Conference PaperPDF Available

Epoxy Resin Based Optofluidic Chips with Integrated Bragg Gratings

Authors:

Abstract

We demonstrate the potential of the epoxy photoresist series EpoCore/EpoClad in a joint photolithography fabrication process of integrated optics and microfluidic channels forming optofluidic sensor chips. The chip comprises integrated optical EpoCore waveguides including planar Bragg gratings on a polymer substrate which are covered by a thick structured EpoClad layer serving as both microfluidic channel network and waveguide cladding. Mercury lamp exposure dose experiments were carried out to identify optimum UV exposure settings for structuring the epoxy photoresist. Waveguide integrated Bragg gratings are fabricated by the phase mask method using coherent UV irradiation from a KrF excimer laser. First results of the reflected intensity interrogation of the grating equipped waveguides reveal that distinct (>30 dB) and narrow (ΔλFWHM < 185 pm) first order Bragg reflections for TE0 and TE1 mode can be obtained. Loading experiments with fluids of varying refractive index induce instant, stable and reproducible shifts of the Bragg reflections (up to ΔλB > 4.2 nm) and thereby prove the great potential of this epoxy material combination for fast, multiple and highly sensitive refractive index sensing of small volume fluids.
EPOXY RESIN BASED OPTOFLUIDIC CHIP WITH INTEGRATED
BRAGG GRATINGS
S. Hessler1,*, B. Schmauss2, R. Hellmann1
1: Applied Laser and Photonics Group, University of Applied Sciences Aschaffenburg, Wuerzburger
Strasse 45, 63743 Aschaffenburg, Germany.
2: Institute of Microwaves and Photonics, University of Erlangen-Nuremberg, Cauerstrasse 9, 91058
Erlangen, Germany
*Corresponding author: steffen.hessler@h-ab.de
Abstract: We demonstrate the potential of the epoxy photoresist series EpoCore/EpoClad in a joint
photolithography fabrication process of integrated optics and microfluidic channels forming optofluidic sensor
chips. The chip comprises integrated optical EpoCore waveguides including planar Bragg gratings on a polymer
substrate which are covered by a thick structured EpoClad layer serving as both microfluidic channel network
and waveguide cladding. Mercury lamp exposure dose experiments were carried out to identify optimum UV
exposure settings for structuring the epoxy photoresist. Waveguide integrated Bragg gratings are fabricated by
the phase mask method using coherent UV irradiation from a KrF excimer laser. First results of the reflected
intensity interrogation of the grating equipped waveguides reveal that distinct (>30 dB) and narrow
(ΔλFWHM < 185 pm) first order Bragg reflections for TE0 and TE1 mode can be obtained. Loading experiments
with fluids of varying refractive index induce instant, stable and reproducible shifts of the Bragg reflections (up
to ΔλB > 4.2 nm) and thereby prove the great potential of this epoxy material combination for fast, multiple and
highly sensitive refractive index sensing of small volume fluids.
Key words: planar Bragg grating, refractive index sensor, optofluidics, microfluidics, EpoCore / EpoClad.
1. Introduction
Microfluidic systems in terms of Lab-on-a-chip (LOC) devices received great attention in their vast development
in the past decade. A variety of LOCs have been widely employed as effective analysis tools for biological and
chemical fluids. Combining such microfluidic channels with integrated optical sensor structures such as, e.g.,
Bragg gratings, or Mach-Zehnder interferometers enables high precision optical measurement and monitoring of
fluid properties [1, 2]. By exploiting the effect of evanescent field interaction of waveguide modes with the
surrounding cladding media, these devices offer an instant detection of changing refractive indices around the
waveguide core.
In literature, evanescent wave refractive index (RI) measurements are commonly carried out using
microstructured fiber Bragg gratings (FBGs) [3, 4, 5, 6] or integrated Mach-Zehnder interferometers [7].
Although showing very high sensing capacities, integrated Mach-Zehnder interferometers exhibit comparably
large dimensions which limit the integration possibility inside a microfluidic network considerably. Employing
planar Bragg gratings as sensing devices allow microfluidic channels to be equipped with a multitude of sensor
elements for complex local fluid measurements.
EpoCore / EpoClad epoxy resins are inexpensive UV-patternable negative photoresists from micro resist
technology featuring low optical losses (~ 0.2 dB/cm @ 850 nm), high glass transition temperatures (> 180 °C)
and an exalted chemical resistance after crosslinking [8]. These properties make the epoxy resins perfectly
suitable for optofluidic systems being applicable at high temperatures or under chemically aggressive ambient
conditions. Moreover, featuring a high refractive index of about 1.574 at an operating wavelength of 1550 nm,
the crosslinked EpoCore potentially provides a wide measurement range for evanescent wave detection of fluids
by changes of effective mode indices. Until today the fabrication processes of EpoCore waveguides for
MOEMS [9], spectroscopy applications [10], flexible neural probes [11] and printed circuit boards [12] have
been reported. However, EpoCore/EpoClad series is in summary still scarcely employed for concrete
applications.
