Power supply selection for electrochemical porous silicon formation

By Vedran Đerek and Pawel Jerzy Wojcik

1. Setup description
Porous silicon is electrochemically produced by applying anodic voltage across the top and bottom of a silicon wafer in the electrolyte containing fluorine anions. A typical two-electrode setup for electrochemical porous silicon formation consists of: silicon wafer (working electrode - anode), platinum electrode (counter electrode -cathode), etching solution (HF based electrolyte), etch cell and power supply. A three-electrode setup is additionally equipped with platinum wire as pseudo-reference electrode. The solvent added to the HF mixture helps in removing bubbles during the etching process, creating a stable contact between the electrolyte and Si and the electrolyte and the counter electrode. During the process the Si is carried off creating H₂ gas and leaving a pore. The pore diameter is determined by applied current density, HF concentration, and Si resistivity [1]. redox.me offers several etch cells differing in the nominal exposure surface area and the option of exposure of the sample to the solution from one and both sides (see Figure 1).

Figure 1. Etch cells available at redox.me: The Small Etch Cell, The Double-Tank Etch Cell, The Standard Etch Cell and The Large Etch Cell

2. Power supply
The most commonly used power sources for anodization of silicon are the Keithley sourcemeters (see Figure 2). Both Keithley 2450 and 2460 sourcemeters are suitable for research on electrochemical porous silicon formation. However, the right choice depends on the currents used in the system.

Most of preparation conditions for porous silicon formation require current densities of up to 100 mA/cm², but there are some applications where currents up to 500 mA/cm² may be needed. Therefore, a nominal exposure area of the cell should be considered as a main parameter when determining a maximum output current needed in the experiment. redox.me offers the etch cells with following nominal exposure surface areas:

• The Small Etch Cell – 0.21 cm²
• The Double-Tank Etch Cell – 1 cm²
• The Standard Etch Cell – 1.20 cm²
• The Large Etch Cell – 8.6 cm²

These values correspond to maximum current densities according to Table 1.

Table 1. Maximum current densities in redox.me cells for Keithley 2450 and 2460

Etch cell model	Maximum current density [A/cm2]
	Keithley 2450	Keithley 2460
The Small Etch Cell	4.762	33.333
The Double-Tank Etch Cell	1	7
The Standard Etch Cell	0.833	5.833
The Large Etch Cell	0.116	0.814

 

It is obvious at a glance that Keithley 2460 will be excellent for any application, while a 2450 is a cheaper option but still good for most of the applications. 

More affordable alternatives might be older Keithley sourcemeter types 2400 and 2401 (see Figure 3), however in that case it's necessary to equip the system with additional PC to record the I-V scans or the V-t behavior (usually with a LabVIEW program), which is essential in porous silicon research.

Figure 3. Keithley 240x sourcemeter

Most affordable option would be one of the programmable power supplies from Keysight series E36xxxx (see Figure 4), such as the high power E36313A, which can also be controlled by a PC (GPIB/USB). Those power supplies also offer data logging out of the box.

Figure 4. Keysight series E36xxxx Power Supply

Both Keithley and Keysight power supplies provide remote sense option, and thus can also be used in a 3-electrode electrochemical configuration, turning them into simple potentiostats. Usage of much cheaper common lab power supplies is possible but highly discouraged due to the low current stability they offer.

References
[1] Sailor M.J. 2012. Porous Silicon in Practice: Preparation, Characterization and Applications. Wiley-VCH Verlag GmbH & Co. KGaA

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