Centro de Microelectrónica Universidad de los Andes -

Tip preparation for the STM-Uniandes

Probes are a fundamental aspect in the application of SPM techniques. They are the first and most important element composing the transducer that allows us detect very small scale interactions. Since a STM uses tunneling electrons as the interaction, both sample and tip have to be of a metallic material so that they have a constant electronic structure throughout their surface.

The microscope's lateral resolution is a very important parameter since the smaller resolution the better the instrument. the lateral resolution is directly related to sharpeness achieved when fabricating probes, and hence their importance.

A scanning tip or probe can be characterized by the largest circle of radius r that can be inscribed into the tip geometry. The lateral ersolution is then estimated as:

The most popular tip fabrication metheds include[21]:

• Mechanical polishing or grinding
• Electrochemical polishing
• Manual cutting

At Uniandes we have developed latter two methods as explained below:

Electrochemical polishing The references section has two papers describing this procedure in more detail.

This method is used to remove material by electrochemically etching the interface between a conductor and an electrolyte by applying a potential difference accross them. This methos usually requires a post cleaning procedure to ensure a good tunneling interaction. It is mainly made using a tungsten (W) wire (9.95% purity 0.50mm diameter) and a less electronegative metal such as cupper or platinum.

The figure below shows the elements building a electrochemical polishing set-up. It includes an electrolyte, cathod, anode and a potenial difference.

This technique is also referred as lamellae drop-off since the etching geometry is in fact a lamellae of elecrtolyte formed around the conductor. The electrolyte is generally KOH or NaOH.

The tungsten wired used has to be carefully cut and placed inside a tip holder, which for our case is done using the metalic tube in a hypodermic needle 20G × 1/2 in, as shown below:

Additionally, this method needs a electropolishing circuit that can detect a very small change in the etching current and swithc off the voltage as soon as the tip has been formed. This circuit is described in the Tip Fabrication tab of the Publications sections. Using this set-up, we have fabricated scanning tips as the ones shown in the images below. From these images we can determine an estimate of thelateral resolution of the instrument. Since their radious os about 300nm, the average lateral resolution expected is of 18nm.

Manual cutting

This method is very similar to mecanically grinding down a wire until reaching a very sharp tip, however in this case instead of using a grinder we are using a fine wire cutters and we are taking advantage of the ductility of the material. THe most common material used in this case is Platinum Iridium (Pt-Ir). Upon applying the right force to a acute cut, very small tips are produce at the very tip of the wire and eventually one of them will have a small enough radius to produce a stable tunneling current when interacting with the sample.

At Uniandes, we used PtIr 80/20 (99.95% pure 0.25mm diameter) and a preocedure similar to the one described in [23]. The same needle 20G × 1/2 in was used as tip holder and a very precise stainless steel wire cutters. THe procedure is schematically shown below.

Once again we have characterized the fabricated tips and we found that the raidus of the tip at the very end was somewhat between the expected range. The images below show SEM images of the fabricated tips using this method and from there we can quantify an approximate radius of 70nm, that could potentially render a lateral resolution of 7,9nm.