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Top 20 Most Read Articles
December 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Double oxidation scheme for tunnel junction fabrication J. Vac. Sci. Technol. B 26, 28 (2008); http://dx.doi.org/10.1116/1.2817629 (4 pages) Online Publication Date: 2 January 2008
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The authors report a method to achieve Al–AlOx–Al tunnel junctions with high specific resistance in a controlled manner using a double oxidation technique. The technique is based on the standard method for oxidation repeated on an additional Al layer. The tunnel junctions were characterized with standard methods, such as comparison of room temperature resistance with liquid helium resistance and the authors found them to be of comparable quality to junctions fabricated with standard single oxidation. Fitting with the Simmons model suggests that both the barrier width and barrier height are consistent with those obtained in a single oxidation step. The junction specific capacitance was determined at low temperature to be 68 fF/μm2. These junctions, employed in low temperature measurements and applications, demonstrate expected and stable conductance characteristics. The double oxidation method is straightforward to implement in a basic setup for tunnel junction fabrication.
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Single atom doping for quantum device development in diamond and silicon J. Vac. Sci. Technol. B 26, 2596 (2008); http://dx.doi.org/10.1116/1.2968614 (5 pages) Online Publication Date: 1 December 2008
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The ability to inject dopant atoms with high spatial resolution, flexibility in dopant species, and high single ion detection fidelity opens opportunities for the study of dopant fluctuation effects and the development of devices in which function is based on the manipulation of quantum states in single atoms, such as proposed quantum computers. The authors describe a single atom injector, in which the imaging and alignment capabilities of a scanning force microscope (SFM) are integrated with ion beams from a series of ion sources and with sensitive detection of current transients induced by incident ions. Ion beams are collimated by a small hole in the SFM tip and current changes induced by single ion impacts in transistor channels enable reliable detection of single ion hits. They discuss resolution limiting factors in ion placement and processing and paths to single atom (and color center) array formation for systematic testing of quantum computer architectures in silicon and diamond.
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J. Vac. Sci. Technol. B 26, 2529 (2008); http://dx.doi.org/10.1116/1.2976600 (5 pages) Online Publication Date: 1 December 2008
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Three-dimensional (3D) alignment with 10 nm order accuracy in 3D electron-beam (EB) lithography has been achieved by means of highly accurate rotation control and mark location using the transmission electron signal. Accurately aligned EB writing from various directions on micron order resists blocks on a small substrate provides great structural flexibility in the creation of 3D nanostructures. As a demonstration of the accuracy, a 3D hydrogen silsesquioxane nanostructure was made by 3D EB writing and two-step development using different developers. Moreover, a 3D poly(methyl methacrylate) nanostructure was made by repeated 3D EB writing and development to demonstrate 3D nanofabrication with great structural flexibility.
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J. Vac. Sci. Technol. B 26, 2604 (2008); http://dx.doi.org/10.1116/1.2978487 (7 pages) Online Publication Date: 1 December 2008
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Nanoimprint lithography presents unique opportunities for patterned media applications due to its advantages of sub-10 nm resolution capability, patterning of a whole disk in a single imprint step with reasonably high throughput, and the relatively low capital cost in comparison to other next generation lithography technologies. However, there are several critical issues that still remain very challenging. This article will briefly discuss these challenges in general and then focus on imprint lithography work including the fabrication of templates and demonstrate the imprinted results. In this work two types of polarities of high-density templates (pillar tone and hole tone) have been fabricated on fused silica substrates for the UV imprint process. The difficulties and limitations in each of the template fabrication processes will be discussed. The authors have successfully demonstrated template fabrication followed by imprinted results with a pitch of 24 nm (1.1 Tdots/in.2) for both tones of templates. Initial imprinted results of dense dot patterns with a pitch as small as 18 nm (2.0 Tdots/in.2) have been achieved. High-resolution scanning electron microscopy images are used as the primary metrology for both the dot size uniformity and the placement accuracy analysis. The difficulties and the limitations in template fabrication, the imprint process, and associated metrology will be discussed.