In this work, we describe a combined fabrication process of an epoxy resin based optofluidic system comprising
of microfluidic channels equipped with optical Bragg grating sensor waveguides. This chip allows precise local
determination and monitoring of fluid refractive indices. Additionally, this is the first report of Bragg gratings in
EpoCore photoresist to the best of the authors’ knowledge.
2. Experimental
2.1. Fabrication process
The epoxy photoresists EpoCore10 and EpoClad10 are structured photolithographically using an EVG620 mask
aligner equipped with an i-line filtered (365 ± 20 nm) mercury vapor lamp. An overview of the single process
steps for the fabrication of the optofluidic chip is depicted in figure 1a. The cyclo olefin copolymer TOPAS6017
with a thickness of 1 mm is used as substrate material. EpoCore10 is spin coated with 5000 rpm for 30 seconds
resulting in a ~5.5 µm layer thickness. In order to find the optimum UV exposure dose for the specific EpoCore
thickness an exposure dose experiment was carried out. As can be seen from figure 1b an optimum exposure
dose for high quality structures amounts ~200 mJ/cm² in contact exposure mode of the amplitude mask. After
developing the EpoCore waveguide structure in a mrDev600 developer bath, Bragg gratings are inscribed
employing the phase mask technique together with a KrF excimer laser. Evaluated writing parameters for a
grating with 503 nm period are a pulse fluence of 18.2 mJ/cm², a frequency of 10 Hz and a total number of 1000
pulses. Afterwards, another spin coating step covers the fabricated structure with a ~20 µm EpoClad layer
serving both as waveguide cladding and microfluidic channels. The channel dimensions and sensor windows are
determined by an additional amplitude mask illumination impinging an exposure dose of 350 mJ/cm². After the
EpoClad layer is developed free of residues, the chip can be sealed by lamination of a further TOPAS6017 sheet
at 195 °C for 30 minutes which simultaneously hard bakes all epoxy resist structures.
Fig. 1. a) process steps for the epoxy optofluidic chip
b) exposure dose experiment for EpoCore
2.2. Interrogation of optical reflection spectra
For the characterization of the Bragg reflections from the integrated Bragg gratings the waveguides are manually
coupled to a single mode fiber connecting the optofluidic chip with a Micron Optics sm125 interrogation system
(wavelength range 1510 1590 nm). In order to quantify the sensitivity of the Bragg grating waveguides to the
surrounding medium, the structures are loaded with DI water having a refractive index of 1.3320 as well as
different sugar solutions with refractive indices ranging from 1.3384 to 1.4298. Furthermore with methyl
salicylate (wintergreen oil, RI = 1.5273) and α-Br-Naphthalene (RI = 1.6470) high index fluids are also tested.
Every loading test was accompanied by a thorough purging step with DI water, isopropanol and nitrogen to
ensure a reproducible and clean sensor window.
3. Results
Using the described process parameters, especially the optimum exposure doses for structuring the EpoCore/
EpoClad photoresist layers results in smooth and defect-free waveguide and microfluidic channels with steep
edges. Likewise, using i-line instead of broadband illumination as well as considerably prolonged baking times
during soft bake and post exposure bake proved to be very beneficial for achieving homogenous structure quality.
A laser scanning surface topography of a thus fabricated sensing window containing a Bragg grating equipped
waveguide is depicted in figure 2a. The waveguide exhibits a thickness of 5.5 µm, a width of 11 µm and a
grating period of 503 nm which is equal to the phase mask. Since these waveguide dimensions allow higher
0
1
2
3
4
5
6
7
0200 400 600
structure thickness t in µm
fluence F in mJ/cm²
center
edge
modes to propagate (multimode waveguide) higher mode Bragg reflections appear as well beside a fundamental
(TE0) mode reflection. Figure 2b shows the spectral reflection interrogation of the fundamental mode of a sensor
structure and constrains that distinct (>30 dB) and narrow (ΔλFWHM < 185 pm) first order Bragg reflections for
different waveguide modes can be obtained.
Fig. 2. a) surface topography of sensing window
including waveguide with Bragg grating
b) first order reflection from the fundamental mode of a Bragg
grating inscribed waveguide.
After the characterization of the fabricated Bragg sensors the shifts of the Bragg reflections from TE0 and TE1
mode are tracked during the loading of the different refractive index fluids. Covering the sensor structure with
these fluids changes the effective mode indices due to modal evanescent field interaction and thus results in
instant shifts of the Bragg reflections. Both TE0 and TE1 mode peaks show high stability and can be fully
restored to initial Bragg wavelengths after purging.
