Microchannel molding: A soft lithography-inspired approach to micrometer-scale patterning
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A new patterning technique for the deposition of sol-gels and chemical solution precursors was developed to address some of the limitations of soft lithography approaches. When using micromolding in capillaries to pattern precursors that exhibit large amounts of shrinkage during drying, topographical distortions develop. In place of patterning the elastomeric mold, the network of capillary channels was patterned directly into the substrate surface and an elastomer membrane is used to complete the channels. When the wetting properties of the substrate surfaces were carefully controlled using self-assembled monolayers (SAMs), lead zirconate titanate thin films with nearly rectangular cross-sections were successfully patterned. This technique, called microchannel molding (CM), also provided a method for aligning multiple layers such as bottom electrodes for device fabrication.
I. INTRODUCTION 1
Soft lithography has proved to be a popular tool among researchers in search of a low-cost means to pattern thin films of polymers, metals, and ceramics. The common feature of this class of techniques is the use of an elastomeric material, most commonly polydimethylsiloxane (PDMS), with a patterned surface. The PDMS can be used either as a mold to impart patterns via physical confinement of a fluid precursor that dries to form the final structured thin film or as a stamp to directly transfer the material to the target substrate. The confinementbased techniques include microtransfer molding (TM),2–4 micromolding in capillaries (MIMIC),5–14 and embossing,9–18 while microcontact printing (CP)19–28 is an example of stamping. Schematic drawings of these techniques are provided in Fig. 1.29,30 To date, the soft lithography techniques have met with much greater success with polymeric and metallic thin films than they have with ceramic materials.29 The deposition of ceramics requires the use of a liquid precursor, typically in the form of a sol or colloidal suspension. These precursors contain a large amount of aqueous and organic material that must be removed during the conversion to a dense, solid phase. A considerable amount of shrinkage occurs during drying and sintering, during which time the contours of the thin film may deviate from the original shape of the mold.13 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0251 J. Mater. Res., Vol. 20, No. 8, Aug 2005
For instance, the use of MIMIC to pattern thin films derived from sol-gels results in the formation of a final sintered topography in which the material has a greater thickness at the lateral edges than in the middle, as shown in the scanning electron microscope (SEM) images in Fig. 2.13 This defect, referred to as the “double-peak” topographical profile, can cause problems when the patterned thin film is used in the fabrication of hierarchical structures requiring pattern alignment for successive depositions of additional layers.13 In many microelectromechanical systems (MEMS) applications, such as microcantilever beam
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