Tutorial:
Environmental Vibration Criteria

Generic Vibration Criteria Curves

These generic criterion curves have been developed on the basis of data from individual systems and measurements made in facilities both before and after vibration problems had been solved. Moreover, they have been used extensively by leading vibration consultants for the semiconductor manufacturing industry for almost 20 years, and have been extended and refined as the industry has moved to narrower line widths.

The curves take into account that equipment used for the most exacting tasks (such as manufacturing semiconductors with smaller device geometries) is stiffer and better-isolated. It is, however, important to note that these criteria are for guidance only. For example, like any useful and general rule of thumb, the criterion curves are reasonably conservative for some specific cases, especially equipment with well-designed built-in vibration control systems. It should also be noted that these criterion curves do not replace high-resolution narrowband spectrum analysis for diagnostic studies. If a comprehensive study is required, both methods should be used.

Fig 7 Vib Crit-S
Generic vibration criterion table

Velocity-Based

The criteria are specified as a set of “1/3-octave-band velocity spectra” that define guidelines for allowable vibration levels for various activities and equipment. One of the major benefits of these criterion curves is that vibration is expressed in terms of root-mean-square velocity instead of units of displacement or acceleration. Various studies have shown that while individual equipment may show unique displacement responses to different frequencies, these points often lie on a curve of constant velocity. Moreover, the threshold constant velocity that affects the performance of equipment within each class tends to be rather uniform. The International Standards Organization (ISO) also uses a velocity-based standard for human exposure to vibration, which is incorporated into the criterion curve chart.

Broadband

Floor vibration in both manufacturing and research environments is typically dominated by random, “broadband” energy, as opposed to pure tone energy consisting of discrete frequencies. Although both broadband and pure tone vibrations will excite system resonances, the degree of their excitation will be different. Because random vibration closely approximates the measured floor vibrations observed in laboratories and manufacturing facilities, a specification based on random vibration more accurately reflects real world applications.

Evaluation of sites and the design goal of many new facilities confirm that “1/3-octave” vibration criteria accurately reflect typical environmental vibration spectra. This means that the bandwidth of the random vibration — the “window” of frequencies which is taken into account — is 23% of the band’s center frequency. A proportional bandwidth is used instead of fixed bandwidth based on a conservative estimate of the system’s ability to damp resonances.

Current Semiconductor Industry Requirements

Much of the current design activity for fab construction is setting goals in the VC-D to VC-E (250 µin./sec to 125 µin./sec). This range is believed to be appropriate to production of line widths of 0.3 µm. Older facilities will increasingly need supplemental vibration control systems to achieve the required level of vibration control. Next generation systems seeking line widths of 0.18 µm and below will almost certainly need active vibration isolation to complement even the most thorough fab construction.

Criterion Curve Max Level(1) micro-in./sec (dB) Detail Size(2) microns Description of Use
Workshop (ISO) 32,000 (90) N/A Distinctly felt vibration. Appropriate to workshops and non-sensitive areas.
Office (ISO) 16,000 (84) N/A Felt vibration. Appropriate to offices and non-sensitive areas
Residential Day (ISO) 8000 (74) 75 Barely felt vibration. Appropriate to sleep areas in most instances. Probably adequate for computer equipment, probe test equipment and lower-power (to 20X) microscopes.
Op. Theatre (ISO) 4000 (72) 25 Vibration not felt. Suitable for sensitive sleep areas. Suitable in most instances for microscopes to 100X and for other equipment of low sensitivity.
VC-A 2000 (66) 8 Adequate in most instances for optical microscopes to 400X, microbalances, optical balances, proximity and projection aligners, etc.
VC-B 1000 (60) 3 An appropriate standard for optical microscopes to 1000X, inspection and lithography equipment (including steppers) to 3 micron line-widths.
VC-C 500 (54) 1 A good standard for most lithography and inspection equipment to 1 micron detail size.
VC-D 250 (48) 0.3 Suitable in most instances for the most demanding equipment including electron microscopes (TEMs and SEMs) and E-Beam systems, operation to the limits of their capacity.
VC-E 125 (42) 0.1 A difficult criterion to achieve in most instances. Assumed to be adequate for the most demanding of sensitive systems including long path, laser-based, small target systems and other systems.

Additional Notes

The information given in this table is for guidance only. In most instances, it is recommended that the advice of someone knowledgeable about applications and vibration requirements of the equipment and process be sought.

  1. As measured in one-third octave bands of frequency over the frequency range 8–100 Hz. The dB scale is referenced to 1 micro-in./sec.
  2. The detail size refers to the line widths for microelectronics fabrication, the particle (cell) size for medical and pharmaceutical research, etc. The values given take into account the observation that the vibration requirements of many items depend upon the detail size of the process.