We have developed a technique of fabricating nanoscale metallic wires by using STM-assisted local CVD. By using this technique, nickel wires as narrow as 18 nm and up to 6 micrometers long can be fabricated routinely. However, due to difficulties in making contact with the leads the electrical properties of these nanowires have rarely been successfully characterized. Here we report the successful fabrication and measurement of nickel nanowires with a four-probe configuration using a H-terminated Si (111) surface for the substrate. The samples are prepatterned with metal contact pads, which can be severely damaged by the tip during writing due to the high electric field at the tip-sample junction. After considerably experimentation we now use tungsten silicide contact pads with a silicon-rich stoichiometry; these pads work quite successfully. The wires we measured are typically tens of nanometers wide, 10 - 20 nm high, and a few microns long. We measured their resistance as a function of temperature from 4.2 to approximately 225 K. At higher temperatures the resistance drops linearly with decreasing temperature, indicating metallic behavior. At lower temperatures (<EQ 40 K) however the smaller wires' resistance increases, while that of the larger ones becomes constant. We believe the resistance rise of the smaller wires may indicate the onset of weak localization. We have also fabricated wires using e- beam lithography with a contamination resist followed by ion milling, and compare our data for the two techniques. Our results show a promising way of fabricating nanoscale electronic devices.
In this work, we have applied mold micromachining and standard photolithographic techniques to the fabrication of parts integrated with 0.4 micron pitch diffraction gratings. In principle, the approach should be scaleable to considerably finer pitches. We have achieved this by relying on the thickness of deposited or grown films, instead of photolithography, to determine the grating pitch. The gratings can be made to extend over large areas and the entire process is compatible with batch processing. Literally thousands of parts can be batch fabricated from a single lot of six inch wafers. In the first stage of the process we fabricate a planarized silicon dioxide pad over which the silicon nitride wave guide runs. The grating is formed by first patterning and etching single crystalline silicon to form a series of trenches with well defined pitch. The silicon bounding the trenches is then thinned by thermal oxidation followed by stripping of the silicon dioxide. The trenches are filled by a combination of polysilicon depositions and thermal oxidations. Chemical mechanical polishing is used to polish back these structures resulting in a series of alternating 2000 angstroms wide lines of silicon and silicon dioxide. The thickness of the lines is determined by the oxidation time and the polysilicon deposition thickness. The silicon lines are selectively recessed by anisotropic reactive ion etching, thus forming the mold for the grating. The mold is filled with low stress silicon nitride deposited by chemical vapor deposition. A wave guide is then patterned into the silicon nitride and the mold is locally removed by a combination of deep silicon trench etching and wet KOH etching. This results in a suspended diffraction grating/membrane over the KOH generated pit.
We have developed a method for encoding phase and amplitude in microscopic computer-generated holograms (microtags) for security applications. An 8 X 8-cell phase-only and an 8 X 8-cell phase-and-amplitude microtag design have been fabricated in photoresist using the extreme-ultraviolet (13.4 nm) lithography tool developed at Sandia National Laboratories. Each microtag measures 80 by 160 microns and contains features 0.2 micrometers wide. Fraunhofer-zone diffraction patterns can be obtained from fabricated microtags without any intervening optics and compare very favorably with predicted diffraction patterns. In this paper, we present the results of a preliminary rigorous coupled-wave analysis of microtags. Microtags are modeled as sub-wavelength gratings of a trapezoidal profile. Only TE polarization is modeled. The analysis in this paper is concerned with the determination of optimal microtag design parameter values and tolerances on those parameters. The parameters are wall-slope angle, grating duty cycle, grating depth, and metal-coating thickness. Our findings indicate that diffraction-efficiency monotonically increases as the gratings are: (1) deepened and (2) coated with metal. Coating with metal achieves a more significant improvement and is easier to implement. The tolerance on the wall slope angle is very loose. The optimal grating duty cycle is between 0.5 and 0.6, depending on the presence of the metallic coating. The application of a protective coating on metal-coated microtags leads to an increase in diffraction efficiency and represents a practical configuration for these elements.
