LT NANOPROBE

Atomically precise electrical probing

SEM Imaging and Tip Navigation at T< 5K

For the navigation of four independent STM probes, simultaneous SEM imaging is indispensable as it bridges dimensions from the mm-scale down to the nm-scale. The SEM enables a large field of view for probe coarse positioning as well as fine positioning and rapid localisation of nanometer-sized structures.

The UHV Gemini column is the ultimate tool for that purpose. It offers unsurpassed resolution under true UHV conditions and at low temperatures. In combination with the LT NANOPROBE, the in-lens Secondary Electron Detector (SED) represents a key advantage: only one small access port is needed in the thermal shield compartment of the microscope stage (at T <5 K). Thus thermal impact is minimised, while still offering a suitable signal for high resolution imaging. Alternatively, other UHV SEM columns or optical microscopes can cover the lower resolution range if sample structures do not require ultimate resolution.

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STM for Probe Approach

STM is the key to advancing probing technology into the sub-nanometer scale. It ensures extremely accurate probe positioning and STM-based safe tip approach of fragile probe tips having diameters in the range of a few ten's of nanometers.

SEM ensures fast and efficient navigation of STM probes from the millimeter- to the nanometer-range. However, the STM tip shadows SEM imaging of structures smaller than the tip apex. Subsequently, STM is employed for atomically precise positioning of the tip and its point of contact.

Tip re-positioning accuracy is important as the probes are approached and positioned sequentially. During the approach process of an individual probe, the other probes need to be retracted to a safety distance of a few 100nm. When the positioning process is finished, all four probes are approached to contact. The outstanding precision of the LT NANOPROBE allows to land on the same atom again.

 

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Electrical Transport Measurements

During STM approach the distance control is based on tunnelling current feedback and therefore requires a dedicated low noise I/V converter. When the tunnelling contact is established, the individual probe-sample distance is well controlled in the nm range.

To establish electrical contact and to control its resistance, the STM feedback is de-activated and the probe is manually approached by setting a piezo scanner z-offset. The individual contact properties are analysed by an IV measurement between the tip and the grounded sample.

Transport measurements in various configurations such as four-point transport or three terminal measurements with one tip acting as (tunnelling) gate, require the I/V converter to be taken out of the signal line. Thus, a pA STM compatible and TTL trigger controlled switching technology is used to route signals of the four probes to external BNC connectors. Using LabVIEW, experimental workflows can be integrated with third party measurement electronics.

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SPM Performance

Each STM module is designed to achieve atomic resolution on metal surfaces with pm stability, enabled by the high intrinsic stability of the STM stage and efficient decoupling from external vibrations by a spring suspension and eddy current damping. During the tip navigation phase, the damping stage can be locked for optimal SEM resolution while still achieving good atomic resolution.

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Atom manipulation

The LT NANOPROBE stability virtually matches state-of-the art performance of well-established single tip low temperature STMs and therefore enables extended experiments such as atom and molecule manipulation and tunnelling spectroscopy.

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QPlus AFM

A suitable AFM technology is required for tip approach on structures on insulating substrates or electronically decoupling layers. The QPlus® AFM detection principle is purely electrical and thus compact enough to be integrated into each probe module, using 3 electrical tip contacts. The concept of the QPlus® AFM with a tip glued to the oscillation prong allows both to mount solid metal tips for transport measurements or to use alternative tip types.

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Low thermal drift

The thermal stability and resulting thermal drift of the whole microscope stage - four probe modules relative to the sample - is crucial for long-term measurements on nm-sized structures. It essentially defines the time window for sequential positioning of the four probes and the measurement time, during which probes can be held in electrical contact at atomic scale dimensions. The excellent drift performance in the few A /h regime also enables single or multi-tip tunnelling spectroscopy experiments.

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Cooling Down to T < 5K

The use of a high-resolution SEM column for tip navigation from above implies an unconventional cryostat concept. A specifically designed bath cryostat with LN2 and LHe reservoirs allows for a measurement time of > 45 hours at T < 5 K and cools the whole microscope stage from below.

LN2 and LHe double shields minimise the thermal impact on the stage. Doors to exchange tips or samples are operated by wobble stick. As the UHV Gemini column uses an in-lens secondary electron detector, the thermal impact during SEM imaging is minimised and the microscope stage main- tains the minimal temperature of T < 5K during SEM operation.

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