AFM cantilevers for electrical measurements

Manufacturers of AFM probes usually offer wide variety of coatings for electrical measurements. Their proper choice for particular scanning mode sometimes becomes the main factor to obtain high quality and informative scans.
In this article we would like to describe in more detail features of the most often used coating materials, their best applications and give some advices about the best-fitting types of cantilevers for various electrical AFM modes.
Properties of standard conductive coatings
Our production of conductive cantilevers works with over 30 different metal or metal alloy targets. Such multiformity helps to produce R&D probes models for very specific applications.
But only very few materials are used for serial production, as their set overlaps more than 90% of possible applications. Let's talk more about each one below.
Au coating has got excellent conductivity and it is the most chemically inert between other metals, used for AFM tips. Maybe such probes will be a very good choice for electrochemical experiments or for some biological objects.
But it is very soft. In our experience, Au-coated cantilevers often degrade fully during just one scan in Spreading Resistance mode. We know some positive experience of its usage for contact conductive AFM - but only for scanning very soft materials by cantilevers of low stiffness constant.
Pt coating is also high-conductive and chemically stable. It can spend years in standard atmosphere conditions and stay still workable without sensible degradation. This coating well suit for Spreading resistance investigations of low-conductive samples and for almost all types of intermittent contact electrical measurements.
Its only shortcoming, in our experience, is that it tends to degrade during contact regime scans under high current. Technically good Pt-coated cantilever can get many scans under 100 - 500 nA current. But for its safety it's better to set working current value of several nanoamperes. Only these conditions can guarantee long lifetime of Pt-coated probes and their high reliability.
W2C+ coating combines great wear-resistance and good conductivity, having standard tip's radius 35 - 45 nm the same time. This material is hard and it stands well against high temperatures, that can be achieved during scans under high current. According to our experiments, W2C-coated probes demonstrate almost zero degradation, getting Spreading resistance scans with current of around 100 mkA. Such properties are very important for investigations of surfaces with high gradient of conductivity in lateral plane.
From the other hand, W2C-coated cantilevers can degrade with time in moisture-loaded air. Two years ago we developed W2C+ modification of this coating with anti-corrosive elements included. Anyway, it's strongly recommended to hold W2C+-coated cantilevers in closed zip-packs with silica gel inside to minimize influence of air humidity.
Doped diamond coating (DCP) is the most strong among conductive ones. It can hold all standard force loads and we didn't notice its degradation under high current. Our customers usually take these cantilevers for Spreading resistance measurements in case probes with other conductive coatings doesn't maintain good tip-sample contact to get reliable data. But application field of these probes is limited for cases, when the one needs getting high quality topography scans. Usually curvature radius of DCP-coated tips is around 100 nm, and topography obtained by them often looks a bit blurry.
Cantilevers for conductive AFM (Spreading resistance mode)
Conductive AFM mode is the most sensitive to tip's quality. If an AFM has no malfunctions and all settings are chosen properly, the most often met reason of missing current is bad tip-sample contact. In turn, 90% of cases of bad contact arise due to damage or time-related degradation of conductive layer at tip's apex.
To avoid this problem, it's very important to choose proper cantilevers and to know their advantages and shortcomings.
For investigations of low conductive samples with uniform conductivity in lateral plane we would recommend HA_C/Pt or HA_FM/Pt probes. HA_C has got the softest bulk, specially designed for scanning in the contact regime. HA_FM levers are stiffer and they may provide higher loading force to achieve stronger tip-sample contact.
Working with Pt coating it's very important to remember, that this conductive layer can be damaged under heating, caused by too strong current flow. It's recommended to maintain current less than 10-20 nanoAmpheres, while getting XY conductivity maps in scanning regime.
Similarly, HA_C/W2C+ and HA_FM/W2C+ will suit well for scanning surfaces with high conductivity or with vast lateral variation of conductivity. They can work under current of 100 mkA and higher without notable degradation for many scans.
Another types of metal coated cantilevers can be chosen for some special applications. Au-coated HA_C probes could suit for soft and sticky biological objects. Conductive probes of higher stiffness, based on HA_NC or HA_HR models may be needed to achieve more close tip-sample contact by increasing pressure. Finally, DCP ones are necessary when all other probes proved their strong degradation on the type of surface that should be investigated.
Cantilevers for oscillatory electrical modes (Kelvin probe mode, EFM, SCM, PFM)
Technically surface potential distribution in Kelvin Probe mode or piezoelectrical map in PFM can be obtained by standard AFM cantilever with no coating from tip side. But electrical properties will be observed with much lower resolution. That's why researchers usually take cantilevers with conductive coating for these modes.
Choice of cantilever for a particular application consists of two steps.
1 - Taking a probe with optimal lever's parameters.
HA_NC (100 - 350 kHz Res. freq. range) will suit well for almost all possible semicontact electrical modes, like Kelvin probe mode, EFM, SEM, etc. Their Q-factor lies in the optimal range for work so with high oscillation amplitude as with smallest ones (below 10 nm). The latter regime can be useful for surfaces with poor fixed particles to avoid their attachment to tip's apex.
HA_HR (200 - 450 kHz Res. freq. range) are the second obvious choice for semicontact electrical modes. Having high Q-factor they can oscillate with the highest amplitudes. Such property can be useful for samples with vast variation of height.
HA_FM (50 - 150 kHz Res. freq. range) are usually being chosen for PFM regime, as their oscillation responce in the contact mode is optimal.
2 - Choosing proper conductive cover.
For semicontact electrical modes Pt coating is the most often used coating, as it has got excellent conductivity and hard enough to work long in the semicontact regime.
For PFM Pt or W2C coating can be chosen. The latter is useful for hard samples, that are usually the reason of fast cantilever's wear off.
So if the question is which AFM probe to take, we could recommend the next ones:
HA_NC/Pt - for Kelvin Probe mode, EFM, SCM;
HA_FM/Pt or HA_FM/W2C - for PFM.
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