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Sensors and Actuators A 195 (2013) 148– 153

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

jo u r n al homep age: www.elsev ier .com/ locate /sna

eterogeneously integrated optical system for lab-on-a-chip applications

uchuan Song, Jing Xiao, Sang-Woo Seo ∗

epartment of Electrical Engineering, The City College of New York, 160 Convent Avenue, New York, NY 10031, USA

a r t i c l e i n f o

rticle history:eceived 14 November 2012eceived in revised form 1 March 2013ccepted 5 March 2013

a b s t r a c t

This paper presents a heterogeneously integrated optical system for lab-on-a-chip applications. Thesystem consists of microfluidic devices, optical waveguides, and a thin-film GaAs photodetector (PD)integrated onto a silicon dioxide (SiO2) coated silicon (Si) substrate. Thin-film format of III-V semicon-ductor based optical devices is used for efficient integration with other functional devices. An integrated

vailable online 22 March 2013

eywords:icrofluidic deviceseterogeneous integrationhotonic sensorshin-film photonic devices

thin-film PD with optical waveguides is successfully implemented to measure dynamical movementsof micro droplets created in a microfluidic device. The proposed integration method provides an alter-native approach to combine various functional devices in a single platform to create fully integratedlab-on-a-chip systems.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Developments of miniaturized lab-on-a-chip systems perform-ng traditional bio/chemical laboratory measurements [1–3] haveeceived a growing interest in recent years. Miniaturized lab-n-a-chip systems provide distinct advantages over conventionalulk systems. These include shorter analysis time, reduced con-umption of reagents, better control of reaction, higher analysishroughput, multiple signal processing, and portability for fieldr point-of-care measurements. However, typical lab-on-a-chipystems demonstrated in the literature require larger externalnterfaces for signal processing and detection [4–9]. For exam-le, optical detection methods are widely implemented into manyicrofluidic bio/chemical analyses. However, most of the detection

onfigurations require bulk optics to guide light into microflu-dic devices and couple back into external detection systems. Thisonfiguration creates various limitations related to reliability, sen-itivity, complexity, size, and cost. Especially, for some applicationshat have limited access to those larger external resources, it woulde desirable to have signal processing and detection units to beo-integrated in a system.

While there are some reported examples of integrated lab-on--chip systems that combine microfluidic and optical components,

ost of them are limited to use passive optical guiding devices such

s optical fibers and waveguides [10–15] and several attempts havehown a potential of the integration of active optical devices with

∗ Corresponding author. Tel.: +1 2126507889.E-mail address: swseo@ccny.cuny.edu (S.-W. Seo).

924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2013.03.001

microfluidic functional devices [16–18]. However, their integra-tion approaches still suffered from high cost, complex packaging,and difficulty related to dissimilar material integration. There stillremains a need for an efficient integration method that can com-bine different functional devices in a cost effective way withoutsacrificing performance of each functional device.

Recently, we have demonstrated heterogeneous integration ofa III-V semiconductor based PD and a microfluidic device on a Siplatform [19]. The potential of heterogeneously integrated sys-tems lies in the use of individually optimized functional devices.Each functional device does not interfere with other devices dur-ing the integration process. In this paper, we further develop ourapproach to incorporate optical waveguides in a heterogeneouslyintegrated system. This approach significantly reduces the prob-lems associated with optical alignment by integrating optical inputwaveguides and detection with microfluidic devices on the samechip.

The approach that we are demonstrating in this paper is usingseparately optimized devices in optical and microfluidic domainon a Si platform. This promises a fully integrated lab-on-a-chip sys-tem containing Si based signal processing unit, III-V semiconductorbased optical detection unit, and polymer based microfluidic unit.

