Lithography was the first fundamentally new printing technology since the invention of relief printing in the fifteenth century. It is a mechanical Plano graphic process in which the printing and non-printing areas of the plate are all at the same level, as opposed to intaglio and relief processes in which the design is cut into the printing block. Lithography is based on the chemical repellence of oil and water. Designs are drawn or painted with greasy ink or crayons on specially prepared limestone. The stone is moistened with water, which the stone accepts in areas not covered by the crayon. An oily ink, applied with a roller, adheres only to the drawing and is repelled by the wet parts of the stone. The print is then made by pressing paper against the inked drawing.
Lithography was invented by Alois Senefelder in Germany in 1798 and, within twenty years, appeared in England and the United States. Almost immediately, attempts were made to print pictures in color. Multiple stones were used, one for each color, and the print went through the press as many times as there were stones. The problem for the printers was keeping the image in register, making sure that the print would be lined up exactly each time it went through the press so that each color would be in the correct position and the overlaying colors would merge correctly.
Microstructures have always been limited by the size of the components that make up a device.“Dip-Pen Nanolithography” (DPN) was developed by Northwestern University as a new tool for preparing nanostructures. The invention of DPN, which has created the world's smallest pen, will catalyze many advances in the emerging areas of nanotechnology, mechanical and molecule-based electronics. Specifically, DPN is the missing link in the nanotechnology arena that will allow development of smaller, lighter weight, faster, and more reliably produced:
1) electronic circuits and devices,
2) high-density storage materials,
3) sensory structures, and
4) micro electro mechanical devices.
DPN is a unique modification of atomic force microscope (AFM) instrumentation. This direct-write technique offers high-resolution patterning capabilities for a number of molecular and biomolecular ‘inks’ on a variety of substrate types such as metals, semiconductors, and monolayer functionalized surfaces. The ability to achieve precise alignment registration of multiple patterns is an additional advantage earned by using an AFM tip to write, as well as read nanoscopic features on a surface. These attributes of DPN make it a valuable tool for studying fundamental issues in colloid, surface science, and nanotechnology, for instance diffusion and capillarity on a surface at the nanometer level, organization and crystallization of particles onto chemical or biomolecular templates, monolayer etching resists for semiconductors, and nanometer-sized tethered polymer structures.
Brief explanation of Atomic Force Microscope (AFM)
An atomic force microscope (AFM) is a new instrument for imaging a sample surface.
As for the structure, an AFM has similarities to a conventional styli profilo meter shown in the following illustration; however AFM can reveal a sample surface precisely up to nanometer size in three dimensions. Due to the sharper tip and small loading force, the lateral resolution in AFM is extremely improved in comparison with the conventional profile meter.
Compare the two
There are two major techniques in AFM. One is DC mode AFM or contact mode AFM, and another is AC mode AFM or dynamic mode AFM.
In DC mode AFM, a tip trace a sample surface with a constant loading force (left illustration). On the other hand, a tip is vibrated vertically while the scanning in AC mode operation (right illustration).
As a cantilever for DC mode AFM, the cantilever should be soft enough to reduce the loading force as much as possible. For a cantilever operated in AC mode, different mechanical features are required. The cantilever has a larger spring constant, higher resonance frequency and higher mechanical Q factor (or quality factor) than those of DC mode cantilevers. In these features, high Q factor is important to achieve stable measurement. Like in the right illustration, vibrating a tip at the resonance frequency of a cantilever and approaching the tip to a sample surface, the vibration will change. Using a cantilever with higher Q factor, larger change occurres. This means that higher sensitivity is accomplished in the deflection sensing in an AFM, and that good regulation of a distance between a tip and a sample is expected.
Characteristics of AFM
The atomic force microscope is one of about two dozen types of scanned-proximity probe microscopes. All of these microscopes work by measuring a local property - such as height, optical absorption, or magnetism - with a probe or "tip" placed very close to the sample. The small probe-sample separation (on the order of the instrument's resolution) makes it possible to take measurements over a small area. To acquire an image the microscope raster-scans the probe over the sample while measuring the local property in question. The resulting image resembles an image on a television screen in that both consist of many rows or lines of information placed one above the other.
