Liquid phase direct laser printing of polymers for chemical sensing applicationsChristos Boutopoulos, Vasiliki Tsouti, Dimitrios Goustouridis, Stavros Chatzandroulis, and Ioanna Zergioti

Citation: Applied Physics Letters 93, 191109 (2008); doi: 10.1063/1.3025596 View online: http://dx.doi.org/10.1063/1.3025596 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/93/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The synthesis of CuO nanoleaves, structural characterization, and their glucose sensing application Appl. Phys. Lett. 102, 103701 (2013); 10.1063/1.4795135 Laser Induced Chemical Liquid Phase Deposition (LCLD) AIP Conf. Proc. 1472, 148 (2012); 10.1063/1.4748082 Tunable emission in surface passivated Mn-ZnS nanophosphors and its application for Glucose sensing AIP Advances 2, 012183 (2012); 10.1063/1.3698310 Integrated carbon fiber electrodes within hollow polymer microneedles for transdermal electrochemicalsensing Biomicrofluidics 5, 013415 (2011); 10.1063/1.3569945 Polymer functionalized piezoelectric-FET as humidity/chemical nanosensors Appl. Phys. Lett. 90, 262107 (2007); 10.1063/1.2748097

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Liquid phase direct laser printing of polymers for chemical sensingapplications

Christos Boutopoulos,1 Vasiliki Tsouti,1,2 Dimitrios Goustouridis,2 Stavros Chatzandroulis,2

and Ioanna Zergioti1,a�

1Physics Department, National Technical University of Athens, Iroon Polytehneiou 9, 15780 Zografou,Athens, Greece2NCSR Demokritos, Institute of Microelectronics, Aghia Paraskevi 15310, Greece

�Received 4 September 2008; accepted 19 October 2008; published online 13 November 2008�

This letter demonstrates the direct laser printing of polymers on capacitive micromechanical arraysfor the realization of a chemical sensor. Each sensor of a single chip array is composed of a thin Simembrane covered by a chemically sensitive polymer layer by means of a direct laser printingtechnique. We present the high spatial resolution deposition of three different sensitive polymermaterials by the liquid phase laser induced forward transfer process. We also show that the optimumsensitivity of the sensors can be achieved by varying the percentage of the coverage of the sensors’membranes with the polymer. © 2008 American Institute of Physics. �DOI: 10.1063/1.3025596�

The fabrication of chemical sensing devices has pre-sented growing interest in recent years as these devices areadvanced tools for environmental monitoring and the foodindustry. Chemical detection based on Si/polymer bimorphshas been demonstrated for various types of sensors such ascantilevers,1 capacitive sensors,2 and ultrasonic transducers.3

One of the most important challenges for Si/polymer bimor-phs fabrication is the deposition of chemical sensing mate-rial. In particular, for multianalyte detection �e-nose�, differ-ent polymer materials may be selectively deposited ondifferent spots of sensor arrays. The conventional ink-jet4

printing method presents several limitations regarding thespatial resolution, solvent properties, polymer molecularweight, and concentration, while alternative printing meth-ods based on polymer lithography5 are time consuming andexpensive.

Laser forward transfer techniques have become impor-tant for high resolution and direct printing of a wide range ofmaterials.6,7 The dry printing of polymer materials by meansof laser induced forward transfer �LIFT� has been demon-strated using nanosecond pulses, for the fabrication of or-ganic light-emitting diodes8 and organic electronics.9 It hasalso been shown that using femtosecond laser pulses thetechnique can achieve submicron resolution for printing ofmetals.10 Here, we present the use of the liquid phase LIFTtechnique for the deposition of polymer materials onto flatsubstrates and onto the sensing sites of a two-dimentional�2D� capacitive sensor array, in order to fabricate a Si/polymer bimorph chemical sensor. The technique is direct,maskless, and offers high spatial resolution printing of awide range of polymers. Moreover, compared to conven-tional ink-jet printing, it is not restricted by the polymer so-lution properties.

