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Monolithic SU-8 Microcavities for Efficient

Fluorescence Collection

Stephen Macken and Daniel Filippini

Linköping University Post Print

N.B.: When citing this work, cite the original article.

Original Publication:

Stephen Macken and Daniel Filippini, Monolithic SU-8 Microcavities for Efficient

Fluorescence Collection, 2009, Procedia Chemistry, (1), 1, 1115-1118.

http://dx.doi.org/10.1016/j.proche.2009.07.278

Copyright: Elsevier

http://www.elsevier.com/

Postprint available at: Linköping University Electronic Press

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-58422

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Procedia

Chemistry

www.elsevier.com/locate/procedia

Proceedings of the Eurosensors XXIII conference

Monolithic SU-8 Microcavities for Efficient Fluorescence

Collection

S. Macken

a

* and D. Filippini

a

a

Division of Applied Physics, IFM, Linköping University, Linköping, Sweden.

Abstract

The fabrication of capped microstructures such as channels or cavities typically involves multiple production steps. In this work we demonstrate a fabrication procedure that enables the generation of capped monolithic microstructures of arbitrary geometry in one single exposure step. The presented method also enables the embedment of metal self-aligned surfaces for use as electrodes or mirrors. The devices furthermore demonstrate a capability of increasing fluorescent collection, as measured by an epi-fluorescent inverted microscope, by up to 15 fold.

Keywords: Fluorescence; microfluidics; SU-8; Fresnel diffraction.

1. Introduction

Microchannels, and microcavities as a special case of a space with a defined geometry, are necessary parts in advanced microfluidics1-2 and microelectromechanical systems (MEMs)3. They are used for reaction chambers4 and opto-fluidic platforms5. There are diverse methods to manufacture such structures among them, e.g. micro stereo lithography6, photolithography7, etching8 and combinations of these methods. In these approaches multiple fabrication steps and sealing procedures are required, since in their final form they are 3D encased spaces.

The fabrication of a separate sealant layer of SU-8 is a common practice often used to assemble these microfluidic systems2. It is produced by underexposing a layer of SU-8 that is attached to a sacrificial layer, which is then placed over an open channel and bonded by further exposure of the SU-8 and then baking when in contact with the open channel2. Other sealing methods involve attaching a coverslip using normal UV curable glue1. This, however, could lead to filling of microchannels and rendering a carefully constructed microsystem useless. Either way the attachment of the sealant is a very delicate procedure that could lead to the partial destruction of the microstructures.

In this work we present the fabrication of partially and totally confined SU-8 (10) microcavities of varied geometries in a single SU-8 exposure step and we demonstrate its’ use for efficient fluorescence collection from indicators within the cavities, which show a fifteen-fold signal increase with respect to the open structures. We characterize fabrication parameters such as exposure time and geometry on mask transferred substrates for the generation of different 3D cavities. Numerical simulations supporting the interpretation of the diffraction profile emanating from the lithographic mask for the creation of the microcavities are also presented.

* Corresponding author. Tel.: +46-(0)13-282702; fax: +46-(0)13-137568. E-mail address: stema@ifm.liu.se.

1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.278

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2. Experimental

Standard microscope glass slides (Menzel-Glaser microscope slides) were cleaned in H2O : H2O2 : HCl (6:1:1) for 10 mins, to remove any residue and provide a good surface for SU-8 attachment. After drying with N2 gas, 150 nm of Cr were deposited through thermal evaporation onto one side of the slides. Following the procedure laid out in 7 a Cr mask was produced on the glass slide to be then spun-coated with SU-8 (to a film thickness between 25-30

µm). By exposing this to photon energies ranging from 144 mJ/cm2 to 252 mJ/cm2 in the manner of Fig. 1(a) various forms of microcavities were produced.

Figure 1: a) Scheme of a mask feature of width b in the experimental conditions used for SU-8 exposure. b) Diffraction profiles at different distances from the substrate. c) Model of cross-linking of SU-8 due to the Fresnel diffraction from two adjacent mask apertures.

3. Results and Discussion

There are a number of benefits to exposing the SU-8 from the glass side through an embedded mask7 ; such that efficient exposure to the SU-8 in contact with the substrate provides proper adhesion while eliminating any tapering at the base of the microstructure. However, it also creates a situation whereby it is possible to control the tapering at the tips of adjacent structures to form a roof on a well-defined and confined microstructure.

Considering now the Fresnel diffraction that will occur as the UV light passes through a b = 20 µm mask aperture along the SU-8 thickness, for z ∈ [0, 30 µm] for UV light (λ=365 nm), the intensity distribution can be calculated and simulated in Matlab9 (figure 1(b)). The coarser illuminating profile, in figure 1(b), is located at what is to become the base of the microstructure, which then spreads out away from the aperture. As expected from diffraction theory10 the calculation also shows that the illuminating pattern extends beyond the aperture and through the “dark” region covered by the mask above the Cr surface.

