NANO LETTERS
Ionic Field Effect Transistors with Sub-10 nm Multiple Nanopores
2009 Vol. 9, No. 5 2044-2048
Sung-Wook Nam,† Michael J. Rooks,‡ Ki-Bum Kim,*,† and Stephen M. Rossnagel*,‡ Department of Materials Science and Engineering, Seoul National UniVersity, Seoul 151-742, Korea, and IBM T.J. Watson Research Center, Yorktown Heights, New York 10598
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Received January 29, 2009; Revised Manuscript Received March 24, 2009
ABSTRACT We report a new method to fabricate electrode-embedded multiple nanopore structures with sub-10 nm diameter, which is designed for electrofluidic applications such as ionic field effect transistors. Our method involves patterning pore structures on membranes using e-beam lithography and shrinking the pore diameter by a self-limiting atomic layer deposition process. We demonstrate that 70∼80 nm diameter pores can be shrunk down to sub-10 nm diameter and that the ionic transport of KCl electrolyte can be efficiently manipulated by the embedded electrode within the membrane.
Electrical manipulation of charged species such as ions,1,2 DNA,3 proteins,4 and nanoparticles5 is important for utilizing complementary metal oxide semiconductor (CMOS) technology in nanofluidic systems. A construction of molecular logic circuit6,7 and a DNA sequencing8,9 in solid state device can be a breakthrough in conventional semiconductor devices (DRAM, Flash, CPU, and so forth), which currently have limitations in dimension shrinkage. In this regard, the combination of “Electronics” and “Nanofluidics” leads to the field of “Electrofluidics”, which utilizes the electrical behaviors of fluids for solid state device applications.10–12 In electrofluidics, the control of surface charge in fluidic channel is a key issue.13–15 The charge on the surfaces of the device makes oppositely charged species accumulate near the channel wall. This screening effect, usually defined by a Debye screening length (λD), allows unique properties in fluidic systems when the channel size becomes small or comparable to the λD for that electrolyte.4,11 The control of surface charge has enabled researchers to build electric circuit elements, such as diodes16,17 and transistors1,4,15 in a nanofluidic system. Furthermore, the development of electrofluidics has significant implications for biohealth applications including sieving18–20 and sensing21 of biomolecular species. For the successful development and integration of electrofluidics, reliable fabrications of nanochannels with molecular-level feature size (sub-10 nm) are essential. Until now, various fabrication methods on solid-state membranes have been suggested to form nanopore structures.12,22,23 * To whom correspondence should be addressed. E-mail: (S.M.R.)
[email protected]; (K.-B. K.) kibum@ snu.ac.kr. † Seoul National University. ‡ IBM T.J. Watson Research Center. 10.1021/nl900309s CCC: $40.75 Published on Web 04/27/2009
2009 American Chemical Society
Focused ion or electron beam drilling has been widely used for fabricating nanopore structures to study the translocations of biomolecular species.22,23 Also, bottom-up approaches have been used to fabricate multipore structures that can be used in molecular sieving or sensing applications.20 However, these methods have inherent problems in terms of integration, process through-put or self-assembly. Moreover, lateral type fluidic channel structures fabricated by both top-down24 and bottom-up1,4 approaches still have limitations to incorporate the molecular-level feature size with delicate structures of CMOS technology. In this report, we suggest a method to fabricate multiple nanopore structures with sub-10 nm feature size. E-beam lithography and atomic layer deposition (ALD) processes were employed to generate nanopore structures with molecular-level feature size (Supporting Information, Figure S1).25 We used a membrane consisting of a metal layer (TiN 30 nm) sandwiched by dielectric films (Si3N4 20 nm). E-beam lithography and reactive ion etching (RIE) processes allowed the formation of nanopore diameters of a few tens of nanometers. The resolution of this top-down drilling process is limited by the combination of many factors such as beam scattering,26 resist chemistry27 and critical dimension loss during pattern transfer,28 which makes it difficult to fabricate sub-10 nm features. The ALD process compensates for this limitation by allowing highly controllable and conformal film deposition at the nanometer scale, which is used to shrink the pore size.25,29 Moreover, the self-limiting process of the precursor molecules allows us a wide process window for uniform nanopore structures.
Figure 1. Principle of electrode-embedded nanopores for IFET. Ionic transport through the nanopores can be manipulated by the surrounding gate dielectric (TiO2, orange) and gate electrode (TiN, yellow). (The green color shows Si3N4).
For electrofluidic applications, we designed nanopores to have an embedded electrode that serves to manipulate KCl electrolyte ions as shown in Figure 1. This concept, namely ionic field effect transistor (IFET), is similar to the semiconductor field effect transistor (FET) except that the channel medium is made of electrolyte ions instead of electrons or holes. In this device, the nanopore ionic channels surrounded by gate dielectric(TiO2)/gate electrode(TiN) allows threedimensional gating, which may cause more efficient controllability on channel conductivity than top- or bottom-gating structures.1,4,15 Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of electrode embedded multiple nanopore structures on a Si3N4 (20 nm)/ TiN (30 nm)/Si3N4 (20 nm) membrane. E-beam lithography followed by RIE generates 70∼80 nm diameter pores. Figure 2a shows that RIE drilling generates clear pore edges with uniform pore size. The pores were confirmed from both frontside and backside of wafers. In order to decrease the pore size, a TiO2 dielectric film was deposited by ALD that provides a conformal film deposition. Figure 2b,c shows SEM and TEM images of nanopore structures that were conformally coated by the ALD TiO2 film. TEM images show that the nanopores are almost filled by the TiO2 film, leaving a seamlike void (1∼2 nm diameter) at the center of the pores. These void structures eventually will serve as nanopores through which the electrolyte will flow. It is worth noting that the ALD process offers a relatively well-controlled self-limiting process for nanopore structures at the sub-10 nm scale (Figure 2e). E-beam lithography and RIE processing can precisely control the feature size of pores when the size is 70∼80 nm. However, this controllability is not effective when the size becomes less than 10 nm. To overcome this limitation, ALD film deposition enables the pores to be shrunk down and to be limited to a molecularlevel feature size as Figure 2e. Since ALD film deposition is based on the reaction of the adsorbed precursors with substrate, the pore shrinking process is stopped if the precursor molecules cannot penetrate into the pore. During this process, the pore size remains near the same size of the precursor molecule. It thus offers a wide process window to generate sub-10 nm nanopores.30,31 Two different kinds of multipores (70 and 80 nm diameters) were shrunk down with a same rate, and finally the pore size became limited at 1∼2 nm diameter. Since the self-limited pore size is determined Nano Lett., Vol. 9, No. 5, 2009
by the ALD precursor molecules (titanium isopropoxide, Ti(OCH(CH3)2)4, ∼1 nm diameter), our approach can generate uniform molecular-level features at the 1∼2 nm scale. In order to explore the possibility of nanopore structures as nanofluidic channels, we measured the ionic conductivity through the pores for different KCl electrolyte concentrations. The fluidic chambers were made with polydimethylsiloxane (PDMS) and Ag/AgCl electrodes were connected on both sides of the chambers. At the high concentration region (Debye length , pore diameter), the ionic conductivity is linearly dependent on the electrolyte concentration, which shows bulk property behaviors (Figure 3). However, as the concentration becomes lower, the ionic conductivity deviates from bulk properties near 10-3 M and saturates to another slope which corresponds to a surface charge governed region (
