Polymeric optofluidic Fabry–Perot sensor by directlaser machining and hot embossing
Jing Wu,1,2 Daniel Day,1,* and Min Gu1
1Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology,Hawthorn, PO Box 218, Victoria 3122, Australia
2Institute of Advanced Nanophotonics, State Key Laboratory of Modern Optical Instrumentation,Zhejiang University, Hangzhou 310027, China
In the development of microfluidic technology towardbiomedical and clinical applications, disposable mi-crofluidics has generated a lot of interest for the pur-pose of reducing possible cross contamination andmisdiagnosis. These devices usually require low costof fabrication, high volume production, good reprodu-cibility, and versatility in design for a wide spectrumof applications. As polymer materials have becamemore prevalent as cost-efficient alternatives to glass,more polymer-based techniques (such as hot emboss-ing [1] and microinjection molding [2]) have beenemployed in the past decade more than the conven-tional glass-based micro-electro-mechanical-systems(MEMS) techniques. Among these techniques, hotembossing is typically used when high precisionand quality are needed and is a simple and economic-al process with high scalability and good repeatabil-ity. Considering the investment and fabrication cost,
conventional photolithography is usually preferredto laser-based lithography techniques for the masterstamp fabrication in hot embossing. The downsidesinclude the requirements for a multistep processand chemical posttreatment, which also affect theproduction rate. Another disadvantage is the shortlife cycle of the master during the embossing process,which is, in some cases less than five times [3]. Moreimportantly, both photolithography and laser-basedlithography lack the flexibility in design modifica-tion. For applications such as DNA sequencing orpoint-of-care diagnostics, the microdevices also re-quire comparably large surface areas or long reactionlength for high-throughput data collection to ensureaccurate diagnosis. As a result, the ideal fabricationmethod should have the ability to deal with largesurface areas and be flexible to enable designmodifications.
In this paper we present a new method of cost-efficient and convenient fabrication of a polymeric-based microfluidic optical sensor by combining directlaser machining and hot embossing. The applicationof lasers has been proven to be important in MEMS
both in system operations like particle manipulation[4–6] and sensing [7–9] or microdevice fabrication[10–14]. In our experiment, the fast, single step,automated process of laser machining provides anappealing solution to several major disadvantagesof conventional hot embossing by increasing the pro-duction rate, improving the reproducibility, reducingthe cost, enabling a robust stamp with longer life,providing large working areas and flexibility in de-sign modification and customization. A proof of con-cept F–P sensor was integrated with a microfluidicchannel in PMMA for optofluidic sensing.
2. Experiment
A. Device Design
As shown in Fig. 1 (the insets), a microfluidic devicewith an F–P cavity incorporated within the micro-channel (shown as the square regions in the middleof the straight microchannel) was produced. The toplayer had holes drilled at the inlet and outlet forconnectors. The fluid was pumped into the micro-channel, which filled the F–P cavity; the refractiveindex was characterized by the transmission signalthrough the F–P cavity. PMMA was selected as thesensor substrate for several reasons in addition tothe low cost advantage of the polymer material.The commercial availability and fabrication simpli-city (low transition temperature) and biological com-patibility make it ideal for laboratorial/industrialmass production.
B. Device Fabrication
The device was fabricated by four steps as illustratedin Fig. 2. First, a 10∶1 ratio of the poly(dimethyl si-loxane) (PDMS) prepolymer and its curing agent(Sylgard 184, Dow Corning) were mixed and spin-coated at a speed of 1500 RPM onto a silicon wafer
(Cemat Silicon S. A.). The wafer was then placedon a hotplate and heated at 75 °C for 20 min. Whenthe PDMS was cured, the master stamp of the chan-nel structure was cut out of the PDMS layer by a CO2laser (Universal Laser Systems). The automated la-ser cutting process takes a few minutes and was con-trolled by a computer. After the cutting process, thePDMS residue was peeled off from the wafer, leavingan inverse pattern of the designed microfluidic de-vice (the master stamp). The wafer was cleaned indistilled water in an ultrasonic bath for 10 min, fol-lowed by several methanol rinses in order to removeany residue.
