An AFM-SIMS Nano Tomography Acquisition System

• Main Author: Swinford, Richard William
• Format: Others
• Published: PDXScholar 2017
• Subjects: Secondary ion mass spectrometry; Atomic force microscopy; Microscopes > Design and construction; Physics
• Online Access: https://pdxscholar.library.pdx.edu/open_access_etds/3485; https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=4494&context=open_access_etds

An instrument, adding the capability to measure 3D volumetric chemical composition, has been constructed by me as a member of the Sánchez Nano Laboratory. The laboratory's in situ atomic force microscope (AFM) and secondary ion mass spectrometry systems (SIMS) are functional and integrated as one instrument. The SIMS utilizes a Ga focused ion beam (FIB) combined with a quadrupole mass analyzer. The AFM is comprised of a 6-axis stage, three coarse axes and three fine. The coarse stage is used for placing the AFM tip anywhere inside a (13x13x5 mm3) (xyz) volume. Thus the tip can be moved in and out of the FIB processing region with ease. The planned range for the Z-axis piezo was 60 µm, but was reduced after it was damaged from arc events. The repaired Z-axis piezo is now operated at a smaller nominal range of 18 µm (16.7 µm after pre-loading), still quite respectable for an AFM. The noise floor of the AFM is approximately 0.4 nm Rq. The voxel size for the combined instrument is targeted at 50 nm or larger. Thus 0.4 nm of xyz uncertainty is acceptable. The instrument has been used for analyzing samples using FIB beam currents of 250 pA and 5.75 nA. Coarse tip approaches can take a long time so an abbreviated technique is employed. Because of the relatively long thro of the Z piezo, the tip can be disengaged by deactivating the servo PID. Once disengaged, it can be moved laterally out of the way of the FIB-SIMS using the coarse stage. This instrument has been used to acquire volumetric data on AlTiC using AFM tip diameters of 18.9 nm and 30.6 nm. Acquisition times are very long, requiring multiple days to acquire a 50-image stack. New features to be added include auto stigmation, auto beam shift, more software automation, etc. Longer term upgrades to include a new lower voltage Z-piezo with strain-gauge feedback and a new design to extend the life for the coarse XY nano-positioners. This AFM-SIMS instrument, as constructed, has proven to be a great proof of concept vehicle. In the future it will be used to analyze micro fossils and it will also be used as a part of an intensive teaching curriculum.