We demonstrate here a new method of fabricating in-plane cylindrical glass nanocapillaries (<100 nm) that does not require advanced patterning techniques (e.g.e-beam or nanoimprint lithography) but the standard photolithography with coarse features (>1 μm).Our method takes advantage of thermal reflow of glass on microstructured silicon and yields self-enclosed optically transparent and highly regular nanocapillaries over large areas.As conceptually described in fig.1,a triangular slender void trapped within a rectangular silicon trench due to non-conformal deposition of glass film can be transformed into a fine cylindrical nanocapillary of a desired size with the precise control of reflow temperature and duration.In the fabrication,rectangular trenches (1.5-3 μm wide,2.5 μm deep) were patterned on (100)-oriented silicon wafers (P-type,100 mm in diameter) through a single-step lithography and deep reactive ion etching.Subsequently,a layer of phosphorus silicate glass (5 μm thick) was deposited through a low-pressure chemical vapor deposition process.The thermal reflow was conducted either in a diffusion furnace (1000 °C) or in a rapid thermal processor (950 °C),both under atmosphere pressure.Fig.2a shows SEM cross-sections of slender voids trapped within a glass filling inside 2.5 μm wide trenches before and after thermal anneal that led to nanocapillaries with consistent size.Numerical simulations verifying this evolution can be found in our recent work [1].Diameters of nanocapillaries formed in different aspect ratio trenches are presented in a plot of anneal time (Fig.2b).The utility of the fabricated nanocapillaries was tested on the stretching of λ-phage DNA (Fig.3).As shown,in the capillaries having a relatively large diameter (400 nm),the molecules preserve their recoiled shape (Fig.3a),whereas those confined in smaller capillaries (200 nm) can be found stretched to length ~8.7 μm (Fig.3b).Further confinement of DNA can be observed in 50 nm capillaries with length ~14.1 μm (Fig.3c),which is in agreement with previous studies [2].In conclusion,the nanocapillaries fabricated by this cost-effective method offer great value for chip-based nanofluidic devices in various biomedical applications.