Tethering molecules to silicon

Researchers at the University of Illinois have succeeded in tethering individual organic molecules at specific locations on silicon surfaces.

‘The semiconductor industry is fast approaching a fundamental limit on how many components can be crammed onto a conventional chip,’ says professor Joseph Lyding, ‘We are exploring what we can make at the atomic level, and how we might merge that with what we perceive as end-of-the-road silicon technology.’

To selectively bind molecules to a silicon surface, Lyding first passivates the silicon bonds with hydrogen, then uses an ultra-high vacuum scanning tunnelling microscope to break individual silicon-hydrogen bonds and dislodge hydrogen atoms from selected sites.

‘We take advantage of the difference in chemical reactivity between clean silicon and hydrogen-passivated silicon,’ says researcher Mark Hersam. ‘By removing individual hydrogen atoms, we create holes in the clean silicon surface. Since these holes — or dangling bonds — serve as effective binding sites, molecules injected in the gas phase will spontaneously self-assemble into the predefined patterns.’

A technique called feedback-controlled lithography gives the patterning process an atomic precision. ‘Feedback-controlled lithography works by actively monitoring the microscope feedback signal and the tunnelling current during patterning, and immediately terminating the patterning process when a bond is broken,’ explains Lyding. ‘By operating the microscope under feedback control, a carefully controlled dose of electrons can be written along a line or over an area to locally depassivate the surface and create templates of individual dangling bonds.’

The researchers demonstrated the feasibility of their technique with three organic molecules: norbornadiene, copper phthalocyanine and carbon-60 ‘buckyballs.’ One advantage of organic molecules is that their end groups can be functionalised for potential electronic or mechanical switching properties. ‘Now the fun can begin as we attempt to take advantage of what chemists can make to add functionality in the form of data storage or processing,’ says Lyding. ‘It may be possible to make molecular devices and switching elements that operate at a hundred trillion times a second.’

While the technology for economically tethering billions of molecular devices on a chip surface does not yet exist, ‘we can make small numbers of the devices and test their function,’ claims Hersam. ‘This is an important step in bridging the gap between molecular electronics and silicon technology.’