We use a fiber-optical interferometer to measure the cantilever deflection. This deflection sensor type only requires placing the end of an optical fiber in close proximity to the cantilever. All electronic components remain outside the cryostat and the UHV system. Moreover, a fiber-optical interferometer sensor directly maps the cantilever deflection, whereas beam-deflection sensors only measure the angular change of the cantilever [42]. A fiber-optical interferometer, thus, permits a precise measurement of the cantilever oscillation amplitude, without the need of a complicated calibration [43-45]. Fiber-optical sensors can obtain sensitivities up to about 1 fm/ using Fabry–Pérot interferometry [46,47]. To date, however, we only implemented a simpler form of the interferometer composed of a cleaved and uncoated fiber end with a reflectivity of typically 4%. This limits the sensitivity of the interferometer to about 89 fm/, (see section “Results and Discussion” for the characterization of the interferometric deflection sensor).

Figure 3a shows a typical setup for a UHV STM or tuning fork-based AFM. Preferably, the low-mass tip is scanned, while the heavier sample and sample receivers are mounted on a xy-positioning unit for the lateral positioning of the sample on a millimeter scale. To avoid stacking the z-positioning unit on top of the xy-positioning unit, the xyz-scan piezo and tip receiver unit are mounted inside a z-positioning unit, permitting the approach of the tip to the sample. Typically, shear piezo stacks are activated with a triangular voltage-versus-time signal to obtain a stick–slip motion of the slider of the positioning unit. In most instruments, the shear piezo stacks are mounted on the instrument body. For the z-positioning unit, typically, two sets of two piezo stacks are rigidly mounted (glued) to the instrument body, while a spring system is used to press the remaining two piezo stacks from the instrument body to the slider, permitting an adjustable clamping force [13]. The latter needs to be sufficiently large to obtain a good mechanical rigidity of the slider while still permitting a stick–slip motion of the slider.

Figure 3: Schematics of the components of (a) a typical, classical STM/tuning fork-based AFM setup with the tip being scanned and (b) our AFM with the sample being scanned.

In a cantilever-based AFM, the deflection sensor (here a cleaved fiber end) must be positioned relative to the cantilever. Scanning the cantilever tip would be impractical in this case, because it would require scanning the entire fiber positioning unit as well as the cantilever. Instead, the cantilever remains fixed to the instrument body, the fiber end is positioned on top of the cantilever, and the sample is scanned relative to the cantilever. This setup, however, requires stacking of the z-positioner on top of the xy-positioning unit or vice versa, making the design of a mechanically rigid instrument more challenging. In addition, the mass of the sample holder and sample holder receiver must be kept to a minimum in order to keep the resonance frequency of the xyz-scan piezo reasonably high, as required for a fast feedback. Furthermore, to avoid instrument downtime due to piezo tube fractures, sample exchange inside the UHV must be performed with minimal force applied to the scan piezo. The schematic setup of our instrument is displayed in Figure 3b. Our cantilever-based AFM instrument is made of two three-axis positioning modules that position (A) the sample relative to the cantilever tip, that is, the sample positioning unit, which is equipped with a sample scan-piezo), and (B) the fiber versus the cantilever back-surface, that is, the fiber positioning unit, which contains a piezo (w-piezo) for fine-tuning the fiber-to-cantilever distance and for keeping the interferometer at one of its most sensitive operating points.

Sample positioning unit

For the sample positioning unit, Pan style positioners [48] are used. Triangular voltage pulse trains are applied to all shear piezo stacks simultaneously. In order to minimize the instrument volume and to maximize its mechanical rigidity, the scan piezo is integrated into the xy-positioning unit, which is contained inside the z-positioning unit, which moves inside the instrument body. Different to conventional z-positioning units as, for example, used in the work of Schwenk et al. [13] and Hug et al. [49], here the shear piezo stacks are attached to the sliding unit. This is one of the many design steps we have undertaken to improve the stability of the tip–sample gap. Because the shear piezos move together with the z-positioner containing the scan piezo with the sample, the mechanical loop from the tip to the sample becomes small in the approached state, whereas in the classical design (Figure 3a), the shear piezos are attached to the body of the instrument leading to the largest mechanical loop in the approached state.