In the second step, a manual embossing systemwas placed onto a hotplate and was preheated to150 °C for 1 h to reach a uniform temperature. Alayer of PMMA was sandwiched between the masterstamp and a piece of glass and inserted into theembossing system. The purpose of the additionalglass layer is to ensure a smooth molding surfacewhen removing the whole structure from the emboss-ing system. The embossing system comprises twowater-cooled platens that are preheated while in con-tact via a hotplate. A calibrated torque wrench isused to apply pressure on the stamp and substratethat are placed between the two platens, by tighten-ing a screw that presses the top platen against thebottom platen. The pattern on the master stamp wasembossed in the PMMA substrate under a pressurecreated by 10Nm torque from a torque wrench, at atemperature of 150 °C for 10 to 15 min, before beingcooled down to room temperature by circulatingwater for 5 min. Once cooled, the sample was re-moved from the embossing system and the PMMAsubstrate was carefully peeled from the masterstamp.
Fig. 1. (Color online) Schematic illustration of the experimental system. The insets show the details of the optofluidic sensor that wasmade up of an F–P cavity (demonstrated in the enlarged top inset) integrated within a PMMA microfluidic channel.
In the third step, in order to improve the cavityquality and to explore for potential sensitivityenhancement, the channel surface in the F–P cavityis coated with thin gold film. The PMMA substratewas put under a shadow mask to coat a gold layeronto the sensing region in the microchannel. Anotherblank piece of PMMAwas also coated with gold to thesame thickness using the shadow mask. Finally,holes were drilled by the CO2 laser at the inletand outlet positions.
In the forth step, the two pieces of PMMA werealigned manually under an inspection microscopewith 4× magnification to achieve a precision withina few tens of micrometers, and a drop of supergluewas placed on each of the four edges to hold thepieces in place during the bonding. The alignedPMMA layers were then placed into the embossingsystem under 10Nm pressure at a temperature of75 °C for 30 min for thermal bonding. Connectorswere then glued to the inlet and outlet openingson the device and connected to a syringe pump viasilicon tubing.
3. Result and Discussion
A. Fabry–Perot Sensing Mechanism
The Fabry–Perot cavity, also referred to as a Fabry–Perot interferometer or an etalon, shown in Fig. 3(a),consists of two parallel flat partially transparentmirrors (two gold coated PMMA surfaces) separatedby a fixed distance with an optical-transparent bulkmaterial in between them. The broadband light inci-dent upon it undergoes multiple reflections betweenthe mirrors, resulting in transmission peaks in theoutput spectrum, which correspond to resonancesof the F–P cavity [shown in Fig. 3(a) inset]. Thewavelengths of the peaks are determined by thecondition
nd ¼ mλ2
; ðm ¼ 1; 2; 3;…Þ; ð1Þ
where d is the distance between two reflective sur-faces, n is the refractive index of the bulk materialin between the mirrors, and λ is the wavelength ofthe light in a vacuum. In our experiment, the fluidchannel runs through the F–P cavity and fills thecavity with fluid of different refractive indices. Theresonance peaks from the F–P cavity are shiftedby the refractive index of the fluid inside the channel.By monitoring the shift of the F–P resonance peakson a spectrometer, the fluid refractive index can bedetermined.
B. System Optimization and Calibration
After devices with different thickness of gold coating(20 and 40nm) were fabricated, they were mountedonto a translation stage and characterized by a spec-trometer (the experimental system as shown inFig. 1). The spectral resolution of the spectrometerin the experiment was 0:2nm. Objective lenses of dif-ferent numerical apertures (NA) were used to opti-mize the system. The transmission spectra fromthe F–P cavities were measured with no liquid insidethe microfluidic channels. The F–P resonance peakswere observed using 0.24, 0.5, and 0:70NA objectivelenses for both samples. Figure 4 shows the calcu-lated finesse (a) and visibility (b) of the cavities fromthe spectral results. Equations (1) and (2) are theequations for finesse and visibility calculations.Comparison between the two figures demonstratesthat the 20 and 40nm samples have higher transmis-sivity but lower visibility with lower NA lenses. Thelower NA lenses have higher transmission efficiencyacross the broad wavelength range used in the illu-mination light resulting in the higher transmissionsignal for the F–P resonances. Taking into considera-tion the finesse and visibility, a higher resolutionF–P signal could be collected when using the higher
Fig. 2. Fabrication procedure for the F–P optofluidic sensor. (a) Spin coating of a PDMS layer onto a silicon wafer, (b) CO2 laser cutting ofthe device template and removal of the unwanted PDMS, (c) embossing of the PDMS template into a PMMA substrate creating the masterstamp, (d) gold coating of the sensing region using a shadow mask, and (e) alignment and bonding of the optofluidic sensor.
