Effect of Various Copper Salt Precursors on Metal-Assisted Chemical Etching of Silicon

Abstract

Metal-assisted chemical etching (MACE) of Si with unstable metals like Cu can be described with reference to the oxidation effect of aqueous Cu2+ metal cations, in combination with the dissolution of Si species by HF. For 1-step MACE, salt precursors introduce possibly oxidizing additional anionic species, such as NO3− ions from Cu(NO3)2, that cannot be ignored when explaining MACE. This study for the first time unites Cu(NO3)2-MACE with isotropic hydrofluoric/nitric/acetic acid (HNA) Si etching, by purposely avoiding extra oxidants like H2O2, and contrasting the etching characteristics with etchants containing CuSO4, Cu(OAc)2, or CuCl2 precursors. A new model involving Cu2+-promoted autocatalysis of reactive nitrogen species (RNSs) is discussed, which explains the high etch rates, the NO3− anion concentration dependence, and the induction period specific to Cu(NO3)2 etchants. These results highlight the importance of Cu salt precursor selection when rationally designing new Cu-containing wet chemical etchants for key applications like MEMS and black silicon.

The Role of the Molecular Hydrogen Formation in the Process of Metal-Ion Reduction on Multicrystalline Silicon in a Hydrofluoric Acid Matrix

“…The widespread explanation of the molecular hydrogen formation resulted from a divalent reaction process [ 17 , 21 , 34 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ] in combination with the hydrogen-free reaction process during tetravalent valence transfer [ 11 , 12 , 15 , 21 , 34 , 40 , 41 , 44 , 55 ] may still fit, to some extent, based on the findings of example 2 of silver deposition. The results of the other metal depositions explained in Section 3.2 , Section 3.3 and Section 3.4 , especially the findings of platinum deposition, support a different and more complex theory for molecular hydrogen formation.…”

“…The unstable Si 2+ species [ 17 , 34 ] is subsequently converted by an H(+I)-based oxidant to the Si 4+ state (SiF 6 2− ) [ 17 , 34 ] with the formation of H 2 . There are different assumptions as to whether the oxidant is H + [ 36 , 37 , 38 , 39 ], H 2 O [ 34 ], HF [ 17 , 34 , 40 ] or the HF dissociation product HF 2 − [ 21 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ] (Equation (3)): …”

“…According to the most prominent theory, the course of the reaction changes from the divalent to the tetravalent mechanism when a certain current density is exceeded [ 17 , 34 ]. The bulk silicon is oxidized to Si 4+ and, subsequently, converted to the formal end product SiF 6 2− in the presence of F − [ 11 , 12 , 29 , 50 , 51 , 52 , 53 , 54 ], HF [ 15 , 21 , 34 , 41 , 44 , 55 ] or HF 2 − [ 21 , 44 ] with the intermediate formation of SiF 4 [ 34 , 40 ] without the formation of molecular hydrogen (Equation (4)): …”

“…Several series of experiments have been carried out based on HF solutions with Ag + , Cu 2+ , AuCl 4 − or PtCl 6 2− and multicrystalline silicon. The metal ions were chosen because they are commonly used in the metal-assisted etching of silicon [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 26 , 27 , 29 , 30 , 31 , 32 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 63 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , …”

“…Several series of experiments have been carried out based on HF solutions with Ag + , Cu 2+ , AuCl 4 − or PtCl 6 2− and multicrystalline silicon. The metal ions were chosen because they are commonly used in the metal-assisted etching of silicon [7][8][9][10][11][12][13][14][15][16][18][19][20][21][22][23][24]26,27,[29][30][31][32][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]63,[67][68][69][70][71][72][73][74]…”