Atomic Force Microscopy (AFM)

Atomic Force Microscopy (AFM) is a powerful form of scanning probe microscopy (SPM) that performs its imaging function by measuring a local property of the surface being inspected, such as its height, optical absorption, or magnetic properties. AFM employs a probe or tip that's positioned very close to surface to get these measurements. AFM operates in two modes, namely, contact mode imaging and non-contact mode imaging.

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Contact mode imaging employs a soft cantilevered beam that has a sharp tip at its end, which is brought in contact with the surface of the sample. The force between the tip and the sample causes the cantilever to deflect in accordance with Hooke's Law, exhibiting a spring constant that typically ranges between 0.001 to 100 N/m. This deflection, as will be explained later, is used by AFM to derive information about the surface of the sample.

The amount of deflection is measured by reflecting light from a laser diode off the back of the beam, and onto a pair or array of position-sensitive photodetectors. The differences between the reflected light received by the individual photodetectors indicate the amount of angular deflection of the cantilever at any given point on the sample. As a cheaper (but less sensitive) alternative to laser detection, the cantilever deflection may be measured using piezoresistive AFM probes that serve as a strain gauge system.

The ability to monitor this deflection allows the AFM to create an image of the sample non-destructively even if the tip is continuously in contact with the sample. To prevent the cantilever tip from damaging the surface of the sample, it is maintained at a constant angular deflection so that the force applied by the tip on the surface is also kept constant. This is achieved using a feedback mechanism that adjusts the distance between the tip and the surface to keep the applied force constant. Applied forces between the tip and the sample typically range from 10^-11 to 10^-7 N.

The AFM's feedback mechanism controls piezoelectric elements that hold the sample and move it in all three axes relative to the tip. The z-movement is used for maintaining the force at the right level, while the x- and y- movements are used for the raster-scanning the tip over the sample. The resulting map s(x,y) of the tip's relative distance from the surface at different x-y values of the scan is then used to form a topographical image of the scanned area of the sample. Thus, contact-mode imaging is primarily used for generating images of a sample's topography.

Non-contact imaging employs a small piezo element mounted under the cantilever to make it oscillate at its resonance frequency. When this oscillating cantilever is brought down to within 10-100 nm from the sample surface, the oscillation gets modified by interaction forces (Van der Waals, electrostatic, magnetic, or capillary forces) between the tip and the sample.

The changes in the oscillation usually involve a decrease in resonant frequency, a decrease in amplitude, and a phase shift. These changes in oscillation characteristics can be used to generate a map that characterizes the surface of the sample. For instance, amplitude modulation can provide information about the sample's topography. Phase shifts can be used to distinguish different surface materials from each other. Frequency modulation can be used to get information about the sample's properties.

One challenge in non-contact imaging is being able to keep the correct tip-to-sample distance while preventing the tip from touching the surface, since there is a maximum distance for the inter-atomic forces to become detectable. Furthermore, the tendency of most samples to develop a liquid meniscus layer in ambient conditions complicates this task.

The advantages of AFM over electron microscopy include the following: 1) it generates true, 3-dimensional surface images; 2) it does not require special sample treatments that can result in the sample's destruction or alteration; and 3) it does not require a vacuum environment in order to operate (it can operate in both air and liquid). On the other hand, its disadvantages include the following: 1) the image size that it provides is much smaller than what electron microscopes can create; and 2) it is slow in scanning an image, unlike an electron microscope which does it in almost real-time.

In the semiconductor industry, AFM is primarily used for the imaging of VLSI cross-sections. Materials that can be imaged by AFM include metals, polymers, photoresists, etc. See also: STM.

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