Abstract The deep reactive ion etching of PMMA in O 2 , O 2 /CHF 3 discharges has been examined as a function of plasma parameters such as pressure, power and relative composition. It is demonstrated that the etching rate initially increases with pressure but decreases after 6.65 Pa. The etching rate increased linearly with power, but the rough surface will occur and sample will be distorted, so power cannot surpass 50 W. The etching rate decreases with CHF 3 ratio in O 2 /CHF 3 discharges. The sidewall undercut becomes serious when the profile depth etched in pure O 2 plasma is more than 100 μm. By addition of proper proportion of CHF 3 to O 2 , the sidewall undercut can be reduced. The sidewall profile is vertical even the etching depth is as deep as 400 μm when etched in O 2 /CHF 3 discharges(40% CHF 3 ), 30 W, 3.99 Pa, and high aspect ratio microstructure is achieved.