Letter www.acsami.org
Thin Nanoporous Metal−Insulator−Metal Membranes Morteza Aramesh,*,†,‡ Amir Djalalian-Assl,‡ Mir Massoud Aghili Yajadda,§ Steven Prawer,‡ and Kostya (Ken) Ostrikov†,∥ †
Institute for Future Environments, School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4000, Australia ‡ School of Physics, the University of Melbourne, Melbourne, VIC 3010, Australia § Manufacturing Flagship, Commonwealth Scientific and Industrial Research Organisation (CSIRO), PO Box 218, Lindfield, NSW 2070, Australia ∥ Plasma Nanoscience Laboratories, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Lindfield, New South Wales 2070, Australia S Supporting Information *
ABSTRACT: Insulating nanoporous materials are promising platforms for soft-ionizing membranes; however, improvement in fabrication processes and the quality and high breakdown resistance of the thin insulator layers are needed for high integration and performance. Here, scalable fabrication of highly porous, thin, silicon dioxide membranes with controlled thickness is demonstrated using plasma-enhanced chemical-vapor-deposition. The fabricated membranes exhibit good insulating properties with a breakdown voltage of 1 × 107 V/cm. Our calculations suggest that the average electric field inside a nanopore of the membranes can be as high as 1 × 106 V/cm; sufficient for ionization of wide range of molecules. These metal−insulator−metal nanoporous arrays are promising for applications such soft ionizing membranes for mass spectroscopy. KEYWORDS: nanoporous materials, plasma-enhanced chemical-vapor-deposition, silicon dioxide, dielectric breakdown, soft ionizing membrane (SIM)
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oxides (AAO) with uniform pore size distribution and very high porosity (>1 × 109 pores/cm2) are ideal for this purpose, but their dielectric properties are not adequate for the task.12 The high concentration of impurities incorporated inside the oxide layer during the electrochemical fabrication process reduces the breakdown voltage of the oxide.13 Although unmodified AAO is not suitable for this purpose, it can still be used as a template material for the growth of other nanoporous materials (which also resolves the challenge of handling fragile thin membranes).14 Here we show how a SIM can be fabricated based on a modified AAO template and report on its breakdown properties. We thus demonstrate a new technique toward scalable fabrication of highly porous, thin silicon dioxide membranes with controllable thickness, pore size and aspect ratios using plasma-enhanced chemical-vapor-deposition (PECVD). The fabricated membranes exhibit excellent dielectric properties with breakdown voltages up to 1 × 107 V/cm (required for SIM), comparable to high quality oxides grown by other methods.15,16 For evaluation of potential applications of the membranes in soft-ionizing membranes, we
espite the wide utilization of nanoporous materials in biological and high-energy applications,1,2 little attention has been paid to their potential applications as soft-ionizing membranes (SIM), especially suitable for mass spectroscopy.3,4 Soft ionization is referred to the high-yield production of ionized molecules from neutral analytes by removal of an electron from each molecule without further fragmentation or dissociation.5 This can be achieved by applying moderate potentials (10−100 V) across a thin (∼100−300 nm thick) insulating (nano-) porous membrane, providing extremely high electric field in a confined space. The small internal volume of the pores enables “soft” ionization of the passing molecules without any fragmentation or secondary ionization, due to the fact that the mean free paths of the molecules are longer than the dimensions of the nanopore region (see the Supporting Information).6−9 Secondary ionization and discharges in ionizing membranes are among the challenges for the current mass spectrometer technology and other in situ analytical techniques such as separation and sensing.10 Despite the great advantages of SIMs for molecular ionization (Table S1), they have not found wide usage because they require complicated, unscaleable and expensive fabrication process.11 The concept of SIMs has been demonstrated with isolated holes on free-standing membranes,5 but ideally a SIM would consist of many holes of uniform diameter, enabling high throughput and sensitivity. Nanoporous anodic aluminum © XXXX American Chemical Society
Received: November 18, 2015 Accepted: February 5, 2016