A further advantage of this design is that the instrument body can be manufactured as a single piece, in the form of a cylindrically shaped molybdenum tube (Figure 4a). As a result, only the sapphire plates, but not the piezo stacks, need to be glued on the inside walls of the body.

Figure 4: CAD drawings of the cylindrical body tube (a) and the leaf spring (b) carrying two of the total of six shear piezo stacks. The top and the cross-sectional views of the z-slider unit are shown in panels (c) and (d), respectively. The z-positioning unit also contains the xy-positioning unit and the xyz-scan tube carrying the sample holder receiver with the sample holder.

Our design with the piezos attached to the moving part, however, requires a spring system that applies a force from the inside towards the sapphire plates mounted on the inside of the instrument’s body tube (Figure 4a,b). Figure 4c,d shows the top and side views of the z-positioning unit containing the xy-positioning unit and the xyz-scan piezo carrying the sample receiver. While four of the six shear piezo stacks are glued to the z-slider unit, the two remaining stacks are glued to a leaf-spring assembly depicted in Figure 4a,b. The central screw (red arrow) pushes the leaf spring against two support cylinders, leading to an outward motion of the piezo stacks, pressing them against the sapphire rail (wide red arrows). With the screw in its released position, the sample z-positioning unit (and also that of the fiber, whichis not shown in Figure 4) can be placed inside the cylindrical body tube (blue arrow in Figure 4a) and the shear piezo stacks can be pressed towards the sapphire rails by tightening the adjustment screw, which is accessible through a hole in the cylindrical instrument body. Note that all piezo-motor adjustment screws are initially tightened under ambient conditions, that is, in air and at room temperature, to a level that the piezo motor slider still reliably moves at a piezo motor voltage pulse amplitude of ±70 V. The screws are then fixed with a small droplet of Torr Seal glue. The slider then still moves after a UHV system bake-out and at low temperatures at a piezo motor voltage pulse amplitude significantly smaller than ±270 V such that a reliable operation of the piezo motors is obtained.

To avoid any cross-coupling of the xy-motion as observed in earlier designs [49], two separated units with confined motions in the x- and y-directions are used here (Figure 4d). Such a stacking of two linear positioning units on top of the z-positioning units in a small building space, however, imposed various design challenges: First, a high mechanical rigidity must be obtained for a good tip–sample gap stability; second, the mechanical loop must be minimized and the design has to be kept as symmetrical as possible to reduce thermal drift; third, the design must allow for a precise adjustment of the pressure of the sliders towards the sapphire rails for the xy-directions.

All these conditions can be fulfilled with a concentric design, where the shear stacks of the x-positioning unit are attached close to the top of the z-sliding unit (Figure 5a). The x- and y-sliders both use three shear piezo stacks and confine the motion along these directions by sliding an Al2O3 sphere attached to the shear stack inside a gap formed by two sapphire cylinders. The shear stacks for the x-direction are glued to the inside close to the top surface of the z-slider (Figure 5a). The x-slider is then arranged below these stacks and contains the three shear stacks of the y-direction, which then move the y-slider. The xyz-piezo is then attached to the top of the latter reaching through a hole in the x-slider to the top of the z-slider, such that the sample holder receiver is sufficiently high that the sample holder can be introduced into it. Both sliders are then pressed against their piezo stacks using a single three-armed leaf spring at the bottom with a sapphire sphere running on a hardened steel plate. Note that initially a sapphire plate was used. However, we found the plate cracked after a few days of piezo motor operation, presumably caused from an ultrasound-actuated contact resonance of the Al2O3 sphere on the sapphire plate arising from the triangular voltage signal applied during piezo motor operation. We found that replacing the sapphire plate by a mechanically more compliant hardened steel plate solved this issue. The sphere is contained in a cage mounted to a fine-thread, and a screw is used to adjust the force acting on the shear stacks of both the x- and y-sliders, facilitating the setting of a force sufficiently large to have a rigid assembly, but small enough to move the sliders at low temperatures, where the range of the shear stacks is significantly reduced.

Figure 5: (a) CAD drawing of the z-positioning unit containing the xy-positioning units with scan piezo and, mounted to it, the sample holder receiver. (b) Schematic drawing of the assembly depicted in (a) highlighting the concentrical design and (c) the corresponding stability triangle. (d) Schematic drawing of a more conventional design, where the scan piezo is mounted on the top of the xy-positioning unit and (e) the corresponding stability triangle.

