Al is not etched chemically by fluorine (e.g., CHF3, CF4) plasmas because
the vapor pressure of AlF3 product is low.
(It is the same story for chromium.)
Depending on the bias, some sputtering can occur, but this is generally
slow.
One thing to watch out for is that sputtered Al can result in micromasking
of tiny spots in the areas you are trying to etch.