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DOI: 10.1021/acsami.5b11182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces have conducted detailed calculations using finite element method approach. Thin nanoporous silicon dioxide membranes were fabricated on top of an AAO membrane (Figures S1 and S2). AAO membranes with the top surface area of 1 cm2, thickness of 100 μm, pore diameter of 150 nm and porosity of ∼1 × 109 pores/ cm2 were prepared by two-step anodization of an aluminum foil in phosphoric acid electrolytes. In principle, by controlling the anodization parameters, the pore size and porosity of AAO substrates can be tuned from 10 to 450 nm and 1 × 109 to 1 × 1011 pores/cm2, respectively (Figure S3).17 Continuous channels in AAO membranes were opened by removing the aluminum from the back of the structure followed by barrier layer opening. AAO membranes were then coated with a thin layer of Chromium, to act as one of the electrical contacts for the SIM, which also turns out to be important in achieving a high-quality silicon dioxide structure. Chromium enhances the adhesion of the grown oxide layer, plus it acts as a catalyst for SiO2 nucleation and uniform island formation.18 SiO2 (100− 350 nm thick) is grown on top of the structure using N2O, SiH4 and N2 precursors in RF plasma at a relatively low temperature (250 °C) (see the Supporting Information). This method is scalable because of facile fabrication of AAO and the capability of modern plasma-enhanced deposition systems, which allows material deposition on wafer scales. As shown in Figure 1a, the grown SiO2 membrane replicates the porous structure of AAO. The SiO2 layer only forms on the
applied across the thin membrane because any uncoated or agglomerated area might lead to failure of the device. For application as SIM, it is desired to promote formation of stoichiometric SiO2 structure, which has the highest dielectric breakdown among other types of SiOx structures (incorporation of other elements, such as hydrogen and nitrogen, would generally result in lower breakdown strength).19 To achieve a good quality SiO2 structure, relatively high ratios of N2O/SiH4 gas flows (∼84) were used. This choice results in complete dissociation of the silane in plasma−due to the higher energy collisions in the gas phase interactions (Table S3), which can increase the ratio of Si−O bonding in the structure with reduced unfavorable incorporation of nitrogen and hydrogen bonds (such as Si−H, Si−N, and N−H).16 To compensate for the negative effect of a high N2O/SiH4 ratio on the film uniformity, we used reduced SiH4 flow (8 sccm) and increased chamber pressure (1000 Torr). The high gas flow ratio of N2O/SiH4 additionally promotes preferential vertical growth of the oxide layer by increasing the density of the excited oxygen atoms and subsequent etching rates. The vertical growth is favorable because otherwise the inner surface of the pores will be blocked for even low thickness of the overlayer.20 It should be noted that the plasma conditions inside the confined space of the nanopores could be significantly different from the plasma conditions on the top surface, which may result in a difference in the gas−solid interactions inside the pores.20,21 This could be the main driving mechanism for the formation of SiO2 only on the top surface, which might have implications also for the modification of other types of nanoporous materials with high aspect longitudinal heterostructures.22,23 The structural properties of the SiO2 films were examined by X-ray photoelectron emission spectroscopy (XPS), IR absorption and refractive index measurements (Figures S5 and 6). The XPS spectrum of the Si 2p core levels shows a single and well-defined peak at 103.8 eV (corresponding to stoichiometric SiO2), whereas no peak was observed in the N 1s scan centered at 398 eV (suggesting lack of nitrogen incorporation). The FTIR absorption spectra of the membranes show a dominant broad absorption at 1054 cm−1, corresponding to Si−O stretching modes in SiO2. The refractive index of the deposited oxide was in the range of 1.43−1.46, corresponding to high-purity SiO2 films. All the analyses suggest that the deposited layer is a high quality SiO2 structure. The grown SiO2 membranes exhibit high dielectric breakdown potentials up to ∼107 V/cm. The dielectric breakdown properties of the as grown silicon dioxide films on Cr-coated AAO were studied at room temperature as a function of DC voltage and thickness of the oxide layer (100−350 nm). Figure 2a shows a schematic of the setup for the dielectric breakdown measurements. A thin layer of gold (∼10 nm) was evaporated as a contact on top of the silicon oxide layer and the potential voltage was applied to the sandwich structure (Au-SiO2−Cr). Paraffin oil, a high dielectric material (see the Supporting Information), was used to fill in the nanochannels of the testing material to avoid air discharges. The voltage was applied to the electrodes and it was continuously increased up to the breakdown voltage with a rate of 10 V/min. Breakdown usually occurs with a visible electric spark, loss of contacts at the gold layer, an abrupt current drop and significant structural damages on the surface (Figure 2b). The
Figure 1. SEM images of thin nanoporous silicon dioxide films coated on top of anodic aluminum Oxide (AAO) template using plasma enhanced chemical vapor deposition. (a) Top view of the coated membrane. (b) Cross-section of the membrane (with a platinum protection layer on top for cross-section preparation using focused ion beam milling). (c) False colored SEM image from the cross-section of the membrane with three distinct layers: AAO, chromium, and SiO2.
