Byong Chon Park

Measurement of microscope calibration standards in nanometrology using a metrological atomic force microscope

Microscope calibration standards in nanometrology were calibrated using a metrological atomic force microscope (metrological AFM) and the validity of calibrated values was shown. The metrological AFM was developed through the modification of a commercial AFM, which replaced the PZT tube scanner with flexure hinge scanners and displacement sensors. These modifications improved the traceability of measured values to metrological primary standards. The grating pitch and step height specimens, which are typical standard artefacts for the calibration of lateral and vertical magnifications of microscopes, were measured using the metrological AFM. The expanded uncertainties (k = 2) of calibrated values were estimated considering the characteristics of the calibration process and were less than 1 nm. The measurement results were compared with those obtained by other metrological methods or the certified values and their consistency was verified by checking the En numbers. These experimental results show that the metrological AFM can be used effectively for the measurements of microscope calibration standards in nanometrology.

An algorithm for stylus instruments to measure aspheric surfaces

A reliable algorithm is developed for the analysis of machined aspheric surfaces with a stylus instrument. This research has been done prior to the evaluation of uncertainties in the aspheric surface analysis. The algorithm considers two factors: the pickup configuration (pivoted arm) and the stylus radius. It also compensates for the sample tilt and the axis offset (the setup error) in the best-fit least-squares process. The algorithm consists of two parts for instrument calibration and aspheric surface analysis, and has been coded by means of C++ and MATLAB. Further it was also applied to the instrument calibration and the aspheric surface measurement, and the results were compared with the instrument-produced ones. The developed algorithm shows better performance over the commercial instrument in both the instrument calibration and the analysis of aspheric surfaces. Besides the uncertainty analysis, the developed algorithm will be a basis for the applications that the commercial instrument cannot provide with its own built-in code.

A simple phase-encoding electronics for reducing the nonlinearity error of a heterodyne interferometer

A phase-encoding electronics capable of compensating for the nonlinearity error in a heterodyne laser interferometer is described. The system consists of the phase demodulating electronics and the nonlinearity compensating electronics. For phase demodulation, we use the phase-quadrature mixing technique. For nonlinearity compensation, the offsets, the amplitudes and the phase of two output signals from the demodulator are adjusted electrically so that their Lissajous figure is a circle. As a result, the correct phase can be obtained. An analysis of the nonlinearity in the heterodyne interferometer and the design of the phase-encoding electronics are presented. The experiment was performed in a Michelson-type interferometer using a transverse Zeeman stabilized He?Ne laser. We demonstrate that this method can encode the phase of a heterodyne interferometer with sub-nanometer accuracy.

The role of an amorphous carbon layer on a multi-wall carbon nanotube attached atomic force microscope tip in making good electrical contact to a gold electrode

Multi-wall carbon nanotube (MWNT) attached atomic force microscope (AFM) tips (MWNT tips) have good potential for use in AFM lithography. Good conducting MWNT tips are needed in such applications. However, characterizing the conductance of MWNT tips is nontrivial: making a good electrical contact between the MWNT and electrode is difficult. We observed that MWNT tips produced by hydrocarbon-deposition attachment usually do not make good electrical contacts to gold electrodes because of the thin and rough amorphous carbon layer on the MWNT that was unintentionally deposited during the attachment. We found that good contacts can be made if a more amorphous carbon layer is deposited to form a thick and smooth amorphous carbon layer on MWNTs. Good contact was made either by transformation of the amorphous carbon layer into a conducting or peel-off layer, exposing the bare MWNT surface. MWNT tips with an exposed MWNT surface showed the well-known high-current-flowing capacity and the stepped-cutting behavior of bare MWNTs. The peeling-off behavior of a thick amorphous carbon layer may be utilized in producing bare-surfaced MWNT tips that have good conductance and therefore are useful for applications.

Atomic force microscopy of steep side-walled feature with carbon nanotube tip

The characteristics of the carbon nanotube AFM tip was investigated as it is used to measure the critical dimensions in the high aspect ratio structures. The research has been done to demonstrate the limitations of the CNT probe in imaging steep or vertical sidewall. Two kinds of samples, silicon dot and the lines in the ArF resist pattern were profiled by using carbon nanotube tip in the tapping mode AFM. There is a large oscillation at the steep sidewall, which cannot be controlled by merely changing scan variables, except by slowing down the scan up to the impractical level. The interaction between the long, slim CNT probe and the vertical sidewall severely limits the usefulness of AFM as a CD metrology tool. To achieve hi-resolution and high aspect ratio imaging simultaneously, a stiffer and/or modifed probe under clever non-contact 2D feedback is needed.

