Writing subwavelength-sized structures into aluminium films by thermo-chemical aperture-less near-field optical microscopy

An optical near-field at the tip of an atomic force microscope probe is utilised to pattern aluminium thin films on glass substrates by photo-thermally induced corrosion in water. Aluminium forms a thin passivating oxide layer when immersed into neutral water at room temperature. Owing to the high energy density of the near-field, the metal below the probe tip can be heated to 100°C due to absorption of the light, which then provokes breakdown of the passivation and metal corrosion. The localised near-field is generated by tip-induced enhancement of an evanescent field originating from a laser beam, that is totally internally reflected at the glass–aluminium–water interface. The process is governed by surface plasmons excited in the aluminium film by the evanescent waves and the field enhancement of the probe tip. Holes of 40 nm diameter and lines below 100 nm width have been written into a 20-nm-thick aluminium film. Applications of the scanning probe lithography process may include the one-step fabrication of point contacts or contact masks for near-field optical lithography and reactive ion etching.     

A versatile multipurpose scanning probe microscope

A combined scanning probe microscope has been developed that allows simultaneous operation as a non‐contact/tapping mode atomic force microscope, a scattering near‐field optical microscope, and a scanning tunnelling microscope on conductive samples. The instrument is based on a commercial optical microscope. It operates with etched tungsten tips and exploits a tuning fork detection system for tip/sample distance control. The system has been tested on a p‐doped silicon substrate with aluminium depositions, being able to discriminate the two materials by the electrical and optical images with a lateral resolution of 130 nm.     

An evanescent-field optical microscope

The classic diffraction limit of resolution in optical microscopy (～γ/2) can be overcome by detecting the diffracted field of a submicrometre-size probe in its near field. The present stage of this so-called scanning near-field optical microscopy (SNOM) is reviewed. An evanescent-field optical microscope (EFOM) is presented in which the near-field regime is provided by the exponentially decaying evanescent field caused by total internal reflection at a refractive-index transition. A sample placed in this field causes a spatial variation of the evanescent field which is characteristic for the dielectric and topographic properties of the sample. The evanescent field is frustrated by a dielectric probe and thus converted into a radiative field. In our case the probe consists either of an etched optical fibre or of a highly sharpened diamond tip. The probe is scanned over the sample surface with nanometre precision using a piezo-electric positioner. The distance between probe and sample is controlled by a feedback on the detected optical signal. The resolution of the microscope is determined by both the gradient of the evanescent field and the sharpness of the tip. Details of the experimental set-up are discussed. The coupling of the evanescent field to the submicrometre probe as a function of probe-sample distance, angle of incidence and polarization has been characterized quantitatively. The observed coupling is generally in agreement with presented theoretical calculations. Microscopy has been performed on a regular latex sphere structure, which clearly demonstrates the capacity of the evanescent-field optical microscope for nanometre-scale optical imaging. Resolution is typically 100 nm laterally and 10 nm vertically. The technique is promising for biological applications, especially if combined with optical spectroscopy.     

Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips

The fabrication of silicon cantilever‐based scanning near‐field optical microscope probes with fully aluminium‐coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic‐ and contact‐mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone‐angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near‐field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near‐field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.     

Photonic nanopatterns of gold nanostructures indicate the excitation of surface plasmon modes of a wavelength of 50–100 nm by scanning near-field optical microscopy

Scanning near‐field optical microscopy images of metal nanostructures taken with the tetrahedral tip (T‐tip) show a distribution of dark and bright spots at distances in the order of 25–50 nm. The images are interpreted as photonic nanopatterns defined as calculated scanning near‐field optical microscopy images using a dipole serving as a light‐emitting scanning near‐field optical microscopy probe. Changing from a positive to a negative value of the dielectric function of a sample leads to the partition of one spot into several spots in the photonic nanopatterns, indicating the excitation of surface plasmons of a wavelength in the order of 50–100 nm in metal nanostructures.     

Photocatalytic deposition of a gold nanoparticle onto the top of a SiN cantilever tip

We propose a new technique for depositing a gold nanoparticle onto the tip of a dielectric support. We employed the photocatalytic effect of titanium dioxide for the deposition. When the titanium dioxide immersed in a solution including gold ions is subject to optical exposure, the excited electrons in the conduction band reduce gold ions into gold metal. Illumination by an evanescent wave generated with a total reflection configuration limits the deposition region to the very tip. In experiments we obtained 100–300 nm gold particles on SiN cantilever tips for atomic force microscopes. The contrast of evanescent interference fringes measured by a near-field scanning optical microscope with this gold nanoparticle probe has proved to be higher than that with a non-deposited SiN probe by a factor of 1.5.     

Application of photon scanning tunneling microscope to measure optical near field for atom manipulation

Photon scanning tunneling microscope has been employed to measure the three-dimensional evanescent optical field of an atom funnel. A 3.8 neV repulsive optical potential has been estimated by a 300 microm long probe with a tip radius of curvature of 21 nm. We have estimated limiting conditions for cold Rb atoms to reflect from the atom funnel. A two-dimensional doughnut-shaped optical near field has also been investigated. An aperture fiber probe is used to profile a focussed TEM(01) beam at the minimum beam waist and measure a dark center of about 10 microm while it is focussed by a converging lens of focal length 8 cm.     

