How to Choose your MAC Lever

Atomic force microscopy (AFM) is a sub-nanometer scale imaging and measurement tool that can be used to determine a sample’s surface topology and measure its mechanical properties. AFM imaging relies on a small AFM probe that is raster scanned over a surface to generate an AFM image. As shown in Figure 1, AFM probes have two major components; a lexible cantilever, which is attached to the probe chip, and a sharp probe tip near the end of the cantilever. The cantilevers generally have either triangular or rectangular geometries. AFM probes can be manufactured from a variety of materials, but most are made of silicon and/or silicon nitride (Si3N4). The tip of the probe comes into contact or near contact with the sample surface. The tip diameter can vary, depending on its speciic application, but it is generally extremely sharp, usually on the order of a few nanometers to tens of nanometers in diameter at the tip apex. Various novel techniques have been developed for creating even sharper tips; for example, carbon nanotubes have been added to the end of the probes. The delection of the cantilever as it is scanned over the sample is monitored and plotted to generate high-resolution AFM images. The images can be obtained under a variety of environments, including ambient and physiological conditions. Although the samples must be immobilized to relatively lat substrates to permit imaging, more complex sample preparation techniques such as staining or ixation can usually be avoided. 

Contact Mode vs Dynamic (AC) Mode AFM

 Contact mode and dynamic (AC) mode are two major AFM imaging modes. In contact mode, the AFM probe tip is in continuous contact or near contact with the sample as it is raster scanned across the surface. Relatively large vertical and lateral forces are conveyed to the sample during contact mode imaging and these forces can often cause damage to delicate samples. In contrast, in AC mode the AFM probe is oscillated up and down at a predetermined frequency as it is scanned over the sample. AC mode can have certain advantages over contact mode; mainly because, as the probe tip oscillates up and down over the sample, it only makes intermittent contact with the sample surface. One result of the intermittent contact, is that the lateral forces and the loading forces between the probe tip and the sample are minimized compare to contact mode. Consequently, AC mode is often preferred for very soft and delicate samples such as polymers and biological materials.  

MAC Levers

 An important component of MAC Mode is an assortment of specially designed, proprietary AFM probes, called MAC Levers. MAC Levers contain a paramagnetic ilm which covers the backside of the cantilever. The paramagnetic ilm permits the cantilever to be driven precisely at its inherent oscillation frequency in air and even in liquids through the application of an external magnetic ield. The coil for the magnetic ield can be placed either above the sample (Top MAC) or below the sample (Bottom MAC). Compared to regular AC mode probes, MAC Levers can be oscillated at more precise frequencies. The result is more accurate and simpler probe tuning, especially in liquids, along with improvements in topography and phase image resolution. Importantly, lower imaging forces and much less damage to soft, delicate samples occur in MAC Mode compared to other AFM imaging modes. In fact, MAC Mode is the gentlest topographic AFM imaging technique. It was speciically designed with delicate materials, such as soft polymers and biological samples, in mind. MAC Mode allows researchers to study samples on the molecular level and image delicate samples which often cannot be resolved with any other AFM imaging technique.

 Types of MAC Levers As mentioned above, MAC Levers are AFM probes that have a proprietary paramagnetic coating on the backside of the cantilever and which are speciically designed, produced and optimized for use in imaging applications utilizing Keysight Technologies Inc.’s MAC Mode. Figure 4 shows a schematic diagram of a MAC lever. There are ive types of MAC levers, TYPE I, TYPE II, TYPE VII, Type VIII and Type IX currently available. Each MAC Lever can be used with Keysight’s MAC Mode and they have been optimized for various applications.

Choosing MAC Levers for Specific Applications

 TYPE II MAC Levers (PN: N9812x) TYPE II MAC Levers have silicon cantilevers and silicon tips. There is one rectangular AFM cantilever on each probe chip. TYPE II MAC Levers are particularly useful for imaging stiffer samples, such as block copolymers, in air or liquid.  

TYPE VII MAC Levers (PN: N9866x) TYPE VII MAC Levers are made from silicon. There is one rectangular AFM cantilever on eachprobe chip. TYPE VII MAC Levers are particularly useful for imaging very soft biological samples in liquid as well as topography and recognition imaging using PicoTREC.  

TYPE VIII MAC Levers (PN: N9867x) T YPE VIII MAC Levers are made from silicon.There is one rectangular AFM cantilevercon each probe chip. TYPE VIII MAC Levers are particularly useful for imaging very soft biological samples in liquid as well as topography and recognition imaging using PicoTREC.  

TYPE IX MAC Levers (PN: N9811x) TYPE IX MAC Levers have silicon cantilevers and silicon tips. There are three different rectangular AFM cantilevers on each TYPE IX chip. Each TYPE IX cantilever has a different spring constant and resonant frequency which can be selected based on the user’s speciic application. This makes TYPE IX MAC Levers especially useful in applications where a a range of spring constants might be required.