivity of spectrum and angle[4]. Applying this feature to the gap detection, this paper shows the GMR grating can easily measure the gap below 100nm. Our method just needs to measure the displacement of the reflection peak instead of the interference intensity resulting from the change of gap. Calibration of the source intensity and the st ability of source power are hence no longer need to be considered. Rigorous coupled-wave analysis (RCWA), which is an exact solution of Maxwell’s equations for the electromagnetic diffraction by grating structures, is used to calculate the models. [5]