Transmission line probe based on a bow-tie antenna

A high transmission probe for scanning near-field optical microscopy is discussed that is based on a bow-tie antenna. The proposed design of the transmission line probe relies on batch-fabricated hollow pyramidal silicon dioxide tips that are partly coated with aluminium to accomplish the tapered dipole antenna. Theoretical calculations of the field distribution were performed to investigate its optical properties. Results were compared with those of conventional aperture tips based on the same silicon dioxide tip configuration, and revealed unique properties with respect to the transmission efficiency.     

High-resolution constant-height imaging with apertured silicon cantilever probes

We present high-resolution aperture probes based on non-contact silicon atomic force microscopy (AFM) cantilevers for simultaneous AFM and near-infrared scanning near-field optical microscopy (SNOM). For use in near-field optical microscopy, conventional AFM cantilevers are modified by covering their tip side with an opaque aluminium layer. To fabricate an aperture, this metal layer is opened at the end of the polyhedral probe using focused ion beams (FIB). Here we show that apertures of less than 50 nm can be obtained using this technique, which actually yield a resolution of about 50 nm, corresponding to λ/20 at the wavelength used. To exclude artefacts induced by distance control, we work in constant-height mode. Our attention is particularly focused on the distance dependence of resolution and to the influence of slight cantilever bending on the optical images when scanning at such low scan heights, where first small attractive forces exerted on the cantilever become detectable.     

Moulded photoplastic probes for near-field optical applications

The inexpensive fabrication of high-quality probes for near-field optical applications is still unsolved although several methods for integrated fabrication have been proposed in the past. A further drawback is the intensity loss of the transmitted light in the 'cut-off' region near the aperture in tapered optical fibres typically used as near-field probes. As a remedy for these limitations we suggest here a new wafer-scale semibatch microfabrication process for transparent photoplastic probes. The process starts with the fabrication of a pyramidal mould in silicon by using the anisotropic etchant potassium hydroxide. This results in an inverted pyramid limited by < 111 > silicon crystal planes having an angle of ∼ 54°. The surface including the mould is covered by a ∼ 1.5 nm thick organic monolayer of dodecyltrichlorosilane (DTS) and a 100-nm thick evaporated aluminium film. Two layers of photoplastic material are then spin-coated (thereby conformal filling the mould) and structured by lithography to form a cup for the optical fibre microassembly. The photoplastic probes are finally lifted off mechanically from the mould with the aluminium coating. Focused ion beam milling has been used to subsequently form apertures with diameters in the order of 80 nm. The advantage of our method is that the light to the aperture area can be directly coupled into the probe by using existing fibre-based NSOM set-ups, without the need for far-field alignment, which is typically necessary for cantilevered probes. We have evidence that the aluminium layer is considerably smoother compared to the 'grainy' layers typically evaporated on free-standing probes. The optical throughput efficiency was measured to be about 10⁻⁴. This new NSOM probe was directly bonded to a tuning fork sensor for the shear force control and the topography of a polymer sample was successfully obtained.     

Shear force imaging of DNA in a near-field scanning optical microscope (NSOM)

A near-field scanning optical module has been constructed as an accessory for a Nanoscope IIIa commercial scanning probe microscope. Distance feedback and topographic registration are accomplished with an uncoated optical fibre scanning tip by implementation of the shear force technique. The tip is driven by a piezoelectric actuator at a resonance frequency of 8–80 kHz. A laser diode beam is scattered by the tip and detected by a split photodiode, with lock-in detection of the difference signal. The amplitude ( r ) and phase (τ) responses were characterized as a function of the calibrated tip–sample separation. Using an r cos τ feedback signal, imaging of pUC18 relaxed circular plasmid DNA spread on mica precoated with cetylpyridinium chloride was achieved. The apparent width (28 ± 5 nm) was approximately four times that achieved by scanning force measurements with the same instrument; the apparent height of the DNA (0.6 ± 0.3 nm) was similar with the two techniques. These results demonstrate the applicability of the shear force signal for imaging biological macromolecules according to topography and in conjunction with the optical signals of a near-field scanning optical microscope (NSOM).     

Upconversion fluorescence imaging of erbium-doped fluoride glass particles by apertureless SNOM

We have imaged fluorescent erbium‐doped fluoride glass particles by apertureless scanning near‐field optical microscopy. The optical excitation has been performed at λ = 780 nm whereas fluorescence emission has been collected around λ = 550 nm. This process, called upconversion by energy transfer, involves two erbium ions and is not linear. Besides an improvement of the lateral resolution, we have observed on some particles that the fluorescence is not homogeneously distributed, but is rather localized in some zones brighter than others. By making tip approach curves, we have also observed that the amount of fluorescence intensity scattered by the tip is increasing when the tip is approaching the sample surface.     

