ABSTRACT
We present a simple and versatile patterning procedure for the reli-able and reproducible fabrication of high-aspect-ratio (104) electri-cal interconnects that have separation distances down to 20 nm and lengths of several hundreds of microns. The process uses standard optical lithography techniques and allows parallel processing of many junctions, making it easily scalable and industrially relevant. To illustrate the advantage of the nanotrench architecture for nano and molecular electronics applications, two different kinds of working devices are presented, spanning the ranges from high-density current systems, up to the mA range, to highly resistive nanoparticles network with sup pA current, and from micro down to nano interconnects systems. Trapping of ferromagnetic
intermediate scale particles for multiscale interconnects purposes is also presented. 6.1 IntroductionNew breakthroughs envisioned for molecular and nano electronics in the area of information storage and processing [1,2] have motivated a significant boost to both fundamental and applied research in the area. Electronic transport through nanoscale down to molecular entities can potentially take advantage of exciting multi-functional properties, with the expectation of dominant quantum mechanical effects and original chemical tuning opportunities. As the field is becoming more mature, it is becoming increasingly clear that the realization of robust single molecule or single nanoparticle devices exhibiting reliable and reproducible behavior is a very challenging experimental and technological task [3-7]. Among the main problems to tackle, which severely limit the possibility of achieving reliable single entity-based devices for scaling up industrial production, are, for instance, their high sensitivity to chemical and electromagnetic environment [8,9], the critical role played by defects or uncontrolled doping in the electronic transport properties [10,11] or the control at nanometer scale precision of the positioning of a single molecular-sized entity in an electronic circuitry. The development of single molecule-based electronic devices for industrial purposes seems, therefore, premature at this time. A possible route to reconcile the molecular scale (below 1 nm) and the top-down nanofabrication route (larger than 10 nm) is to use a multi-scale approach, where a cluster (metallic, semimetallic, or semiconductor) of typically 10 nm size [12] or a conducting microsphere, of typical 102−103 nm size, is used as an intermediate bridge. One can also take advantage of bottom-up fabrication methods for incorporating macroscopic amounts of identical molecules into devices or to assemble well-organized monolayers of nanoparticles [13−15]. Key advantages are simplicity, ability to cover large areas, and cheap manufacturing costs. Large numbers of interconnected molecules also ensure device robustness through ensemble averaging [16,17]. Nanoscale entities such as functionally ordered arrays of nanoparticules also represent an alternative opportunity for designing new devices such as gas sensor, electro-optic devices, or magneto-electronic nanodevices [18−20]. The
transport and magneto-transport mechanisms taking place are often very sensitive to electrode/nanoparticle molecular interface, which can create artifacts or difficulties in unambiguous interpretation of the data. Devices based on multiple connected nanoparticles should be less sensitive to such artifacts and improve the reproducibility of the measurements from one sample to the other. Hence, there is a need for a versatile configuration of device, compatible with bottom-up fabrication methods and made by standard optical lithography techniques to achieve low manufacturing costs. Planar electrode geometry, directly inspired by top-down nanofabrication techniques, provides several key advantages. First, the integration of a three-terminal configuration device capable of gating the flow of carriers can be easily implemented. Furthermore, planar substrates are also ideally compatible with bottom-up fabrication methods involving nanoparticle layers such as Langmuir-related techniques. Finally, the open direct access to the middle molecules or nanoparticle layers optimizes chemical and optical control or excitation. This is particularly attractive if we want to gain access to the intrinsic properties of the molecules for the purpose of realizing multi-functional devices. Lateral geometry also provides unique advantages versus the more ubiquitous vertical geometry [21−23], where metal electrodes sandwich an organic film. Such geometry suffers from the key problem of possible formation of leaking metallic paths during the deposition of the top electrode [24,25], which can only be avoided by adding an intermediate highly conductive polymer film [16]. We present a recently developed process [26] to design wide electrodes (potentially up to mm scale), separated by a short distance (down to 20 nm) corresponding to typical thicknesses in organic vertical structures. There is an extensive literature on nanofabrication techniques for realizing closely spaced electrodes, and it is clear that such dimensions should be easily achievable by e-beam lithography tools. One should, however, emphasize that the critical issue of obtaining high-aspect-ratio (HAR) gaps below 50 nm is an extremely challenging task, necessitating state-of-the-art electron beam equipment, which might suffer from reliability issues when extremely high aspect ratios are desired. More importantly, the use of a sacrificial layer becomes necessary if aspect ratio exceeding 10−100 are needed. Layers made of aluminum oxide [27], chromium oxide [28] or molecular monolayers [29] are reported in
the literature. All these techniques suffer from the requirement to have a robust etching process, which needs to be critically selective and critically efficient. Realizing HAR gap devices becomes of key importance when materials with intrinsically large resistivity are investigated. In the field of nanoelectronics, materials with high sheet resistance values are often encountered. Hopping transport makes low-temperature measurements critical and becomes a practical limitation to studies. Conversely, when low resistance samples are investigated, series resistance of the leads, and their intrinsic resistivity, might limit the measurement quality and reliability. There is, therefore, a need for realizing planar electrode interconnects with large aspect ratios, exhibiting good interface resistance, and compatibility with low and high impedance measurement systems. One industrially attractive approach that does not rely on electron beam lithography, maximizes the reliability for HAR trenches, and minimizes contamination is the so-called shadow edge evaporation method [26,30]. This technique has been used previously to make nanogaps of very small widths (around 100 nm) to contact single entities [31,32] or to make a template for nanowire and nanochannel patterning [30,33].We have developed a process [26] using only standard optical lithography to make HAR gap devices consisting of two planar electrodes separated by a “nanotrench” 20 to 100 nm wide and up to 100 µm in length, i.e., with a ratio up to 1:5000. This ratio can be straightforwardly increased by 1−2 orders of magnitude by using an inter-digitized geometry or by patterning longer electrodes. This is because the process is based on a simple geometrical effect which can be effective for any length. It would be only limited by the optical lithography capability and intrinsic wafer defects. Avoiding the use of more sophisticated lithography techniques makes our fabrication strategy more relevant for industrial up-scaling. Our goal is to provide compelling evidence of the adequacy of the shadow-edge evaporation approach for making trenches with no detrimental structural defects for applications in electronics. We demonstrate the interest and reliability of this approach by presenting two examples of applications for nanoparticle and microparticle electrical properties, showing that no leakage or shorts are present in our structures, and that the electrical contacts are of good quality and suitable for a wide range of current measurements, going from sub-pA measurements up to tens of mA currents.
