Rivas Cardona, Juan Alberto (2006-08). Development of a microfluidic device for patterning multiple species by scanning probe lithography. Master's Thesis.
Scanning Probe Lithography (SPL) is a versatile nanofabrication platform that leverages microfluidic "ink" delivery systems with Scanning Probe Microscopy (SPM) for generating surface-patterned chemical functionality on the sub-100 nm length scale. One of the prolific SPL techniques is Dip Pen Nanolithography(TM) (DPN(TM)). High resolution, multiplexed registration and parallel direct-write capabilities make DPN (and other SPL techniques) a power tool for applications that are envisioned in micro/nano-electronics, molecular electronics, catalysis, cryptography (brand protection), combinatorial synthesis (nano-materials discovery and characterization), biological recognition, genomics, and proteomics. One of the greatest challenges for the successful performance of the DPN process is the delivery of multiple inks to the scanning probe tips for nano-patterning. The purpose of the present work is to fabricate a microfluidic ink delivery device (called "Centiwell") for DPN (and other SPL) applications. The device described in this study maximizes the number of chemical species (inks) for nanofabrication that can be patterned simultaneously by DPN to conform the industrial standards for fluid handling for biochemical assays (e.g., genomic and proteomic). Alternate applications of Centiwell are also feasible for the various envisioned applications of DPN (and other SPL techniques) that were listed above. The Centiwell consists of a two-dimensional array of 96 microwells that are bulk micromachined on a silicon substrate. A thermoelectric module is attached to the back side of the silicon substrate and is used to cool the silicon substrate to temperatures below the dew point. By reducing the temperature of the substrate to below the dew point, water droplets are condensed in the microwell array. Microbeads of a hygroscopic material (e.g., poly-ethylene glycol) are dispensed into the microwells to prevent evaporation of the condensed water. Furthermore, since poly-ethylene glycol (PEG) is water soluble, it forms a solution inside the microwells which is subsequently used as the ink for the DPN process. The delivery of the ink to the scanning probe tip is performed by dipping the tip (or multiple tips in an array) into the microwells containing the PEG solution. This thesis describes the various development steps for the Centiwell. These steps include the mask design, the bulk micromachining processes explored for the micro-fabrication of the microwell array, the thermal design calculations performed for the selection of the commercially available thermoelectric coolers, the techniques explored for the synthesis of the PEG microbeads, and the assembly of all the components for integration into a functional Centiwell. Finally, the successful implementation of the Centiwell for nanolithography of PEG solutions is also demonstrated.