Focused electron beam induced deposition of high purity gold dots enabling surface functionalization for biomedical applications

Abstract

Moore`s law has provided the economical basis for the spectacular growth of semiconductor industry in the last decades. Physical limitations with shrinking minimum feature size further will soon lead to prohibitively expensive production of future nanometer sized structures. New mask-less process technologies like the resist-free direct-write Focused Electron Beam Induced Deposition (FEBID) method provide increased freedom in three dimensional design while simultaneously allowing for new applications of micro and nanostructures not accessible to current lithographic methods. Gold surfaces provide an excellent base for biomolecular immobilization to functionalized self-assembled monolayers. This work explores the possibility of FEBID to deposit high purity gold within a scanning electron microscope for future biomolecular functionalization of sensor surfaces. Traditionally FEBID gold contains up to 90 at. % of carbon contamination, which severely limits practicality of the FEBID gold process for a lot of applications, including the self-assembly of alkane thiols on the surface. Therefore, this work first reproduces a published deposition process using the dimethyl-(trifluoroacetylacetonate)-gold(III) precursor. Using an electron beam energy of 3 keV and varying current, 1x1 μm squares with a height between 100 nm and 1000 nm were deposited, featuring a as-deposited gold purity of 39.5 at. %, onto silicon and germanium substrates within a scanning electron microscope. By introducing post-deposition electron irradiation of the structures as a new, additional step, a purity increase to 49 at. % was achieved. The same process in oxidizing atmospheres of hydrogen peroxide and water vapor showed an even further increase in gold content to 74 at. % and 82 at. %, respectively. This novel post-deposition treatment allows for localized purification of the produced Au-structures. As an alternative, co-injecting water vapor at the time of the gold deposition was found to further increase gold purity up to 87 at. %, without the need for additional post-deposition treatment. This SEM EDX result was also confirmed by TEM EELS measurements of 82 at. % gold purity. The water co-injection process thereby yields the highest reported FEBID gold purity from a commercially available precursor to date. To verify the coating procedure for biomolecular functionalization via an activated selfassembled alkane thiol on gold, infrared spectroscopy, ellipsometry as well as fluorescent microscopy were employed and confirmed the successful coating of lithographical gold microstructures. Although the FEBID post-deposition ex-situ cleaned gold deposits showed weaker levels of fluorescence, immobilized biomolecules could also be detected.