18-05-2012, 03:26 PM
Dip pen nanolithography
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Dip pen nanolithography is a promising new tool in the world of nanotechnlogy; it based upon existing technologies and has exciting applications, stretching from fields such as biology to semiconductors. Dip pen nanolithography (DPN) relies on the power of theatomic force microscope to pattern directly on a range of substances with a variety of "inks". This technology was developed in 1999 at Northwestern University by Professor Chad Mirkin. Since then, it has matured substantially and is even available commercially through the company NanoInk, demonstrating that this technology is beginning to find its niche among other scanning-probe techniques.
What is DPN?
Dip-Pen Nanolithography (DPN) is an Atomic Force Microscope (AFM)-based direct-write technique that allows researchers to carefully fabricate hard and soft nanostructures on different surfaces. It works by transferring molecules of an "ink" solution from the tip of an AFM probe to the substrate. DPN technology is extremely versatile in that it has a range of useful properties: it can build molecular structures, functionalize surfaces, or even be used to image a surface. DPN's unique characteristics make it an exciting development in the world of nanofabrication.
The simplest way to imagine the process of dip-pen nanolithography is to envision a conventional ink pen moving over a surface, depositing ink as it moves along. The head of the AFM probe is coated in the ink that is to be deposited, and the molecules of the ink are delivered to the surface through a solvent meniscus that forms between the head of the probe and the substrate. This is a simple and elegant way to directly deposit molecules at ambient temperatures, and has made DPN a very attractive tool for scientists. It minimizes the need for chemical etching or other procedures that can be destructive (1).
This image shows a moving AFM head depositing "ink" molecules on the substrate through the water meniscus
A significant advantage of using DPN technology is that it is compatible with both organic and inorganic inks. Molecules such as DNA and proteins can be deposited on materials as diverse as metals, semiconductors, and other functionalized surfaces. The ability to work with both hard and soft matter on length scales below 100 nanometers distinguishes DPN from other lithographic techniques. Although this is an advantage in that it is a relatively simple direct-write process, it does expose some of DPN's limitations. Ink/substrate combinations must be chosen carefully, so that the ink does not agglomerate or diffuse to an unreasonable degree. In order to create stable structures, the interaction between the ink molecules and the surface must be such that the molecules are able to anchor themselves to their deposition location. With the precision of the AFM and certain ink/substrate combinations, it is possible to reach resolutions as high as 10-15 nanometers(2).
Advantages of DPN
While DPN is relatively new compared to other lithographic and printing techniques, it has many unique advantages. DPN offers the ability to place selected molecules exactly where desired with sub-100 nm precision. In addition, the advantage of being able to use organic and inorganic substances as "inks" is a major benefit over some of the other existing technologies and allows for greater applications in biology, the life sciences, and other fields. In fact, it is possible to deposit organic inks upon inorganic substrates, and visa versa. Using a multi-probe array, DPN can write with both organic and inorganic inks simultaneously. This is a benefit of DPN that is not shared by other forms of lithography (1).
With DPN, one can build layers on top of other already formed layers, making the technique quite versatile. With the ability to precisely deposit molecules, DPN offers a complete lithographic package. The best resolution (essentially, the thinnest line of "ink" that can be drawn, see image) reached experimentally is approximately 15 nm, over 5000 times thinner than a human hair (2). Using a sharp tip on an atomically flat surface, these ultra-high resolutions can be obtained. With this type of resolution, it is possible to build complex arrays in smaller areas, allowing increased functionality without increased size, a trait desirable in our quickly miniaturizing world.
This image shows the incredible resolution that can be achieved with DPN
DPN directly writes onto surfaces rather than using masks to etch the surface, so the molecules remain where they are placed. The direct-write technique also prevents some of the damage that can be done from radiation, which results from such lithographic techniques as electron-beam lithography and photolithography. On such a small scale, even small perturbations can have drastic consequences. In addition, depending on the ink/substrate combination, DPN has a wider tolerance for working conditions, such as humidity and temperature, over other nanometer-scale lithographic techniques (2). This helps to make the process more accessible since vacuum chambers and other condition-controlling devices can be prohibitively expensive.
Besides these benefits, an important commercial aspect of DPN is expense. The cost of the equipment provides the bulk of the cost of operation for DPN, and this cost is significantly less than many competing devices. After the initial purchase, the remaining costs are comprised of tips and inks, neither are very expensive, especially when compared to other cost-of-operation goods such as lithographic masks (3). For instance, it is possible to purchase a DPN ink for just $200 from Nanoink, while lithographic masks can cost hundreds of thousands.