Invited Paper NS+AS+BI+SP-WeM11
Scanned Probe Based Nanofabrication on Silicon: Progress, Challenges and Technology Spin-Offs

Wednesday, October 30, 2013, 11:20 am, Room 203 B

Atomic-scale nanofabrication on Si(100) surfaces can be achieved by using hydrogen as an atomic layer electron resist. Electrons from a STM probe can create atomically precise patterns of clean silicon under ultrahigh vacuum conditions. These patterns can serve as templates for selective chemistry, including the atomically precise placement of molecules on the silicon surface. By performing these studies under UHV conditions it is also possible to obtain information about the hydrogen desorption process. Two desorption regimes are observed. The first involves the direct excitation of the bonding-to-antibonding transition and requires an electron energy of ~6.5 eV. In this regime a constant quantum desorption efficiency is observed irrespective of the electron current. At electron energies insufficient to excite the direct bonding-to-antibonding transition, desorption also occurs but with a strong current-dependent desorption efficiency. In this regime, the STM electrons inelastically excite the Si-H vibrational modes, which have a long lifetime of ~10 ns. Thus, if the excitation rate, which depends on the electron current, exceeds the quenching rate by the lattice, then the Si-H bond moves up its vibrational state ladder until desorption occurs from the hot ground state. Atomic resolution patterning is a side benefit of this vibrational heating process since tunneling electron energies are involved. Following a suggestion by Avouris, these experiments were extended to deuterated Si(100) surfaces where a giant kinetic isotope effect was observed. In the single-particle desorption regime, when excitation to the antibonding level occurs deuterium accelerates more slowly away from the silicon surface, thereby enhancing the probability that the bond will reform. Experimentally, deuterium is observed to be about two orders of magnitude more difficult to desorb than hydrogen. In the vibrational heating regime the deuterium desorption probability decreases by many orders of magnitude due to the short Si-D vibrational lifetime (< 1 ns). The STM isotope experiments set the stage for the discovery that deuterium can be used to dramatically reduce hot carrier degradation in silicon CMOS technology. Hot carriers at the transistor gate dielectric-silicon interface desorb the hydrogen used to passivate silicon dangling bonds. Deuterating the interface increases chip hot carrier lifetimes by more than an order of magnitude. Consequently, deuterium processing is now used in advanced chip manufacture. We have also developed a novel tip sharpening method for STM and AFM probes that results in sub-5 nm radii probe tips with ultra-hard conductive coatings.