Engineeringtalk
- November 2006
Edited by the Engineeringtalk editorial team
Vibration isolation is key to accuracy
As nanotech applications become more diverse, the need for reliable vibration control has become increasingly critical, says Jim McMahon.
It wasn't too long ago that making the decision where to locate your scanning probe microscope was a simple one - put it in the basement where the ambient vibration was minimised. But recently, with nanotechnology applications growing exponentially, scientists and engineers are putting their equipment in a multitude of locations where vibration noise is significantly high. Scanning probe microscopes, interferometers and stylus profilers are being sited in locations that pose a serious challenge to vibration isolation.
Additionally, in an effort to keep their nano-equipment costs as low as possible by cutting out the peripherals, many academics and industries are not adequately providing for vibration isolation on their ultra-sensitive nano-equipment that they are putting into their facilities. Although high-budget installations (valued in the hundreds-of-thousands of dollars) typically incorporate adequate vibration isolation, this is not the case with many lesser-budget setups (those spending under US $120,000 for equipment), which represents the area of most rapid growth in the nanotechnology universe. It is estimated that 40-50% of these sites, in both academia and industry, are initiated with inadequate vibration isolation.
The above is influenced to some degree by the fact that those
specifying nano-equipment do not always fully grasp the extreme
sensitivity of the instruments, and that they require proper
site selection and vibration isolation. With any type of microscope
or other nano-instrument, even a high-powered optical microscope,
you have got to put noise isolation there or you will end
up with diffused and fuzzy imaging, and sometimes no image
at all, resulting in reduced operability of a facility's nano-equipment.
"Vibration isolators are one of those necessities that
people are not really focused on when they go to purchase
an instrument like an AFM (atomic force microscope)",
says George McMurtry, CEO of NanoAndMore USA, a distributor
of AFM probes and nanotechnology peripherals. "But it
is different with the bigger scanning electron microscopes
and transmission electron microscopes, because you are dealing
with a very expensive piece of gear that technically needs
all sorts of isolation in order to work properly".
"They are more apt to talk about it right up front". "But when you get into the smaller instruments like white light interferometers, laser interferometers, stylus profilers, and atomic force microscopes, then you get problems with site preparation", continues McMurtry.
"In many cases there is not a lot of site prep done,
despite the fact that they may be sitting on the 4th floor
of a building, and without isolation will end up getting really
poor images".
"When we are talking on the phone with clients, they
ask us what probe will solve a specific problem that they
are experiencing", Says McMurtry. "Sometimes no
probe will solve the problem, they first have to solve their
noise problem". "And that means looking at some
sort of mechanical isolation".
As instrumentation gets more and more complex, and measurements are being done at a smaller and smaller level, those vibrations that are present will start to dominate, and the need for more effective isolation increases. Isolators have been used since the beginning with atomic force microscopes in the 1980s, but there weren't that many AFMs then, and most of them were in basements. The use of nano-instrumentation has grown dramatically, and the need for increased isolation has followed that trend.
"There are so many more people using AFMs in so many different environments that isolators are needed more often", says Mark Flowers, President of Nanoscience Instruments, a distributor of atomic force microscopy products. "In the early days you could put your AFM in the basement of your building, now people want to use their AFMs on the 3rd floor". "But in the basement you are going to have a much better environment, and you could get by with not a sophisticated isolator". "Many times the consumer is unaware of the need for isolation", continues Flowers.
"We discuss with the user the type of environment the equipment will be going into, what applications they are going after, and then determine what they will need in an isolator". "For example, we are seeing a great growth in the education market". "There are lot of initiatives out there to expose undergraduates and high school students to nanotechnology". "But, they don't necessarily know what they need to sustain proper isolation".
The vibrations are usually very subtle. What you will not feel with your hands or feet would cause considerable noise and disturbance to an AFM or interferometer. The noise is caused by a multitude of things, it is not just originating from one spot. Every single building is making noise. Depending on how high up off the ground you are, and how old the building is, you are going to get a constant vibration.
Within the building itself you have things that are going to create even more vibrations, such as the heating and ventilation system, fan s, pumps that are not properly isolated, and elevators. These mechanical devices create a tremendous amount of vibration in the building, and depending on how far away the instruments are from it, they may or may not be adversely affected. External to the building, your equipment may be influenced by vibrations from adjacent traffic, wind, construction, and other elements.
These internal and external influences cause lower frequency
vibrations, which raise havoc with nano-instrumentation. The
wind blows and you get a little movement. Just the sway of
it is around 2Hz, and causes a substantial resonance. A train
near the building can cause movement in the cement slab, nothing
that a viewer would ever feel, but for instrumentation purposes
it has disastrous consequences.
