Ken Huffenus and Maurice Gertel
Rapid advances in the optoelectronics and telecommunications industries are translating into demand for new levels of vibration damping and control.
Early efforts to isolate precision equipment from harmful vibrations included large masses suspended from bungee cords, huge granite blocks mounted on tennis balls, and similar homemade systems. In many cases, these systems reduced vibrations in precision systems, but were not very consistent and took a long time to build. Engineers required an inexpensive, repeatable vibration isolation system that could be integrated into precision equipment.
They still do, but the need to understand the potential side effects of vibration has become even more critical with the recent influx of new devices for the optoelectronics, semiconductor, and telecommunications industries. This translates into a demand for new levels of vibration damping (reduction of the amplitude of an input vibration) and control.
Understanding vibration
In most precision equipment, the key vibration-related problem is structural, with mechanical resonance acting as the destructive agent. Consider, for example, an idealized rigid base with four cantilevered beams and tip masses (m). The beams represent internal supports and have different stiffness (k). The masses represent elements mounted on the base, such as optical components. Each element has a different natural frequency (f), which is a function of its own stiffness and mass (f = 0.159(k/m)1/2).
Subject this equipment to a constant sinusoidal vibration sweep, from below the lowest natural frequency to above the highest, and the maximum response motions of each tip mass occurs when its natural frequency coincides with the disturbance frequency. This defines the condition of resonance to be avoided or controlled.
The ratio of the resonant peak response to the input vibration amplitude (Q) is a function of a system's internal damping. High damping, which is desirable, produces a low Q. If, for example, the acceptable safe vibration amplitude is 1.75 mil, the Q for all of the elements of a system must be below that level. Elements with a Q greater than 1.75 will be adversely affected by environmental vibration. This is typical of a majority of optical equipment and illustrates why protection such as vibration isolation is required.
System analysis
An analytical approach to specifying vibration isolation in electronic equipment is a three-step process. Engineers must first define the vibration environment and then define the equipment sensitivity to vibration. Preferably this should be a spectrum of vibration amplitude at the equipment supports versus vibration frequency.
Then it is necessary to determine the reduction of vibration environment spectra required to protect the equipment. In concept, a simple overlay of the spectra from steps one and two should provide acceptable frequency ranges for establishing a minimum required isolator transmissibility function.
To isolate vibrations, engineers must first identify their sources, including machinery that may not even be in the same room. In addition, environmental vibrations, such as seismic activity or from a nearby highway, create low-frequency disturbances that are transmitted through a building structure (usually 5 to 15 Hz). In multilevel buildings, the structure itself will sometimes sway at a very low frequency (<2 Hz). Damping this motion requires a pendulum isolation system, which will allow greater freedom in the horizontal plane.
Many manufacturing processes in the semiconductor and optics industries are in cleanroom environments where vibrations from foot traffic and ventilation systems are readily transmitted through raised floors. These vibrations occur at low frequencies (usually 10 to 20 Hz). For example, any HVAC equipment will create a resonance at approximately 60 Hz that is visible on a microscopic display unless damped before reaching the device.
Quality control equipment in heavy manufacturing environments is also subject to low-frequency vibrations caused by nearby stamping presses or other heavy machinery. These vibrations occur anywhere above 25 or 30 Hz and can have very large amplitudes. Even machinery in lighter manufacturing environments can generate vibrations that filter throughout the facility.
Most buildings can in fact be characterized as having a low-level broadband random vibration spectrum with multiple, superimposed, strong discrete "tonal" frequencies from motors or generators, for example. Horizontal vibrations are approximately 25% of the magnitude of vertical vibrations, so an effective isolation system must work best in the vertical plane.
Determining the exact nature of the vibration in a specific area requires a vibration survey. This involves placing accelerometers where sensitive equipment will be installed and collecting the vibration data. The information is then downloaded to a Fourier analyzer that transforms the input signal from the accelerometer into data that can be manipulated by a program like Matlab and entered into a spreadsheet. Vibration isolation equipment must reduce the transmitted vibrations at all of the identified frequencies.
There are a variety of design solutions to make new equipment more resistant to environmental vibration. These generally involve combinations of increasing component and structure stiffness, increasing internal damping or energy dissipation, and selectively reducing weight. The objective is to increase component and structure natural frequencies to at least 100 Hz.
