Micromachined SFM Probes for High-Frequency Electric and Magnetic Fields
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Figure 1. Micromachined coaxial SFM tip with -10 nm inner conductor tip radius and -100 nm shield diameter 13 .
Figure 2. Integration of coaxial tip and broadband transmission line along SFM cantilever
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For these measurements we chose an ultrafast distributed circuit, a nonlinear transmission line (NLTL) which not only exhibits clearly distinct waveforms along its structure but also approximates the switching waveforms of advanced digital circuits 15 . We prepared the tip/cantilever assembly with a 90 'C bake to drive off water, followed by a drop of HMDS applied with a brush fiber to the cantilever and body to promote adhesion of photoresist. Using a small drop of AZ-5214 photoresist to cover the cantilever and body provides a thick, low-stress dielectric suitable for insulating the conductive cantilever from a metal shield. We evaporated a 300A Ti/2000A Au shield onto the cantilever and tip to create a controlled impedance structure which can be contacted with 0.2 mm diameter flexible coaxial cable 16 . Bringing this contact out to a 50 GHz sampling oscilloscope completes the local probe. We bring the tip into light contact with a Au or alumina surface to remove the shell of metal and photoresist at the tip, exposing the SFM tip as a center conductor. With this assembly we are able to probe local picosecond electric fields along the NLTL using a sampling oscilloscope without intervening amplification. There are limitations on bandwidth imposed by the connectors and cabling, indicated in Fig. 4, which can be overcome with better assembly approaches and localized instrumentation, such as diode samplers and detectors.
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Figure 3. Spatially resolved waveforms along intermediate section of nonlinear transmission line measured with coaxial SFM probe tip. 0F
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-80 -100 40 60 Frequency (GHz) Figure 4. Spectrum of output signal from nonlinear transmission line as measured by coaxial SFM probe.
We expect these frequency limitations to be resolved with fully integrated tips, cantilevers and probe bodies, as discussed in the next section for magnetic field probing. While we are resolving experimental limitations, it is also important to understand the field strengths and distributions at the probe tip. To do this we have performed full-wave three dimensional field calculations 17 on the tip geometry and material of Fig. 1, as shown in Figs. 5 and 6. In Fig. 5 we plot the total electric field strength integrated over three dimensions as the tip is positioned over a sample with 100 nm conductors and spaces at a 10 nm height measuring a frequency of 20 GHz. In this plot, the tip is located > 5 pim from the energized conductor, so the field strength is low, but it is the relative strength of the field that is most important here. The expected "lightning rod" field-concentrating effect is clear, yet as shown in Fig. 6, a cut through 23
the plane normal to the
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