Try some ultra sonic cleaning in acetone or a surfactant.  Part of the problem
may be that you have a conformal layer that you are trying to lift off.  In the
event that the PMMA is hardened you might try some Nanostrip (I think aluminum
oxide will be OK) or N-methyl pyrrollidone (NMP).

Good luck,
   Michael

>>> "Jun Huang"  8/29/2008 9:52 AM >>>
Hi, all,

  The structure of our device composed of silicon/Patterned PMMA(~200
nm)/ALD Al2O3 (~40 nm). I have tried to dip the device in acetone to do
lift-off but the Al2O3 layer on the entire substrate surface does not
lift-off.  It seems that either acetone can not penetrate the Al2O3 layer or
the PMMA layer is hardened.  The deposition temperature for ALD  Al2O3 is ~
150 C which  should not  be high enough to harden PMMA.

  Does anyone have any suggestions on the lift-off process?