|Direct-Write of Silicon and Germanium Nanostructures|
Nanostructured materials (nanowires, nanotubes, nanoclusters, graphene) are attractive possible alternatives to traditionally microfabricated silicon in continuing the miniaturization trend in the electronics industry. To go from nanomaterials to electronics, however, the precise one-by-one assembly of billions of nanoelements into a functioning circuit is required—clearly not a simple task. An interdisciplinary team from the University of Washington, in collaboration with the ALS and the Pacific Northwest National Laboratory, has devised a strategy that could make this task a little easier. They have demonstrated the ability to directly "write" nanostructures of Si, Ge, and SiGe with deterministic size, geometry, and placement control. As purity is essential for electronic-grade semiconductors, the resulting patterns were carefully evaluated for carbon contamination using photoemission electron microscopes at ALS Beamlines 7.3.1 and 11.0.1.
Traditional nanofabrication strategies for integrated circuits often involve multiple lithography and alignment steps or complex wafer bonding procedures. In our pursuit of ever-smaller and more powerful electronic devices, scanning probes have emerged as attractive tools for nanofabrication in light of their ability to locally deliver desired stimuli or chemicals to a small area of a sample surface. Specifically, atomic force microscopy (AFM) has been employed in several nanofabrication schemes. However, until now, such techniques have been limited to writing carbon-based nanostructures that can be used as etch resists but do not add any other material functionality to the silicon substrate.
For solid-state nanostructures, high-field lithography is a true one-step scheme because the raw material for the nanostructures (Si and Ge—the main materials in circuit elements) is locally synthesized by a large electric field during writing, and post-patterning processing is not required. In brief, an AFM tip traces the desired shapes along the silicon substrate while immersed in an inorganic liquid precursor containing Ge or Si: diphenylgermane (DPG) for Ge and diphenylsilane (DPS) for Si. A moderate sample bias (~12 V) induces a large electric field (>109 V/m) between the AFM tip and the substrate. Electrons tunnel from the tip into the precursor molecules, which initiates a chemical reaction that fragments the molecules and deposits the desired material onto the substrate. The voltage bias that triggers the reaction can be easily turned on or off.
In this fashion, arbitrary shapes of silicon and germanium as small as 25 nm are routinely produced at a rate of 1 μm/s. Line widths are limited by the tip radius and are directly related to write speed and bias, with slower write speeds and higher bias generally producing wider and taller lines. Complex architectures of Si and Ge heterostructures can be easily fabricated in a single direct-write session without the need for tip–sample realignment. To increase the throughput of this process, the team now also uses nanostructured stamps to mimic multiple tips working in parallel.
When making small structures, materials characterization becomes a challenging pursuit. To this end, the researchers employed Beamlines 7.3.1 and 11.0.1 to perform near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy. NEXAFS spectra are recorded by two photoemission electron microscopes (PEEM-2 and PEEM-3). PEEM can elucidate the chemical and elemental composition of the structures with a spatial resolution as good as 25 nm. For the fabricated germanium nanostructures, spectra collected with PEEM-3 show a strong peak that corresponds to the Ge L3-edge (1218 eV). Because carbon is an unwanted impurity in electronics fabrication, the carbon content of the silicon and germanium nanostructures can be estimated by comparison with structures made of pure carbon using PEEM-2. The peaks corresponding to graphite-like carbon (285.3 eV) and diamond-like carbon (290 eV) were analyzed. The results indicate that the germanium and silicon nanostructures contain at most trace carbon amounts.
Research conducted by J.D. Torrey, S.E. Vasko, A. Kapetanovic, V. Talla, M.D. Brasino, and M. Rolandi (University of Washington); Z. Zhu (Pacific Northwest National Laboratory); and A. Scholl (ALS).
Research funding: National Science Foundation, Intel, 3M Nontenured Faculty Grant, and University of Washington New Faculty Seed Funds. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publications about this research: J.D. Torrey, S.E. Vasko, A. Kapetanovic, Z. Zhu, A. Scholl, and M. Rolandi, "Scanning probe direct-write of germanium nanostructures," Adv. Mater. 22, 4639 (2010); S.E. Vasko, A. Kapetanovic, V. Talla, M.D. Brasino, Z. Zhu, A. Scholl, J.D. Torrey, and M. Rolandi, "Serial and parallel Si, Ge, and SiGe direct-write with scanning probes and conductive stamps," Nano Lett. 11, 2386 (2011).
ALS Science Highlight #230