A BIG Name in Nanoscience

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The Western Nanofabrication Facility

With the establishment of Surface Science Western in 1981 and the Canadian Synchrotron Radiation Facility at the University of Wisconsin-Madison (Madison, WI) in 1982, the University of Western Ontario (UWO) (London, ON) was one of Canada’s pioneers in nanoscience long before the word “nano” became all pervasive. In 1986, a large contingent from Atomic Energy of Canada Ltd. (Mississauga, ON) led by Ian Mitchell, PhD and Peter Norton, PhD relocated to UWO to create Interface Science Western, a unique research program that exploits ion beams from particle accelerators. Their analytical methods touch directly on nanoscale.

Some of the most recent and largest infrastructure investments at UWO have been dedicated to nanoscience research. In 2000, Mitchell spearheaded a successful $7-million Canada Foundation for Innovation (CFI) (Ottawa, ON) proposal to build the Western Nanofabrication Facility (Nanofab). Its official opening took place in September 2004. Ontario funding, via the Ontario Photonics Consortium Project, has been invaluable in assisting with operating costs since lab start up.

The Nanofab, located in UWO’s physics and astronomy building, is a 550-m2 clean room that houses a suite of state-of-the-art instruments capable of supporting basic and applied research in the design and analysis of nanomaterials and devices. Silvia Mittler, PhD is Nanofab’s director, and day-to-day operations are co-ordinated by two scientists, lab manager Richard Glew, PhD and senior research scientist Todd Simpson, PhD, as well as lab technician Nancy Bell. The Nanofab staff helps train users in the areas of deposition, patterning, analysis, and characterization. They also provide technical advice and perform service measurements and fabrication for a variety of internal and external clients.

The Nanofab is equipped with state-of-the-art photon-, ion- and electron-based lithographic equipment, as well as standard and specialized deposition and characterization tools. The leading instruments used for patterning include photolithography (with the help of a mask aligner), e-beam lithography, and focused ion beams (FIB). The latter two setups are equipped with their own scanning electron microscopes (SEM) for viewing fabricated periodic or random nanostructures. An additional optical holography lab, located in the chemistry department, complements Nanofab’s activities and is routinely used to fabricate periodic patterns. Nanofab’s reactive ion etching (RIE) capabilities can be used for silicon samples as well as for other materials such as silicon oxides and nitrides, and to generate 3-D nano- and microstructures. Lateral resolutions from several µm down to 50 nm, and depth resolutions from a few nm to several µm are possible. Aspect ratios for silicon of ~100: 1 can be achieved.

Metals (Au, Al, Cr), silicon and its oxides and nitrides, conducting indium tin oxide, and organic materials such as polymers can all be deposited on substrates at various thicknesses by different methods including sputtering, plasma-enhanced chemical vapour deposition, e-beam deposition, or spin coating. A Langmuir-Blodgett trough is also available to fabricate ultra-thin organic films by mono- or multilayer deposition.

Once the micro- or nanoscale structures have been fabricated, the Nanofab’s electron and optical microscopy capabilities can be used to image and measure the structures’ spatial dimensions. In addition, X-ray analyses are able to yield elemental compositions, while various other instruments are on hand to provide refractive indices and topological thickness maps with a height resolution of about 100 nm. The Nanofab is also equipped with a two-photon confocal microscope that can be used for either fluorescence microscopy or 3-D photo-patterning. Here, samples are mounted on a relatively unique high-precision table.
Users can also take advantage of the infrastructure for sample cleaning, CO2 drying, annealing and precision cutting. The FIB will soon be equipped with a lift-out tool that will allow user to pick up and move ultra-thin sample slices for further analysis.

Current Nanofab activities assist research projects being conducted by members of the faculties of science, engineering, and medicine and dentistry. The following are just a few examples of the work being carried out, and provides a glimpse of the myriad of possibilities available for potential users.

Photonic Crystals
Photonic band gap materials design and fabrication are synergistic pursuits at the interface of photonics and nanomaterials activities. Photonic crystals (PCs) are periodic structures in the order of the wavelength of light with alternating high- and low-refractive indices showing photonic band gaps, where certain wavelengths are prohibited from propagation. There is a worldwide effort in this field because, in addition to their potential as perfectly reflecting mirrors, PCs with implanted defects can act as a variety of optical devices without threshold including lasers, waveguides, sensors, and filters. Complete band gaps have been observed or predicted for a number of periodic dielectric lattices in 2- and 3-D.

One of the main applications of 2-D PCs is manipulating and guiding light in planar photonic circuits with very small foot-print areas. Even though PCs offer exciting wave-guiding possibilities in 2- and 3-D, the practical use of PC waveguides is limited due to poor coupling efficiency between the PC waveguides and other optical components such as conventional index-guided waveguides. Coupling poses a challenge because PC waveguides exhibit a significantly different mode profile and propagation mechanism compared to traditional waveguides that use index confinement. Jayshri Sabarinathan, PhD and colleagues from the department of electrical and computer engineering are interested in implementing planar PC waveguides in practical applications. Therefore, it is essential that they be able to analyze and improve the coupling mechanism and transmission in these waveguides.

