Nicolaie Moldovan

Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices

MEMS devices are currently fabricated primarily in silicon because of the available surface machining technology. A major problem with the Si-based MEMS technology is that Si has poor mechanical and tribological properties J.J. Sniegowski, in: B.

Design and Operation of a MEMS-Based Material Testing System for Nanomechanical Characterization

In situ mechanical characterization of nanostructures, such as carbon nanotubes and metallic nanowires, in scanning and transmission electron microscopes is essential for the understanding of material behavior at the nanoscale. This paper describes the design, fabrication, and operation of a novel microelectromechanical-systems (MEMS)-based material testing system used for in situ tensile testing of nanostructures. The device consists of an actuator and a load sensor with a specimen in between. Two types of actuators, in-plane thermal and comb drive actuators, are used to pull the specimens in displacement control and force control modes, respectively. The load sensor works based on differential capacitive sensing, from which the sensor displacement is recorded. By determining sensor stiffness from mechanical resonance measurements, the load on the specimen is obtained. Load sensors with different stiffness were fabricated. The best resolutions were achieved with load sensors that are designed for testing nanotubes, reaching 0.05 fF in capacitance, 1 nm in displacement, and 12 nN in load. For the first time, this MEMS-based material testing scheme offers the possibility of continuous observation of the specimen deformation and fracture with subnanometer resolution, while simultaneously measuring the applied load electronically with nano-Newton resolution. The overall device performance is demonstrated by testing freestanding cofabricated polysilicon films and multiwalled carbon nanotubes.

A microelectromechanical load sensor for in situ electron and x-ray microscopy tensile testing of nanostructures

We report on the performance of a microelectromechanical system (MEMS) designed for the in situ electron and x-ray microscopy tensile testing of nanostructures, e.g., carbon nanotubes and nanowires. The device consists of an actuator and a load sensor with a gap in between, across which nanostructures can be placed, nanowelded, and mechanically tested. The load sensor is based on differential capacitance measurements, from which its displacement history is recorded. By determining the sensor stiffness, the load history during the testing is obtained. We calibrated the device and examined its resolution in the context of various applications of interest. The device is the first true MEMS in which the load is electronically measured. It is designed to be placed in scanning and transmission electron microscopes and on x-ray synchrotron stages.

Fracture strength of ultrananocrystalline diamond thin films—identification of Weibull parameters

The fracture strength of ultrananocrystalline diamond (UNCD) has been investigated using tensile testing of freestanding submicron films. Specifically, the fracture strength of UNCD membranes, grown by microwave plasma chemical vapor deposition (MPCVD), was measured using the membrane deflection experiment developed by Espinosa and co-workers. The data show that fracture strength follows a Weibull distribution. Furthermore, we show that the Weibull parameters are highly dependent on the seeding process used in the growth of the films. When seeding was performed with microsized diamond particles, using mechanical polishing, the stress resulting in a probability of failure of 63% was found to be 1.74 GPa, and the Weibull modulus was 5.74. By contrast, when seeding was performed with nanosized diamond particles, using ultrasonic agitation, the stress resulting in a probability of failure of 63%, increased to 4.13 GPa, and the Weibull modulus was 10.76. The tests also provided the elastic modulus of UNCD, whi...

Mechanical properties of ultrananocrystalline diamond thin films relevant to MEMS/NEMS devices

The mechanical properties of ultrananocrystalline diamond (UNCD) thin films were measured using microcantilever deflection and membrane deflection techniques. Bending tests on several free-standing UNCD cantilevers, 0.5 μm thick, 20 μm wide and 80 μm long, yielded elastic modulus values of 916–959 GPa. The tests showed good reproducibility by repeated testing on the same cantilever and by testing several cantilevers of different lengths. The largest source of error in the method was accurate measurement of film thickness. Elastic modulus measurements performed with the novel membrane deflection experiment (MDE), developed by Espinosa and co-workers, gave results similar to those from the microcantilever-based tests. Tests were performed on UNCD specimens grown by both micro and nano wafer-seeding techniques. The elastic modulus was measured to be between 930–970 GPa for the microseeding and between 945–963 GPa for the nanoseeding technique. The MDE test also provided the fracture strength, which for UNCD was found to vary from 0.89 to 2.42 GPa for the microseeded samples and from 3.95 to 5.03 for the nanoseeded samples. The narrowing of the elastic modulus variation and major increase in fracture strength is believed to result from a reduction in surface roughness, less stress concentration, when employing the nanoseeding technique. Although both methods yielded reliable values of elastic modulus, the MDE was found to be more versatile since it yielded additional information about the structure and material properties, such as strength and initial stress state.

