5.6 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Probe tip | 3/5 | https://en.wikipedia.org/wiki/Probe_tip | reference | science, encyclopedia | 2026-05-05T06:25:51.740066+00:00 | kb-cron |
==== Lamella method ==== In the double lamella method, the lower part of the metal is etched away, and the upper part of the tip is not etched further. Further etching of the upper part of the wire is prevented by covering it with a polymer coating. This method is usually limited to laboratory fabrication. The double lamella method schematic is shown in Fig. 5.
=== Single atom tip preparation === Transitional metals like Cu, Au and Ag adsorb single molecules linearly on their surface due to weak van der Waals forces. This linear projection of single molecules allows interactions of the terminal atoms of the tip with the atoms of the substrate, resulting in Pauli repulsion for single molecule or atom mapping studies. Gaseous deposition on the tip is carried out in an ultrahigh vacuum (5 × 10−8 mbar) chamber at a low temperature (10K). Depositions of Xe, Kr, NO, CH4 or CO on tip have been successfully prepared and used for imaging studies. However, these tips preparations rely on the attachment of single atoms or molecules on the tip and the resulting atomic structure of the tip is not known exactly. The probability of attachment of simple molecules on metal surfaces is very tedious and required great skill; as such, this method is not widely used.
=== Chemical vapor deposition (CVD) === Sharp tips used in SPM are fragile, and prone to wear and tear under high working loads. Diamond is considered the best option to address this issue. Diamond tips for SPMs are fabricated by fracturing, grinding and polishing bulk diamond, resulting in a considerable loss of diamond. One alternative is depositing a thin diamond film on Silicone tips by CVD. In CVD, diamond is deposited directly on silicon or W cantilevers. A is shown in Fig. 6. In this method, the flow of methane and hydrogen gas is controlled to maintain an internal pressure of 40 Torr inside the chamber. CH4 and H2 dissociate at 2100 °C with the help of the Ta filament, and nucleation sites are created on the tip of the cantilever. Once CVD is complete, the flow of CH4 is stopped and the chamber is cooled under the flow of H2. A schematic diagram of a CVD setup used for diamond tip fabrication for AFM application is shown in Fig. 6.
=== Reactive ion etching (RIE) fabrication === A groove or structure is made on a substrate to form a template. The desired material is then deposited in that template. Once the tip is formed, the template is etched off, leaving the tip and cantilever. Fig. 7 illustrates diamond tip fabrication on silicon wafers using this method.
=== Focused ion beam (FIB) milling === FIB milling is a sharpening method for probe tips in SPM. A blunt tip is first fabricated by other etching methods, such as CVD, or the use of a pyramid mold for pyramidal tips. This tip is then sharpened by FIB milling as shown in Fig. 8. The diameter of the focused ion beam, which directly affects the tip's final diameter, is controlled through a programmable aperture.
=== Glue === This method is used to attach carbon nanotubes to a cantilever or blunt tip. A strong adhesive (such as soft acrylic glue) is used to bind CNT with the silicon cantilever. CNT is robust, stiff and increases the durability of probe tips, and can be used for both contact and tapping mode.
== Cleaning procedures == Electrochemically etched tips are usually covered with contaminants on their surfaces which cannot be removed simply by rinsing in water, acetone or ethanol. Some oxide layers on metallic tips, especially on tungsten, need to be removed by post-fabrication treatment.
=== Annealing === To clean W sharp tips, it is highly desirable to remove contaminant and the oxide layer. In this method a tip is heated in an UHV chamber at elevated temperature which desorb the contaminated layer. The reaction details are shown below. 2WO3 + W → 3WO2 ↑ WO2 → W (sublimation at
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1075K) At elevated temperature, trioxides of W are converted to WO2 which sublimates around 1075K, and cleaned metallic W surfaces are left behind. An additional advantage provided by annealing is the healing of crystallographic defects produced by fabrication, and the process also smoothens the tip surface.
=== HF chemical cleaning === In the HF cleaning method, a freshly prepared tip is dipped in 15% concentrated hydrofluoric acid for 10 to 30 seconds, which dissolves the oxides of W.
=== Ion milling === In this method, argon ions are directed at the tip surface to remove the contaminant layer by sputtering. The tip is rotated in a flux of argon ions at a certain angle, in a way that allows the beam to target the apex. The bombardment of ions at the tip depletes the contaminants and also results in a reduction of the radius of the tip. The bombardment time needs to be finely tuned with respect to the shape of the tip. Sometimes, short annealing is required after ion milling.
=== Self-sputtering === This method is very similar to ion milling, but in this procedure, the UHV chamber is filled with neon at a pressure of 10−4 mbar. When a negative voltage is applied on the tip, a strong electric field (produced by tip under negative potential) will ionize the neon gas, and these positively charged ions are accelerated back to the tip, where they cause sputtering. The sputtering removes contaminants and some atoms from the tip which, like ion milling, reduces the apex radius. By changing the field strength, one can tune the radius of the tip to 20 nm.