Sidewall lithography of micron-sized features in high-aspect-ratio meso-scale channels using a three-dimensional assembled mask
© Ji et al.; licensee Springer. 2014
Received: 16 April 2014
Accepted: 22 July 2014
Published: 15 August 2014
We report a fabrication technique for dense, detailed lithographic patterning on high-aspect-ratio vertical sidewalls, and illustrate its use through the fabrication of etched array of micro-dimples on the sidewalls of aluminum channels. A photoresist etch mask was spray-coated inside the channel, and patterned using a silicon-fabricated three-dimensional mask which includes a diffuser and a reflector to uniformly redistribute the vertically incident source light in the channel. All the lithographic processes from photoresist coating to development have been carried out without disassembling the channel. Arrays of 107.5 μm-deep circular dimples were isotropically etched onto the sidewalls of a 31 mm-tall, 3.2 mm-wide aluminum channel.
In applications such as three-dimensional (3-D) electrical interconnect formation or surface texturing , the ability to fabricate micro-patterns on straight vertical sidewalls is very attractive, yet extremely challenging. Despite the recent advances in 3-D microfabrication technologies, little progress has been made in the development of a lithographic patterning process on 3-D surfaces, especially on vertical sidewalls. Unless the substrate is inclined or the direction of the incident light is deflected, direct exposure or patterning of the vertical sidewall is almost impossible and the level of difficulty or complexity rises if the sidewall is a part of a narrow or high-aspect-ratio channel. Techniques such as shadow mask deposition , inclined laser patterning , or metal transfer micro-molding  have been investigated. Morishita et al. fabricated multi-height shadow mask to improve the pattern definition at the bottom and vertical sidewall of the trench compared to conventional planar shadow mask . Rajaraman et al. used laser ablation process to partially scribe the conformally deposited polymer layer covering the vertical sidewall of the 3-D metal electrode . Zhao et al. used combination of multiple molding and metal deposition processes to pattern metal layer on the vertical sidewall of the structure . Although these processes provide practical means to serve their needs, available pattern geometries and process flexibility are limited.
In this research, we report a new sidewall patterning technology based on a conventional photolithography process using spray-coated photoresist and silicon fabricated 3-D mask. In contrast to the previous sidewall patterning approaches, we have developed a direct patterning process by redirecting the highly directional light from the exposure tool. Analogous to the lithography process on a conventional two-dimensional surface where substrates are aligned and exposed subsequently, batch fabrication approach can be utilized in the sidewall patterning process using the proposed 3-D mask. As the channel measures 31 mm-tall and 3.2 mm-wide, exposure light incident on the narrow channel entrance on the top surface has been diffused and redistributed throughout the channel height by two reflecting surfaces. To our knowledge, direct patterning on the inner vertical sidewalls of a centimeter scale flow channel by lithographical means has not been attempted up to the present.
3-D mask design and fabrication
This paper reports a direct lithography process on a vertical sidewall using a silicon deep-etched and assembled 3-D mask. To overcome the directionality of the UV source in conventional exposure tool, we have fabricated a 3-D silicon structure whose inner cavity mimics the light-guide plate used in edge illuminated liquid crystal displays. Sidewalls of a 31 mm-deep and 3.2 mm-wide flow channel have been patterned by exposing the spray coated photoresist with the fabricated 3-D mask. Using the patterned photoresist as the etch mask, arrays of 107.5 μm-deep circular dimples have been fabricated successfully. To our knowledge, this is the first direct lithography process to pattern the inner vertical sidewalls of a centimeter scale flow channel. The process can be utilized in various types of 3-D structure fabrication processes including surface feature generation and transducer formation on vertical sidewall of the channel or structure.
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