Optical Cleanliness Specifications and Cleanliness Verification


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Optical Cleanliness Specifications and Cleanliness Verification
Irving F. Stowers, ScD Lawrence Livermore National Laboratory, P0 Box 808, MS L-495, Livermore, CA 94550
ABSTRACT Optical cleanliness is important to NIF because it results in beam obscuration and scatter losses which occur in the front-end (containing over 20,000 small optics) and the large-aperture portions of the laser (containing —7,300 optics in 192 beamlines). The level of particulate cleanliness necessary for NIF, is similar to the scatter loss due to surface roughness. That
is, the scatter loss should not exceed 2.5x105 per surface.
Establishing requirements for optical and structural surface cleanliness needs consideration of both particulate and organic thin-film cleanliness. Both forms of cleanliness may be specified using guidelines specified in Military Standard 1246C and are referred to as cleanliness Levels. This Military Standard is described briefly and displayed in tables and charts. The presence of organic thin-films on structural surfaces is of particular concern if the contaminated surface is near solgel coated optics (solgel coatings provide an antireflection (AR) quality); or the optic is in a vacuum. In a vacuum, organic contaminant molecules have a much high probability of transporting from their source to a solgel-coated optic and thereby result in the rapid change in the transmission ofthe antireflection coating.
Optical surface cleanliness can be rapidly degraded if a clean optic is exposed to any atmosphere containing an aerosol of small particles. The use of cleanrooms, as described in Federal Standard 209C, minimizes the settling of particulate contaminants and is described using charts and tables. These charts assist in determining the obscuration and scatter loss that can be expected when a clean surface is exposed to various Classes of cleanrooms due to particulate settling.
Keywords: cleanliness, aerosols, cleanroom, particulate settling, light scattering, beam obscuration.

1. SCATTER RELATED REASONS FOR ESTABLISHING HIGH CLEANLINESS REQUIREMENTS
The NIF laser system is conceptually divided into small aperture front-end and large aperture high-fluence optics. The smaller front-end optics number over 20,000 and precondition the laser light before entering the 192 symmetric beam-lines.
Because of the large number of serially arranged optical components, it is necessary to achieve and maintain very high levels of surface cleanliness to essentially eliminate all surface scattering losses. Surface cleanliness has been assigned an integrated scatter loss budget of 0.1% for the front-end optics and an additional 0.2% for the large aperture optics. This results in a typical surface scatter loss budget, due to contaminants, of 2.5x105 per surface. For comparison, the scatter loss per surface due to surface roughness is ofa comparable value of 5.Ox1O5.
Large aperture optics on NIF have a cleanliness requirement of Level 50-A/b as installed and will be removed for refinishing if dirt and damage caused obscuration exceeds 2.5x104 or any single damage site reaches 2-mm in size. The smaller front-end optics must be initially cleaned to Level 100-A and will be removed for refmishing if the accumulated dirt and damage caused obscuration exceeds 2.5x10 or any single damage site reaches 250-tm in size.
Optical and structural surface cleanliness is further specified as initial cleanliness (immediately after cleaning), as-assembled cleanliness, and end-of-life cleanliness. These cleanliness Levels are defined in Table 1.
2. DAMAGE RELATED REASONS FOR ESTABLISHING HIGH CLEANLINESS REQUIREMENTS
Ample evidence exists that particulate contamination initiates damage on both bare and coated optical surfaces in the presence of high intensity laser light[1}. More recently, it has been found that flashlamp light is sufficient to create aerosols within laser amplifier cavities and that the these aerosol particles subsequently settle onto laser amplifier slabs and initiate

Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

Part of the SPIE Conference on Optical Manufacturing and Testing Ill

Denver, Colorado • July 1999

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pitting damage to the optical surface[2]. Stowers, et al, have found that the damage resulting from contaminants can be 7.7 times larger than the contaminant causing the damage.

Large optical surfaces
Small optical surfaces Structural surfaces enclosing large optics Structural surfaces enclosing small optics

Surface cleanliness Level as-cleaned
Level 50—A/1O
Level 100-A Level 83-AJ1O Level 300-A

Surface cleanliness Level as-assembled Level 50-A/1O
Level 100-A Level 100-A/1O
Level 300-A

Surface cleanliness Level at end-of-life
1 damage site of 2-mm size per surface or per surface obscuration of
2.5x104 1 damage site of 250 jim
size per surface or obscuration of 2.5x104
Level 100-A/1O
Level 500 (visibly dirty)

Table 1 Cleanliness Levels in the as-cleaned, as-assembled, and end-of-life conditions for small optics, large optics, and structural surfaces.

