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Multi-tip SPM Instrument Development

Ultra CompactMulti-Tip Scanning Probe Microscope/ Nanoprober for electrical characterization at the nanoscale

Multi-tip SPM Instrument Development

Since microelectronics evolves into nanoelectronics, it is essential to perform electronic transport measurements at the nanoscale. The standard approach to this is to use lithographic methods for contacting nanostructures. In a final nanoelectronic device, this method will be used. However, in research and development stages other methods to contact nanoelectronic devices may be more suitable. An alternative approach for the contacting of nanostructures is to use the tips of a multi-tip scanning probe microscope, in analogy to the test leads of a multimeter used at the macroscale. The advantages of this approach are: (a) in situ contacting of “as grown” nanostructures still under vacuum allows to keep delicate nanostructures free from contaminations which can be induced by lithography steps performed for contacting. (b) Flexible positioning of contact tips and different contact configurations are easy to realize, while lithographic contacts are permanent. (c) Probing with sharp tips can be non-invasive (high ohmic), while lithographic contacts are invasive (low ohmic).

Multi-tip scanning probe microscopy instruments can be used for:

  • Local potential measurements on the nanoscale
  • Controlled nondestructive measurements in spectroscopy mode
  • Four point measurements with free positionable local probes on structured samples

4 tip stm 2Photo of the ultra-compact TetraProbe multi-tip scanning probe microscope. Four independent STM units are integrated within a diameter of 50 mm, resulting in an unsurpassed mechanical stability, enabling atomic resolution imaging with each tip.

The figure below shows the internal structure of two of the four modular scanning units. Each unit comprises a KoalaDrive, a nanomotor particularly developed in order to make scanning probe microscopes ultimately small. The KoalaDrive, which is described in detail here, is used for the coarse tip-approach towards the sample. The tip is mounted under 45° relative to the vertical direction in order to allow the positioning of all tips at one region on the sample. The KoalaDrive is fixed to a horizontal plate (black in the figure) which is moved in the horizontal directions via slip-stick motion. The plate rests on three balls fixed to three tube piezo elements. Saw-tooth signals on these piezo elements allow an inertial slip-stick motion (coarse motion) of the plate in the horizontal xy-directions. The fine xyz-scanning of the tips is per- formed by these three piezo elements, as well. More details can be found at Rev. Sci. Instrum. 83, 033707 (2012).

koala_scannerSchematic side view of the internal structure of the TetraProbe instrument. For details, refer to the text.

Four of these scanning units are integrated inside a housing of 50 mm outer diameter, allowing for a completely in- dependent motion of all four tips. The whole instrument is built ultrahigh vacuum compatible. The tips and the sample can be changed without breaking the vacuum. With the sample holder placed on top of the housing, it is closed completely leading to a good electric shielding from outer disturbances. The sample holder can be moved in xy-directions over several mm by shear piezo elements on top of the housing. In order to bring the tips to a desired position for the subsequent electrical measurements, it is important that the motion of the four tips and the sample can be observed, either by an optical microscope, or by a scanning electron microscope as shown in the bottom of the figure above. A scanning electron microscope (SEM) image of two tips brought close to the sample under study is shown in the figure below. Imaging with the secondary electrons leads to a shadow effect (dark shadow image of the tip apex) giving access to the tip sample distance. With the KoalaDrive as key element, it was possible to develop an ultra-compact multi-tip scanning tunneling microscopy (STM) instrument with a drift of less than 0.2 nm/minute at room temperature and with atomic resolution for all four tips. Recently, a startup company named mProbes has been founded, which offers this instrument and others.



Performing electrical measurements with a four-tip microscope demands more than to have four tips and to be able to position them to desired places. Concerted measurements of currents and voltages with all four tips have to be performed on a real time basis. Our electronics allows operating each tip either as (biased) current probe, or as voltage probe as shown in the figure below. Before an electrical measurement, all four tips are positioned to the desired positions on the sample. Subsequently, the tips are approached towards the sample by a desired distance, and different I/V ramps are applied between different tips (and/or the sample), as shown for a specific example in Fig. 3 b). In the simplest case a current is injected between the two outer tips and a potential difference is measured between the inner tips (classical four-point measurement). However, also various kinds of other measurements can be performed.


Important features of the TetraProbe multi-tip scanning probe microscope are:

  • Modular and compact design based on the KoalaDrive® nanopositioner
  • Each tip can be positioned independently
  • Tip coarse positioning with optical microscope/SEM control
  • Simultaneous tunneling and scanning with all four tips
  • Atomic resolution with each tip
  • Tip exchange and sample exchange in situ
  • Software controlled switching between current probe and voltage probe for each tip
  • Software allows virtually any possible "concerted" spectroscopic measurements involving the four tips and the sample
  • Multi-Tip Scanning Force Microscopy with the Needle Sensor and low temperature version available

7x7atresAtomically resolved image of the Si(111)-7x7 surface

Real time optical microscope movie of the positioning of the sample and four tips on a structured wafer. The structured rectangles have a size of 30 μm x 50 μm.