Room temperature bonding of micromachined glass devices for capillary electrophoresis
Introduction
Microfluidic devices etched in glass substrates provide an on-chip fluidic network in which chemical reactions, sample injection, and separation of reaction products can be pumped and driven using electrokinetic phenomena [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Devices fabricated in glass have used a cover plate to close the channels in the etched plate, which was bonded at high temperatures to allow softening and flow of the glass plates [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. It would be convenient to lower the temperature required for bonding, or even to make the bonding step reversible, in order to reduce fabrication time and cost, and increase the flexibility of device usage.
Oxidized silicon wafer bonding has been extensively studied [12], [13]. Frequently, room temperature, intimate contact of the wafers gives a cold welded bond that is strong enough to allow further handling of the bonded structure without additional treatment. Glass bonding should be similar to oxidized semiconductor wafer bonding, however, the microscale roughness of Si is less than 5 Å [13], while that of glass is 50–70 Å for fired or mechanically polished glass [14]. Consequently, it is not obvious that the intimate contact of glass required for formation of a strong cold weld can be achieved. We recently reported that temperatures could be lowered to 440°C with extensive cleaning [16], while Wang et al. [17] have reported a chemical additive used in bonding silicon can be used to lower the bonding temperature of glass to about 90°C.
In this paper, we report a simple method to bond glass at room temperature that is based on rigorous cleaning [11], [16], [18]. This method was successfully used to bond a wide range of the same or different types of commercially available glass, without the need for thermal treatment. Microfluidic devices bonded with this method did not leak under normal operating conditions with either pressure or electrokinetically driven flow and showed good separation performance.
Section snippets
Materials and reagents
Borosilicate glass (Pyrex, Borofloat) was from Paragon Optical (Reading, PA). Photomask glass was from Agfa-Gevaert (Belgium) and cover-slip glass (Corning 0211) was from Corning Glass (Parkridge, IL). Microscope slides and Sparkleen detergent were from Fisher Scientific (Edmonton, Canada). Hollow diamond drill bits were from Lunzer (Saddle Brook, NJ). Crystal bond was from Aremco (Ossining, NY). The beads were Spherisorb ODSI (Phase Separations, Flintshire, UK), a porous C-18, Silica bead with
Conditions for the absence of interference fringes
The glass types tested for room temperature bonding (RT) are listed in Table 1. The first criteria of importance are the glass smoothness and its preparation during manufacture of the plates. The Pyrex used was ground and polished, while the borofloat, microscope slide, and cover slip glass (Corning 0211) were manufactured by a float process. The manufacturer's procedure for the photomask glass was not available, but it was either rolled or else ground and polished.
All of the glasses tested
Conclusion
The most important factors for successful room temperature bonding were the cleanliness and flatness of the glass surfaces. The separation performance and the durability of these RT bonded devices were comparable to that of devices prepared by high temperature bonding (>400°C). The availability of a low temperature bonding process that does not require additional chemical treatments should prove significant, greatly increasing the flexibility available in fabrication. Greater range in the
Acknowledgements
We thank the Natural Sciences and Engineering Council of Canada for support. NC thanks the Alberta Microelectronic Centre for a Research Fellowship and for the use of their facilities and G. McKinnon for helpful contributions. We are grateful to P. Myers of Phase Separations, UK, for donating the silica beads. DJH thanks M. Schmidt, M. Gray, and D. Sobek of MIT for valuable discussions.
References (20)
- et al.
Micromachining of monocrystalline silicon and glass for chemical analysis systems: a look into next century's technology or just a fashionable craze?
Trends Anal. Chem.
(1991) - et al.
Low temperature bonding for microfabrication of chemical analysis devices
Sens. Actuators, B
(1997) - et al.
Capillary electrophoresis and sample injection systems integrated on a planar glass chip
Anal. Chem.
(1992) - et al.
Planar glass chips capillary electrophoresis: repetitive sample injection, quantitation and separation efficiency
Anal. Chem.
(1993) - et al.
Glass chips for high-speed capillary electrophoresis separations with submicrometer plate heights
Anal. Chem.
(1993) - et al.
Micromachining a miniaturized capillary electrophoresis-based chemical analysis system on a chip
Science
(1993) - et al.
Micromachining of capillary electrophoresis injectors and separators on glass chips and evaluation of flow at capillary intersections
Anal. Chem.
(1994) - et al.
Effects of injection schemes and column geometry on the performance of microchip electrophoresis devices
Anal. Chem.
(1994) - et al.
Electroosmotic pumping and valveless control of fluid flow within a manifold of capillaries on a glass chip
Anal. Chem.
(1994) - et al.
Micromachining chemical and biochemical analysis and reaction systems on glass substrates