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Surface Modification

Atomic Force Microscopy in Nanoscience and Nanotechnology: Surface Modification

Deliberately creating structures having nanometer sizes is a scientific and technical challenge that has been addressed for several centuries. Clearly, physicists are able to combine atoms, chemists create molecules and biologists create biomolecules with sizes on the nanometer scale. A greater challenge is to engineer structures in the mesoscopic scale of 10 - 1000 nm. Methods for creating mesoscopic structures include optical lithography, and ebeam lithography. Although successful, these techniques are limited in their use to few researchers because of the expense, greater than 1 million dollars. With the Atomic Force Microscope (AFM), it is now possible to create mesoscopic sized devices for a fraction of the cost of optical and ebeam techniques.
The atomic force microscope is the most widely used scanning probe instrument in nanoscience and nanotechnology. The AFM combines surface profilometry and scanning tunneling microscopy and can operate under ambient conditions on both insulating and conducting materials. In general, the AFM can be used to measure, modify, or manipulate surface structures. This technical note focuses on surface modification - that is, nano-manufacturing processes. The AFM is well suited for R&D and "proof-of-concept" demonstrations of fabrication and patterning techniques in the nanoscale regime.
Surface structures can be modified by a passive probe (e.g. surface indentation) or by several types of active probes. Active probes could employ electrical, chemical, optical, or diffusion processes to change the surface.

Mechanical Surface Modification

The Atomic Force Microscope was initially developed to image the surfaces of insulating materials. However, by accident, it was discovered that the AFM probe could cause mechanical modifications to a surface. Such mechanical modifications can now be used proactively to alter surface topography.
Two types of mechanical surface modifications are possible. In the first, the probe is pushed into the surface. In the second, a line is scratched into the surface. The size of the features depends on the following factors: Figure 1 illustrates and example of an AFM used to indent ~0.4 µm diameter depressions in a material's surface.
Figure 1: Mechanically modifying a surface. After indenting a 3X3 array of depressions ~ 0.4 µm in diameter and ~ 50 nm deep; image depth range: 103 nm. The image has an area 5.2 µm square.

Electrical Surface Modification

Electrically conducting AFM probes can be used to chemically modify a surface to "draw" an image. For example, applying an electrical bias between the conducting probe and a substrate can locally oxidize selected regions of the surface to form patterns.
Figure 2: By placing a bias between an electrically conductive probe and a surface, the surface can be modified.
The dimensions of the pattern drawn by electrically conducting techniques depends on:
Figure 3: These patterns were drawn with an electrically conductive AFM probe using Anodic Oxidation. At the left the, line widths are 50 nm. At the right, two line widths were drawn using two different biases between the probe and sample.
For growing an oxide on silicon, the line-width of the pattern ranges up to tens of nanometers. The thickness can be controlled in the range of 10-50 nm. When the writing is done in ambient air, the line width depends on the relative humidity, because water adsorbed at the tip-substrate interface focuses the electric field and also acts as the anodization medium. The oxide thickness was found to depend on the electric field strength. Figure 3 shows two patterns written in silicon oxide with an AFM.

Nanolithography by Molecular Deposition

Another way to create a nanopattern is by dip-pen nanolithographyT. With DPNT method, ink molecules are adsorbed on the AFM tip and transferred by diffusion through the liquid meniscus (formed between the probe tip and the substrate surface) onto the substrate. The DPNT process is patented by NanoInk, Inc. NanoInk is the sole manufacturer of DPNT process solutions. The technique is illustrated in Figure 4. The liquid can be water or another suitable solvent in which a material (the "pigment") is dissolved. The ink could also be in liquid form and require no solvent. Inks include organic compounds such as alkane thiols, dendrimers, polymers, or large biomolecules such as antibodies, proteins, or DNA. (A dendrimer - from Greek dendra for tree - is a small, high-molecular weight globular molecule built up from branched units forming a tree-like structure.)
Figure 4: Illustration of the AFM probe being used to deposit molecules on a sample's surface. As the probe moves across the surface, molecules move down the probe and attach to the surface.
Figure 5 shows patterns formed by the DPNT process on a gold substrate using open loop control of the AFM tip. Note that the box drawn around the LLL pattern is not closed - the open loop control did not allow accurate location of the endpoint.
Advanced DPNT experiments involve the overlay of patterned ink layers with the precise deposition of molecules, and this process requires more complex instrumentation. NanoInk has developed a dedicated DPN WriterT tool called NSCRIPTORT. Built upon advanced PNI scanner technology, NSCRIPTORT offers a sophisticated, user-friendly DPNT experience with integrated environmental control.
Figure 5: Lateral force microscope image of a pattern written with the DPNT process. This pattern was written with an AFM that does not have X-Y calibrations sensors. As a result, at the right upper corner of the image two of the lines did not connect as intended.
Image courtesy of Brandon Weeks - Lawrence Livermore National Labs -52000-01-12

AFM Lithography Instrumentation

Any atomic force microscope can be used for creating nanometer-sized patterns on a surface. However, the quality and complexity of the patterns depends on specific scanning probe hardware and software. The method of patterning is determined by the types of probes, substrates, and the specific software performance capabilities used to drive the scanning probe tool.
Hardware: The most critical hardware feature that is required is an X-Y calibration system. Because the piezoelectric ceramics used in AFM have unwanted characteristics such as creep and hysteresis, calibration sensors are necessary to guide the motion of the probe. Without the calibration sensors, the probe moves in an unpredictable motion, and it is hard to write complex patterns. As an example, in figure 5 the AFM did not have calibration sensors, and the lines at the upper right of the pattern did not connect as intended. Conversely, Figure 6 is an illustration of a complex pattern drawn with the DPN™ process. Because the instrumentation had X-Y calibration sensors, a higher quality pattern was possible.
Figure 6: Lateral Force Microscope (LFM) images of patterns created using DPN™. With X-Y calibration sensors the lines at the edges of the boxes intersect as expected. Image written with a NanoInk DPN Writer™ using PNI scanner technology. Line widths vary from 60 nm - 100 nm.
Software: Software is used for defining the pattern that will be drawn with an AFM and then for drawing the pattern on a materials surface. Figure 7 shows a control window that may be used for creating patterns with an AFM. The pattern may be drawn on a section of the screen as a combination of dots and lines or it may be imported from a bit-map file. The software allows writing of the patterns by applying a specific force or a specified voltage. Also, the rate of scanning can be specified.
Figure 7: A typical control window for modifying surfaces with an AFM. Each of the buttons has a drop down menu that facilitates selection of parameters.
Advanced DPN™ patterning involves the overlay of complex pattern layers with the precise deposition of molecules, and this process requires more complex instrumentation. NanoInk has developed a dedicated DPN™ Writer tool called NSCRIPTOR™. Built upon advanced PNI scanner technology, NSCRIPTOR™ offers a sophisticated, user-friendly DPN™ experience with integrated environmental control.

 
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