J. R. Barnes

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.

Self-texturing of nitrogenated amorphous carbon thin films for electron field emission

The electron field-emission process for nitrogenated amorphous carbon (a-C:H:N) thin films deposited using a magnetically confined hydrocarbon plasma is examined. The morphology of the films obtained using an atomic force microscope is compared to the field-emission properties. Beyond a chemical composition of 14 at. % nitrogen, the mirror smooth a-C:H:N films become self-texturing, and multiple “domelike” cathodes of nanometer scale are observed. The dimensions of these “domelike” cathodes varies with time, and after a 15 min deposition have dimensions of approximately 50 nm base diameter and 20 nm in height. When the electronic field emission of these textured films (N content 15 at. %) are measured, there is an enhancement in the emitted current density of ∼2 orders of magnitude at an electric field of 20 V/μm, in comparison to the untextured films with a nitrogen content of 11 at. %.

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...

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...

Characterization of electron beam induced modification of thermally grown SiO2

We used local probe techniques to characterize electron beam (e‐beam) induced changes in thin oxides on silicon. Primary effects of the 1 nm wide, 300 keV e beam included the formation of positive charges trapped in the SiO2, physical restructuring in the oxide, and deposition of carbonaceous compounds. Charges remained stable in thicker oxides (460 nm) and appeared as changes in the contact potential or microwave response with widths down to 100 nm. In thinner oxides (20 nm) the amount of charge was smaller and less stable; below 7 nm no charge was detected. Physical changes in the oxide, evident as a swelling of irradiated areas, accounted for the etching selectivity of these regions.

Magnetic force microscope study of local pinning effects

A magnetic force microscope is used as a highly localized probe of the magnetic properties of Co‐Pd multilayer films. The stray field from the magnetic tip is used to induce magnetization changes in the sample on a scale of the order of ∼100 nm. These changes are characterized by discontinuous jumps in the force or force gradient acting on the tip due to its interaction with the magnetostatic stray field from the sample. Both reversible and irreversible domain‐wall motion is seen.

A near-field optical microscope with normal force distance regulation

Near‐field imaging is a means of exceeding the diffraction limit in optical microscopy to yield subwavelength resolution optical images of a sample surface. In order to achieve such high resolution, it is necessary to scan the measurement probe above the surface at a height of only a few nanometers which requires careful control of the separation between tip and sample. In the implementation of the near‐field optical microscope (NFOM) reported here, the distance regulation scheme is based on an inverted noncontact atomic force microscope (AFM) in which a cantilever is used as the sample substrate and imaging is performed with a fiber optic tip. In this way, both the benefits of AFM and NFOM are realized simultaneously.