Unlike traditional microscopes, scanned-probe systems do not use lenses, so the size of the probe rather than diffraction effects generally limit their resolution.
Measurement of topography with a force probe
AFM operates by measuring attractive or repulsive forces between a tip and the sample.
Concept of AFM and the optical lever: a cantilever touching a sample
In its repulsive "contact" mode, the instrument lightly touches a tip at the end of a leaf spring or "cantilever" to the sample. As a raster-scan drags the tip over the sample, some sort of detection apparatus measures the vertical deflection of the cantilever, which indicates the local sample height. Thus, in contact mode the AFM measures hard-sphere repulsion forces between the tip and sample.
In noncontact mode, the AFM derives topographic images from measurements of attractive forces; the tip does not touch the sample.
AFMs can achieve a resolution of 10 pm, and unlike electron microscopes, can image samples in air and under liquids.
In principle, AFM resembles the record player as well as the stylus profilometer. However, AFM incorporates a number of refinements that enable it to achieve atomic-scale resolution:
High-resolution tip-sample positioning
Cantilever deflection detection by Laser beam deflection method
AFMs can generally measure the vertical deflection of the cantilever with picometer resolution. To achieve this most AFMs today use the optical lever, a device that achieves resolution comparable to an interferometer while remaining inexpensive and easy to use.
The optical lever (figure 1) operates by reflecting a laser beam off the cantilever. Angular deflection of the cantilever causes a twofold larger angular deflection of the laser beam. The reflected laser beam strikes a position-sensitive photo detector consisting of two side-by-side photodiodes. The difference between the two-photodiode signals indicates the position of the laser spot on the detector and thus the angular deflection of the cantilever.
Because the cantilever-to-detector distance generally measures thousands of times the length of the cantilever, the optical lever greatly magnifies motions of the tip. Optical lever detection can theoretically obtain about- ~2000 fold magnification.
AFM cantilevers have high flexibility
A high flexibility stylus exerts lower downward forces on the sample, resulting in less distortion and damage while scanning. For this reason AFM cantilevers generally have spring constants of about 0.1 N/m (figure 2).
It would take a very long time to image a surface by dragging a Slinky over it, because a Slinky cannot respond quickly as it passes over features. That is, a Slinky has a low resonant frequency, but an AFM cantilever should have a high resonant frequency.
Schematic illustration of the meaning of "spring constant" as applied to cantilevers. Visualizing the cantilever as a coil spring, its spring constant k directly affects the downward force exerted on the sample.
The equation for the resonant frequency of a spring:
This shows that a cantilever can have both low spring constant and high resonant frequency if it has a small mass. Therefore AFM cantilevers tend to be very small. Commercial vendors manufacture almost all AFM cantilevers by microlithography processes similar to those used to make computer chips.
Inexpensive and reasonably Sharp tips by Micromachining
Force microscopists generally use one of three types of tip.
The "normal tip" (figure 4a; is a 3 µm tall pyramid with ~30 nm end radius.)
The electron-beam-deposited (EBD) tip or "supertip" (figure 4b); improves on this with an electron-beam-induced deposit of carbonaceous material made by pointing a normal tip straight into the electron beam of a scanning electron microscope. The supertip offers a higher aspect ratio (it is long and thin, good for probing pits and crevices) and sometimes a better end radius than the normal tip.
Finally, Park Scientific Instruments offers the "Ultralever" (figure 4c), based on an improved microlithography process. Ultralevers offers a moderately high aspect ratio and on occasion a ~10 nm end radius.
(a) (b) ©
Three types of AFM tip. (a) normal tip (3 µm tall); (b) supertip; © Ultralever (also 3 µm tall).
Tube piezoceramics position the tip or sample with high resolution
Piezoelectric ceramics are a class of materials that expand or contract when in the presence of a voltage gradient or, conversely, create a voltage gradient when forced to expand or contract. Piezoceramics make it possible to create three-dimensional positioning devices of arbitrarily high precision. Four electrodes cover the outer surface of the tube, while a single electrode covers the inner surface. Application of voltages to one or more of the electrodes causes the tube to bend or stretch, moving the sample in three dimensions
Exploded view of a tube scanner
AFMs use feedback to regulate the force on the sample
The AFM feedback loop. A compensation network monitors the cantilever deflection and keeps it constant by adjusting the height of the sample (or cantilever).
