Gaoliang Dai

Metrological large range scanning probe microscope

We describe a metrological large range scanning probe microscope (LR-SPM) with an Abbe error free design and direct interferometric position measurement capability, aimed at versatile traceable topographic measurements that require nanometer accuracy. A dual-stage positioning system was designed to achieve both a large measurement range and a high measurement speed. This dual-stage system consists of a commercially available stage, referred to as nanomeasuring machine (NMM), with a motion range of 25 mm×25 mm×5 mm along x, y, and z axes, and a compact z-axis piezoelectric positioning stage (compact z stage) with an extension range of 2 μm. The metrological LR-SPM described here senses the surface using a stationary fixed scanning force microscope (SFM) head working in contact mode. During operation, lateral scanning of the sample is performed solely by the NMM. Whereas the z motion, controlled by the SFM signal, is carried out by a combination of the NMM and the compact z stage. In this case the compact z...

Accurate and traceable calibration of two-dimensional gratings

Accurate and traceable calibration of lateral standards (1D and 2D gratings) is a basic metrological task for nano- and microtechnology. Both the mean pitch and the uniformity of the gratings should be measured quantitatively. Although optical diffractometers are effective for measuring the mean pitch, they are not able to measure the uniformity of gratings. In this study, the calibration of gratings is performed using a metrological large range scanning probe microscope with optimized measurement strategies. Two different kinds of data evaluation methods, a gravity centre method and a Fourier transform method, have been developed and investigated. Cosine error, a significant error source of the measurement, is analysed and corrected. Calibrations on several 1D gratings have been carried out. The calibrated mean pitch values have an excellent agreement with those measured by optical diffractometry. Nevertheless, irregularities of the gratings were only deduced from the SPM results. Finally, the usage of the 1D/2D gratings for the calibration of a typical SPM is illustrated.

Accurate and traceable measurement of nano- and microstructures

An approach to establishing rigorous nano- and microdimensional metrology using scanning probe microscopes (SPMs) and metrological profilometers is presented. An overview on calibrations of nanostructures—such as step height, one- and two-dimensional gratings, feature width, nanoroughness and geometry of a nanohardness indenter—and microstructures—such as microgroove, microroughness and geometry of a macrohardness indenter—is given in this paper.

A high precision micro/nano CMM using piezoresistive tactile probes

To satisfy various demands of micro- and nanoscale-dimensional metrology, a coordinate measuring system based on a nano positioning and measuring machine (NMM) has been built. The measuring system is able to perform measurements by using sensors such as scanning force microscopes, stylus profilometers, optical fixed focus sensors and assembled cantilever probes. In recent years, two kinds of tactile micro/nano CMM probes have been developed and coupled to the system. In such a way, the function of the device has been expanded from a metrological SFM to a micro/nano CMM. In this paper, the development of the micro/nano CMM is reported. The design ideas concerning the key components of the CMM, such as positioning stage, probe and software, are introduced. The characterization of the probe is described in more detail. Measurements on a typical test artefact have been demonstrated as an example.

Atomic force probe for sidewall scanning of nano- and microstructures

An atomic force microscope (AFM) probe applicable for sidewall scanning has been developed. In its configuration, a horizontal AFM cantilever is microassembled with a vertical AFM cantilever. An AFM tip located at the free end of the vertical cantilever and extending horizontally is capable of probing in a direction perpendicular to sidewalls. The bending, torsion, or deformation of the horizontal cantilever is detected when the tip is brought into contact, intermittent contact, or noncontact with sidewalls. Measurement results taken at the sidewalls of microtrenches, microgears, and line edge roughness samples are presented.

A metrological large range atomic force microscope with improved performance

A metrological large range atomic force microscope (Met. LR-AFM) has been set up and improved over the past years at Physikalisch-Technische Bundesanstalt (PTB). Being designed as a scanning sample type instrument, the sample is moved in three dimensions by a mechanical ball bearing stage in combination with a compact z-piezostage. Its topography is detected by a position-stationary AFM head. The sample displacement is measured by three embedded miniature homodyne interferometers in the x, y, and z directions. The AFM head is aligned in such a way that its cantilever tip is positioned on the sample surface at the intersection point of the three interferometer measurement beams for satisfying the Abbe measurement principle. In this paper, further improvements of the Met. LR-AFM are reported. A new AFM head using the beam deflection principle has been developed to reduce the influence of parasitic optical interference phenomena. Furthermore, an off-line Heydemann correction method has been applied to reduce the inherent interferometer nonlinearities to less than 0.3 nm (p-v). Versatile scanning functions, for example, radial scanning or local AFM measurement functions, have been implemented to optimize the measurement process. The measurement software is also improved and allows comfortable operations of the instrument via graphical user interface or script-based command sets. The improved Met. LR-AFM is capable of measuring, for instance, the step height, lateral pitch, line width, nanoroughness, and other geometrical parameters of nanostructures. Calibration results of a one-dimensional grating and a set of film thickness standards are demonstrated, showing the excellent metrological performance of the instrument.

