  Reduction of the inter-probe distance in multi-probe and double-tip STM down
to the nanometer scale has been a longstanding and technically difficult
challenge. Recent multi-probe systems have allowed for significant progress by
achieving distances of around 30 nm using two individually driven, traditional
metal wire tips. For situations where simple alignment and a fixed separation
can be advantageous, we here present the fabrication of on-chip double-tip
devices that incorporate two mechanically fixed gold tips with a tip separation
of only 35 nm. We utilize the excellent mechanical, insulating and dielectric
properties of high quality SiN as a base material to realize easy-to-implement,
lithographically defined and mechanically stable tips. With their large contact
pads and adjustable footprint these novel tips can be easily integrated with
most existing commercial combined STM/AFM systems.
