Success stories
Dr Maan Alkaisi : Nano-lithographer
Dr Alkaisi at work in the Lithography Lab.
Dr Maan Alkaisi is the calm director of a hive of nano-industry, his slow steady voice and calm manner belying the pulse of activity surrounding him. Maan is a true engineer. “I always look for useful applications of the ideas and technologies we develop”. His area of expertise is lithography- basically making very very small structures, a handy skill to have in this age of shrinking technology. The diversity of his research is evidence that the walls dividing areas of science such as biology and physics are crumbling under the influence of nanotechnology.
The lithography lab
The security door of the lithography lab at Canterbury University opens to a menagerie of clean white coats and booties, thoughtful scientists and the pulsing heartbeat of strange machinery- an impressive yet strangely casual and friendly scene. Thanks to the MacDiarmid Institute the lab has been fitted out with some very impressive equipment. The MacDiarmid sponsored RAITH 150 electron beam lithography (EBL) machine can write features into a chip with resolution down to 20nm (that’s a few atoms wide). The Karl Suss MA6 mask aligner, also MacDiarmid sponsored, is the best on the market. Symbols of the value being placed in research, these machines are having a huge impact on the quality of work and the confidence of researchers. An air of quiet pride and positivity fills the lab.
The lithographic process
The principle of lithography is incredibly simple; similar to the sort of stencil painting and potato-printing children do at school. The complications lie in the scale. You know how frustrating it can be removing the backing from a fiddly piece of double-sided tape. Imagine trying to do this on a nano-scale.
The following diagram outlines the basic lithography process.
The layer of silicon nitride is insulating so it isolates the metal tracks laid on top of it. Peeling the imaging layer off when the substrate is covered in metal as (in stage 6a) is a tricky business. It tends to pull the delicate metal pattern with it leaving a bare substrate. Maan has devised a clever solution. Two imaging layers are applied in stage 3, the first is low molecular weight and the second high molecular weight. Electrons fired into the substrate in stage 4 scatter further into the low than the high molecular weight imaging layer. The result is a little tab that can be used to safely lift off the imaging layer.
Using these simple techniques a huge range of structures can be created.
The Bio-nanotech Group
As testimony to the eclectic nature of nanotechnology, Maan (an electrical engineer) has been chosen to co-ordinate the Bio-nanotech group in the MacDiarmid Institute. This group provides the means for biologists and medical researchers to work with chemists, physicists and engineers to create nano-scale tools, which could revolutionise medical research methods. The combination of expertise, hi-tech equipment and kiwi ingenuity could give New Zealand a unique advantage in bio-nanotechnology.One important bio-nanotech venture is the microfluidics program described in the article on Jeff Tallon.
Maan is particularly involved in a project with medical researcher John Evans from the Department of Obstetrics and Gynaecology at the Christchurch School of Medicine. They are designing ‘biochips’, which will allow ‘real time’ viewing of hormones being released by biological cells. This project is especially close to Maan’s heart. “It's an application to the lithography techniques that we’re developing here that will help people, which is what we all work for”
The project has been an expansive experience forcing the researchers involved to look outside their specialist fields.
“As an engineer I can’t do it on my own and the biologists, medical people and chemists can’t do it on their own but when we all get together we manage”
The project involves “developing a platform for capturing living biological cells and separating them according to their type or being alive or dead.”
Once the cell is captured, an Atomic Force Microscope (AFM) can be used to take high-resolution images. These microscopes are perfect for imaging live cells. They are capable of resolution down to a few nanometres but unlike electron microscopes do not require a vacuum. The cell can be surrounded by fluid containing all the necessary ingredients to keep it alive. AFM imaging will allow researchers to watch as hormones emerge from holes opening up in cell walls.
As an electrical engineer Maan has to try to understand biological cells in terms of charging, discharging, attracting, repelling and positive and negative fields.The cell needs to be anchored in place otherwise the microscope probe will drag it while scanning. Maan has been able to force cells to certain locations by applying voltage to a system of metal electrodes laid across the surface of the platform. The cell membrane is full of ions and can be treated as a dielectric material. “You can attract or repel cells according to the charge you apply.” If the cells die or are infected the ion concentration changes so electric fields can be used to sort cells.
Bio-chips are made using a combination of the methods shown in the diagram above. Electrodes are defined using additive pattern transfer and cavities for the cells to sit in are etched using subtractive pattern transfer.
On the Left: AFM image of a trapped biocell.
On the right: AFM image of a trapped bead showing two electrodes and the etched cavity on the biochip.
"With lithography we can... programme the AFM tip to go to specific cells. Not like normal procedures which rely on statistics." Researchers will be able to deliver different chemicals to specific cells then go back to the same cell and observe the effect.
"This level of control will hopefully be useful to biologists."
Solar cells
Tiny features etched onto the surface of solar cells can trap light which can vastly improve the cell's efficiency. After spending three years perfecting the lithographic techniques for surface texturing Maan's PhD student discovered an ideal shape for light absorption; a microscopic pillar-like structure, which absorbs 99.96% of incident light. Unfortunately the etching process damages the crystal structure and this reduces the current induced in the cell and therefore the efficiency. Using a technique called ‘wet etching’ involving chemical reactions in solution they were able to ‘shave’ off a lot of the damaged surface. The final efficiency is close to the world record.
Metallic Nano-transistors
Transistors are the fundamental building blocks of integrated circuits in computers. They act like switches, controlling the flow of current. Traditionally they have been made out of thin layers of semiconducting material and features defined using ‘optical lithography’ but the demand for increased information density is pushing this technology to its limits. Maan has developed metal transistors that are potentially more compact, easier to produce and cheaper. Unlike traditional transistors they are one dimensional, consisting of a simple pattern of metal tracks laid onto a substrate. The figure below shows a field effect metallic transistor in its two modes of operation.
Voltages applied to the side gates control the flow of current from the source to the drain; when no voltage is applied (as in figure a) current flows; when a negative voltage is applied (as in figure b) the electric field set up repels electrons, which reduces the current. This gives the transistor’s two modes of operation. The ultimate aim is to reduce the current in the second mode to zero – the ideal situation for a transistor.
Low cost, high-resolution lithography
The electron beam in the new Raith 150 EBL machine can achieve resolution down to 20nm. This is impressive but every feature has to be drawn individually making the process time consuming and expensive. Maan has been investigating alternative methods. One of these, called ‘nanoimprint’ lithography is a stamping process, similar to stepping in mud and leaving a footprint behind. A mould is pressed into the imaging layer to emboss the pattern. Moulds can be made using electron beam lithography, then used again and again – perfect for mass production. This method is ideal for one-dimensional structures like metallic transistors.
Maan is working on another new lithography method with his colleague Richard Blaikie. You can read more about this technique, called ‘ENFOL’ (Evanescent Near Field Lithography) in the article on Richard Blaikie.
