Atomic Force Microscopy :

Introduction

AFM designed to measure the topography of a nonconductive sample. The AFM has undergone several enhancements over the years, allowing it to measure the local resistivity, temperature, elasticity, tribology, as well as allowing studies beyond the limitations of conventional optics.The underlying principle of AFM is the detection of the bending of this cantilever spring as a response to external forces. In the case of adhesive interaction between the tip and a surface, these forces are of the order 0.1 - 1 nN. To measure such small forces one must use not only very sensitive force-measuring springs but also very sensitive ways for measuring their bending.In order to detect this bending, which is as small as 0.01 nm, a laser beam is focused on the back of the cantilever. From there the laser beam is reflected towards a position-sensitive photodetector. Depending on the cantilever deflection the position of the reflected beam changes. The photo-detector converts this change in an electrical signal.

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 curvature on 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 etc. Along with force, additional quantities may simultaneously be measured through the use of specialized types of probes. 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 tip or sample is mounted on a 'tripod' of three piezo crystals, with each responsible for scanning in the x,y and z directions. In 1986, the same year as the AFM was invented, a new piezoelectric scanner, the tube scanner, was developed for use in STM. Later tube scanners were incorporated into AFMs. The tube scanner can move the sample in the x, y, and z directions using a single tube piezo with a single interior contact and four external contacts. An advantage of the tube scanner is better vibrational isolation, resulting from the higher resonant frequency of the single-crystal construction in combination with a low resonant frequency isolation stage. A disadvantage is that the x-y motion can cause unwanted z motion resulting in distortion.

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 (non-contact or "tapping") modes where the cantilever is vibrated