The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution of fractions of ananometer, more than 1000 times better than the optical diffraction limit. The precursor to the AFM, the scanning tunneling microscope, was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s, a development that earned them the Nobel Prize for Physics in 1986. Binnig,Quate and Gerber invented the first AFM in 1986. The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale. The information is gathered by “feeling” the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning.
The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvatureon the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke’s law. Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces,capillary forces, chemical bonding, electrostatic forces, magnetic forces (see magnetic force microscope, MFM), Casimir forces, solvation forces, etc. As well as force, additional quantities may simultaneously be measured through the use of specialised types of probe (see scanning thermal microscopy, photothermal microspectroscopy, etc.). Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. Other methods that are used include optical interferometry, capacitive sensing or piezoresistive AFM cantilevers. These cantilevers are fabricated with piezoresistive elements that act as a strain gauge. Using a Wheatstone bridge, strain in the AFM cantilever due to deflection can be measured, but this method is not as sensitive as laser deflection or interferometry.
If the tip was scanned at a constant height, a risk would exist that the tip collides with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample. Traditionally, the sample is mounted on a piezoelectric tube, that can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample. Alternatively a ‘tripod’ configuration of three piezo crystals may be employed, with each responsible for scanning in the x,y and z directions. This eliminates some of the distortion effects seen with a tube scanner. In newer designs, the tip is mounted on a vertical piezo scanner while the sample is being scanned in X and Y using another piezo block. The resulting map of the area s = f(x,y) represents the topography of the sample.
The AFM can be operated in a number of modes, depending on the application. In general, possible imaging modes are divided into static (also called contact) modes and a variety of dynamic (or non-contact) modes where the cantilever is vibrated.
The new microscope, reported by researchers at Georgia Tech and Stanford University, could be a dramatic improvement on atomic force microscopy, one of the main tools of nanotechnology. Much of nanotechnology is made possible by scanning tunneling microscopy, including atomic force microscopes (AFM), which provide detailed information about the atomic and molecular features of materials and nano devices. AFM, which use cantilever probes with perpendicular, ultrasharp tips to scan a material line by line, have also been used to fabricate nanostructures. But the process is slow, a drawback that has limited it largely to the research lab.
The new version of AFM uses a probe that scans materials 100 times faster than existing AFM. What’s more, the technology can simultaneously measure characteristics such as stiffness and stickiness while imaging the material. That type of information can, for example, help engineers design computer chips that use new nanomaterials, says Levent Degertekin, lead researcher on the project and professor of mechanical engineering at Georgia Tech.
The new probe replaces a conventional AFM cantilever with a drum-like membrane from which a tip extends that scans the material. In one scanning mode, as the tip moves above a surface, it lightly taps the material. With each tap, the instrument gathers precise information about both the tip’s position and the forces acting on it, sensing the shape of the material and how stiff and sticky it is, as the tip comes into contact, then pulls away. The new probe is faster than conventional AFM because, instead of using bulky actuators, it moves the tip by using electrostatic forces between the membrane and an electrode.
The design of the membrane-based probes makes them relatively easy to arrange in arrays in which each probe can move independently, Degertekin says. One possible application of such an array is fast parallel printing, in which each probe tip is used something like the nib of an old-fashioned fountain pen — an existing AFM technique called dip-pen lithography. This sort of printing might be used for future generations of electronics that have features too small to be made with current techniques, he says. A more immediate application is in printing arrays of biomolecules for biological assays.
The new device, which can be retrofitted into an existing AFM, could be in use by researchers within a year, says Degertekin. “People can take our device, put it on their own AFM, and they can achieve much better results,” he says. More advanced AFM machines in future years can take better advantage of the full range of the probe’s capabilities.