David M. Tanenbaum

Mechanical properties of suspended graphene sheets

Using an atomic force microscope, we measured effective spring constants of stacks of graphene sheets (less than 5) suspended over photolithographically defined trenches in silicon dioxide. Measurements were made on layered graphene sheets of thicknesses between 2 and 8nm, with measured spring constants scaling as expected with the dimensions of the suspended section, ranging from 1to5N∕m. When our data are fitted to a model for doubly clamped beams under tension, we extract a Young’s modulus of 0.5TPa, compared to 1TPa for bulk graphite along the basal plane, and tensions on the order of 10−7N.

The OE-A OPV demonstrator anno domini 2011

Polymer solar cells were prepared in large numbers using roll-to-roll methods and were subsequently integrated into the Organic Electronics Association (OE-A) demonstrator in the year 2011 and presented as a small credit card sized lamp with a flat outline. The lamp comprised the polymer solar cell together with printed circuitry, discrete components and flexible lithium polymer batteries. The number of discrete steps required for the manufacture of the lamp was 35 and more than 10 000 units of the demonstrator was manufactured. We describe the efforts towards increasing the technical yield which was 89% overall and discuss the compromises that had to be made to achieve the high technical yield for a process that was as automated as possible. All the steps in the preparation of the solar cell, the circuitry and the overlays were performed using full roll-to-roll methods. The mounting of the discrete components, such as LED, diode and Zener diode, was performed in sheets of 15 units using a fully automated SMD mounting machine. The mounting of the batteries, contacts and final testing was done manually. The lamination into the final lamp and the final laser cutting into the discrete lamps were performed using automated systems.

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

High resolution electron beam lithography using ZEP‐520 and KRS resists at low voltage

ZEP‐520 and KRS resist systems have been evaluated as candidates for use in low voltage electron beam lithography. ZEP‐520 is a conventional chain scission resist which has a positive tone for over two orders of magnitude in exposure dose. KRS is a chemically amplified resist which can be easily tone reversed with a sensitivity ∼8 μC/cm2 at 1 keV. Both resist systems are shown to have sensitivities ∼1 μC/cm2 for positive tone area exposures to 1 keV electrons. A decrease in contrast in 50 nm thick resist layers is seen when exposure voltage is lowered from 2 to 1 keV, indicating nonuniform energy deposition over the resist thickness. High resolution single pass lines have been transferred into both Si and SiO2 substrates at both low and high voltages in each resist system without using multilayer resist masks. The ZEP‐520 and KRS resists are shown to have resolutions of 50 and 60 nm, respectively, at 1 kV, within a factor of 2 of their high voltage resolutions under identical development conditions. A cusp shaped etch profile in Si allows high aspect ratio 20 nm wide trenches to be fabricated using these resists on bulk Si. Low voltage exposures have been used to pattern gratings with periods as small as 75 and 100 nm in ZEP‐520 and KRS, respectively. Low voltage exposures on SiO2 show no indications of pattern distortion due to charging or proximity effects.

Charge Induced Pattern Distortion in Low Energy Electron Beam Lithography

Charge induced pattern distortions in low voltage electron beam lithography in the energy range of 1 to 5 kV were investigated. Pattern distortion on conducting substrates such as silicon was found to be small, while significant pattern placement errors and pattern distortions were observed in the case of electrically insulating substrates caused by charge trapping and deflection of the incident electron beam. The nature and magnitude of pattern distortions were found to be influenced by the incident electron energy, pattern size, electrical conductivity, and secondary electron emission coefficient of the substrate. Theoretical modeling predicts the electron beam deflection to be directly proportional to the trapped surface charge density and inversely proportional to the accelerating voltage.

Titanium nitride coated tungsten cold field emission sources

Titanium nitride (TiN) thin film coatings were studied by field emission microscopy and spectroscopy. Coated tungsten tips were found to be capable of emitting extremely high currents at low extraction voltages (∼1 mA at 900–1700 V). Current fluctuations for >400 μA total emission from a single tip were 7% rms, measured over ∼ 1 h. Electron energy distributions measured <0.4 eV (full width at half‐maximum). Since TiN thin films are commonly used in the microelectronics industry, TiN coatings have the potential for being a relatively simple and widely accessible method for improving the performance of cold field emission sources.

Dual exposure glass layer suspended structures: A simplified fabrication process for suspended nanostructures on planar substrates

We have developed and demonstrated here a simplified flexible fabrication process for glass nanomechanical systems. This process uses a single layer of spin on glass (SOG) material with two negative tone electron beam exposures at two different exposure energies to define the suspended and support structures, respectively. After development the SOG can be converted into glass. The process is additive and can be applied to any flat substrate. We have fabricated a variety of glass nanomechanical oscillators and measured their mechanical resonances using a mechanical piezoelectric driving force and optical interferometric detection. Suspended structures were fabricated with thickness of less than 50 nm and lateral dimensions of less than 100 nm supported anywhere from 150 to 800 nm above the substrate. Resonance frequencies for glass wires with both ends fixed (cross section 110 nm×180 nm) and lengths of 4–9 μm range from 7 to 30 MHz, with quality (Q) factors of over 1000. Annealing the structures in an oxygen ambient roughly doubles both the frequencies and the Q factors.

Patterning of Octadecylsiloxane Self-assembled Monolayers on Si(100) using Ar(3P0,2) Atoms

We report the use of metastable Ar(3P0,2) atoms and a physical mask to pattern octadecylsiloxane self-assembled monolayers grown directly on silicon surfaces. The damage to the monolayer is confirmed using lateral force microscopy, changes in hydrophilicity, and x-ray photoelectron spectroscopy analysis. Metastable atom exposures sufficient to uniformly damage the monolayer should allow pattern transfer to the underlying Si(100) substrate following chemical and plasma etching. With optical manipulation of the incident metastable atoms, this technique could provide the basis for massively parallel nanoscale fabrication on silicon.

Fabrication of arrayed glassy carbon field emitters

Glassy carbon has desirable properties for electron field emission such as surface inertness, electrical conductivity, and thermal stability. In addition, a uniform thick substrate with a polished surface is easily obtainable. This enables one to apply large scale integrated circuit processing for fabricating arrayed tips. By using oxygen reactive ion etching, cusps over 3.5 μm in height and 2.5 μm in base diameter are fabricated with a tip radius of under 10 nm. The process is assisted by the formation of a layer of etch products which protects the newly forming tip from bending and over etching. The field emission current up to 50 μA from the glassy carbon tips is obtained by applying high voltage to a mesh anode. The current which passed through the mesh anode is collected at another electrode and measured. The Fowler–Nordheim plot suggests the existence of nm scale structure on the tip. This favorable result indicates glassy carbon substrate is a good substrate for field emitter arrays.

Low Energy Electron Beam Top Surface Image Processing Using Chemically Amplified AXT Resist

High resolution processes are demonstrated with a positive-mode chemically amplified AXT top surface imaging resist system exposed with a low energy electron beam. Top surface imaging is an ideal match to low energy electron beam lithography because it allows thick resist layers to be patterned despite the limited penetration depth of the electron beam. The three key steps of the process are exposure, silylation, and etch development. All three steps influence the final process sensitivity, contrast, and resolution. The AXT has a poly(hydroxy styrene) base resin, and has been formulated both with and without a dye used to enhance optical absorption. We have achieved sub 100 nm resolution both with and without a postexposure bake. Critical area doses below 1 μC/m2 are demonstrated. The edge roughness and density of etch residue from silylation defects have been compared for a variety of oxygen plasma etch systems.