WP1: Active probe as a high-speed nanometrology tool

The aim of this work package is to develop innovative active self-sensing and self-actuating high-speed AFM probes which are applicable for high resolution and high-speed (HS) AFM measurements. Active probes consist of an AFM tip and an active cantilever. Important active cantilever parameters will be traceably calibrated, e.g. the function of the load force or the tip displacement. Developing cantilever calibration methods for stiffness, their piezoresistive deflection detector and thermomechanical deflection actuator will involve electrical impedance spectroscopy (IS) and interferometric measurements of the active high-speed cantilevers developed in Task 1.1 and the associated control electronics developed in Task1.2. This has never been performed before. The results obtained will enable the fabrication of dedicated high-speed cantilevers as well as their measurement and control electronics.

Task 1.1 will design and fabricate cantilever structures for HS-SPM operation at increased resonance frequency (~1 MHz) with moderate stiffness (<20 N/m) and minimised mass. Diamond tip probes will be accurately milled by focused ion beam milling to achieve the required aspect ratio (tip height of ca. 10 μm and tip apex of 20 nm).

Task 1.2 will design and fabricate active and flexible HS cantilever control and front-end measurement electronics that will be capable of generating the frequency bandwidth up to the 10 MHz needed to excite the resonance vibration in cantilevers in their higher eigenmode regime. It will be suitable for use with the end-user metrological scanning platforms of the partners.

Task 1.3 will develop metrology methods and techniques for the calibration of active HS cantilevers for cantilever stiffness with an uncertainty of <10 %, and their piezoresistive deflection detector and thermomechanical deflection actuator (both with uncertainties of <10 %).

Task 1.4 will install 20 calibrated HS probes from Task 1.3 on the scanning stages of the SPMs that are available at PWR and NA to establish a correlation between scan speed, surface geometry, and material composition before the WP1 HS probes are transferred to other partner labs for further testing and use in test measurements in WP4.

WP2: Long stroke high-speed hybrid SPM scanner

The aim of this work package is to develop a long-stroke (≥ 10 mm x 10 mm), high-speed (up to 10 mm/s or faster) and high accuracy (~1 nm) hybrid SPM scanner as a core component of a high-performance SPM. As existing conventional stages do not provide a sufficient range of travel in combination with high motion dynamics and nanometre positioning accuracy to satisfy the required scanner characteristics, an innovative design concept has been proposed. The SPM scanner will include a hybrid combination of three stages: a 3-axes monocrystalline piezo stage, a 6-axes polycrystalline piezo stage (Task 2.2) and a 6-axes large-stroke magnetic levitation (MagLev) stage (Task 2.3). To confirm the performance of these stages a high-precision, high-speed 6-DOF interferometric metrology system will be developed (Task 2.4).

The development of the MagLev stage has been identified as the main challenge in this work package due to the requirements for high-quality sensor systems (e.g. optical incremental sensors and fibre-optic interferometers) and complex control algorithms (e.g. model-based multiple input multiple output (MIMO) state space control) to achieve the necessary positioning performance and tracking accuracy. The project’s MagLev stage will have multiple (at least 6 to 8) internal high-resolution positioning sensors, providing the advantages of compact size and low-cost. However, as the internal positioning sensors may have a limited accuracy for stage positioning, the WP will also develop a high precision 6-DOF interferometric metrology system to further enhance the positioning accuracy (1 nm) of the scanner. This 6-DOF interferometric metrology system will also be used to characterise the positioning performance of the developed stages and has the additional potential for use as the direct positioning control of the MagLev stage and/or piezo stages (Tasks 2.5 and 2.6).

Task 2.1 will define the architecture, modules and components for the complete HS-SPM hybrid scanner, and it will then create an initial CAD design. Task 2.2 will define the specification requirements, architecture, and components for the hybrid piezo stage based on a 6-axes polycrystalline piezo stage and a 3-axes monocrystalline piezo stage. After review and approval by the consortium, the hybrid piezo stage will be manufactured and tested within Task 2.2. In the same way, Task 2.3 will develop, manufacture and test the long-stroke, high-speed and highly accurate MagLev stage.

Task 2.4 will develop a high-precision, high-speed 6-DOF interferometric metrology system based on three multi-beam interferometers located along the x-, y- and z-axes. Then, Task 2.5 will plan and perform the characterisation of the HS-SPM scanner incorporating the hybrid piezo stage and the MagLev stage developed in Tasks 2.2 and 2.3 respectively using the 6-DOF interferometric metrology system developed in Task 2.4. Finally, Task 2.6 will enhance the HS-SPM scanner’s performance by directly coupling the measurement results of the 6-DOF interferometric metrology system to the controller of the HS-SPM scanner.