With this concentric design, dimensional changes in the height of the shear stacks and sliders with temperature are at least partially compensated by those of the scan piezo. Together with the highly symmetric design along the x and y axes, this further reduces the thermal drift. Moreover, any mechanical excitation of the instrument, for example, from floor or acoustic vibrations may cause a wiggling motion of the slider of the size δ away from the supporting shear piezo stack (Figure 5b), which will translate into a later motion of δ/2 (Figure 5c). This is much smaller than the mechanically amplified motion of occuring in the classical stacked xy-motor design depicted in Figure 5d,e.

Fiber positioning unit

The same type of z- and xy-positioning units are also used to approach the fiber to the rear side of the cantilever and to position it along and perpendicular to the cantilever axis. Note that the xy-positioners for the fiber are tilted by the same 12° angle (Figure 6) as the cantilever to permit the y-positioning of the fiber parallel to the long axis of the cantilever. Similar to xy-positioners of the sample, the x- and y-positioners of the fiber can be independently adjusted without any cross-coupling. This permits a reliable positioning of the fiber either above the central axis of the cantilever or towards the cantilever edges to pick up torsional cantilever deflections (see section “Performance of the SPM”).

Figure 6: CAD sketch of the fiber z-positioning unit containing the x- and y-positioning unit. The assembly can be placed inside the cylindrical instrument body. After tightening the adjustment screw, the spring-loaded z-shear piezo stack and, consequently, the z-shear piezo stack attached to z-positioning unit will be pressed towards the sapphire rails on the inside of the cylindrical instrument body. The geometrical cantilever-to-fiber configuration is also highlighted. The cantilever and the fiber are tilted by 12° relative to the sample.

In order to maximize the sensitivity of the interferometric cantilever deflection measurement, a fiber-to-cantilever distance between two adjacent interference extrema must be selected and kept constant. This fine-positioning is performed by the w-piezo tube, which is implemented in the form of a stack of individual piezo plates (Figure 6).

Sample and cantilever holders

UHV AFM instrumentation typically permits the in situ exchange of samples and (cantilever) tips. For this, sample and cantilever are mounted on corresponding holders (Figure 7a–c and Figure 7d–f, respectively). For efficient UHV AFM experimental work, it is favorable to have a conveniently large number of different sample and cantilever holders. Such holders with electrical contacts, however, are complex, and their fabrication and assembly typically require considerable efforts. For this reason, all our sample/cantilever holders use the same four laser-cut metal parts as base plates (m1–m4) connected via a simple ceramic center piece (Figure 7f), on top of which different assemblies can be arranged, for example, to carry a sample button heater (Figure 7a–c) or a shaker piezo for the mechanical excitation of the cantilever oscillation (Figure 7d–f).

Figure 7: (a, b) Top and side view CAD sketches of a sample holder with a button heater for sample preparation. (c) A hat-shaped Au(111) single crystal mounted in a sample holder containing a button heater. (d, e) Top and side view CAD sketches of a cantilever holder with a shaker piezo integrated into the holder below the cantilever. (f) A cantilever holder with a mounted (glued) cantilever. The wire on the top right to the m1 contact plate is for the measurement of the tunneling current. The wire on the top left contacts the cantilever shaker piezo, while the wire on the bottom left provides the ground and shields the cantilever excitation voltage from the cantilever. (g) Typical sample/cantilever receiver design used in earlier instruments [45] where the sample/cantilever holder are clamped down by springs. (h) New sample/cantilever receiver design used here, permitting a force-free introduction/removal of the sample/cantilever from the corresponding receiver. (i, j) Manipulator with a rotatory hex-key end piece that can be moved along its long axis to clamp a sample/cantilever holder for a safe transport between the chamber transport system and the sample/cantilever holder receiver in the AFM (k).