Improving the performance of interferometers in metrological scanning probe microscopes

The traceability of metrological scanning probe microscopes (MSPMs) is achieved in most cases by laser interferometers. Different means have been adopted to account for the nonlinearity of those interferometers. The thorough investigation of an existing MSPM shows the necessity of interferometrical position measurement with real time full-bandwidth nonlinearity correction. The paper demonstrates that the ellipse parameters of Heydemann nonlinearity correction are sufficiently stable and position independent. This is used in reducing the signal processing time by calculating the ellipse parameters in advance and fixing them during real time nonlinearity correction. As a result, a real time signal processing system with the ability of executing Heydemann correction in 0.32??s and a complete demodulation in 2.2??s is designed and implemented. It reduces the residual nonlinearity of interferometers from about 3.5 to <0.3?nm. Some measurement results of a flatness standard illustrate the effectiveness of this new method.

Development of a 3D-AFM for true 3D measurements of nanostructures

The development of advanced lithography requires highly accurate 3D metrology methods for small line structures of both wafers and photomasks. Development of a new 3D atomic force microscopy (3D-AFM) with vertical and torsional oscillation modes is introduced in this paper. In its configuration, the AFM probe is oscillated using two piezo actuators driven at vertical and torsional resonance frequencies of the cantilever. In such a way, the AFM tip can probe the surface with a vertical and a lateral oscillation, offering high 3D probing sensitivity. In addition, a so-called vector approach probing (VAP) method has been applied. The sample is measured point-by-point using this method. At each probing point, the tip is approached towards the surface until the desired tip–sample interaction is detected and then immediately withdrawn from the surface. Compared to conventional AFMs, where the tip is kept continuously in interaction with the surface, the tip–sample interaction time using the VAP method is greatly reduced and consequently the tip wear is reduced. Preliminary experimental results show promising performance of the developed system. A measurement of a line structure of 800 nm height employing a super sharp AFM tip could be performed with a repeatability of its 3D profiles of better than 1 nm (p–v). A line structure of a Physikalisch-Technische Bundesanstalt photomask with a nominal width of 300 nm has been measured using a flared tip AFM probe. The repeatability of the middle CD values reaches 0.28 nm (1σ). A long-term stability investigation shows that the 3D-AFM has a high stability of better than 1 nm within 197 measurements taken over 30 h, which also confirms the very low tip wear.

Self-calibration of scanning probe microscope : mapping the errors of the instrument

In this paper, a self-calibration method has been employed for mapping the errors in the xy-plane and the squareness error between the z-axis and xy-plane of the SPMs. The self-calibration method has advantages such as a rather simple calibration setup, being capable of extracting both artefact-related and SPM-related errors, and a high calibration performance limited only by the stability of the instruments. Two commercial SPMs have been investigated using the proposed method. The obtained results agree well with those obtained from classical calibration methods by means of highly accurate transfer standards and metrological devices. The self-calibration method could be a kind of effective complementary method for mapping more errors of SPMs.

SEM linewidth measurements of anisotropically etched silicon structures smaller than 0.1 µm

Scanning electron microscopy (SEM) is a standard method for linewidth (CD) metrology. For structure sizes smaller than 0.1 µm the information volume of the scanning electron probe is of the same order of magnitude as the structure size and the resulting SEM signal profile is a superposition of structural information from the whole structure. Evaluation of top and bottom linewidths needs to take into account the electron diffusion in the solid state. SEM linewidth measurements at anisotropically etched silicon structures with an exact edge slope angle of 54.7° and a top linewidth smaller than 0.1 µm were performed by a low-voltage SEM metrology system. Different algorithms were applied for linewidth evaluation which were especially adapted for measurements at small structures. The results of SEM linewidth evaluation were compared among each other and to AFM measurements performed by a large-range scanning probe microscope.