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Scanning Past the Confocal Microscope Barrier

Confocal laser scanning microscopy has evolved into 'a technology that can measure, profile, and image in horizontal and vertical planes with submicron accuracy. Commercially available for the past 10 years, the technique has propelled optical imaging beyond what was previously perceived to be the optical diffraction barrier.

The technology is currently being used in many scientific and industrial imaging and measuring applications in the semiconductor, photomask, ceramic, metal, glass, and liquid fields. As the semiconductor industry continues to push dimensions into the submicron range, one of the non-vacuum, non-electron-beam techniques that has evolved is the confocal laser scanning microscope for imaging and measuring submicron defects and critical dimensions on photomasks and wafers.

Laser scanning optical microscopes consist of an optical microscope with a laser-light source, a high-resolution CCD camera, and a precision-drive motor on the z axis of the microscope. The laser, which is scanned in the x direction with an acoustic optical deflector and in the y direction with a galvanometer mirror, is projected down through the optics from the c-mount position of standard optical microscopes.

The laser beam scans the sample at TV rates. Laser light reflected from the sample surface travels back through the same optical path as the projected laser beam, and is received by a high-resolution CCD camera. The laser-generated image is then displayed on a standard TV monitor. Apertures are used in the optical path of the laser to remove the defocused light rays, so that only focused images are displayed. This horizontal, focused slice, called a confocal slice, is created by the aperture. Because the aperture also permits only the brighter center portion of the laser beam to be used, horizontal resolution is improved.

A precision-drive motor moves the stage vertically in 100-angstrom increments, which dissects the horizontal band of focused light rays and rapidly collects a thin series of focused horizontal planes which are stored in memory.

These 100-angstrom horizontal slices are then digitized. Vertical measurements and non-contact surface profiles can be derived from this information with a resolution of 300 angstrom, vertically. Since the digitized x, y, and z information is stored in memory, computer software can reconstruct the image in a 3-D format. A deconvolution of the image can be used to produce a 3-D confocal image with 50-nm resolution.

Color confocal laser scanning microscopes have been developed that produce a true color image of a sample. This has been achieved by projecting three different scanned lasers - red, green, and blue - simultaneously down the optical path.

Portable confocal laser scanning microscopes have also been developed, which allow each user to physically place the microscope on top of samples too large to put under the objective of a standard laboratory optical microscope.

A portable microscope has been used to measure micro cracks on the inside of steel-walled boilers. These cracks have been caused by heat and pressure over time. The depth of the cracks indicates the useful life left in the boiler.

Applications for confocal laser scanning microscopes include measuring the depth of corrosion on metal containers that the government has used to store poisonous gases since the 1940s. This measurement is being made to estimate the life left in the metal containers before leakage occurs. The instrument has also been used to measure and profile the meniscus on the surface of a liquid. There is enough reflected light in the confocal slice at the surface of the liquid to produce an amazingly clear image and profile of the meniscus.

Confocal microscopy has also been used to measure the depth of pit marks on the windows of the space shuttle, which are caused by small meteorites and space debris. Although the shuttle windows are more than 2.5-cm thick, NASA has tight limitations on the depth of pit marks, which defines the reusability of the shuttle windows.

Since the confocal slice consists only of focused light rays from a thin plane, scattered light and defocused light is eliminated from each image. This means highly reflective, non-conductive surfaces such as ceramics can be seen and imaged without coating. These surfaces would be impossible to image with an optical microscope.

Confocal laser scanning microscopes are being used for high-resolution 3-D imaging of semiconductors. They can also show magneto-resistive head submicron veils which can be removed by CO(2) gas systems. The presence of these veils can be rapidly determined with high-resolution images generated by 3-D confocal scanning laser microscopes.

- Tom Pomposo, Lasertec Inc.
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Author:Pamposo, Tom
Publication:R & D
Geographic Code:1USA
Date:Jul 1, 1998
Words:740
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