NA lenses. The higher NA objective lenses enable thecapture of higher spatial frequencies transmittedthrough the F–P, which could result from diffusereflections in the sensor caused by: (1) the increasedsurface roughness of the PMMA/gold surface com-pared with high quality mirrors, (2) debris/contami-nation in the sensor from the fabrication process, or(3) low quality and nonuniform thickness gold coat-ing resulting in pinholing. The finesse and visibilityof the 40nm sample were higher than the 20nm sam-ple with all the objective lenses used.
Measurements with the 0:5NA objective lens haveshown better finesse and visibility compared to lowerNA cases [presented in Figs. 3(b) and 3(c)], whilethere was a slight drop in the visibility in the mea-surements with the 0:7NA objective lens. In the fol-lowing experiments, the 0:5NA objective lens waschosen to work with the optofluidic devices for refrac-tive index detection. The finesse, F, of the resonantcavity is given by
F ¼ λ22ndδλ ; ð2Þ
Fig. 3. (Color online) (a) Schematic illustration of the principle of an F–P cavity. The output spectrum of the transmission signal shows aseries of peaks with respect to different wavelengths. (b) The transmission spectra of the samples with different gold coating thickness (20,30, 30, and 40nm) using objective lenses of (b) NA ¼ 0:14 and (c) NA ¼ 0:5.
Fig. 4. Finesse (a) and visibility (b) calculated from the spectra collected with different NA objective lenses of the sensors with 20 and40nm gold coating.
where δλ is the FWHM of the resonant peak. Thevisibility, V, of the resonant cavity is
V ¼ Imax − Imin
Imax þ Imin; ð3Þ
where Imax and Imin are the maximum and minimumintensities of the resonances in the transmissionspectra.
The optofluidic sensor with 20nm gold coating wasselected to be theoretically studied and experi-mentally tested for the fabrication calibration. Theexperimental transmission signal from a device of20nm gold coating was compared with theoreticalcalculated spectra (normalized). As shown inFig. 5(a), for both theoretical and experimental re-sults, there exist five peaks from 625 to 660nmwavelength range. The distance between adjacentpeaks is about 6:5nm. As shown in the figure, thevariation between theoretical peaks to experimentalpeaks is within 2nm within the measured wave-length range, with both peaks almost overlappedwith each other at around 650nm. The experimentresult agrees well with the theoretical calculation
for the microfluidic device with 20nm gold coating;however, discrepancies between the “idealistic”theoretical case and the experimentally measuredF–P data are likely to result from any of the follow-ing: (1) reduced quality or nonuniform thickness ofthe gold coated surfaces resulting a difference be-tween the actual and theoretical values for reflection,transmission, and absorption of the gold layer; (2) at-mospheric disturbances, such as pressure, tempera-ture, and/or humidity; and (3) the temperaturedependence of the properties of the PMMA substrateand/or gold layer as a result of heating due to theillumination light.
