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Vibration Isolation System Design for Advanced Metrology System

Vibration Isolation System Design for Advanced Metrology System
When one stage of vibration isolation isn't enough. Combining active and passive vibration control systems can offer the ultimate solution in vibration isolation.

Custom Vibration Isolation System
Figure 1 Custom two stage vibration isolation platform with STACIS active control and MaxDamp passive control. And a custom designed stainless steel platform.

To solve the most challenging vibration problems for inspection, metrology, and e-beam tools, one level of vibration isolation is often not enough.

Pneumatic isolators, with a soft spring, are ideal for attenuating vibration at 10Hz and above. This typically includes vibration from nearby machinery, large transformers, fans, rotors, as well as on-board vibration such as structural resonances from the frame, pumps, fans, and other electronics and or mechanical structures in the machine.

Serial hard-mount active isolators are unmatched for attenuating vibration originating in the floor below a frequency of 10Hz, and even below 1Hz. Downward looking inertial sensors measure floor vibration and with a closed loop feedback control system, drive a piezoelectric actuator to cancel vibration - the often-impactful vibration in floors and buildings from nearby footfall, automobiles and trains, even seismic activity and building structural resonances.

Figure 2 Transmissibility curve of MaxDamp vs. regular Gimbal Piston showing the relative reduction in amplification at the resonant frequency for both cases.

For manufacturers of advanced metrology equipment, like Taylor Hobson, vibration control is growing in importance. As optical metrology technology advances and surface measurement geometries approach the nanometer scale, it is important to plan ahead and design a foundation, or support structure, for these tools that will mitigate vibration across a wide frequency range, including those frequencies below 10Hz.

This was the goal from the early stages of design for the Taylor Hobson LUPHOScan 850 HD while also maintaining the high load capacity for the largest and highly versatile non-contacting 3D metrology tool.

With over 130 years of experience, Taylor Hobson engineers know that optical metrology is highly sensitive to vibration, particularly vibration below 150 Hz, and requires pneumatic isolators. With the large, automated inspection arm on the LUPHOScan 850 HD, it was also clear that additional damping would be required. Using a proprietary internal fluid, TMC’s MaxDamp isolators provide a combination of excellent high frequency vibration isolation with the additional damping to reduce the effects of onboard motion such as that from a rotary or translational stage.

While MaxDamp® isolators have much less amplification at their natural frequency compared to that of the all-air Gimbal Piston pneumatic spring (roughly 4x vs 8x) this system does not provide attenuation of floor vibration below 5Hz. For additional attenuation, it is critical to follow certain rules when adding a 2nd level of vibration isolation.

Figure 3 LUPHOScan 850HD with machine frame mounted on vibration isolation platform

This is a perfect application for TMC’s STACIS® active vibration isolation. First, STACIS is a hard mount isolator. In other words, it is 50-100x stiffer than pneumatic isolators. This impedance mismatch is required to eliminate the “sprungmass” effect. Furthermore, STACIS is an active vibration cancellation system with sub-Hz inertial sensors and high-capacity piezoelectric actuators in a closed loop feedback system which is decoupled from the payload it supports.

This low frequency active vibration cancellation system provides isolation starting as low as 0.6Hz and 10x reduction of floor vibration starting at 2 Hz. It also is a “rigid” support which is important for an instrument with built-in pneumatic isolators which provide 10x reduction at 10Hz, and even more at higher frequencies. In addition the LUPHOScan 850HD is designed to hold very large asymmetric substrates, up to 350 kg (770 lbs.). These substrates are spun around for measurement at speeds up to 90 degrees/sec, meaning the instrument must remain stable under significant dynamic loads. Some measurements can take up to 20 minutes, so a stiff, stable platform is essential to the proper function of the LUPHOScan 850HD.

Stacked in this way, the two levels of isolation can each be optimized without influence from the other. The end result is the LUPHOScan 850 HD metrology tool with one of the most advanced vibration isolation systems commercially available.

Figure 4 Typical transmissibility of an integrated vibration control system consisting of passive MaxDamp stacked on top of an active STACIS platform

This was not an “off the shelf” vibration isolation system. TMC engineers worked closely with Taylor Hobson engineers to design a customized system to not only meet these stringent vibration isolation requirements, but also to fit a specific set of mechanical criteria. Pneumatic isolators grow in footprint to support heavier loads. If too large, the internal pressures will be too low which will adversely impact performance. TMC rightsized the MaxDamp isolators specific to the LUPHOScan850 HD stage and its capabilities for large-size optics. Similarly, STACIS modules are available in three capacities, and it is important to specify a model which is neither over-, nor under-loaded. STACIS modules are available in standard sizes with capacities up to 1815 kg (4000 lbs.) each.

The LUPHOScan 850 HD also has a unique shape to enable it to be an extremely versatile 3D metrology tool. Because of this, TMC needed to design a custom shaped, highly damped stainless steel laminate platform to support the MaxDamps and the main stage of the tool, while also accommodating the asymmetric payload and its large size.

For the LUPHOScan 850 HD, the two stages of isolation, for both very low frequency as well as high frequency vibration, are ideal for enabling the highest level of precision even in high vibration environments. And the engineering expertise combined with manufacturing capabilities to provide custom built solutions made TMC the perfect partner when developing this breakthrough in 3D optical metrology.

Figure 5 Completed LUPHOScan 850HD metrology tool

To download the application note, click here.