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Nanoimprint lithography: An old story in modern times? A review J. Vac. Sci. Technol. B 26, 458 (2008); http://dx.doi.org/10.1116/1.2890972 (23 pages) Online Publication Date: 27 March 2008
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Nanoimprint lithography (NIL) is a high throughput, high-resolution parallel patterning method in which a surface pattern of a stamp is replicated into a material by mechanical contact and three dimensional material displacement. This can be done by shaping a liquid followed by a curing process for hardening, by variation of the thermomechanical properties of a film by heating and cooling, or by any other kind of shaping process using the difference in hardness of a mold and a moldable material. The local thickness contrast of the resulting thin molded film can be used as a means to pattern an underlying substrate on wafer level by standard pattern transfer methods, but also directly in applications where a bulk modified functional layer is needed. Therefore it is mainly aimed toward fields in which electron beam and high-end photolithography are costly and do not provide sufficient resolution at reasonable throughput. The aim of this review is to play between two poles: the need to establish standard processes and tools for research and industry, and the issues that make NIL a scientific endeavor. It is not the author’s intention to duplicate the content of the reviews already published, but to look on the NIL process as a whole. The author will also address some issues, which are not covered by the other reviews, e.g., the origin of NIL and the misconceptions, which sometimes dominate the debate about problems of NIL, and guide the reader to issues, which are often forgotten or overlooked.
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J. Vac. Sci. Technol. B 10, 1237 (1992); http://dx.doi.org/10.1116/1.585897 (30 pages)
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The status of research on both wurtzite and zinc‐blende GaN, AlN, and InN and their alloys is reviewed including exciting recent results. Attention is paid to the crystal growth techniques, structural, optical, and electrical properties of GaN, AlN, InN, and their alloys. The various theoretical results for each material are summarized. We also describe the performance of several device structures which have been demonstrated in these materials. Near‐term goals and critical areas in need of further research in the III–V nitride material system are identified. |
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Metal transfer assisted nanolithography on rigid and flexible substrates J. Vac. Sci. Technol. B 26, 2421 (2008); http://dx.doi.org/10.1116/1.2993172 (5 pages) Online Publication Date: 1 December 2008
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A nanolithography technique based on metal transfer printing is demonstrated. The transferred metal conveniently acts as an etch mask for pattern transfer to a substrate. This lithography technique can be used on flexible plastic substrate as well as on rigid substrate due to the low pressure and temperature used in the metal transfer process. Dense nanosize metal particle arrays with different shapes such as square, diamond, and nanobar were created with high yield over large area, and localized surface plasmon spectra of those particle arrays were measured. Pattern linewidth was reduced to 50 nm in metal grating by depositing metals on the poly(dimethysiloxane) grating sidewall using a shadow evaporation process.
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J. Vac. Sci. Technol. B 26, 2587 (2008); http://dx.doi.org/10.1116/1.2978877 (5 pages) Online Publication Date: 1 December 2008
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This article describes the fabrication process for a silicon single-electron transistor (Si-SET) with tunnel barriers made of thermal silicon dioxide. The fabrication method uses electron-beam lithography, dry etching, and chemical mechanical polishing to make devices with well-defined geometry and good yield. The method introduces chemical mechanical polishing as a key processing technique in the production of nanometer scale features required for the device. The fabricated SETs show charging energies in excess of 20 meV and operating temperatures above liquid nitrogen. All the process steps are fully compatible with the current complementary metal oxide semiconductor (CMOS) technology and combined with the higher operating temperature of the SETs, the fabrication method paves the way for designing of hybrid CMOS-SET architectures. Si-SET can also be a key enabling device for the manufacturable implementation of quantum-dot cellular automata.
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Band offsets of wide-band-gap oxides and implications for future electronic devices J. Vac. Sci. Technol. B 18, 1785 (2000); http://dx.doi.org/10.1116/1.591472 (7 pages)
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Wide-band-gap oxides such as SrTiO3 are shown to be critical tests of theories of Schottky barrier heights based on metal-induced gap states and charge neutrality levels. This theory is reviewed and used to calculate the Schottky barrier heights and band offsets for many important high dielectric constant oxides on Pt and Si. Good agreement with experiment is found for barrier heights. The band offsets for electrons on Si are found to be small for many key oxides such as SrTiO3 and Ta2O5 which limit their utility as gate oxides in future silicon field effect transistors. The calculations are extended to screen other proposed oxides such as BaZrO3. ZrO2, HfO2, La2O3, Y2O3, HfSiO4, and ZrSiO4. Predictions are also given for barrier heights of the ferroelectric oxides Pb1−xZrxTiO3 and SrBi2Ta2O9 which are used in nonvolatile memories. © 2000 American Vacuum Society. |
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Potential of a rotary stage electron beam mastering system for fabricating patterned magnetic media J. Vac. Sci. Technol. B 26, 2611 (2008); http://dx.doi.org/10.1116/1.3013277 (8 pages) Online Publication Date: 1 December 2008
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To fabricate master templates of high-density patterned magnetic media, the authors developed a high-resolution and high-throughput rotary stage electron beam mastering system. They accomplished the fabrication of discrete track media groove patterns with 45 nm track pitch, and also a dense bit array with both track and bit pitch of 35 nm for bit patterned media under the conditions of a bit rate of 1.22 MHz/bit at 50 kV acceleration voltage. The high resolution and high throughput are derived from using a continuous stage movement flyback lithography (CSFL) function. CSFL is enabled by differentiated capabilities, such as an electron optical column that achieves a large beam current at a small beam diameter, and a friction-driven slider stage that provides a highly accurate positioning capability and a dynamic focus correction feature and r-theta stage driving. The CSFL capability, working in conjunction with a blankingless beam shift lithography (BLSL), is very effective for fabricating various kinds of servo pattern elements, such as grooves and dots, grooves and right-angled grooves, grooves and half-pitch-shift dot arrays, and various pit length patterns, to construct effective servo patterns.
