Defect-Free AFM Scratching at the SiÕSiO 2 Interface Used for Selective Electrodeposition of Nanowires J. Michler, a R. Gassilloud, a,b Ph. Gasser, a L. Santinacci, b, * ,c and P. Schmuki b, ** ,z a Materials Measurement Research Group, Swiss Federal Laboratory EMPA, CH-3602Thun, Switzerland b Departement of Materials Science, University of Erlangen-Nuremberg, D-91058 Erlangen, Germany We demonstrate selective electrodeposition of Pd into atomic force microscopy AFM nanoscratches produced in thermal oxide covered p-type Si100 without creating substantial damage in the silicon substrate. A 10 nm thick thermal SiO 2 film was scratched about 5-7 nm deep with a diamond tip of an AFM. Then, etching in HF was used to remove uniformly 4-5 nm SiO 2 , thus to expose the Si within the nanoscratches while still maintaining an oxide layer on the rest of the surface. Pd was selectively electrodeposited into these scratches. The underlying silicon exhibits no significant damage induced by scratching. © 2004 The Electrochemical Society. DOI: 10.1149/1.1643791 All rights reserved. Manuscript submitted July 28, 2003; revised manuscript received August 27, 2003. Available electronically January 22, 2004. Approaches to produce nanostructures have become increasingly important over the past few years. Many techniques are based on scanning probe microscopies such as scanning tunneling microscopy STM and atomic force microscopy AFM. Local oxidation of silicon surfaces by STM or AFM has been used for surface pattern- ing in the sub-100 nm range. 1-3 AFM can also be used to nanoma- chine surfaces in the micro and nanoscale, e.g., in scratching and wear experiments. 4 This type of study showed that it is possible to produce nanosize grooves on silicon surfaces using an AFM equipped with a single-crystalline diamond tip mounted on a stiff cantilever spring constant  10 N/m). Recently it has also been demonstrated that it is possible to exploit AFM-induced nano- scratches produced through the native oxide layer on silicon for selective electrochemical copper deposition. 5 Also thicker 10 nm oxide films can serve as an efficient mask, i.e., may be used to suppress electrodeposition outside the scratched locations. The latter approach has been used to achieve selective electrochemical depo- sition of copper and palladium on oxide-covered silicon surfaces with a submicrometer lateral resolution. 6 Most recently, organic monolayers were used as an AFM scratch resist for Cu immersion plating on Si. 7 However, one common drawback of all these scratch- ing approaches is that deformation of the single-crystalline substrate takes place. Depending on the scratching parameters, different crys- tal planes are exposed and defects such as dislocations can be intro- duced leading to an undesired undefined structure of the metal/ silicon interface. In this work the goal was to avoid any structural deformation of the silicon surface by the following measures: ( i ) A two-step ap- proach was used to define the pattern in the SiO 2 . In the first step, the oxide was only partially removed by scratching. In the second step, the remaining oxide layer inside the scratches was removed by a timed HF etch leaving oxide coverage on the rest of the surface. ( ii ) The loads on the tips used for scratching the oxide were con- trolled and kept below the dislocations Tresca shear stress and phases transformation hydrostatic pressure threshold of silicon. All experiments were carried out on silicon 100 wafers p-type:  = 1-10  cm) that carried a 10 nm thick thermal oxide layer. Samples were prepared by cleaving pieces of 1.2  1.2 cm. These pieces were degreased by sonicating in acetone, isopropanol, metha- nol, then rinsed with deionized water and blown dry in an Ar jet. AFM scratching and imaging were performed with a PicoSPM mi- croscope from Molecular Imaging driven by a Nanoscope E control- ler from Digital Instruments. The tip was a single-crystal diamond tip mounted on a stainless steel cantilever spring constant = 250 N/m). For scratching a force of 50 N was employed and for imaging a force of 200 nN was used. Scratches were made by scanning the sample in the x or y direction at 40 m/s for a deter- mined number of cycles ( N ) to create a measurable wear trace on the surface. After scratching, about 3-5 nm of oxide layer was re- moved by dipping the samples in 1% HF for 30 s. This step is designed to remove the remaining oxide in the scratch but leave sufficient oxide on the rest of the surface to act as a resist. Addition- ally, the exposed area in the grooves is H-terminated and thus pas- sivated. The electric contact to the Si electrode was established by smear- ing InGa eutectic on the back side of the samples. The samples were then pressed against an O-ring of the electrochemical cell leaving 0.785 cm 2 exposed to the electrolyte. For Pd deposition the electro- lyte consisted of 0.01 g/L PdCl 2 + 0.1 M HCl and was prepared from analytical grade chemicals and deionized water. Electrodepo- sition was carried out with a potentiostat Jaissle 1002 T-NC and a conventional three-electrode setup using a Pt mesh as a counter electrode and a Ag/AgCl electrode ( E = 236.3 mV vs. a saturated calomel electrode, SHE as a reference electrode. For deposition, the samples were polarized first during 5 s at -800 mV vs. Ag/AgCl and then during 30 s at -400 mV vs. Ag/AgCl. Cross-sectional transmission electron microscopy XTEM samples were prepared using a dual-beam focused ion beam FIB FEI Strata DB 235 installation. To protect the surface of the samples during the ion milling process, a 150 nm thick layer of platinum was deposited over the surface by electron beam induced deposition within the FIB instrument, followed by ion beam deposition of 1.5 m platinum as top layer. Scanning electron microscopy SEM micrographs were acquired with the Strata DB 235 that was equipped with a field emission gun. A Philips CM30 transmission electron microscope operated at an acceleration voltage at 200 kV was used to study the microstructure. Figure 1 shows a typical AFM profile of scars produced by AFM scratching. The width and depth of the scratches are approximately 300 and 5-7.5 nm, respectively. The open angle of the V-shape scratch corresponds to about 175°. As the thickness of the oxide layer originally was 10 nm, scratching did not remove the complete oxide layer. By a subsequent HF etch of 30 s, about 3-5 nm SiO 2 were removed. This led to an oxide-free and H-terminated Si surface within the scratches, whereas on the rest of the surface approxi- mately 5 nm of oxide was left. The etch rate was determined from a series of calibration experiments using the same thermal SiO 2 layer, etching in 1% HF and determining the thickness as a function of etch time from Auger electron spectroscopy AES sputter depth profiles. In Fig. 2 an SEM image after electrodeposition of Pd on the nanoscratched and etched surface is shown. Clearly, deposition of Pd takes place with a high selectivity within the scratches. No Pd crystallites are present outside the scratched area. However, the * Electrochemical Society Student Member. ** Electrochemical Society Active Member. c Present address: MADIREL Laboratory, University of Provence, CNRS, Marseilles, France. z E-mail: schmuki@ww.uni-erlangen.de Electrochemical and Solid-State Letters, 7 3 A41-A43 2004 0013-4651/2004/73/A41/3/$7.00 © The Electrochemical Society, Inc. A41