In our arrangement with the exposure through the glass, the tip of the microstructure is 30 µm from the mask aperture, so any SU-8 at this depth requires approximately 170 mJ/cm2 to cross-link. As diffracted light exposes the SU-8 inside the masked region it can lead to negative sidewall profiles of the developed microstructures, depending on the exposure dose11. By introducing a second aperture at a distance d = 30 µm from each other will thus create the situation illustrated in figure 1(c). This figure shows that the diffracted light from each side will overlap producing an exposure profile able to cross-link the SU-8 inside the masked region and above the Cr mask, but not close to the substrate, which in turn leads to the creation of microcavities between adjacent microstructures.

Figure 2, column 1, demonstrates results of this fabrication procedure for (a) square, (b) hexagonal and (c) circular periodical apertures, illuminating 30 µm high SU-8 (10), which creates 3D microcavities by cross-linking of the SU-8 in the masked regions. To then estimate the exposure pattern at z = 30 µm for a 2D periodic mask with the considered aperture geometries we follow Poons’ approach12. The results of which show how the calculated exposure patterns support the observed microcavities.

For instance, the square mask creates a diffraction pattern at 30 µm from the substrate that extends perpendicular to the aperture sides creating a low exposure region at the centre of four adjacent squares. A similar effect is seen when hexagonal structures are examined except now a triangular area is under exposed. Finally, the circular structure, which provides the most homogeneous intensity pattern of the UV light, is used here to illustrate the

S. Macken and D. Filippini / Procedia Chemistry 1 (2009) 1115–1118 1116

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closed cavities. By comparing these to the SEM pictures, of figure 2 column 1, a clear validation of the modeling can be seen with model features being replicated in experimental results and that the degree of connectivity between the microstructures is clearly linked to the exposure supplied to the cross-linking process (the intensity scale in the simulation and profiles is non-linear in order to underscore the intensity distribution outside the apertures). Thus by designing a 3D exposure strategy capped monolithic cavities of diverse geometries can be created in a single SU-8 exposure procedure.

To test the effectiveness of microcavities to contain substances and aid the detection of fluorescent indicators, 0.5 mM of rhodamine, dissolved in acetone, was dropped at the edge of circular structures and allowed to fill the channels through capillary action13 and compared with open structures coated in the same solution. Figure 3 are fluorescence microscope images, taken in the rhodamine spectral region, of open structures (a1), open structures with a mirror behind (a2) and of capped structures with an embedded Cr mirror (a3 the cavities demonstrated here).

The fluorescence signal is much larger in the closed microcavities than from normal open pillar microstructures and this is in part due to the presence of Cr on the base of the microcavities, which is seen in figure 3(a2), which shows a higher fluorescence signal between the pillars when a mirror is present than when it is not. However, this accounts for only a minor contribution of the total intensity boost achieved with the microcavities, so to better illustrate this difference between the fluorescent signals a profile plot along the white lines of the images (a1) and (a3) is shown in figure 3(b). Due to the intensity of the red fluorescent light, from the rhodamine, the red channel of the camera was easily saturated therefore the values presented in figure 3(b) are from the green channel of the pictures, which partially overlaps with the emission band of a rhodamine filter thereby allowing access to the gain in fluorescent signal using a non-saturated channel while under identical camera settings.

The profiles of figure 3(b) show two distinct areas of increased fluorescence G1 and G2; firstly at the edge of the pillar structures, forming the ring of figure 3(a), providing an approximately three-fold increase in fluorescence collection (G1) and secondly in between these structures, in the microcavities, where a fifteen-fold increase in fluorescence collection can be seen (G2).

As put forward in14 the majority of fluorescent light from a dipole will be emitted into the dielectric with the higher refractive index, at angles above the critical angle of the dielectric15, 16, 17. This poses a problem in that light emitted above the critical angle of a substrate will remain confined within the substrate. However due to the orientation of the surfaces with regard to the emission direction it is possible for the fluorescent light, in our case, to escape confinement to a larger extent and provide the greater fluorescence signal.

In summary, SU-8 microcavities can improve fluorescent collection. While at the same time they demonstrate the feasibility of producing in a single SU-8 exposure step closed microstructures thereby simplifying the manufacture of microfluidic systems with self-aligned metal surfaces.

Figure 2: Column 1: SEM images of partially closed cavities formed using a) square apertures, b) hexagonal apertures and completely closed cavities formed using c) circular apertures. Column 2: selected symmetry of partially closed cavities formed using d) square apertures, e) hexagonal apertures and completely closed cavities formed using f) circular apertures. Column 3: 2D model of diffraction pattern for the considered symmetries in column 2. Column 4: Intensity profile in the selected segments indicated in column 2.

Figure 3: 0.5 mM rhodamine on 25 µm wide 30 µm tall: a1) open circular pillars, a2) open circular pillars with a mirror in the back and a3) closed circular structures with embedded Cr mirror. b) Profiles taken from indicated white line, across a1) and a3), comparing fluorescent signal in the green channel for open (dashed black line) and closed (solid black line), circular patterned SU-8.