If the equipment already exists, redesign for vibration tolerance is not practical. The simplest solution is to place the equipment on a vibration-free table or platform supported by low natural frequency, active-air mounts, or air legs. Ideally, the table should behave as an "infinite impedance" relative to the equipment and its internal components and not react to any resonance in the equipment. Performance will be maximized when the isolator natural frequency is 1/5 the lowest equipment frequency, and the table or platform weight is at least five times the equipment weight (see Fig. 1).
Inserting a low-frequency isolation system significantly reduces the response for the idealized equipment shown in Fig. 1. In this illustration, the isolation system has a natural frequency of 2 Hz that has been selected to occur at 1/5 the lowest component frequency. The isolator response to the same constant 1-mil vibration amplitude is shown, together with the superimposed component responses and specified allowable vibration amplitude of 0.02 in./s2. The isolator response ratio to a constant amplitude vibration disturbance is commonly referred to as the isolator transmissibility characteristic. Here, the isolator response has in effect transformed the constant 1-mil vibration disturbance into a new environment with a large amplitude at 2 Hz, which is not critical. It then drops off rapidly at higher frequencies to reduce the disturbance at the internal component resonances, which is critical. With this addition of isolation, the peak response of each component is now safely below the allowable level of 0.02.
The right equipment for the job
One of the more flexible design options involves active-air isolation. Self-leveling pneumatic isolators have unique properties that make them ideally suited to protecting sensitive equipment. The compressibility of air follows simple laws of physics and provides low-stiffness low-frequency support in a convenient pressure range of 20 to 80 psi. The load capacity of the air isolator is simply a function of its cross-sectional area, and unit isolators are available with load ranges of 100 to 400 lb up to 6000 to 24,000 lb. Isolation of small equipment to large inertia masses is feasible with the same isolation efficiency.
The air-isolation mount consists of an air spring chamber and an air-damping chamber (see Fig. 2). The dual-air-chamber design provides a unique isolation characteristic. Essentially, the design functions as a spring-damper system that is much softer and has a much lower natural frequency than a steel spring. The effectiveness of this design depends on an internal orifice that controls the airflow between the two air chambers. Careful regulation of the orifice produces the optimum transmissibility curve, which provides high damping at low frequencies to control resonance and low damping at high frequencies to provide maximum isolation roll-off of 12 dB per octave. Conventional highly damped isolators can only achieve 6 dB per octave.
One advantage of an active-air isolation system is that it can be retrofitted to a device in the form of an isolation table or tabletop platform. Pneumatic mounts can also be integrated into an original-equipment design.
The optimum vibration isolation strategy design solution will vary from application to application. Wafer inspection stations, for example, subject prototype silicon wafers to many tests to verify the critical properties. Some of these tests involve placing a small needle on submicron features that exist on silicon wafers. Any disturbances during the inspection process can cause serious damage to the expensive prototype wafers. One option is to locate the probe station so it can be placed on a pneumatic isolation table.
Another option for semiconductor test equipment is a modular mount system to support the wafer itself and any other components that are sensitive to vibrations. This subassembly can then be integrated into a rigid steel frame also housing any displays or controls included with the system. This method of isolation provides the same performance, without being visible. Probe stations can be complex and incorporate such items as computers, positioners, and video systems. Modular isolation mounts allow for a compact, total solution.
Most microscopes also incorporate some form of vibration isolation. Currently, they are capable of viewing submicron particles either within a silicon wafer or a cell. When working at these high magnifications, any vibration in the room will be visible on the display. Lower frequency disturbances cause the image to "jitter." At higher frequencies, the image will appear blurred or constantly out of focus. Short-term effects include inconsistent data. Long-term effects can be excessive wear and premature aging of internal parts.
Larger microscopes, such as SEMs (scanning electron microscopes), can incorporate modular air-isolation mounts. The microscope's column is placed on a custom platform supported by isolation mounts that can handle up to 20,000 lb. Smaller desktop microscopes do not require custom supports. Upright and inverted microscopes used for electro-physiology can be placed on an isolation table or tabletop platform to remove local vibrations. The lab tables and platforms incorporate the same isolation technology and fit easily into a crowded lab environment.
Ken Huffenus is a sales engineer and MAURICE GERTEL is chairman at Kinetic Systems Inc., 18 Arboretum Rd, Boston, MA 02131; e-mail: [email protected].