When specimens are larger than about 1 mm by 1 mm in size, and imaging must be accomplished by point-by-point measurements across the specimen, such measurements are often made using scanning stage laser microscopes. In scanning stage laser microscopes, the sample is moved in a raster scan under a stationary focused laser beam. Such microscopes have good spatial resolution, and can image both large and small specimens, but are slow. Because of this ability to image large specimens, scanning stage microscopes are frequently used for recording Optical Beam Induced Current (OBIC) or Optical Beam Induced Voltage images or maps of semiconductor materials and devices. An early scanning stage OBIC microscope was described by Oliver and Dixon.sup.1, and the use of such a microscope for OBIC imaging of infrared focal plane arrays was described by Moore.sup.2 et al. Scanning stage microscopes have also been used for photoluminescence (PL)imaging or mapping of semiconductor devices and wafers, as described by Hovel.sup.3 and by Moore and Miner.sup.4. If a scanning stage microscope is optimized for imaging large specimens, the stage must move large distances very quickly, and this is incompatible with the precision movement required for high resolution imaging of small areas on the same specimen. Some scanning stage mapping systems attempt to overcome this disadvantage by mounting small, high resolution stages on top of large, high speed low resolution stages, so that both high resolution, small-area scans can be accomplished as well as rapid, low resolution large-area scans. This technique has several disadvantages: the two sets of stages require separate controllers, and four stages are required, adding extra cost, and the high resolution stages add extra mass that must be moved by the high speed, low resolution stages.
Another technique for recording high resolution PL images of large semiconductor specimens is described by Carver.sup.5, who uses a scanning beam to record high resolution images of a 250 micron by 250 micron area, and then translates the specimen to image other areas. Because this technique measures PL across the whole specimen at high resolution, it results in very large data files and is slow. FNT .sup.1 B. A. Oliver and A. E. Dixon, "Laser beam induced current measurements of minority carrier diffusion length", Canadian Journal of Physics 65, 814-820, 1987. FNT .sup.2 C. J. L. Moore, J. Hennessy, J. Bajaj, and W. E. Tennant, "Finding Faults in Focal Plane Arrays", Photonics Spectra, p.161, September 1988. FNT .sup.3 H. J. Hovel, "Scanned photoluminescence of semiconductors", Semicond. Sci. Technol. 7, A1-A9, 1992. FNT .sup.4 C. J. L. Moore and C. J. Miner, "A Spatially-Resolved Spectrally-Resolved Photoluminescence Mapping System", J. Crystal Growth 103, 21-27, 1990. FNT .sup.5 G. E. Carver, "Scanned photoluminescence with high spatial resolution in semi-insulating GaAs and InP", Semicond. Sci. Technol. 7, A53-A58, 1992.