Each device is optimized based on its preferred technologyand integrated with other devices with a post processing on ahost substrate. For example, a microfluidic device is optimizedthrough its low-cost fabrication process such as a soft lithogra-phy technique. PDs are fabricated on GaAs-based semiconductor

material structures. Thin-film device processing can be extendedto different III-V semiconductor materials to accommodate diversetarget wavelengths that will be implemented in the detectionunits. Thin-film format of the device allows three-dimensional,

F. Song et al. / Sensors and Actuato

Samples or Reagents

Optica l detection

Microfluidic functions

Sign al process ing

Light gu iding

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ig. 1. Typical configuration of a microfluidic lab-on-a-chip employing opticaletection methods.

ayer-by-layer integration of microfluidic, photonic, and electronicunctions. Since PDs are only a few micron thick, their integrationso not interfere with subsequent microfluidic device integrationnd they can be closely integrated within a few tens of micronsith microfluidic channels to enhance optical coupling efficiency

rom an optical measurement (such as absorption, fluorescence,cattered light, and transmitted light), which enhances system sen-itivity. In contrast to that conventional external optical detectionystems require careful optical alignments to microfluidic systemsnd stable environmental condition, the demonstrated integratedystem has optical devices embedded inside the system to provideeliable operation for highly portable applications. Optical align-ent between excitation and detection is done with a polymer

ased optical waveguide. As a demonstration of the feasibility of ourroposed integration method, the integrated system implementshe functionalities of microfluidic droplet detection on a single sub-trate through a network of polymeric microfluidic channels, opti-al waveguides, and an integrated thin-film PD. The basic designnd implementation of the demonstrated integration structure cane applied for various optical sensor systems on a chip. The demon-tration of co-integration of microfluidic and optical devices on a Siubstrate opens the door to fully integrated lab-on-a-chip systems.

. Design and fabrication

.1. Lab-on-a-chip concept

Optical detection methods are widely used in many microflu-dic systems due to their well-established procedure and highensitivity. These include fluorescence detection [20,21], opti-al spectroscopy [22], absorption detection [21], refractive indexetection [23], surface plasmonic detection [24], etc. A typicalicrofluidic system employing optical detection methods requires

ptical light sources, light guiding structures, microfluidic func-ional devices, optical detection, and electrical signal processingnits as shown in Fig. 1. In this paper, we are demonstrating aeterogeneously integrated microfluidic chip with an optical detec-or and optical waveguides. Optical detection can be tailored withptimized III-V semiconductor based PDs targeting specific detec-ion wavelengths, and flexible optical guiding and splitting can bechieved using integrated optical waveguides. Since the proposed

ntegration allows an intimate interface to Si based electronics,otentially, electrical signal processing can be co-integrated withhe detection structure. This will represent an important stepoward a fully integrated lab-on-a-chip system with various fluidic,ptical, and electrical functions.

rs A 195 (2013) 148– 153 149

2.2. Sensor chip design

Fig. 2(a) shows the structure of the overall sensor chip that wedesign. The structure has a separate top microfluidic functionallayer and a bottom optical detection and guiding layer on a Siplatform. The top microfluidic functional layer shown in Fig. 2(b)is designed to generate micro droplets using polydimethylsilox-ane(PDMS) material. A thin-film GaAs PD shown in Fig. 2(c) isseparately fabricated and heterogeneously integrated onto electri-cal contact pads defined on a SiO2/Si substrate. The whole sensorchip is encapsulated by aligning the PDMS microfluidic layer onthe top of the optical detection layer on a Si substrate as shownin Fig. 2(d). The details of the fabrication steps will be followedin the below sections. To make a stable bond between the sensorchip with a PD and PDMS microfluidic channels, we apply a leak-tight reusable sealing method. Fig. 2(e) shows a photograph of thepackaged sensor.

