Confocal microscopy expands the horizons of the inspection industry to the nanometer range by removing out-of-focus elements and by providing a nearly diffraction-free focal plane. Add software to this mix and you get crisp, 3-D images.
Because today’s micro-electronic structures typically involve multiple metal and dielectric layers in a very small geometry, using conventional microscopes to view them at high magnification provides a limited depth of focus. Confocal microscopy views these structures with high resolution and superior depth of focus.
How does the confocal microscope manage to supply these images? It passes light through a pinhole and, using an objective lens, focuses it to a point on the sample. But only when the specimen is exactly in the focal plane of the objective and illumination source is it in a confocal position.
When the sample is in the focal plane, the reflected light is focused back at the pinhole and the maximum amount of light returns through the hole. Light not in the focal plane is reflected, partially defocused or blocked at the pinhole, leaving a high-contrast image.
To furnish the 3-D likeness, a sharply focused image must be formed by capturing several thin, horizontal layers. The data from these horizontal slices is sorted and compiled by computer to create a multidimensional, brilliantly clear reproduction of the sample.
From the 3-D view, you obtain a variety of views and measurements with a resolution that can be as much as twice that of the conventional microscope. The real power of the confocal microscope, however, is the increased contrast that results because the out-of-focus objects appear black, improving the visibility of the desired image.
Imaging Techniques
The confocal microscope uses several technologies to provide an image: the spinning or Nipkow disk, the raster scanning stage, laser scanners and acousto-optical scanners. The stage-scanning microscope moves the sample past the optical axis while the Nipkow and laser types hold the sample stationary.
The Nipkow instrument spins a disk containing several thousand pinholes arranged in a spiral pattern (Figure 1). When the disk spins, the light illuminates the holes in the disk and a corresponding number of spots on the sample. The light reflected from a spot on the sample passes back and is focused onto the same pinhole from which it entered. The reflected light returns through these holes and a confocal image can be viewed.
The spinning disk uses a high-intensity light source including halogen, xenon and mercury. The scattered light is transformed into color information corresponding to the varying heights of the substrate topography. The color variation is a result of chromatic aberrations inherent to the objective. Each layer is characterized by a single color, causing any defects or irregularities to be enhanced and easily seen.
The disk-scanning technology can generate 1,000 frames/s. And as you may imagine, it requires the pinholes be in the right place at precisely the right time for light to pass through correctly. Any misregistration or aberration in the objective lens and the light beam misses the pinhole, losing the signal.
For the laser-scanning microscope (Figure 2), the sample is usually stationary, except for depth scanning along the Z axis. It uses vibrating or rotating mirrors to direct the laser across the sample in the X-Y plane. A computer digitizes the signal from the sample and reconstructs the image.
Many laser wavelengths are available for sample investigation. Wavelengths of <450 nm are not a good choice for samples with passivation or oxide on the surface of the semiconductor because the light is absorbed. A longer wavelength, such as 633 nm, is better because it passes easily through the passivation.Laser confocal microscopy operates much more slowly than the disk-scanning type, typically at 1 frame/s. But the laser focuses all the light energy on one spot, providing more than enough energy for fluorescence imaging with high resolution in the nanometer range. A good application is the imaging of biological fluorescent samples, such as tissue slices or proteins.
The acousto-optical scanner is comprised of a rectangular optical crystal with piezoelectric transducers attached to one end. It can scan at a slow rate for the best spatial resolution and a fast rate for the best temporal resolution.
When the transducers of the acousto-optical instrument are driven at high frequencies, an acoustic wave is generated to deflect the laser beam. The acoustic wave can deflect the laser at a very fast rate, scanning up to 480 frames/s so you can view rapidly occurring events.
The trade-off for such high speed is the need for large amounts of data storage. For example, some applications use up to 10 MB of data per second. When this happens, special considerations must be made for acquiring and processing large quantities of data, including the use of optical disk drives.
Aspects to Consider
The ability to see deep into a sample is one of confocal microscopy’s finest attributes. The working distance of the objective theoretically determines how deeply within the sample the confocal system can scan, which makes it a crucial capability of the instrument.
