an ol ▁elimin ates ▁hydro gen ▁and ▁ens ures ▁complete ▁in fil tr ation ▁of ▁H F ▁solution ▁within ▁the ▁por es . ▁Sub sequently , ▁uniform ▁distribution ▁of ▁por os ity ▁and ▁thick ness ▁is ▁improved . ▁ ▁St ain ▁et ch ing ▁ ▁It ▁is ▁possible ▁to ▁obtain ▁por ous ▁sil icon ▁through ▁st ain - etch ing ▁with ▁hydro flu or ic ▁acid , ▁nit ric ▁acid ▁and ▁water . ▁A ▁publication ▁in ▁ 1 9 5 7 ▁revealed ▁that ▁st ain ▁films ▁can ▁be ▁grown ▁in ▁dil ute ▁solutions ▁of ▁nit ric ▁acid ▁in ▁concentr ated ▁hydro flu or ic ▁acid . ▁Por ous ▁sil icon ▁formation ▁by ▁st ain - etch ing ▁is ▁particularly ▁attract ive ▁because ▁of ▁its ▁simplicity ▁and ▁the ▁presence ▁of ▁readily ▁available ▁cor ros ive ▁re ag ents ; ▁namely ▁nit ric ▁acid ▁( H NO 3 ) ▁and ▁hydro gen ▁flu or ide ▁( H F ). ▁Furthermore , ▁st ain - etch ing ▁is ▁useful ▁if ▁one ▁needs ▁to ▁produce ▁a ▁very ▁thin ▁por ous ▁Si ▁films . ▁A ▁publication ▁in ▁ 1 9 6 0 ▁by ▁R . ▁J . ▁Arch er ▁revealed ▁that ▁it ▁is ▁possible ▁to ▁create ▁st ain ▁films ▁as ▁thin ▁as ▁ 2 5   Å ▁through ▁st ain - etch ing ▁with ▁H F - H NO 3 ▁solution . ▁ ▁B ottom - Up ▁Syn thesis ▁▁ ▁Por ous ▁sil icon ▁can ▁be ▁synth es ized ▁chem ically ▁from ▁sil icon ▁t et rach lor