Submicron micromachining technology for liquid phase chromatography separation
05/01/2008
Executive OVERVIEW
Liquid phase chromatography is undoubtedly the most powerful technique to separate and identify molecules present in a given sample mixture. Chromatography is, however, also the separation process that is by far the most sensitive to the microstructure of the device in which it is performed. Any degree of structural heterogeneity inevitably leads to a loss in performance. Until now, chromatography has been performed in random packed beds of spherical particles???a technique that can benefit from the use of fine structure glass or silicon on the submicron scale available with state-of-the-art micromachining processes. However, this paper describes the fabrication of a liquid phase chromatograph using advanced submicron micromachining technology, which leads to enhancements of the separation performance.
Computational fluid dynamics calculations of liquid phase chromatography have shown that the change from a random structure to a perfectly ordered structure increases the separation speed by a factor of ~4. Given that the particles currently used have a size on the order of 3-5μm, and given that the analysis time also scales with the second power of the particle size, the production of microfabricated pillar arrays with a diameter of ~1μm (and an interpillar distance of =0.5μm) would therefore offer an additional factor of at least 9 in potential separation speed increase as compared to current technology used [1].
To investigate both the separation enhancement and the fabrication technology limits, a liquid phase on-wafer chromatograph with 56 channels having a width of 50-150μm and a length of ~4cm was designed and fabricated using advanced micromachining technology (Fig. 1). The channels are homogenously filled with pillars of width ranging from 1-5μm, while the gap between the pillars is typically ~1μm, but in extreme cases, it is only slightly larger than 0.1μm.
A fully micromachined liquid phase chromatograph has been fabricated in the following process flow: A Si oxide layer is deposited on 200mm Si wafers and subsequently patterned using a deep-UV stepper, enabling patterning of submicron structures. The Si oxide is patterned and used as a hard mask for the next deep reactive ion etching (DRIE) step, which defines the pillars with an aspect ratio up to ~25 and the separation channel (Fig. 2). In the next steps, the fluidic interconnects are processed using lithography, oxide etch, and DRIE of the channels to a depth of ~100μm. Next, the wafer is bonded (using anodic bonding) to a 200mm glass wafer to close the fluidic channels. Finally, the access holes are etched from the back side of the Si wafer using DRIE.
The performance of the device is characterized by injecting a band of tracer molecules, following the band during its pressure-driven flow through the micropillar array using a motorized translation stage and a fluorescence microscope equipped with a high-resolution CCD camera and measuring the broadening of the injected tracer band. The result shows that the combination of the micromachining process and the state-of-the-art microfluidic simulation enables an optimal peak broadening without side wall distortion.
The separation of a mixture of three coumarin dyes into three separate bands by the application of a mono-molecular layer of hydrophobic octododecylsilane molecules on the surface of the micropillars, was successfully demonstrated.
Conclusion
The first actual demonstration of the fabrication of a liquid phase chromatograph using advanced submicron micromachining technology can lead to enhancements of the separation performance. A speed of analysis increase of a factor 5???10 was demonstrated.
Acknowledgments
This paper has been modified from its initial version previously published in the IEDM 2007 conference proceedings. Agnes Verbist, IMEC, Leuven, Belgium; and Hamed Eghbali and David Clicq, Vrije U. Brussel, Brussels, Belgium, are also co-authors.
References
- J. Billen et al., J. Chromatogr. A, 2005, 1073, pp. 53-61.
- D. Sabuncuoglu Tezcan et al., Proc. IEDM 2007, pp. 839-842.
Deniz Sabuncuoglu Tezcan received her BS, MS, and PhD in electrical engineering from Middle East Technical U., Ankara, Turkey, in 1995, 1997, and 2002, respectively. She is a research engineer specializing in process development and integration of various projects at IMEC; e-mail [email protected].
Piet De Moor received his PhD in physics from Leuven U. in 1995 and joined IMEC in 1998. Currently, he is leading a team at IMEC developing 3D integration technology and associated microsystems applications using aggressive wafer thinning, through Si wafer interconnects and wafer-to-wafer bonding.
Wim De Malsche received his degree in bioengineering in 2004 at the Vrije Universiteit Brussel (VUB, Belgium) and is working toward a PhD in both the mesoscale chemical systems group at the U. of Twente, Enschede, The Netherlands, and the transport modeling and analytical separation science group of the VUB’s department of chemical engineering (CHIS).
Joris Vangelooven received his degree of Bioengineer in 2005 at the VUB and is currently doing a PhD at the CHIS group of the VUB.
Gert Desmet is a professor in biochemical engineering and analytical biochemistry at the VUB. He is leading the transport modeling and analytical separation science group in the department of chemical engineering of the same university. His research focuses on the miniaturization of separation methods and on the investigation of flow effects in chromatographic systems.