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We demonstrate the stimulated Raman nanoscopy of a small number of molecules in a plasmonic gap, excited without resonant electronic enhancement, measured using near-field photon-induced forces, eliminating the need for far-field optical detection. We imaged 30 nm diameter gold nanoparticles functionalized with a self-assembled monolayer (SAM) of 4-nitrobenzenethiol (4-NBT) molecules. The maximum number of molecules detected by the gold-coated nano-probe at the position of maximum field enhancement could be fewer than about 42 molecules. The molecules were imaged by vibrating an Atomic Force Microscope (AFM) cantilever on its second flexural eigenmode enabling the tip to be controlled much closer to the sample, thereby improving the detected signal-to-noise ratio when compared to vibrating the cantilever on its first flexural eigenmode. We also demonstrate the implementation of stimulated Raman nanoscopy measured using photon-induced force with non-collinear pump and stimulating beams which could have applications in polarization dependent Raman nanoscopy and spectroscopy and pump-probe nano-spectroscopy particularly involving infrared beam/s. We also discuss using photon induced forces as a technique to sort and select best performing metal coated tips for further use in tip-enhanced experiments.

We discuss a new measurement technique called thermal proximity sensing (TPS) and its application to the inspection of magnetic hard-disk surfaces. This method uses the magneto-resistive (MR) readback element of a hard-disk file in a novel thermally sensitive mode to actively sense the air-bearing gap spacing while the disk spins under the slider. We present model calculations that describe the origin of the thermal sensitivity and its range of applicability. By collecting height information as a function of position, we have obtained high-resolution images of disk topography with sensitivity in the subnanometer range. Initial results of the technique, obtained on a test stand, showed an exceptional imaging ability for surface features. We have also obtained useful surface structure data from in situ measurements of disk surface defects in an operating hard-disk file.

We present the design and theory of operation of a new fiber‐optic detection mechanism for force microscopy. The technique comprises a differential fiber Michelson interferometer, in which most spurious signals due to environmental variations are automatically rejected. Any residual fluctuations are canceled by the action of a feedback mechanism, which also ensures operation at maximum sensitivity. This detection technique is capable of high sensitivity response to both high and low frequency tip variations, and can thus be used in the attractive and repulsive modes. A number of results are presented on the applications of a force microscope utilizing this detection mechanism. Both ac and dc modes of operation are demonstrated.

We present a force microscope which uses a feedback‐controlled differential fiber interferometer for the measurement of the deflection of the force‐sensing cantilever. Due to the differential principle, and the feedback control, the influence of thermal and mechanical drifts or fluctuations is minimized. Topographic images in both modes, the attractive ac mode and the repulsive dc mode, have been taken to demonstrate a first performance of the instrument.

The contact potential difference between a reference tip and sample can be measured with high spatial resolution using a modified version of the scanning force microscope. The instrument is comparable to a Kelvin probe but allows the simultaneous imaging of contact potential difference and, in addition, topography. The contact potential difference is not only dependent on the material, i.e. work function, but also on the condition of the surface. Therefore changes in contamination, stress, temperature, the crystalline structure, oxide-layer properties and (trapped) charges can significantly alter the contact potential difference. Initial results concerning these parameters will be presented.

Measurement of dopant density in silicon with lateral resolution on the 100 nm scale has been demonstrated with a ‘‘near field’’ capacitance microscope. The technique is based upon the measurement of local capacitance between a 100 nm tip and the sample surface as the tip is scanned over the surface. The capacitive microscope used to image the dopant is capable of measuring capacitance variations of 3×10−22 F/(Hz)1/2, and previously has demonstrated a 25 nm resolution on metallic surfaces. Dopant imaging is achieved by scanning the tip under feedback control close to a silicon surface, and measuring the capacitance variations. Several types of measurements have been made on the submicrometer scale. First, direct two‐dimensional visualization of the dopant in silicon has been achieved by this nondestructive, noncontacting technique. Second, capacitance versus voltage (C–V) measurements have been made with high spatial resolution, providing the means for the measurement of many of the properties of silicon ...