top surface of AAO, while no growth was observed within the internal surface of nanopores (Figure 1b). The thickness (100− 350 nm) of the grown oxide layer can be controlled by deposition time (∼1−5 min). Increasing the thickness generally leads to a reduction of the pore size, and typically the pores are blocked when the thickness exceeds 300 nm (Figure S4). It is possible to modify the structure and properties of the grown oxide layer by controlling the plasma parameters. The expected influence of the plasma parameters are listed in Table S2. To fabricate a nanoporous membrane with good insulating properties for application as SIM, the plasma growth should have three characteristics: (i) the contribution of impurities and formation of unfavorable chemical bonding should be minimized during the growth to achieve high dielectric properties, (ii) the oxide growth should be preferentially vertical to allow replication of porous structure up to suitable thickness without closure of the pores, and (iii) the growth should be uniform to allow controlling the thickness with minimum variance, which is crucial when a high electric field is B
DOI: 10.1021/acsami.5b11182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 2. (a) Schematics of the setup for breakdown voltage measurements, where a positive potential was applied on to gold layer and the Chromium layer underneath was grounded. Paraffin oil was used to fill in the pores underneath the gold electrode area (nanopores are not depicted in this schematic). (b) SEM image of the structure after breakdown. The “breakdown” pattern resembles “Lichtenberg figures” produced by electron injections, leading to structural progressive damages from the contact point (the red spot). (c) I−V curve for thin silicon dioxide membranes with different thickness: nanoporous oxides with 100 and 250 nm thickness, and a nonporous oxide with 350 nm thickness. (d) −Ln(I/E2) vs 1/E reveals the conduction mechanism in membranes with different thicknesses. The 100 nm thick sample has different conduction mechanism from the other two samples. (The arrow on the right-hand side indicates the direction of current increase).
“breakdown” pattern resembles “Lichtenberg figures” (Brownian trees in surface discharges) produced by electron injection. The pattern is very dense at the center (contact point) and it expands radially from the center in a series of discontinuous steps. This similarity of the “breakdown” pattern suggest that the observed permanent structural damage is mainly due to the local Joule heating at high voltages, which can cause disassociation of dielectric molecules and subsequent rupture of chemical bonds. Figure 2c shows the current-electric field characteristics of both porous and nonporous oxide films with different thickness. The measured “breakdown” electric field for different films (100−350 nm thick) was within the range of ∼0.8−1 × 107 V/cm, which is very close to the breakdown value of highquality PECVD grown SiO2 films reported by others.19 The breakdown voltage of the 250 nm-thick porous film was ∼1 × 107 V/cm, which is similar to the nonporous film (350 nm thick) and higher than the porous 100 nm thick film (∼8 × 106 V/cm). Before the breakdown point, some current leakage is observed through the thin oxide layer which eventually rises continuously until breakdown occurs. The leakage current can be due to various physical phenomena, such as electron tunnelling, thermionic emission, impact ionization, or hopping conduction.24 However, the leakage current can also be influenced by local parameters and geometry of the material. A higher leakage current was observed for the porous film compared to the nonporous film, with the highest leakage current for the 100 nm thick film. One possible explanation for
the higher leakage current in the porous structures (compared to nonporous film) could be the unavoidable partial diffusion of gold nanoparticles inside the nanopores during the evaporation step. The presence of the gold nanoparticles inside the nanochannels may induce surface conduction along the pores, which is expected to be more significant for the thinner membranes due to the smaller surface area in the pores. In addition to the gold nanoparticles, oil molecules inside the membrane channels could also contribute in leakage current by partial ionization. Field-assisted molecular ionization of the filling medium (oil) can take place at voltages lower than the breakdown voltage, which generates charged molecules and mediates charge transfer between the metallic electrodes.25 To reveal the conduction mechanism in thin films, we have plotted −Ln(I/E2) against 1/E in Figure 2d. For thicker membranes, due to the linear shape of the plot at high fields (E > 2 × 106 V/cm), it is assumed that the electronic conduction mechanism mainly follows Fowler-Nordheim model, i.e. tunnelling of electrons from the metal electrode to conduction band of the oxide layer (Table S4).26−29 The slopes of the linear plots are different with the higher slope for the nonporous film, which might be due to thermal effects or impact ionization of the oil molecules inside the nanopores. The conduction mechanism in the thinner porous membrane, i.e., 100 nm thick, differs significantly from thicker oxides, which might be due to a combination of effects (Table S4).30 It is presumed that the nanopore effects have significantly influenced the conduction mechanism in the ultrathin membrane. Ultrathin nanoporous dielectrics are susceptible to C
DOI: 10.1021/acsami.5b11182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
of the sample. The nonuniformity of the film may create electrically weak regions, which can cause early breakdown. Figure 3e shows the effect of the film thickness on the breakdown properties of the membrane. Ultrathin membranes (
