ide , ▁using ▁self - form ing ▁salt ▁by products ▁as ▁templates ▁for ▁p ore ▁formation . ▁The ▁salt ▁templates ▁are ▁later ▁removed ▁with ▁water . ▁ ▁D ry ing ▁of ▁por ous ▁sil icon ▁ ▁Por ous ▁sil icon ▁is ▁system atically ▁pr one ▁to ▁presence ▁of ▁cra cks ▁when ▁the ▁water ▁is ▁ev apor ated . ▁The ▁cra cks ▁are ▁particularly ▁evident ▁in ▁thick ▁or ▁highly ▁por ous ▁sil icon ▁layers . ▁The ▁origin ▁of ▁the ▁cra cks ▁has ▁been ▁attributed ▁to ▁the ▁large ▁cap ill ary ▁stress ▁due ▁to ▁the ▁minute ▁size ▁of ▁the ▁por es . ▁In ▁particular , ▁it ▁has ▁been ▁known ▁that ▁cra cks ▁will ▁appear ▁for ▁por ous ▁sil icon ▁samples ▁with ▁thick ness ▁larger ▁than ▁a ▁certain ▁critical ▁value . ▁Bel let ▁concluded ▁that ▁it ▁was ▁impossible ▁to ▁avoid ▁crack ing ▁in ▁thick ▁por ous ▁sil icon ▁layers ▁under ▁normal ▁ev apor ating ▁conditions . ▁Hence , ▁several ▁appropriate ▁techniques ▁have ▁been ▁developed ▁to ▁minim ize ▁the ▁risk ▁of ▁cra cks ▁formed ▁during ▁dry ing . ▁ ▁Super crit ical ▁dry ing ▁Super crit ical ▁dry ing ▁is ▁re put ed ▁to ▁be ▁the ▁most ▁efficient ▁dry ing ▁technique ▁but ▁is ▁rather ▁expensive ▁and ▁difficult ▁to ▁implement . ▁It ▁was ▁first ▁implemented ▁by ▁Can ham ▁in ▁ 1 9 9 4 ▁and ▁involves ▁super he ating ▁the ▁liquid ▁p ore ▁above ▁the ▁critical ▁point ▁to ▁avoid ▁inter fa cial ▁t ension . ▁ ▁Free ze ▁dry ing ▁Free ze ▁dry ing ▁procedure ▁was ▁first ▁documented