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4. Conclusions

In order to close microstructured channels and chambers, creating in essence a microcavity, a multiple stepped procedure is normally required. In this paper we have shown that by designing the 3D exposure profile and illuminating from the substrate side capped monolithic structures can be achieved in a single SU-8 exposure and with the additional benefit to naturally embed self aligned metal electrodes inside, where the Cr mask could be replaced by any other metal, and be used for example for electrochemical detection, dielectrophoretic pumps and electro wetting18, 19, 20

We have also demonstrated a clear application of this technique resulting in up to a fifteen-fold increase in fluorescence collection by using microcavities to observe fluorescent fluids, which could represent a measuring region in a regular fluidic structure.

Acknowledgements

This work was supported by grants of the Swedish Research Council (VR) and the Swedish Foundation for Strategic Research (SSF) through the program Nanosense [A3 05:204].

References

1. L’Hostis E, Michel PE, Fiaccabrino GC, Strike DJ, de Rooij NF and Koudelka-Hep M. Microreactor and electrochemical detectors fabricated using Si and EPON SU-8. Sens. Act. B 2000;64:156

2. Song Y, Kumar CSSR and Hormes J. Fabrication of an SU-8 based microfluidic reactor on a PEEK substrate sealed by a ‘flexible semi-solid transfer’(FST) process. J. Micromech. Microeng. 2004;14:932.

3. Roberts K, Williamson F, Cibuzar G and Thomas L. The Fabrication Of An Array Of Microcavities Utilizing SU-8 Photoresist As An Alternative ‘LIGA’ Technology. University/Governmet/Industry Microelectronics Symposium 1999. Proceedings of the thirteenth biennial 20-23

June 1999;139-41.

4. Yoona S, Parka S and Kim Y. A micromachined microcalorimeter with split-flow microchannel for biochemical sensing applications. Sens. Act. B 2008;134:1:158.

5. Ruano-Lòpez JM et al. A new SU-8 process to integrate buried waveguides and sealed microchannels for a Lab-on-a-Chip. Sens. Act. B 2006;114:542.

6. Zhang X, Jiang XN and Sun C. Micro-stereolithography of polymeric and ceramic microstructures. Sens. Act. A. 1999;77:2:149. 7. Peterman MC, Huie P, Bloom DM and Fishman HA. Building thick photoresist structures from the bottom up. J. Micromech, Microeng.

2003;13:380.

8. Zhanga H, Amroa NA, Disawala S, Elghaniana R, Shileb R and Fragalab J. Microstructure array on Si and SiOx generated by micro-contact printing, wet chemical etching and reactive ion etching. App. Sur. Sci. 2006;253:4:1960.

9. K.M. Abedin, M.R. Islam and A.F.M.Y. Haidera. Computer simulation of Fresnel diffraction from rectangular apertures and obstacles using the Fresnel integrals approach. Optics & Laser Technology 2007;39:237-46.

10. Hecht E. Optics, Addison Wesley Publication, 4th edn., 2002;10.3:485-509.

11. Kim S, Yang H, Kim K, Lim Y, Pyo H. Study of SU-8 to make a Ni master-mold: Adhesion, sidewall profile, and removal. Electrophoresis 2006;27:3284-96.

12. Poon T. Contempory optical image processing with MATLAB. Elsevier, 1st edn., 2001, 58-65

13. Courbin L, Denieul E, Dressaire E, Roper M, Ajdariand A and Stone HA. Imbibition by polygonal spreading on microdecorated surfaces. Nat. Mat. 2007;6:661- 4.

14. Polerecký L, Hamrle J and MacCraith BD. Theory of the radiation of dipoles placed within a multilayer system. App. Opt. 2000;39:22. 15. Macken S, Lundström I and Filippini D. Optical properties of microstructures for computer screen photoassisted experiments. App. Phy. Lett.

2006;89:254104.

16. Macken S, Di Natale C, Paolesse R, D’Amico A, Lundström I, Filippini D. Towards integrated devices for computer screen photo-assistedmulti-parameter sensing. Anal. Chim. Acta 2009;632:1:143-7.

17. Filippini D, Åsberg P, Nilsson P, Inganäs O, Lundström I. Computer screen photo-assisted detection of complementary DNA strands using a luminescent zwitterionic polythiophene derivative. Sens. Act. B 2006;113:1:410-8.

18. Seo J et al, Development of inlaid electrodes for whole column electrochemical detection in HPLC, Lab on a Chip; 2009, DOI: 10.1039/b822045j.

19. Li Y, Dalton C, Crabtree HJ, Nilsson G and Kaler KVIS. Continuous dielectrophoretic cell separation microfluidic device. Lab on a Chip 2007;7:239-48.

20. Walker S and Shapiro B. A control method for steering individual particles inside liquid droplets actuated by electrowetting. Lab on a Chip 2005;5:1404-7.

S. Macken and D. Filippini / Procedia Chemistry 1 (2009) 1115–1118 1118

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