For the first time, we demonstrate sublithographic memory read/write operation using micro to nano addressing block (MNAB) decoders. Test structures are fabricated with integrated one-time programmable oxide ROM elements addressed using MNAB devices that have 4 sub-50 nm silicon fins at 140 nm period. Functional operation is obtained for all 4-bit ROM sequences and over different ROM cell areas

Over the past few years, a number of techniques, including self-assembly, nanoimprint lithography and spacer-based frequency doubling, have been explored to pattern line and space structures that are considerably denser than possible with conventional photolithography, with excellent critical dimension control. In order to build useful circuits with them, an efficient way of interfacing these nanoscale lines with the photolithographically defined peripheral circuits is needed. While many solutions to this problem have been proposed, for the first time, we report a solution (MNAB) that is (a) fully silicon-process compatible, (b) non-stochastic (i.e. is completely deterministic,) and (c) does not require any critical alignment. The MNAB uses discrete analog potentials to deplete and switch off certain nanoscale lines enabling selection of the remaining nanoscale line. Simulation results show that individual nanoscale silicon lines can be addressed with selectivities (ratio of current in conducting line to that in depleted lines) exceeding 100times at features as small as 10 nm using this concept. Experimental results on various silicon prototypes show that selectivities exceeding 100times can be easily obtained even on sub-20 nm features

A new form of read out for high density read-only memory is presented whereby a data density of 400 bits/μm2, corresponding to 256 Gbits/in.2, can be accessed at data rates in the tens of MHz range. The technique is based on detecting the modulation in light scattering from a sharp scattering object due to the dipole-dipole coupling between the probe and surface being scanned using a sensitive homodyne interferometer. Theoretical considerations indicate that data densities in the 100 Tbits/in.2 range could be accessed at data rates of 100 MHz using this technology.

A new form of optical spectroscopy is demonstrated whereby the local scattering from the interaction between a nanometer size probe tip and a sample is measured as a function of wavelength. We show images of viruses and molecules at molecular spatial resolution and demonstrate how the image contrast varies with wavelength. To first approximation, the contrast as a function of wavelength varies in the same way as the local polarizability of the sample

In this paper, we have demonstrated a new technique to detect and image defects on a disk surface using product data heads.

A near field optical microscopy method and apparatus eliminates the necessity of an aperture for scanning a sample surface (18) and greatly reduces the detected background signal. A small dimension tip (14), on the order of atomic dimension, is disposed in close promity to the sample surface (18). A dither motion is applied to the tip (14) at a first frequency in a direction substantially normal (22) to the plane of the sample surface (18). Dither motion is simultaneously applied to the sample (20) at a second frequency in a direction substantially parallel (24) to the plane of the sample surface (18). The amplitude of the motions are chosen to be comparable to the desired measurement resolution. The end (12) of the tip (14) is illuminated by optical energy. The scattered light from the tip (14) and surface (18) is detected at the difference frequency for imaging the sample surface (18) at sub-wavelength resolution without the use of an aperture. Alternatively, the tip (14) is maintained stationary and the sample (20) undergoes motion in the two directions.

The past six years has seen a tremendous growth in scanned probe microscopies of various sorts. In this letter, we add a new capability to this family−mapping of thermal conductivity variations on a nanometer scale. We show how our new probe technique can be used to measure thermal conductivity of conductors and thin insulating films deposited on top of conductors. Our results also demonstrate for the first time, the capability of the technique to image subsurface details of samples. As the thermal conductivities of different materials can vary by over three orders of magnitude, we suggest this as an important new contrast mechanism for studying materials on the nanometer scale.

Measurements of the contact potential difference between different materials have been performed for the first time using scanning force microscopy. The instrument has a high resolution for both the contact potential difference (better than 0.1 mV) and the lateral dimension (

Near-field scanning microscopy at microwave frequencies (1) and its extension to the visible region (NSOM)(2, 3) at around 50 nm resolution has attracted much attention (4–7). Recently, a Scanning Interferometric Apertureless Microscope (SIAM) was introduced (8, 9, 14) where the scattered electric field variation due to a vibrating and scanning probe tip in close proximity to a sample surface is measured by encoding it as a modulation in the phase of one arm of an interferometer. Here, we review the SIAM technique and present images of various samples at a resolution of 1 nm - almost two orders of magnitude superior to other NSQM’s. A basic theory based on coupled dipoles is put forward and compared with experiments. It shows that the contrast mechanisms are fundamentally different to those in regular near-field optical microscopes due to a unique dipole-dipole mechanism. Furthermore, the theory predicts the ability to measure complex susceptibility down to the atomic level.

