Chemical Etching in Acidic Solutions

Acidic silicon etchants are mainly used for two purposes for the delineation of crystal defects, as discussed in Section 2.5, or to remove silicon in an isotropic manner. Isotropic etching adds another degree of freedom to the design of micromechanical structures, because all alkaline etches are anisotropic. Most isotropic etchants for silicon were developed in the early days of silicon crystal technology and exhaustive reviews on this topic are available [Tu3, Rul]. A brief summary is given below. 

Silicon is stable in acidic solutions that do not contain fluoride because the silicon surface is passivated by a native oxide. If only H F is present in an aqueous solution the etch rate remains low, showing values below 0.1 rim miri 1 on single crystalline silicon depending on the OFT concentration [Hu2]. This low etch rate 

The etch rate is further increased if H202 is added to the solution, as shown in Fig. 2.5 b. At such low rates the reaction is controlled by the kinetics of the reaction at the interface and not by diffusion in the solution. This etching solution is therefore found to be perfect to remove micro- and mesoporous silicon selectively from a bulk silicon substrate or to increase the diameter of meso- or macropores in an well-controlled, isotropic manner [Sa3], 

Addition of HN03 to HF increases the etch rate by several orders of magnitude, to values as high as 1500 pm min-1 for a mole ratio of 4.5 HF to 1 HN03 [Sc8, 

One of the interesting features of this etching system is its electrochemical component [Tu4]. The initial step of the reaction is proposed to be reduction of HNO3 and hole injection into the VB of silicon  

---

Formation of microporous structure of exposed adhesive surfaces by chemical etching in acid solution (Cr0j/H2S04 or CrOj/HBF)... 

---