Megasonic cleaning steps into the spotlight
09/01/2001
New technologies could also help reduce chemical consumption in the cleanroom
Hank Hogan
SAN FRANCISCO--IN MEGASONIC cleaning, as the name implies, megahertz sound waves are used to scrub surfaces clean of particles and contamination. The advantages for cleanroom industries, such as semiconductor manufacturing, could loom large. There could be reductions, for instance, in chemical consumption.
Two recent product announcements, one from Verteq Inc. (Santa Ana, CA) and another from ProSys Inc. (Campbell, CA), may make megasonic cleaning sparkle even brighter.
 Verteq's Goldfinger Mach2 (above) uses megasonic technology to clean wafer surfaces, while ProSys's microprobe (right) monitors the effectiveness of megasonic cleaning. |
"By the use of sonic energy, we're able to get by with extremely dilute forms of the same chemistry, which normally would have to be used concentrated," says Michael Olesen, a senior member of the technical staff at Verteq, which recently unveiled its Goldfinger Mach2 single-wafer cleaning system.
The rapid-fire sound fluctuations in megasonic cleaning, he explains, give rise to pressure waves and small bubbles in a liquid, or cavitation. It's the cavitation that powers the clean. Some of the advantages to this bubbly approach are that there's no physical contact involved and part of the cleaning energy can come from the sound itself. Thus, even unreactive water can be used to blast contamination away.
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However, megasonic cleaning and its scouring bubbles can also blast away fine lines on semiconductor chips, lifting those delicate features right off the surface. According to Olesen, these problems arise from the size of the bubbles, the concentrated nature of the sonic energy, and the amount of pressure generated. The first factor is driven by the frequency of the sonic transducer. The second and third components are an interaction between transducer power, equipment design, chemistry and environmental conditions.
Verteq's answer to this indiscriminate cleaning problem is its soon-to-be-patented sonic transducer design, which is found in the Goldfinger Mach2. Innovative technology allows the Goldfinger transducer to diffuse the sonic energy and filter out harmful sound frequencies.
"We have qualified data that we can produce cleans on 120 nanometer structures, poly gate stack structures, which really are one of the most sensitive structures, without damage," asserts Olesen.
ProSys is another manufacturer of megasonic cleaning equipment. This, however, isn't a new megasonic cleaner. Rather, the ProSys MicroProbe is a tool for monitoring and evaluating the effectiveness of both megasonic and ultrasonic cleaning baths.
Doug Swanson, president and chief operating officer of ProSys, points out that today's instruments measure sonic-induced cavitation indirectly. The most popular methods involve pressure measurement. However, says Swanson, this isn't a direct reading of cavitation. So, ProSys has licensed a technology based on light. The company won't reveal the source of the license or the exact nature of the technique. Swanson does say it depends on photon emission, which enables the monitoring of cavitation in real time.
The tool, which Swanson says has a target price under $10,000, will have to be calibrated for use in particular environments and cleaning solutions. The first units, which will ship this month, are portable and can be used in conjunction with a laptop. The MicroProbe can monitor both megasonic and ultrasonic baths, providing information that could increase cleanliness and cut costs.
"You could monitor the depletion of the fluid," says Swanson. "That's a big, big cost savings to most people."