WP3: Data processing and software

The aim of this work package is to develop the methodology and Gwyddion software tools needed for handling high-speed data sets. These will be used in the hybrid HS-SPM prototype and also in the CMI HS-AFM system that will be upgraded in this project and to make the software generally available as open source software. Existing HS-AFM systems typically use custom built data processing toolchains based on fast image retrieval (e.g. 10 frames/s–1000 frames/s) and stitching. The methods are rarely transferrable to other instruments and in most cases were not designed for metrological purposes. As an alternative a more general method based on metrological XYZ data set generation was recently proposed. However, this is not often used by HS-AFM owners as it places far higher demands on sensors and data storage. Even if some of the current data processing methods are already available as open source software, such as Gwyddion, developed by CMI, the support for HS-AFM data processing is very simple and does not yet include many state-of-the art approaches. The complexity of data processing and the missing methodology is one of the barriers for wider use of HS-AFM systems.

In WP3, different advanced concepts for data sampling in HS-AFM systems will be addressed:
• conventional stitching-based scanning,
• general xyz data set acquisition and
• a compressed sensing approach.
The necessary algorithms will be developed and wherever applicable will be made open source via Gwyddion data processing.
To further support the development of HS-AFM/hybrid HS-SPM metrology systems, an analysis of the uncertainties related to the sampling and data fusion will be undertaken (target uncertainty component related to data fusion should be lower than 30 % of the total uncertainty when a single frame is taken). There are other uncertainty sources that need to be addressed, like filtering unwanted components, e.g. from higher eigenmodes in optical pickup. Methodology for estimating such uncertainty components will be developed in WP3 and will be usable for the uncertainty analysis of individual HS-AFM systems within WP4.

Task 3.1 will develop a method including uncertainty estimation for linking traceable slow scan images with faster non-traceable ones for addition to Gwyddion open source software.

Task 3.2 will develop open-source experiment control and data processing software tools based on the combination of adaptive scanning and general XYZ handling approaches.

Task 3.3 will adapt data sampling methods, based on compressive sensing, for applications in high-speed imaging.

Task 3.4 will evaluate the performance of the different data sampling and data processing approaches developed in this work package and will extend their functionality to multi-functional imaging (dimensional + electrical, and dimensional + mechanical parameters).

WP4: Prototype Met-HS-SPM and industrial applications

The aim of this work package is to develop at least one prototype of the highest accuracy long-stroke, high-speed SPM by incorporating the SPM sensors, scanners and software modules developed in the WP1 -WP3, respectively. In addition, two other instruments, one at VTT and another at CMI, will be upgraded and used for the purposes of testing individual modules, the project performance comparison and the measurement of stakeholder samples (silicon samples and optical surfaces). In total, three instruments will be used for the WP4 measurements:

- One high-speed SPM prototype will be built at PTB using the newly designed hybrid PI stage, developed in WP2, the probing concepts derived from WP1 and the software derived from WP3. This instrument will be used for demonstrating the accuracy achieved in the project.
- One custom built high-speed AFM, based on commercially available components, will be constructed and used at VTT to test the probes and electronics developed in WP1 under the environmental operational conditions of a typical laboratory high-speed system.
- An existing high-speed AFM instrument at CMI, based on the passive detection scheme upgraded by the introduction of the interferometric system developed in A2.4.5, will be used for the development of scan path and data fusion techniques in WP3 and tested prior to implementation on the prototype HS-SPM in WP4.

The performance of the highest accuracy SPM instrument will then be systematically and thoroughly characterised for noise and measurement accuracy using physical standards that are used in metrology (gratings, step height standards). Finally, the instrument will be used to confirm the localised functional properties of nanostructures contained on at least 2 different industrial samples (e.g. an ultra-smooth optical surface, a 3D calibration artefact from CZ SMT). This testing will provide the evidence of the development of a fully characterised long-stroke, high-speed SPM system. This system will be capable of real-time traceable quantitative multi-sensing measurements in a production process. This is needed by industry to characterise the localised functional properties of nanostructures.

Task 4.1 will build prototype HS-SPM(s) incorporating the high-speed probes with piezoresistive cantilevers (WP1), stages (WP2) and open source experiment control and data processing software (WP3).

Task 4.2 will fully characterise the developed prototype SPM(s) concerning, e.g. short-term stability, long-term stability, noise level, maximum measurement speed, and tip wear.

Task 4.3 will test and verify the measurement capability of existing/upgraded HS-AFMs (CMI) and the prototype HS-SPM for the multifunctional properties (e.g. dimensional, mechanical or electrical) of nanostructures.

Task 4.4 will devise a technical protocol and use it to verify the metrology performance via comparisons of (i) the prototype HS-SPM against the “classical” (i.e. short-range, slow) metrological SPMs available at NMI partners, and (ii) the metrological HS-SPMs of partner NMIs (i.e. the MAFM of VTT and the HS-SPM of CMI) against the SPMs of industrial partners and/or stakeholders.

Task 4.5 will perform measurements on representative industrial samples (e.g. a 3D calibration artefact, an ultra-smooth optical surface, and a silicon sample) to demonstrate the dimension, optical, electrical and thermal measurement capabilities of the developed HS-SPM prototype.