Microfabrication of cantilever styli for the atomic force microscope

Abstract

Atomic force microscopy (AFM) is a newly developed high resolution microscopy technique which is capable of mapping forces near surfaces or, by means of these forces, the topography of the surface itself. In one mode of operation, AFM can resolve individual atoms on both conducting and insulating surfaces. A crucial component for the AFM is a flexible force-sensing cantilever stylus, whose properties should include, among other things: a sharp tip, a low force constant, and a high mechanical resonance frequency. These requirements can be met by reducing the size of the cantilever stylus through microfabrication techniques and employing novel methods to construct a sharp tip. Presented here are a number of microfabrication processes for constructing cantilever styli with properties ideal for the AFM. These fabrication processes include (1) a method for producing thin film SiO2 or Si3N4 cantilevers without tips, (2) a method for producing Si3N4 cantilevers with integrated pyramidal tips formed by using an etch pit on the (100) surface of Si as a mold, (3) a method for producing SiO2 cantilevers with conical tips formed by a combination of isotropic and anisotropic plasma etching of a small Si post, and (4) a method for producing SiO2 cantilevers with integrated tetrahedral tips formed by anisotropically etching a corner of a small Si post to a sharp point. Each of these processes uses a (100) Si wafer as a substrate and relies on conventional batch fabrication techniques. The quality (i.e., sharpness) of the tips produced by the above methods matches or exceeds that of conventional tips used in the AFM or scanning tunneling microscope (STM). Alternative methods for producing tips by evaporation of material through an orifice or by selective chemical vapor deposition of W metal into a pyramidal etch pit in Si have been demonstrated, but these methods have not yet been successfully used in cantilever assemblies.

On the Adhesion between Fine Particles and Nanocontacts: An Atomic Force Microscope Study

“…There are currently many methods available for the determination of the spring constant of AFM cantilever. The mechanical properties and characterizes of cantilever are very important, because we can calculate spring constant of cantilever with Eq.3.2 (rectangular cross section) [160][161][162][163] Here, E is elastic modulus, is width, t is thickness, and w L is length in Fig.3.7. The cantilevers are available in a range of force constants, their small size leads to high resonant frequencies, they are relatively easy to use, and the tips are relatively sharp and durable.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…The equation is valid for a rectangular cantilever. A high resonance frequency is also important to be able to scan fast because the resonance frequency limits the time resolution [38,39]. Cantilevers have different top and bottom faces.…”

“…(2.3). For V-shaped cantilevers the spring constant can at first approximation be written in the parallel beam approximation as [38] …”

“…Nowadays microfabricated tips, which is the subject of this section, or particles attached to the end of a cantilever, which will be discussed in Section 2.5, are used. Commercially available microfabricated tips are made from silicon nitride [38] or silicon [109][110][111][112][113]. Both materials are oxidized under ambient conditions.…”

Peer Reviewed: Atomic Force Microscopy Probes Go Electrochemical

“…Today, AFM probes with various reproducible physical characteristics are routinely prepared from silicon and silicon nitride through microfabrication (4). An attractive feature of AFM is that the surface structure of a wide range of materials can be investigated, irrespective of sample conductivity.…”