General and Special Probes in Scanning Microscopies

Abstract

Scanning probe microscopy (SPM) provides nanometer-scale mapping of numerous sample properties in essentially any environment.\rThis unique combination of high resolution and broad applicability has led to the application of SPM to many areas of science\rand technology, especially those interested in the structure and properties of materials at the nanometer scale. SPM images\rare generated through measurements of a tip–sample interaction. A well-characterized tip is the key element to data interpretation\rand is typically the limiting factor.\r\r\r\r\r\r\rCommercially available atomic force microscopy (AFM) tips, integrated with force-sensing cantilevers, are microfabricated\rfrom silicon and silicon nitride by lithographic and anisotropic etching techniques. The performance of these tips can be\rcharacterized by imaging nanometer-scale standards of known dimension, and the resolution is found to roughly correspond to\rthe tip radius of curvature, the tip aspect ratio, and the sample height. Although silicon and silicon nitride tips have a somewhat\rlarge radius of curvature, low aspect ratio, and limited lifetime due to wear, the widespread use of AFM today is due in large\rpart to the broad availability of these tips. In some special cases, small asperities on the tip can provide resolution much\rhigher than the tip radius of curvature for low-Z samples such as crystal surfaces and ordered protein arrays.\r\r\r\r\r\rSeveral strategies have been developed to improve AFM tip performance. Oxide sharpening improves tip sharpness and enhances\rtip asperities. For high-aspect-ratio samples such as integrated circuits,\r\rsilicon AFM tips can be modified by focused ion beam (FIB) milling. FIB tips reach 3° cone angles over lengths of several\rmicrons and can be fabricated at arbitrary angles.\r\r\r\r\r\r\r\rOther high resolution and high-aspect-ratio tips are produced by electron-beam deposition (EBD), in which a carbon spike is\rdeposited onto the tip apex from the background gases in an electron microscope. Finally, carbon nanotubes have been employed\ras AFM tips. Their nanometer-scale diameter, long length, high stiffness, and elastic buckling properties make them possibly\rthe ultimate tip material for AFM. Nanotubes can be manually attached to silicon or\r\rsilicon nitride AFM tips or grown onto tips by chemical vapor deposition (CVD), which should soon make them widely available. In scanning tunneling microscopy\r\r\r\r(STM), the electron tunneling signal decays exponentially with tip–sample separation, so that in principle only the last few\ratoms contribute to the signal. STM tips are, therefore, not as sensitive to the nanoscale tip geometry and can be made by\rsimple mechanical cutting or electrochemical etching of metal wires. In choosing tip materials, one prefers hard, stiff metals\rthat will not oxidize or corrode in the imaging environment.