During the last decade, the scanning probe microscope (SPM) has developed to a point where it is now an accessible, easy-to-use nanotechnology research tool. The original scanning tunneling microscope has evolved into many new instruments, the first being the atomic force microscope (AFM), which have been used to observe nearly every physical property of a surface that it is possible to measure.
The first applications of SPMs included the measurement of surface texture and step heights and, after 1980, basic research in surface science; exploration in engineering, physical, and life sciences; and nanoscale process development. We now have a host of instruments and imaging modes. Lateral force microscopy, a materials sensing mode, measures the lateral or twisting deflections of the probe arising from forces parallel to the surface. If a surface is perfectly flat but has an interface between two different materials, it is often possible to image the change in surface properties such as ‘stickiness’, composition, and chemical makeup. Other sensing modes include the electrostatic force microscope and the magnetic force microscope. In a modern AFM, many of these operating modes are integral to the instrument system.
In the early 1990s, the AFM was an expensive, technically challenging instrument. The research focus was on the microscope itself rather than the results that it could achieve. If a researcher wanted to examine a sample, he or she had to seek out an AFM expert and schedule time for an all-day project. This expert would be known for ‘doing AFM’: “I do AFMs. What do you need to look at?” Although being able to operate the AFM was a valuable skill, there were undoubtedly many good ideas that were not pursued because of the complications involved. Today, although the AFM is still evolving, the focus is on the results. In addition to incredible images, today's AFMs provide precise and accurate measurements. Applications range from nanoscience and biotechnology to process development and control. For example, AFMs are essential for DVD production quality control.
When the cost drops and an instrument becomes easy to use, it moves from a service organization to the individual worker, which opens up creativity and a vast array of new possibilities. Two centuries ago, an optical microscope was a rarity; its effective use required training and skill. Now it is a commonplace laboratory tool that is easily used with minimal training. Similarly, the AFM has developed (in a couple of decades rather than centuries) from a complex, expert-only instrument to an intuitive, PC-driven, tabletop R&D and manufacturing support tool. We may have lost the unique, esoteric appeal (along with the concomitant expense) of a specialist instrument, but we have gained an easy-to-use nanotechnology manufacturing and research tool.
In spite of this trend, scanning probe instruments are often still marketed as expensive, high-end instruments for the well-equipped research laboratory. This is not the case. Commercial systems are available that provide a ‘means to an end’ at about half the cost of their high-end counterparts. The ‘SPM for everyone’ is possible because of developments in computer control and position feedback. Precision motion control for these instruments has improved and modern AFMs use calibration sensors to control probe positioning. As a result, step height measurements in the semiconductor industry can be made to within a nanometer. Various data collection modes are available including continuous, vibrating, step-and-repeat, and lateral force scanning. Probe tips have progressed from electroformed tungsten to chemically deposited, single crystal silicon pyramids and carbon nanotubes. Intuitive software, based on recent advances in gaming graphics and digital image manipulation, can simultaneously generate vivid, three-dimensional images of surface topography and physical properties.
The promise of nanoengineering is driving the development of a number of AFM-related technologies. Combining an AFM with force-feedback control will permit the ‘feeling’ of the shape of molecules and surface atoms. Glass or inexpensive (and disposable) plastic probes will have shapes and sizes tailored to the required resolution. By imaging precisely patterned samples called ‘tip characterizers’, it will be easy to tell when a probe is broken or defective. Atomic-scale reference standards will become available. Arrays of probe tips operating in tandem will speed up data acquisition and, perhaps, be used as molecular-scale random access memory elements. Other future applications include dip-pen lithography – literally writing at the molecular scale; the direct manipulation of atoms and biomolecules; and, ultimately, single-atom computer memories. It is risky to predict the products that may evolve from these technologies. After all, when the laser was invented, no one could have guessed all its applications.
[1] Paul West is chief technology officer of Pacific Nanotechnology, Inc., California, USA.