integrated sensitive on-chip ion field effect transistors based on wrinkled ingaas nanomembranes

NANO EXPRESS Open Access

Integrated sensitive on-chip ion field effecttransistors based on wrinkled InGaAsnanomembranesStefan M Harazim*†, Ping Feng, Samuel Sanchez, Christoph Deneke, Yongfeng Mei, Oliver G Schmidt

Abstract

Self-organized wrinkling of pre-strained nanomembranes into nanochannels is used to fabricate a fully integratednanofluidic device for the development of ion field effect transistors (IFETs). Constrained by the structure andshape of the membrane, the deterministic wrinkling process leads to a versatile variation of channel types such asstraight two-way channels, three-way branched channels, or even four-way intersection channels. The fabrication ofstraight channels is well controllable and offers the opportunity to integrate multiple IFET devices into a singlechip. Thus, several IFETs are fabricated on a single chip using a III-V semiconductor substrate to control the ionseparation and to measure the ion current of a diluted potassium chloride electrolyte solution.

BackgroundThe integration of ion field effect transistors (IFETs)into solid state micro-/nanofluidic systems for accuratetransport control of charged species such as ions, pro-teins, or DNA in “Lab on a Chip” systems (LoC) ishighly important in life sciences [1-5]. Recently, there isan increasing interest in IFETs due to the high demandin micro total analysis system (μ-TAS) devices dealingwith bio-health, bio-sensing, and bio-physical applica-tions [3,6,7]. Those μ-TAS devices should automate theentire analytical process, from sample processing andpreparation to sensing and analysis within a small,cheap, and easy to handle system. To realize structuresfor simultaneous sample treatment on a single chip, it iscrucial to combine (1) a semiconductor material as sub-strate material for microelectronic components and (2)a fabrication technique which ensures the integrationand alignment of the nanofluidic channels within thechip [8].Over the last few years, different production techni-

ques have been developed to prepare nanochannels forion control such as nanotubes [9-12], porous mem-branes [4] or ion channels [13] to name a few. The per-meability of nanofluidic channels for ions can be

modified by the channel size, surface charge distributionwithin the channel, and external electrical fields [2,5].For extrinsic ion current control within the nanochan-nel, a field effect transistor electrode structure designhas to be integrated [1,5,14]. New technological methodsare necessary to achieve the challenging goal of fabricat-ing accurately aligned single nanochannels, includingthe microelectronic integration into a μ-TAS device.Other fabrication techniques have not succeeded in thecombination of all of these μ-TAS requirements.Furthermore, the fabrication of nanochannels with onlyone dimension in the nanometer range does not provideeasy single molecule treatment. The random alignmentto the substrate of some nanochannel fabrication meth-ods leads to difficulties in the reproducibility of IFETproduction. As an alternative solution or to overcomethese limitations, self-deterministic wrinkling in thenano regime on semiconductor materials were investi-gated recently and successfully tested for their fluidiccapabilities. Due to the high integration state and theirgood alignment to the substrate, it was predicted thatthey could eventually be potential candidates in new labon a chip applications [15]. Herein, we present a newmethod of manufacturing nanofluidic channels for IFETdevices on a semiconductor material by combining ourrecently developed technique called “release and bondback of layers” (REBOLA) with microfluidic fabricationtechniques [8,15,16]. The original REBOLA process

* Correspondence: [email protected]† Contributed equallyInstitute for Integrative Nanosciences, IFW Dresden, Helmholtzstrasse 20,01069 Dresden, Germany

Harazim et al. Nanoscale Research Letters 2011, 6:215http://www.nanoscalereslett.com/content/6/1/215

