Inﬂuence of metallic catalyst and doping level on the metal assisted chemical etching of silicon A. Backes ⁎, A. Bittner, M. Leitgeb, U. Schmid Institute of Sensor and Actuator Systems, Vienna University of Technology, Floragasse 7, 1040 Vienna, Austria abstract article info Article history: Received 24 August 2015 Received in revised form 8 November 2015 Accepted 10 November 2015 Available online xxxx The presented work shows a study of the boundary condition between metal and silicon, in metal assisted chem- ical etching. This is achieved by varying silicon doping type and concentration as well as metal type and oxidation agent concentration. First, the etch rate dependence of silver particles, on n- and on p-doped samples is investi- gated revealing different etch rates depending on doping concentration. Additional experiments using an etch so- lution containing no oxidation agent show an impact of the metal–semiconductor combination on the etch process. In this case the higher work function of Pt particles compared to Ag leads to an etching independent of silicon doping. © 2015 Published by Elsevier Ltd. Keywords: Metal assisted chemical etching Silver particles Platinum particles Etch rate Doping concentration In the widely accepted model for metal assisted chemical (MAC) etching described by Chartier et al. [1], a positive charge is injected via the metal (catalyst)–semiconductor contact into the silicon surface. To understand the inﬂuence of different metals, the Fermi level E f of the semiconductor has often been compared with the redox potentials of the etch solution [2,3]. Within this work, we vary systematically the metal–semiconductor junction properties in order to clarify the impact on the MAC etching performance. Typical metals used for MAC offer work functions φ m towards vacuum that are either located in the band diagram above the conduction band edge of silicon like Pt, Pd and Au or between the valence and the conduction band edge like Ag [4–8]. The difference Δ if between the metal ionization Energy φ m and the Fermi level E f varies for a given metal–semiconductor combination, if the position of E f is changed. For Pt–Si, only negative values are possi- ble while for Ag–Si the contact potential (CP) can be either positive or negative, as E f changes with doping concentration and temperature [9]. For the combination of intrinsic silicon and silver at 300 K, the value of Δ if is about +0.3°eV. This value increases with p-doping, as the Fermi level is lowered, and decreases with n-doping, turning even into negative values. Therefore, the accumulated charges at the inter- face are either positive or negative depending on the pre-sign of Δ if . As a consequence, locally different space-charge regions build up at the silicon surface with different boundary conditions, since the redox potential of the etch solution presents a condition different from the metal covered regions. To investigate the inﬂuence of the Schottky con- tact on the etching it is desirable to limit the overall reaction by the charge transition between catalyst and silicon. For this reason an etch- ant with a low H 2 O 2 concentration was chosen governing the etch rate by the proton supply, which originates from the H 2 O 2 decay [1]. Those etchants produce straight pores penetrating perpendicular into the silicon surface with high aspect ratios [10]. Additionally experi- ments without H 2 O 2 were carried out to eliminate the inﬂuence of the additional proton source to reveal the inﬂuence of the contact potential. Furthermore, a clean semiconductor surface is important to exclude parasitic effects on the barrier height, like residual oxides or any other contaminations [9]. The most common deposition techniques for the catalyst are sputtering [1] and evaporation [11], particle deposition through metal salt reduction [12] or sedimentation of dispensed parti- cles [5], respectively. In this study, sputter deposition is applied follow- ed by a thermal dewetting step. This is for two reasons: Sputtering offers the possibility for an in-situ cleaning of the substrate surface and ther- mal dewetting allows creating particles on the substrate without any additional chemical treatment. For this study, eleven single-side polished, (100)-oriented silicon wafers were used having different properties. Five of these samples were boron doped with a concentration ranging from 10 12 to 10 19 cm -3 . The six n-doped samples offer a phosphorous concentration between 10 13 and 10 19 cm -3 . Table 1 gives a detailed summary of the sample details as provided by the silicon manufacturer. The Fermi level E f , depending on doping concentration N D (N A ) and type, was cal- culated using a full ionization approximation. The corresponding values for the intrinsic carrier concentration n i and intrinsic Fermi level E i were taken from [9]. Prior to further processing the samples were cleaved into pieces of 10 × 10°mm 2 and rinsed with acetone and 2-propanol, followed by Scripta Materialia 114 (2016) 27–30 ⁎ Corresponding author. http://dx.doi.org/10.1016/j.scriptamat.2015.11.014 1359-6462/© 2015 Published by Elsevier Ltd. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat