Micro-Electro-Mechanical Systems technology, commonly known with the acronym MEMS, refers to the fabrication of devices with dimensions on the micrometer scale. The most important characteristic of MEMS is that electronic components are often integrated with micro-mechanical ones, movable or stationary, resulting in very complex miniaturized machine. The low power consumption requirements that derive from their small component size has made these system very precious, as evident from their numerous commercial applications, for instance ink jet printer heads, accelerometers for airbags, chemical sensors and many others. These structures are created via fabrication processes and equipment devoted for the Integrated Circuit (IC) industry. The fabrication of MEMS commonly involves bulk micromachining, surface micromachining or LIGA process.
Bulk micromachining defines microstructures by etching directly into the bulk material such as silicon crystal. In surface micromachining, microcomponent structures are defined via both additive (growth or deposition techniques) and subtractive process (etching process). Both techniques allow the use of integrated circuit technology. The LIGA process is very similar to bulk and surface micromachining; the bigger difference arises from the radiation source employed to transfer desired micro-features onto the substrate. Both surface and bulk micromachining use the UV radiation while the LIGA the X-ray radiation that permit to penetrate higher layers of resist and then to fabricate taller structures. However, specific techniques can be combined with the previous ones to create features in the nano-scale range, as it will be described later in the fabrication process of self-standing metallic nano-gap MEMS structures.
More importantly, the micromachinable materials and the range of processes now extend far beyond just those found in the IC industry. Medicine and biology are amongst the most promising and most challenging fields of application for MEMS. This does not come as a surprise considering that the technology has the capability to fabricate minimally invasive yet highly functional devices that match the size range of many structures found in the human body. Examples include pressure sensors that are small enough to fit through 1-^m catheters or pacemakers that have incorporated micro-scale accelerometers to pace the heart in proportion to the patient activity. These small and tiny devices, also referred to as Biomedical Micro-Electro-Mechanical Systems (Bio-MEMS), hold promises for precision surgery with micrometer control, for rapid screening of common diseases and genetic predispositions, and for implantable drug delivery systems for controlled release of drugs in dose and time.
The development of new, affordable, disposable analytic microchips is changing diagnostics. Examples of analytical functions that are benefiting from such developments include blood supply screening, analysis of biopsy samples and body fluids, minimally invasive and non-invasive diagnostic procedures, rapid identification of disease, and early screening. These systems will eventually perform diagnostic procedures in a multiplexed format that incorporates multiple complementary methods. Ultimately, these systems will be combined with other devices to create completely integrated analysis and disease-treatment systems. There are a number of mechanisms to provide timed release of drugs, such as micro-encapsulation and transdermal release (discussed earlier). Implantable Bio-MEMS are preferred for therapies that require several daily injections, such as for diabetes treatment. If the drug level is monitored in real time, it could also be adapted to metabolic variations. In treatments like chemotherapy, the device can be implanted where the drug is most needed. Among these techniques, implantable pumps have the advantage that the drug therapy can be delivered at the optimal time and concentration to a specific site.
4.6.1. Self-standing Metallic Nanogap MEMS Structures for Nano Trapping Application
The investigation of electronic transport through individual molecules or nano-scale objects has attracted considerable interest as an important step towards the fabrication of molecular devices whose functions are specifically defined by the electrical junctions in which single molecules or small molecular assemblies. An attractive approach to study single molecular properties is based on use of nano-gap electrodes, namely a pair of electrodes with a nano-meter gap [128-130]. By using this approach, single molecular samples are measured under conditions similar to those of real devices. In view of this, techniques for fabricating metal electrodes separated by nanometre-scale gaps are essential. Over the past few years, a number of methods for fabricating narrow-gap electrodes have been developed which includes mechanical break junctions [131-133] and electrochemical plating [130-134] for atomic-scale precision in the gap size. Alternatively, shadow evaporation [135, 136] and electro-migration-induced breaking of electron-beam lithography defined thin-metal nano-wires [137-139] permit only a crude control of the gap size. To use these nano-gaps as test-beds for single molecules requires the ability to tune the size of the gap.
In an alternative approach, we have focused our attention on NEMS like devices which are capable of adjusting the relative intra-electrode gap size of a pre-programmed gap by means of electro-thermal actuation which develop in adjacent conductors when currents flows into them. For this, we have fabricated nano-gap tip structures with intra-tip gap size falling into the sub-20 nm resolution. Electron beam lithography (EBL) and electroplating process have been used to fabricate two- and three-terminal nano-devices consisting of tip-shaped free standing electrodes with tip separation variable ranging between 10-100 nm. A silicon nitride (Si3N4) membrane, coated by 1.2 ^m of PMMA, is exposed at 30 KeV with a beam current of 1 nA. The exposure dose was calculated including proximity correction effects. The patterned samples were then electroplated in a commercial Nickel bath at 55 °C. The thickness of the deposited layer was 0.8-0.9 |j.m. An ion milling process with Argon was carried out to remove the base plating film from the substrate. The final step was to create the self-standing structure by removing the Si3N4 membrane. This standard EBL process followed by electroplating had allowed us to obtain nano-tips with separations as smaller as 50 nm. In order to obtain separations in 10-20 nm range between nano-tips, before the EBL process, we used a defocused electron-beam to brush the PMMA layer for a time t ranging from 15 to 50s depending on the resist thickness. Thus the resist is partially exposed and, during the following EBL process, the Gaussian shape of the e-beam allows the resist near the edges of the tip to reach the right dose to be developed. We did not evaluate the effect of defocused e-beam brushing exposure time (t) on the reduction of separation between tips. However experimental results showed that for t ranges from 20-50 s, the inter-electrode distance reduced almost linearly from 40nm down to 10nm; for t higher than 60s, the tip separation could easily be reached 5 nm. A careful calibration of the brushing time between 40 and 60 s thus had allowed the reduction of inter tip gap to the 5-10 nm range. All the fabricated nano-devices were inspected by plane-view scanning electron microscopy (SEM) in order to establish the viability of the whole technological
process and the separation between the nano-tips. In figure 4.37, SEM micrographs of several fabricated self-standing nano-gap electrodes tips with intra-tip gap in 5-70 nm range are shown.
Fabricated structures use resistive heating (Joule effect) to generate thermal expansion and movement. The power needed to adjust the relative distance between adjacent features is relatively small: for instance, by applying to the circuits a current of few tens of mA, the driving force overcomes the intrinsic elastic force of our nickel/gold microstructures, and a self standing electrodes change their distance by an amount of about 100 nm.
With our designed adjustable nano-electrode, we are exploring the possibility to make electrical measurements as a function of the length of stretched molecules. The electrodes, when covered or made by gold as shown in figure 4.38, can be useful to trap molecules functionalized with thiolic groups due to the highly stable bound between gold a sulphur.
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