Figure 3 shows the Bragg wavelength shifts of the two modal Bragg reflections correlated to the loaded
refractive indices. Comparing the total shifts for a respective RI loading reveals a much higher sensitivity of the
TE1 mode. Higher order modes possess a larger mode field diameter and thus larger evanescent field than the
fundamental mode thereby explaining the higher refractive index sensitivity with increasing mode order. TE0
Bragg shift is approximated by a linear fit and TE1 Bragg shift by an exponential fit. Derivations of these
functions deliver the RI sensor sensitivity in terms of wavelength shift per refractive index unit (nm/RIU). TE0
mode sensitivity attains 3.37 nm / RIU due to the linear behavior, whereas TE1 mode sensitivity increases
exponentially with increasing refractive index. A maximum sensitivity of 38.4 nm / RIU is observed at
RI = 1.5273 for methyl salicylate, which excels TE0 mode sensitivity by a factor >11. Assuming a detectable
wavelength resolution of 1 pm, TE0 mode and higher order TE1 mode accordingly have maximum RI resolution
values of 2.97 10−4 and 2.6 ⋅ 10−5 , respectively. As expected, testing the Bragg structure with
α-Br-Naphthalene (RI = 1.6470), which has a higher RI than EpoCore results in outcoupling of the light and a
complete loss of the Bragg signals, again proving the high surface sensitivity of the fabricated Bragg structure.
1,0 1,1 1,2 1,3 1,4 1,5 1,6
0
1
2
3
4
5
y = -3.3645 + 3.3691 * x
Bragg peak of TE1 mode
Bragg peak of TE0 mode
exponential fit
linear fit
wavelength shift (nm)
refractive index @ 633 nm
y = 0.8252 * exp(x/0.8449)
+ 1.5691E-11 * exp(x/0.0598)
- 2.6802
Fig. 3. Refractive index sensing with the optofluidic chip.
3. Conclusions
In this paper, epoxy resin series EpoCore / EpoClad is proposed for the combined fabrication of integrated Bragg
grating sensors inside microfluidic channel networks as an optofluidic chip. A reliable fabrication process is
developed which enables flexible modification of the waveguide as well as the microfluidic layout by
deployment of appropriate amplitude masks. The fabricated refractive index sensor structure permits multimodal
guidance thus leading to TE0 and TE1 mode Bragg reflections. Testing the unmodified Bragg grating sensor
window achieved a maximum sensitivity of 38.4 nm / RIU with an associated RI resolution of 2.6 ⋅ 10−5 for the
higher order mode.
References
[1] Psaltis, D., Quake, S. R., & Yang, C. (2006). Developing optofluidic technology through the fusion of
microfluidics and optics. Nature, 442(7101), 381-386.
[2] D. Erickson, S. Mandal, A. H. J. Yang, and B. Cordovez, “Nanobiosensors: optofluidic, electrical and
mechanical approaches to biomolecular detection at the nanoscale,” Microfluidics and Nanofluidics 4, 33
52 (2008).
[3] Chryssis, A. N., Lee, S. M., Lee, S. B., Saini, S. S., & Dagenais, M. (2005). High sensitivity evanescent
field fiber Bragg grating sensor. Photonics Technology Letters, IEEE, 17(6), 1253-1255.
[4] Fang, X., Liao, C. R., & Wang, D. N. (2010). Femtosecond laser fabricated fiber Bragg grating in microfiber
for refractive index sensing. Optics letters, 35(7), 1007-1009.
[5] Zhang, Y., Lin, B., Tjin, S. C., Zhang, H., Wang, G., Shum, P., & Zhang, X. (2010). Refractive index
sensing based on higher-order mode reflection of a microfiber Bragg grating. Optics express, 18(25), 26345-
26350
[6] Liang, W., Huang, Y., Xu, Y., Lee, R. K., & Yariv, A. (2005). Highly sensitive fiber Bragg grating
refractive index sensors. Applied Physics Letters, 86(15), 151122.
[7] B. J. Luff, J. S. Wilkinson, J. Piehler, U. Hollenbach, J. Ingenhoff, and N. Fabricius (1998). “Integrated
optical Mach-Zehnder biosensor,” J. Lightwave Technol. 16, 583592.
[8] Micro Resist Technology GmbH, EpoCore/EpoClad datasheet. Available at: http://www.microresist.de
[9] Guan, T., Ceyssens, F., & Puers, R. (2013). An EpoClad/EpoCore-based platform for MOEMS fabrication.
Journal of Micromechanics and Microengineering, 23(12), 125005.
[10] Malak, M., Philipoussis, I., Herzig, H. P., & Scharf, T. (2013, September). Polymer based single mode
optical waveguide for spectroscopy applications. In SPIE Optical Engineering+ Applications (pp. 884615-
884615). International Society for Optics and Photonics.
[11] Fiedler, E., Haas, N., & Stieglitz, T. (2014, August). Suitability of SU-8, EpoClad and EpoCore for flexible
waveguides on implantable neural probes. In Engineering in Medicine and Biology Society (EMBC), 2014
36th Annual International Conference of the IEEE (pp. 438-441). IEEE.
[12] Prajzler, V., Nekvindova, P., Hyps, P., & Jerabek, V. (2015). Properties of the Optical Planar Polymer
Waveguides Deposited on Printed Circuit Boards. Radioengineering, 24(2).
... Highly stable Bragg gratings with a maximum length of 5 mm are inscribed into the epoxy waveguides by KrF excimer laser irradiation through a uniform +1/-1 phase mask with a grating period of 990 nm inducing local refractive index modifications in the waveguide core. The optimized laser parameters for phase mask assisted writing of Bragg gratings in EpoCore waveguides are described elsewhere in detail [12][13][14][15] . ...
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