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relied on wrinkling large strained nanometer thickmembranes onto semiconductor substrate materials,which allowed us to fabricate complex networks ofnanochannels. We improved the REBOLA process tofabricate aligned single nanochannels of different types,instead of producing networks, to be utilized in IFETdevices. Consequently, the nanochannels are integratedinto a microfluidic device unit which includes thephotolithographically defined electrodes for ion detec-tion as well as the microchannel system for liquid trans-port. A self-limiting atomic layer deposition (ALD) ofAl2O3 isolates the electrodes electrically from the sub-strate and fine-tunes the inner diameter of the nano-channel to match the channel height with that of theDebye length of the liquid. The Debye length expressesthe thickness of the electrical double layer at the solid-liquid interface, where the electrical neutrality of anelectrolyte is broken and is equal to approximately30 nm for a 10-4 M ion solution [17]. The integration ofwrinkled nanomembranes into a LoC system is pre-sented. Furthermore, the nanochannels are suitable forion current manipulation and IFET applications, whichwas tested with a 10-4-M KCl solution as electrolytewith differing electrode parameters (from +1 V to -1 V).

ResultsDevice fabricationThe device structure consists of three assembling parts:(1) the nanofluidic channel fabrication, (2) integration ofelectrical components, and (3) microfluidic channel fab-rication for the liquid reservoirs on either side of the

nanochannel, including a top poly-dimethylsiloxane(PDMS) sealing layer.The basic substrate for wrinkling consists of a stack of

three layers which was grown by molecular beam epitaxy(MBE) onto the GaAs substrate. First, a 200-nm thickGaAs film was grown as a buffer to provide a smooth andcrystalline surface for the second 80-nm thick AlAs sacrifi-cial layer. Finally, a 20-nm thick In0.2Ga0.8As functionallayer was grown on top of the sacrificial layer. The fabri-cated stack of layers was processed into a square-shapedmesa structure with a lateral side length of 6 μm by photo-lithography and wet chemical etching as shown in Figure1a. A diluted HF etchant selectively removes the AlAssacrificial layer and released the InGaAs nanomembranefrom the GaAs buffer (Figure 1b). Afterwards, due to theinternal stress of the wrinkling layer, the InGaAs mem-brane began to bond back to the substrate and wrinkledinto a straight channel-like structure (Figure 1c) [16,18].After the nanochannel fabrication, the electrical isola-

tion of the entire substrate, including the nanochannel,was carried out by an ALD method by depositing a15-nm thick Al2O3 layer. The electrodes were thendeposited in separate steps starting with the gate elec-trode on top of the channel. After the gate electrodeintegration by photolithography processing and electronbeam evaporation of 80 nm of gold, a second ALD stepisolated the gate electrode completely. The purpose ofthe second Al2O3 layer is not only to ensure the electri-cal isolation but also to tune the final inner diameter ofthe nanochannel. The ALD method allows for a homo-genous coating of nanochannels with lengths in the

Figure 1 Sketch of the general wrinkling process in three steps. (a) Pre-defined wrinkling structure by photolithography and wet chemicalprocessing, (b) selective HF etching of the sacrificial AlAs layer and starting release of the strained InGaAs layer, (c) bond back and wrinkling ofthe released functional InGaAs layer.


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micrometer regime [19]. This coating is of significantimportance since ions with a given concentration have adefined Debye length, which needs to be in the sameorder as the nanochannel dimension [20,21]. The origi-nal height of the nanochannel after wrinkling, measuredby AFM, was 95 nm. After the first ALD deposition of15 nm of Al2O3, the inner height had been diminishedto 65 nm. By the coating of a 20-nm Al2O3 layer fromthe second isolation step, the channel height finallyreached a total height of 25 nm. After the gate electrodewas isolated and the channel size fine-tuned to thedesired value, the deposition of the source and drainelectrode was also performed by a photolithography andelectron beam evaporation step. The source and drainelectrodes were separated by 1 μm to that of the nano-channel openings, whereas the gate electrode partiallycovered the top of the nanochannel (66% of the length).Figure 2 shows optical images of two IFET devicesincluding the electrodes and the microfluidic system.Each nanochannel array shown in Figure 2a provides alarge number of wrinkled InGaAs membranes (165wrinkled membranes per array) which provide variouschoices of nanochannels to be selected for the IFET fab-rication. Almost similar nanochannels of each arraywere selected to prepare the IFET devices. Figure 2adepicts two arrays of nanochannels containing the pro-cessed electrodes and microfluidic structures.The microchannels were defined by photolithography