C. Fabry–Perot Optofluidic Sensing
After sensor fabrication and calibration, detection offluid refractive index was demonstrated by the opto-fluidic sensor. Fluids of different refractive indicesranging from 1.33303 up to 1.43087 were made fromglycerine-water solutions of different concentrations(as shown in Table 1) based on the technical instruc-tions for OPTIM glycerine from Dow Chemical Com-pany [15]. The refractive index of the glycerine-watersolutions was confirmed via comparison with cali-brated refractive index oils (Cargille Labs) using arefractometer. Devices containing F–P cavities with20 and 40nm gold coating were used in the measure-ments. The solutions of different refractive indiceswere spectroscopically characterized by their trans-mission signal through the F–P cavities in the micro-channels. A basement measurement was takenbeforehand with the empty microchannels (n ¼ 1)before any solution was pumped into the microchan-nels. Between each measurement the microchannelswere rinsed with deionised water. Figure 5(b) showsan example of the modulation of refractive index overthe resonance shifts. The two arrows indicate thatthe corresponding F–P resonance peak was shiftedby the refractive index change.When the fluid refrac-tive index increases from 1.33303 to 1.34729, the F–Presonance peak shifted toward longer wavelengths,from 645 to 652nm in the transmission spectra.
Figure 6 presents the position of the F–P reso-nance peak positions from spectroscopic measure-ments determined by the fluid refractive indices.The positions of the F–P resonance peaks shift to-ward longer wavelengths as the refractive index ofthe fluid is increased (from 1.33303 to 1.43087).
Fig. 5. (a) Experimental and theoretical calculation of the devicewith the 20nm gold coating. (b) Measured transmission spectra ofthe 40nm gold coated device using fluids of two different refractiveindices (n ¼ 1:33303 and 1.34729).
Table 1. Refractive Index of Glycerine-WaterSolutions (20 °C)
The positions of the F–P resonances were recordedand plotted as a function of the refractive index. Itis observed that the resonance peak shift has a linearrelationship to the fluid refractive index in the micro-channel. Different results were observed by the de-vices of 20 and 40nm thicknesses of gold coatings.The explanation relies on the phase change of lightby metal absorption. In a simple F–P model, usuallythe mirrors are assumed to be perfect (highly re-flected and no absorption). But in real case, the metalfilms coated on the mirror surfaces DO absorb andcause phase changes to the light propagating in theF–P cavity, thus, a variation in the film thicknesswould have an impact on the sensing result basedon interferometry. Additionally, as the mechanicaland optical properties of the polymer and gold coat-ings are temperature dependent, variations in thespectral measurements could occur. To reduce thetemperature effect the devices are mounted onto athick metal plate on the translation stage that actsas a heat reservoir.
The sensor sensitivity is defined as the detectablechange in fluid refractive index as a function of theF–P resonance shift (Δn=Δλ) in the spectroscopicmeasurements. In our experiments, a sensitivity of469nm=RIU is achieved using the optofluidic sensorwith the 20nm gold coating. This sensitivity enablesthe limit of detection of a change in the refractive in-dex of 2:13 × 10−3 per 1nm shift in the F–P resonancespectrum. As the FWHM of the resonance peaks is ofthe order of 2:5nm, a 1nm shift is easily detectable.The minimum detectable wavelength change is de-pendent on the resolution of the spectrometer andthe finesse of the cavity. By improving the resolvingpower of the spectrometer or increasing the quality ofthe F–P, the resolution could be further improved.During typical biological or chemical sensing, the an-ticipated shift in the resonance spectrum is likely tobe less than the peak-to-peak distance of the reso-nances, which is approximately 10nm. However,should the shift become larger than 2π, there is
the possibility that ambiguities in the measurementmay occur. To address this, the device could be mod-ified to incorporate a multicavity F–P or simulta-neous measurement of a separate reference cavity.
4. Conclusion
In summary, a new method has been presented thatintegrates a microfluidic device and an optofluidicsensor by combining direct laser machining andhot embossing processes. A gold coated F–P optoflui-dic sensor was fabricated in PMMA for fluid refrac-tive index detection. The experimental performanceof the sensor was confirmed by theoretical cal-culation, and a sensitivity of 469nm=RIU wasachieved, which equates to a limit of detection of2:13 × 10−3 RIU. This technique ensures the fabrica-tion of hot embossing masters at an increased pro-duction rate, with a better flexibility in designmodification and a potentially large surface area pat-terning ability. It is suitable for the research and de-velopment of disposable microfluidic chips and holdsgreat potential of commercialization with efficienthigh-volume production. This fabrication techniquecan be applied to the production of more complextwo-dimensional patterns, rather than the singlemicrochannel demonstrated in this paper, such asthe fabrication of straight or curved multichanneldevices. There is also the possibility of producingthree-dimensional devices based on layering of thereplicated PDMS layers from different PMMA mas-ter stamps. With further refinements, this techniquecould extend to the integration of optical, elec-trical, and fluidic components to various polymermicroplatforms.
