Probe tip
A probe tip in scanning microscopy is a very sharp object made from metal or other materials, like a sewing needle with a point at one end with nano or sub-nanometer order of dimension. It can interact with up to one molecule or atom of a given surface of a sample that can reveal authentic properties of the surface such as morphology, topography, mapping and electrical properties of a single atom or molecule on the surface of the sample.
The proliferation of the thrust in probe-based tools began after the invention of the scanning tunneling microscope and atomic force microscopy by Gerd Binnig and Heinrich Rohrer at the IBM Zurich research laboratory in 1982. It opened a new era for probing the nanoscale world of individual atoms and molecules as well as studying surface science, due to their unprecedented capability of characterizing a wide range of unique properties such as mechanical, chemical, magnetic and optical functionalities of various samples at nanometer-scale resolution in a vacuum, ambient or fluid environment. The utilization of sharp probe tips enabled the researcher to see inside the microscopic world from within the macroscopic world. The increasing demand for sub-nanometer probe tips is attributable to their robustness and versatile applicability because of their direct application to the numerous fields of science that includes nanolithography, nanoelectronics, biosensor, electrochemistry, semiconductor, micromachining and biological cells studies. The significant number of applications for the topographic surface characterization of the materials and biological specimen in various fields of science made researchers and scientists realise that it is imperative to have reproducible mass production of probe tips with sharp apexes.
Probe tip size and shape in microscopy are important parameters providing direct connection between resolution and imaging quality. Resolution and imaging mechanism may depend on geometry and composition of the tip and the surface being probed. Tip size, shape and reproducibility are extremely important in order to monitor and detect the interaction between the surfaces.
This article describes the fabrication, characterization and application of sharp tips. A wide range of tip fabrication techniques including cutting, grinding, pulling, beam deposition, ion milling, controlled crashing, field emission, field evaporation, fracture and electrochemical etching/polishing are discussed. Both limitations and advantages are also provided for various tip fabrication methods. The history and development, working principles, characterization and applications of the recent advancement of sharp tips are also described.
History and development
The discovery of a sharp probe tip has always been of significant interest among researchers considering its importance in the material, life and biological sciences for mapping the surface structure and material properties at molecular or atomic dimensions. The history of the tip can be tracked back to the nineteenth century during the invention of the phonautograph in 1859. The phonautograph is the predecessor of the modern gramophone. It was invented by Scott and Koenig. It consisted of a parchment diaphragm with an attached stylus, along with a hog's hair, which was used to trace a wavy line on a lamp-blacked surface. In the later development of the gramophone, along with other replacements, the hog's hair was replaced by a needle to reproduce sound. In 1940, a pantograph was built utilizing a shielded probe and adjustable tip. A stylus was free to slide vertically to be in contact with the paper. In 1948, a tip was employed in the probe circuit to measure peak voltage. The fabrication of electrochemically etched sharp tungsten, copper, nickel and molybdenum tips were reported by Muller in 1937. The revolution for sharp tips occurred in producing a variety of tips with different shape, size, aspect ratio composed of tungsten wire, silicon, diamond and carbon nanotubes occurred with Si-based circuit technologies. This allowed the production of tips for numerous applications in the broad spectrum of nanotechnological fields. Following STM, came the invention of atomic force microscopy by Gerd Binnig, Calvin F. Quate, and Christoph Gerber in 1986. In their instrument they used a broken piece of diamond as the tip sticking it to a hand-cut gold foil cantilever. Focused ion and electron beam techniques for the fabrication of strong, stable, reproducible Si3N4 pyramidal tips with 1.0 μm length and 0.1 μm diameter were reported by Russell in 1992. Ground breaking advancement came through the introduction of microfabrication methods for the fabrication of precise conical or pyramidal silicon and silicon nitride tips. Later, numerous research experiments were explored to fabricate comparatively less expensive and more robust tungsten tips, with a need to attain less than 50 nm radius of curvature.The new horizon in the field of fabrication of probe tips was revealed when the carbon nanotube, which is basically an approximately 1 nm cylindrical shell of graphene, was introduced. The use of single wall carbon nanotubes is less vulnerable to breaking or crushing during imaging due to their flexibility. Probe tips made up of carbon nanotubes can be efficiently used to get high-resolution images of both soft and weakly absorbed biomolecules like DNA on the surface at molecular resolution.
