An introduction to Atomic Force Microscopy Modes

The machinery comprising the AFM can be utilized for active manipulation as well as passive observation. That is, it can become a toolbox for altering the physical and chemical properties of the SUT, not just a microscope. The tools in that toolbox include:

• Lithography
• Chemical deposition – The AFM as generalized printer/plotter
• Electrochemistry

Lithography
The term “lithography” originally meant “writing on stone”, a technique long-used for printing of hand-drawn illustrations, as in books and magazines. In modern times it has taken on a connotation of the creation of micro-structures on semiconductor devices; conventional printing technology long ago left it behind. The exact definition in the nanoscale context is still in flux, but it retains its basic meaning of image creation.

Like modern printing, nano-scale AFM lithography uses one, or the other, or both of the two basic techniques: bit-mapped imagery and vector-drawn imagery. As in computer printers, software renders the desired image described by a source file (e.g. .bmp or .jpeg). It moves the probe and indents, scratches, or deposits “ink” of some kind (Figure 4-1). Resolution of such AFM lithography, of course, usually depends upon the probe diameter and noisiness of the PZT positioning process. The primary drawback to AFM lithography is low scan rate.

Simple Scratching. What is a drawback for AFM microscopy can be an asset for surface etching. If the force exerted by the probe is high enough, and the SUT is softer than the probe, the probe will scratch the surface.

Anodic Oxidation. In 1989 it was demonstrated that the localized current from a small probe, as in a scanning tunneling microscope, can modify a surface through localized electrochemistry. The most common example is anodic oxidation. In this particular example, the width of the oxidized line that is created depends on the number times it is traced.Chemical deposition – The AFM as generalized printer/plotter. The AFM can be used as a sort of generalized plotter, applying a variety of “inks” at the nanometer scale, for a variety of applications. The vehicle for the deposition can be:

• pyramidal scanning probe tips
• hollow probes

Many applications have been envisioned in which precise deposition or fabrication or lithographic mask creation at the nano-scale are needed.

The fluid deposited may also be a so-called “molecular glue” which forms a template or mask for subsequent chemical nano-scale fabrication.

Dip Pen Nanolithography^® (DPN^®) is a proprietary scanning probe lithography technique invented in Dr. Chad Mirkin's laboratory at Northwestern University, in which a microscopic pen (e.g. the tip of an atomic force microscope cantilever) is coated with an “ink” (a chemical compound or mixture) and put in contact with the “paper” (a substrate). Both the name and its abbreviation are now trademarks of the Nanoink corporation. “Dip pen” is an archaic name for a quill pen, by analogy to the procedure.

Electrochemical experiments
An electrochemical cell can be added to an AFM enabling electrochemistry experiments in situ. The AFM can then be used to study electrochemical changes in surface properties without thesurface being exposed to air. With the addition of a galvanostat, the surface topography can be investigated as function of the surface potential relative to a reference cell.