The study of electrochemicl porosification of silicon has allowed the production of different kinds of porosities in it, ranging from micropores to macropores, and directed by cristallographic planes, or electrical current lines. Additional studies have been on the use of different kinds of electrolytes, to etch faster, or with lower branching. Pore modulation has been also an important development, for a range of applications from photonics to energy storage devices. The modulation can be of either different porosities or different pore diameters, and has been accomplished by the use of different current densities [1], or by the use of different light intensities (mainly for n-type Si) [2].

In particular, the use of light allows the production of electron-hole pairs in semiconductors. This is necessary in n-type Si, since electronic holes are part of the etching reaction, and they are scarce in n-type wafers. Having the availability of more of fewer electronic holes, one can etch wider or narrower pores respectively [1].

Metal Assisted Chemical Etching (MACE) is an alternative etching method, which does not use a power source (it is an electroless method). Nobel metal (commonly Au, Ag or Pt) particles or films are deposited on the surface of Si, and the metallized samples are immersed in an etching solution afterwards. The regions with metal are commonly etched much faster, thus they work as a catalyst [3]. At the metal-semiconductor interface, space-charge-region (SCR) is formed, accelerating electronic holes formed at the electrolyte-metal interface. In this way, there is a higher availability of holes for etching in (or close to) the regions where the metal is present.

Having an internal electric field (given by the SCR) in MACE, which is external in the case of electrochemical etching, it is strightforward to think that light may work in a similar way in both etching methods. However, there are just a few reports on the use of light for MACE, and it has been used just at the front-side (the etched face). There are no reports on the use of back-side illumination for MACE. In the present work, evidences of pore modulation or etching conditions through the use of alternating illumination at the back-side will be presented. Illumination induces either lateral or vertical etching, depending on the doping type of Si.

[1] E. Quiroga-González, J. Carstensen, C. Glynn, C. O’Dwyer, H. Föll, Phys.Chem.Chem.Phys., 16, 255 (2014).

[2] S. Matthias, F. Müller, J. Schilling, U. Gösele, Appl. Phys. A, 80(7), 1391 (2005).

[3] X. Li and P.W. Bohn, Appl. Phys. Lett.77, 2572 (2000).