Electrochemical Etching of n-Type Silicon in Fluoride Solutions

Abstract

The interaction between hydrogen-passivated silicon surfaces and fluoride solutions is complex and leads to a wide range of surface morphologies, ranging from atomically flat Si(111):H(1 ϫ 1) surfaces to porous layers. The hydrogen-passivated surface is stable in fluoride solutions or strong base and the morphology of the surface is dependent on pH and solution chemistry. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Under open-circuit conditions, etching of the silicon surface occurs through both chemical and electrochemical processes at a rate that increases with increasing pH. At low pH, the etch rate is negligible and the morphology of the surface is unchanged. In basic fluoride solutions at about pH 7-8, the etching process is anisotropic and results in the formation of atomically flat Si(111):H(1 ϫ 1) terminated surfaces. 2,3,[9][10][11][12][13][14]16,19 While considerable progress has been made in understanding the evolution of surface morphology, the etching mechanism remains poorly understood. Experimentally it is known that the silicon surface remains hydrogen terminated during etching and it is generally assumed that etching proceeds through sequential reaction of Si-H groups with F Ϫ or OH Ϫ to form Si-X. Subsequent steps involve chemical reaction of the Si-Si backbonds with H 2 O or HF. However, this model is inconsistent with the complicated relationship between the anodic etching current and pH. 21,22 In this paper we show that the pH dependence of the etching current and the density of intermediates can only be explained by considering all species present in solution.Experimental All experiments were performed on n-type Si(111) (100 ⍀ cm, N D ϭ 4 ϫ 10 14 cm Ϫ3 ) wafers (Wacker Siltronic). Prior to each experiment wafers were RCA cleaned, 23,24 and then rinsed with acetone, ethanol, and deionized water. The native oxide was removed by immersing in concentrated HF for 1 min followed by a final rinse in deionized water before immersing in the etching solution.Solutions were prepared from NH 4 F with the addition of NH 4 OH or HF to adjust the pH. The fluoride ion concentration was varied from 10 Ϫ3 to 10 M, the HF concentration varied from 10 Ϫ7 to 1 M, and the pH range was 2-10. The concentrations of the relevant species in solution were determined from the equilibrium constants 25,26 and by use of a fluoride-ion-specific electrode.Results and Discussion Figure 1 shows a typical cyclic voltammogram for n-Si(111) in 1 M NH 4 F at pH 6.8 at a scan rate of 50 mV s Ϫ1 . All voltammograms in the pH range 2-10 exhibited the same features. From the open-circuit potential (OCP), the current increases and exhibits a maximum before decreasing to a value that is essentially independent of potential. On the reverse scan the current density remains potential independent to potentials close to the open-circuit value. In most cases the potential at which the current density decreased to zero corresponded to the OCP. Subsequent scans exhibited the same features, although at higher pH (>9) ...

The mechanism of Si etching in fluoride solutions

“…19 Species such as (HF) 2 and HF 2 À have been implicated. [101][102][103][104][105] However, when kinetics studies are performed, it is important that the proper equilibrium constants are used to calculate solution compositions. 88,106,107 In particular, consensus now maintains that (HF) 2 is not formed in aqueous solutions of HF and, consequently, any mechanism relying upon this species is incorrect.…”

Open-Circuit Study of Stain Etching Processes Leading to the Formation of Porous Silicon Layers

“…23 The reaction rate is increased with the presence at those surface sites of holes reaching the surface from the silicon bulk driven by the inverse band bending existing under accumulation layer conditions. The etching rate must be a function of the surface density of sites weakened by the existence of broken bonds or holes.…”

“…Once the holes are created by the presence of nitrate ions the silicon dissolution mechanism follows through a number of steps, which have been already proposed and discussed. [18][19][20]23,[30][31][32] In step 2 the injected holes are driven by the field acting in the space charge layer, V scl , toward the surface at a rate given by Eq. 4.…”

“…It is well established that the silicon surface remains covered with hydrogen during the etching process and that the corrosion takes place through attack of the Si-H bonds by species like F Ϫ , OH Ϫ , HF, HF 2 Ϫ or even ͑HF͒ 2 according to the pH of the attacking solution. [21][22][23] The object of this work is to contribute to the knowledge of the growth mechanism of the porous layer by using the results of the measurements of the rest potentials, E Iϭ0 ͑i.e., the potential under zero current conditions͒, as a function of the concentration of the oxidizing agent. An increase in the concentration of the oxidizing agent, NO 3 Ϫ ions, gives rise to a shift in the positive direction of the open-circuit potential (E Iϭ0 ).…”

The Role of Band Bending in Affecting the Surface Recombination Velocities for Si(111) in Contact with Aqueous Acidic Electrolytes

“…If they do exist at open circuit, the low S values measured for Si in contact with these solutions are therefore dominated by the effects of a large, persistent value of. Trap-state densities between 10 10 −10 12 cm -2 are present on the surface of Si/1 M NH 4 F(aq) contacts adjusted to have a pH in the range between 3 and 11 . Assuming k n ,s = k p ,s = 10 -8 cm 3 s -1 implies that S values should be between 100 and 10 000 cm s -1 .…”