The presence of a feedback loop is one of the subtler differences between AFMs and older stylus-based instruments such as record players and stylus profilometers. The AFM not only measures the force on the sample but also regulates it, allowing acquisition of images at very low forces.
The feedback loop in above figure consists of the tube scanner that controls the height of the entire sample; the cantilever and optical lever, which measures the local height of the sample; and a feedback circuit that attempts to keep the cantilever deflection constant by adjusting the voltage applied to the scanner.
One point of interest: the faster the feedback loop can correct deviations of the cantilever deflection, the faster the AFM can acquire images; therefore, a well-constructed feedback loop is essential to microscope performance. AFM feedback loops tend to have a bandwidth of about 10 kHz, resulting in image acquisition times of about one minute.
Working of DPN
Dip-Pen Nanolithography (DPN) is a scanning probe nanopatterning technique in which an AFM tip is used to deliver molecules to a surface via a solvent meniscus, which naturally forms in the ambient atmosphere. When an AFM is typically used, the device’s
stylus is positioned on or close to the specimen surface. This minute distance allows interaction of an AFM cantilever with the sample surface. As the AFM tip is moved over the surface, the device’s movements are translated into topological information of the sample. However, a side effect of the minute tip-to-surface separation exists; water vapor condenses from the surrounding air and forms a droplet between the tip and the substrate.
DPN takes advantage of the before mentioned side effect to create patterns on a desired substrate. A standard AFM cantilever is coated with a single type of organic species; proteins, oligonucleotides, etc. The modified stylus equipped AFM uses the water droplet, formed at the tip-substrate interface, as a medium to transport the coated molecules to the sample surface.
Figure 1. Illustration of molecular deposit of DPN tip. The desired molecules are deposited through a water meniscus medium to the surface.
Using the DPN technique, numerous grids of proteins, magnetic nanoparticles, and
DNA can be created (Figure 2).
Figure 2. Images of dots (2a) and lines (2b) of magnetic nanoparticles created using DPN.
Although DPN allows a great deal of flexibility with respect to pattern designs and deposition molecules, there is a fundamental limitation; an individual stylus can only be coated with one type of molecule or one combination of molecules. Therefore, only one-component patterns can be produced per run. If other molecules or molecular combina-tions are desired, theAFM tip must be replaced. Likewise, there are a limited number of species on the tip. Once those molecules are used up, the tip must be recoated.
To address the abovementioned issues, DPN version 2 will hypothetically work like a fountain pen (Figure 3a). The device, currently being developed group, consists of reservoirs of desired molecules in solution. The inks then flow to a customized AFM tip. The reservoirs can be switched on the fly, by an incorporated micro fluidic valve. The only limit for species deposition will be the speed at which the solution reaches the tip. At this time, the actual characteristics of the “Fountain-Pen Nanolithography” (FPN) tip, channels, and reservoirs are in the design process. Various geometries, materials, and fabrication processes are being tested for possible prototype development (Figure 3b). Therefore, the future FPN will depend on µF channel properties to dictate the device’s performance.
Figure 3a) Illustration of possible FPN device. Microfludic channels will transport molecular inks to the tip for species deposition.
b) Illustration of Massively Parallel Multi-tip Nanoscale Manipulator with Fluidic capability.
An important requirement for creating stable nanostructures is that the transported molecules anchor themselves to the substrate via chemisorption. When T-substituted alkanethiols are patterned on a gold substrate, a monolayer is formed in which the thiol headgroups form relatively strong bonds to the gold and the alkane chains extend roughly perpendicular to surface.
A) AFM image showing lattice-resolved monolayer of octadecanethiol patterned on gold via DPN.
Attributes of DPN
Creating nanostructures using DPN is a single step process, which does not require the use of resists. Using a conventional Atomic Force Microscope it is possible to achieve ultra-high resolution features–as small as 15 nm linewidths and ~ 5 nm spatial resolution, Figure 4a. For nanotechnology applications, it is not only important to pattern molecules in high resolution, but also to functionalize surfaces with patterns of two or more components.
One of the most important attributes of DPN is that because the same device is used to image and write a pattern, patterns of multiple molecular inks can be formed on the same substrate in very high alignment, Figure 4b.