Sample and cantilever receivers

These sample/cantilever holders can be transported through the UHV system using the linear manipulator. In most instruments, the receivers for the sample or cantilever holders use clamping springs to fix the holders in their positions (Figure 7g). However, the introduction of the sample/cantilever holder into the corresponding receiver requires overcoming frictional forces, which may lead to a deformation of the holding springs and, consequently, to a loose fixation of the sample/cantilever holder in its receiver. Moreover, the sliding motion will also create wear particles, which may contaminate the surface of the sample or the inside of the instrument. Generally, such receiver designs compromise between a sufficiently large clamping force and the frictional forces that need to be overcome to exchange the sample/cantilever.

Here, we designed a new type of sample/cantilever receivers containing an adjustable clamping spring to overcome these inherent problems (Figure 7h). When the sample/cantilever holder is introduced or removed from the receiver, the clamping spring is in a lower position, not touching the sample/cantilever holder, such that the latter can be introduced or moved without applying forces to the receiver. The fixation of the sample/cantilever holder is then performed by rotating the fixation screw, which pushes the clamping spring against the sample/cantilever holder (Figure 7h). The required rotary motion can be applied via a customized magnetic-feedthrough manipulator, which includes a rotatable hex-key end piece (Figure 7i,j). This end piece can further be moved along its axis, permitting the clamping of a sample/cantilever holder and, thus, allowing its safe and rapid transport between the linear manipulator head and the corresponding receivers in the AFM (Figure 7k).

Note that we have tested different designs for the screw-activated clamping mechanism. We found the mechanism to be reliable (permits operation for more than a year with lots of sample/cantilever holder exchanges) with a conical screw coated by tungsten disulfide running in a thread of the receiver (fixation screw and thread piece in Figure 7h). The screw or the part with the thread can easily be replaced in the case of extensive wear. The conical end of the screw then presses on a sapphire inlay glued to the bottom part of the clamping spring.

The fixation of the sample/cantilever holder inside the corresponding receiver also leads to an electrical contact between pads on the sample/cantilever holder and contact pins on the receiver. We typically use three (out of the four) contact pins on the holder top, but can also use two contact pins on the clamping springs and, hence, have a total of five electrical contacts. Because four top contacts overdefine the plane of the sample/cantilever holder, the holder typically has a smaller thickness in one of the front contact areas, such that only one of the front electrical pins makes contact with the holder. A modified design of our holder with more (spring-loaded) electrical contacts from the top has been recently described by Schwenk and co-workers [13].

The sample holder receiver, which is fixed to the top of the scan piezo tube is manufactured from DISPAL Aluminum 225 [50] to reduce its mass. For the cantilever receiver, which is not scanned but directly mounted to the tubular Mo body of the instrument, Mo is used.

Modular wiring design

In order to facilitate instrument service, modification, or repair, every module of the microscope has a separate wiring branch and can thus be easily removed from the microscope without having to remove wires or connectors from the module.

For the sensitive signal inputs and outputs, such as STM current and sample bias voltage, coaxial cables Lakeshore CC-SS-100 [51] with a SMA connector at their ends are used. These are wired to the two front electrical contact pins (Figure 7d–f). For all other contacts and also the wiring for the scan piezo, piezo motors, piezo for the mechanical actuation of the cantilever oscillation, temperature sensor (below the sample holder), and heaters, silver-coated Cu wires (DABURN 2451 [52]) are used. For electrical screening, wires carrying opposite voltages (X+ and X−, Y+ and Y− for the scanner as well as W+ and W− for the w-piezo) are twisted. Furthermore, groups of twisted pairs are contained in a CuBe braid with a custom-built multi-pin plug at the end, which is then plugged into the corresponding socket on the bottom plate of the LHe tank of the cryostat (Figure 2b).

From the multi-pin socket at the cryostat bottom, the wire bundles for specific instrument modules are reordered into functional groups, for example, one group containing all wires for the piezo positioners, for sample scan and w-piezo, for electrical contacts to the sample and cantilever, and for instrument heaters and temperature sensors.

