Nano-scale large area gap control for high throughput electrically induced micro-patterning
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Micro- and nano-scale patterning is essential to the fabrication of various kinds of devices including electronic circuits, optical devices, optoelectronic devices, thin film heads for magnetic storage, displays, etc. There are several current and emerging applications that specifically require regular arrays of repeating patterns such as gratings, posts, and holes. At the nano-scale, for replication using lithography techniques such as optical lithography and imprint lithography, the cost of making the master can be a significant portion of the fabrication cost, particularly if small batches of customized parts are required. High resolution patterning using electric fields allows the creation of micro- and nano-scale structures using low resolution masters. Most of the literature to date has focused on using high glass transition temperature polymers that need to be heated to induce the patterning process. While this allows the ability to use a wide variety of materials, it leads to poor throughput as it can take several minutes to complete the patterning of one device region. The patterning speed can be increased by using photocurable, low viscosity monomers instead of high glass transition temperature polymers. Process control requires a tool that can control the parallelism of the gap between a conductive wafer and template to the nanometer level over large areas. The tool must have high resolution orientation and position control and high apparent stiffness to prevent the electric field from pulling the template and wafer together. In this research, high stiffness mechanism designs were investigated first. Such designs proved impractical, with travel, stiffness, and maximum side load requirements difficult or extremely expensive to meet. Therefore, a novel precision machine concept was explored. A parallel mechanism that is simultaneously actuated by piezo actuators and by voice coils was studied. Feedforward compensation of the applied electric force using voice coils was used to reduce the need for a stiff mechanism. The result was the Hybrid Active Gap Tool (HAGT), a 3-RPS parallel mechanism which has the ability to significantly enhance the quality of electrically induced patterning. Performance of the Hybrid Active Gap Tool was validated using a set of gap control experiments. The new design and control system resulted in very high precision orientation alignment needed for gap control. Without voice coil compensation, the tool has a stiffness of less than 3N/µm . With voice coil compensation, the apparent stiffness of the tool varies from a minvi imum of 30N/µm up to nearly infinite stiffness and into negative stiffness if overcompensation is intentionally used. Voice coil compensation allows the tool to meet the stringent performance requirements of the patterning process without the need for a high stiffness mechanism. Gaps as small as 400nm were maintained with the electric field applied and the gap changed by less than 5nm from the nominal 400nm during the process. Smaller gaps can be achieved with improvements in template mesa height calibration and better understanding of piezo actuated mechanism designs.