It is also now well known that high-intensity laser light is sufficient to dislodge particles from structural surfaces which forms an aerosol that transports particles from structural surfaces to nearby optical surface[3}. If it were not for this laser displacement of contaminants, most contaminants would remain tightly attached to structural surfaces and not transport onto optical surfaces.
The evolution of large lasers has necessitated the need to find more cost-effective optical fabrication processes. One of these is the recent application of thin solgel coating used as anti-reflection coatings on surfaces aligned normal to the laser. Solgel, however, is a material of enormous surface area per gram of coating. It has the unfortunate property that it acts as a getter for organic molecules that may be in inert gases inside laser cavities. The addition of a few monolayers of organic material to the surface of a solgel coating can result in a change in the transmission of the coating of up to several percent. In fact, solgel coated optical surfaces in vacuum (at iO Ton) can loose transmission at the rate of up to 0.1% per day in the presence of an inert gas containing a very low concentration of mid-atomic-weight organic matter (organic matter with an atomic weight of 100-200 amu seems to result in the most rapid change in solgel coating transmission). The rapid change in the transmission of solgel coatings in vacuum is exacerbated by large mean-free-path between gas molecules which allows molecules leaving a contaminated surface to be transported nearly ballistically to nearby solgel coated surfaces. At atmospheric pressure this transport mechanism is thwarted by the small mean-free-path of the gas molecules. However, the transport of mid-weight organic molecules still seems to occur at atmospheric pressure, it simply occurs at a substantially lower rate. Solgel coating which remain open and exposed to the air in a Class I 00 cleanroom also suffer from the same degradation in transmission and we have repeatedly measured transmission change ofO.l% per month in high quality cleanrooms.
3. SURFACE CLEANLINESS STANDARDS
MIL-STD-1246C Product Cleanliness Levels and Contamination Control Program[41 defines surface cleanliness Levels due to particles and thin-films. It has been found that the cumulative size distribution of surface particle contaminants generally follows a log10 concentration versus (logio diameter)2 function. A cleanliness Level therefore represents an area concentration of particles exceeding a particular size. Each specific surface cleanliness Level is named for the largest particle size expected to be found on 1 ft2 [or 0. 1 m2] of surface area. Thus a surface with a Level 1 00 distribution of contaminants, willhave only one (1) particle of 100 jim diameter on each one (1) square foot of surface and an analytically defined number of smaller particles down to 1 jim diameter.
For a particular cleanliness Level (defined by a line in Figure 1) the cumulative concentration is given by the equation below.
particles diameter 10(o.926x(1ogo (cleanliness Level)—Iogo(diameter[/LinJ)))
ft2

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m For a surface with a cleanliness of Level 100, the concentration of 100 .tm and larger particles is found to be 1/ft and the
concentration of 5 and larger particles is I ,785 I ft.
MIL-STD 1246C also defines cleanliness Levels associated with thin-film contaminants. Thin-film cleanliness is called NonVolatile Residue (NVR) and is defined as "material remaining after evaporation of a liquid". The thin-film cleanliness Level is defined as the mass of the contaminant per ft2 [or per 0.1 m2] and is shown in Table 2. In practice, the NVR cleanliness Level is written as an attachment to the end of the particulate cleanliness Level [e.g. Level 100-AI1O]. As an indication of the relative cleanliness of Level A/1O, a 0.37 nm thick layer of carbon is equivalent to an A/1O cleanliness Level.
01)
ci E A
C,' U)
0

1 10 25 50 100150200 300 500 750 100015002000
Particle size, itm
Figure 1 Surface cleanliness chart derived from MIL-STD-1246C. A Level 100 cleanliness Level allows only 1 particles /ft of 100 im size or larger and simultaneously allows 1,785 particles / ft2 of 5 im size or larger. These two points are shown as small circles • on the Level 100 cleanliness line.