For example, 3-D computer simulations (Figure 1a) were carried out to analyze a structure having a maximum transmission of 78%. With these designs in hand, they used the Nanofab’s infrastructure to successfully fabricate photonic crystal-slab waveguides integrated with conventional-slab waveguides (Figure 1b).
Rob Lipson, PhD from the department of chemistry is interested in interference lithography because this approach is emerging as one of the simplest, fastest, and cheapest methods of creating large-scale periodic structures for PC fabrication. The outputs of the most common lasers have micron to sub-micron wavelengths, dimensions that correspond to those most desired for PC applications and periodicities.

The dimensionality of the patterns created by interference lithography depends on the number of laser beams used. Two-beam lithography to write 1-D gratings is well established. Three and four beams are required to generate 2- and 3-D patterns, respectively. However, one limitation of interference lithography is the difficulty associated with controlling the structures that can be produced with a given experimental arrangement. The structure and periodicity of a pattern are determined by the wavelength of the laser light, and/or the angles at which the different beams interfere.

According to Huygen’s Principle, when a beam of light passes through a diffractive element (mask), each point on the aperture is regarded as a source of secondary wavelets. At a given distance and location from the mask, these secondary wavelets can constructively or destructively interfere to produce bright and dark areas. Depending on the structure of the mask, different diffraction patterns can be formed with dimensions that vary strongly in distance between the diffractive element and the recording sample. A photoresist layer is one such example. The optical proximity effects due to near-field diffraction provide a strong a priori degree of control on the microstructures that can be generated. Lipson’s group calls this approach Diffraction Element Assisted Lithography (DEAL).

In a series of experiments, a 2-D triangular mask manufactured by e-beam lithography was used as the diffractive element to pre-pattern the coherent ultraviolet output of a Nd:YAG laser in the near-field prior to its capture in a photoresist. The diffraction patterns were recorded in a ~1-2 µm thick SU-8 photoresist on a silicon substrate.
The two DEAL patterns generated when the distance between the mask and photo resist was set to z = 4012 ± 0.1 :m and z = 4000 ± 0.1 :m are shown in Figures 2 and 3.
The diffraction pattern can be calculated. The resultant normalized colour simulations of the intensity distributions, shown in Figure 2a and 3a, are in excellent agreement with the experimental results (atomic force microscope (AFM) images: Figure 2b and 3b; SEM images: Figure 2c and 3c). In Figure 2a the wavelets from the three gratings making up the triangular mask are in phase while in Figure 3a the phases of two of the wavelets are identical, but that of the third wavelet differs by p. In the former case, the DEAL pattern is a 2-D array of islands while in the latter, a similar array was generated but this time made of air holes.

Nano-probes
High-resolution studies of living cells require new imaging and sampling techniques. Norton and colleagues from the department of chemistry are interested in nanofabricated probes for near field scanning optical microscopy (NSOM) and sub-attoliter sampling. They are attempting to develop new high light throughput NSOM probes that are also capable of trapping and delivering sub-attoliter volumes of material from and to the membrane of a live cell. These volumes should contain as few as one molecule of interest; for example, a pharmaceutical. The effect of the molecule’s removal or delivery to a specific location on the membrane localized by fluorescent labelling to within ~ 50 nm will be followed by NSOM.

The high throughput tips are formed from optical fibres and are metalized with aluminium. FIB methods are then used to drill an aperture or a cylindrical cavity of 50 nm diameter by 100 nm depth into the tip. Through judicious choice of diameter and depth, the light can be more or less confined to this cavity, potentially permitting single-molecule spectroscopy at the membrane. Figure 4 shows an SEM image of the cross-section of a cavity drilled in a metalized tip.

Zhifeng Ding, PhD and his group (chemistry department) have recently built a scanning electrochemical microscope (SECM) that combines electrochemistry with AFM and NSOM. SECM measures the rate of faradic reactions at an ultramicroelectrode (nano-fabricated tip) during its movement in a solution above a substrate. By measuring the electrode current, the electrochemical reaction rate of the species contained in the solution gap between the tip and the substrate can be found as a function of the gap spacing and the reactive nature of the substrate. SECM is useful in a wide range of applications, including fast heterogeneous kinetics, imaging of biological molecules, and characterization and fabrication of interdigitated arrays. Of particular interest is the use of SECM to perform “chemical images” where differences in reaction rates can be observed at different locations. Figure 5 shows an SECM image of two COS 7 live cells. Information about the cell metabolism is anticipated.

The same kind of live cells can be labelled with a fluorescent probe and then imaged with the Leica fluorescence microscope located in the Nanofab (Figure 6). While the fluorescence image gives intracellular structure information, the SECM image supplies rich information on the extra cellular chemical reactions. Both techniques are complementary.
Research conducted in the Nanofab is financed by various funding agencies including the Ontario Photonics Consortium through the Ontario Research and Development Fund, the Canadian Institute for Photonic Innovations (Quebec City, QC), the Natural Sciences and Engineering Research Council of Canada’s (Ottawa, ON) NanoInnovation Platform program, Materials Manufacturing Ontario (Mississauga, ON), the Emerging Materials Knowledge Network, AuTEK, the Academic Development Fund of Western, CFI and OIF.

Silvia Mittler, PhD is director of UWO’s Nanofab, as well as a professor cross appointed in the departments of electrical and computer engineering, chemistry, and medical biophysics at UWO. Mittler is also the Canada Research Chair in Photonics of Surfaces and Interfaces.