Article pubs.acs.org/JPCC
Robust, High-Density Zinc Oxide Nanoarrays by Nanoimprint Lithography-Assisted Area-Selective Atomic Layer Deposition Vignesh Suresh,† Meiyu Stella Huang,† M. P. Srinivasan,*,† Cao Guan,‡ Hong Jin Fan,‡ and Sivashankar Krishnamoorthy*,§ †
Department of Chemical and Biomolecular Engineering, National University of Singapore (NUS), E5, 4 Engineering Drive 4, Singapore 117576 ‡ Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University (NTU), Singapore 637371 § Patterning and Fabrication group, Institute of Materials Research and Engineering (IMRE), Agency for Science Technology and Research (A*STAR), 3, Research Link, Singapore 117602 S Supporting Information *
ABSTRACT: Polymer templates realized through a combination of block copolymer lithography (BCL) and nanoimprint lithography (NIL) are used to direct atomic layer deposition (ALD) to obtain high-quality ZnO nanopatterns. These patterns present a uniform array of ZnO nanostructures with sub-100 nm feature and spatial resolutions, exhibiting narrow distributions in size and separation, and enhanced mechanical stability. The process benefits from the high lateral resolutions determined by the copolymer pattern, controlled growth rates, material quality and enhanced mechanical stability from ALD and repeatability and throughput from NIL. The protocol is generic and readily extendible to a range of other materials that can be grown through ALD. By virtue of their high feature density and material quality, the electrical characteristics of the arrays incorporated within MOS capacitors display high hole-storage density of 7.39 × 1018 cm−3, excellent retention of ∼97% (for 1000 s of discharging), despite low tunneling oxide thickness of 3 nm. These attributes favor potential application of these ZnO arrays as charge-storage centers in nonvolatile flash memory devices. processes have been listed in the ITRS roadmap12 as promising techniques to drive further miniaturization at low-cost. Both techniques use polymers to create templates to produce nanopatterns through directed growth, deposition, or etching processes.13 Preparation of high-resolution NIL molds using BCL14,15 has been recently reported and is a win−win combination for both the techniques: For NIL, this allows achieving high-resolution molds on arbitrarily large areas at low cost and breaking free from low-throughput options such as ebeam lithography. For BCL, it enables repeatability; further enhancement in throughput, for example, reduced processing; scalability using step and flash imprint lithography (SFIL) options; freedom in the choice of organic templates (not necessarily restricted to one of the blocks of the copolymer); and freedom over the type of substrate material used. In this article, we employ a combination of BCL and NIL specific toward producing selective area growth templates for ALD of ZnO. ALD is known to be a thin-film growth tool for creating high-quality thin films at ultrahigh resolutions. The process shares similarity with the CVD in terms of vapor phase
1. INTRODUCTION Nanopatterns of diverse materials are of much interest as a means of enhancing performance, enabling miniaturization, or as determining components of functional devices or interfaces. Generic capabilities toward producing material patterns at highfeature resolutions and density using low-cost and highthroughput are actively sought in this direction. The geometric attributes of the nanopatterns, viz. density, size, shape, composition, and material quality, play key roles in the functional properties of the resulting patterns. Several topdown and bottom-up techniques have attempted to meet these goals, with varying degrees of success. Among these techniques, some examples of those that have shown to cater to macroscopic arrays are nanoimprint lithography (NIL),1,2 laser interference lithography,3 photolithography,4 and nanostencils5 among top-down methods, and block copolymer lithography (BCL),6−8 nanosphere lithography,9 and anodized alumina10,11 among bottom-up methods. Besides the feasibility of delivering templates at high resolutions, considerations such as process compatibility with existing manufacturing tools and ability for integration within devices have played key roles in influencing the preferred choice of techniques, especially for nanoscale devices. BCL and NIL have proven value in fabrication down to sub-10 nm length scales, and both © 2012 American Chemical Society
Received: July 19, 2012 Revised: September 17, 2012 Published: October 1, 2012 23729
dx.doi.org/10.1021/jp307152s | J. Phys. Chem. C 2012, 116, 23729−23734
The Journal of Physical Chemistry C
Article
chemical reactions to form inorganic films yet markedly differs in the high degree of control over the growth rates attainable and its high sensitivity to the surface functionality. The latter aspect has been exploited to achieve patterned arrays by selectively masking certain areas of the surface. Patterns of polymers16−21 and self-assembled molecular layers22−27 created using photolithography,17,18,23,24 soft-lithography,21,22,25,26 and BCL16,20,27 have been used as ALD masks. The use of BCL and NIL as means of producing a template of polymers for ALD has not been previously investigated. We demonstrate this for ZnO arrays with high-resolution and quality and investigate their electrical properties toward charge-trap flash memories. Whereas BCL by itself has been shown to produce templates suitable for selective area ALD, its combination with NIL brings in additional advantages. Specifically, one could attain the polymer templates suitable