Ž . Materials Science and Engineering C 15 2001 109–112 www.elsevier.comrlocatermsec Novel porous silicon formation technology using internal current generation A. Splinter ) , J. Sturmann, W. Benecke ¨ ( ) Institute for Microsensors, -actuators, and -systems IMSAS , UniÕersity of Bremen, P.O. Box 330440, D-28334 Bremen, Germany Abstract A novel porous silicon formation technique that combines the advantages of thick layer anodization and electroless stain etch will be shown. A current generated by a galvanic element of silicon and a precious metal on the backside of a silicon wafer in a hydrofluoric acid Ž . Ž . HF rhydrogen peroxide H O rethanol electrolyte is utilised to generate porous silicon. In this case, the silicon operates as anode and 2 2 the metal as cathode for current generation. This current is similar to the external current needed for anodization. Besides the standard Ž . Ž . porous silicon etch solution HF for electrochemical silicon dissolution and ethanol to reduce surface tension , an oxidizing agent, H O , is used to support the etch process and to generate a higher etch rate. Different kinds of metallization and etching solutions were 2 2 investigated. This innovative technology enables to generate porous silicon layers of 10 mm without an external current. The porous structure achieved with this new technology is comparable with pores generated with anodization. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Electrochemical cell; Internal current 1. Introduction Presently, two porous silicon formation technologies are published: the anodization into an electrochemical cell w x wx 1,2 , and stain etch 3 without external current into an Ž . hydrofluoric acidrnitride acid HF–HNO solution. For 3 anodization, an external current is necessary to achieve porous silicon thickness of up to 100 mm. Stain etch is an electroless process, and the porous layer thickness is lim- ited to a few micrometers. For both technologies, the general pore formation mechanism and the resulting pore configurations are the same: dependence on the doping- type, p- or n-type and doping level, and influence of HF concentration. The resulting pores are in the range of 1 to Ž . Ž . 4 micropores , 4 to 50 mesopores or 50 nm, up to the Ž . micrometer range makropores . A novel porous silicon formation technique that com- bines the advantages of thick layer anodization and electro- less stain etch, which is based on the same pore formation mechanism, will be shown. ) Corresponding author. Tel.: q 49-421-218-2846; fax: q 49-421-218- 4774. Ž . E-mail address: splinter@imsas.uni-bremen.de A. Splinter . 2. Theoretical background 2.1. GalÕanic element The basic idea for a new porous silicon fabrication technology is a galvanic element for current generation. A galvanic element is formed into a conductive elec- trolyte by a junction of a silicon substrate and a precious Ž metal layer on top of the wafer especially gold or plat- . wx inum 4. Silicon has a lower standard potential in comparison with precious metal. This potential difference induces a current into the silicon. This internally generated current has the same dimen- sions as an externally applied current used by porous Ž . silicon anodization Fig. 1 . Thereby, silicon has the negative potential and dissolves with an electrochemical reaction into an HF-based elec- trolyte. 2.2. NoÕel porous silicon fabrication technology A galvanic element of a silicon wafer with a precious metal layer, which generates an internal current as shown above, is used for porous silicon formation. 0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0928-4931 01 00263-6