Nanoscale dispensing of single ultrasmall droplets

Nanoscale dispensing (NADIS) is a novel technique to deposit material at micrometric and submicrometric dimensions. It has great flexibility in feature shape and choice of deposited material. Due to its expected low cost and short turn-around time, it has the potential to be an interesting tool complementary to standard lithographic processes. Furthermore, NADIS has a great potential in the creation of high-density microarrays used in proteomics or genomics. The key feature of NADIS is the deposition of a liquid through an aperture created in a scanning force microscopy probe tip. The liquid is loaded into the hollow back of the pyramidal probe tip. Upon contact, liquid is transferred from the tip to the substrate surface. The transfer of liquid occurs without any external pressure. The control of the NADIS probe displacement is achieved using a standard atomic force microscope. Two different approaches to fabricate tips with apertures were investigated. The first approach relies on opening the tip during the microfabrication of the probe, whereas the second approach implies a modification of commercially available probes by focused ion beam milling. Both kinds of probes have shown their ability to perform successfully the dispensing of droplets. Nanoscale dispensing has been demonstrated for deposition of ultrasmall single droplets with volumes down toattoliters in a controlled way and with high lateral accuracy. The smallest droplet spacing that has been achieved was less than 500 nm. The size of the droplets and the possible droplet density are largely dependent on the aperture diameter and on the surface wettability. Nanoparticles and fluorescent molecules were also dispensed. In such cases, the liquid is used as a transport medium for the substances to be deposited. By moving the NADIS probe during contact on the substrate, it was possible to write features such as lines with sizes that can be as small as 400 nm. Some theoretical aspects are discussed, in particularly the capillary forces associated with axisymmetric liquid menisci. Experimentally measured capillary forces during the dispensing are compared with theoretically determined values.