of SU-8 10 photoresist, which is mechanically stable andbiocompatible [22-24]. By using this high viscous, nega-tive photoresist and optimized spin-coating parameters,the microfluidic channels reach a height of 10 μm. Toseal the entire channel system, a PDMS layer was pre-pared and placed on top of the microchannel system(see Figure 3). This top layer avoids rapid liquid eva-poration and contains also connectors to attach anexternal micropump system to the device [25,26].

Versatility of wrinklingThe improved REBOLA technique uses small strainedInGaAs membranes on mesa structures which allows forthe exact on-chip placement of the nanochannels. In orderto have an accurate and reproducible channel alignmentand to identify the most suitable wrinkling behavior forthe fabrication of straight nanochannels, the wrinkling ofdifferent InGaAs membrane shapes were investigated. Weaimed for the fabrication of square, circular, and stripe-shaped membranes as shown in Figure 4. Mesa structureswith circular or stripe shapes lead to straight channelstructures, but they also offer a bigger variety of channeltypes with different alignments. For an example, a circularshape with a diameter of 6 μm gives a more random distri-bution of the channel orientations and even different typeschannels themselves (Figure 4a). The most observed

channel types are straight channels; nonetheless, three-way channels as well as four-way channels appear in anumerous quantity. For stripe-like mesa structures with awidth of about 6 μm and a length of several tens of micro-meters, parallel channels can be realized (Figure 4b). Onlysquare-shaped InGaAs layers wrinkle almost in the sameorientation every time and, therefore, this kind of structurehas been used as the base pattern for the device assem-bling (Figure 4c). This preferred wrinkle orientation relieson the crystal structure of the sacrificial AlAs layer andthe different HF etching rates along the crystal axis [8,18].Due to the MBE layer growth, the AlAs crystal structure issimilar to the substrate which is GaAs(001). The etchingrates are highest perpendicular to the <110> direction ofthe crystal. Assuming that the edges of the functionalInGaAs membrane are aligned to the <110> direction ofthe substrate, the two faster under-etched edges will bondback to the substrate prior to the two edges with a loweretching rate. This etching rate preference, fixes the wrinkleorientation of any square-shaped nanomembrane parallelto the <110> direction. A focused ion beam (FIB) cut of astraight two-way nanochannel is shown in Figure 4d,where the internal structure of the InGaAs wrinkles canbe observed. The benefits of the self-organized wrinklingof strained InGaAs nanomembranes into nanofluidicchannels are the integration into a semiconductor sub-strate with accurate control of location, channel type, andthe orientation on the chip.

Measurement preparationThe sample was mounted onto a chip carrier whichwas plugged into the measurement stage. The mea-surement itself was controlled by a PC software andwas operated by the semiconductor parameter analyzer4156C (Agilent, Santa Clara, CA, USA). All measure-ments have been done at room temperature. Prior tothe ion current measurement, the system was success-fully tested to have no electrical or liquid leakage,which was carried out without liquid in the system toensure electrical isolation between each electrode andbetween the electrodes and the substrate. Al2O3 isstable using the current measurement conditions witha maximum electrical field strength of about 2 V/μm.The breakdown voltage for similar systems was investi-gated recently and is more than one order of magni-tude higher [27]. The liquid leakage tests have beeninvestigated with pure DI water and the same KClsolution as for the ion current measurement. The con-ductivity was always in the expected range so that noleakage was present [10].