NVR Cleanliness
Level
A/100
A/SO
A/20 A/10 A/S A/2
A B
C D
E

Limit, NVR mg/ft2 (or .tg/cm2)
0.01
0.02 0.05
0.1
0.2 0.5 1.0 2.0 3.0
4.0 5.0

Table 2 Thin-film (NVR) cleanliness Levels as defined in MIL-STD 1246C. The A/10 Level is equivalent to a single monolayer of contaminant.

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4. SURFACE CLEANLINESS VERIFICATION
Although MIL-STD-1246C defines cleanliness Levels is does not define how to measure it. Measuring the anal
concentration of particles on surfaces can be done either directly or indirectly. Direct examination of very clean surfaces such
as Level 100 with 1,785 particles/ft O.Ol9particles/mm2 5pm will require the examination of 53 mm2 at lOOx
magnification to statistically locate a single 5 jim diameter particle. Since a count of 1 particle after examining 53 areas is not statistically significant, at least 200 mm2 may need to be examined to achieve a variance of 2 4h/2• In contrast, indirect counting techniques concentrate the particles through liquid flushing or wiping of large areas onto relatively small filter areas followed by counting under a microscope. This mechanical concentration can be expected to result in a 50 to lOOx increase in particle concentration and thereby 1) reduce the counting time, 2) improve the counting statistics, and 3) increase the particle concentration so that it is significantly above the background noise ofcontaminants on the filter paper. Indirect sampling can be done by flushing a surface with a suitably clean solvent, pouring the contaminated solvent through a membrane filter, and then examining the filter paper under a microscope. If the flushed surface area is significantly larger than the surface area of the filter paper then a relatively large concentration ratio can be achieved. LLNL has developed a filter wiping technique that utilizes a clean dry membrane filter to "swipe" a proscribed area and then the filter paper is examined under a microscope. Unlike the direct examination technique previously described, the swiping distance is adjusted (depending upon the cleanliness Level being verified) to achieve at least 1 particle of 5 xm size in every mm2 of microscope viewing area. Utilizing very clean filter paper with 0.1 particles I mm2 5jtm and by adjusting the swiping distance to several feet, it is possible to measure particle cleanliness to Level 50. The examination procedure and counting statistics are described in MEL98-012 Surface Cleanliness Validation by Swzpingfor NIF Components[5].
The NVR cleanliness Level can be verified by an indirect sampling process described in MEL98-O15 Measurement of Nonvolatile Residue for NIF Components[6J. The examination process utilizing a very high quality methylene chloride solvent which is used to wash at least 1 ft2 of surface area and the contaminated run-off fluid is captured in a precleaned bottle. In a cleanroom hood, the fluid containing the NVR residue is concentrated through solvent evaporation and eventually placed on a preweighed cup and weighed on an ultra-microbalance. The weight is divided by the area flushed and reported as mg/& [or j.tg/cm2J. By using good laboratory practices, a background level of 0.02 mg/ft2 can be achieved. Thus, NVR cleanliness Levels ofA/lO can be reliably measured.
5. AIRBORNE CLEANLINESS SPECIFICATIONS
The NIF laser bay, switchyand and target bays are designated Class 10,000. The cleanrooms that perform precision cleaning and assembly are designated as Class 100, and supporting facilities are designated Class 1,000. In contrast, the inside of the laser cavity is designated Class 1. Airborne cleanliness is designated by "Class" which is a measure of the number of particles/ft3 of a size 0.5 j.tm diameter. Details of the metric equivalent classifications can be found in FED-STD-209E Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zone[7]. The chart which designates the standard cleanroom Classes is shown in Figure 3. Interestingly, the choice of 0.5 im for the definition of Class is based on the observation that High Efficiency Particle Air (HEPA) filters tend to have their lowest filtration efficiency at or near 0.5 jim because this is the cross-over point between two different particle capture mechanisms within the filter (diffusion due to Brownian motion dominates the capture mechanism for smaller particles whereas inertial effects dominant for larger particles). These filters actually become more efficient both above and below this cross-over point and designating filter efficiency at this point represent a conservative design philosophy.
6. RELATIONSHIP BETWEEN AIRBORNE CLEANLINESS AND SURFACE CLEANLINESS
