Wollaston probes

The Wollaston probe is a relatively massive structure compared with most inert probes used in other forms of AFM. These usually incorporate hard ultrasharp tips, made from silicon or silicon nitride, whose contact radius may be as small as 10 nm or so, mounted at the end of a relatively simple stiff elastic cantilever. The spatial resolution of such probes is therefore far superior to that of the Wollaston probe, whose high and variable spring constant (5-20 N/m) and complexity also render it unsuitable for all except the contactmode imaging described above (various alternative AFM imaging modes are described below). 

Figure 7.6. The effect of the Wollaston probe geometry on the two-dimensional shape of a circular particle as acquired in the resulting image.

At the time of writing, there are two major types of probe, both of which are described above the Wollaston and the nanoprobes. The Wollaston probe is very robust, can be used at temperatures of 00 °C, and can provide the calorimetric measurement required for the thermal force-distance curves described above as well as AC and DC thermal imaging. Its disadvantages are that the spatial resolution is of the order of a micro-meter and can be used only in contact mode. 

At the time of writing there is only one manufacturer of micro/nano-TA equipment Anasys Instruments, (www.anasysinstruments.com). They supply the hardware and software for local thermal analysis and thermal imaging that can be interfaced with the most popular types of atomic force microscope. More recently they have launched an instrument based on an optical microscope, the Vesta system, which is simpler to use than an atomic force microscope but the spatial resolution is limited to approximately 1.5 micrometers, see Fig. 7.22. The Wollaston probes are supplied by Veeco (www.veeco.com) and are, therefore, compatible only with Veeco AFMs. The nanoprobes are supplied by Anasys Instruments and can be used with most popular makes of AFM. In addition, Anasys Instruments supply calibration kits containing temperature standards. More recently they have launched an instrument based on an optical microscope, the Vesta system, which is simpler to use than an atomic force microscope, but the spatial resolution is limited to approximately 1.5 pm. 

A photomicrograph of a Ihcrmal probe is shown in Figure 31-16. The most common type of thermal probe is the resistive probe based on a Wollaston wire. This... 

Figure 2. Schematic diagrams of resistive SThM probes a) Wollaston wire type [34,35], b) micro-machined coated Si cantilever [37] and c) data storage doped Si probe [55].

In 1994, Dinwiddie and PyUdd [34,35] described the first combined SThM/AFM probes that employed resistance thermometry to measure thermal properties. These were fashioned from Wollaston process wire. This consists of a thin platinum/5% rhodium core (about 5 pm in diameter) surrounded by a thick (about 35 pm) silver sheath. The total diameter of the wire is thus about 75 pm. A length of wire is formed into a V and the silver is etched away at the apex to reveal a small loop of Pt/Rh which acts as a miniature resistance thermometer (Figure 2(a)). A bead of epoxy resin is added near the tip to act... 

Buzin Al, KamasaP, PydaM, Wunderlich B (2002) Application of Wollaston Wire Probe for Quantitative Thermal Analysis, Thermochim Acta, 381, 9-18. 

Hammiche and co-workers [285] described a technique in which a miniaturized Wollaston wire resistive thermometer is used as a probe to record IR absorption spectra by detecting photothermally induced temperature fluctuations at the sample surface. These authors claimed that such an approach opens the way to spatial resolution extended beyond the diffraction limit by a few hundred nanometers. As an alternative, Palanker et al. [280] suggested to use tipless probing. 

A photomicrograph of a thermal probe is shown in Figure 31-16. The most common type of thermal probe is the resistive probe based on a Wollaston wire. Thb wire has a thick coating of silver on top of a thin core of platinum or a platinum-rhodium alloy. At the tip of the probe, the silver is etched away to expose the bare wire. Micromachined probes have also been developed. With these probes, almost all of the electrical resistance is located at the tip. As a result, when an electric current is applied, only the tip becomes hot. The electrical resistance of the tip is also a measure of the temperature. 

Cantilever mount Wollaston wire Platinum core Probe tip... 

