Find and identify wafer contaminants

Combine fluorescence with infrared microscopy to learn the chemical makeup of particles.

Thomas J. Tague, Jr., CooperSurgical, Shelton, CT -- Test & Measurement World, 4/1/2001

The semiconductor industry has been waiting for an inspection technique that will let users see particle contaminants on integrated wafers and help determine the contaminants’ chemical make-up. New infrared microscopes that allow simultaneous visual observation and collection of infrared (IR) radiation for chemical spectral analysis go a long way toward meeting this need. To enhance the usefulness of an IR microscope/ spectrometer, researchers have added fluorescence illumination. Fluorescence illumination makes viewing and identifying contaminants easier and more accurate.

Fluorescent compounds absorb light and immediately emit it at longer wavelengths than those absorbed. If you’ve seen a glowing poster or shirt exposed to “black” light, you’ve seen fluorescence. People who analyze materials based on their fluorescent characteristics divide samples into two categories: those that exhibit primary fluorescence and those that rely on secondary fluorescence.

Primary fluorescence occurs in minerals and many chemicals due to the structure of the compounds. When your sample doesn’t exhibit primary fluorescence, you can use a fluorescent dye, or fluorochrome, that binds to samples. The attached fluorochrome reveals the presence of the sample by secondary fluorescence, which has made fluorescence microscopy a common analytical tool.

Fortunately for analysis purposes, most contaminants of interest to semiconductor manufacturers exhibit natural fluorescence. Fluorescence works particularly well when you need to detect contaminants or materials against a nonfluorescent or dark background—typical of the surface of a wafer or individual IC. Fluorescence also works well when you must detect small quantities—as few as 50 molecules of a contaminant per square micron.

By combining darkfield fluorescence illumination with IR spectroscopy, inspectors can quickly locate contaminants and identify their composition. That information helps locate and eliminate the source of contamination on a production line.

To see how well fluorescence and IR microscopy techniques would work together, we ran tests using a Continuum IR microscope from Spectra-Tech (Shelton, CT) attached to a Nicolet Instrument Corp. (Madison, WI) Magna 560 IR spectrometer. The spectrometer and microscope function as a single unit. As an excitation source, we used a mercury-arc lamp and Olympus America (Melville, NY) fluorescence cubes to filter out all but a narrow range of wavelengths.

The Olympus U-MNV filter passes violet wavelengths of 400–410 nm, and the U-MWG filter passes green wavelengths of 510–550 nm. Experiments determined which wavelength band produces the most fluorescence, but we found ultraviolet (UV) radiation produces the best “yield” of fluorescence for contaminants. We captured visual microscopy images using a video camera and a standard PC equipped with a frame-grabber card.

We used a single dual-remote masking aperture to define the sample for IR data collection and to minimize diffraction. The remote mask defines the analysis area and blocks spectral information outside the area of interest. We set the aperture to 30x8 µ for reflection microanalysis in the Spectra-Tech µView software package. By using a mouse to draw a box around the area of interest, we set the aperture size. The software averages the infrared spectra over 128 scans and then uses a baseline to correct the spectrum for analysis.

a)         b)

Figure 1. This figure shows photographic images of an integrated wafer under (a) normal reflection illumination and (b) ultraviolet fluorescence illumination.

When we examined a semiconductor wafer using normal brightfield illumination, we observed the image shown in Figure 1a. In this view, it’s difficult to see small contaminant particles on the IC structures. Figure 1b shows the same wafer under darkfield UV fluorescence illumination. The darkfield illumination used for fluorescence microscopy “hides” the nonfluorescing surface features, thus making it easy to identify and mask each particle for IR data collection.

The IR spectrum (Figure 2a) of the yellow particle from Figure 1b shows absorption peaks caused by low molecular weight polystyrene ( Figure 2b). The spectrum in Figure 2c results from mathematically subtracting the polystyrene spectrum from the raw spectrum. We compared the spectrum in Figure 2c to a library of spectra and found that it closely matches that of an alkyl urea resin (Figure 2d).

Figure 2. (a) This IR spectrum was collected from the yellow particle shown in Figure 1b. (b) This reference spectrum of 800-molecular weight polystyrene was obtained from the Hummel Polymer Reference Library. The spectrum in (c) is the subtraction of (b) from (a). We compared the spectrum in (c) to reference spectra and decided it most closely matched that of (d), an alkyl urea resin.

Don’t worry about interpreting the spectra of contaminants. Several companies including Nicolet provide libraries of IR spectra. And during production, the number of possible contaminants is usually limited and their sources easily determined. In most cases, you can customize your own library of spectra to include the primary compounds that could contaminate wafers or ICs.

The particles we examined represent typical contaminants found in semiconductor processing, but you also can apply IR analysis and fluorescence illumination when inspecting PCBs. For example, an analysis that shows the presence of residual water near solder joints may indicate the presence of flux. The ability to quickly locate and identify organic contaminants demonstrates that fluorescence illumination coupled with IR spectroscopy provides a powerful tool for analyzing semiconductors, integrated wafers, and circuit boards. T&MW

For more information

Foster, Barbara, Optimizing Light Microscopy for Biological and Clinical Laboratories, Kendall/Hunt Publishing Co., Dubuque, IA, 1997.

Messerschmidt, Robert G., and Matthew Harthcock, editors, Infrared Microspectroscopy: Theory and Applications (Practical Spectroscopy Series, Vol. 6), Marcel Dekker, New York, NY, 1988.

Thomas J. Tague, Jr. , works as a scientist at CooperSurgical and has 13 years of experience in the areas of spectroscopy and microscopy. He received his PhD in physical chemistry from the University of Utah and a BS in chemistry from the University of Texas at San Antonio.