Optical Microscopy

Optical microscopy, or light microscopy, refers to the inspection of the sample at higher magnification using an instrument known as an optical or light microscope. During optical or light microscope inspection, the specimen is positioned perpendicularly to the axis of the objective lens. Light is then shown on the sample, which reflects some light back to the lens. The image seen in the microscope depends not only on how the specimen is illuminated and positioned, but on the characteristics of the specimen as well.

Fig. 1. A high-end microscope with an image capture system (left) and an ordinary optical microscope (right)

A basic light microscope has the following parts: 1) a lamp to illuminate the specimen; 2) a nose piece to hold 4-5 objectives used in changing the viewing magnification; 3) an aperture diaphragm to adjust the resolution and contrast; 4) a field diaphragm to adjust the field of view; 5) an eye piece to magnify the objective image (usually by 10X); and 6) a stage for manipulating the specimen.

Optical microscopes are commonly classified as either low-power or high-power microscopes. Low-power microscopes are those which typically magnify the specimen at 5X to 60X, although some can magnify up to 100X. High-power microscopes, on the other hand, typically magnify the specimen at 100X to 1000X.

There are three modes by which optical microscopy is commonly conducted, namely, brightfield illumination, darkfield illumination, and interference contrast (Nomarski).

Brightfield illumination is the normal mode of viewing with an optical microscope. This mode provides the most uniform illumination of the sample. Under this mode, a full cone of light is focused by the objective on the sample. The image observed results from the various levels of reflectivities exhibited by the compositional and topographical differences on the surface of the sample.

Under darkfield illumination, the inner circle area of the light cone is blocked, such that the sample is only illuminated by light that impinges on its surface at a glancing angle. This scattered reflected light usually comes from feature edges, particulates, and other irregularities on the sample surface. Darkfield illumination is therefore effective in detecting surface scratches and contamination.

Interference contrast (Nomarski) makes use of polarized light that is divided by a Wollaston prism into two orthogonal light packets. These slightly displaced light packets hit the specimen at two different points and return to the prism through different paths. The differences in the routes of the reflected packets will produce interference contrasts in the image when the packets are recombined by the prism upon their return. Surface defects or features such as etch pits and cracks that are difficult to see under brightfield illumination can stand out clearly under Nomarski mode.

When performing optical microscopy, the following must be observed:

1) The specimen must be positioned perpendicular to the axis of the objective. Otherwise, some regions of the specimen within the viewing area will be out of focus.

2) The mode of operation must be chosen well to meet the desired results. If needed, more than one mode of operation may be employed to characterize the area of interest.

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