Each phase lasts for several seconds. The passivation layer protects the entire substrate from further chemical attack and prevents further etching. However, during the etching phase, the directional ions that bombard the substrate attack the passivation layer at the bottom of the trench (but not along the sides). They collide with it and sputter it off, exposing the substrate to the chemical etchant. These etch/deposit steps are repeated many times over resulting in a large number of very small isotropic etch steps taking place only at the bottom of the etched pits. To etch through a 0.5 mm silicon wafer, for example, 100–1000 etch/deposit steps are needed. The two-phase process causes the sidewalls to undulate with an amplitude of about 100–500 nm. The cycle time can be adjusted: short cycles yield smoother walls, and long cycles yield a higher etch rate. Applications RIE "deepness" depends on application: in DRAM memory circuits, capacitor trenches may be 10–20 µm deep, in MEMS, DRIE is used for anything from a few micrometers to 0.5 mm. in irregular chip dicing, DRIE is used with a novel hybrid soft/hard mask to achieve sub-millimeter etching to dice silicon dies into lego-like pieces with irregular shapes. in flexible electronics, DRIE is used to make traditional monolithic CMOS devices flexible by reducing the thickness of silicon substrates to few to tens of micrometers. What distinguishes DRIE from RIE is etch depth: Practical etch depths for RIE (as used in IC manufacturing) would be limited to around 10 µm at a rate up to 1 µm/min, while DRIE can etch features much greater, up to 600 µm or more with rates up to 20 µm/min or more in some applications. DRIE of glass requires high plasma power, which makes it difficult to find suitable mask materials for truly deep etching. Polysilicon and nickel are used for 10–50 µm etched depths. In DRIE of polymers, Bosch process with alternating steps of SF6 etching and C4F8 passivation take place. Metal masks can be used, however they are expensive to use since several additional photo and deposition steps are always required. Metal masks are not necessary however on various substrates (Si [up to 800 µm], InP [up to 40 µm] or glass [up to 12 µm]) if using chemically amplified negative resists. Gallium ion implantion can be used as etch mask in cryo-DRIE. Combined nanofabrication process of focused ion beam and cryo-DRIE