Multifunctional hydrogel nano-probe techniques uncovered a new scope for initiating a completely new concept for the fabrication of tips and their extended ease of applicability for inorganic and biological samples in both air and liquid. The biggest advantage of this mechanical method is that the tip can be made in different shapes, such as hemispherical, embedded spherical, pyramidal, and distorted pyramidal, with diameters ranging from 10 nm – 1000 nm for applications including topography or functional imaging, force spectroscopy on soft matter, biological, chemical and physical sensors. Table 1. Summarizes various fabrication, material and application of tips.
| Fabrication Method | Material | Application | References |
| Grinding, cutting, fracture, center aligned | Diamond, | Nanoindentation, 2D profiling in semiconductor, doping type and concentration of native silicon oxide | |
| Beam Ion Milling | Diamond | Local electrical characterization of thin metal–oxide–semiconductor dielectrics, conducting AFM | |
| Field ion microscope | SiOx, Si3N4, quartz | Nanoelectronics, bond strength in biomolecules | |
| etching | W, W, Ag, Pt, Ir, Au | Semiconductor, nano-patterning, metal surface imaging | |
| Hydrogel | Poly- diacrylate | Biological soft and hard sample, dip-pen nanolithography | |
| RIE-Reactive-ion etching | Diamond, | Forces, optical properties | |
| Glue | Polymers, carbon nanotube | Charge density waves on the surface of conducting material, imaging of single atom | |
| Single atom functionalized | Single CO2 molecule attached to metal tip | Bond-order, catalysis, chemical structure | |
| Electron beam deposition | Silicon | Lithography, high resolution imaging | |
| Chemical vapor deposition | CNT, diamond | Electronic devices, Semi-conductor |
Tunneling current and force measurement principle
The tip itself does not have any working principle for imaging, but depending on the instrumentation, mode of application, and the nature of the sample under investigation, the probe tip may follow different principles to image the surface of the sample. For example, when a tip is integrated with STM, it measures the tunneling current that arises from the interaction between the sample and the tip. In AFM, short-ranged force deflection during the raster scan by the tip across the surface is measured. A conductive tip is essential for the STM instrumentation whereas AFM can use conductive and non-conductive probe tip. Although the probe tip is used in various techniques with different principles, for STM and AFM coupled with probe tip is discussed in detail.Conductive probe tip
Somewhat, the name implies STM utilizes tunneling charge transfer principle from tip to surface or vice versa thereby recording the current response. This concept originates from particle in a box concept, that is, if potential energy for a particle is small, electron may be found outside of potential well which is a classically forbidden region. This phenomenon is called tunneling.Expression derived from Schrödinger equation for transmission charge transfer probability is as follow:
where
Non-conductive probe tip
Non-conductive nano-scale tips are widely used for AFM measurements. For non-conducting tip, surface forces acting on the tip/cantilever, are responsible for deflection or attraction of tip. These attractive or repulsive forces are used for surface topology, chemical specifications, magnetic and electronic properties. The distance dependent forces between substrate surface and tip are responsible for imaging in AFM. These interactions include are van der Waals forces, capillary forces, electrostatic forces, Casimir forces and solvation forces. One unique repulsion force is Pauli Exclusion repulsive force is responsible for single atom imaging as in references and Figures 10 & 11.Fabrication methods
Tip fabrication techniques fall generally into two classifications: mechanical and physicochemical. In the early stage of development of probe tips, mechanical procedures were popular because of the ease of fabrication.Mechanical methods
A few reported mechanical methods to fabricate tips include cutting grinding and pulling. For example, cutting a wire at certain angles with razor blade or wire cutter or scissor. Another mechanical method for tip preparation is fragmentation of bulk pieces into small pointy pieces. Grinding a metal wire/rod into a sharp tip was also a method used. These mechanical procedures usually leave rugged surfaces with many tiny asperities protruding from the apex which led to atomic resolution on flat surfaces. However, irregular shape and large macroscopic radius of curvature result in poor reproducibility and decreased stability especially for probing rough surfaces. Another main disadvantage of making probes by this method is that it creates many mini tips which lead to many different signals, yielding error in imaging. Cutting, grinding and pulling procedures can only be adapted for metallic tips like W, Ag, Pt, Ir, Pt-Ir and gold. Non-metallic tips cannot be fabricated by these methods.In contrast, a sophisticated mechanical method for tip fabrication is based on the hydro-gel method. This method is based on bottom-up strategy to make probe tips by a molecular self-assembly process. First, a cantilever is formed in a mold by curing pre-polymer solution, then it is brought into contact with the mold of the tip which also contains pre-polymer solution. The polymer is cured with ultraviolet light which helps to provide a firm attachment of the cantilever to the probe. This fabrication method is shown in Fig. 2.
