S. J. O’Shea

A femtojoule calorimeter using micromechanical sensors

We describe a highly sensitive new type of calorimeter based on the deflection of a ‘‘bimetallic’’ micromechanical sensor as a function of temperature. The temperature changes can be due to ambient changes, giving a temperature sensor or, more importantly, due to the heat absorbed by a coating on the sensor, giving a heat sensor. As an example we show the results of using the sensor as a photothermal spectrometer. The small dimensions and low thermal mass of the sensor make it highly sensitive and we demonstrate a sensitivity of roughly 100 pW. By applying a simple model of the system the ultimate sensitivity is expected to be of the order of 10 pW. The thermal response time of the cantilever can also be determined, giving an estimate of the minimum detectable energy of the sensor. This we find to be 150 fJ and again from our model, expect a minimum value of the order of 20 fJ.

Conducting atomic force microscopy study of silicon dioxide breakdown

A summary is given of an experimental method used to obtain the local dielectric strength of thin insulators using atomic force microscopy with conducting tips. This technique is applied to 7–15 nm thick SiO2 films grown on either crystalline silicon or polysilicon substrates. The dielectric breakdown of the oxides over small areas (∼5×10−16 m2) follows that observed in the intrinsic breakdown of conventional metal–oxide–semiconductor structures, with a maximum breakdown field of 13.2±0.8 MV/cm. On the polycrystalline samples variation in dielectric strength between individual grains can be observed, with the oxide over some grains breaking down entirely. A difficulty when working in air is that sample or tip contamination and induced growth of material under the tip lead to changes in either the effective barrier height or local material thickness which are not related to the SiO2 film. These effects are discussed in detail.

Lateral stiffness of the tip and tip-sample contact in frictional force microscopy

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 calcu...

Solvation forces near a graphite surface measured with an atomic force microscope

Solvation force interactions in a liquid near a solid wall (graphite) were investigated using an atomic force microscope. The general features of the data show the distinctive oscillatory force curve associated with solvation forces, with a mean periodicity approximately equal to the minimum dimensions of the molecules. Moreover, the graphite surface can still be imaged with atomic resolution; which suggests that the technique can be used for the detailed study of short range forces over specific parts of various surfaces.

Characterization of tips for conducting atomic force microscopy

The conductivity of a variety of atomic force microscopy tips was investigated by measuring both current‐voltage characteristics and force profile data for the tip in contact with graphite. This allows the conductivity to be studied as a function of tip‐sample distance. It was found that etched gold wires or levers coated with a conducting diamond film were the best tips to use for experimentation on hard (SiO2) surfaces. Metal‐coated levers, particularly gold, were found to wear rapidly such that the very end of the tip became insulating.

Spatially resolved electrical measurements of SiO2 gate oxides using atomic force microscopy

Using a modified atomic force microscope (AFM) with a conducting cantilever, we have investigated the dielectric strength of SiO2 gate oxide films. This has been achieved by spatially resolving the prebreakdown tunneling current flowing between the silicon substrate and tip. During AFM imaging a voltage ramp was applied to the tip at each image point so as to determine the local threshold voltage required to generate a small tunneling current in the oxide, without causing an irreversible electrical breakdown. For an oxide 12‐nm thick this voltage was found to vary by more than a factor of 2.7 over an area of 0.14 μm2, with a maximum value of 40.5 V. This suggests that the breakdown strength of conventional metal‐oxide‐silicon capacitors may not be limited by the intrinsic dielectric strength of the oxide, but by imperfections or nonuniformities in the Si/SiO2 structure. By preventing irreversible oxide breakdown during scanning, we can image the dielectric properties of oxide films with a lateral resoluti...

Atomic force microscopy stress sensors for studies in liquids

Simple, sensitive, and fast sensors can be constructed from standard atomic force microscopy cantilevers. For use in liquid environments, it is preferable to use changes in surface stress as the measurement basis of the sensor. We have constructed such a sensor in an electrochemical cell so that simultaneous cyclic voltammograms and strain measurements can be made. This is demonstrated with examples of underpotential deposition of Pb on Au(111) and electrocapillary effects in KCl.

Atomic force microscope study of boundary layer lubrication

An atomic force microscope is used to investigate the lubrication properties of a simple surfactant (n‐dodecanol) on mica at 24 °C. The liquid becomes strongly layered as it is confined between the tip and the mica and a bilayer structure was often observed. The lubrication properties of the adsorbed molecules are clearly evident in the very low friction forces observed at appreciable applied loads (∼100 MPa). It is only when the last layer of molecules is removed that a significant friction signal is observed. It appears that the tip‐mica interaction is dominated by adhesive forces and hence the measured friction and normal forces are strongly influenced by the effective contact area of the tip.

FORCE MICROSCOPY IMAGING IN LIQUIDS USING AC TECHNIQUES

Two ac techniques for imaging under liquids using atomic force microscopy are investigated. In the first method, the sample is oscillated with a sinusoidal displacement, whereas in the second method, the cantilever is oscillated with a sinusoidal magnetic force. Both techniques are successful for topographic imaging under liquids with the tip in repulsive contact with the sample. Of the two methods, the cantilever driven technique is found to be less noisy. In addition to topographic imaging, noncontact magnetic force imaging under liquid is demonstrated.

Magnetic force microscopy of Co-Pd multilayers with perpendicular anisotropy

The domain structure of sputtered Co‐Pd multilayer films of varying thickness has been investigated by magnetic force microscopy. The domains appear as stripe domains, typical of perpendicularly oriented films. The size of the domains was strongly influenced by the thickness of the film. The domain repetition lengths give an additional experimental parameter which has been used to provide a stronger test of a theoretical model developed for ferromagnetic multilayer films [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)]. It is found that the experiment and theory are broadly in agreement provided that the increased magnetization of the multilayer caused by polarization of the Pd is accounted for. There is a noticeable difference between the variation of the measured and theoretical domain repetition lengths with film thickness. This is attributed to the effects of domain‐wall pinning which is not considered in the model. It is estimated that the characteristic length of the films is 55 A and the domain‐wall energy is 14 mJ/m2.The domain structure of sputtered Co‐Pd multilayer films of varying thickness has been investigated by magnetic force microscopy. The domains appear as stripe domains, typical of perpendicularly oriented films. The size of the domains was strongly influenced by the thickness of the film. The domain repetition lengths give an additional experimental parameter which has been used to provide a stronger test of a theoretical model developed for ferromagnetic multilayer films [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)]. It is found that the experiment and theory are broadly in agreement provided that the increased magnetization of the multilayer caused by polarization of the Pd is accounted for. There is a noticeable difference between the variation of the measured and theoretical domain repetition lengths with film thickness. This is attributed to the effects of domain‐wall pinning which is not considered in the model. It is estimated that the characteristic length of the films i...