Metal electrode integration on macroporous silicon: pore distribution and morphology
© Scheen et al.; licensee Springer. 2012
Received: 23 April 2012
Accepted: 16 July 2012
Published: 16 July 2012
In this work, a new approach for the one-step integration of interdigitated electrodes on macroporous silicon substrates is presented. Titanium/gold interdigitated electrodes are used to pattern p-type silicon substrates prior the anodization in an organic electrolyte. The electrolyte characteristics, conductivity, and pH have been found to affect the adherence of the metal layer on the silicon surface during the electrochemical etching. The impact of the metal pattern on size distribution and morphology of the resulting macroporous silicon layer is analyzed. A formation mechanism supported by finite element simulation is proposed.
Silicon anodization in hydrofluoric acid (HF)-based organic electrolyte is a simple and effective technique to obtain macroporous silicon (MPS) on p-type substrates. Macroporous silicon has been demonstrated as an interesting material for a number of applications such as silicon micromachining [1, 2], photonic , and sensing [4, 5]. The integration of metal electrodes on macroporous silicon surface is of paramount importance in case of sensing applications . In silicon microtechnologies, metal electrodes are usually obtained by patterning metal layers evaporated or deposited on the substrate surface. Due to the high macroporous silicon surface roughness, the patterning of electrodes with dimensions below 5 μ m is a challenge. An alternative approach is to pattern the metal electrodes on the silicon surface prior the electrochemical etching. In this case, the presence of the metal on the surface affects pore distribution and morphology. In this work, the macroporous silicon characteristics are investigated as a function of the geometry (width and pitch) of metal pattern and modeled using finite element simulations.
Metal interdigitated electrodes were patterned on the surface of p-type silicon substrates (10 to 20 Ω·cm, (100) orientation) using standard lift-off. The electrodes consist of 1400-μ m long Au ribbons (100-nm thick). Au was chosen because it is HF resistant. A 5-nm thick Ti layer was used to promote the Au surface adherence. Ti was chosen instead of Cr because the combination of Cr and Au forms an electrochemical local element, giving rise to the metal layer detachment after few minutes of electrochemical etching . The metal was evaporated all over the wafer using a dual e-beam evaporator (e-gun Balzers, Liechtenstein). The nondesired metal layers on the wafer were removed by dissolving the prepatterned underlying resist in acetone. Three different geometries (widths and pitches, WP (μ m)) were tested on the same sample: WP1 = 2-2, WP2 = 5-2, and WP3 = 2-5. The prepared samples were then electrochemically etched to form the macroporous silicon layer. All the experiments were carried out at room temperature, and the solution was stirred to avoid inhomogeneities due to the gas accumulation on the surface. A platinum grid was used as the cathode, while the anode was the silicon substrate. A 200-nm thick aluminum layer was evaporated on the wafer back side, and an electrical contact was made by placing an aluminum plate in contact with it. A mixture of HF and N,N-dimethylformamide (DMF) (HF (48%):DMF = 1:4) was used as electrolyte. DMF is not stable in presence of acids like HF and is, thus, partly hydrolyzed back into formic acid and dimethylamine . The reaction is exothermic and gives rise to the increase of the solution temperature. Solution pH and conductivity were measured after the cooling of the solution at room temperature. A pH of 4 and a conductivity of 2 mS/m were measured. The hydrolysis of the DMF is not limited to the preparation moment. As a consequence, the properties of the solution were observed to change to reach a threshold value. For a 20-day-old solution stored in a closed bottle, it was measured with a pH of 6 and a conductivity of 30 mS/m, indicating that a bigger fraction of the DMF was hydrolyzed. Both as-prepared and 20-day-old solutions were used as electrolytes.
The enhanced conductivity of the old solution allows to use higher current densities for lower applied voltage. In both cases, the macroporous silicon layers were formed, keeping constant the etching current density Jetch at a value lower than the critical current density Jps=182 mA/cm2 at which the silicon surface electropolishing is produced .
Results and discussion
The different characteristics of the electrolyte (pH =6, conductivity = 30 mS/m) affected the etching parameters as well as the morphology of the resulting macroporous layer. A lower etching voltage, Vetch≈1.5 V, was necessary to sustain Jetch = 91 mA/cm2. Even in this case, a slow increase of the etching voltage as a function of the etching time was observed. A different etching profile was observed in close proximity of the masking layer. The metal layer was sufficiently adherent to the silicon substrate to sustain the etching process and the sample cutting for analysis.
A new approach for the integration of metal electrodes on macroporous silicon was shown. The impact of etching parameters as electrolyte characteristics and etching current on the formation of silicon macropores on Ti/Au prepatterned substrate was analyzed. It was shown that the electrolyte consisting of an as-prepared HF/DMF mixture induces catalytic phenomenon at the silicon-metal interface and the consequent metal underetching and detachment. On the other hand, electrolytes consisting in 20-day-old HF/DMF mixtures, in which the DMF hydrolization gives rise to increased conductivity and pH, allow the integration of metal interdigitated electrodes on macroporous silicon. In both cases, the pore distribution and morphology are affected by the presence of the metal pattern. Pores grow faster in close proximity of the metal strip edges, leading to the formation of walls in the middle of the uncovered area for small pitches. The experimental results were in agreement with the COMSOL simulation results that showed a current density distribution influenced by the enhanced electric field at the metal edges.
Future work will be devoted to study the influence of the electrolyte composition, surface state, and metallic materials on the catalytic behavior of the metal layer, with the important consequence on the adhesion of the metal layer at the silicon surface.
GS is a PhD student of the Université Catholique de Louvain (UCL). MB is post doctorate research assistant at the UCL. LAF is an associate professor at the UCL. All authors are members of the group Sensors, Microsystems and Actuators Laboratory of Louvain (SMALL) chaired by LAF.
This work is supported by the GaSePoC project funded by INNOV’Iris and by the MINATIS project co-funded by the European Regional Development Fund (ERDF) and the Walloon region of Belgium. GS acknowledges the F.R.S.-FNRS (Belgium) for financial support.
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