Physio-chemical procedures
Physiochemical procedures are fabrication methods of choice these days which yield extremely sharp and symmetric tips with more reproducibility compared to mechanical fabrication-based tips. Among physicochemical method, the electrochemical etching method is one of the most popular methods. Etching is a two or more step procedure. The "zone electropolishing" is the second step which further sharpens the tip in a very controlled manner. Other physicochemical methods include chemical vapor deposition, and electron beam deposition onto pre-existing tips. Other tip fabrication methods include field ion microscopy and ion milling. In field ion microscopy techniques, consecutive field evaporation of single atoms yields specific atomic configuration at the probe tip which yields very high resolution.Fabrication through etching
Electrochemical etching is one of the easiest, least expensive, most practical, reliable and most widely accepted metallic probe tip fabrication method with desired quality and reproducibility. Three commonly used electrochemical etching methods for tungsten tip fabrication are: single lamella drop-off methods, double lamella drop-off method and submerged method. Various cone shape tips can be fabricated by this method by minor changes in the experimental setup. A DC potential is applied between the tip and a metallic electrode immersed in solution ; electrochemical reactions at cathode and anode in basic solutions are usually used. The overall etching process involved is written here:Anode;
Cathode:
Overall:
Here, all the potentials are reported vs. SHE.
The schematics of the fabrication method of probe tip production through the electrochemical etching method is shown in Fig. 3.
In the electrochemical etching process, W is etched at the liquid, solid and air interface, as shown in Fig. 3. Etching is called static if the W wire is kept stationary. Once the tip is etched, lower part falls due to the lower tensile strength than the weight of lower part of wire. The irregular shape is produced by the shifting of the meniscus. However, slow etching rates can produce regular tips when the current flows slowly through electrochemical cell. Dynamic etching involves slowly pulling up the wire from the solution, or sometimes the wire is moved up and down producing smooth tips.
Submerged method
In this method a metal wire is vertically etched reducing the diameter from 0.25 mm ~ 20 nm. Schematic diagram for probe tip fabrication with submerged electrochemical etching method is illustrated in Fig 4. These tips can be used for high quality STM images.Lamella method
In the double lamella method, the lower part of metal is etched away, and the upper part of tip is not etched further. Further etching of the upper part of 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. Gaseoss deposition on the tip is carried out in an ultrahigh vacuum chamber at a low temperature. 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. Therefore, this method is not widely used.Chemical vapor deposition (CVD)
Sharp tips used in SPM are fragile and prone to damage and wear and tear easily under high working load. Diamond is considered the best option to address this issue. Diamond tips for SPM application are fabricated by fracture of bulk diamond, grinding and polishing diamond. But, these methods result in considerable loss of diamond. Another strategy to prevent this loss is coating of Silicone tips with thin diamond film. These thin films are usually deposited by CVD. In CVD, diamond is deposited directly on silicon or W cantilever. A schematic diagram for chemical vapor deposition set up is shown in Fig. 6. In this method, flow of methane and hydrogen gas is maintained in such a way that pressure inside the chamber is maintained at 40Torr. CH4 and H2 are dissociated at elevated temperature of 2100 °C with the help of Ta filament. Nucleation sites are created on the tip of the cantilever. Once CVD is complete, CH4 flow is stopped and the chamber is cooled under flow of H2. Schematics of CVD set up for diamond tip fabrication for AFM application are shown in Fig. 6.Reactive ion etching (RIE) fabrication
In the RIE method, first a groove or structure is made on a substrate followed by the deposition of the desired material in that template. Once the tip is formed, the templating structure is etched off leaving the tip and cantilever. A schematic for diamond tip fabrication on silicon wafers through this method has been described in Fig. 7.Focused ion beam milling
Focused ion beam milling is a sharpening method for probe tips in SPM. In this method, first a blunt tip is fabricated by other methods, for example, a pyramid mold can be used to fabricate a pyramidal tip, CVD method or any other etching method. Then, this tip is sharpened by FIB milling as shown in Fig. 8. The focused ion beam diameter is controlled through a programmable aperture which directly correlates with the tip diameter.Glue
This method is used to attach carbon nanotubes on a cantilever or blunt tip. A strong adhesive is used to bind CNT with the silicon cantilever. CNT are robust, stiff and increase durability of probe tips and can be used for both contact and tapping mode.Cleaning procedures
Electrochemically etched tips are usually covered with contaminant 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
