  Van der Waals heterostructures are an excellent platform for studying
intriguing interface phenomena, such as moir\'e and proximity effects. Surface
science techniques like scanning tunneling microscopy (STM) have proven a
powerful tool to study such heterostructures but have so far been hampered
because of their high sensitivity to surface contamination. Here, we report a
dry polymer-based assembly technique to fabricate van der Waals
heterostructures with atomically clean surfaces. The key features of our
suspended dry pick-up and flip-over technique are 1) the heterostructure
surface never comes into contact with polymers, 2) it is entirely solvent-free,
3) it is entirely performed in a glovebox, and 4) it only requires temperatures
below 130$^{\circ}$. By performing ambient atomic force microscopy and
atomically-resolved scanning tunneling microscopy on example heterostructures,
we demonstrate that we can fabricate air-sensitive heterostructures with
ultra-clean interfaces and surfaces. Due to the lack of polymer melting, the
technique is further compatible with heterostructure assembly under ultra-high
vacuum conditions, which promises ultimate heterostructure quality.
