Replace heatsink net6501

Hello all,

I'm installing a new heatsink on my net6501 and I'm wondering whether I
should clean the cpu with alcohol before applying the new 'paste' and
heatsink? I've replaced a heatsink before but then a nice chip
appeared.. now I see a small pcb with a mirror on it... so before I ruin
the CPU I though better ask advice first.

Here's a link to a picture of the CPU.
http://temp.muffins.diskstation.me:8080/IMG_20150702_150737.jpg

Thanks in advance and kind regards,

Maarten