In the arena of microscopy techniques, the scanning probe microscope, with its many derivitives, is still in its infancy. Binning and Rohrer won the 1986 Nobel Prize for work performed with the scanning tunneling microscope. Shortly thereafter, atomic force microscopy was realized. For convenience, these two techniques have been lumped under the header of scanning probe microscopy. A discussion of these technologies follows.

SPM techniques all use piezo-electric ceramics and computers. The differences among the techniques comes down to the 'probe' that is being used to 'sense' the sample, and what phenomenon the 'probe' is 'sensing.' How the piezo-electric ceramics are utilzed on a certain manufacturers instrument most likely comes down to their attempt at avoiding copyright infringement. The Digital Instruments Nanoscope III System that I am most familiar with uses piezo-electric ceramics to scan (move) the specimen in relationship to the probe. Other companies might use the piezo material to move the probe in respect to the sample. What are piezo-electric ceramics?

Piezo-electric ceramics are materials whose crystal planes slip on one another when a current is applied across them. This slippage causes shrinkage or elongation of the crystal (depending on the current applied). Segmented tubes of this material are made. The tubes are usually segmented into five zones, two being x, two being y, and one z axis. Two x and y axes are needed so that they can 'fight' themselves. As the current to one x axis is increased, the current to the other x axis is decreased at the same rate. Thus, one side of the tube is expanding while the the other side is contracting. This causes the tube to 'lean,' scanning the sample back and forth under the probe. The y component of the scan is achieved in the same fashion. The current applied to the x and y portions determine the area of the specimen that is scanned. The z portion of the tube is used to keep the sample in close proximity to the probe when sample surface detail changes. The current to the z portion can be used to map the surface of the sample. An image formed by this current is referred to as a height mode image.

We now know about the scanning action, and it's time to start discussion of the many 'probes' one can employ. Since scanning tunneling microscopy (STM) was first, let's start there. The 'probe' used in STM is most often a very sharp section of Pt/Ir wire. A sample is placed onto the scanner, and the wire is brought into close proximity of the sample surface. The computer is allowed to take control of the wire/sample separation, bringing them closer together until a predetermined tunneling current is established between the wire and sample surface. This current is usually on the nano-to pico-ampere scale. Once this current is established, we have two modes of operation by which to form an image of the sample. One mode is the constant current mode. This tells the computer that is controlling the scanning to make necessary corrections to the z portion of the scanner so that the tunneling current is always the same. Tunneling current might change due to surface roughness (increasing the gap between the wire and the surface), or change in atomic number of the sample (one area might be a better conductor). The other mode of operation would be the constant height mode, where the sample is held the same distance below the wire as the scan is performed, and the image is formed from the variations in tunneling current that this causes. Scanning tunneling can provide atomic force microscopy (AFM) techniques. STM requires that a sample be conductive, where this is not a requirement of AFM.

The scanners and computers used for STM can be used for Atomic Force Microscopy (AFM). AFM can be either contact mode AFM, or tapping mode AFM (tapping is a registered trademark of Digital Instruments, Inc). Both of these modes make use of a laser beam being reflected off of the back of a cantilever to generate an image of the sample surface. This laser is housed in the AFM head. In this head are the laser, the cantilever, a mirror, and a four quadrant photodiode array. The laser has x and y positioning so that it can be positioned on the back of the cantilever. Proper positioning of the laser enhances its reflectance toward the mirror. The reflected laser strikes the mirror. The angle of the mirror can be changed so that the laser hits the photodiode array. The photodiode array is divided into four quadrants. In which quadrant, and where in the quadrant the laser strikes determines the voltage that comes out of the photodiode array. One can tell the computer that the voltage coming out of the photodiode array must remain constant. Therefore, if the scanning cantilever encounters a surface change on the specimen, it must move specimen up/down with the z portion of the scanner so that cantilever deflection doesn't occur (deflection of cantilever=position change of laser on photodiode=change in voltage output). This type of data (not allowing the cantilever to be deflected and using the voltage change of the z portion of the piezo scanner) is referred to as height mode. Deflection mode imaging is when the sample is allowed to cause deflection of the cantilever. This deflection causes the laser to change position on the photodiode array, changing the voltage output of the photodiode array. An image can be made using this changing voltage. It should be readily apparent that deflection and heigh mode images can not be optimized at the same time.

Normal contact mode AFM utilizes the top and bottom portions of the four quadrant photodiode array. Lateral Force Microscopy (LFM), or also called frictional force microscopy, makes use of the left and right hand portions of the photodiode array. The cantilevers used for contact AFM are bi-legged silicon nitride semi-conductor material that has a pyrimidal tip. Normally, the sample is scanned perpendicular to the base of the pyrimidal tip. The software allows one to choose the angle that the sample is scanned in respect to the tip. If one scans in a direction parallel to the base of the pyramid, differences in 'stickiness' of the sample can cause the cantilever to torque, deflecting the reflected laser light between the left and right quadrants of the photodiode array. This deflection, as measured by the photodiode array output, can be used to map different frictional characteristics of the sample's surface.

During contact mode AFM imaging, the cantilever is physically touching the sample. This contact can alter surface structures. Tapping or 'non-contact' mode avoids this, and has become the most popular mode of SPM operation. Ideally, the tip of the cantilever used in tapping never makes contact with the sample being imaged. The design of the cantilever used for tapping is different from those used for contact. The tapping mode cantilever is a single 'plank' or spring board of silicon. The laser is focused on the back of the cantilever in much the same fashion as in the contact mode. In the cantilever holder, there is a stack of piezoelectric material, the resonance frequency of the cantilever can be found. As the cantilever bounces, the laser reflectance moves over a fairly constant line length on the photodiode array. With the cantilever vibrating at it's oscillating frequency, it is brought into close proximity of the sample surface. Air that is trapped between the cantilever and the sample surface dampens the vibration of the cantilever, changing the laser line length on the photodiode array. Different line lengths cause the different voltage outputs of the photodiode array. The most common approach to forming an image is to have the software set up so that it moves the sample up or down to keep the coltage output of the photodiode array constant. The voltage to the piezo responsible for moving the sample is used to create an image of the surface of the sample.