Reuben Hale, P.E.
Ph: (510) 507-1300
Acoustic Noise Sensitivity - Sensitivity to acoustic noise is very important and is often not well understood. Acoustic noise problems are often misattributed to floor vibration. Indeed, the "Achilles Heal" of many high resolution tools, with good vibration isolation, is the mechanical vibration driven by acoustic noise. The structural dynamics and geometry of the isolated structure determine its acoustic sensitivity. We understand this puzzle and have the tools and experience to engineer solutions.
Acoustic sensitivity testing is often not performed, and when it is, it is often done poorly. Often a total SPL level is given that may match the tool manufacturing site, with no consideration to the frequency dependence of the tool's sensitivity.
Response Dynamics has developed testing methodologies to give the best estimates of acoustic sensitivity we have seen in our 30-plus years of work. We understand the challenges to testing in a semi-reverberant environment, and how to mitigate and take into account the many acoustic room modes that will effect the testing.
We have performed these tests on many high resolution tools including:
- Scanning Electron Microscopes
- Atomic Force Microscopes
- Precision Timing Electronics
We have developed techniques to translate our test data to a meaningful and effective acoustic noise specification that can be used for site evaluation and customer acceptance. Our sensitivity testing often also involves vibration and magnetic field tests. We sometimes perform these tests to base line a tools acoustic noise, vibration, or magnetic field sensitivity before making improvements to the tool to improve its performance and make it less sensitive to disturbance.
The Acoustic Specification - The frequency dependent sensitivity estimates must often be translated in to a specification for the customer. The vibration, acoustic, and magnetic field specification serves as an understanding between the manufacturer and the customer. If the tool has a performance problem and the site no longer meets the specification then the customer may take responsibility to analyze the issue and fix the facility problem. If the tool has a performance problem due to facility disturbance and yet the levels are within the specifications, then the tool manufacturer may be asked fix the tool, or take the tool back.
We have found that the worst scenario is when the specification is too lax and the disturbance becomes an intermittent problem. At this point the customer has gone through all the preparation work, installed the tool, and gone through training, only to find that the tool does not perform well and that the manufacturer is to blame because they really didn't know the sensitivity of their product. On the other hand, a specification that is too hard to meet may cause the customer to choose a competitor's product that has a less stringent specification.
Thus, we feel that the best approach is to create a realistic specification that will accommodate the widest variety of installation sites and yet still provide confidence that it can be used to reject the sites that really would result in poor tool performance and unhappy customers. To accomplish this, thorough testing must be done on multiple tools to insure that the sensitivity measured is representative of the actual population of manufactured tools, given system variations, critical sensitive structures and processes.
There are often people not familure with acoustic sensitivity who will wonder how a 500 lb air isolated granite slab, or steel SEM chamber can be moved by acoustics noise. A simple back-of-the-envelope calc can help put it in perspective: Consider a single tone acoustic exitation at, say 40 Hz, with a modest 75 dB sound level acting on an air isolated granite base with area of 1 m^2, weighing 200 kg, we would expect about 9 nanometers of granite moiton. An optical structure with an unforturnately placed resonance at 40 Hz with 1% critical damping would have about 50x amplification and result in about 445 nanometers of optics distrubance. Stiffen the optical structure to get a 100 Hz resonance and double the damping are we get about 35 nanometers of disturbance.
- Sensitivity Testing Topics Include:
Overview, Floor Vibration Sensitivity, Acoustic Noise Sensitivity, Magnetic Field Sensitivity