Low temperature bonding for microfabrication of chemical analysis devices

https://doi.org/10.1016/S0925-4005(97)00294-3Get rights and content

Abstract

A low temperature bonding process was developed for the fabrication of microchip devices for liquid and heterogeneous phase chemical analysis. Photolithographically etched microchannels on glass substrates were closed by bonding a glass cover plate using a spin-on sodium silicate layer as an adhesive. Good channel sealing was achieved by curing at 90°C for 1 h or room temperature overnight. The fluidic performance of the device was evaluated by monitoring the electroosmotic flow on the chip. The results compared well with those obtained from devices made by high temperature direct bonding of the substrate and cover plate. The dielectric and mechanical strength for bonds, created using the low and high temperature methods, were compared. A dielectric strength of 400 kV cm−1 was obtained for the sodium silicate bonding and 1100 kV cm−1 for the high temperature bonding. Mechanical strength measurements gave a surface energy value of ≈2.7 J m−2 for sodium silicate bonding, compared to 6.5 J m−2 for direct bonding. The mechanical strength of glass bonds obtained with sodium silicate at low temperature was comparable to that reported for the sodium silicate bonding of silicon wafers at >200°C or by conventional direct bonding of oxidized silicon at 1400°C. The low temperature bonding performance is adequate for microfabricated fluidic devices that employ electrokinetic transport phenomena. The reduced temperature of the bonding process will allow chemical surface modification prior to bonding.

Introduction

Microfabrication of miniature analytical instruments for chemical and biochemical applications has recently attracted considerable interest. Various microdevices have been fabricated that perform liquid phase analysis including capillary electrophoresis 1, 2, 3, electrochromatography [4], micellar electrokinetic capillary chromatography [5]and DNA separations 6, 7, 8. Both chemical reactions and separations have been carried out in monolithic integrated devices 9, 10. The devices were fabricated by etching the channel manifolds into glass substrates using standard photolithographic and wet chemical etching methods. Closed channels were initially formed by bonding a glass cover plate to the substrate in a muffle furnace at 620°C [1]. A direct bonding technique was also developed, by which the glass substrate and cover plate were first surface hydrolyzed and joined by hydrogen bonding and then annealed at 500°C [3]. However, the high temperature bonding process hindered the fabrication of devices that contain temperature-sensitive materials, such as immobilized chemical arrays, structural features such as electrodes and waveguides, or materials with different coefficients of thermal expansion. Consequently, bonding is desired at a process temperature lower than 100°C.

There are a number of low temperature techniques presently in commercial use for wafer scale bonding in the microelectronics industry, as alternatives to the silicon direct bonding method [11], which requires a process temperature above 1100°C to achieve good bond strength. Among these are anodic bonding [12], eutectic bonding [13]and the use of low melting temperature glasses for bonding [14]. Anodic bonding is commonly used to bond glass to silicon, silicon to silicon, or glass to metal with electric field assistance while heat is applied (typically 300°C). Eutectic bonding of metals is performed with applied pressure at the alloy melting temperature, which is lower than that of the pure constituents. Various glasses with low melting points, such as boron-doped glass (450°C) [15]and glass frit (400–600°C) 13, 14have been used as an intermediate layer for silicon-to-silicon wafer bonding. Quenzer and Benecke [16]used a spin-on sodium silicate layer to bond two wet-oxidized silicon wafers together at 200–300°C. Similar studies were conducted by Yamada et al. 17, 18who used commercially available spin-on glass containing Si(OH)x, where 2<x<4, as an adhesive to bond silicon wafers with silicon dioxide or silicon nitride surface films at 250°C.

A major commercial application of sodium silicate is as a binder for sand and other mineral particles 19, 20. Iler has described the properties of aqueous silicate in detail [21]. Silica is slightly soluble in water due to silicic acid, Si(OH)4 formation through hydrolysis:SiO2x+2H2OSiO2x−1+SiOH4Supersaturated solutions of Si(OH)4 in pure water are thermodynamically unstable because condensation polymerization takes place through dehydration leading to the deposition of silica and formation of colloidal particles. At high pH (>9), silicate ions are formed:Si(OH)4+OHHSiO3+2H2OHSiO3+OHSiO2−3+H2OThus, the concentration of Si(OH)4 is greatly lowered by conversion to ionic species. This leads to an increased solubility of silica in water, to the point of complete dissolution [22]. Diluted solutions of sodium silicate solution (usually written as SiO2·Na2O) appear to be quite stable at pH>12.

A supersaturated solution of Si(OH)4 can be formed by lowering the pH of an aqueous silicate solution. Low temperature condensation reactions between Si–OH groups to form Si–O–Si bonds have been demonstrated in the formation of glass sol-gels [26]and also, in crack healing in glass. For example, Michalske et al. studied the closure and repropagation of healed cracks in silicate glass at different relative humidities at room temperature [27]and the adhesion of hydrated silicate films in between cracked glass surfaces at different temperatures [28]. In both cases, the reopening of healed cracks provided evidence of high energy bond formation. They suggested a bonding mechanism in which hydrogen bond formation initially connects the two surfaces, followed by siloxane bond formation at low temperatures.

In this paper, we present a study of glass-to-glass bonding, using sodium silicate as an adhesive layer at process temperatures lower than 100°C for the fabrication of miniaturized chemical analysis devices.

