Nanofabrication with Block Copolymer Templates
We have exploited the highly-regular microdomain structures formed by block copolymers in the creation of templates for nanoscale lithography--a two-dimensional templating procedure, in contrast to the three-dimensional approach used in restricting crystallization. The chemical differences between the blocks constituting the film can be used to create a mask (for example, by selectively etching away polydiene microdomains with ozone), allowing the block copolymer pattern to be transferred to the underlying substrate through reactive ion etching. We have termed this patented process “block copolymer nanolithography”; it is applicable to a broad range of substrates, and moreover, the mask or patterned substrate can act as a template for the growth of regular arrays of nanodispersed materials. We have successfully patterned Si, Ge, and Si3N4 substrates, and have extended this method to fabricate metal dots and lines, and compound semiconductor quantum dots, as shown in the figure below:
Examples of structures produced by block copolymer nanolithography; all dots are approximately 25 nm in diameter. Left: tapping-mode atomic force microscope (TM-AFM) image of an array of GaAs quantum dots (with Dan Dapkus, USC). Center: scanning electron microscope image of an array of holes (aspect ratio near unity) in a Si wafer. Right: TM-AFM image of an array of metallic Au dots fabricated in a trilayer process.
The parallel nature of this method lends itself to the fabrication of large-area arrays; we routinely pattern areas as large as 50 cm2. The individual nanostructures have very narrowly-distributed sizes, controllable through the block copolymer molecular weight; typical diameters are 20 nm with a 30 nm spacing, yielding 300,000 lines per cm, or 1011 dots per cm2. If the block copolymer film is deposited an annealed quiescently, it forms a polygrain structure, which is faithfully replicated in each of the images shown above. For some applications, this polygrain structure is satisfactory, but for others, long-range order of the microdomains is required. For such cases, shear-aligned block copolymer thin films are the templates of choice.
Shear-aligned monolayers of cylinder-forming block copolymer can serve as the template for arrays of metal nanowires, with a pitch of order 50 nm. Like the classic gold wire grids used for polarizing infrared radiation, such metal nanowire arrays will polarize light—but because of their small spacing, they will polarize not only infrared, but visible and ultraviolet (UV) light as well. An aluminum nanowire grid, and the principle of polarization, are illustrated below.
Top: Schematic of how a metal wire polarizer works. Provided the light is well below the plasma frequency of the metal– meaning that the electrons in the metal can respond quickly to the light’s electric field– then E-polarized light is blocked (left), while H-polarized light is transmitted. Bottom: plan-view scanning electron microscope image of a grid of aluminum wires with 35 nm pitch, supported on a transparent quartz substrate.
While the community has had, for many decades, compact transmission polarizers which are excellent for infrared (gold wire grid) and red (Polaroid sheet) light, we do not today have such a device for blue or UV light. Today, the most advanced production photolithography for microelectronic devices uses 193 nm UV light from an ArF excimer laser. At such short wavelengths, polarization of the light becomes critical for achieving minimum feature size; obtaining s-polarized light at the wafer will be even more critical for the 193 nm immersion lithographic processes which are on the horizon. Thus, a compact transmission polarizer for UV light could be very useful in operating today’s and tomorrow’s photolithography stations. We have fabricated aluminum nanowire grids with square-centimeter areas, and demonstrated their ability to polarize down to 193 nm. We have also developed a model for their polarization characteristics, which reveals that the polarization of the transmitted light switches near the metal’s plasma frequency.
Supported by the National Science Foundation through the Princeton Center for Complex Materials
Recent/Current People and Projects:
Young-Rae Hong - Broadband Polarizers from Aligned Cylinder-Forming Block Copolymers
Ed Matteo GS - Fabrications and Characterization of Lipid Vesicle Arrays
Chris Copplestone (Oxford) - Inducing Perpendicular Microdomain Alignment
Jose Vedrine - Ordered Dot and Hole Arrays from Aligned Sphere-Forming Block Copolymers
Tom Pickthorn (Oxford) - Gold Nanowire Fabrication Using Shear-Aligned Block Copolymer Thin Films
Vincent Pelletier *05 - Physics and Technology of Sheared Cylinder-Forming Diblock Copolymer Thin Films
Selected Recent Publications:
V. Pelletier, K. Asakawa, M.W. Wu, D.H. Adamson, R.A. Register, and P.M. Chaikin, “Aluminum Nanowire Polarizing Grid: Fabrication and Analysis”, Appl. Phys. Lett., 88, 211114 (2006); also in Virtual J. Nanoscale Sci. Tech., 13(23), (June 12, 2006).
M.L. Trawick, D.E. Angelescu, P.M. Chaikin, and R.A. Register, "Block Copolymer Nanolithography", Chapter 1 in Nanolithography and Patterning Techniques in Microelectronics, D.G. Bucknall, ed., (cambridge: Woodhead Publishing, Ltd., 2005), pp.1-38.
C. Harrison, M. Park, R. Register, D. Adamson, P. Mansky, and P. Chaikin, "Method of Nanoscale Patterning and Products Made Thereby", U.S. Patent 5,948,470, issued September 7, 1999.