The form of SThM most relevant to the subject of this discussion is carried out using near-field electrical resistance thermometry, and this method has been adopted in the work reported in this chapter. This is because miniaturized resistive probes have the considerable advantage that they can be used both in passive mode as a thermometer and as an active heat source. This enables local thermal analysis (L-TA see text below) as well as SThM to be carried out. At present the most common type of resistive probe available is the Wollaston or Wollaston Wire probe, developed by Dinwiddle et al. (1994) and first used by Balk et al. (1995) and Hammiche et al. (19%a) The construction details of this probe are illustrated in Fig. 7.3. A loop of 75-pm-diameter coaxial bimetallic Wollaston wire is bent into a sharp V-shaped loop. The wire consists of a central 5-pm-diameter platinum/10% rhodium alloy core surrounded by silver. The loop is stabilized with a small bead of epoxy resin deposited approximately 500 pm from its apex. The probe tip or sensor is made... 

Figure 73. A Wollaston resistive thermal probe (not to scale), including a section through Wollaston process wire.

Figure 7.5. Diagrams illustrating effects of shape of probe tip on spatial resolution (a) Wollaston wire (b) low-aspect-ratio pyramidal tip (c) high-aspect-ratio pyramidal tip.

Figure 7.11. Wollaston thermal probe temperature calibration curves. The soUd line was constructed by melting organic crystals of known T. The broken Une was constructed by holding the probe just out of contact with the variable temperature stage at various temperatures.

Very recently, a tuneable CO2 laser has been combined with an AFM to form an aperture-less nearfield imaging system to obtain contrast in infrared absorption on a scale of about 100 nm [299]. However, the tuneable range of the CO2 laser is limited to a region of the IR spectrum that is not particularly informative for most IR chromophores ( 2300 cm ). For many applications coupling of a tuneable IR diode laser to an infrared microscope [300] is more attractive. Hammiche et al. [301] have used a Wollaston resistive thermometer as a photothermal probe to record IR spectra of polymers. Anderson [302] has indicated that an AFM/FTIR microscope without specialised tips can provide surface topography and chemical mapping at high spatial resolution. Direct infrared detection at a surface with the use of an AFM was tested both with filter and FTIR spectrometers (Fig. 5.6). Nowadays, IR spectroscopy at... 

Figure 6.7. Resistively heated AFM probes used for thermal imaging and thermal analysis. (A) Wollaston wire probes used in the commercial microTA system. (B) Microfabricated silicon probes with integral heaters at the base of the tip used in the commercial nanoTA. (Figure 6.7A from Anasys Instruments [134] reproduced with permission. Figure 6.7B from Hammiche and Pollack [157] unpublished.)...

Scanning thermal microscopy (SThM) is a contact AFM technique that allows spatial mapping of temperature or thermal conductivity across a sample surface in addition to topography. Most thermal probes utilize a temperature-sensitive resistor placed on the end of the tip. These resistor probes can be fabricated from a V-shaped Wollaston wire made of a platinum inner core and outer sheath of silver, in which the silver sheath is etched away at the V-shaped tip. Eigure 19 shows a Wollaston wire probe. In passive mode, the tip is scanned across a heated sample under constant-force feedback (contact mode) and a small current is passed through the probe to sense the tip resistance. The resistance value at any point is a measure of the local temperature, and thus a temperature map and topographic image may be produced simultaneously. 

FIGURE 19 (A) Electron micrograph of a Wollaston wire SThM probe. (B, C) Topographic and temperature maps of an active integrated circuit. The bright region in the temperature map (C) reveals a hot spot due to flow of electrical current. [Courtesy of Thermo Microscopes, Inc.]... 

Temperature mapping can be used to examine electronic devices that are passing current and dissipating energy. Figure 19 shows topographic and temperature maps of a silicon device recorded by SThM using a Wollaston wire probe. SThM has also been applied to a variety of polymer thin-film samples. Typical temperature resolution is tens of millikelvins. Spatial resolution is naturally a function of the probe sharpness and is about 100 nm. 

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