Preventing Nanoscale Wear of Atomic Force Microscopy Tips Through the Use of Monolithic Ultrananocrystalline Diamond Probes

Nanoscale wear is a key limitation of conventional atomic force microscopy (AFM) probes that results in decreased resolution, accuracy, and reproducibility in probe-based imaging, writing, measurement, and nanomanufacturing applications. Diamond is potentially an ideal probe material due to its unrivaled hardness and stiffness, its low friction and wear, and its chemical inertness. However, the manufacture of monolithic diamond probes with consistently shaped small-radius tips has not been previously achieved. The first wafer-level fabrication of monolithic ultrananocrystalline diamond (UNCD) probes with <5-nm grain sizes and smooth tips with radii of 30-40 nm is reported, which are obtained through a combination of microfabrication and hot-filament chemical vapor deposition. Their nanoscale wear resistance under contact-mode scanning conditions is compared with that of conventional silicon nitride (SiN(x)) probes of similar geometry at two different relative humidity levels (approximately 15 and approximately 70%). While SiN(x) probes exhibit significant wear that further increases with humidity, UNCD probes show little measurable wear. The only significant degradation of the UNCD probes observed in one case is associated with removal of the initial seed layer of the UNCD film. The results show the potential of a new material for AFM probes and demonstrate a systematic approach to studying wear at the nanoscale.

Nanofountain-probe-based high-resolution patterning and single-cell injection of functionalized nanodiamonds.

Nanodiamonds are rapidly emerging as promising carriers for next-generation therapeutics and drug delivery. However, developing future nanoscale devices and arrays that harness these nanoparticles will require unrealized spatial control. Furthermore, single-cell in vitro transfection methods lack an instrument that simultaneously offers the advantages of having nanoscale dimensions and control and continuous delivery via microfluidic components. To address this, two modes of controlled delivery of functionalized diamond nanoparticles are demonstrated using a broadly applicable nanofountain probe, a tool for direct-write nanopatterning with sub-100-nm resolution and direct in vitro single-cell injection. This study demonstrates the versatility of the nanofountain probe as a tool for high-fidelity delivery of functionalized nanodiamonds and other agents in nanomanufacturing and single-cell biological studies. These initial demonstrations of controlled delivery open the door to future studies examining the nanofountain probes potential in delivering specific doses of DNA, viruses, and other therapeutically relevant biomolecules.

Elasticity, strength, and toughness of single crystal silicon carbide, ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon

In this work, the authors report the mechanical properties of three emerging materials in thin film form: single crystal silicon carbide (3C-SiC), ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon. The materials are being employed in micro- and nanoelectromechanical systems. Several reports addressed some of the mechanical properties of these materials but they are based in different experimental approaches. Here, they use a single testing method, the membrane deflection experiment, to compare these materials’ Young’s moduli, characteristic strengths, fracture toughnesses, and theoretical strengths. Furthermore, they analyze the applicability of Weibull theory [Proc. Royal Swedish Inst. Eng. Res. 153, 1 (1939); ASME J. Appl. Mech. 18, 293 (1951)] in the prediction of these materials’ failure and document the volume- or surface-initiated failure modes by fractographic analysis. The findings are of particular relevance to the selection of micro- and nanoelectromechanical systems m...

Roughning and smoothing dynamics during KOH silicon etching

Abstract We studied the influence of surface preparation prior to KOH etching and of surfactants added to the etchant over the etching rates and roughness of the Si (111) and Si (100) surfaces. The investigated etchants were 25% KOH at 70°C, and 25% KOH with small amounts of anionic, cationic and non-ionic surfactants. The surface preparation refers to the use of the following solutions for the native oxide removal: HF:H2O 1:10 (followed by DI water rinsing and drying), HF:C2H5OH 1:10 (dried without any further rinsing), and 10% HCl in HF:H2O 1:1 (also dried without rinsing). The evaluations were made by mechanical profilometry and AFM. No difference between the samples dipped in HF–water and HF–alcohol could be observed. The etching rate of the samples dipped in HCl-containing solutions were greater, while their roughness was diminished. We analyzed the influence of surfactants on the roughness and the anisotropy. The etch rates increased when using cationic and anionic surfactants and decreased with non-ionic ones. The anisotropy is modified by surfactants. Tentative explanations for the roughning mechanisms are proposed.

A multi-ink linear array of nanofountain probes

After the successful design and fabrication of a single-probe nanofountain pen, a second-generation device with two on-chip ink reservoirs feeding a linear array of 12 microfluidic cantilever probes was manufactured. The new device excels by sharper and more uniform tips, more robust fabrication and improved performance in writing and imaging. Its capabilities in writing sub-100 nm features and with two different inks delivered in liquid phase were demonstrated. Their applications range from an affordable method of nanopatterning with sub-100 nm resolution to microspotters for bio-assay generation and tools for combinatorial nanoscale biochemical experiments.