2.3. Fabrication and integration of a sensor chip with a thin-filmPD and optical waveguides

Thin-film GaAs metal–semiconductor–metal (MSM) PDs aregrown, fabricated and optimized on a separate GaAs growth sub-strate, with the removal of the substrate to form thin-film devices.The as-grown material MSM structure is: 1 �m undoped GaAsabsorption layer/200 nm AlAs sacrificial etch layer/GaAs growthsubstrate. The MSM PDs are fabricated with a standard lift-offphotolithography, followed by Ti\Au (25 nm\200 nm) Schottkymetallization. The MSM PD has 47 �m long fingers with 2 �mwidth and 3 �m spacing and a detection area of 50 �m × 50 �m.After metal definition, mesas are formed using H2SO4:H2O2:H2O(1:8:200) down to the AlAs sacrificial layer. Next, Apiezon Waxcovers the MSM PDs to protect them during GaAs substrate sep-aration process using 10% HF. After the substrate is separatedfrom the PDs, the MSM PDs are temporally bonded to a trans-fer diaphragm. The device thickness is around 1 �m. An exampleof a thin-film GaAs MSM PD on a transfer diaphragm is shownin Fig. 2(c). For the proposed integrated system, an integrationsubstrate is prepared using a 3 �m thick thermally grown SiO2layer (an electrical insulation layer) on Si substrate. Electrical con-tact pads (Cr\Au: 25 nm\200 nm) for PD integration are vacuumdeposited and patterned on the substrate. Using the heterogeneousintegration process [25], a thin-film GaAs MSM PD is inverted andintegrated onto electrical contact pads on the substrate. After inte-grating the MSM PD onto the substrate, 10 min annealing at 150 ◦Cis performed to ensure stable metal-to-metal bonding. The topog-raphy of the integrated substrate is relatively flat since the PDthickness is only around 1 �m. Next, we spin and bake a thin layerpolymer (SU-8) on the top of the substrate as an insulation layerbetween the microfluidic channel and the PD. Fig. 2(d) shows aninverted MSM PD integrated on electrical contact pads on a SiO2/Sisubstrate. Since the fingers of the integrated MSM PD face downand the back-side of the MSM PD faces on the microfluidic channel,the inverted MSM PD eliminates conventional finger shadowingeffect for front illumination, which degrades the sensitivity of theconventional MSM PD [26]. In a separate effort, the heterogeneousintegration process has been improved to accommodate automaticalignment and parallel integration utilizing fluidic assisted self-assembly method. The results are reported elsewhere [27,28]. Then,on the top of the substrate with the integrated thin-film PD, we pat-tern optical waveguides using SU-8 polymer as a waveguide coreand air as a waveguide cladding through a standard photolitho-

graphy process. The thickness of the waveguides is around 40 �m.Different widths of optical waveguides (40, 60, and 80 �m) are alsotested for efficient guiding of light. A convex lens structure at thewaveguide output end is also defined to reduce the divergence of

150 F. Song et al. / Sensors and Actuators A 195 (2013) 148– 153

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ig. 2. (a) The structure of the integrated sensor chip. (b) Microfluidic channel junctihin-film GaAs MSM PD integrated onto electrical pads on SiO2/Si substrate and op

he exit light from the waveguide output. The designed lens has5 �m of a curvature radius. Based on theoretical estimation usingn optical beam propagation method (Rsoft BeamProp software29]), the lens structure effectively focuses the exit light from theaveguide. The focal length at � = 532 nm is expected to be around

25 �m for SU-8 and oil interface.

.4. Fabrication of microfluidic channel network and sealingethod

To demonstrate the intimate integration of a microfluidic chan-el network and a thin-film PD, we design and fabricate a functionalicrofluidic device using PDMS polymer material. The structure

s shown in Fig. 2(a) and (b). This microfluidic channel networkas three inlets and one outlet. The central channel of the three

nlet channels is for the target detection liquid sample (which is aye solution) and two outer channels are used as immiscible car-ier liquid (which is oil). The two liquids are then forced to flowhrough a narrow orifice which is located at the downstream of thehree channels. By changing hydrodynamic conditions of the liq-ids (mostly by flow rate change through external syringe pumps),he target sample liquid is focused into a narrow stream whichhen breaks at the orifice. Droplets of the target liquid sample areormed in the outlet channel. The formation of droplets also highlyepends on the wetting condition of the microfluidic channel. Theydrophobic surface of the channel wall is required to ensure thefficient generation of dye droplets in oil carrier liquid.

The fabrication of microfluidic channels is based on the softithography using replica molding. First, a master mold is createdsing a conventional photolithography by patterning a 50 �m thickU-8 layer on a Si wafer. Second, as a releasing layer, hexamethyl-isilazane is spin-coated on top of the master mold. Next, a mixturef PDMS prepolymer and curing agent (Sylgard 182 from Dow Corn-ng) with a weight ratio of 10:1 is poured on the master moldnd cured for 30 min at 125 ◦C. Then, PDMS replica is peeled fromhe master mold and cut into a suitable size. Next, inlet and out-

et ports of the microfluidic channels are punched. Fig. 2(a) showshe structure. Without any surface treatment, the PDMS surface isydrophobic which is beneficial for our droplet generation. Afterhat, PDMS microfluidic channels are aligned and attached on top

droplet generation. (c) A thin-film GaAs MSM PD before integration. (d) An invertedaveguides structure. (e) A photograph of the packaged sensor.

of the integrated inverted thin-film MSM PD. Finally, we apply theplastic plate sealing method to improve the bonding of interfaceand injection.