In the preceding chapters we have seen how the scanning tunneling microscope (STM) and the scanning force microscope (SFM) have been applied to a wide range of problems in chemistry, physics and biology. The STM has taught us that it is possible to stabilize and scan a fine probe tip (only one atom across at its very end) by using piezoelectric scanners coupled with electronic feedback techniques [6.1]. In order to achieve this sort of precise control of the tip—sample spacing however, one needs to be able to derive an electronic feedback signal that varies rapidly as the tip—sample distance is varied. In the STM we monitor the rapid (almost exponential) decrease in tunnel current with increasing tip—sample spacing to achieve this end. However, as we will see here, the same scanning and feedback principles can also be applied to other types of interactions than the tunneling current between tip and sample. All these new microscopies have the characteristic that their resolution is not determined by any wavelength that is used for the interaction as in conventional microscopy (the so called Abbe limit [6.2]) but rather by the size of the interacting probe that hovers over the sample surface to scan the image. These microscopies therefore come under the general class of super-resolution microscopy.

As device sizes get smaller and smaller, conventional optical techniques for characterisation are reaching their limit. There is an ever increasing need to perform such measurements on the nanometer scale. The Scanning Tunneling Microscope and the Atomic Force Microscope has taught us how to position a tip with angstrom accuracy over a sample surface and scan it in X, Y and Z in order to form an image. It is possible to use this principle to image other physical properties of a surface than the density of electronic states provided by the Tunneling Microscope. In this talk, we shall review several extensions of the STM and AFM that we have demonstrated recently within our group. These include high resolution magnetic force imaging, c/v imaging, thermal imaging, photovoltage microscopy, chemical potential microscopy, optical and phonon absorption microscopy. The principles of operation of these microscopies will be described together with representative results obtained on various samples.

The design and operation of an atomic force microscope employing a stable fiber-optic detection system are discussed, and a number of results are presented.

The Scanning Capacitance Microscope has been used to profile insulating, semiconducting and conducting surfaces with resolution down to 25 nanometers. In the case of semiconducting materials, it has been demonstrated that lateral dopant profiling can be achieved with 100 nanometer spatial resolution‐well beyond that which can be achieved by conventional techniques. Measurements on semiconductors indicate that the technique should be useful for device characterization. In this paper, we review the basis of Scanning Capacitance Microscopy and present some of the more recent results.

The Kelvin probe force microscope, introduced some years ago, has opened up several avenues of investigation. In this letter, we demonstrate that the technique is capable of distinguishing constituents of a metal alloy through their work-function differences. The intermetallics in the alloy are clearly resolved. We discuss the basic principles of the measurement technique and present our results on aluminum/copper surfaces. The limits of the technique are also discussed.

Measurement and control of high-aspect-ratio structures such as dynamic random-access memory trenches is an important step in the manufacture of modern memory devices. We present a novel technique based on infrared interferometry that has been implemented in manufacturing and is capable of measuring sub- 0.25-mum -wide and 10-mum -deep trenches nondestructively and with an accuracy of better than 0.1mum .

A technique is described for the measurement of surface photovoltage (SPV) on semiconducting samples using the attractive‐mode scanning force microscope. The use of SFM potentiometry1 permits microscopic measurement of surface photovoltage to be made in air with a spatial resolution of a few tens of nm and voltage resolution of better than 1 mV. Thus, with a spatial resolution only 1 order of magnitude worse than scanning tunneling microscopy measurements,2 atomic force microscopy (AFM) is seen to be a good choice in measurements on devices or other structures where measurement under ambient is desirable or tolerable. Historically, SPV has permitted the observation of a wide range of features in semiconducting samples, including mapping of dopant concentration, sodium accumulation, local charge, dislocations, precipitates, stacking faults, grain boundaries and microcracks, local resistivity, recombination time, band gap shifts (for instance in heterostructures), and stress.3 Experimental results will be presented which demonstrate the application of AFM/SPV to microcracking, dopant profiling and dislocation structure.

