Etch Rate Characterization of PMMA Via CO2 Laser for Hybrid Micromolding Process

This paper presents fabrication of PMMA  micromolds for hybrid soft lithography using laser CO₂ ablation system. The system has a CO₂ laser system with a maximum rated power of 60 watts that produces an invisible infrared beam at a wavelength of 10.6 μm. The laser system works with vector and raster modes to produce lines and filled shapes, respectively. In this work, the system was set to vector mode for single pass ablation. The control factors that determine the channel depth and width are laser power and speed of ablation, which are set as a percentage via the control software. A CO₂ laser emits infrared radiation at a wavelength of 10.6 μm. PMMA has a high absorptance of about 0.92 in the infrared (Absorptance is different from  absorbance and is defined as: The ratio of the radiant flux absorbed by a body to that incident upon it.). PMMA also combines a low heat capacity with a low heat conductance, which means that any absorbed heat will result in a rapidly rising temperature. This means that the CO₂-laser beam ablates the PMMA photothermally. Unlike UV lasers, a CO₂ laser emits radiation continuously. Wherever the focused laser beam meets the PMMA surface, the temperature of the irradiated spot rises rapidly, that the PMMA first melts and then decompose, leaving an V-shape groove. The channels have a V groove due to the temperature distribution within the PMMA substrate [1] The relationship between laser power and the channel depth will be discussed. Initial study indicates that the channel depth increases linearly with the power and decreases exponentially with the beam speed. We observed that the effect of beam speed and power on the channel/groove width is not significant.  However, it varies almost linearly with both beam speed and power. Hence, by varying beam speed and power we can achieve different depths and aspect ratios.

This novel low cost, large scale, micromold manufacturing technique can be extended to 12 “ x 24” PMMA Sheets [2].  This allows us to batch fabricate very large numbers of micromolded devices at the same time, or very large continuous structures such as long lines for electronic routing on large substrates. Not only does this potentially reduce manufacturing cost through larger batch fabrication, it enables the construction of new very large area devices fabricated by hybrid micromolding process defined by Khosla et al (Hybrid fabrication process for combining micromolded nanocomposite with undoped PDMS polymer) [3, 4], such as electrode arrays, electrical wires, and micromagnets etc.

References:

1.  Ajit  Khosla, “Micropatternable multi-functional nanocomposite polymers for flexible soft MEMS applications” PhDThesis, http://summit.sfu.ca/item/12017
2. A. Khosla, and B. L. Gray; “Micro-patternable Multifunctional Nanocomposite Polymers for Flexible Soft NEMS and MEMS Applications” ECS Trans. 2012 volume 45, issue 3, 477-494.
3. A. Khosla and B. L. Gray, "New technologies for large-scale micro-patterning of functional nanocomposite polymers", Proc. SPIE 8344, 83440W (2012)
4. A. Khosla, B. L. Gray, "Fabrication of multiwalled carbon nanotube polydimethyl- siloxnenanocomposite polymer flexible microelectrodes for microfluidics and MEMS," Proceedings of SPIE Vol. 7642, 76421V  (2010).