Helmut Wolff

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.

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.

An atomic force microscope for the study of the effects of tip–sample interactions on dimensional metrology

An atomic force microscope (AFM) has been developed for studying interactions between the AFM tip and the sample. Such interactions need to be taken into account when making quantitative measurements. The microscope reported here has both the conventional beam deflection system and a fibre optical interferometer for measuring the movement of the cantilever. Both can be simultaneously used so as to not only servo control the tip movements, but also detect residual movement of the cantilever. Additionally, a high-resolution homodyne differential optical interferometer is used to measure the vertical displacement between the cantilever holder and the sample, thereby providing traceability for vertical height measurements. The instrument is compatible with an x-ray interferometer, thereby facilitating high resolution one-dimensional scans in the X-direction whose metrology is based on the silicon d220 lattice spacing (0.192 nm). This paper concentrates on the first stage of the instruments development and presents some preliminary results validating the instruments performance and showing its potential.

Design and three dimensional calibration of a measuring scanning tunneling microscope for metrological applications

A scanning‐tunneling microscope (STM) of the scanning‐sample type with transducers for the measurement of the position in all three axes has been developed. Motions in the X‐, Y‐, and Z‐axis are straight and rectangular to a high degree and the capacitance transducers are calibrated in situ by plane mirror laser‐interferometry. With these qualities as part of the design, the Abbe error may be minimized. X‐Y‐capacitance transducers and X‐Y‐piezo actuators are part of analog servo loops, thus providing positioning in the X‐Y‐plane to a desired coordinate. The STM is mainly built from commercially available parts.

New developments at Physikalisch Technische Bundesanstalt in three-dimensional atomic force microscopy with tapping and torsion atomic force microscopy mode and vector approach probing strategy

A new three-dimensional atomic force microscopy (3D-AFM) for true 3D measurements of nanostructures has been developed at Physi- kalisch Technische Bundesanstalt (PTB), the national metrology institute of Germany. In its configuration, two piezo actuators are applied to drive the AFM cantilever near its vertical and torsional resonant frequencies. In such a way, the AFM tip can probe the surface with a vertical and/or a lateral oscillation, offering high 3D probing sensitivity. For enhancing mea- surement flexibility as well as reducing tip wear, a vector approach probing (VAP) method is applied. The sample is measured point by point using this method. At each probing point, the tip is approached toward the surface in its normal direction until the desired tip-sample interaction is detected and is then immediately withdrawn from the surface. Preliminary experimental results show promising performance of the PTB system. The measure- ment of an IVPS 100 sample using a flared AFM tip showed a repeatability of its 3D profiles better than 1 nm (p-v). A single crystal critical dimension reference material having features with almost vertical sidewalls was also measured using a flared AFM tip. These results show that the feature has average left and right sidewall angles of 89.5 and 89.4, respectively. However, the nonuniformity of the feature width within the measurement window of 1 μm may be up to 10 nm. The standard deviation of the average middle CD values from 10 repeated measurements is 0.1 nm. In addition, an investigation of long-term measurement stability was performed on a PTB photomask. The results changed at a rate of about 0.00033 nm per line, which confirms the high measurement stability and the very low tip wear of the system.

Atomic force microscope cantilever based microcoordinate measuring probe for true three-dimensional measurements of microstructures

An atomic force microscope (AFM) cantilever based coordinate measuring probe applicable for true three dimensional measurements of microstructures is presented. The probe has a shaft glued to a commercial AFM cantilever and a probing sphere located at the free end of the shaft. A conventional optical lever method is applied to detect the bending, torsion, and the dynamic behavior of the AFM cantilever which will change dramatically when the probe sphere is in proximity to the object to be measured. The probe has advantages such as small probing force (<1μN), high probing sensitivity allowing nanometer measurement resolution, easy exchange of probing elements, and low cost. Measurements of a microgear and a fuel injection nozzle using the developed probe are demonstrated, indicating its promising application potential.

Atomic force microscope cantilever as an encoding sensor for real-time displacement measurement

A tuning fork-based atomic force microscope cantilever has been investigated for application as an encoding sensor for real-time displacement measurement. The algorithm used to encode the displacement is based on the direct count of the integer pitches of a known grating, and the calculation of the fractional parts of a pitch at the beginning and during displacement. A cross-correlation technique has been adopted and applied to the real-time signal filtering process for the determination of the pitch during scanning by using a half sinusoidal waveform template. For the first investigation, a 1D sinusoidal grating with the pitch of 300 nm is used. The repeatability of displacement measurements over a distance of 70 µm is better than 2.2 nm. As the first application, the real-time displacement of a scanning stage is measured by the new encoding principle as it is moved in an open-loop mode and closed-loop mode based on its built-in capacitance sensor.

Atomic force microscope cantilevers as encoders for real-time forward and backward displacement measurements

Atomic force microscope cantilevers have been investigated for their use as the encoder for real-time high-resolution displacement measurements, when paired with a 1D sinusoidal grating of well-known pitch. For a known one-directional (forward or backward) displacement measurement, the decoding algorithm is based on directly counting the integer periods of the grating and calculating the fractional parts at the beginning of the displacement and at the actual position by using one cantilever. Using two cantilevers arranged in the quadrature phase shift positions on the grating makes the measurement of two-directional (forward and backward) displacements possible. The decoding algorithm directly unwraps the phase between two encoded signals. Cross-correlation filtering and the differentiation process of two encoded signals are found to be very successful to guarantee the implementation of real-time displacement measurements by suppressing noise and reducing the offset and tilt of the encoded signals.

A micro-SPM head array with exchangeable cantilevers

In this paper a MEMS based micro-SPM head array is proposed to enhance the performance of the currently available nano-measuring machines and effectively reduce the measurement time for large specimen. It consists of 1 × N ( N = 7 in our case) micro-SPM heads/units, realized in one chip by MEMS technique. And it can be easily extended to a micro- SPM head matrix. The main part of the micro-SPM head is the MEMS-positioning stage, which is realized on the basis of an electrostatic lateral comb-drive actuator. In order to take the advantage of the high lateral resolution of conventional cantilevers, a flexible cantilever gripper was designed to be integrated into the MEMS-positioning stage within the SPM head. Conventional cantilevers can be mechanically mounted onto the MEMS-positioning stage or dismantled from the MEMS-positioning stage after the tip is worn out. In this way, the well-designed and calibrated MEMS-positioning stage can be repeatedly and efficiently utilized. The structure design and simulation of mechanical and electrical performances of the mico-SPM head will be detailed in this paper. First experimental results proved the feasibility of the cantilever gripper design.