Near Field Scanning Optical Microscopy and Spectroscopy of Electronic Materials and Structures
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ABSTRACT A Near Field Scanning Optical Microscope (NSOM) with spectroscopic capability is applied to imaging semiconductor and microelectronic structures. NSOM combined with spectroscopic analysis provides physical and chemical information of thin films and defects with ultra high spatial resolution. We have studied epitaxial and bulk samples and partially fabricated Si0 2/Si CMOS structures to investigate the spatial resolution and imaging modes of NSOM. Reflected intensity contrast in NSOM yields images of defect networks in InGaAs/InAlAs/GaAs epitaxial layers and shows thickness variations in SiO 2 films on Si. Surface topological changes observed in NSOM demonstrate a spatial resolution of significantly better than 0.25 [im. Fluorescence imaging is examined for chemically identifying materials and defects.
I.
INTRODUCTION
Increases in density of integration and concomitant reductions in minimum feature size have driven spatial resolution requirements for analytical and metrological instrumentation to well below the limit set by far field diffraction of visible light. Analytical and metrological instruments with spatial resolutions adequate for the current generation of CMOS all rely on ion or electron methods. However, these instruments are destructive and have a high cost of ownership. The near-field scanning optical microscope (NSOM) is a scanned probe microscope (SPM) capable of both imaging and spectroscopy at visible and near visible wavelengths with spatial resolution in the 20-50 nm range.1,2 The breakthrough enabling the advancement of NSOM was the introduction of the adiabatically tapered fiber optic probe tip capable of high transmission efficiency (105) while exhibiting tip diameters as small as 20 nm. 4 NSOM has previously been applied to imaging biological surfaces and for single 7 P1 molecule • detection. ' It has also been used for chemical sensing, for semiconductor 8-11 11 and for surface modification and nanofabrication.12,13 We have studied epitaxial spectroscopy, and bulk samples and Si0 2/Si structures as vehicles to examine spatial resolution and imaging modes in NSOM.
II.
EXPERIMENTAL
The instrument used in this work is a modified Topometrix, Inc., Aurora NSOM and is shown diagrammatically in Figure 1. The current system employs the tripod piezoelectric XYZ sample scanner, piezoelectric tip dither tube, support electronics, and software from the Aurora platform. The system was modified to allow flexibility of laser sources, and to provide high collection efficiency and spectral analysis of collected light. Key components to the system are a wavelength filtered laser source, single mode fiber optic delivery system, near field tip and 183 Mat. Res. Soc. Symp. Proc. Vol. 406 01996 Materials Research Society
Single Element 0
Laser Tip and Dithering
Dithring• [••.•Spectrometer
Quad Detector Sample and X,Y,Z Stage
Figure 1. Schematic diagram of the NSOM spectroscopy instrument. dithering piezo, XYZ sample scanners, far field collection optics, and spectroscopic detection system. Sho
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