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Bad Vibrations

Even the slightest vibrations can damage silicon wafers, especially at the latest technology nodes. That's why vibration management is becoming an increasingly important issue for fab managers. Reactec commercial director Tim Kent reports.

As feature size and particulate performance targets become more demanding, vibration levels are becoming a significant limiting factor to equipment performance. Responses from the industry vary: many manufacturers are overlooking solvable problems, while others have invested time and effort to the point of negative returns. One thing is certain: there is a competitive advantage to be gained from reduced vibration levels. However, a measured response is critical.


Particulate performance
When working with silicon wafers up to 300mm diameter, the slightest vibrations can cause particles to come free from the wafer. The damaging effects of the particulate performance are well documented, but can cause considerable wastage of valuable materials, delays in throughput and impacts on yield quality.
Unwanted vibrations can come from a variety of sources: pumps, fans, motors and vacuum devices, as well as other equipment in the cleanroom itself, all create vibrations which in turn will increase the potential danger of putting particulates into the air.


Feature size
As the industry reduces the feature size on semiconductors, the greater the impact of vibration becomes. And as the pace gathers towards meeting 65nm challenges and beyond, manufacturers are finding vibration issues increasingly on the agenda. Tolerances are now so fine that any movement can lead to considerable wastage of valuable components or materials. Vibrations can effect the precision positioning of elements such as laser/ion beams or wafers themselves. Low frequency vibrations (<150Hz) tend to cause most problems and can be the most difficult to remove.
In the 1990s, many companies employed specialised vibration control engineers to solve these problems, but they are very rare today. There has never been a one-size-fits-all solution and the characteristics and sources of the vibration must be examined carefully. There may be multiple sources of vibration, each of which may require a different solution.
There are numerous off-the-shelf vibration products and materials, though these do need to be selected with a certain amount of care. Quite often, manufacturers have implemented measures that they feel intuitively should constitute a solution, but have in fact compounded a problem. In other cases, the sheer complexity of vibration problems has led some manufacturers to skirt around the issue rather than tackling it head-on.
Fortunately many vibration problems can be cured quickly and cost-effectively. According to the nature of the vibration problem, the solutions can range from passive (springs and rubber dampers) to electronically controlled devices. These high-tech tools fit into two categories, active and semi-active, and use feedback techniques to compensate for vibration.
Active devices generally create a counterforce or "anti-wave", whereby a force is generated with symmetrical characteristics, out of phase, to the object vibrating. Semi-active devices are also adaptable, but are used more commonly where variable material properties are needed in one device, such as on damper providing different damping levels for different environments or operations.
Active and semi-active technologies have become realistic options in recent years through huge gains in accuracy and cost. Integrated sensors can be attached to equipment to monitor vibration levels to work alongside smart materials. Smart materials are a special class of material that have advanced to a stage where they can be easily retrofitted into equipment, and can change properties to match the environment, such as changing the effectiveness stiffness.


The benefits of vibration reduction/elimination can be summarised as follows:
l Feature size can be reduced
l Beam/wafer positioning stabilised
l Performance improvement
l Reduce wastage levels by the elimination of particulates
l Time savings: prevent months elapsing when trying to identify and resolve individual problems due to vibration
l Relax expensive design constraints of the cleanroom (eg less expensive floors)
l Reduce time to market for equipment.














Solutions to vibration problems can range from springs and rubber dampers (above) to electronically controlled devices that use feedback techniques to compensate for vibration (below)


 







 










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