Floor Vibration Criteria and Isolation for Precision Stages: A Practical Selection Guide

What floor vibration criteria actually mean

Vibration criteria are a way of stating, at each frequency, how much floor motion is acceptable for a given class of work. They are usually expressed as a velocity or acceleration limit, and they are graded: a general office, a normal laboratory and a nanometre-scale imaging or patterning tool occupy different grades. The reason the criteria are frequency-dependent is that a tool responds differently to slow sway than to fast buzz. A rigid stage on a good frame may tolerate low-frequency motion and be disturbed mainly by higher frequencies from pumps, fans and foot traffic.

The mistake to avoid is selecting a criterion by reputation. A tool that holds an overlay budget of a few tens of nanometres on a glass substrate has different needs from a scanning probe microscope, and two tools in the same room may need different criteria because their internal stiffness and their sensitivity bands differ. Derive the criterion from the process requirement, then check it against the measured site spectrum.

Measure the site before you choose a platform

Site measurement is not complicated, but it must be done honestly. A calibrated accelerometer or seismometer is placed at the exact position where the tool will stand, at the mounting height and on the same floor structure, and data are logged over a full day and night so that the quiet and the busy periods are both captured. Foot traffic during working hours, a lift, a compressor cycling and an air handler starting can each dominate a particular frequency band, and a short daytime measurement can miss the worst case altogether.

The result is a spectrum, and spectra must be read with the tool's own response in mind. A disturbance at a frequency where the stage structure is stiff matters far less than the same amplitude near a resonance of the stage, the platform or the frame. Where the machine's modal behavior is known, the site spectrum can be weighted by it, so the isolation decision targets the frequencies that actually reach the tool point rather than the loudest number on the chart.

• Measure at the intended tool position, at mounting height, on the real floor
• Log through a full day and night to capture the worst case
• Note the sources active at each time: services, lifts, traffic, adjacent equipment
• Weight the spectrum by the tool's own stiffness and resonances

Passive or active isolation: choosing by frequency

Passive isolation, typically pneumatic air springs with damping, is simple, robust and effective above its natural frequency, but it amplifies motion near that frequency. If the site has a strong low-frequency component near the isolator's natural frequency, a passive platform can make things worse. Active isolation uses sensors and actuators to cancel motion across a wider band, including lower frequencies, at the cost of power, complexity and a service requirement. Neither is universally better; the choice follows the measured spectrum and the band where the tool is sensitive.

When the floor is not the whole problem

Not every measured error is floor vibration, and treating a servo problem as a vibration problem wastes money. Internal sources such as a vacuum pump, a chiller, a cable dress that pulls on the carriage, or a linear motor that is not tuned will produce scatter that no floor isolation can remove. The clean way to separate them is to measure at the same time as the positioning error: if the stage's error correlates with the floor spectrum at the same frequencies, the disturbance is external; if it does not, the source is inside the machine or in the control loop.

Structural design inside the tool matters as much as the platform beneath it. A stiff bridge, well-damped structural paths and short, rigid load paths from the stage to the frame raise the machine's own resonances and reduce its response to whatever floors through. A well-damped tool on an active platform will always beat a compliant tool on the same platform.

Specifying and verifying the installed platform

A useful specification states the site spectrum, the chosen criterion, the isolation type and its attenuation band, the total moving mass and the acceptance test. It should also state the conditions at which performance is claimed: production speed, with services running, at the intended temperature, and at the intended location on the floor.

Verification belongs on the installed and running tool. Measure vibration at the tool point and positioning error at the same time, with the tool performing its real task, and compare the result with the criterion and with the factory acceptance data. Where margin is thin, the record shows which frequency band to attack, whether through a change of isolation, a change of location, or a change in when the critical work is performed.