A prior art scanning beam imaging system for macroscopic specimens (a macroscope) was described by Dixon and Damaskinos.sup.6, and one embodiment is shown in FIG. 1. This scanning-beam system includes a confocal detector for reflected light or fluorescence imaging, and an additional non-confocal detector, usually used for reflected light or photoluminescence imaging. A large scan area is achieved by using a telecentric laser scan lens instead of a microscope objective. Laser beam 103 from laser 102 passes through a beam expander and spatial filter comprised of lens 104, pinhole 106 and lens 108, and is expanded to match the size of the entrance pupil of laser scan lens 128 (the laser scan lens used in this embodiment is a telecentric f*theta lens with a flat focal plane). The beam is deflected in the X-Y plane by first scanning mirror 1141, which rotates about an axis parallel to the Z-direction. Lens 116 of focal length f1 is placed a distance f1 from scanning mirror 114. Lens 118, of focal length f1, is placed a distance 2f1 from lens 116, and a distance f1 from scanning mirror 120. This brings the scanning beam back to the center of second scanning mirror 120 as a parallel beam. Second scanning mirror 120 rotates about an axis parallel to the X-direction, and imparts a deflection in the Y-Z plane. Laser scan lens 128 is placed such that the center of it's entrance pupil coincides with the center of scanning mirror 120, and it focuses the incoming scanning beam through beamsplitter 152 to a diffraction-limited spot in macroscopic specimen 130. Light reflected, scattered or emitted from the focus spot in the specimen travels back toward laser scan lens 128, and part of this returning light is reflected by beamsplitter 152 towards condenser lens 154 and detector 156 so that a non-confocal image can be obtained using detector 156. Part of the light transmitted by beamsplitter 152 is collected by laser scan lens 128, passes back through the scan system, and is partially reflected by beamsplitter 112 toward lens 136, pinhole 138 (at the focal point of lens 136) and detector 140. Only light which is part of a parallel beam entering lens 136 will be focused to pass through pinhole 138 and be detected. Thus the combination of lens 136, pinhole 138 and detector 140 act as a confocal detector, detecting only light originating at the focus spot in specimen 130, and rejecting light from any other point. This allows the macroscope to perform optical image slicing. The signal from detector 140 (or detector 156) is digitized using a frame grabber 160 as the raster scan proceeds, and the image is displayed on a high resolution computer screen 162. For low resolution, large area OBIC images, OBIC amplifier 158 (current-to-voltage converter) is attached to the specimen to measure OBIC, and the output signal from the OBIC amplifier is digitized and displayed. The macroscope does an excellent job of imaging large specimens, but its zoom capability is limited by the small numerical aperture (NA) of the laser scan lens, which results in a spot size that is large compared to that used in a confocal scanning laser microscope. FNT .sup.6 A. E. Dixon and S. Damaskinos, "Scanning Laser Imaging System", U.S. Patent Application, August 1993.
When specimens are smaller than about 1 mm by 1 mm, a scanning beam laser microscope is often used to image the specimen using reflected light, fluorescence, photoluminescence, OBIC, and other contrast mechanisms. A prior art confocal scanning beam microscope is shown in FIG. 2. In this microscope, a light beam 103 from laser 102 passes through a spatial filter and beam expander comprised of lens 104, pinhole 106 and lens 108. The beam passes through beamsplitter 112 and through a scan system comprising first scanning mirror 114, lenses 116 and 118, second scanning mirror 120, and lenses 200 and 202, as shown. This results in a scanning beam that enters microscope objective 204 with the beam crossing the optic axis of the microscope at the position of the entrance pupil of microscope objective 204, which focuses the beam to a diffraction-limited spot in specimen 130. The position of the focal plane is changed by moving focusing stage 208 in the axial (Z) direction. Light reflected, scattered or emitted from the specimen is collected by the microscope objective, travels back through the scan system (and is descanned), and is partially reflected by beamsplitter 112 into the confocal detection arm comprised of lens 136, pinhole 138, and detector 140, and is detected. For high resolution OBIC measurements, an optical beam induced current amplifier (current-to-voltage converter) is connected to the specimen, and OBIC contrast is displayed on the microscope's computer screen. Scanning beam laser microscopes provide high resolution and rapid scan, but the scan area is limited to the field of view of a microscope objective.
Another prior-art embodiment of confocal scanning-beam optical microscopes is the class of microscopes known as Nipkow Disk microscopes. The microscopes in this class were described by Gordon Kino.sup.7, and a particularly simple and useful embodiment is the real time scanning optical microscope described in FIG. 2 of Kino's paper. These microscopes are different from the confocal microscope already described in that a large number of pinholes in a rotating disk are the source of a large number of scanning beams which are focused on the specimen simultaneously, and reflected or fluorescent light beams from these focused spots are detected simultaneously. FNT .sup.7 G. S. Kino, "Efficiency in Nipkow Disk Microscopes", in "The Handbook of Biological Confocal Microscopy", pp. 93-99, IMR Press, Madison, Wis. 53706, Edited by J. Pawley, 1989.