Differential confocal microscopy for stable scanning-microsphere assisted direct laser writing

The integration of microspheres within the instruments of optical metrology and mask-less lithography could already show a significant enhancement of their lateral resolution. Exposing complex large structures exploiting this high resolution requires the lateral movement of the microsphere over the substrate. Challenging remains the accurate lateral and axial positioning of the microsphere ensuring the constant exposure conditions at every point. Preserving the advantage of optical instruments to not actually contact the specimen, the microsphere must be kept at a nanometer-close, yet constant distance from the surface. Here, we introduce the, to our best knowledge, novel approach to combine the principle of the differential confocal microscope with a scanning microsphere. This produces a differential signal towards the surface allowing a nanometer-sensitive and fast control of the axial position of the microsphere above the substrate. In preliminary experiments we show the repeatable pick-up of microspheres and their precise lateral scanning using a nanopositioning and nanomeasuring machine as well as axial depth responses and differential signals from the realized microsphere assisted differential confocal probe.