---
title: "Probe tip"
chunk: 4/5
source: "https://en.wikipedia.org/wiki/Probe_tip"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:41:57.526681+00:00"
instance: "kb-cron"
---

== Coating ==
The surface of silicon-based tips cannot be easily controlled because they usually carry the silanol group. The Si surface is hydrophilic and can be contaminated easily by the environment. Another disadvantage of Si tips is the wear and tear of the tip. It is important to coat the Si tip to prevent tip deterioration, and the tip coating may also enhance image quality. To coat a tip, an adhesive layer is pasted (usually chromium layer on 5 nm thick titanium) and then gold is deposited by vapor deposition (40-100 nm or less). Sometimes, the coating layer reduces the tunnelling current detection capability of probe tips.

== Characterization ==
The most important aspect of a probe tip is imaging the surfaces efficiently at nanometre dimensions. Some concerns involving credibility of the imaging or measurement of the sample arise when the shape of the tip is not determined accurately. For example, when an unknown tip is used to measure a linewidth pattern or other high aspect ratio feature of a surface, there may remain some confusion when determining the contribution of the tip and of the sample in the acquired image. Consequently, it is important to fully and accurately characterize the tips. Probe tips can be characterized for their shape, size, sharpness, bluntness, aspect ratio, radius of curvature, geometry and composition using many advanced instrumental techniques. For example, electron field emission measurement, scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning tunnelling spectroscopy as well as more easily accessible optical microscope. In some cases, optical microscopy cannot provide exact measurements for small tips in nanoscale due to the resolution limitation of the optical microscopy.

=== Electron field emission current measurement ===
In the electron field emission current measurement method, a high voltage is applied between the tip and another electrode, followed by measuring field emission current employing Fowler-Nordheim curves 
  
    
      
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. Large fields-emission current measurements may indicate that the tip is sharp, and low field-emission current indicates that the tip is blunt, molten or mechanically damaged. A minimum voltage is essential to facilitate the release of electrons from the surface of the tip which in turn indirectly is used to obtain the tip curvature. Although this method has several advantages, a disadvantage is that the high electric field required for producing strong electric force can melt the apex of the tip, or might change the crystallographic tip nature.

=== Scanning electron microscopy and transmission electron microscopy ===
The size and shape of the tip can be obtained by scanning electron microscopy and transmission electron microscopy measurements. In addition, transmission electron microscopy (TEM) images are helpful to detect any layer of insulating materials on the surface of the tip as well as to estimate the size of the layer. These oxides are formed gradually on the surface of tip soon after fabrication, due to the oxidation of the metallic tip by reacting with the O2 present in the surrounding atmosphere. Scanning electron microscopy (SEM) has a resolution limitation of below 4 nm, so TEM may be needed to observe even a single atom theoretically and practically. Tip grain down to 1-3 nm, thin polycrystalline oxides, or carbon or graphite layers at the tip apex, are routinely measured using TEM. The orientation of tip crystal, which is the angle between the tip plane in the single-crystal and the tip normal, can be estimated.

=== Optical microscopy ===
In the past, optical microscopes were the only method used to investigate whether the tip is bent, through microscale imaging at many microscales. This is because the resolution limitation of an optical microscope is about 200 nm. Imaging software, including ImageJ, allows determination of the curvature, and aspect ratio of the tip. One drawback of this method is that it renders an image of tip, which is an object due to the uncertainty in the nanoscale dimension. This problem can be resolved by taking images of the tip multiple times, followed by combining them into an image by confocal microscope with some fluorescent material coating on the tip. It is also a time-consuming process due to the necessity of monitoring the wear or damage or degradation of the tip by collision with the surface during scanning the surface after each scan.

=== Scanning tunneling spectroscopy ===
The scanning tunneling spectroscopy (STS) is spectroscopic form of STM. Spectroscopic data based on curvature is obtained to analyze the existence of any oxides or impurities on the tip. This is done by monitoring the linearity of the curve, which represents metallic tunnel junction. Generally, the curve is non-linear; hence, the tip has a gap-like shape around zero bias voltage for oxidized or impure tip, whereas the opposite is observed for sharp pure un-oxidized tip.

=== Auger electron spectroscopy, X-ray photoelectron spectroscopy ===
In Auger electron spectroscopy (AES), any oxides present on the tip surface are sputtered out during in-depth analysis with argon ion beam generated by differentially pumped ion pump, followed by comparing the sputtering rate of the oxide with experimental sputtering yields. These Auger measurements may estimate the nature of oxides because of the surface contamination. Composition can also be revealed, and in some cases, thickness of the oxide layer down to 1-3 nm can be estimated. X-ray photoelectron spectroscopy also performs similar characterization for the chemical and surface composition, by providing information on the binding energy of the surface elements.
Overall, the aforementioned characterization methods of tips can be categorized into three major classes. They are as follows: