Integration of microfluidics with grating coupled silicon photonic sensors by one-step combined photopatterning and molding of OSTE

We present a novel integration method for packaging silicon photonic sensors with polymer microfluidics, designed to be suitable for wafer-level production methods. The method addresses the previously unmet manufacturing challenges of matching the microfluidic footprint area to that of the photonics, and of robust bonding of microfluidic layers to biofunctionalized surfaces. We demonstrate the fabrication, in a single step, of a microfluidic layer in the recently introduced OSTE polymer, and the subsequent unassisted dry bonding of the microfluidic layer to a grating coupled silicon photonic ring resonator sensor chip. The microfluidic layer features photopatterned through holes (vias) for optical fiber probing and fluid connections, as well as molded microchannels and tube connectors, and is manufactured and subsequently bonded to a silicon sensor chip in less than 10 minutes. Combining this new microfluidic packaging method with photonic waveguide surface gratings for light coupling allows matching the size scale of microfluidics to that of current silicon photonic biosensors. To demonstrate the new method, we performed successful refractive index measurements of liquid ethanol and methanol samples, using the fabricated device. The minimum required sample volume for refractive index measurement is below one nanoliter. © 2013 Optical Society of America OCIS codes: (130.6010) Sensors; (130.6622) Subsystem integration and techniques; (160.5470) Polymers. References and links 1. Y. S. Sun, J. P. Landry, Y. Y. Fei, X. D. Zhu, J. T. Luo, X. B. Wang, and K. S. Lam, “Effect of fluorescently labeling protein probes on kinetics of protein-ligand reactions.” Langmuir 24, 13399–13405 (2008). 2. T. Kodadek, “Protein microarrays: prospects and problems,” Chemistry & Biology 8, 105–115 (2001). 3. G. Zheng, F. Patolsky, Y. Cui, W. U. Wang, and C. M. 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Introduction
The combination of a biological recognition element with a physical transducer makes biosensing a powerful tool for biological and medical analysis.Although widely used, label-based biosensing has several limitations, such as the risk of interference with the reaction under study [1], labeling heterogeneity [2], and a lack of real-time kinetics measurements for quantifying reaction rates.Label-free biosensing addresses these limitations by providing real-time physically quantifiable information without labels.
Silicon based sensors have recently shown utility for important biological and chemical labelfree sensing applications.Silicon nanowire sensors [3], and transistor based next generation DNA sequencing devices [4], are examples of such sensors showing low detection limits, high scalability, simple fabrication, and mass production capability in dense arrays at low cost.
Silicon photonic waveguide based biosensors share these characteristics.These sensors detect refractive index changes within their evanescent field upon biomolecule binding with high sensitivity, and can be fabricated in compact arrays using standard lithography techniques.Several such biosensors have been reported: Ring resonators [5] have been shown to be scalable to large sensing arrays [6], and yield volume refractive index sensitivities up to 70 nm/RIU [5].Spiral-path interferometers have shown sensitivity of 163 nm/RIU [7].Moreover, the use of surface grating couplers enables the probing of light everywhere on chip [8].Grating couplers can be added to the sensing circuits without an additional lithography step, yielding a one-step fabrication process for grating coupled photonic sensor chips on SOI substrates [9].These reports show that silicon photonic label-free biosensing has the potential to become a highly scalable and low cost sensing technique.However, for chemical and biological sensing, the integration of liquid handling systems consumes a considerably larger wafer area than that of the photonic footprint, and thus the scaling benefit is lost.
Molded polydimethylsiloxane (PDMS) constitutes the current academic solution for silicon photonic biosensor microfluidics [10], but drawbacks such as large wafer area consumption, long curing times, adsorption of small biomolecules into the PDMS, and lack of bonding techniques compatible with surface biofunctionalization, make industrial application questionable [10].These limitations are apparent in [11], in which stamping with an epoxy glue for bonding results in channel clogging.Moreover, PDMS based soft-lithography molding of vias (through holes) is hampered by squeeze-film formation, thus necessitating a second low resolution via fabrication step by hole punching [12].
To address these limitations, the Off-Stoichiometry Thiol-Ene (OSTE) polymer was introduced [13] and microfluidic integration on silicon at wafer level demonstrated [14].More recently, by a UV-initiated thiol-ene reaction, vias have been photopatterned within a few seconds [15], and a low temperature dry-bonding technique enabled by a room-temperature thiol-isocyanate click reaction [16], permitting bonding to biofunctionalized silicon [17] and gold [18].Moreover, we showed integration of OSTE onto a silicon photonic interferometer sensor, by coupling light to the chip and verifying the interference in the output power [19].
Here, we demonstrate the integration of OSTE on silicon photonics by performing refractive index measurements with a silicon photonic ring resonator chip.We present a one-step integration of microfluidics onto silicon photonic sensors able to match the size scale of the liquid handling system with that of the silicon photonics, by using a self-bonding OSTE polymer microfluidic layer that is structured using a combined photolithography and micromolding process.Figure 1 illustrates the process: 1) By a photolithography of OSTE, we open optical vias with the same size scale as optical fibers above grating couplers, enabling light coupling anywhere on the sensing chip.2) For rapid prototyping, we mold tube connectors directly into the microfluidic layer.These tube connectors can be substituted by compact manifold connectors, if needed, for a higher integration density at wafer level.3) We dry bond the microfluidic layer to patterned silicon surfaces by click chemistry.