Measurement of 100-nm polystyrene sphere by transmission electron microscope

The mean diameter of the 100-nm polystyrene sphere was measured with the transmission electron microscope (TEM) with a view to the development of the accurate and visual sizing technique for nm scale particles. To minimize the sizing error due to the inaccurate instrument-stated magnification and the edge location uncertainty, the 300-nm spheres were mixed with 100-nm spheres to provide an internal calibrator, whose diameter is accurately known. The diameters of 100-nm spheres were determined by comparing with that of 300-nm spheres in the same negatives (TEM pictures). For the correction of the electron beam-induced shrinkage effect, the dependence of the shrinkage on the accelerating voltage, beam intensity, and the exposure time was examined and analyzed. Based on such an investigation, and with the several data manipulation routines, the mean diameter was determined with the expanded uncertainty of approximately 2% at the 95% confidence level. The measured value is consistent with those obtained by other laboratories using different techniques for the same spheres.

Evaluation of carbon nanotube probes in critical dimension atomic force microscopes

Abstract. The decreasing size of semiconductor features and the increasing structural complexity of advanced devices have placed continuously greater demands on manufacturing metrology, arising both from the measurement challenges of smaller feature sizes and the growing requirement to characterize structures in more than just a single critical dimension. For scanning electron microscopy, this has resulted in increasing sophistication of imaging models. For critical dimension atomic force microscopes (CD-AFMs), this has resulted in the need for smaller and more complex tips. Carbon nanotube (CNT) tips have thus been the focus of much interest and effort by a number of researchers. However, there have been significant issues surrounding both the manufacture and use of CNT tips. Specifically, the growth or attachment of CNTs to AFM cantilevers has been a challenge to the fabrication of CNT tips, and the flexibility and resultant bending artifacts have presented challenges to using CNT tips. The Korea Research Institute for Standards and Science (KRISS) has invested considerable effort in the controlled fabrication of CNT tips and is collaborating with the National Institute of Standards and Technology on the application of CNT tips for CD-AFM. Progress by KRISS on the precise control of CNT orientation, length, and end modification, using manipulation and focused ion beam processes, has allowed us to implement ball-capped CNT tips and bent CNT tips for CD-AFM. Using two different generations of CD-AFM instruments, we have evaluated these tip types by imaging a line/space grating and a programmed line edge roughness specimen. We concluded that these CNTs are capable of scanning the profiles of these structures, including re-entrant sidewalls, but there remain important challenges to address. These challenges include tighter control of tip geometry and careful optimization of scan parameters and algorithms for using CNT tips.

Ion Beam Bending of Nano Scale Materials in Free Space

We report an ion beam bending process to control the shape of nanometer scale materials. 5–30 keV gallium ion beam was found to plastically bend diverse materials having nanometer scale geometries of free-standing tube, spike and plank, along the ion beam so that their free end is directed toward the ion source, in vacuum space free of any applied electric or magnetic field. With the process, we could create a bent tip for scanning force microscope and hooks for nano-manipulation, out of straight spikes carved with nano-scale dimensions.

Calibration of two-dimensional nanometer gratings using optical diffractometer and metrological atomic force microscope

The pitch and orthogonality of two-dimensional (2D) gratings have been calibrated by using an optical diffractometer (OD) and a metrological atomic force microscope (MAFM). Gratings are commonly used as a magnification standard for a scanning probe microscope (SPM) and a scanning electron microscope (SEM). Thus, to establish the meter-traceability in nano-metrology using SPM/SEM, it is important to certify the pitch and orthogonality of 2D gratings accurately. ODs and MAFMs are generally used as effective metrological instruments for the calibration of gratings in nanometer range. Since two methods have different metrological characteristics, they give complementary information for each other. ODs can measure only mean pitch value of grating with very low uncertainty, but MAFMs can obtain individual pitch value and local profile as well as mean pitch value, although they have higher uncertainty. Two kinds of 2D gratings, each with the nominal pitch of 700 nm and 1000 nm, were measured, and the uncertainties of calibrated values were evaluated. We also investigated the contribution of each uncertainty source to the combined standard uncertainty, and discussed the causes of main ones. The expanded uncertainties (k = 2) of calibrated pitch values were less than 0.05 nm and 0.5 nm for the OD and the MAFM, and the calibration results were coincident with each other within the expanded uncertainty of the MAFM.

Multiwalled carbon nanotube field emitter as an electron source for a microcolumn

A multiwalled carbon nanotube (MWCNT) field emitter is developed as the electron source for a microcolumn-based field-emission scanning electron microscope (SEM). A MWCNT is first attached onto a tungsten (W) support tip using the nanomanipulator in the SEM. Then, an electrical bias is applied between the MWCNT field emitter and W tip to improve the contact between them, which lowers the threshold voltage for field emission. An emission current stability test showed that the emission current is stable. The fabricated MWCNT emitter exhibits a high emission current of 12 μA and sample current of around 2 nA, even at a low tip bias of 350 V in the microcolumn. The tip bias is much lower, and the sample current is higher than the equivalent values reported for W tips. The authors acquired images of a 1000-mesh copper grid using the microcolumn-based SEM with a MWCNT field emitter as the source. Our results suggest that MWCNTs should be considered a promising candidate as an electron source for microcolumns.