  Layer-by-layer assembly of van der Waals (vdW) heterostructures underpins new
discoveries in solid state physics, material science and chemistry. Despite the
successes, all current 2D material (2DM) transfer techniques rely on the use of
polymers which limit the cleanliness, ultimate electronic performance, and
potential for optoelectronic applications of the heterostructures. In this
article, we present a novel polymer-free platform for rapid and facile
heterostructure assembly which utilises re-usable flexible silicon nitride
membranes. We demonstrate that this allows fast and reproducible production of
2D heterostructures using both exfoliated and CVD-grown materials with perfect
interfaces free from interlayer contamination and correspondingly excellent
electronic behaviour, limited only by the size and intrinsic quality of the
crystals used. Furthermore, removing the need for polymeric carriers allows new
possibilities for vdW heterostructure fabrication: assembly at high
temperatures up to 600{\deg}C, and in different environments including
ultra-high vacuum (UHV) and when the materials are fully submerged in liquids.
We demonstrate UHV heterostructure assembly for the first time, and show the
reliable creation of graphene moir\'e superlattices with more than an order of
magnitude improvement in their structural homogeneity. We believe that broad
adaptation of our novel inorganic 2D materials assembly strategy will allow
realisation of the full potential of vdW heterostructures as a platform for new
physics and advanced optoelectronic technologies.