Section snippets

Experimental

Standard 75×25×1 mm glass microscope slides (Corning, No.2947 or Becton Dickinson, Gold Seal No.3011) and 22 mm circular 0.15 mm thick or 25×25×0.15 mm square cover plates (VWR Scientific, circular and Corning, square) were used for the bonding experiments. The generation of the channels on the glass substrate involves standard photolithographic procedures followed by wet chemical etching as previously described [3]. Fig. 1 shows an example of the microchip design with the simple cross-channel

Bonding

Freshly diluted solutions from newly purchased, concentrated sodium silicate solution provided strong bonding between two glass surfaces with thermal processing at 90°C for 1 h or at room temperature overnight. A bonding wave was observed once the two glass surfaces were brought into initial contact, similar to the silicon direct bonding process [31], and within a few seconds, the contacting wave had spread over the entire area. The cover plate can be removed using a razor blade after the

Conclusions

Sodium silicate bonding has proved to be a simple and effective method for low temperature fabrication of glass microchip devices for chemical and biochemical assays. Strong bonds between glass surfaces and good channel sealing have been achieved at 90°C or room temperature. There is no significant difference in the electroosmotic performance of devices made by sodium silicate bonding and those made by high temperature direct bonding. Although the low temperature bonding presented here resulted

Acknowledgements

The authors gratefully acknowledge Dr G.E. Jellison, Jr for his contributions to the ellipsometry measurements. This research was sponsored by the US Department of Energy (DOE), Office of Research and Development. Oak Ridge National Laboratory is managed by Lockheed-Martin Energy Systems, for the US Department of Energy under Contract DE-AC05-96OR22464. Also, this research was sponsored in part by an appointment for H.Y.W. to the ORNL Postdoctoral Research Associate Program. These postdoctoral

HongYing Wang received her PhD degree in 1994 in the Department of Macromolecular Science at Case Western Reserve University. This work was carried out while she was a postdoctoral research associate in the Chemical and Analytical Sciences Division, Oak Ridge National Laboratory (ORNL) from 1994 to 1996. She is currently a senior Polymer Scientist at Soane BioSciences, CA. Her research interests are in microfabrication of miniature analytical instruments for chemical and biochemical

References (37)

  • A.T Woolley et al.

    Ultra-high-speed DNA sequencing using capillary electrophoresis chips

    Anal. Chem.

    (1995)
  • S.C Jacobson et al.

    Integrated microdevices for DNA restriction fragment analysis

    Anal. Chem.

    (1996)
  • S.C Jacobson et al.

    Precolumn reactions with electrophoretic analysis integrated on a microchip

    Anal. Chem.

    (1994)
  • W.P Maszara

    Silicon-on-insulator by wafer bonding: A review

    J. Electrochem. Soc.

    (1991)
  • T.R Anthony

    Dielectric isolation of silicon by anodic bonding

    J. Appl. Phys.

    (1985)
  • W.H. Ko, J.T. Suminto, Semiconductor integrated circuit technology and micromachining, in: W. Gopel, J. Hasse, J.N....
  • W.P. Eaton, S.H. Risbud, R.L. Smith, Wafer bonding by low temperature melting glass, Proceedings of The First...
  • A Yamada et al.

    SOI by wafer bonding with spin-on glass as adhesive

    Electron. Lett.

    (1987)
  • Cited by (0)

    HongYing Wang received her PhD degree in 1994 in the Department of Macromolecular Science at Case Western Reserve University. This work was carried out while she was a postdoctoral research associate in the Chemical and Analytical Sciences Division, Oak Ridge National Laboratory (ORNL) from 1994 to 1996. She is currently a senior Polymer Scientist at Soane BioSciences, CA. Her research interests are in microfabrication of miniature analytical instruments for chemical and biochemical applications and polymer applications in electrophoresis media and microanalysis tools.

    Robert S. Foote received his PhD in chemistry from Duke University in 1972. He is currently a staff scientist in the Chemical and Analytical Sciences Division of ORNL. His primary interests are in the development of microinstrumentation for biochemical assays.

    Stephen C. Jacobson received his PhD in chemistry from the University of Tennessee in 1992. Following a postdoctoral fellowship at ORNL, he joined the research staff there in 1995. Research interests include liquid separations and miniaturization of chemical instrumentation.

    Joachim H. Schneibel conducted research into mechanisms of superplastic deformation at Oxford University, where he received a D Phil in 1979. After postdoctoral studies at the Massachusetts Institute of Technology he joined ORNL in 1981. His research interests and experience include experimental and theoretical studies of superplastic deformation, diffusional creep, sintering, creep cavity growth, grain boundary sliding, and crack propagation.

    J. Michael Ramsey received his PhD in chemistry from Indiana University in 1979. After completion of graduate school he was awarded a Eugene P. Wigner Postdoctoral Fellowship at ORNL. He became a permanent staff member at ORNL in 1981. Presently, he is a senior Staff Scientist and leader of the Optical Spectroscopy Group in the Chemical and Analytical Sciences Division. His research interests include miniature chemical instrumentation, ultrasensitive laser-based detection techniques, resonant multiphoton ionization, nonlinear spectroscopies, diode laser-based chemical instrumentation, and real-time chemical characterization of aerosols.

    1

    Present address: Seagate Technology, Inc., Scotts Valley, CA 95066, USA.

    View full text