Recently, high‐resolution mapping of dopant concentration has been demonstrated with the scanning capacitance microscope (SCM). Here, we demonstrate that a similar measurement can be made with the atomic force microscope using the previously demonstrated capacitive force sensing mode. By applying appropriate bias to the force tip, depletion‐induced capacitive variation is mapped over regions of varying dopant density. This method has a predicted sensitivity comparable to the SCM, and in addition allows imaging of trapped charge, as well as an independent measurement of the surface topography. Results of first‐order model calculations are presented which give estimates as to the limits in sensitivity and resolution of this method

A new microscope, the scanning chemical potential microscope (SCPM), has demonstrated the measurement of thermoelectric potential variations with atomic resolution. These measurements can be attributed to atomic scale variations in the surface chemical potential gradient (∂μ/∂T). This capability was experimentally demonstrated by measuring the thermoelectric voltage produced at the tunnel junction between a tip and a heated sample with a modified scanning tunnelling microscope (STM). Registered images of STM current and thermoelectric voltage were acquired by temporally multiplexing current and voltage data from line to line, providing a direct spatial comparison of current and voltage contrast. The results on graphite and molybdenum disulfide demonstrate that the tunnelling and thermoelectric contrast is spatially different on an atomic scale. The experiments are described, and the results of some first order classical calculations are presented.

When harmonics are produced in the scanning acoustic microscope, the resultant images have a complex appearance which requires interpretation. A theoretical calculation of the distribution of harmonic radiation has been made for the case of a focused beam scanning across the edge of an object such as a Mylar sheet suspended in water. The results show that most of the harmonic signal is generated in the water, and that diffraction causes the appearance of fringes in the image. Experimental results agree with the theory.

SUMMARY We review the principle of differential imaging and its application to scanning tunnelling microscopy (STM). It is shown that placing a lateral dither on an STM tip at high frequency provides the means for transfering topographic information to a frequency range where noise is small. Differential STM imaging on graphite and gold is demonstrated. A simple relation between the differential image and the conventional topographic image is described.

SUMMARY The main features of the scanning acoustic microscope are reviewed with particular reference to applications in biology and non-destructive evaluation. Recent developments on imaging the interior of solids are discussed. Proper interpretation of recorded images can lead to maps of velocity and density via the V(z) response. Further improvements in resolving power (using cryogenic liquids or high pressure gases) should make the acoustic microscope a powerful tool in many areas of science and technology.

Several articles have been written on the subject of scanning acoustic microscopy. Nevertheless, apart from some specific papers dealing with reflection microscopy, little has been said about the various contrast mechanisms involved in the process of image formation, and the effects of parameters such as acoustic antireflection coatings on the lens surface and absorption in the water on the quality of the recorded image. This paper is devoted to an analysis of these problems. It is shown that in the amplitude mode of operation, the acoustic microscope acts as a phase contrast microscope and is sensitive to the local phase gradients in the object. Furthermore, it is shown that the effect of an acoustic antireflection coating on the lens surface is to substantially reduce the side lobe amplitude of the focal distribution at the expense of a slight increase in the width of the main lobe. Experimental observations agree with theory.

In this paper we describe a new technique, based on Fourier optics, to explain the propagation, as well as loss, of three‐dimensional thermal waves in isotropic, homogeneous materials. Using this, the dependence of temperature distribution at any arbitrary infinite parallel plane on the aperture distribution is derived. In addition, the temperature distribution at the aperture plane, due to a given perpendicular source is formulated, by applying the boundary conditions in the spatial frequency domain. A scattering matrix theory is developed to analyze the propagation of thermal waves in multilayered structures. This directly relates the heat source characteristics to the temperature distribution at any level. The contrast mechanism in the subsurface mode of operation is explained, and the dependence of the response of a typical system on depth is explored. In addition, the theoretical amplitude and phase images of a cylindrical void are presented; the results are in good agreement with the published exper...