At elevated temperature, trioxides of W are converted to WO2 which sublimates around 1075°K, 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, a 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 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 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.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 wear and tear of the tip. It is important to coat Si tip to prevent tip deterioration, and the tip coating may also enhance image quality. First an adhesive layer is pasted and then gold is deposited by vapor deposition. 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 nanometer 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, transmission electron microscopy, scanning tunneling 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 resolution limitation of the optical microscopy.Electron field emission current measurement
In electron field emission current measurement method, a high voltage is applied between tip and another electrode followed by measuring field emission current employing Fowler-Nordheim curves. 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 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, 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. SEM has a resolution limitation of below 4 nm, TEM may be needed to observe even a single atom theoretically and practically. Tip grain down to 1-3 nm or thin polycrystalline oxides or carbon or graphite layers at the tip apex are routinely measured using TEM. The orientation of tip crystal i.e. the angle between the tip plane in the single-crystal and the tip normal can be estimated.Optical microscopy
In the past, optical microscope has been only used to investigate if the tip is bent 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 taking images of tip multiple times followed by putting them together into image by confocal microscope with some fluorescent material coating on the tip. Also, it is a time-consuming process considering the necessity of monitoring the wear or damage or degradation of tip by the collision with the surface during scanning the surface after each scan.Scanning tunneling spectroscopy
The scanning tunneling spectroscopy is spectroscopic form of STM in which spectroscopic data based on curve is obtained to analyze the existence of any oxides or impurities on the tip by monitoring the linearity of the curve which represents metallic tunnel junction. Generally, cure is non-linear and 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, any oxides present on the tip surface is 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 and/or composition can 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 in three major classes. They are:
- Imaging tip using microscopy- is used to take image of tip with microscopy except scanning probe microscopy e.g. scanning tunneling microscopy, atomic force microscopy are reported in the references.
- Using known tip characterizer- shape of tip is deduced by taking image of a sample of known measurement which is known as tip characterizer.
- Blind method- tip characterizer of either known or unknown measurement is used.
Applications
In another application, multiwall carbon nanotube and single wall carbon nanotube tips were used to image amyloid β derived protofibrils and fibrils by tapping mode AFM. Functionalized probes can be used in Chemical Force Microscopy to measure intermolecular forces and map chemical functionality. Functionalized SWCNT probes can be used for chemically sensitive imaging with high lateral resolution and to study binding energy in chemical and biological system. Probe tips that have been functionalized with either hydrophobic or hydrophilic molecules can be used to measure the adhesive interaction between hydrophobic-hydrophobic, hydrophobic-hydrophilic, and hydrophilic-hydrophilic molecules. From these adhesive interactions the friction image of patterned sample surface can be found. Probe tips used in force microscopy can provide imaging of structure and dynamics of adsorbate at the nanometer scale. Self-assembled functionalized organic thiols on the surface of Au coated Si3N4 probe tips have been used to study the interaction between molecular groups. Again, carbon nanotube probe tips in conjunction with AFM can be used for probing crevices that occur in microelectronic circuits with improved lateral resolution. Functionality modified probe tips has been to measure the binding force between single protein-ligand pairs. Probe tips have been used as a tapping mode technique to provide information about the elastic properties of materials. Probe tips are also used in the mass spectrometer. Enzymatically active probe tips have been used for the enzymatic degradation of analyte. They have also been used as devices to introduce samples into the mass spectrophotometer. For example, trypsin-activated gold probe tips can be used for the peptide mapping of the hen egg lysozyme.
Atomically sharp probe tips can be used for imaging a single atom in a molecule. An example of visualizing single atoms in water cluster can be seen in Fig. 10. By visualizing single atoms in molecules present on a surface, scientists can determine bond length, bond order and discrepancies, if any, in conjugation which was previously thought to be impossible in experimental work. Fig. 9 shows the experimentally determined bond order in a poly-aromatic compound which was thought to be very hard in the past.