法律意见书格式Generic Vibration Criteria for Vibration-Sensitive Equipment
过去进行时的句子
Colin G. Gordon
Colin Gordon & Associates, 411 Borel Avenue Suite 425, San Mateo, CA 94402 USA
描写云的优美句子
ABSTRACT
The vibration criterion (VC) curves, commonly ud in the design of facilities which hou vibration-nsitive instruments and tools, were developed by the author and his colleagues, in the early 80’s, published by SPIE in 1991 and by IEST in 1993. Each of the criterion curves A through E is associated with a “line width” or “detail size” which was an attempt by the authors to describe the capabilities of the tools with which each curve might be associated. In the years since the curves were developed there have been substantial developments in tool design and isolation. In this paper the curves are reviewed in the context of prent-day tools and process. Changes are propod where the might be justified.
Keywords: Vibration Criteria, facility design, fabrication tools, microelectronics fabrication, medical and pharmaceutical rearch, vibration-nsitive equipment.
1. INTRODUCTION
The criterion curves were developed in the 1980’s in respon for a need for design standards to accommodate a wide range of tools and instruments ud by the microelectronics, medical and biopharmaceutical industries. The curves and descriptors that accompanied them were necessarily “generic” in the n that they were intended to meet the needs of all tools within each category as best the authors could judge bad on experience mingled with tool-specific specifications (often incomplete) provided by manufacturers.
As time pasd the curves were extended to reflect the needs, or projected needs, of technology and tool developments, primarily in the microelectronics industry. The most recent technical paper on the curves, describing their history and justification, was prented at a SPIE conference in November 1991 and published in the proceedings of that conference (Ref. 1). A paper by Amick (Ref. 2) discuss the VC curves and other generic criteria and many of the issues involved in processing vibration data. The VC curves are published in design documents published by the Institute of Environmental Sciences and Technology (Ref. 3) and the American Institute of Steel Construction (Ref. 4).
The VC curves are now widely accepted throughout the world as a basis for designing and evaluating the performance of microelectronics fabrication facilities where continuity of vibration-free tool performance is esntial. In this paper the curves and their descriptors are reviewed in the light of continued and projected future tool developments taking into account experience on past and prent projects. Some of their limitations are discusd.
2. THE VC CURVES
In their prent form the criteria take the form of a t of one-third octave band velocity spectra, labeled vibration criterion curves VC-A through VC-E. The are shown in Figure 1, together with the International Standards Organization (ISO) guidelines for the effects of vibration on people in buildings. The criteria apply to vibration as measured in the vertical and two horizontal directions. The application of the criteria as they apply to people and vibration-nsitive equipment are described in Table 1.
A general description of the curves, and their intended method of u, is as follows:
1) The vibration is expresd in terms of its root-mean-square (rms) velocity (as oppod to displacement or
acceleration). It has been found in various studies that while different items of equipment (and people) may exhibit maximum nsitivity at different frequencies (corresponding to internal resonances), often the points of maximum nsitivity lie on a curve of constant velocity.
2) The u of a proportional bandwidth (the bandwidth of the one-third octave is twenty-three percent of the band
center frequency) as oppod to a fixed bandwidth is justified on the basis of a conrvative view of the internal damping of typical equipment components. Experience shows that in most environments the vibration is dominated by broadband (random) energy rather than tonal (periodic) energy. In such an environment measurement bandwidth is critically important.桃花面
3) The fact that the criterion curves allow for greater vibration velocity for frequencies below 8 Hz reflects experience
that this frequency range, in most instances, lies below the lowest resonance frequency of the tool structure.
Relative motions between the components are, therefore, harder to excite and the nsitivity to vibration is reduced.
4) For a site to comply with a particular equipment category the measured one-third octave band velocity spectrum
must lie below the appropriate criterion curve of Figure 1.
The equipment criterion curves have been developed on the basis of data on individual items of equipment and from data obtained from measurements made in facilities before and after vibration-related problems were solved. The curves are generic in the n that they are intended to apply to broadly defined class of equipment and process. They are intended to apply to the most nsitive equipment within each category that is defined.
The criteria assume that bench-mounted equipment will be supported on benches that are rigidly constructed and damped so that amplification due to resonances is limited. They take into account the fact that certain types of equipment (such as SEM’s) are often supplied by the manufacturer with built-in vibration isolation.
The criteria are for guidance only. The “detail sizes” given in Table 1 appear to reprent experience at the time of writing. They reflect the fact that the quality of design and of built-in isolation in most equipment tends to improve as dimensional requirements become more stringent. In some instance
s the criteria may be overly conrvative becau of the high quality of built-in isolation. Thus, for instance, many steppers ud in photolithography are, currently, relatively innsitive to vibration.
When measuring for compliance with the criteria one must take into account the “nature” of environment that is being measured:夜船吹笛雨潇潇
1) When the environment is relatively constant in time and spatially uniform—generated for instance by continuously
running mechanical systems (fan, pumps, etc.) or by heavily traveled highways—it is generally adequate to measure the “energy average” vibration levels. Levels can be measured at multiple locations, if the area being evaluated is large, and the collective data can be summarized statistically. It is considered reasonable to classify the VC performance bad on the “average plus one standard deviation” level at each frequency.