Fabrication
Figure 2 shows a schematic cross section of the microfluidic layer fabrication and bonding to the silicon chip.We used a 70% thiol excess OSTE polymer (OSTE-70) [13] for the microfluidic layer.Along with the general OSTE properties discussed above, OSTE-70 provides an excess  of thiol that permits a strong bond to a silanized silicon surface [14].
We poured the OSTE-70 prepolymer into a PDMS mold defining the chip outline and fluid connectors (Fig. 2(a)), and then sandwiched the polymer between the PDMS and a glass mask.The glass mask contains chromium patterns that define vias, and 50 µm thick SU8 reliefs that define microchannels, and thus acts simultaneously as a mold and a photolithography mask (Fig. 2 eye to the connector molds in the PDMS mold.After a 13 s UV exposure at 13 mW/cm 2 , using a collimated NUV lightsource with wavelength peaks at 365, 405, and 436 nm (OAI, USA), the OSTE layer was released and subsequently developed in butyl acetate for 30 s (Fig. 2(c)).
During development, unexposed prepolymer dissolves, leaving open vias for optical and fluidic connections, as seen in the photographs in Fig. 3.We functionalized the surface of the silicon photonic chip by dipping it for 10 min into 5% (by mass) of 3-(triethoxysilyl)propyl isocyanate (Sigma-Aldrich) in toluene.We dried the chips for 10 min at 70 • C, and aligned the microfluidic layer to the chips using a long working distance microscope objective and a micropositioning stage, followed by bonding.The bonding takes advantage of the excess thiol functional groups on the OSTE surface that covalently bond the fluidic layer to the silicon chip, by a click reaction [16] during 10 min at 70 • C (Fig. 2(d)).We have already reported a reduction of the bond time down to 5 min and temperature down to 37 • C using this method [14].As illustrated in Figs.3(a) and 3(b), the optical vias are separated by only 75 µm wide bond areas from the 100 µm wide microfluidic channel.This tight spacing allows grating coupler separation of only 500 µm, thus reducing the required wafer footprint of the silicon photonics.Due to the limited accuracy of punching, punched vias in PDMS are commonly separated by close to 1 mm [20].PDMS via to channel spacing down to 200 µm has been recently shown using a polymerization inhibition technique [21], but this method leaves the PDMS surface unpolymerized, and has only been used to perforate thin membranes and not combined with connector molding, as shown here.
A particular feature of the OSTE-70 is its low glass transition temperature of 37 • C [14].This allows us to safely unbond the OSTE layer by raising the temperature to 50 • C and pealing the layer off the silicon surface.This permits reuse of the silicon chips.For permanent bonding, an OSTE formulation with a higher glass transition temperature can be used [13].

Optical measurements and results
To demonstrate the usefulness of the integrated chip for refractive index based sensing, we measured the resonance wavelength shifts of a silicon photonic ring resonator sensor (40 µm diameter [9]), upon the injection of a dilution series of ethanol and methanol in water.The measurement setup is shown in Fig. 3(c wide and 50 µm high microchannels above the 40 µm diameter ring, the minimum required sample volume for refractive index measurement is below one nanoliter.After alignment of the optical fibers with the grating couplers, light emitted from a laser source is coupled into the chip, and thus the resonance peaks can be observed with a wavelength domain component analyzer (Agilent Technologies 86082A).
We mixed ethanol and methanol in DI water at different concentrations, yielding six solutions with a range of refractive indexes from 1.333 to 1.358 RIU [22].We flowed the samples through the chip at a flow rate of 3.5 µL/min, with a DI water flush between each of them.The resonance shifts observed for the ethanol solutions are shown in Fig. 4(a).We observe a red shift from 1552.8 nm to 1554.1 nm, and an increase in Q from 17 200 to 17 300, as the ethanol concentration increases.The increase in Q is caused by the reduced infrared absorption of the water in the sample at higher ethanol concentration [23].Figure 4(b) shows the measured resonance wavelength shift for all the samples, as a function of refractive index.The slopes of the linear fits for ethanol and methanol coincide, yielding a volume refractive index sensitivity of 50.5 nm/RIU, in good agreement with previously reported similar devices [5].Although the slopes for both solutes are consistent, we observe a slight offset, most likely due to a difference in temperature around 10 • C between the two solution series.

Conclusions
We have presented a novel one-step microfluidic integration technique for silicon photonic waveguide based sensors with surface grating couplers.Using the lithographic capability of the OSTE polymer, we combine lithography and molding to enable the integration of photolithographed vias for optical probing and microfluidic channels in a single step.Moreover, dry bonding of OSTE to silanized silicon photonic chips is compatible with biofunctionalized surfaces.We show leakage free bonding to patterned silicon, together with refractive index measurements using a grating coupled ring resonator sensor chip, fabricated by these means.By using this process, extendable to the wafer scale, we can match the size scale of the OSTE based microfluidics to the current size scale of label-free photonic biosensors.

Fig. 1 .
Fig.1.The integration scheme: The top mold is a glass mask combining chromium patterns for photolithography of vias with SU8 reliefs for microchannel molding.The insets show photographs of the reliefs and via patterns on the mask.The bottom PDMS mold defines the chip outline and the fluidic connectors.After UV curing, development, and dry-bonding of the microfluidic layer to the photonic chip, the integrated chip is ready for measurements.

Fig. 2 .
Fig. 2. A schematic cross-section of the fabrication of the OSTE microfluidic layer and its bonding to a photonic silicon chip: (a) OSTE-70 is poured into a PDMS mold.(b) A UV cure through a glass mask polymerizes exposed parts by a thiol-ene reaction.(c) The fluidic and optical vias are developed in butyl acetate.(d) A silanized silicon photonic chip is then aligned and bonded to the fluidic layer and cured for 10 min at 70 • C.

Fig. 4 .
Fig. 4. (a) Resonance spectra for different concentrations of ethanol in water.The red shift increases as the ethanol concentration increases.(b) The measured resonance wavelength shift shows a linear dependence on the refractive index of the injected solution.