Short Wavelength Sources and Imaging

Summary

Short wavelength optical sources, particulary coherent sources, have long fascinated the scientific community. This interest is due both to the challenges involved in realizing such sources, and to the fact that short wavelength, high energy photons are useful tools for investigating new scientific areas and solving technical problems. Short wavelength sources are used to unravel the complexities of atomic and ionic energy levels, to diagnose high density plasmas, to probe materials and surfaces, and to initiate and monitor chemical reactions; short wavelength light is required to fabricate and inspect nanostructures and high density integrated circuits.

Our program has focused on the development of vacuum ultraviolet (VUV) sources, both broad band and coherent, and on one application of those sources: high contrast imaging techniques, particularly for biological objects. Microscopy using VUV radiation offers high resolution, high contrast, and chemical sensitivity. Because photons interact with matter in a fundamentally different way than high energy electrons, VUV microscopy provides a unique way of observing and measuring biological structures with minimal sample preparation, compared to electron microscopy.

Our overall goal is to develop imaging techniques that can approach diffraction-limited resolution independent of the wavelength of the illumination. Such a system gives the user the flexibility to choose the wavelength to optimize contrast and resolution requirements. To this end, we are focusing on contact printing and holography, both of which avoid the limitations of optical components. These techniques, however, do not provide magnification and thus high resolution recording films are required. Films of high molecular weight polymers, such as poly(methylmethacrylate) or PMMA, are sensitive at short wavelengths and have a spatial resolution under 5 nm. Short wavelength light initiates a photochemical reaction that fragments the polymer chain, resulting in regions of low molecular weight corresponding to the incident light intensity. The polymer fragments are preferentially removed by brief immersion in a solvent, leaving a surface contour map of the image. An atomic force microscope (AFM) measures this surface and produces a digital map that can be processed numerically, using the film characteristics and imaging geometry, to produce magnified images in various forms.

Our source development is built on my earlier collaboration with Prof. Steve Harris at Stanford University; results at Rice include: the first measurement of gain in the ionic excimer Cs(2+)F(-), one of a new class of short wavelength lasers that could prove useful for imaging; development and characterization of a laboratory-scale, high repetition rate VUV laser source for imaging, the Xe(2+) Auger laser; and the investigation of the efficiency of various laser-produced plasma target materials for pumping the Xe Auger laser (gold produces twice the laser output as stainless steel, reducing exposure times). Imaging project accomplishments include: measurement of PMMA response characteristics in the 60 to 130 nm range; demonstration that AFM readout of PMMA film provides improved sensitivity and precision compared to previous methods; recording simple holograms of test spheres; and imaging biological cells on PMMA using both our laser-produced plasma source and the Xe 109 nm laser; regions of different contrast are evident.

This work has been supported by the U.S. Air Force Office of Scientific Research, and the National Science Foundation. For further information, please consult the publications below, or contact me at: young@ece.rice.edu

Publications

Resist Characteristics of Poly(Methyl Methacrylate) at Vacuum Ultraviolet Wavelengths for High Resolution Imaging
I. E. Ferincz, C. Toth, and J. F. Young, J. Vacuum Sci. Technol. B Vol. 15 No. 4, July/August 1997 (in press). Abstract

Comparison of Laser-Produced Plasma Target Materials for Pumping the 109 nm Xe(2+) Auger Laser
T. Dennis, H. M. Duiker, J. Wu, C. Toth, and J. F. Young, IEEE J. Selected Topics in Quantum Electronics Vol. 1, 867-871 (1995). Abstract

A Reasonably Practical XUV Laser for Applications
T. S. Clement, C. Toth, J. Wu, and J. F. Young, IEEE Journal of Quantum Electronics Vol. 30, 2136-2140 (1994). Abstract

Optical Gain in the Ionic Excimer Cs(2+)F(-) Excited by Soft X-Rays from a Laser-Produced Plasma
C. Toth, J. F. Young, and R. Sauerbrey, Optics Letters Vol. 18, 2120-2122 (1993). Abstract

Laser-Produced Plasmas as Short Wavelength, Incoherent Optical Sources
J. F. Young, in Methods of Experimental Physics: Atomic, Molecular, and Optical Physics Vol. 3, F. B. Dunning and R. G. Hulet, eds. (Academic Press, due 1997). Abstract

Vacuum Ultraviolet Optical Microscopy
J. F. Young, H. M. Duiker, I. Ferincz, and T. Dennis, SPIE Proceedings Vol. 2524, 201-207 (1995). Abstract

Short Wavelength Microscopy
J. F. Young, IEEE Lasers and Electo-Optics Society Newsletter Vol. 8 No. 3, 1-15 (June 1994).