DiscussionThe current chip structure includes three independentoperating IFET devices. To prove the suitability of the


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presented IFET devices, a 10-4-M KCl solution had beenadded to reservoir 1 and 2 of each single device (seeFigure 5). The source electrode was always grounded toVS = 0 V, whereas the drain electrode had a bias ofeither VD = +1 V (to attract the negative chloride ions)or VD = -1 V (to attract the positive potassium ions).The measurement of the ion current is a steady state

measurement, meaning that no polarization effects dis-turb the detection of the ion current. In Figure 6a, fourconductance curves are displayed to demonstrate thefast relaxation capabilities of the presented device. ForVG = -1 V and VD = -1 V the conductance decayed fastbelow 5% of the initial value in the first 5 s. The mea-surement is displayed from 3 s on, whereas at 0 s the

Figure 2 Top view on two IFET devices. (a) Optical microscope images of two devices containing one wrinkled nanochannel each, theelectrodes and the microfluidic channels. (b) and (c) are the zoomed images on one device in two different Z-positions focusing on thenanochannel (b) and on the electrodes (c). The unlabeled scale bars are 5 μm.


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bias was already applied to the electrodes. The datapoint after 55 s of each curve represents the channelconductance without polarization effects. All ion currentmeasurements were taken in the steady state after 55 s.The ion current manipulation had been driven in a

sweep mode starting with VG = -1 V to VG = +1 V in0.5-V steps. The ion permeability of the nanochannelfor a specifically charged species can be tuned by thegate electrode while keeping all other electrode para-meters constant. The measurement took place always atthe drain electrode. Figure 6b shows the ion currentmodulation of Cl- (red circular data points) and K+

(black square data points) ions with changing of thegate potential. In case that there is no gate effect (VG =

0 V), the same ion current is expected, independently tothe chosen source and drain parameters, since potas-sium and chloride ions have almost the same mobilityin a liquid (76.2·10-7 m2/sV for K+ and 79.1·10-7 m2/sVfor Cl-) [20]. Figure 6b shows that at VG = 0 V, the ioncurrent for Cl- and K+ is in the same order at about 28pA. By increasing the gate potential to VG = +1 V, posi-tive ions will be repelled and negative ions will beattracted to move through the nanochannel. This gatepotential modification results in a lower K+ and anincreased Cl- ion current. A mirrored behavior can beobserved when the gate potential is decreased to VG =-1 V. The weak effect on the chloride ion current mod-ulation at lower gate potentials can be explained by the

Figure 3 Schematic of the microfluidic setup. (a) A processed transistor structure, including the electrodes and the wrinkled nanochannel. (b)SU-8 microchannel walls defining the microfluidic channels on the chip structure. (c) Sealing of the microchannel system by using a flexiblePDMS top layer.


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permeability of PDMS for water molecules during themeasurement. During this time, water molecules willdiffuse into the PDMS, which can slightly increase theion concentration in the reservoirs. Therefore, theDebye length decreases, and the gate potential loses theefficiency for ion current manipulation.

The presented IFET devices have a comparatively fastion current detection capability in the lower pA regime[10]. The feasibility of accurate nanochannel alignmenton semiconductor substrates, and the high integrationstate of all components are strong criteria for futurecontinued investigations on wrinkled nanomembranesfor more complex IFET systems.

ConclusionsThe fast and easy fabrication of nanochannels by the com-bination of the versatile REBOLA technique and standardphotolithography leads to semiconductor films whichform nano-sized wrinkles with useful fluidic capabilities.Different nanomembrane shapes have been prepared toinvestigate the most suitable wrinkling structure for IFETdevices. Rectangular-shaped membranes wrinkle intostraight two-way channels with a fixed orientation relativeto the crystal structure of the substrate. Indeed, a circular-shaped membrane creates a higher variation of channeltypes, which can be used to fabricate more complex fluidiccircuit, on-chip structures, but the square-shaped struc-tures wrinkle always the same, which is more useful forreproducible IFET assembling. To demonstrate the feasi-bility of the integrated nanochannels for IFET on-chipdevices, samples with several IFETs have been fabricatedand were successfully tested for ion separation using KClas a model electrolyte solution. The usage of a semicon-ductor material as a substrate and the highly integratedstate of all components, including accurate channel posi-tioning and definable channel orientation, might be highlydemanded for the next integration level of “Lab on aChip” devices. The near future approach is to adapt thesystem for single molecule detection, which should find ahuge number of applications in bio-analytic μ-TASs. Lateron, light can be used for controlling the wrinkling beha-vior of the nanomembrane in order to obtain more com-plex and deterministic wrinkle structures [28].

