Etch Rate Reduction of Heavily Doped Materials

ETCH RATE REDUCTION OF HEAVILY DOPED MATERIALS 

For a given type and orientation the etch rate of silicon in alkaline solutions is largely independent of doping concentration up to about 10 Vcm. At a doping level of about 2 x lO Vcm the etch rate of boron-doped silicon drastically decreases with increasing dopant concentration Reduction by as much as three orders of 

Doping with other elements such as Ge and P (n-type dopant) also results in etch rate reduction but at higher concentrations than B as shown in Fig. 7.30. ° It has been reported that ion implantation of species such as As to a level of lO Vcm reduces the etch rate. On the other hand, doping with As, P, and Sb was found to change little from 10 to 10 /cm in 23.4% KOH + 13.3% IPA. ° Carbon implantation to concentrations greater than 10 /cm causes drastic etch rate reduction as shown in Fig. 7.30. The effect is more pronounced in EDP than in KOH. 

The occurrence of etch rate reduction on highly boron doped materials appears to be independent of doping methods, whether by solid-source diffusion, epitaxial growth, or ion implantation. However, the boron concentration at which significant reduction occurs is different for different methods of doping. The critical boron concentration for etch rate reduction to occur is affected by the defect density in different doped materials. It is found that for similar boron concentrations the amount of etch rate reduction in KOH solutions decreases with increasing defect densityIn 

FIGURE 7.30. Normalized 100 silicon etch rates as a function of dopant concentration for boron, phosphorus, germanium, and carbon in KOH and EDP solutions. After Seidel et al ° and Lehmann et al (Reproduced by permission of The Electrochemical Society, Inc.) 

---

Etch Rate Reduction of Heavily Doped Materials. 783... 

---