Sidewall Imaging of Microarray-Based Biosensor Using an Orthogonal Cantilever Probe

We proposed a novel atomic force microscopy (AFM) technique based on the orthogonal cantilever probe (OCP) for measuring the sidewall of microstructure/nanostructure. The OCP mainly consists of a horizontal arm and a vertical arm with a protruding tip, where the vertical arm is orthogonally assembled on the end side of the horizontal arm. Therefore, the interaction force between the tip and the sample can be detected by the torsion signal of the OCP. This new technique is capable of scanning the steep sidewall in the torsional mode of the OCP and mapping the horizontal surface by the bending deformation of the OCP. Theoretical calculation and finite-element analysis (FEA) were first used to design and determine the reasonable parameters of the OCP. Then, the designed OCP was prepared by focused ion beam (FIB) milling and microassembly technique, and the dynamic and static properties of the OCP were studied for obtaining its performance parameters. A gold-silicon interface and a microcomb structure were used to verify its ability to image with high precision. Finally, a biosensor based on the 3-D microarray was imaged by the OCP to study its uniformity of the surface modification on three representative positions (sidewall, top, and bottom). Meanwhile, the top and bottom of the microarray were also measured by the conventional AFM method with a commercial probe, which verified the credibility and powerful capabilities of the proposed OCP. Experimental results showed that the sidewall roughness of the biosensor is significantly smaller, which means that the surface modification effect is different from other positions.