Imagine trying to measure a very few angstroms or nanometres
of displacement, and you have got to have an absolutely stable
surface on which to rest your instrument. If you don't, any
of that vibration transferred into the mechanical structure
of your instrument will cause vertical noise and a fundamental
inability to measure these kinds of high-resolution features.
An isolator is used to solve a problem, and how bad the problem
is determines the solution you need.
Since the 1960s air tables have been used for isolation.
Basically cans of air, they are still the most popular isolators
used. But, air tables with resonant frequencies at 2 to 2.5Hz
can typically only handle vibrations down to about 8 to 10Hz,
not quite low enough for optimum performance with modern nano-equipment.
For purposes of clarity in scanning probe microscopes and
interferometers, air tables are an inefficient isolation solution.
The air systems have been adequate up until a few years ago
when better isolation was required. As some may recall from
the early years of nanotechnology, research scientists were
fond of suspending their very expensive AFMs from bungee cords
hanging from the ceiling, and sustained acceptable vibration
isolation. Although some are still employing this technique,
these numbers are dwindling - many aren't willing to take
that risk any longer, and have switched over to other isolation
systems.
One of those is active isolation, also known as electronic force cancellation. Active isolation uses electronics to sense the motion, and then puts in equal amounts of motion electronically to compensate, effectively canceling out the motion. Their efficiency is fine for application with the latest nanotechnology, as they can start isolating as low as 0.7Hz, quite sufficient for isolating the lower frequencies that are so damaging to image clarity with SPMs and interferometers. However, if you can get things mechanically isolated without having to use energy, such as in the form of electricity, then you are inherently better off. If you don't have to have some form of supplied energy to run your isolator, then you will not be so negatively influenced by problems of electronic dysfunctions and power modulations, which can interrupt scanning.
Negative-stiffness vibration isolation systems have become a growing choice for nanotechnology applications. Not only are they a highly workable vibration solution, but their cost is significantly less - up to one-third the price of active systems - making it an economical solution to cost-conscious administrators. "This is a passive approach for achieving low vibration environments and isolation against sub-Hertz vibrations", Says Dr David L Platus, inventor of negative-stiffness mechanism vibration isolation systems, and president and founder of Minus K Technology. "These isolation systems enable vibration-sensitive instruments, such as scanning probe microscopes, micro-hardness testers and scanning electron microscopes to operate in severe vibration environments, such as upper floors of buildings and clean rooms". "The images and data produced are many times better than those achievable with pneumatic isolators".
Negative-stiffness isolators employ a unique - and completely mechanical - concept in low-frequency vibration isolation. Vertical-motion isolation is provided by a stiff spring that supports a weight load, combined with a negative-stiffness mechanism (NSM). The net vertical stiffness is made very low without affecting the static load-supporting capability of the spring. Beam-columns connected in series with the vertical-motion isolator provide horizontal-motion isolation. The horizontal stiffness of the beam-columns is reduced by the "beam-column" effect. (A beam-column behaves as a spring combined with an NSM.) The result is a compact passive isolator capable of very low vertical and horizontal natural frequencies and very high internal structural frequencies. The isolators (adjusted to 0.5Hz) achieve 93% isolation efficiency at 2Hz; 99% at 5Hz; and 99.7% at 10Hz.
"Improved vibration isolation directly correlates to improved instrument performance", says Patrick O'Hara, President and CEO of Ambios Technology, a manufacturer of SPMs, stylus profilers and optical interferometers used in nanotechnology. "When you are trying to measure atomic scale features, mechanically stable support structures are critically important". "Up until the advent of probe microscopes, and some of the other very high-resolution imaging and data acquisition techniques, air isolators were adequate for most of applications - but not any longer".
"What negative-stiffness isolators provide is really quite unique to the field of nanotechnology", continues O'Hara. "In particular, the transmissibility of a negative-stiffness isolator - that is the vibration that transmits through the isolator as measured as a function of floor vibrations - which is substantially improved over air or active isolation systems". "Although active isolation systems have fundamentally no resonance, their transmissibility does not roll off as fast as negative-stiffness isolators". "So, at building and seismic frequencies the transmissibility of active isolators can be 10x greater than negative-stiffness isolators". "This causes substantial adverse measurement and imaging artifacts in the data". "Air isolators have the added disadvantage that their 2 to 2.5Hz resonance effects a significant loss in isolation capability below about 5Hz". "Negative-stiffness isolators are clearly the most efficient choice for probe microscopes".