The method for achieving the desired bottom corner rounding depends sginificantly on the chemistry used for shallow trench etching. Chlorine based plasmas show a tendency for microtrenching, while HBr based plasmas result more easilty in square or rounded trench bottoms (slide 1).
For the HBr based process, the trench bottom becomes more rounded when the pressure is increased (slide 2) and the ion energy is increased.
For chlorine based plasmas, a significant bottom corner can be obtained by oxygen addition. Silicon oxychloride is formed on the trench sidewall. For sufficiently high oxygen flows, this passivation layer can extend towards the etch front and eliminate the microtrench. When too much oxygen is added to the gas mixture, the oxide layer starts to cover the etch front. Chlorine can attack the silicon only in a few areas which causes bottom roughness (slide 3).
The bottom corner rounding for chlorine based plasmas can be controlled by adjusting the pressure and the oxygen flow (slide 4).
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