Advances in High-Speed Atomic Force Microscopy

High-speed atomic force microscopy (HS-AFM) is a scanning probe technique capable of recording processes at the nanometre scale in real time. By sequentially increasing the speed of individual microscope components, images of surfaces can be recorded at up to several images per second. We present a HS-AFM platform composed of custom¿built measurement head, controller and software, scanners and amplifiers that is shared with the community in an open¿hardware fashion. A new scanner design combined with an advanced control system is shown. The simple addition of a secondary actuator to widely available tube scanners increases the scan speed by over an order of magnitude while allowing for a 130 ¿m × 130 ¿m wide field of view, which is not possible with traditional high¿speed scanner designs. Controllers beyond standard proportional-integral controllers are capable of significantly increasing imaging speed by anticipating resonances. Such filters are cumbersome to design with conventional methods. It is shown how convex optimization can be used to design optimal controllers with guaranteed stability for atomic force microscopy in an automated fashion. By integrating two lasers into the small spot¿size optics of an AFM readout head we are able to use the first laser for detecting the deflection of the smallest, and thus fastest currently available high¿speed cantilevers, while using the second for photo¿thermal actuation. Using this instrument, we demonstrate multi¿frequency atomic force microscopy (MF-AFM) at previously not accessible frequencies of more than 20 MHz. By employing the driving laser not for resonant excitation as is usual in dynamic AFM, a new imaging mode, photothermal off-resonance tapping (PORT) is presented. By repeatedly thermally bending the cantilever below it¿s resonant frequency, the surface is probed at a rapid rate. The resulting force is extracted from the deflection of the cantilever in time¿ domain at real time and used for feedback and image generation. The dynamic and static force contributions in both PORT and state of the art high-speed amplitude modulation atomic force microscopy (AM-AFM) are measured and analyzed in detail. It is shown that by decoupling the driving frequency from the resonant frequency the dynamic tip¿sample impact forces can be drastically reduced when compared to resonance based AFM modes. SAS-6 is a centriolar scaffolding protein with a crucial role in the duplication of centrioles, which are the main microtubule organizing organelle of eukaryotic cells. Defects in centriole duplication are associated with cancer and microencephaly. To understand these defects, is therefore important to understand the kinetics of SAS-6. In¿vitro, SAS-6 polymerizes into rings of between eight and ten monomers. Using the new PORT mode we are able to study the dynamic assembly of SAS-6. It is shown how SAS-6 rings can not only assemble by canonical one-by-one addition, but can form as a fusion of larger, already assembled fragments. Finally, it is shown how PORT can be used to observe fast processes of and on living cells. The adhesion and detachment of thrombocyte cells is studied. Membrane disruptive effects are shown on gram¿negative as well as gram¿positive bacteria.

Design and characterisation of low-cost thick-film piezoresistive force sensors for the 100 mN to 100 N range

Giancarlo Corradini, Vincent Fahrny, Thomas Maeder, Peter Ryser, Sven Nico Stauss

Two simple low-cost thick-film force sensor designs are optimised, characterised and tested in this work, combining calculations of sensor element and joint stresses, joint strength characterisation and measurement of complete sensors. All sensors are based on standard hybrid substrates. Our results show that a single sensor geometry cannot cover such a wide range of forces in an optimal way. For small forces (100 mN to ca. 2 N), simple cantilever force sensors are an excellent solution, achieving reasonable precision with a very simple design and compatibility with an SMD (surface mount device) assembly process with solder or conductive glue. Characterisation of solder joint strength shows that such joints can reliably withstand the bending moments resulting form the loading of the cantilever sensor up to ca. 2 N. Above this force, both solder joint and cantilever strength become critical in the cantilever design. Therefore, a 3-point or 4-point bending beam geometry must be selected, thereby extending the force range to ca. 100 N.

2005

Data-Driven Controller Design for Atomic-Force Microscopy

Georg Fantner, Christoph Manuel Kammer, Alireza Karimi, Adrian Pascal Nievergelt

A novel method to design data-driven, fixed-structure controllers with H2 and H∞ performance objectives is presented. The control design problem is transformed into a convex optimization problem with linear matrix inequality constraints, which can be solved efficiently with standard solvers. The method is used to design a data-driven controller for an atomic-force microscope. The closed-loop performance of the calculated controller is validated on a real setup.

2017

Design of a high-bandwidth tripod scanner for high speed atomic force microscopy

Maja Dukic Pjanic, Georg Fantner, Nahid Hosseini, Chen Yang, Jie Zhao

Tip-scanning high-speed atomic force microscopes (HS-AFMs) have several advantages over their sample-scanning counterparts. Firstly, they can be used on samples of almost arbitrary size since the high imaging bandwidth of the system is immune to the added mass of the sample and its holder. Depending on their layouts, they also enable the use of several tip-scanning HS-AFMs in combination. However, the need for tracking the cantilever with the readout laser makes designing tip-scanning HS-AFMs difficult. This often results in a reduced resonance frequency of the HS-AFM scanner, or a complex and large set of precision flexures. Here, we present a compact, simple HS-AFM designed for integrating the self-sensing cantilever into the tip-scanning configuration, so that the difficulty of tracking small cantilever by laser beam is avoided. The position of cantilever is placed to the end of whole structure, hence making the optical viewing of the cantilever possible. As the core component of proposed system, a high bandwidth tripod scanner is designed, with a scan size of 5.8 µm × 5.8 µm and a vertical travel range of 5.9 µm. The hysteresis of the piezoactuators in X- and Y-axes are linearized using input shaping technique. To reduce in-plane crosstalk and vibration-related dynamics, we implement both filters and compensators on a field programmable analog array. Based on these, images with 512 × 256 pixels are successfully obtained at scan rates up to 1024 lines/s, corresponding to a 4 mm/stip velocity.

2016