Under no circumstances should the designations Class and Level be used interchangeable. Class refers to the maximum expected particle concentration in a volume of gas whereas Level refers to the maximum particle concentration on a surface. Again, a Class 1 00 cleanroom is not required to achieve or maintain a Level 100 surface, and in fact, maintaining a Level 100 surface is a Class 100 cleanroom is dependent on several variables, the most important being the time of exposure to the air in the cleanroom. The only way to guarantee the maintenance of a Level 100 surface in a cleanroom is to cover it or to place the critical component within a container with an even lower airborne particle concentration or Class.
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7. AIRBORNE CLEANLINESS VERIFICATION
Measuring and verifying airborne cleanliness is easily accomplished using commercially available instruments made possible, in part, by the investment made by the electronics industry and the air pollution industry. Airborne particle counters are available from a large number of vendors and they all work on roughly the same principal; the gas being tested is drawn through a small cell through which a focused light beam passes. The presence of a single particle trips the scattered light sensor which counts the event and the intensity of the light scattered off of the particle determines its equivalent spherical diameter and thereby allows the instrument to assign the counting event to a particular size bin. Current instruments are generally able to measure particles exceeding 0.1 pm in diameter and may report the presence of particles as large as 25 pm. When set to display a concentration of all particles/ft3 equal to or exceeding 0.5 pm these instruments will directly display the Class ofthe aerosol in the gas being sampled.
8. CONCLUSIONS
A high degree of surface cleanliness (both particulate and organic) is necessary in laser systems to reduce scattered light and to minimize damage initiated by the presence of particulate matter on optical surfaces. LLNL has established a series of specifications for the surface cleanliness of optical surfaces as well as the structural surfaces surrounding the optics. The Level of particulate and thin-film (NVR) contaminants that are allowed on as-cleaned, and as-installed optics are discussed as well as the level of cleanliness expected at end-of-life. The process for verifying surface cleanliness is discussed and references are given for all pertinent government and LLNL documents. Through experimentation, we have verified that the specified cleanliness Levels are achievable under laboratory conditions and should be achievable under production conditions.
9. ACKNOWLEDGEMENT
The author wishes to thank to following individuals who contributed to the technical content of this paper: Sudhir Jam, Douglas Ravizza, Alan Burnham, John Ertel, Sue Frieders, Thomas McCarville, Dave Camp and James Fair.
10. REFERENCES
[1] F. Y. Génin, K. Michlitsch, J. Fun, M. R. Kozlowski, and P. Krulevitch, "Laser-induced Damage ofFused Silica at 355 and 1064 nm Initiated by Aluminum Contamination Particles on the Surface", in Laser-induced Damage in Optical Materials, SPIE Vol. 2966, 126 (1996). [2] I.F. Stowers, J.A. Horvath, J.A. Menapace, A.K. Bumham, and S.A. Letts, "Achieving and Maintaining Cleanliness in NIF Amplifiers", SPIE Vol. 3492, 1998. [3] Park, H.K.; Grigoropoulos, C.P.; Leung, W.P.; Tam, A.C., "A Practical Excimer Laser-based Cleaning Tool for Removal of Surface Contaminants", IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part A, Dec. 1994, vol.17, (no.4):631-43. Park, H.K.; Grigoropoulos, C.P.; Yavas, 0.; Leung, W.P.; and others, "Efficient Excimer Laser Cleaning for Removal of Surface Contaminants", Proceedings of 1994 Conference on Lasers and Electro-Optics and The International Electronics Conference CLEO/IQEC, Anaheim, CA, USA, 8-13 May 1994, Opt. Soc. America, 1994. p. 426-7. [4] Institute of Environmental Sciences and Technology, 940 E. Northwest Highway, Mt. Prospect, IL 60056, tel. 847-255-
156 1.
[5] Available from LLNL or the author as document N1F5002426. [6] Available from LLNL or the author as document N1F5002325. [7] Institute of Environmental Sciences and Technology, 940 E. Northwest Highway, Mt. Prospect, IL 60056, tel. 847-2551561. [8] Hamberg, Otto, "Particulate Fallout Predictions for Clean Rooms", Journal of Environmental Sciences, Vol. 25, No. 3, May-June 1982, p. 15-20.
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Optical Cleanliness Specifications and Cleanliness Verification