But, working distance can be a cumbersome limitation if you are viewing IC chips mounted in a package. The chip is below the level of the top of the package and you must peer down amid a variety of densely packed features, much like looking down the skyscrapers of a city to study a tiny feature on the sidewalk. There is a practical limit to good imaging, which is the working distance minus the difference in the heights of the features.1
When looking for a confocal microscope, especially one with real-time performance, consider these features outlined by Buddy Bossmann of Carl Zeiss:
o A halogen light source, which is less costly than a mercury arc lamp. However, when a mercury lamp is required, the microscope system should directly control the intensity of the lamp.
o Microscope integration—the ability of a single keystroke to integrate microscope functions to provide a confocal image. This greatly reduces training time and allows you to easily change between viewing modes.
o Automatic focus.
o Programmability—the ability to program all objective and reflector combinations as well as the aperture size and lamp intensity from the keyboard.
o Support of multiple video ports for direct viewing and image management. It should also be RS-232 compatible.
o Software for reconstructing images for defect review and characterization.
o Design of confocal light path based on an understanding of the objective chromatic aberration. This allows the color image to be fully optimized and color sectioning to be correlated to the sample. Image reconstruction is also more accurate when the objective lens is properly designed.
Contacting the companies that manufacture confocal microscopes is another way to get the information you need. They can tell you what equipment you need, especially the appropriate objective lenses with the right working distance, wavelength correction and numerical aperture.
Reference
1. Scott, M., “Choosing Optics for Confocal Microscopy,” American Laboratory, April 1992, pp. 32-37.
Acknowledgments
Material for this article was provided by Buddy Bossmann and Terry Seregely of Carl Zeiss; Kirk Czymmek, Ph.D., Applications Scientist at Noran Instruments; and Guoquing Xiao, Principal Scientist at Technical Instrument.
Confocal Microscopy Products
Confocal Module Microscope
Provides Automated Controls
The Confocal Scan Module (CSM) combines with the company’s Axiotron 2 Microscope to provide an automated white-light imaging system. The CSM uses a Nipkow spinning disk to optically section image height information. Scattered light is transformed into colors indicating height, and enhanced optics and reflectors increase light efficiency. A keypad controls the five-position nosepiece, the four-position reflector, the motorized aperture diaphragm and illumination levels. An internal motorized and programmable aperture diaphragm allows the contrast to be varied; two reflectors enable the use of brightfield and DIC confocal modes. Carl Zeiss, (800) 233-2343.
Confocal Module Integrated
With Imaging Optics
The Polycon Real-Time Confocal Microscope integrates confocal modules with improved resolution and enhanced sample contrast over previous models. The confocal image and standard illumination techniques can be compared by toggling a lever. The unit is equipped with an integrated tilting viewing head. Water immersion objectives improve definition. A 100-W DC mercury burner is used for confocal imaging. A port accommodates TV cameras and a 35-mm camera. Leica, (800) 248-0123.
Confocal System Provides
3-D Imaging in Real Time
The Optiphot 200C Confocal Inspection System provides imaging of 3-D samples in real time. It offers sub-half-micron resolution through the use of the company’s CF® optics, which are diffraction-limited, aberration-free and combine high numerical apertures and long working distances. A built-in aperture slider with varying diameter pinholes allows you to adjust the system according to the numerical aperture of the objective. The light source is either a mercury or a halogen lamp. Nikon, (516) 547-8531.
Add-On Accommodates
3-D Inspection in Real Time
The Ultra-fast 3-D Package is an option for the company’s ODYSSEY® XL with INTERVISION™ Confocal Laser Scanning Microscopy System. It uses a piezoelectric-driven objective lens controller to acquire 15 optical image slices/s. A lens positioner placed between the objective lens and nosepiece turret moves the lens in rapid, repeatable 100-m m steps. The positioner adapts to Carl Zeiss and Nikon microscopes. The software synchronizes optical sectioning with the digital capture capabilities of the system. Noran Instruments, (608) 831-6511.
Confocal Metrology System
Measures Photomasks
The KMS 310 Confocal Measuring System measures photomask structures at the development, etch and pelliclized stages. It features automated alignment, regular or random dense-array pattern recognition, automatic focus and illumination control, and digital image calibration. The system performs 500 measurements/h on an 8″ ´ 8″ motorized stage with a repeatability of <2 microns. Technical Instrument, (415) 431-8231.Copyright 1996 Nelson Publishing Inc.
April 1996