Wollaston Wire probe

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

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 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 Figure 6.7B from Hammiche and Pollack [157] unpublished.)...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. 

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. 

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].

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... 

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 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.]... 

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... 

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

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