by Robert P. Donovan
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Past columns have discussed the measurement of particles entrained in various mediums, using well-established optical scattering instruments. Of more direct impact and interest to a manufacturer, however, are the particles on a product surface. And indeed commercial instruments — surface scanners — are readily available for measuring particles on surfaces, the most common instruments being those based on light scattering or, alternatively, optical imaging.
However, both these optical instrument designs are limited with respect to the minimum size of particle that can be detected. These limitations are becoming more important with the shrinking critical dimensions of each new generation of device. The limitations exist when counting particles on bare silicon wafers and become even more complex when scanning patterned wafers and wafers with trenches.
One problem is that even bare wafer surfaces are not optically flat but have a measurable roughness. This surface roughness contributes a scattering signal of its own, which establishes a background noise level that the light intensity signal from a surface particle must exceed in order to be detected and counted. The most commonly used light scattering instruments, compatible with at-line measurements, cannot distinguish particles smaller than about 0.1 micron from this rough-surface component of state-of-the-art wafers. One has to resort to scanning electron microscopy (SEM) to obtain information about smaller surface particles and SEM is seldom used as a production tool.
There have been a number of attempts to do better with light scattering instruments. One group in Germany, for example, used a fluorescent microscope in a series of experiments to document the size dependence of particle deposition on surfaces.1 They published data based on fluorescent polystyrene latex (PSL) particles of nominally 0.08 micron diameter. This detection capability pushed the sensitivity of a surface scanner modestly but, unfortunately, most real particles do not fluoresce so that the impact of this technique in manufacturing has been limited.
A more contemporary but more subtle approach has been to analyze light scattering signals in sufficient detail to be able to distinguish the light scattering signals of particles from those of surface structure or roughness. Looking at multiple scattering angles with various polarizations of incident light, some instrument manufacturers now claim particle detection limits as low as 0.07 micron. Such surface scanners are more sophisticated than simple single-beam scattering designs and have pushed the light signal processing approach near its limits.
A different approach is now called for.
What is needed is an instrument that is a surface particle analog to the condensation particle counter (CPC) which has been successful in counting aerosol particles in the 0.01- to 0.1-micron size range and lower (see CleanRooms, May 2000, p.8). I recall a promising set of experiments carried out by Venu Menon [0.18 micron program manager, Texas Instruments Inc.] while at the Research Triangle Institute. Menon's technique was to grow small surface particles into large surface particles by simply flooding the wafer surface with a volatile liquid such as isopropyl alcohol (IPA) and allowing that liquid to evaporate [US patent 5061068].
The last bit of IPA to evaporate was a puddle surrounding each surface particle, creating a transient surface scattering center much larger in diameter than that of the particle in the middle of the puddle. This procedure could be confirmed by depositing easily countable particles, 0.3-micron in diameter, on a wafer, making a map of that deposition pattern and then raising the detection threshold of the scanner to 0.5-micron, causing the particle pattern to disappear.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM. His Sandia project work is developing technology for recycling spent rinse waters from semiconductor wet benches.
References
- Schmidt, F., K. G. Schmidt and H. Fissan, “Methods for Detection of Submicron Particles by a Lightmicroscope,” J. Aerosol Sci. 21, 1990, pp S535-S538.