we have developed an uncomplicated method of plasma etching deeply recessed features such as deep trenches , of at least 5 μm in depth , in a silicon - containing substrate , in a manner which generates smooth sidewalls , having a roughness of less than about 1 μm , typically less than about 500 nm , and even more typically between about 100 nm and 20 nm . features having a sidewall taper angle , relative to an underlying substrate , typically ranges from about 85 ° to about 92 ° and exhibiting the smooth sidewalls are produced by the method . in one embodiment , a stabilizing etchant species is used constantly during the plasma etch process , while at least one other etchant species and at least one polymer depositing species are applied intermittently , typically periodically , relative to each other . in another embodiment , the stabilizing etchant species is used constantly and a mixture of the other etchant species and polymer depositing species is used intermittently .