2) When the environment is not constant in time—impacted for instance by walkers (footfall excitation), or nearby
trucks—it may be necessary to measure the “maximum rms” (sometimes called “peak hold”) vibration levels.
A comprehensive discussion of data processing methods is given in Ref (2).
3. OTHER GENERIC CRITERIA
A number of different criteria for vibration-nsitive tools have been developed over the years. Few of them are truly generic in the n that they can be ud to embrace the requirements of a wide range of tool types. Two candidates, one existing and one under development are described below.
3.1 Medearis Time Domain Method
Medearis (Ref. 5) recommends generic criteria for vibration-nsitive equipment bad on “time domain” as oppod to “frequency domain” peak-to-peak displacement measurements. The frequency range of measurements is not defined. He suggests limits of 2.5 microns (100 microinches) and 7.5 microns (300 microinches) for microelectronics facilities and science laboratories, respectively. His criteria are not reconcilable with the fact that most tool makers, recognizing that their equipment is not equally nsitive at all frequencies, provide siting specifications in the form of frequency domain spectra.
3.2 Ahlin Respon Spectrum Method
Ahlin (Ref. 6) is currently developing a new measurement and evaluation methodology bad on the concept the respon spectrum, ud extensively by structural dynamicists in ismic design engineering. The methodology has promi but is, currently, neither supported by instrumentation nor experience.
4. CRITICAL REVIEW OF THE VC METHODOLOGY
The VC criteria curves have been ud extensively by the microelectronics industry and rearch communities for the past fifteen years. In some cas, tool makers have adopted the criteria as a basis for “siting” specifications for their equipment. The author and his colleagues have ud the criteria as the basis of design for veral hundred chip fabrication facilities. This experience shows, clearly, that the criteria work—that tool operational problems, due to vibration, can be avoided if vibration conditions on the floor of the facility comply with the criterion curve appropriate to the process.
有一种声音在记忆深处In spite of their widespread u, some aspects of the VC curves have been questioned. Some of the issues raid are discusd in the following paragraphs.
4.1 Spectral-Domain versus Time-Domain
Vibration-related problems with tools generally ari at the resonance frequencies of structural components within the tool. It is at such frequencies that components vibrate most strongly, with the likelihood of image distortion due to differential motions between components. The frequency nsitivity of tools is recognized by most tool vendors who, almost without exception, specify siting requirements in the frequency domain. Examples of current tool specifications are given in Figures 2, 3 and 4*. Time-domain amplitude limits, which consider the sum-total of all frequencies without discrimination, make little n unless specific frequency ranges are quoted for the measurement system.
4.2 Velocity versus Displacement (or Acceleration)
The three common metrics of displacement, velocity and acceleration are absolutely related to each other by frequency. Basically, therefore, each metric is equally valid and one can convert from one metric to the other by post-processing of the measured data. Velocity appears as a convenient metric for at least three reasons:
1) Photolithography process are dependent upon maintaining a stable image during the exposure time. The limit,
therefore, is a limit on image velocity. One can argue also that visual interpretation of images en through a microscope are dependent upon the velocity of image movement, rather than displacement.
2) Many vendor specifications are given in terms of velocity, others are given in displacement or acceleration.
Velocity is a “happy compromi”.
3) Velocity lies midway, in terms of conversion, between displacement and acceleration and reprents, therefore, a
convenient choice.
* Interpretation of vendor specifications is not always easy since, often, bandwidth information is not given and frequency limits are not clearly stated. Sometimes the specifications are not bad on the results of physical testing.
4.3 One-Third Octave Band versus Narrowband Spectra
Vibration environments on floors and “greenfield” sites are, generally, dominated by random broadband energy. Pure tone components may ari only due to poorly isolated mechanical equipment (fans, pumps, etc.). The bandwidth of the filters (or effective filters) ud in frequency-domain measurements are critically important when broadband energy is involved; the broader (wider) the filter the higher will be the measured levels.
The VC curves are bad on one-third octave band measurements, where the filter width bears a constant proportion (0.23) to the band center frequency. U of the one-third octave methodology is defendable for a number of reasons:
1) It adequately describes the respon of a resonator to broadband excitation assuming a reasonable value of
damping.
2) It is ud by a number of tool vendors in their siting specifications.
3) It substantially simplifies the complexity of a typical vibration spectrum, especially one containing significant tonal
components.
Narrowband spectra, on the other hand, are invaluable when diagnosing the caus and sources of floor vibration.
4.4 Shape and Frequency Range
The VC curves extend from 4 Hz to 100 Hz (80 Hz in many data reports). At the time the curves were developed the 4 Hz lower limit made n since equipment, at that time, exhibited negligible nsitivity to low frequency inputs. Tho few vendors who quoted vibration specifications for their tools rarely defined requirements below about 5 Hz.
