Atomic Force Microscopy-Induced Nanopatterning of Si„100… Surfaces L. Santinacci,* ,a T. Djenizian,* and P. Schmuki** Department of Materials Science, University of Erlangen-Nuremberg, Chair for Surface Science and Corrosion (LKO), D-91058 Erlangen, Germany In this study, we investigate the possibilities of selectively electrodepositing Cu on surface defects created in p-type and n-type Si100 by scratching the surface with the tip of an atomic force microscope AFM. Nanosized grooves were produced on Si surfaces with a diamond-coated AFM tip at heavy forces. Cu was electrodeposited on these grooved surfaces from a 0.01 M CuSO 4 + 0.05 M H 2 SO 4 electrolyte under various conditions. The results clearly show that defects created on H-terminated p-type Si100 lead to an enhanced reactivity, i.e., preferential Cu deposition at such defects is possible. However, a much higher degree of selectivity of the deposition is obtained if AFM-induced grooves are produced on surfaces that carry a native oxide layer. The masking effect of this insulator film is demonstrated by selective Cu electrodeposition into scratches on oxide-covered p- and n-type silicon. After an optimization of electrochemical parameters, we achieved the deposition of uniform and well-defined nanostructures. The process presented here opens new perspectives for selective electrodeposition and direct patterning of Si surfaces. © 2001 The Electrochemical Society. DOI: 10.1149/1.1389341 All rights reserved. Manuscript submitted January 21, 2001; revised manuscript received April 20, 2001. Available electronically August 10, 2001. In recent years, copper electrodeposition has become of great interest to the microelectronics and microsensor industries. Copper layers are used as electrical interconnections ranging from several micrometers down to a few nanometers thick. 1 More recently, cop- per deposition on insulating interlayers has been introduced as a replacement for aluminum in ultralarge-scale integration metalliza- tion for improved interconnects. 2-8 But direct metal deposition on silicon is also of interest to electronic device technology, for ex- ample, to build Schottky diodes on semiconductor surfaces. 9-11 Patterned metal deposition on semiconductor surfaces is typi- cally carried out using different indirect methods such as photoli- thography combined with metal evaporation, electrodeposition, or molecular beam epitaxy. In contrast to indirect patterning tech- niques, which demand masking, direct patterning approaches have been far less explored. Recently, it was demonstrated that porous Si growth can be electrochemically initiated preferentially at surface defects created in an n-type semiconductor substrate by Si 2+ focused ion beam FIB bombardment. 12,13 Selective copper deposition has also been performed on a p/n junction showing preferential deposi- tion at the p/n-border region, 14,15 which has been ascribed to differ- ences in the reactivity due to the doping type and concentration. 15 More recently, it was shown that after FIB defect writing in p-type Si, selective metal electrodeposition in the defective regions is possible. 16 In this case, the introduction of defects on the semicon- ductor surface using an FIB allows one to realize patterning in the submicrometer range. Other nanopatterning approaches are based on scanning probe microscopy atomic force microscope, AFM; scan- ning tunnel microscope, STM approaches. These tools have been widely used for surface imaging with atomic resolution see, e.g., Ref. 17 and 18. But AFM can also be used to investigate the me- chanical properties of surfaces, or to nanomachine surfaces in the micro- and nanoscale, e.g., in scratching and wear. 19 It has been demonstrated that it is possible to obtain nanoscratches on a silicon surface using an AFM equipped with a monocrystalline diamond tip. 20 Other works have investigated in situ modification of surfaces by in situ STM. The process involves metal deposition from the solution on the tip and subsequent transfer of metal from the tip to the substrate by an appropriate tip approach. 21-23 The technique was used first on single crystalline metal substrates and then on semicon- ductor surfaces. 24 Furthermore, Mu ¨ller et al. 25 reported that terminal azide groups of a self-assembled monolayer on a glass slide surface were converted to amino groups in hydrogen-saturated isopropyl alcohol only in the regions scanned with a Pt-coated AFM tip. Other work has shown that electrochemical dissolution of a p-GaAs100 electrode in H 2 SO 4 solution is accelerated by scanning with an AFM tip. 26 This in situ AFM study has also shown that electrodeposition of Cu on the p-GaAs surface selectively occurs at the modified sites. The aim of the present study is to follow up on the above- mentioned FIB work, i.e., to investigate the possibility of selectively electrodepositing metals on surface defects on semiconductors. However, in this work surface defects are introduced by AFM. Nanosized scratches are produced by scanning a Si surface with diamond-coated AFM tip at heavy forces. The reactivity of these mechanical defects is then studied to explore the possibility of se- lective copper electrodeposition in the defective regions of the sur- face. Experimental Experiments were carried out on silicon 100 wafers p-type: 1 to 10  cm; n-type: 2 to 6  cm. Prior to the experiments, a 250 nm layer of SiO 2 was grown on these wafers. Into this oxide film, arrays of square openings with a size of 200  200 m were etched using classical photolithography. This prepatterning was carried out as an aid for locating the nanostructures subsequently deposited in the openings. The patterned wafers were cleaved to samples of 0.7  0.7 cm, and the pieces were degreased by subsequently soni- cating in acetone, isopropanol, and methanol and then rinsing with distilled water. If not noted otherwise, prior to the experiments, the samples were dipped in 1% HF for 1 min to remove the air-formed native oxide layer. After an AFM treatment as described below, the sample was dipped again in 1% HF for 1 min and rinsed in distilled water. The back contact to the Si electrodes was established by smearing InGa eutectic on the back side of the sample. The samples were then pressed against an O-ring of the electrochemical cell leav- ing 0.113 cm 2 exposed to the electrolyte, which consisted, for cop- per deposition, of 0.01 M CuSO 4 + 0.05 M H 2 SO 4 . The electrolyte was open to air prior to and during the measurements. All solutions were prepared from analytical grade chemicals and deionized water. The electrochemical cell was placed in a black box to avoid uncon- trolled photoelectrochemical effects. AFM scratching and imaging were carried out with an Auto Probe CP from Park Scientific Instruments. Because of their high hardness, diamond-coated tips provided by Park Scientific Instru- ments were used for these experiments. Undoped silicon 111 shows a plastic deformation at 20 N. 20 To apply sufficient loads for deformation, the cantilever force constant had to be high 17 N/m. AFM imaging was carried out in contact mode at a constant force of 100 nN. The images were treated using the manufacturer’s software * Electrochemical Society Student Member. ** Electrochemical Society Active Member. a On leave from: Department of Materials Science, Swiss Federal Institute of Tech- nology Lausanne EPFL, CH-1015 Lausanne, Switzerland. Journal of The Electrochemical Society, 148 9 C640-C646 2001 0013-4651/2001/1489/C640/7/$7.00 © The Electrochemical Society, Inc. C640