for selective area ALD within a few minutes of processing. In contrast, a BCL-only processing would demand several steps repeated every time a template is desired. A typical NIL process would involve spin-coating of a resist, followed by a rapid thermal or UV-assisted imprinting sequence, and is simpler to implement. Another significant advantage is the reusability of NIL molds for several imprinting sequences. For example, the high-resolution NIL molds employed in this work could be used in excess of 50 times without loss of reproducibility. ZnO is among extensively investigated materials in the context of their use in LEDs,28 memory devices,29 antireflection coatings,30 solar cells,31 and piezoelectric actuators.32 Patterned arrays of ZnO have been reported using different approaches,1,2,10,11,33,34 whereas those that produce macroscopic arrays at high resolution are limited. ZnO by vapor−liquid− solid (VLS) growth1,35 allows inexpensive production of highdensity nanowire arrays using self-assembled catalysts. The process is better suited for high aspect ratios than for welldefined shapes at low-aspect ratios. In addition to fabricating well-defined ZnO patterns, we investigate their charge trapping and retention behavior to assess their potential in catering to charge-trap flash (CTF) memories. CTF memories are nonvolatile memories that store information as charges within trap states in a continuous thin film. This is an alternative to the nanocrystal flash (NCF) memories,36−38 where charges are stored within isolated nanocrystals. The memory window of the device relates to the charge-storage capacity of the device, which translates as the density of trap states available to store charges. The longevity of the stored memory relates to the retentivity of charges, which in turn translates as the proportion of trap states with deep potential energy wells, and a low-level leakage.39 The commonly used USB flash memory devices are CTF memories that employ a silicon-oxide-nitride-oxide-silicon (SONOS) stack with Si3N4 as the charge-trap layer.39 The classical SONOS suffers from the limitation of a low dielectric constant of 7.5 for silicon nitride together with the leakage of charges from the shallow trap levels due to its small conduction band offset with Si.40,41 This may require the use of a thicker SiO2 tunnel oxide layer to prevent charge leakage and thereby enhance charge retention. The thicker oxide, however, needs higher program/erase voltages and lowers writing speeds.42 As a means of countering these drawbacks, ZnO provides promising advantages because it is known to possess a rich density of discrete charge-trap levels due to oxygen vacancies and zinc interstitials.43 The trap states in ZnO nanostructures are energetically deeper than in SONOS.44,45 Moreover, its
large conduction band offset with SiO2 allows better retention of charges within ZnO.40 In addition to these material advantages, the isolated nature of ZnO features within nanopatterns carry potential advantages of reducing lateral charge conduction and minimizing charge leakage due to localized defects in the tunneling oxide, thereby enhancing charge retention. Consequently, nanopatterns of ZnO can be expected to encompass advantages of both CTF and NCF memories. We demonstrate in this work that the discrete atomic layer deposited ZnO nanopatterns realized through a combination of BCL and NIL approaches exhibit high chargestorage capacity on par with state of the art SONOS memories39,46 and enhanced retention for low tunnelling oxide thickness of 3 nm at low operating voltages.
2. EXPERIMENTAL SECTION 2.1. Materials. Poly(methyl methacrylate) (PMMA) of molecular weight 75 000 g/gmol was purchased from Microresist Technologies (Berlin, Germany) and used without further purification. 2-Propanol and acetone were obtained as anhydrous solvents with purity >99% from Sigma-Aldrich. Prime grade silicon wafers were obtained from Silicon Valley Microelectronics (Santa Clara, CA). 1H,1H,2H,2H-Perfluorododecyltrichlorosilane with a purity of 97% was obtained from Sigma Aldrich. Silicon wafers with thermally grown oxide were obtained from Global Foundries, Singapore. Point Probe Plus silicon tips for tapping mode imaging measurements with atomic force microscopy were purchased from Nanosensors (Neuchatel, Switzerland). 2.2. Methods. The silicon substrates were diced and cleaned by ultrasonicating in acetone, followed by 2-propanol, and finally exposed to UV/ozone (UV-1, SAMCO, Kyoto, Japan) for 10 min. Ellipsometry (WVASE 32, J.A.Woollam, Lincoln, NE) was used to measure thickness of the oxide layers. The NIL molds were fabricated as previously reported8 using reverse micelle arrays coated from 0.5% (w/w) solution of the copolymer from m-xylene spin coated (CEE model 100CB spinner, Brewer Science, Rolla, MO) at 5000 rpm on SiO2(25 nm)/Si substrates as etch masks using C4F8/CH4 and SF6/C4F8 plasmas for the SiO2 and Si etching, respectively. AFM (Nanoscope IV Multimode AFM, Veeco Instruments, Plainview, NY), FESEM (JEOL 6700F, Tokyo, Japan), and TEM (Philips CM300, Amsterdam, The Netherlands) were used to characterize the films during each step of fabrication. The TEM was equipped with DX4 EDS system and Gatan Filter that enables high-resolution elemental analysis. Thin films (∼100 nm) of PMMA (Microresist Technologies, Germany) used as NIL resists were obtained by spin-coating the resist formulation (as-purchased) at 3000 rpm. NIL molds were first treated with O2 plasma, followed by 1H,1H,2H,2H-perfluorododecyltrichlorosilane in vapor phase within a desiccator at a pressure of
