404 Prakt. Met. Sonderband 38 (2006) 
2. EXPERIMENTAL 
For the application of SPM techniques like SNOM (scanning near-field optical microscopy) 
or TERS (tip enhanced Raman spectroscopy) in nano-optical device inspection, metallized 
tips with small tip radius (< 100 nm) are needed to achieve high field enhancement effects 
[3, 4, 6]. In this chapter, deposition, tailoring of tip shape using FIB milling and a new tip 
characterization technique are described. 
2.1 TIP PREPARATION APPLYING WET CHEMICAL ETCHING 
To obtain SPM tips with small radii of curvature as base material for dedicated optically 
active tips, tungsten wires were etched electro-chemically as represented in principle in 
Fig. 1a. This technique is able to provide tips with high quality in a reproducible way. The 
commonly used configuration for tip etching [7] was slightly modified to adapt the 
technique to the requirement for optical applications. A diluted KOH solution (3 mol/lit: 56 g 
KOH in 333 ml water) and an applied voltage between 1.8 VV to 2.1 V were chosen for the 
tungsten tip etching [8]. To optimize the preparation procedure, the tungsten wires were 
bent about 200 um below the ring electrode before etching. This approach enables to 
mount the lower part of the wire after etching directly to a tuning fork (Fig. 1b). 
The tips are mounted to the tuning fork in such a way that the total mass of tip and glue 
added onto the tuning fork is minimized, in order to reduce the shift of the tuning fork’s I 
resonance frequency and, consequently, to maintain a high sensitivity during feedback in ft 
SPM applications. S 
{ah = 
; _ - lomella 
0.125 mm wire © 
| 
m =D 
8.4 electrode 
Fig. 1: Principle of tungsten tip fabrication using a silver ring with about 7 mm diameter as 
cathode, and a tungsten wire 0.125 mm diameter as anode (a, left; modified after [71). 
SEM micrograph of a tuning fork with mounted tip for nano-optical applications (b, right). 
We used Epo Tek (batch D3926) Epoxy cured with UV light (20 sec) for gluing the tip onto 
the tuning fork. This glue technique enables a stable mounting of the tips on the tuning 
fork, and to remove non-optimized tips reversibly from the tuning fork without damaging Ir 
the tips. Fig. 2a shows a chemically etched W tip with a tip diameter of about 13 nm. v 
After carefully covering the tuning fork with a metal, the tips were sputter-coated by a thin t 
metal film (Ag or Au) as a prerequisite to obtain the near-field enhancement effect e. g. for ) 
TERS experiments. This coating results in an increase of the tip diameter (Fig. 2b).