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Direct-Write of Silicon and Germanium Nanostructures Print

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.

From Sand to Processor

Modern electronic integrated circuits are made of silicon. Silicon is the most abundant element in the earth's crust and is found in common sand. It takes several laborious and expensive steps to go from sand to a processor. Silicon is extracted from the sand and molded into single-crystal ingots. These ingots are sliced into very thin (less than 1-mm) wafers that form the base for as many as a trillion circuit elements (transistors). To fit so many on a single wafer, a process called photo-lithography is used to achieve features as small as 20 nm. In photolithography, a desired component shape is transferred with light onto a polymer layer on top of the wafer. The areas under the polymer are protected from subsequent processing, which etches the exposed silicon away. The protecting polymer is then removed and the cycle is repeated to make more features, in effect, chiseling out tiny transistors. As a result of this fabrication process, large amounts of polymer and expensive single-crystal silicon are disposed of and go to waste. The Rolandi group has devised a strategy to fabricate transistors by adding—instead of removing—silicon (and germanium) nanostructures at precise wafer locations. This strategy holds the potential for fabricating transistors in fewer steps with less material waste. Reduced energy consumption and waste could lead to "greener" electronic manufacturing.

Direct-write PacMan

The Pac Man and one Pac Dot are actual nanostructures created by the direct-write process. The Pac Man was written in germanium with a -12 V tip bias at 1 μm/s tip speed. The Pac Dot was written in silicon with a -12 V tip bias at 5 μm/s. The height of the structures is about 5 nm. Scale bar = 5 μm. A similar image resulting from this work was featured on the cover of Advanced Materials, Volume 22, Issue 41 (November 2, 2010).

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.

Direct-write process

Left: Schematic of the AFM tip and substrate geometry and the chemical synthesis of Si and Ge nanostructures. Right: Precursor molecules for Si (diphenylsilane) and Ge (diphenylgermane).

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.

NEXAFS spectra

Left: Representative electron-yield image (x-ray energy = 1222 eV) of Ge structures written from diphenylgermane. Center: NEXAFS spectrum acquired from one of the Ge structures on the left. The spectrum shows the onset of the 1218-eV edge that corresponds to the Ge L3 excitation. Right: Spectra acquired at the onset of the C K-edge (285.7 eV) for Ge (red), Si (green), and C (black) nanostructures. This data validates that at most, only trace amounts of C is present in the Si and Ge structures.



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


ALSNews Vol. 321