Spatial distribution and polarization dependence of the optical near-field in a silicon microfabricated probe

This paper reports on the spatial distribution and polarization behaviour of the optical near-field at the aperture of a Si micromachined probe. A sub-100 nm aperture at the apex of a SiO₂ tip on a Si cantilever was successfully fabricated by selective etching of the SiO₂ tip in a buffered-HF solution using a thin Cr film as a mask. The aperture, 10–100 nm in size, can be reproducibly fabricated by optimizing the etching time. The optical throughput of several apertures was measured. For a 100 nm aperture, a throughput of 1% was approved. The probe shows a very high optical throughput owing to the geometrical structure of the tip. The spatial distribution of the near-field light is measured and simulated using a finite difference-time domain method. The polarization behaviour of apertures with different shapes was analysed using a photon counting camera system.     

Propagation of surface plasmon polariton in nanometre-sized metal-clad optical waveguides

Using a local anodic‐oxidation method with a probe tip of a scanning near‐field optical microscope (SNOM) as the electrode, we have fabricated an oxide core with subwavelength dimensions on metal. The propagation of the surface plasmon polariton (SPP), which is excited at the interface between the oxide core and the metal clad, has been investigated using the same SNOM. Altering the wavelength of input light from 532 nm to 830 nm, the propagation length of the SPP extends from 2 µm to 6 µm. We carried out a simulation of the SPP propagation, and obtained a similar wavelength dependence.     

Performance of visible and mid-infrared scattering-type near-field optical microscopes

We describe the principles of two scattering‐type near‐field optical microscopes (s‐SNOMs), one operating at 633 nm wavelength, the other at selectable wavelengths in the range 7.3–11.3 µm, and compare the measurement experience. Both use interferometric detection of scattered radiation, and are therefore capable of amplitude and phase‐contrast imaging. In this study both instruments use the same or even identical commercial probe tips, and measure a single, three‐component, test sample. Our results show that the imaging process of s‐SNOM is wavelength‐independent, namely, that the resolution is determined by the properties of the tip only, and that the contrast is given by the complex refractive index of the sample, predictable from a simple, analytical model of tip–sample interaction. A novel, ‘edge‐darkening’ artefact is described which may appear in s‐SNOM and that is wavelength‐independent.     

Silicon technology-based micro-systems for atomic force microscopy/photon scanning tunnelling microscopy

We developed silicon nitride cantilevers integrating a probe tip and a wave guide that is prolonged on the silicon holder with one or two guides. A micro-system is bonded to a photodetector. The resulting hybrid system enables us to obtain simultaneously topographic and optical near-field images. Examples of images obtained on a longitudinal cross-section of an optical fibre are shown.     

Photoplastic near-field optical probe with sub-100 nm aperture made by replication from a nanomould

Polymers have the ability to conform to surface contours down to a few nanometres. We studied the filling of transparent epoxy‐type EPON SU‐8 into nanoscale apertures made in a thin metal film as a new method for polymer/metal near‐field optical structures. Mould replica processes combining silicon micromachining with the photo‐curable SU‐8 offer great potential for low‐cost nanostructure fabrication. In addition to offering a route for mass production, the transparent pyramidal probes are expected to improve light transmission thanks to a wider geometry near the aperture. By combining silicon MEMS, mould geometry tuning by oxidation, anti‐adhesion coating by self‐assembled monolayer and mechanical release steps, we propose an advanced method for near‐field optical probe fabrication. The major improvement is the possibility to fabricate nanoscale apertures directly on wafer scale during the microfabrication process and not on free‐standing tips. Optical measurements were performed with the fabricated probes. The full width half maximum after a Gaussian fit of the intensity profile indicates a lateral optical resolution of ≈ 60 nm.     

Dynamic etching method for fabricating a variety of tip shapes in the optical fibre probe of a scanning near-field optical microscope

A novel etching method for an optical fibre probe of a scanning near-field optical microscope (SNOM) was developed to fabricate a variety of tip shapes through dynamic movement during etching. By moving the fibre in two-phase fluids of HF solution and organic solvent, the taper length and angle can be varied according to the movement of the position of the meniscus on the optical fibre. This method produces both long (sharp angle) and short (wide angle) tapered tips compared to tips made with stationary etching processes. A bent-type probe for a SNOM/AFM was fabricated by applying this technique and its throughput efficiency was examined. A wide-angle probe with a 50° angle at the tip showed a throughput efficiency of 3.3 × 10⁻⁴ at a resolution of 100 nm.     

Apertureless near-field Raman spectroscopy

We report on the tip‐enhanced Raman spectra of C60 obtained on a custom‐built apertureless scanning near‐field optical microscope. A commercial atomic force microscope tip coated with 100 nm thickness of gold was used to enhance locally the Raman signal and permit topographic and spectral information to be acquired simultaneously. We present preliminary data which demonstrate the tip enhancement effect using C60 as a test sample.