The paper presents a new concept of a micromachined integrated sensor for combined atomic force/near field optical microscopy. The sensor consists of a microfabricated cantilever with an integrated waveguide and a transparent near field aperture tip. The advantage compare to the fiber based near field tips is the high reproducibility of the aperture and the control of the tip-sample distance by the AFM-channel. The key process consists in a novel micromachined aperture tip. The aperture tip is fabricated in a reliable batch process which has the potential for implementation in micromachining processes of scanning probe microscopy sensors and therefore leads to new types of multifunctional probes. For evaluation purposes, the tip was attached to an optical fiber by a microassembly setup and subsequently installed in a near-field scanning optical microscopy. First measurements of topographical and optical near-field patterns demonstrate the proper performance of the hybrid probe.
We have applied a new generation of short cantilevers with high resonant frequencies to tapping mode atomic force microscopy of a process in situ. Crystal growth in the presence of protein has been imaged stably at 79 lines/s (1.6 s/image), using a 26 micrometers long cantilever with a spring constant of 0.66 N/m at a tapping frequency of 90.9 kHz. This high scan speed nearly eliminated distortion in the step edge motion and allowed imaging of finer features along the step edges. Atomic force microscopy with short cantilevers therefore allows higher resolution imaging of crystal growth in space as well as time.
We have designed and built an atomic force microscope (AFM) with optical beam deflection detection providing a focused spot size of 1.6 micrometers in diameter. This small spot size was implemented with a variable focus adjustment that allows us to re-focus on each cantilever. This design opens up the usage of a new range of small cantilevers with low-noise characteristics. We have microfabricated novel aluminum cantilevers with dimensions as small as 9 micrometers in length and 2.5 micrometers in width and have characterized them with this new AFM. The resonance frequency of the smallest cantilever was 2.5 MHz in air and 0.94 MHz in water. We demonstrated the imaging capabilities of the AFM head by imaging abalone nacre with a 10 micrometers long cantilever using the tapping mode in liquid at a drive frequency of 442 KHz.
This paper addresses the problem of determining the absolute force constant of Atomic Force Microscope cantilevers. In the method presented, the cantilever under test is deflected against a reference cantilever of known spring constant. The relative deflection of the two cantilevers is related to their spring constants. The novelty of our approach is in the use of a micromachined reference cantilever of a precisely controlled force constant. Preliminary results show that our method is capable of measuring the force constant of cantilevers in the range of 0.1 to 10 N/m with an accuracy of better than 20%. The error is dominated by the non-linear effects in the force versus distance curves used for the measurement.
A process relying on the molding technique for the fabrication of diamond cantilevers with diamond tips integrated on silicon wafers for scanning probe microscopy applications is described. Either hot filament or microwave CVD diamond deposition and standard techniques of silicon micro-machining are employed. The deposition of well- developed tips depends critically on the pretreatment applied to enhance nucleation density; abrasive treatment with diamond powder as well as the bias-enhanced nucleation turned out to be successful. With optimized processes, well- shaped tips with a radius of curvature in the order of 30 nm can be obtained. They consist of high quality diamond according to micro-Raman spectroscopy. The definition of the cantilever area is another critical step which can be solved by proper process design. The fabrication of conductive tips/cantilevers is possible by boron doping. Finally, first performance tests of the diamond tips and cantilevers are presented.
The largest array (12 X 12) of microelectromechanical probe tips with integrated actuators and capacitive sensors for scanning probe microscopy has been designed, fabricated, and characterized. Each array element consists of a single crystal silicon tip on a torsional cantilever with out-of- plane interdigitated electrode capacitors. In addition, the size of each array element is about 150 micrometers by 150 micrometers with a tip-to-tip spacing in the array of 200 micrometers . Give these dimensions, the packing density of the devices is about 2500 units/cm2. The out-of-plane torsional design allows for significant improvement in performance (larger tip displacement and increased sense capacitance) and a higher density of devices per unit area as the minimum feature size decreases. Applications such as information storage, molecular manipulation, and nanolithography require high density, parallel arrays for reasonable throughput.