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Recent advances in processing of ZnO J. Vac. Sci. Technol. B 22, 932 (2004); http://dx.doi.org/10.1116/1.1714985 (17 pages) Online Publication Date: 26 April 2004
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A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures. © 2004 American Vacuum Society. |
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Directed self-assembly of individual vertically aligned carbon nanotubes J. Vac. Sci. Technol. B 26, 2524 (2008); http://dx.doi.org/10.1116/1.2990788 (5 pages) Online Publication Date: 1 December 2008
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The deposition of high-aspect-ratio particles such as carbon nanotubes may be done in sub-100-nm windows in insulating thin films over metal using electrophoresis. Surface charge on the insulator causes the windows to become nanoscopic electrostatic lenses. Under certain conditions only one nanotube will be deposited at the base of a window. Finite element analysis shows that the number of deposited nanotubes is controlled by the electric field and the geometry of the windows and nanotubes. This discovery enables the process integration of carbon-based electronics with more traditional technologies such as complementary metal oxide semiconductor using the current generation of lithography and process technology. Devices such as vertical field effect transistors and interconnected nanoprobe arrays may now be fabricated in the metal levels to facilitate three-dimensional polylithic circuit architectures.
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Si-containing block copolymers for self-assembled nanolithography J. Vac. Sci. Technol. B 26, 2489 (2008); http://dx.doi.org/10.1116/1.2981079 (6 pages) Online Publication Date: 1 December 2008
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Block copolymers can self-assemble to generate patterns with nanoscale periodicity, which may be useful in lithographic applications. Block copolymers in which one block is organic and the other contains Si are appealing for self-assembled lithography because of the high etch contrast between the blocks, the high etch resistance of the Si-containing block, and the high Flory–Huggins interaction parameter, which is expected to minimize line edge roughness. The locations and long range order of the microdomains can be controlled using shallow topographical features. Pattern generation from poly(styrene)-poly(ferrocenyldimethylsilane) and poly(styrene)-poly(dimethylsiloxane) block copolymers, and the subsequent pattern transfer into metal, oxide, and polymer films, is described.
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Gas-assisted focused electron beam and ion beam processing and fabrication J. Vac. Sci. Technol. B 26, 1197 (2008); http://dx.doi.org/10.1116/1.2955728 (80 pages) Online Publication Date: 11 August 2008
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Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.
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Improvement in linewidth roughness by postprocessing J. Vac. Sci. Technol. B 26, 2265 (2008); http://dx.doi.org/10.1116/1.3013860 (6 pages) Online Publication Date: 1 December 2008
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In order to meet the linewidth roughness (LWR) requirements for the 16 nm node, postprocessing methods need to be investigated to reduce the LWR after the lithography step. We present the results of five different techniques applied to a single extreme ultraviolet photoresist. The results show that rinse has the most promise in achieving the nearly two time LWR improvement needed. However, other techniques such as etch/trim, hardbake, vapor smoothing, and ozonation give at least 10%–20% LWR reduction and could be further optimized. Some of the physical based techniques which melt the photoresist reduce the midspatial frequency (50–10 nm period) roughness, whereas chemical based techniques reduce the low order spatial frequencies ( ∼ 500–50 nm period). Hence, a combination of techniques may be the ultimate solution.