3. Results and discussion

A schematic diagram outlining the operating principle of thesensor system is shown in Fig. 3. Laser light (at � = 532 nm) froma silica optical fiber is coupled to an integrated polymer wave-guide which is formed by SU-8 as the core and air as the cladding.This coupled light is guided through the waveguide and shines ondroplets formed in the microfluidic channel by using the externalsyringe pumps. Fig. 3(a) and (b) shows the top and cross-sectionalviews of the sensing area with an integrated MSM PD, respec-tively. The droplets are composed of Rhodamine 6G in methanol,which can generate fluorescence light upon excitation from an inte-grated waveguide. Since fluorescence intensity is expected to bestronger for higher excitation intensity, the fluorescence intensityin a droplet will be higher near the waveguide output area as indi-cated in yellow in Fig. 3(a) and (b). As a droplet passes across theintegrated thin-film MSM PD, which is integrated right under themicrofluidic channels network, photocurrent change from the inte-grated MSM PD is measured and recorded as a series of pulsescorresponding to the droplets. The number of droplets across thePD traveling the outlet channel can be determined by counting thenumber of the measured signal pulses, and the width of a dropletcan be estimated with the width of its measured electrical signal.With an integrated filter, which will be our future work, this sys-tem can identify fluorescent tagged particles/cells in microfluidicchannels.

To detect a target liquid sample, the integrated PD is connectedto a low noise current amplifier (Stanford Research Systems SR570)via the electrical contact pads. The output signal from the currentamplifier is acquired in a computer through a National Instrumentdata acquisition card (NI PCI-6220) at 10,000/s sampling rate. The

fluidic inlet and outlet are connected via tubing to syringes loadedwith the liquids. The flow rates of the inlet liquids are controlled byHarvard Syringe Infusion Pump 22 which covers a flow rate rangefrom at �l/h to ml/min. By adjusting the flow rates of the inlet

F. Song et al. / Sensors and Actuators A 195 (2013) 148– 153 151

Fig. 3. (a) Top view and (b) Cross-sectional view of a schematic diagram of thesensing area with an integrated MSM PD. (c) A captured image when light beam iscct

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oupled into a waveguide and shone on a passing droplet formed in the microfluidichannel. (For interpretation of the references to color in text, the reader is referredo the web version of the article.)

iquids, droplets are formed at the junction. In this study, immis-ible liquids used are dye dissolved methanol and mineral oil. Theineral oil is pumped into the side inlet channels, while the central

nlet channel is filled with a solution of Rhodamine 6G (0.1 g per–3 l of methanol). By controlling the flow rates of liquids, the liq-id in the central inlet channel is focused into a narrow stream andhe breakup of the stream is occurred in the narrow orifice region.he droplet characteristics are determined by the density andiscosity of a liquid and interfacial surface tension created by theiquid–channel wall interface. Fig. 3(c) shows an example image of

roplets with a mixture of mineral oil and Rhodamine 6G solutionnd optical excitation through an integrated optical waveguide.

Fig. 4 shows the characteristics of an inverted thin-film MSMD, which is integrated on a SiO2/Si substrate. The resulting dark

Applied Voltage (V)

Fig. 4. Measured dark current and photocurrent of an integrated inverted MSM PD.

current and photocurrent from the PD as a function of bias voltageare measured with a Keithley 2400 source measurement unit.Measured dark current is 0.402 nA at 5 V. Photo responsivity at� = 532 nm of the inverted thin-film PD is calibrated with a New-port calibrated PD (918-SL) and a Newport power meter (1930-C).The measured responsivity of the integrated PD at � = 530 nm is0.09 A/W. The responsivity can be further improved by applying ananti-reflection coating on the surface of the PD or thicker absorbingGaAs layer.

