Boon Ping Ng

Improve performance of scanning probe microscopy by balancing tuning fork prongs

This paper presents an approach for improving the Q-factor of tuning fork probe used in scanning probe microscopes. The improvement is achieved by balancing the fork prongs with extra mass attachment. An analytical model is proposed to characterize the Q-factor of a tuning fork probe with respect to the attachment of extra mass on the tuning fork prongs, and based on the model, the Q-factors of the unbalanced and balanced tuning fork probes are derived and compared. Experimental results showed that the model fits well the experimental data and the approach can improve the Q-factor by more than a factor of three. The effectiveness of the approach is further demonstrated by applying the balanced probe on an atomic force microscope to obtain improved topographic images.

Near-field ellipsometry for thin film characterization.

A near-field ellipsometry method is presented for nano-scale thin film characterization. The technique fuses the topographic and ellipso-metric optical measurements that are simultaneously obtained by a scanning near-field optical microscopy (SNOM). It is shown that the proposed near-field ellipsometry is able to attain nano-scale lateral resolution and correct artifacts in characterization. The effectiveness of the proposed method is verified by simulation and experimental studies.

Interactions of higher order tip effects in critical dimension-AFM linewidth metrology

A major challenge in critical dimension atomic force microscope width metrology is accounting for the effects of the tip on the apparent features in an image. The overall effect of the tip is to broaden the apparent width of lines and narrow the apparent width of trenches as a result of the geometrical interaction of the tip with the surface. To a first approximation, this effect can be well-modeled as a constant bias in width measurements, independent of the specific feature characteristics, by the width of the tip. Beyond this approximation, there are a number of smaller tip effects related to the measurement details. Some of these result from the details and interactions of the tip shape and feature shape, resulting in small variations of the tip bias from feature to feature and measurement to measurement. These effects are sometimes called shape effects, secondary effects, or higher order tip effects. One source of higher order effects is due to the lateral dithering of the tip, which increases the effective tip width. However, in addition to this known effect, the tip dither may also affect the apparent edge heights of the tip flare. Although the apparent tip width is expected to depend on lateral dither, the apparent edge height, at least in principle, would not be expected to depend on lateral dither. Our analysis suggests that this apparent dependence results from the interaction of the fine details of the tip flare shape and the dither envelope with the specific algorithm used to estimate the edge height. Consequently, both the sign and the magnitude of the dependence are specific to every tip. This is one example of the inter-dependencies that can be present when evaluating higher-order tip effects. Although these are often small, accurate metrology in some applications may require consideration of possible interactions between such smaller, secondary tip effects. The authors illustrate this with examples from photomask metrology, where measurement of chrome features on binary masks requires some attention to secondary tip effects.A major challenge in critical dimension atomic force microscope width metrology is accounting for the effects of the tip on the apparent features in an image. The overall effect of the tip is to broaden the apparent width of lines and narrow the apparent width of trenches as a result of the geometrical interaction of the tip with the surface. To a first approximation, this effect can be well-modeled as a constant bias in width measurements, independent of the specific feature characteristics, by the width of the tip. Beyond this approximation, there are a number of smaller tip effects related to the measurement details. Some of these result from the details and interactions of the tip shape and feature shape, resulting in small variations of the tip bias from feature to feature and measurement to measurement. These effects are sometimes called shape effects, secondary effects, or higher order tip effects. One source of higher order effects is due to the lateral dithering of the tip, which increases the ef...

Effects of lateral tip control in CD-AFM width metrology

Critical dimension atomic force microscopes (CD-AFMs) use flared tips and two-dimensional sensing and position control of the tip-sample interaction to enable scanning of features with near-vertical or reentrant sidewalls. Sidewall sensing usually involves lateral dither of the tip, which was the case in the first two generations of CD-AFM. Current, third-generation instruments also have a fast dither tube actuation (FDTA) mode where a control algorithm and fast response piezo actuator are used to position the tip in a manner that resembles touch-triggering of coordinate measuring machines (CMMs). All methods of tip position control, however, induce an effective tip width that may deviate from the actual geometrical tip width. When lateral dithering is involved, this effect is readily understood as the addition of a dither envelope to the geometrical tip width.The effective tip width is a key correction parameter for accurate feature width measurements and is typically estimated using a tip calibration procedure. However, the possibility exists of small errors in the estimated tip width due to variations and dependencies of the effective width on tip, tool, material, and environmental parameters. We are investigating this possibility through a systematic study of the dependence of the apparent width on measurement mode, dither amplitude, tip type, and sample composition. While we believe that there are potential effects that should be considered carefully, we also conclude, particularly for silicon features, that most potential biases can be removed by performing the calibration and measurement exercises under the same conditions.