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J. Vac. Sci. Technol. B 26, 2447 (2008); http://dx.doi.org/10.1116/1.2991516 (4 pages) Online Publication Date: 1 December 2008
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For gas-flow aligned growth of carbon nanotubes (CNTs), it is important to minimize interaction of the growing CNTs with the substrate. The authors present a method to fabricate thin catalyst films on top of protruding hydrogen silsesquioxane (HSQ) patterns. Self-alignment of the catalyst film with the HSQ pattern is achieved by exposing two layers of resist, polymethyl methacrylate (PMMA) on top of HSQ, simultaneously. By selecting appropriate development parameters for PMMA and HSQ, a common exposure dose can be applied. After a standard lift-off process HSQ is developed and CNTs are grown on the protruding HSQ patterns resulting in gas-flow aligned CNTs that can be further processed, e.g., for the fabrication of CNT based transistors.
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Control of DNA motion in microchannels integrated with dual electrodes J. Vac. Sci. Technol. B 26, 2578 (2008); http://dx.doi.org/10.1116/1.2991859 (5 pages) Online Publication Date: 1 December 2008
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The authors have fabricated a microfluidic system for the control of DNA motion consisting of 400 or 900 nm wide Si channels covered by a bonded glass layer containing integrated dual electrodes. By applying an ac potential to an electrode pair on both sides of the channels, precise lateral positioning of DNA has been demonstrated for the first time. By modifying the amplitude or frequency of the ac potential applied to the electrode pairs, they control DNA motion within the channel, causing it to increase speed, decrease speed, reverse directions, or halt. The precise control of DNA average position within channels is expected to be an important step in the development of microfluidic single molecule analysis systems.
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Elemental analysis with the helium ion microscope J. Vac. Sci. Technol. B 26, 2103 (2008); http://dx.doi.org/10.1116/1.2993262 (4 pages) Online Publication Date: 1 December 2008
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The newly developed helium ion microscope is an instrument well suited to high resolution surface specific imaging with several unique contrast mechanisms. In addition to its imaging capabilities, the focused helium ion beam (subnanometer in size) has recently been used for elemental analysis. The scattering probability, angular distribution, and recoil energy combine to provide valuable information about the specimen being analyzed.
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Imprinted large-scale high density polymer nanopillars for organic solar cells J. Vac. Sci. Technol. B 26, 2562 (2008); http://dx.doi.org/10.1116/1.2981076 (5 pages) Online Publication Date: 1 December 2008
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Nanoimprint with a large-scale nanoporous Si mold is developed to fabricate high density periodic nanopillars ( ∼ 1010/cm2) in various functional polymers. A anodic alumina membrane is first obtained using electrochemical anodization. The membrane is used as a mask for a two-step plasma etching process to obtain a Si mold of 50–80 nm wide and 100–900 nm deep pores. The mold is used in nanoimprint lithography to fabricate ordered and high density polymer nanopillars and nanopores in SU-8, hydrogen silsesquixane, polymethylmethacrylate, poly(3-hexylthiophane) (P3HT), and phenyl-C61-butyric acid methyl ester (PCBM). Then, the imprinted P3HT nanopillars were used to make bulk heterojunction solar cells by depositing PCBM on top of the pillars. Imprinting provides a way to precisely control the interdigitized heterojunction morphology, leading to improved solar cell performance.
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Damage mechanism in low-dielectric (low-k) films during plasma processes J. Vac. Sci. Technol. B 26, 1926 (2008); http://dx.doi.org/10.1116/1.3010721 (7 pages) Online Publication Date: 10 November 2008
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Plasma is extensively used for the etching/ashing of low-dielectric (low-k) films. However, since low-k films, such as SiOC films, are vulnerable to plasma irradiation, they are severely damaged during plasma processes, such as the extraction of methyl groups from low-k films. As a result, plasma irradiation increases the dielectric constant of low-k films and reduces the reliability of Cu/low-k interconnects. In previous work, the authors achieved highly selective and low-damage etching processes for low-k films by using their developed neutral beam process instead of the conventional plasma process. They have now investigated the damage mechanism in low-k films (porous SiOC films) during plasma processes by clarifying the effects of ions, radicals, and photons in plasma. First, they compared the damage in SiOC films etched by the conventional plasma process and the neutral beam process. Their results show that plasma processes change the structure of the SiOC film deeply within the film (over 100 nm in depth) and increase the film’s dielectric constant, whereas the neutral beam process maintains the structure of the SiOC film. Additionally, they found that when a combination of the neutral beam process and a 172 nm excimer lamp is used, photon irradiation enhances the extraction of methyl groups from the SiOC film by breaking Si–C bonds in the film. These results show that photon irradiation plays a very important role in the damage mechanism in low-k films during plasma processes.
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