Integration of field emitters into scanning probe microscopy sensors using focused ion and electron beams

Abstract

Scanning probe microscopy (SPM) enables high resolution imaging of sample surfaces. Probes with tip radii of a few nanometer facilitate the measurement of topography and surface roughness with sub nanometer vertical resolution. Lateral movement is precisely controlled and enables scanning with sub nanometer resolution and accurate positioning of the probe at a predefined position on a previously imaged surface. The integration of a controllable electron source into the tip of a silicon probe additionally allows the exposure of well defined areas with low energy electrons. Electron emission as well as electron dose can be controlled by variation of the gate voltage. Potential areas of application are lithography, electrical testing, and surface analysis by electron beam based spectroscopy, like inverse photoemission spectroscopy. In this work, field emitter structures were integrated into the tip of silicon probe sensors using focused ion and electron beam techniques. Material processing by focused particle beams has already been used for the repair of damaged or melted tips in field emitter arrays (FEA) and for the processing of emitter gate structures intended for use as electron sources in vacuum electronics. The focused ion beam was used for the processing of gate aperture by physical sputtering. The insulating layer was partially removed as damage and gallium contamination induced into substrate and insulator material during FIB processing deteriorate emission performance. Different methods, e.g. wet etching, in situ etching with gases, for removing residual insulator material and gallium contamination were investigated. Platinum emitters with diameters less than 100 nm were fabricated into the gate opening by electron beam induced deposition. Leakage current emerging after deposition of emitter structure was investigated and assigned to the deposition of precursor material at the side walls during deposition process. Different shapes of emitters have been fabricated to increase stability and improve electrical properties of the field emitters. Geometry of the silicon probes was adapted to the integration of an emitter gate structure with an diameter in the range of micrometers. Processing of the silicon probes was altered to replace the apex by plateaus with diameters between 1 /spl mu/m to 15 /spl mu/m. Figure 2 shows a flattened probe with a plateau diameter of approx. 3 /spl mu/m. The silicon probes were thermally oxidized with a thickness of 1 /spl mu/m and then deposited by platinum with a thickness of 0.3 /spl mu/m. Electrical contacts for gate and emitter voltage were processed by optical lithography. Measured electron emission currents for emitter gate structures processed by focused ion and electron beams were about 0.5 /spl mu/A/tip for a gate voltage of 70 V. The turn-on voltage was about 40 V.