Tapping mode scanning capacitance microscopy is a new technique to study local capacitances. It is a combination of the tapping mode scanning force microscopy and the scanning capacitance microscopy. With the tapping motion of the tip, tip-sample distance is accurately regulated. In addition, topographic images are simultaneously obtained together with the capacitive images. In this paper, feasibility of the quantitative capacitance determination is described. Experimental results with a silicon nitride film are compared with the theoretical curve. The effective electrode area (lateral resolution) and the tip-sample capacitance were calculated.
The spatial and time resolved characterization of electronic devices by scanning probe microscopy demands the fabrication of proximal probes with well defined properties. To fulfill these requirements micromachining is the most appropriate technique, as it allows probe fabrication in a batch process with highest reproducibility. In this paper we describe the development of electrical and thermal near-field probes which can be employed for high frequency scanning force microscopy (HFSFM) and scanning thermal microscopy (SThM) respectively. Both probes have been completely fabricated in a micromachining batch process based on an almost identical technological design. For electrical imaging by HFSFM a coplanar wave guide probe was developed. The probes wave guide properties have been characterized by network analysis. A novel thermal probe consisting of a Schottky diode at the tip of a silicon cantilever was developed for SThM. Preliminary results on electrical and thermal characterization will be presented.
The probe forming capability of a microfabricated silicon electrostatic electron lens is under investigation. The lens measures 7 mm by 9 mm by 1.64 mm and consists of three silicon electrodes separated by Pyrex optical fibers. A test structure was designed to house the micromachined lens and a commercially available electron emitter as well as deflectors and an electron detector. Images of a 1000 mesh gold TEM wire grid at a working distance of 4 mm are being obtained at magnifications greater than 10,000 X. Data from the images will be analyzed to estimate the quality of the electron beam.
Multiplexing near-field scanning optical microscopy (NSOM) by the use of a nanoarray with parallel imaging is studied. The fabrication, characterization, and utilization of nanoarrays with approximately 100 nm diameter apertures spaced 500 nm center-to-center is presented. Extremely uniform nanoarrays with approximately 108 apertures were fabricated by electron beam lithography and reactive ion etching. The nanoarrays were characterized by atomic force microscopy and scanning electron microscopy. In this paper we utilize these nanoarrays in a laser-illuminated microscope with parallel detection on a charge-coupled device. Detection of B-phycoerythrin molecules using near- field illumination is presented. In principle, our system can be used to obtain high lateral resolution NSOM images over a wide-field of view (e.g. 50 - 100 micrometers ) within seconds.
The adaptation of a Digital Instruments DimensionTM 3000 atomic-force microscope to provide a near-field scanning optical microscopy capability is described. The enabling technology for the adaptation is the bent optical fiber probe. The design and operation of this probe to measure evanescent fields emerging from optical waveguides is described.
A combined SNOM/SFM aperture probe is presented which is based on a conventional scanning force microscopy cantilever. Probe fabrication was performed in a batch process which allows to get reproducible mechanical and optical properties. For SNOM applications a tip is integrated at the very end of the cantilever which consists of a hollow metal pyramid with a miniaturized aperture of about 60 nm. To select the appropriate tip material the transmissivity of different metals were investigated in the visible range. The SNOM/SFM probes were characterized both mechanically and optically, e.g. the transmission of apertures is measured as a function of their size. To determined the lateral resolution in the optical transmission mode measurements on test samples are shown. Additionally, a novel probe design is introduced where the geometry of the single aperture tip is altered to obtain a double aperture tip.
We describe the design, fabrication and characterization of a biaxial torsional scanning mirror for use in a microfabricated confocal optical microscope. The mirror, fabricated using wafer bonding and surface micromachining techniques, is a gimbal structure consisting of a silicon plate, measuring 500 micrometers X 500 micrometers X 25 micrometers , which is suspended in a silicon frame by silicon nitride hinges. It is actuated electrostatically, and is capable of raster scanning with a line scan frequency of 2.7 kHz and with a scan range of +/- 1.5 degree(s) about both orthogonal axes. Both resonant and non-resonant scanning are discussed. Sample images acquired with a frame rate of 10 Hz using a microfabricated microscope with the biaxial scanning mirror are presented.