A new optical technique is described for measurement of absolute distance. The approach is based upon a wavelength multiplexed heterodyne interferometer with FM demodulation. By temporally multiplexing discrete wavelengths in a heterodyne interferometer, a complete elimination of interferometric range ambiguity can be achieved while maintaining the high range sensitivity and resolution of interferometry. The basic theory is presented and an algorithm is described for measurement of range over meter distances with submicrometer resolution. The experimental implementation of the wavelength multiplexed interferometer is described and ranging results with 2 μm resolution from 20 cm are presented. A scanned three‐dimensional map of a surface contour with 3‐mm topography is also presented.

A modified version of the atomic force microscope is introduced that enables a precise measurement of the force between a tip and a sample over a tip‐sample distance range of 30–150 A. As an application, the force signal is used to maintain the tip‐sample spacing constant, so that profiling can be achieved with a spatial resolution of 50 A. A second scheme allows the simultaneous measurement of force and surface profile; this scheme has been used to obtain material‐dependent information from surfaces of electronic materials.

A laser probe technique is used for the visualization of acoustic surface vibration fields. The sensitivity achieved exceeds that attained by other methods by more than an order of magnitude. The system can be used for measurements on acoustic resonators and surface wave acoustic devices. In addition it can form the basis of an acoustic holography system. Acoustic holograms at 50 MHz have been recorded, and optically reconstructed. The resolution obtained is close to the theoretical limit for that frequency.

Noise stemming from mechanical vibration, electronic noise, or low frequency (1/f power spectrum) inherent in the tunneling process, often limits the resolution, speed, or range of application of scanning tunneling microscopy (STM). We demonstrate a technique for minimizing the effect of these noise sources on the STM image. In our method, the tunneling tip is vibrated parallel to the sample surface at a frequency f0, above that of the feedback response frequency. Two signals are obtained simultaneously: the conventional topography, and a differential image corresponding to the amplitude of current modulation at f0. The resultant ac signal can be simply related to the normal STM topographic image, with significant improvement in the signal‐to‐noise ratio.

We describe a refined technique for magnetic force microscopy (MFM) which makes possible magnetic imaging with 25 nm resolution. Previous MFM work has relied on a servo system which used the amplitude of the response of an electropolished iron wire cantilever tip to an applied, near‐resonance excitation to maintain a constant force gradient between tip and sample. The present results have been obtained with a redesigned instrument which uses a fast lock‐in detector to servo on the phase, as opposed to the amplitude of the tip response. With these changes and the use of sharp electrochemically etched Ni tips, it is possible to servo stably on the very weak force gradients encountered in MFM, within 20 nm of the sample surface, and thus obtain 25 nm resolution.

We propose a novel mechanically scanned B‐scan system for imaging the interior of solids with diffraction‐limited resolution. By utilizing an additional spherical surface in conjunction with a conventional acoustic lens, it is possible to reduce spherical aberration, (which would otherwise seriously degrade the imaging performance), to a negligible value. In typical situations, it is possible to scan the focus through a depth of several hundred wavelengths.

Apparatus is provided for investigating surface structures irrespective of the materials involved. A fine scanning tip is heated to a steady state temperature at a location remote from the structure to be investigated. Thereupon, the scanning tip is moved to a position proximate to, but spaced from the structure. At the proximate position, the temperature variation from the steady state temperature is detected. The scanning tip is scanned across the surface sturcture with the aforesaid temperature variation maintained constant. Piezo electric drivers move the scanning tip both transversely of, and parallel to, the surface structure. Feedback control assures the proper transverse positioning of the scanning tip and voltages thereby generated replicate the surface structure to be investigated.

High‐resolution images of domains written in a magnetic thin film have been obtained for the first time using force microscopy. The sample consisted of 500‐A‐thick Tb19Fe81 with magnetization of 109 emu/cm3. Micron‐sized magnetic domains were thermomagnetically written in the sample using a focused laser beam. Domain images were obtained by observing the magnetic interaction of the sample with a small vibrating magnetized iron tip. Typical observed force gradients were in the range 0.8×10−4–6×10−4 N/m and the forces were in the range 10−12–10−11 N. The spatial resolution of the technique was on the order of 1000 A. This was sufficient resolution to see irregularities in those laser written marks which were recorded using low bias field.

