H. Kumar Wickramasinghe

High‐resolution capacitance measurement and potentiometry by force microscopy

We demonstrate the usefulness and high sensitivity of the atomic force microscope (AFM) for imaging surface dielectric properties and for potentiometry through the detection of electrostatic forces. Electric forces as small as 10−10 N have been measured, corresponding to a capacitance of 10−19 farad. The sensitivity of our AFM should ultimately allow us to detect capacitances as low as 8×10−22 F. The method enables us to detect the presence of dielectric material over Si, and to measure the voltage in a p‐n junction with submicron spatial resolution.

Strength of the electric field in apertureless near-field optical microscopy

Enhancement γ of the electrical field at the end of a tip relative to the incident field in a focused radiation beam is calculated by the finite-element time-domain (FETD) method. First, the reliability of the FETD method is established by calculating the electric field on simple structures like thin cylinders, spheres, and ellipsoids, and comparing the results with analytical solutions. The calculations on these test structures also reveal that phase retardation effects substantially modify γ when the size of the structure is larger than approximately λ/4, λ being the radiation wavelength. For plasmon resonance, in particular, phase retardation severely reduces the resonance and the expected field enhancement for a gold tip. The small value of γ=4 calculated by FETD is about an order of magnitude smaller than the value found in recent published work. Resonance effects can be recovered for special tips, which have a discontinuity or a different material composition at the end of the tip. Some tuning of th...

Method for imaging sidewalls by atomic force microscopy

We demonstrate a new method for imaging vertical and near vertical surface features by atomic force microscopy (AFM). It is based on an attractive force mode AFM, equipped with a special boot‐shaped tip, coupled with a measurement of slope and with a special tracking technique. Surface profiling is achieved through a novel servo and scanning system. Mapping sidewall profile opens the door to measurement of critical dimensions (width and wall angles) of lines and trenches in integrated circuits, with high accuracy.

Thermally assisted recording beyond traditional limits

This work demonstrates that current magnetic storage density limitations due to superparamagnetic effects can be overcome by thermally assisted writing. Specifically, we write magnetization patterns on thin magnetic films that correspond to storage densities of ∼400 Gbit/in.2. Simple thermal diffusion considerations predict potential storage densities of >1 Tbit/in.2 accompanied by recording speeds of >1 GHz.

Toward accurate metrology with scanning force microscopes

Scanning force microscopes (SFMs) have become valuable instruments for inspection of surfaces on a submicron scale in the semiconductor industry. Their ability to track the position of a surface with an accuracy of the order of 1 nm opens the door to applications in micro‐ or nanometrology. Standard SFMs, using a conical shaped tip, are being applied to measurements of the height or depth of shallow features, such as via holes or small lines and trenches, that cannot readily be characterized by scanning electron microscopy or optical techniques. Recent technical developments of the SFM have provided new capabilities for accurate measurement of the line and trench width using a flared tip and an improved scanning and tracking method for the tip. Laboratory tests as well as industrial tests have revealed that repeatability in measuring critical dimensions approaches 1 nm. If careful attention is given to issues of calibration, particularly for the motion of the tip and for dimensioning of the tip itself, SF...

Ultrafast molecule sorting and delivery by atomic force microscopy

An atomic force microscope (AFM) is tailored to perform ultrafast electrophoretic differentiation of molecules on populations of <0.1zeptomoles (10−22moles) on the surface of a probe tip. The driving force for differentiation is a large electric field applied over the length of an AFM tip that results in enhanced differential mobilities stemming from the confinement of the water layer on the tip surface. In a demonstration on DNA oligonucleotides, a 5-mer and a 16-mer exhibit migration times of 15 and 5ms, respectively, approximately five orders of magnitude faster than in conventional capillary electrophoresis.

Nanoscale quantitative stress mapping with atomic force microscopy

A nanometer scale noninvasive method for quantitatively mapping tensile strain in metals or semiconductors is demonstrated. The technique is based on the Kelvin probe force microscopy detection of changes in the electronic work function of a material resulting from the tensile strain. Measurements are quantified using a simple microlever mechanical system by recording changes in the work function as a function of the applied strain. A linear relationship of the work function on the tensile stress is observed with a stress sensitivity of 1kPa. Finally, the stress distribution in a strained silicon membrane is imaged.

Resolution test for apertureless near-field optical microscopy

A test configuration for apertureless near-field optical microscopy permits characterization of near-field signals free from significant topography effects. Using a second tip as the scanned object, a high-contrast dipole–dipole signal is observed when the two tips are closely spaced, by 10 nm or less. A spatial resolution on the order of 5 nm is demonstrated. The measured signal is also material dependent: an inverted contrast is recorded when one of two high-dielectric (silicon) tips is replaced by a metallic (nickel) tip. Measurement results are accounted for by theory, which also confirms the nature of the detected signal: it results from interference between a near-field dipole–dipole field scattered by the end of the tips, and a reference field scattered by the main body of the tips. Different illumination sources are investigated. In all cases, the component of the electric field parallel to the tip axes provides the major source of signal and contrast.

Scanned Probes Old and New

The Scanning Tunneling Microscope (STM) has stimulated a range of new microscopies which essentially use the same scanning and feedback principles to obtain nanometer resolution images. In this contribution, we review the history of scanned probe techniques and discuss some of the new directions that have evolved with particular reference to work done in our own group.

Detection of high- and low-frequency vibrations using a feedback-stabilized differential fiber optic interferometer

Highly sensitive and stable detection of minute ac and pseudo-dc (i.e. , very low frequency) vibrations is performed by means of a differential fiber optic Michelson interferometer. Any residual instabilities are eliminated by means of an ac feedback control loop. The stabilization technique is not affected by the variations in the reflectivity of the sample and by launch inefficiencies into the fiber. This approach also provides a simple, quantitative method for calibrating sample vibrations. Results are presented on the operation of the system in stabilizing its output as well as on the detection of the vibration of a silicon sample.