With the increasing u of pneumatic isolation systems (air springs) as an integral part of the tool, concern has incread about vibration conditions at the resonance frequencies (1 to 3 Hz) typical of the isolators. At resonance, the isolator amplifies the floor vibration and the possibility exists that tool operation could be affected. Figures 2, 3 and 4 show veral specifications that extend below 4 Hz.
There is less concern about the upper frequency limit of the VC curves. Some manufacturers do ext
岑港之战end their requirements above 100 Hz but there is significant evidence that vibration at the frequencies is rarely a problem.
In their prent form the VC curves impo less stringent vibration limits for frequencies below 8 Hz. In the 4 to 8 Hz range the limit, in effect, is that of constant acceleration instead of constant velocity.
Arguments have been put forward in recent years that, in the ca of pneumatically isolated equipment, not only should the range of the curves be extended downwards, to 1 Hz say, but that the constant velocity limit should be retained throughout the range. A suggested revision to the shape of the generalized VC curve has been published by Ungar, Sturz and Amick (Ref. 7). It is shown in Figure 5. No change is suggested in the ca of tools that do not u pneumatic isolation.
5. GENERIC CURVE SELECTION
黄瓜蛋汤的做法Table 1 attempts to describe the application of the VC curves in terms of “detail size” (line width in the ca of microelectronics fabrication) and the general type of equipment for which each curve is appropriate.
The detail sizes are clearly approximate and dependent upon the degree of sophistication ud in th
e structural and isolation design of the individual tool. One must constantly bear in mind the fact that the curve descriptors are intended to apply to the most nsitive tools within each category. Since tools are constantly changing and new tools introduced, the “most nsitive” limit is a constantly moving target.
A few words need to be said about the detail sizes assigned to the most stringent curves, VC-D and VC-E. It is a fact that the tool maker must develop his tool in his own facility, which typically will have a slab-on-grade floor located, often, in a light-industrial zone with heavily-traveled roads and highways. Silicon Valley in Northern California is such an area. It is likely that the tool maker’s floor will carry ambient vibration amplitudes in the range VC-E to VC-D. One can argue, therefore, that prent and future tools can never be more nsitive than allowed by the VC-E to VC-D range. If the basic tool itlf is more nsitive than the curves the tool vendor will, necessarily, need to incorporate additional isolation as an integral part of the tool.
The “Description of U” texts are very generally written. Slightly different versions of the texts have been introduced by Murray, Allen and Ungar (Ref. 4) and Ungar, Sturz and Amick (Ref. 7). Clearly, the descriptors should not be ud blindly. Advice should be sought from the tool maker(s) as regards their requirements, and allowance should be made for future technology and fut
ure tools over the planned life of the building.
A number of issues must be considered when lecting the appropriate criterion curve for a new or retrofitted building. Some of the are discusd below:
5.1 Ba versus Operating Condition
“Ba” vibration conditions apply to the vibration environment prior to installation and/or start-up of tools and equipment ud in “normal” production. Tools can often create vibration associated, for example, with the vacuum pumps ud by electron microscopes. The ba building condition is the most critical condition since it reprents a “lower limit” of performance in the same way that a building HVAC noi impos a lower limit on office noi. As tools are introduced, vibration amplitudes will generally increa to an extent dependent upon the care taken in vibration isolation of the tools and their accessories (pumps, etc.). Examples of “before” and “after” environments are shown in Figures 7 and 8.
5.2 Facility Maturation
Almost inevitably, vibration environments will deteriorate as time pass due to aging of vibration ge
nerating equipment, misalignment of isolation hardware, growth of vibration-generating activities around the plant, etc. The worst effects of maturation can be measured and, perhaps, controlled by installing a computer-bad monitoring system but some degree of deterioration is almost inevitable. Some parts of the changes shown in Figures 7 and 8 are certainly due to aging.
5.3 Multiple Criteria and Layout Flexibility
The microelectronics industry, especially, is subject to rapid change both in terms of tools and process layout. In the past there has been a tendency by this industry to design all parts of the fabrication cleanroom to a uniformly strict vibration criterion. This allows maximum flexibility of prent and future layout; the most nsitive tools can be placed anywhere within the cleanroom envelope. In an attempt to reduce facility costs there has been a recent move towards minimizing the area of floor designed to the most stringent criterion and using a more relaxed criterion for non-nsitive fabrication. Of cour, as a result, the layout flexibility of the facility is reduced.
5.4 Facility Costs
The choice of vibration criterion affects the cost of the facility. Facility costs have become a major parameter in new facility planning in the past veral years. Thus, whereas veral years ago facilit
y cost was less important than issues of flexibility, future technology trends and conrvatism, now the situation is markedly changed, at least in some areas.
5.5 Future Trends
It has been argued earlier that future tools and instruments, no matter their capabilities in terms of resolution and processing technology, will never require vibration environments significantly better (lower) than the VC-E to VC-D range. It is very likely that the 0.1 micron limit specified for VC-E will be extended downwards as time pass. Performance in the VC-E to VC-D range is exceptionally difficult to achieve on column-supported floors of conventional design.