A) Ultra-high resolution pattern of mercaptohexadecanoic acid on atomically flat gold surface.
B) DPN generated multi-component nanostructure with two aligned alkanethiol patterns.
Applications of DPN
We are currently using DPN to probe fundamental surface science questions as well as to create technologically relevant nanostructures. Part of the process of investigating these technological applications requires that we develop methods, which will allow parallel patterning in addition to the serial capabilities of DPN.
Applications of DPN can be classified as:
DPN technology could be used to create many small-scale sensors and power assemblies mounted on a single chip for use on micro-satellites or mounted within an unmanned aerospace vehicle (UAV). The savings in launch weight provides for significant savings in launch costs.
DPN is currently using to probe fundamental surface science questions as well as to create technologically relevant nanostructures.
It catalyzes many advances in the emerging areas of nanotechnology and molecule-based electronics. This advance will enhance the possibility of future Air Force weapon systems becoming smaller, lighter, and less expensive.
It is used to produce solid-state nanoresists, organic and bioorganic circuits, nanoprinted catalysts etc.
Figure 5. Some of the potential applications of DPN, center: micro fabricated multiple AFM probe.
A Multipen Plotter for Parallel Patterning
Initial DPN experiments have involved a single AFM probe for formation of patterns on a substrate in a serial fashion. Creation of many patterns in duplicate is thus a slow process. The throughput of DPN patterning may be significantly increased if a large and dense array of DPN pens is used to create features in parallel. Initial experiments in which two DPN pens operated in parallel were used to demonstrate this concept. Significantly, it has been determined that the DPN linewidth is not a strong function of the contact force within a limited range.
This important characteristic of DPN eliminates the need to perfectly align the pen arrays to the substrate surfaces, and therefore, the need for complicated multiple pen feedback system. Current efforts in this area are focused on the design and testing of a prototype microfabricated 32-pen array.
A) Schematic of two-pen DPN plotter.
Patterning on Semiconductor Surfaces via DPN
Using DPN a method of patterning high resolution (sub-100 nm) organic patterns on silicon oxide (SiOx) and gallium arsenide (GaAs) surfaces has been developed. The choice of molecular ink is crucial to successful patterning. The surface coating agents that are commonly employed for these types of substrates (trichloro or trialkoxysilanes) are rapidly hydrolysed under standard DPN conditions (30% humidity) and thus polymerize on the AFM tip before they can be transferred to the surface.
DPN- Generated Templates for Combinatorial Fabrication and Study of New Particle-Based Materials.
DPN is well suited for nanofabrication of customized structures in arrays consisting of several to thousands components which can be combinatorially screened for a certain process, for instance catalysis or cell adhesion. DPN allows one to systematically vary the lattice parameters of a 2-dimensional chemical or biochemical template array, including spacing, dot size, orientation, and chemical composition. Subsequent selective interaction, for instance, of certain particles with the template can be used to initiate the process of 3-dimensional colloidal crystal growth. The ability to form these types of crystals based on particles of a size on the order of the wavelength of light with a high degree of control over the lattice parameters and integrity of the structure is extremely important for the study and fabrication of photonic band gap materials for use in optical communications devices.
In conventional DPN, a probe tip is coated with a liquid ink, which then flows onto the surface to make patterns wherever the tip makes contact. Dozens of research groups worldwide are working on DPN applications, but the technique – which uses the AFM tips to both sense surface patterns and write new patterns – has been limited by an inability to turn the ink flow on and off. Existing dip pens apply ink as long as they remain in contact with a surface.
The thermal DPN (tDPN) method solves that problem by using easily-melted solid inks and special AFM probes with built-in heaters that allow writing to be turned on and off at will. The tDPN technique could be used to produce features too small to be formed with light-based lithography, and as a nanoscale soldering iron for repairing circuitry on semiconductor chips. The technique could also provide a new tool for studying basic nanotechnology phenomena.
The researchers have so far produced lines about 95 nanometers wide and are optimising their process to make smaller features.
The ability to sense surface features and put down new patterns with the same AFM tip could be useful in repairing errors in the tiny patterns on circuits or masks used in semiconductor manufacture.