Yesterday one oracle database(220.127.116.11.0) was hung after I sent "shutdown immediate" command. I found a process ora_emn0 background process was started before shutting down. emn is Event Monitor Process for Advanced Queue Notification, which we don't use it on this instance. See Note 105067.1 to find What is the EMN0 process?
Why does it cause the hang of shutdown? After searching in metalink, I found one note describes this issue ( Database Shuts Down After the Message: Restarting dead background process EMN0). It is related to a known Bug 1800676 in this release.
SQL> shutdown abort
SQL> shutdown immediate
SQL> alter trigger AURORA$SERVER$SHUTDOWN disable;
Set _system_trig_enabled=false in the init.ora and restart the database.