Hi Yawei, your layer is probably too thick, try:

spin on HDMS 4000 rpm
bake hotplate 120C few minutes
spin on AZ5214 4000 rpm 55 sec  th=1.4 um
bake on hotplate 2min10sec 90C
NUV exposure 1.8 - 2.5 sec contact 9 mW/cm2
bake on hotplate 30sec 120C critical time
cool down a minute
NUV flood exposure  7 sec
develop in pure MF321 1min 15 sec
rinse in DI

works well for features over 1 micron, greetings Carel Heerkens