No abstract

In atomic force and frictional force microscopy, quantitative interpretation of lateral stiffness at the tip-sample contact requires a detailed understanding of all factors contributing to the frictional force as measured in a typical experiment. We used a scanning transmission electron microscope to image and determine the geometry of the tip apex of a variety of atomic force microscope cantilevers. On the basis of this measured structure, we then used finite element analysis to model the lateral stiffness of the tip and found that the tip stiffness is often smaller than the lateral stiffness of the cantilever. Furthermore, we analyzed the stiffness of the tip sample contact and found that for sharp tips the contact stiffness can also be comparable to the lateral stiffness of the cantilever. If these two effects are ignored, significant errors can result in the calculation of lateral forces. We demonstrated the effects of lateral tip and contact stiffness experimentally and used the measurements to calculate the radius of the tip-sample contact.

Poly(methylmethacrylate) (PMMA) which is commonly used as a positive resist can also be used in a negative manner with exposure at higher dose levels. In this paper we investigate the full potential of this resist for high-resolution pattern definition. We show that although the point spread exposure distribution is similar to that for positive PMMA, features of the order of 10 nm are easily achieved. These resist structures can be transferred into the underlying materials using plasma etch techniques to a similar degree of resolution. The density of resist features is also greater than for positive PMMA with features of ∼15 nm in width being possible on a 30 nm pitch. The instability of the resist structures at extreme linewidths has been identified as a potential problem in utilizing the process.

Using an atomic force microscope equipped with a conducting diamond tip, the surface potential on a current carrying gold nanowire was measured with microvolt potential sensitivity and nanometer spatial resolution. Potentiometry images illustrate the stages of failure of nanowires subjected to current stressing. During this failure process, a discontinuity in the potential gradient and an enhanced resistance region were observed at the failure site until a complete fracture was formed. By increasing the repulsive force and accurately positioning the tip, gold could be manipulated into the nanoscale fracture so that the electrical conductivity of the nanowire was regained.

We explored the applicability of a system of self-assembled monolayer (SAM) resists on gold, recently developed by Tam-Chang et al. [Langmuir 1995, 11, 4371−4382], to electron-beam lithography carried out at high (>1000 eV) and low (<15 eV) energies. Lithography using high-energy electrons to make transformations of the short-alkyl-chain, amide-containing monolayer used in this system required doses of electrons >30 μC/cm2, whereas contamination from the chamber in moderate vacuum (10-6 Torr) interfered with the process and provided equally useful resist layers against a cyanide etch of the gold in the absence of monolayers. Low-energy electron lithography of the same monolayer using a scanning tunneling microscope (STM) as the source proved more reliable and allowed the formation of 30−40 nm structures wherever the STM tip passed over the surface with sufficient voltage and current. Our data highlight some of the difficulties encountered when using self-assembled monolayer resists as components in “positive” electron-beam lithography on gold and suggests constraints on using SAMs as ultimate resists.