Detecting Megasonic Bubbles on Computer Chips

In the multibillion-dollar semiconductor industry, there has been no reliable way to monitor silicon wafers as they undergo dozens of crucial "megasonic" cleaning steps, in which the wafer is immersed in a liquid and blasted with very-high-frequency (megahertz) sound waves. By generating scrubbing bubbles in the liquid, megasonic cleaning does an excellent job of removing impurities such as very small particles.

However, the process (possibly through the action of overzealous "killer bubbles") can inadvertently damage circuit components and thereby reduce yields of computer chips. Collateral damage from megasonic cleaning only stands to worsen in the future as new processors shrink further: for example, the new Apple Power Mac G5 has 90-nm features.

At last week's meeting of the Acoustical Society of America in San Diego, Gary W. Ferrell (gferrell@us.sez.com) of SEZ America, Inc., a Silicon Valley office of an Austrian electronics firm, described a new optical probe for monitoring--and potentially reducing--the side effects of megasonic cleaning. Ferrell and coworkers take advantage of the fact that megasonic cleaning generates "multibubble sonoluminescence" (MBSL), the emission of light from multiple bubbles as they collapse in the liquid.

Therefore, the team has developed "sonoluminescence imaging" which maps the location of the collapsing bubbles. By comparing the location of the collapsed bubbles with optical images of removed particles, they can currently monitor the removal of 100-nm-and-larger objects in the chip. Already, they have used sonoluminescence imaging to increase the efficiency of megasonic cleaning.

With their new tool, the researchers also aim to make megasonic cleaning more uniform throughout the chip. Their optical probe is possibly the first practical application of sonoluminescence, which up to now has resided primarily in the realm of basic science. (Paper 2pPA6 at meeting; abstract at http://asa.aip.org/web2/asa/abstracts/search.nov04/asa283.html)