Reflection-based near-field ellipsometry for thin film characterization

This paper presents a near-field ellipsometry method for nano-scale thin film characterization. The method is based on a reflection configuration of near-field optical detection. In the method, a new set of ellipsometry equations is established by taking into consideration the near-field sample-probe interaction and the topography of the thin film. Experimental and simulation results have shown that the proposed near-field ellipsometry is able to attain precise thin film characterization with nano-scale lateral resolution.

Artifact removal by intrinsic harmonics of tuning fork probe for scanning near-field optical microscopy.

This paper presents a new method to reduce the topographical artifacts in scanning near-field optical microscopy (SNOM) images. The method uses the harmonics caused intrinsically by the nonlinearity in the oscillation of the SNOM probe even when the probe is working in a normal condition without extra excitation. Using these intrinsic harmonics, the gradient of the received SNOM signal with respect to the probe motion is obtained. Then, taking advantage of a SNOM capable of simultaneously obtaining both the topographical and optical signals, topographical artifacts are calculated from the product of the gradient and the topographical signal, and then removed from the received SNOM signal. The effectiveness of the proposed method is demonstrated experimentally.

An improved dynamic model of tuning fork probe for scanning probe microscopy

This paper presents a two coupled oscillators model to describe the dynamics of a tuning fork with a probe attached. The two coupled oscillators are unbalanced only in their effective masses and the damping ratios. By applying a frequency domain system identification approach in experimental investigation of various probe attachment cases, a good accuracy of the model is demonstrated. The effectiveness of the model is further demonstrated in quantitative analysis of the noise performance and the sensitivity of force sensing with a tuning fork probe. Compared with existing models, the proposed model can more accurately characterize the dynamics of a tuning fork probe.

Contour Metrology using Critical Dimension Atomic Force Microscopy

Abstract. The critical dimension atomic force microscopy (CD-AFM) has been proposed as an instrument for contour measurement and verification since its capabilities are complementary to the widely used scanning electron microscopy (SEM). Although data from CD-AFM are three dimensional (3-D) in structure, the planar two-dimensional data required for contour metrology are not easily extracted from CD-AFM data. This is largely due to the limitations of the CD-AFM method for controlling the tip position and scanning, in which the relevant sidewall data are only obtained in one lateral axis. To use CD-AFM for contour metrology, the extracted profiles must include actual sidewall data from both lateral axes. Using two images acquired from orthogonal scan directions, profile extraction, and a method to combine those profiles, a technique for obtaining contours with the CD-AFM is developed. The main sources of error for this technique are described. The contours derived from CD-AFM were compared with those obtained using the SEM. Our results show that CD-AFM has the potential to make important contributions to semiconductor contour metrology.

Nano-Imaging Collagen by Atomic Force, Near-Field and Nonlinear Microscope

As the most abundant protein in the human body, collagen has a very important role in vast numbers of bio-medical applications. The unique second order nonlinear properties of fibrillar collagen make it a very important index in nonlinear optical imaging based disease diagnosis of the brain, skin, liver, colon, kidney, bone, heart and other organs in the human body. The second-order nonlinear susceptibility of collagen has been explored at the macroscopic level and was explained as a volume-averaged molecular hyperpolarizability. However, details about the origin of optical second harmonic signals from collagen fibrils at the molecular level are still not clear. Such information is necessary for accurate interpolation of bio-information from nonlinear optical imaging techniques. The later has shown great potential in collagen based disease diagnosis methodologies. In this paper, we report our work using an atomic force microscope (AFM), near field (SNOM) and nonlinear laser scanning microscope (NLSM) to study the structure of collagen fibrils and other pro-collagen structures.

Shear-force atomic force microscope by using the second resonance regime of tuning fork probe

An imaging scheme of shear-force atomic force microscope is proposed by exploiting the second resonance regime of the tuning fork probe. Theoretical analysis and experimental results demonstrate that the imaging scheme can deliver better sensitivity and higher resolution of topographic imaging.