Interferometer system

The layout of the fiber-optical interferometer system is depicted in Figure 8a. A similar interferometer system has been developed by Miyahara and co-workers [53]. To perform the interferometry, we use a Sony SLD201 V3 laser diode with a wavelength of 785 nm coupled via an optical insulator to a Au-coated monomode optical fiber having a core diameter of 5 μm [54] delivering a maximum of 9.3 mW into the fiber at a drive current of 140 mA. To keep the temperature of the laser diode constant, it is mounted onto a Thorlabs TCLDM9 [55] thermoelectric cooler block, and the laser diode is operated at constant current. A combined laser diode and temperature controller (Thorlabs ITC502 [55]) controls both the current and the temperature. In contrast to earlier designs, which relied on a 50:50 fiber-optical 2 × 2 coupler, the increased power of the laser diode permits [43,44] the use of a 98:2 fiber-optical 2 × 2 coupler with the laser diode connected to one of the two 2% branches. Thus, for the 9.3 mW maximum input power, only 1.4%, that is, 127 μW reaches the fiber end in the AFM, because of additional losses in the optical connectors. This minimizes the light coupled to UHV/cryostat system (blue shaded area in Figure 8a) containing the AFM and, thus, a potential heating effect. It also maximizes the intensity of the light reflected back from the fiber-end/cantilever assembly to the measurement photodiode, which leads to about 50 μW on the measurement photodiode, which is part of a 10 MHz bandwidth current-to-voltage converter.

Figure 8: (a) Setup of the interferometer system. (b, c) Amplitude and phase as a function of the frequency for mechanical and optical cantilever excitation, respectively. (d) Wide frequency range mechanical excitation spectrum of the cantilever showing the first and second flexural and first torsional resonances. (e) Interferometer signal as a function of the fiber position across the cantilever (displayed schematically by the gray area). (f) Measured oscillation amplitudes of the cantilever for the first flexural (red) and first torsional oscillation modes (blue), respectively. The torsional oscillation modes vanish if the fiber is positioned above the central axis of the cantilever.

The interferometer system can be equipped with an additional laser diode (LP633-SF50 [55]) with a wavelength of 635 nm coupled into the fiber with a 2-color-combiner (NR73A1 [55]), allowing for an optical excitation of the cantilever oscillation. We found that a mechanical excitation of the higher cantilever oscillation modes can become challenging when other resonances arising from the mechanical setup of the cantilever holder with its excitation piezo are located close to the cantilever resonance. Figure 8b,c shows the measured amplitude and phase of the second flexural cantilever resonance excited mechanically (by the shaker piezo on the cantilever holder) or optically (using a DC and AC current for the 635 nm laser diode to oscillate its light intensity), respectively. Note that the additional color filter placed in front of the photodiode prevents the backreflected 635 nm light to reach the photodiode, such that only the interference of the 785 nm laser light is used to map the cantilever deflection. For the specific cantilever, the dependence of amplitude and phase on the excitation frequency expected for a harmonic oscillator becomes disturbed significantly by a nearby mechanical resonance of the cantilever holder for a mechanical excitation of the cantilever (Figure 8b). Because the cantilever resonance frequency changes when the cantilever interacts with the surface, that is, in AFM operation mode, the 180° phase shift from the cantilever resonance can overlap with the phase shift arising from the mechanical resonance, leading to a failure of the phase-locked loop to track the cantilever’s resonance frequency [53,56]. In such a case, optical excitation is preferred. In contrast to the mechanically excited cantilever (Figure 8b), an optical excitation (Figure 8c) leads to clean harmonic oscillator-like phase and amplitude versus frequency curves.

Note that the 10 MHz bandwidth of the photodiode current-to-voltage converter permits the measurement of higher flexural and torsional modes occurring at frequencies well beyond 1 MHz (Figure 8d). To measure torsional cantilever oscillation modes, the fiber needs to be positioned outside the long cantilever axis, close to the boundary of the cantilever [57]. Figure 8e shows the measured interferometer signal as a function of the fiber position across the cantilever. For a cantilever width w of 30 μm, we can estimate the laser spot size to be about 10–15 μm on the cantilever. Figure 8f shows the measured size of the first flexural (red curve, left vertical axis) and torsional (blue curve and right vertical axis) cantilever oscillation mode with frequencies of 2.959 kHz and 2.206 MHz as a function of the position of the fiber across the cantilever. While the flexural mode oscillation signal (red curve in Figure 8f) remains roughly constant (with a slight dip in the middle of the cantilever similar to that observed in the interference signal from Figure 8e), the torsional mode signal vanishes at the center of the cantilever (blue curve in Figure 8f). The absence of the signal at the center of the cantilever can also serve as a signature to clearly identify a torsional oscillation mode.