The authors would like to acknowledge SwinburneUniversity of Technology through the SwinburneUniversity Postgraduate Award and the CooperativeResearch Centre for Polymers. Zhejiang Universityis J. Wu’s current address and the work in this paperwas fully completed at Swinburne University ofTechnology.
References1. H. Becker and U. Heim, “Hot embossing as a method for the
fabrication of polymer high aspect ratio structures,” Sens.Actuators A, Phys. 83, 130–135 (2000).
2. R.-D. Chien, “Micromolding of biochip devices designed withmicrochannels,” Sens. Actuators A, Phys. 128, 238–247(2006).
3. M. B. Esch, S. Kapur, G. Irizarry, and V. Genova, “Influence ofmaster fabrication techniques on the characteristics of em-bossed microfluidic channels,” Lab Chip 3, 121–127 (2003).
4. M. P. MacDonald, G. C. Spalding, and K. Dholakia, “Micro-fluidic sorting in an optical lattice,” Nature 426, 421–424(2003).
5. J. Enger, M. Goksor, K. Ramser, P. Hagberg, and D. Hanstorp,“Optical tweezers applied to a microfluidic system,” Lab Chip4, 196–220 (2004).
6. M. Ozkan, M. Wang, C. Ozkan, R. Flynn, and S. Esener,“Optical manipulation of objects and biological cells in micro-fluidic devices,” Biomed. Microdevices 5, 61–67 (2003).
7. E. Eriksson, J. Scrimgeour, J. Enger, and M. Goksor, “Holo-graphic optical tweezers combined with a microfluidic device
Fig. 6. Measured F–P resonant peak positions for two samples(with 20 and 40nm gold coatings) using glycerine-water solutionsof different refractive indices.
for exposing cells to fast environmental changes,” Proc. SPIE6592, 65920P (2007).
8. H. Mushfique, J. Leach, H. Yin, R. Leonardo, M. Padgett, andJ. Cooper, “3Dmapping of microfluidic flow in laboratory-on-a-chip structures using optical tweezers,” Anal. Chem. 80,4237–4240 (2008).
9. J. Wu, D. Day, and M. Gu, “Shear stress mapping in microflui-dic devices by optical tweezers,” Opt. Express 18, 7611–7616(2010).
10. A. Marcinkevičius, S. Juodkazis, M. Watanabe, M. Miwa, S.Matsuo, H. Misawa, and J. Nishii, “Femtosecond laser-assisted three-dimensional microfabrication in silica,” Opt.Lett. 26, 277–279 (2001).
11. Y. Cheng, K. Sugioka, and K. Midorikawa, “Microfluidic laserembedded in glass by three-dimensional femtosecond lasermicroprocessing,” Opt. Lett. 29, 2007–2009 (2004).
12. Y. Cheng, K. Sugioka, K. Midorikawa, M. Masuda, K. Toyoda,M. Kawachi, and K. Shihoyama, “Three-dimensional micro-optical components embedded in photosensitive glass by afemtosecond laser,” Opt. Lett. 28, 1144–1146 (2003).
13. F. He, Y. Cheng, L.-L. Qiao, C. Wang, Z.-Z. Xu, K. Sugioka, andK. Midorikawa, “Two-photon fluorescence excitation with amicrolens fabricated on the fused silica chip by femto-second laser micromachining,” Appl. Phys. Lett. 96, 041108(2010).
14. J. Wu, D. Day, and M. Gu, “A microfluidic refractive indexsensor based on an integrated three-dimensional photoniccrystal,” Appl. Phys. Lett. 92, 071108 (2008).
