Center for Materials for Information Technology an NSF Materials Science and Engineering Center Nanolithography Lecture 15 G.J. Mankey

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Writing and Printing To go far beyond the Raleigh limit of optical processes, other probes must be applied. By definition a nanostructure is one with nanometer dimensions, but in practice true nanoscience implies a paradigm shift of the lithography process to below 100 nm. Sub 100-nanometer writing and printing requires the use of x-rays, electron beams, focused ion beams, atomic force probe tips, or a nanoimprint mask. Printing processes are x-ray lithography and nanoimprint lithography. The writing processes all involve scanning the probe across the surface writing a single spot at a time and are thus time consuming and expensive.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Why Nano? Semiconductors and magnetic storage devices will get smaller. A current bit on a hard drive is 300 nm by 100 nm. The future evolution of this technology will require nanoscale readers, writers and possible media. To be marketable, these devices must be inexpensive and reliable. The challenge for physics is not only to make it possible, but routine! ref: W. Eberhardt in Frontiers in Surface and Interface Science (2002).

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Resolution Limit of e-beams The wavelength of an electron is The diffraction limited beam has a diameter d = 0.6 / , where  is the beam divergence (~0.01). A 10 kV beam has = 0.01 nm and this simple analysis gives a 1 nm spot. In general, the spot sizes are added in quadrature, as shown in the diagram. ref: Handbook of Microlithography, Micromachining and Microfabrication, SPIE (1997)

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Electron-Solid Interactions Increasing electron beam energy arbitrarily will not result in a smaller spot in the resist. Higher energy electrons produce more secondary electrons which travel through the solid and expose a larger volume of resist. This is similar to photoelectron blur in x-ray lithography which ultimately puts a limit on the smallest achievable feature. Features ~ 10 nm are theoretically possible, but rarely demonstrated. ref: Handbook of Microlithography, Micromachining and Microfabrication, SPIE (1997)

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Two Layer Resist Processing The best results of e-beam have been achieved for making a pattern of a single layer of evaporated or sputtered material. The two-layer process produces an undercut structure which functions as a shadow mask for the deposition process. After deposition from a collimated source, the resist is removed with a solvent. ref: Handbook of Microlithography, Micromachining and Microfabrication, SPIE (1997)

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Resist Contrast Resist contrast determines sensitivity to process parameters such as exposure time and resist thickness. Higher contrast resist usually has a more vertical sidewall angle. The contrast is defined as: Film Retention (%) Log (Electron Dose) D1D1 D2D2 High Contrast Low Contrast ref: Handbook of Microlithography, Micromachining and Microfabrication, SPIE (1997)

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Dot Arrays Fabricated by EBL AFM images of dot arrays of different period fabricated by e-beam lithography, followed by thermal evaporation and lift-off. The pitch, P, is varied systematically while the raster rate and beam current are constant. Under the experimental conditions, uniform arrays are obtained when the period is greater than 45nm. The density of the dots is up to 300 Gdot/in 2. Image size is one square micrometer. P = 40 nmP = 45 nm P = 50 nmP = 60 nm

Center for Materials for Information Technology an NSF Materials Science and Engineering Center X-Ray Lithography X-rays are produced by synchrotron radiation from a bending magnet in a high energy electron storage ring. Resist, usually PMMA, is exposed through an x-ray mask in proximity or in contact with the wafer. This technique has the advantage that very high aspect ratios can be achieved, since the x-rays are highly penetrating. ref: Handbook of Microlithography, Micromachining and Microfabrication, SPIE (1997)

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Resolution Limit of XRL The wavelength of an x-ray is given by = 1.2/E(keV), so one would expect higher energies to be more desirable. Blur, caused by the excitation of photoelectrons and the associated shower of secondary electrons limits the patterning ability to about 20 nm. XRL has been applied to MEMS, where the high aspect ratio is a distinct advantage. ref: Handbook of Microlithography, Micromachining and Microfabrication, SPIE (1997)

Center for Materials for Information Technology an NSF Materials Science and Engineering Center AFM Lithography The AFM tip is used to "bulldoze" through a top layer of resist. Submersion in developer produces an undercut in the lower layer of resist. The top layer then serves as a shadow mask for deposition. After removal of resist, the deposited nanostructure remains. For nanocontact printing, the initial structuring is performed in parallel with a contact mask. AFM Bulldozing Developing DepositingLift Off Undercut