Nano-Cantilevers for an Ultra-Sensitive Bio-Assay
J. Zheng, K. Klein, R. Pucha, S.V. Sitaraman, D. Merlin, S. Rajalakshmi, D.S.R. Sarma, A. Gerwirtz and S.K. Sitaraman
Georgia Institute of Technology, US
biosensors, cantilever array, piezoresistive, micro and nano-scale, nanotechnology
Micro and Nano-scale cantilever arrays are being developed for an ultra sensitive bio-assay (Figure 1). The advantage of these cantilevers is that they eliminate the use of laser for deflection detection, which is impractical for nano-scale and array cantilevers. This bio sensor array has the potential to offer high-throughput detection of proteins, DNA, RNA, peptides and whole-cell for a broad range of applications ranging from disease diagnosis to biological weapons detection. For example these cantilevers can bond specific reagents to detect and measure the presence of particular antigens and/or complementary DNA sequence with a smaller size and at much earlier stages of disease progression compared to current medical diagnostic technologies. These highly sensitive electron beam lithography (EBL) based micro and nano cantilevers can facilitate simultaneous analysis of multiple samples for multiple analytes and improved measurement confidence through increased statistical data. Design 1: One design of the cantilever array is shown in Figure 1 and Figure 2(a). The cantilever is a free standing structure with a multilayer of thin films, which consists of a coating layer, a passivation layer, a piezoresistive material layer, and the silicon base. The coating layer will selectively bond with the target molecules, or will carry some biomaterials that bond with target molecules. The resultant surface tension force change will deform the cantilever and the embedded piezoresistive materials. The two legs of the cantilever form a closed loop for measuring the change of the resistance or the voltage applied. The deflection will be monitored. For some molecule which does not have strong surface tension effect after bonding to the coating material, the cantilever could be used to sense the mass change. In this case, only the tip region of the cantilever is covered with the coating material, as shown in Figure 1(b). The mass of molecules bonded at the tip region will have the strongest effect on the bending of the cantilevers. Design 2: Another design of the cantilever array consists of pre-deformed micro and nano-springs as shown in Figure 2(b) and Figure 3. In figure 3, the spring is pre-deformed by depositing layers of stress-engineered thin films. For example, changing the chamber pressure during chromium (Cr) sputtering, a tensile or compressive stress can be formed in Cr. A compressive to tensile stress gradient can be formed by the sequential deposition of compressive and tensile Cr layers such that the released structure will assume the desired pre-deformed shape. In this approach, there is no need to etch the bulk silicon. Micro-scale stress-engineered springs have been successfully applied as probing and interconnect technology in microelectronic packaging . Advantage: The embedded piezoresistive material for deflection detection eliminates the use of laser beam, which is impractical for nano-scale and array cantilevers. 1-10Å deflection has been reported in similar cantilever used in atomic force microscopy . The array design of the cantilevers has the flexibility to incorporate different sizes of cantilevers, thus facilitating simultaneous analysis of multiple samples and improved measurement confidence. This paper will present the fabrication process of the cantilever array. Different cantilevers from 20nm to 40um wide will be fabricated. The shape of the cantilever will be optimized considering the dimension of the cantilevers (length, width, thickness and the width of the cantilever leg). The deflection of the control cantilevers will be compared against the deflection of the reference cantilevers when antigens bind from a serum containing antigens.
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Nanotech 2005 Conference Program Abstract