Recent progress in the functionalization of atomic force microscope ...

2236 Zhou et al.: Recent progress in the functionalization of AFM probes 2236FIG. 4. a Probe having a sharp end, formed by dry etching the siliconnitride cantilever using a mask that extends only part of the way to the topof the pyramidal AFM tip. The sensor wire runs around the edge of thereleased probe, and is self-aligned to the edge since excess metal disappearswhen the probe is released. b Thermocouple using double self-alignmentso that the size of the junction is limited by the hole behind the apex of thetip and the junction is self-aligned to the end of the tip as before. The probeis shown before the silicon substrate has been removed by a final wet etchprocess. c Double self-aligned thermal resistor probe fabricated using thesame method as the thermocouple in b. The probe is shown after release.vantages. First, they must be made individually, which istime consuming and results in lack of reproducibility betweenprobes. Second, if a fiber is to be pulled in such a waythat the aperture is small, it is hard to prevent the taper angleat the end of the fiber from being rather shallow. The effectof this is that such probes lose a significant amount of thelight in the cutoff metallic optical wave guide leading up tothe aperture, as well as suffering the large losses associatedwith the aperture itself. Finally, the fiber is rather stiff alongits length, so that force regulation must be accomplished bythe use of shear-force regulation rather than by measurementof the deflection of a cantilever, which is more reliable. Thisobjection has been overcome to some extent by the use of abent fiber probe.More recently SNOM probes based on the use of micromachinedAFM probes have been realized. 20,21 EBL probefunctionalization technology is well suited to the productionof such probes. The process which has been adopted to dateinvolves the definition of a simple physical hole at the apexof the AFM probe by use of dry etching. 22 After release theentire probe, including the aperture, is coated with aluminumusing a vacuum evaporator. Previous attempts to fabricatesuch probes using a palladium layer in which an aperturewas defined before release 5 were unsuccessful due to stressesin the rather thick Pd layer which were necessary for adequatescreening. A particular problem with this technique,however, is that the etching of a small hole through a siliconnitride layer that is thick enough to act as an adequate AFMprobe is difficult. For this reason, the silicon nitride aroundthe tip is first thinned from approximately 500 nm in thicknessto some 200 nm before aperture definition the ovalshape is approx 50 m across around the pyramid in Fig.J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999

2237 Zhou et al.: Recent progress in the functionalization of AFM probes 2237FIG. 5. a Overview of a SNOM probe with a large aperture. The cantilever shape has been extended around the base of the pyramid to form a shield againstscattered light. The silicon nitride over the pyramidal tip has been thinned to enable the aperture which is narrow to be etched all the way through. b Apexof a pyramid showing a 150 nm diam SNOM aperure. c Effect of metallization granularity on the aperture shape. The aperture is seen to be rough on thescale of the grain size, approximately 50 nm. Note the smooth shape of the underlying silicon nitride substrate. d Somewhat smoother aperture profileresulting from the use of a cladding metal containing 5% w/w of copper.5a. Since light scattering around the probe was also foundto be troublesome, the cantilever has been extended for somedistance to form a screen around the probe. Such probes havebeen successfully scanned without force feedback for extendedperiods. Due to their low vertical stiffness and thelarge size of the probe apex region the local force on any partof the probe is low. Wear on the probe or on the substrateusually GaAs has therefore been found to be reduced to acompletely insignificant level regardless of the external forceapplied to the probe. This significantly simplifies operationof the SNOM microscope, since deflection measurement systemsgenerally based on bright optical sources such as diodelasers are not required. Due to the lack of any cutoff regionin front of the probe aperture itself, measurements of totallight throughput for a 150 nm aperture Fig. 5b in red light633 nm of approximately one part in 10 3 has been routinelyobtained. It should be noted that these throughputs were obtainedin a simple confocal transmission system having illuminationand collection optics operating at approximate numericalapertures NAs of 0.2–0.3. The SNOM probe itselfis capable of accommodating an illuminating beam of 0.55NA, which would lead to approximate quadrupling of theilluminating intensity at the aperture.The limitation on resolution of the probes is defined bythe granularity of the coating metal 99.999% pure aluminum.As may be seen from Fig. 5c, the aperture is roughJVST A - Vacuum, Surfaces, and Films

2238 Zhou et al.: Recent progress in the functionalization of AFM probes 2238FIG. 6. a 100 nm junction bismuth Hall probe magnetometer fabricated on a silicon tip. Leads to the junction are made of gold to reduce access resistanceand to allow bonding. b Junction size of a Hall probe fabricated as a lithographic test. The junction size is estimated to be 50 nm. c Completeelectromagnetic Hall probe consisting of a single-turn electromagnetic coil at the apex of an AFM probe. d Close-up view of the probe shown in c. Thesingle-turn coil consists of an approximately 50 nm wide wire which reaches a maximum diameter of approximately 250 nm. The wires leading to the coilare written as close together as possible approx 100 nm spacing center to center to minimize stray magnetic fields.on the scale of the metal grain size 50–70 nm, even thoughthe nitride aperture is far better defined than that. Recentexperiments using an alloy (Al 0.95 Cu 0.05 ) having similar opticalproperties to those of pure aluminum are promisingFig. 5d.C. Magnetic sensorsThe use of silicon cantilevers allows the operation offunctionalized probes in the attractive regime. Examples oftwo such probes are shown in Fig. 6. Figure 6a shows aHall-bar probe, fabricated from bismuth, situated at the apexof a silicon attractive-mode probe. Such probes are expectedto have advantages over the more commonly used Hallprobes, which are fabricated on flat, solid substrates, sincethe former have better access and are also able to operate ina controlled fashion in close proximity to the specimen usingforce feedback. Functional Hall probe sensors have been fabricateddown to a 100 nm junction size Fig. 6a, and litho-J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999