W2C cantilevers crash test reveals unique stability

30.12.2015

It’s well-known that results of Spreading resistance or LAO measurements depend strongly on a quality of tip-sample contact. In most cases its bad state is caused by a damage of tip’s end conductive layer due to its physical or electrical stress. In our opinion there are the two main schemes of possible cover’s crush: Under a high potential difference polarized material of probe’s end attracts strongly to sample’s surface. As the result – conductive cover flows from tip’s apex. Conductive layer wears off during cantilever’s motion in the contact regime. This process is especially affected by high temperatures at tip-sample contact point due. They arise due to respectively high current that flows through a very small contact area. A new conductive W₂C cover of Etalon probes should combine low thickness (around 20-30 nm) and high resistivity to all possible negative influences appearing during measurements. To verify our suggestions we have made special “crush” tests with several probes of HA_FM/W₂C series (http://www.ntmdt-tips.com/products/view/ha-fm-w2c-1-1). All the measurements were fulfilled by NT-MDT AFM’s Solver PRO and NTEGRA Prima. We took a piece of HOPG as a sample as it has got very high conductivity and allows to transfer very high current through a tip-sample contact area with our standard bias voltages. To examine tip’s apex resistivity to high voltage between a sample and a cantilever we carried out series of I(V) curves over the range -10V … +10V. Each curve took 4 seconds so that voltage changed slowly enough. On the images below you may see the first and the hundredth curves:

As it looks from the images, there was no degradation detected after 100 I(V) curves in the same point. The similar test was carried out for scanning process in spreading resistance mode. Several scans of 20x20 um size topography with parallel imaging of current distribution were taken. During some of them bias voltage was so high that current detector worked in its saturation regime. That means that real current flowing between a probe and a sample exceeded 10 mkA. Anyway, nor probes’ degradation during that scans, neither during next ones (with softer parameters) was detected. More of all, further scans demonstrated good current contrast that differed from topography. That just approves that current images were correct and a tip was workable. Topography (left one) and current (right one) images with an extremely high bias voltage.

Current image of the same area during the next scan. It has got a similar non-conductive area as on the previous current scan but differs significantly from topography:

Accounting on the results of our measurements, now we may approve that W₂C conductive layer forms really strong conductive tip apex, which is wear-resistive and may even work for some time under extremely high voltage or current condition. We don’t recommend anybody to repeat these experiments continuously, stressing the probes. But after that result we may definitely say that W₂C line of conductive cantilevers may provide stable and long work in standard measurements’ condition.