Defect and Junction Delineation

The OCP etch rate of p-type and highly doped n-type Si electrodes in HF-HNO3 mixtures increases by an order of magnitude under sufficiently anodic bias [Le20]. In the cathodic regime significant dark-currents are observed for p-type electrodes, as shown in Fig. 4.12. This is ascribed to hole injection from the electrolyte [Kol4]. Note that hole injection is not observed in aqueous HF free of oxidants. 

It is known that HF-HN03-based solutions etch highly doped substrates faster by a factor of about three compared to moderately doped ones. A higher selectivity is reported on addition of chemicals that reduce the HNOz concentration, like H202 or NaN3 [Mul], However, this report suffers from the fact that the etch rate was measured for separate wafers of a homogeneous doping density. For pp+ or nn+ structures, which are not spatially separated, only a low selectivity is observed, because of the autocatalytic behavior of the etchant. 

Masking is required for many micromechanical applications. While Si3N4 is only suitable for a small etching depth because of its significant etch rate in HF, noble metals like gold are sufficient mask materials. In contrast to alkaline etchants, organic materials like certain resists or even some adhesive tapes are well suited to protect the silicon surface in isotropic etchants. 

Note the different magnifications. A high density of etch pits produce a haze-like appearance of the wafer surface. 

Etchants for defect and junction delineation are usually composed of HF and an oxidizing agent such as HN03 [Dal, Gr4, Ka4, Nel], K2Cr207 [Se5] or Cr03 [Sil, Jel, Sc7, Ya4, Me5]. Alkaline solutions are rarely used for defect delineation [Mal2], An etch pit will form on a silicon surface if the dissolution rate is enhanced locally. Enhancement of the etch rate may occur for various reasons  

---

The FIF-CrOs etching system is widely used for defect sensitive etching and delineation of junctions between silicon layers of different doping concentrations. The etch rate of silicon in pure FIF solution is very low due to the lack of holes at the OCR Addition of CrOs increases the etch rate due to the increase of surface hole concentration resulting from the reduction of Cr. CrOs dissolves in water to form... 

---