In this work, three different polymers were printed onstandard flat substrates and on sensor substrates by means ofliquid phase LIFT. The flat substrates consisted of low tem-perature oxide �LTO� on silicon �100�, while the sensor sub-strates were arrays of LTO on silicon micromechanical ca-

pacitive sensors. The LTO layer has passivation propertiesand is commonly used for electrical isolation and protectionof sensor structures from corrosive environments. Further-more, the hydrophilic LTO top layer of the substrates enablesthe deposition of homogenous polymer droplets. The poly-mer solutions were prepared in the following concentrations:�a� 2% w/w poly �2-hydroxyethyl methacrylate� �PHEMA�in etlyl-lactete, �b� 10% w/w polyacrylic acid �PAA� in 3:1H2O:glycerol, and �c� 10% w/w poly �4-vinylpyridine� in3:1 H2O:glycerol �P4VP�.

The target material was prepared by spin coating thepolymer solutions onto quartz plates, coated with a 40 nmthick Cr layer, to attain a uniform layer thickness. The Crintermediate layer plays a crucial role in the transfer processas it acts as a radiation absorber.6 It prevents the direct ex-posure of the polymer solutions to the laser irradiation andthus ensures that the transferred polymer patterns maintaintheir delicate physical and chemical properties. Figure 1shows a schematic illustration of the LIFT process. Theexperimental apparatus is composed of a pulsed Nd:YAG�yttrium aluminum garnet� laser �266 nm wavelength, 4 nspulse duration� and a high power imaging micromachiningsystem, which has been previously described in detail.11

The sensor array was arranged in a 16�16 matrix, thusallowing up to 256 different sensing sites. The fabrication ofthe sensor array was based on the silicon wafer bonding of asilicon on insulator wafer and another wafer with a straincompensated SiGeB epitaxial layer.12 The membranes werepassivated by a LTO layer of 0.5 �m thickness. Each mem-brane’s circumference was supported onto a SiO2 layer witha corresponding circular cavity underneath. A capacitivechemical sensor was thus formed between the membrane anda fixed electrode formed in the underlying substrate usingphosphorus doping. Finally aluminum deposition and pat-terning of the contacts to the membrane and the fixed elec-trode completes the device �Fig. 1 inset�. Typical values forthe membranes’ thickness and the gap between the two elec-trodes were 1.5 and 0.5 �m, respectively. When the sensor isexposed to humidity or volatile organic compounds �VOCs�,the polymer absorbs the corresponding molecules and its me-chanical properties change resulting in a change of the mem-

a�Author to whom correspondence should be addressed. Electronic mail:zergioti@central.ntua.gr.

APPLIED PHYSICS LETTERS 93, 191109 �2008�

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brane deflection and thus the measured capacitance.2 Simu-lations of the sensors performance based on finite elementanalysis revealed that partial coverage of the membraneswith polymers optimizes their sensitivity.13 A 2D finite ele-ment model �FEM� that exploits the axial symmetry of thecircular membranes was developed, utilizing plane82 andshell51 elements �ANSYS software from ANSOFT�. The highspatial resolution of the LIFT technique enabled the partialcoverage of the Si membranes of the capacitive sensor arraywith polymers and subsequent test of the sensor sensitivityfor various coverage percentages.

In the present work, we investigated the polymer print-ing on flat LTO/Si substrates using the liquid phase LIFTtechnique. Figure 2�a� shows a pattern of P4VP polymerdroplets deposited on a flat LTO/Si substrate by varying thelaser pulse energy in such a way that each column corre-sponds to a different laser pulse energy. Three different re-gions are observed. Below 1.0 �J, there is no transfer ofpolymer droplets. For higher laser pulse energies, between1.2 and 3.0 �J, the transferred polymer droplets are wellaligned with a regular circular shape. Finally, between 3.4and 4.3 �J, the deposition becomes unstable, resulting innoncircular droplets and characteristic splashes. These threeregions could be related to three different mechanisms of theliquid phase LIFT that have been previously investigated bymeans of time resolved imaging.14,15 According to the sug-gested mechanisms, the absorption of the laser pulse pro-duces a vapor pocket at the target-liquid film interface. Atlow laser pulse energies, the expansion of the vapor pocket isnot sufficient to detach any material from the liquid volume�subthreshold regime�. At intermediate laser pulse energies, apart of the liquid volume is detached through a jet formation

