Hi Huynh, you can use cloning rings and trypsnize only the cells within the ring :)

http://www.sigmaaldrich.com/catalog/product/sigma/c1059?lang=en®ion=NL

Thank you, Mr Gomes

but is it ok if i wanna take cells in basement layer only, not cells forming colony? :(

Yes, the rings glues to the bottom of the plate. Whatever cells you want you can collect them using the ring. It is just a matter of what you are choosing 

Mr Gomes

Thank you so much, now i understood. I will fix it. :D