Propagation losses of the fabricated polymer waveguides aremeasured as shown in Fig. 5. The measurement is conducted byusing a microscope camera, which is sensitive at � = 532 nm. Laserlight (at � = 532 nm) is coupled into an optical waveguide with asingle mode fiber. The scattered light from the top of the opticalwaveguide is captured using a CCD camera through a microscope.The measured waveguide loss includes material and structuralloss parameters (such as surface roughness, material loss, modalpropagation loss, etc.) of the waveguide. Assuming that the scat-tered light is proportional to the light in the optical waveguide,the optical propagation loss can be estimated from this measure-ment. Three different widths of optical waveguides are fabricatedand characterized. We obtained 5.8 dB/cm, 4.3 dB/cm and 2.5 dB/cmof the optical waveguide propagation losses of SU-8 waveguideswith 40 �m, 60 �m, and 80 �m width, respectively. Our fabricatedwaveguides are highly multi-mode waveguides for the excitationlight at � = 532 nm. The measured propagation loss is inversely pro-portional to the width of the polymer waveguide, which is alsoobserved and explained in other published results [30–32] espe-cially for the case that the waveguide width is much larger thanthe operating wavelength.

Fig. 6 shows the measured electrical signal from the integratedPD. To better understand the behavior of the measured voltagechange in the integrated PD when a droplet passes across the PD,a series of captured images at different positions of a droplet areincluded as A, B, C, D, and E in Fig. 6. First, a light beam from a laserdiode at � = 532 nm is coupled into a waveguide through an opticalfiber. For a demonstration, we choose an optical waveguide with60 �m width, which gives a good fiber coupling and low opticalpropagation loss. When a droplet passes by the PD in the channelduring the illumination of an excitation laser light, the scattering,absorption, and fluorescence from the droplet affect the measuredphotocurrent of the PD. Since the optical excitation is done through

an integrated waveguide, the measurement can effectively reduceany errors due to an optical misalignment. Without any droplet onthe integrated PD indicated as position A in Fig. 6, some of excitedlight is coupled into the integrated PD and creates photocurrent. In

152 F. Song et al. / Sensors and Actuators A 195 (2013) 148– 153

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Fig. 6. Measured signal versus time from the integrated PD when droplets pass the

ig. 5. Optical loss measurements for SU-8 waveguides with different widths. Thehickness of waveguides is around 40 �m. The widths are 40 �, 60 �m, and 80 �mespectively.

ur measurement, the voltage resulting from the photocurrent inhis case shows around 3.2 V. When a droplet is approaching to thenput of an excitation optical waveguide, the measurement volt-ge is increased to 3.5 V shown in position B. At this position, thexcitation light from the waveguide effectively generates stronguorescence closely on the detection area of the integrated PD andome of scattered excitation light is still coupled on the integratedD. Therefore, the PD signal is increased. Once a droplet fully cov-rs the integrated PD as indicated in position C, the excitation lights more effectively absorbed by the droplet and generates stronguorescence at the side of the droplet, which is a little bit farther

rom the detection area of the integrated PD. Therefore, the mea-ured signal drops to a lower value. When we compare this caseith a larger droplet covering the integrated PD at position E, theeasured PD signal is further reduced due to the separation of the

trong fluorescence further away from the detection area of thentegrated PD. As a droplet moves away from the excitation optical

aveguide and PD indicated as position D in Fig. 6, the fluorescences generated more closely on the detection area of the integratedD and scattered excitation light at the boundary of the droplet isdded to the measured PD signal. Therefore, the measured voltages again increased. By observing the measured voltage, we can count

nd identify relative size of droplets. Integrating thin-film filters onhe top of the PD, the proposed configuration will allow an efficientn-chip optical detection system identifying fluorescent, scatter-ng, and absorption of a target substance in microfluidic devices.

integrated PD in the fabricated microfluidic channel. Captured images are shown indifferent locations of a droplet in the microfluidic channel.

4. Conclusions

A heterogeneously integrated optical system with microfluidicdevices on a Si platform has been demonstrated. The basic designand implementation of the demonstrated integration structure canbe applied for various optical sensor systems on a chip. Thin-filmformat of optical device structure and layer-by-layer integrationmethod may provide an alternative approach to achieve fully inte-grated lab-on-a-chip systems, which combine different functionaldevices in optical, microfluidic, and electrical domains.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.sna.2013.03.001.

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