�jetting regime�. Recently, it has been observed that the jetoriginates from the tip of a balloonlike formation on thesurface of the liquid layer.16 As soon as the jet reaches thesubstrate, a single and uniform droplet is formed. At higherpulse energies, a plume is observed that leads to nonuniformdroplets and splashes �plume regime�. The effect of thetarget-substrate distance variation on the deposition processwas also studied. At intermediate laser fluences, the target-substrate distance was varied from 50 to 1000 �m and nonoticeable effect on the droplet size and shape was observed,indicating that the transfer mechanism is the previously de-scribed phenomenon of jet-formation.16

Multianalyte sensing requires the deposition of differentpolymers on each membrane of the sensor array. Figure 2�b�shows the average polymer droplet diameter as a function ofthe laser pulse energy and fluence for the PHEMA, PAA, andP4VP polymer solutions. The laser spot radius at the targetwas 15 �m and the target-substrate distance was kept at200 �m. It is observed that the polymer droplets size pre-sents an approximately linear dependence on the laser pulsefluence for the studied polymer solutions. This is an expectedresult as the increase of the nanosecond laser pulse energyincreases the modified region at the Cr absorption layer-liquid film interface that causes the vapour pocket15 and thetransfer of the liquid. Comparing our results with the litera-ture on liquid phase LIFT performed by nanosecond17–19 andfemtosecond20 laser pulses, we can observe the same trend,i.e., as the laser pulse energy increases there is a steady in-crease in the droplet diameter.

FIG. 1. �Color online� Schematic illustration of the LIFT process. The insetshows a cross section view of the sensor. FIG. 2. �Color online� �a� Optical microscopy image of P4VP polymer

droplets deposited by increasing the laser energy. �b� Diameter of the liquidphase deposited polymer droplets plotted as a function of the laser pulseenergy and fluence for three different polymer solutions.

191109-2 Boutopoulos et al. Appl. Phys. Lett. 93, 191109 �2008�

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Furthermore, functionality tests were performed usingthe developed sensors. We used the LIFT technique to de-posit the polymer materials on the sensor arrays at variablepercentage coverage of the membranes. Each sensor array iscomposed of 16�16 sensors, with a membrane diameter of200 �m and a 1.5 �m thickness. The selection of the spe-cific dimensions was decided on the criterion of the maxi-mum sensitivity based on previously reported work employ-ing the FEM.21 The effect of the polymer droplet diameter onthe sensor sensitivity was experimentally studied by depos-iting PHEMA spots of various diameters on different mem-branes of the same array. It is noted that membrane coverageexpresses the ratio of the polymer spot radius to the sensormembrane radius. The main challenge for the technique wasto be controllable and repeatable in homogenously coveringthe sensor membranes with polymers at the desired percent-age �Fig. 3�a��.

To test the functionality and the sensitivity of the system,a series of experiments was performed by detecting widelyused VOCs, such as ethanol and methanol, and humidityat different concentrations between 5000 and 20 000 ppm.Figure 3�b� shows the measured capacitance variations forvarious water vapor concentrations as a function of thePHEMA percentage coverage of the sensor’s membrane. Inthe same graph, the results of the FEM are also plotted. Thecapacitance variations are normalized to the maximum valuefor each vapor concentration. It is observed that the sensorcapacitance change is maximized when the sensor membrane

coverage is about 70%, regardless of the analyte concentra-tion, which is in excellent agreement with the correspondingsimulated values by the FEM.

In summary, our work demonstrates that the LIFT pro-cess is widely applicable and is unique in its ability to de-posit sensitive polymer material resulting in high spatial res-olution patterns. Furthermore, we have used this technique tooptimize the sensitivity of a capacitive micromechanical sen-sor array by the selective deposition of polymer on its sensorelements. This simple, direct, one-step process may be ap-plied more widely for depositing different materials with ap-plications in microelectronics engineering.

The project is funded by the EU, under the FP6 project,IST, STRP, «Integrated polymer-based micro fluidic microsystem for DNA extraction, amplification, and silicon-baseddetection».

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FIG. 3. �Color online� �a� Optical microscopy images of sensors partiallycovered with P4VP polymer. �b� Experimental �points� and simulated �con-tinuous line� normalized sensor capacitance variation plotted as a functionof membrane coverage with PHEMA polymer for different concentrations ofwater analyte.

191109-3 Boutopoulos et al. Appl. Phys. Lett. 93, 191109 �2008�

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