First, What kind of resist do you use? Pre-bake 1
minute in 100 degree C, are you sure that you have
drive all the solvent out from the resist? and what
about your light exposure? you need to fine the right
light intensity so that the PR is not over-expose or
under-expose. In your case, it probably under-expose.
Developing PR for a long time is not desire, unless
you have the control parameter.

>my spin speed is 4000rpm with a ramp of 30000r/s, and the prebake
>temperature is 100 degree c with a time of 1 minute, but there is always
>some photoresist remained no matter how long the develop time, why? Is
>the pre bake temperature too high and the the photoresist been
>destroyed?