Novel Diffraction Based Deflection Profiling For Microcantilever Sensor Technology

Abstract

A novel optical diffraction based technique is proposed and demonstrated to measure deflections of the order of ~1nm in microcantilevers (MC) designed for sensing ultra-small forces of stress. The proposed method employs a double MC structure where one of the cantilevers acts as the active sensor beam, while the other as a reference. The active beam can respond to any minute change of stress, for example, molecular recognition induced surface stress, through bending (~1nm) relative to the other fixed beam. Optical diffraction patterns obtained from this double slit aperture mask with varying slit width, which is for the bending of MC due to loading, carries the deflection profile of the active beam. A significant part of the present work explores the possibility of connecting diffraction minima (or maxima) to the bending profile of the MC structure and thus the possibility to measure induced surface stress. To start with, it is also the aim to develop double MC sensors using PHDDA (Poly – Hexane diol diacrylate) because this material has the potential to achieve high mechanical deformation sensitivity in even moderately scaled down structures by virtue of its very low Young’s modulus. Moreover, the high thermal stability of PHDDA also ensures low thermally induced noise floors in microcantilever sensors. To demonstrate the proposed optical diffraction-based profiling technique, a bent microcantilever structure is designed and fabricated by an in-house developed Microstereolithography (MSL) system where, essentially one of the microcantilevers is fabricated with a bent profile by varying the gap between the two structures at each cured 2D patterned layer. The diffraction pattern obtained on transilluminating the fabricated structure by a spherical wavefront is analyzed and the possibility of obtaining the deflections at each cross section is ascertained. Since the proposed profiling technique relies on the accurate detection and measurement of shifts of intensity minima on the image plane, analysis of the minimum detectable shift in intensity minima for the employed optical interrogation setup with respect to the minimum detectable contrast and SNR of the optical measurement system is carried out, in order to justify the applicability of the proposed minima intensity shift measurement technique. The proposed novel diffraction based profiling technique can provide vital clue on the origins of surface stress at the atomic and molecular level by virtue of the entire bent profile due to adsorption induced bending thereby establishing microcantilever sensor technology as a more reliable and competitive approach for sensing ultra-low concentrations of biological and chemical agents.