25 August 2000 Acoustic agitation for enhanced development of LIGA PMMA resists
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Proceedings Volume 4174, Micromachining and Microfabrication Process Technology VI; (2000) https://doi.org/10.1117/12.396462
Event: Micromachining and Microfabrication, 2000, Santa Clara, CA, United States
The development of exposed PMMA resist for the LIGA process is very difficult and time-consuming when the resist thickness is large and feature aspect ratios exceed about four. This is due mainly to limitations on the development rate imposed by the diffusive transport of PMMA fragments away form the dissolution surface. Development rates under these conditions can be enhanced significantly by high- frequency acoustic agitation. To study this enhancement, analytical solutions describing the periodic flow field are used to evaluate the time-mean Reynolds stresses that drive streaming fluid motion. The resulting steady flow and transport rates within a feature are then computed by solving the Navier Stokes and species transport equations. For typical acoustic frequencies and power levels of 1 MHz and 6 w/cm2 the streaming flow within the feature is torroidal, with inflow along the feature walls at maximum speeds approaching 1 mm/min, coupled with a slower outflow along the feature center. The computed increase in transport, relative to diffusion, is typically on the order of 4 to 6 for feature aspect ratios ranging from 3 to 10 and for polymer fragment diffusivity on the order of 10-11 m2/s, provided that the feature width is greater than about 10 microns. In smaller features, the streaming speed may be suppressed by overlapping viscous boundary layers on opposing feature walls. Higher frequencies help reduce the boundary layer thickness but may lead to less efficient multi-cellular flow patterns when the acoustic wavelength is less than the feature depth.
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Robert H. Nilson, Robert H. Nilson, Stewart K. Griffiths, Stewart K. Griffiths, } "Acoustic agitation for enhanced development of LIGA PMMA resists", Proc. SPIE 4174, Micromachining and Microfabrication Process Technology VI, (25 August 2000); doi: 10.1117/12.396462; https://doi.org/10.1117/12.396462

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