We have modified a transmission acoustic microscope by replacing the input lens with an optical counterpart. The input to this system comes from a mode‐locked and Q‐switched Nd : YAG laser at 1.06 μm. Acoustic signals arising from thermoelastic coupling are generated in metallic films at frequencies determined by the modulation envelope of the optical beam. The output acoustic lens and transducer responsive to sound waves at 840 MHz detect the acoustic energy which comes from the heated volume (∼2‐μm diameter) near the focal region. The sample is mechanically scanned through the focal point in a raster pattern to record the photoacoustic images. We suggest this as a method for collecting new information on microscopic structures.

We describe a new type of jet plating that uses a focused high‐frequency sound field directed through the center of the jet. The acoustic jet results in an improved morphology for gold and copper depositions although plating rates are not enhanced. Four‐point probe measurements of acoustic‐jet plated copper lines show up to a 50% decrease in electrical resistivity compared to jet plated deposits.

A new technique is described whereby one can extract the nonlinear photothermal properties of a sample in photothermal microscopy. The basic scheme relies on chopping a pump heating beam at ω and detecting the temperature variation of the sample using a linear temperature sensor (in our case a mirage detector) tuned to 2ω. Harmonic images of cracks and other samples show very high contrast when compared with the fundamental image.

We present data which show that the magnetic force microscope is capable of detecting the component of the magnetic field parallel to the surface of a sample under study. Images of bits in a Co‐alloy thin‐film disk and of laser‐written bits in a TbFe film were taken with a magnetized tip tilted at 45° with respect to the surface normal. In both cases the asymmetric part of the image of a domain is interpreted in terms of gradients in the in‐plane component of the magnetic field. The bits written in the Co‐alloy disk were decorated with small magnetized particles, allowing identification of the domain boundaries and the asymmetric component of the force microscope image due to in‐plane magnetization.

When the scanning acoustic microscope is used in a defocussed mode to measure surface acoustic wave properties, circular surface wave fronts are generated on the specimen which propagate to a diffraction‐limited focus. Thus, high resolution surface wave images are simply formed and a wide range of established nondestructive testing techniques become available to the acoustic microscope. Images obtained in the reflection mode are presented which demonstrate the unexpectedly high spatial resolution.

We describe a new method for imaging magnetic fields with 1000 A resolution. The technique is based on using a force microscope to measure the magnetic force between a magnetized tip and the scanned surface. The method shows promise for the high‐resolution mapping of both static and dynamic magnetic fields.

Measurement of dopant density in silicon with lateral resolution on the 200 nm scale has been demonstrated with a near‐field capacitance technique. The technique is based upon the measurement of local capacitance between a 100 nm tip and a semiconducting surface. Lateral dopant imaging is achieved by the measurement of the voltage‐dependent capacitance between tip and sample due to the depletion of carriers in the semiconductor, as the tip is scanned laterally over the surface. Measurements of dopant density have been demonstrated over a dopant range of 1015–1020 cm−3. Capacitance‐voltage measurements have been made on a submicrometer scale.

We briefly review some of the history and technology of scanning microscopy. The common threads that connect these different scanning microscopies are higlighted. In the case of far-field scanning microscopy, the resolving power is governed by the Abbe criterion. In the case of near-field scanning microscopy, the Abbe limit can clearly be surpassed. The invention of the Scanning Tunneling Microscope has provided some of the confidence necessary to broaden and enhance the capabilities of near-field scanning microscopies. The paper concentrates on the history and technology of selected scanning near-field and far-field microscopies.

We describe a thermally thin temperature sensor for use with a restricted class of sample. Such a sensor would be useful in certain photothermal spectroscopic studies and, in particular, to absorption studies on a microscopic scale. The sample under investigation must be thermally thin and in good thermal contact with the sensor, under such conditions, any changes in the temperature of the sample results in a corresponding change in the sensor. We present results obtained for human blood using the photothermal microscope, demonstrating the feasibility of recording absorption images and spectra from microscopic objects.

A review is made of progress in high resolution near field thermal imaging. The Scanning Thermal Profiler, initially demonstrated as a surface profiler, has profiled aluminum films with sub 100 nanometer resolution. More recently, the thermal sensor has been used to map current induced heating of a surface with comparable resolution. Finally, photothermal imaging of laser heated surfaces has been demonstrated, with image structure well below 100 nanometers.