MethodsFilm growth, structure reproducibilityThe III-V semiconductor layers were epitaxially grownon GaAs(001) substrates. After the growth of a 200-nmGaAs buffer, 80-nm AlAs and 20-nm In0.2Ga0.8As weregrown. Several samples were lithographically patternedto ensure the reproducibility of nanochannel formation.The sample dimension is always 7 by 7 mm in lateraldimension. A sample contains 12 of the so-called nano-channel arrays with 165 wrinkling structures each (seealso Figure 3a). This is to increase the chance to havethe same-shaped nanochannel with the same fluidic cap-abilities on every new sample. For reproducibility, allsamples are cut along the <110> GaAs direction and thephotolithographic pattern has been always aligned to thesubstrate structure in the same way.

Figure 4 Versatility of wrinkling. Alternative wrinkling structuresleading to different channel arrangements such as (a) multipleopenings for circular mesa structures and (b) parallel alignment ofchannels for stripe-like mesa structures. (c) An SEM image of asquared GaAs mesa structure with a wrinkled In0.2Ga0.8As layer ontop. (d) An SEM/FIB cut image of one wrinkled nanochannel. Thecarbon cover protects the wrinkled structure during the FIB cut.Unlabeled scale bars are 2 μm.


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Wet etching risks and safetyThe HF etchant is highly toxic. Special safety clothes arestrongly recommended. The local waste disposing pro-cedure is to be obeyed.

Electrical measurement device and conditionsThe semiconductor parameter analyzer Agilent 4156Chas been used under standard environment conditions,such as room temperature and normal pressure. Themeasurement device has been driven in the “sweep mea-surement mode.” Light impact on the sample as well asacoustic vibration has to be avoided at all times becauseof its semiconductor behavior and the high electricalsensitivity of the measurement device.

AcknowledgementsThe authors thank Elliot Smith for the fruitful discussions and Emica Coric forthe FIB cut and SEM images. This work was funded by the VolkswagenFoundation (I/84072).

Authors’ contributionsSMH carried out the wrinkling studies, device assembling, electricalmeasurements and wrote the manuscript with contribution from SS andOGS. PF participated during the electrical measurements. CD did the MBEgrowth of the substrates. OGS and YF initiated the project and provided theoriginal idea.

Competing interestsThe authors declare that they have no competing interests.

Received: 1 November 2010 Accepted: 14 March 2011Published: 14 March 2011

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Figure 5 Schematic diagram representing a cross section ofone transistor device. The two reservoirs are mainly separated bythe SU-8 photoresist barrier and are only connected by thenanochannel. The top PDMS lid seals the entire system and avoidsrapid liquid evaporation. The electrodes are electrically isolated byan Al2O3 layer (bright blue).

Figure 6 Electrical measurements. (a) Transistor conductanceover time at different electrode configurations. Biases were appliedat 0 s to the electrodes and the measurement started after 2 s. Thefast decay of the polarization effects enables the measurement ofthe real current without additional effects after 55 s. (b) Ion currentmodulation by changing the gate bias while keeping VS = 0 V andsetting VD = +1 V for detecting Cl- ions (red points) and VD = -1 Vfor detecting K+ ions (black points).


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doi:10.1186/1556-276X-6-215Cite this article as: Harazim et al.: Integrated sensitive on-chip ion fieldeffect transistors based on wrinkled InGaAs nanomembranes. NanoscaleResearch Letters 2011 6:215.

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integrated sensitive on-chip ion field effect transistors based on wrinkled ingaas nanomembranes

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