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title:Particles On Surfaces : Detection, Adhesion, and Removalauthor:Mittal, K. L.publisher:CRC Pressisbn10 | asin:0824795350print isbn13:9780824795351ebook isbn13:9780585381565language:EnglishsubjectParticles--Congresses, Surfaces (Technology)--Congresses.publication date:1995lcc:TA418.78.S96 1992ebddc:620.43subject:Particles--Congresses, Surfaces (Technology)--Congresses.

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Particles on Surfaces:Adhesion Induced Deformations

D. S. Rimai, Lawrence P. DeMejo, Ray Bowen, and Jeffrey D. Morris

Eastman Kodak Company, Rochester, New York

The adhesion of fine particles to substrates is a complex, dynamic problem. The surface forces between the particles and the substrates can cause both materials to deform. In some instances the adhesion induced stresses are sufficiently large so as to result in plastic deformation of the materials. This paper briefly reviews past research of surface force induced deformations. Direct evidence of such deformations between micrometer size particles and substrates, obtained using scanning electron microscopy at high tilt angles, is shown. Evidence for the occurrence of both elastic and plastic response to the stresses generated by the adhesion forces is presented. The thermodynamic work of adhesion between solids is calculated from the adhesion induced contact radius.

Evidence of hysteresis in the load-displacement relationship associated with adhering and removing a particle on a surface is presented. Finally, unexplained behavior such as the contact radius varying as the particle radius to the 3/4 power for polyurethane substrates is discussed.

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Figure 5A plot of the log of the contact radius as a function of the log of the particle radius for R < 60 m. The slope of the least squares fit line is approximately 2/3, which is consistent with the predictions of models which assume elastic response to the adhesion induced stresses.

Figure 6A plot of the contact radius as a function of particle radius to the 2/3 power for glass particles having radii less than 60 m in contact with a polyurethane substrate. The least squares fit line through the data is seen to intersect the origin, suggesting the absence of any significant externally applied load.


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Contamination on Optical SurfacesConcerns, Prevention, Detection, and Removal

Jean M. Bennett

Naval Air Warfare Center, China Lake, California

Contamination on optical surfaces is bad because it produces scattering and degrades the performance of an optical system in which it occurs. This paper discusses types of contamination, some methods for their prevention, how to detect contamination, and suggested methods for removing it.

INTRODUCTION

Contamination on optical surfaces is a subject that is of concern to optical system designers, optics manufacturers, and the users of optics. Contamination degrades the performance of the optics by increasing scattering and decreasing throughput. Dust on lens surfaces in a microscope that are nearly in focus in the image plane will form dark blobs on the picture. If dust is present on the surface of a reticule in a pointing optical system or a measuring microscope, it may interfere with the measurement. Dust on the windshield of a car (or pits in the windshield) can scatter sunlight into a drivers eyes when the car is heading into a low sun shortly after sunrise or before sunset. These are some examples showing why contamination in optical systems is bad. In this paper, types of contamination will be discussed in more detail. The effects of contamination on optical measurements, prevention of contamination, techniques for detecting contamination, and methods for removal of contamination are all covered in this paper.

TYPES OF CONTAMINATION

Contamination on optical surfaces can be in the form of (1) particulates or (2) thin continuous or partially continuous films. Particulate contamination is most often in the form of dust: lint from clothing or lens tissue, or fine particles of


airborne sand or other inorganic material. Tiny particles of abrasive such as cerium oxide, aluminum oxide, or rouge (iron oxide) can become imbedded in a surface during the polishing operation. In the case of optical systems that contain baffles particles can slough off the baffles and settle on optical surfaces. Henion et al.1 comment on particulate debris produced by pulsed electron bombardment of optical baffle materials. For optics aboard a spacecraft, particles can come from outgassing of spacecraft materials or from space dust. Miller et al.2 describe contamination on the front surfaces of optics produced by small rocket plumes.

Thin film contamination comes from a variety of sources, among them airborne pollutants such as water vapor that can deposit as a monolayer on an optical surface. Champetier and Giguere3 describe blue haze, a particular type of contamination that formed on metal mirrors and appeared to be produced by atmospheric pollutants in the Los Angeles basin. A particularly insidious form of thin film contamination is the organic film that results from outgassing of certain types of plastic containers holding finished optics. Strip coatings that are poured onto polished optical surfaces to protect them and are subsequently removed by stripping often leave a thin film or sometimes small patches of material on the surface. Bennett et al.4 studied the residues left on optics by several types of commercially available strip coatings. In a vacuum chamber, foreign material can be deposited on a surface during a glow discharge cleaning operation. This material is knocked off the walls of the vacuum chamber by the energetic electrons in the glow discharge. Dried liquid residue will remain on a surface following an improper cleaning operation; for example, tiny droplets may be left following spray cleaning. Gas from an aerosol can is often used to blow particles off an optical surface. The spray from the can should be filtered to prevent liquid droplets from falling on the surface. These are exceedingly difficult to remove. Finally, people can be the direct cause of contamination by leaving fingerprints on an optical surface or by talking toward a surface and depositing small particles of spit on it.

EFFECTS OF CONTAMINATION

All types of contamination adversely affect the performance of an optic or an optical system. Particulates on a surface will cause unwanted light scattering, with the light going in directions other than where it was intended. Scattered light that does not follow the direction of the primary beam will produce image blurring or add a haze over the entire image plane, depending on the range of scattering angles. Scattered light will also reduce the energy throughput of beam-directing systems. The specular reflectance of particle-contaminated mirrors will be reduced and the transmission of contaminated lenses will also be reduced. Although these effects are generally small, in some instances they can be very important. For example, in a high power laser system, light scattered back into the laser can depump the laser with catastrophic results. Figure 1 illustrates the large increase in scattering that can occur when an ultralow scatter ring laser gyro mirror becomes coated with particles. The graph on the left shows the integrated fractional scatter in parts per million from a particle-covered mirror. The graph on the right shows that the scatter was reduced by about a factor of eight after the mirror was cleaned. The article by Young5 discusses the effect of particulate contamination on mirror scattering levels.


Figure 1.Integrated fractional scatter in parts per million from a multilayer-coated superpolished Zerodur mirror before (left) and after cleaning (right) (after Rahn, Naval Weapons Center, unpublished).

When surface profiles are taken with mechanical contact profilers, either in the form of conventional mechanical profilers or as one of the scanning probe microscopes making measurements on the atomic level,6 particles sticking to the surface will produce unwanted peaks in the surface profile. Particles on a surface that is going to be coated with a single or multilayer film will act as nucleation sites and produce larger cone-shaped defects in the film that have increased scattering over that of the bare substrate.7-10

Contaminant layers on top of high quality mirrors can increase scattering. Contaminant layers on surfaces intended to be coated with optical films will affect the adhesion of the films. These same contaminant layers can also increase the absorption of deposited films. Contaminant films can greatly influence measurements of the ellipsometric parameters and since ellipsometric measurements are very sensitive to thin surface films. Mechanical contact profilers will not perform properly if a contaminant film, such as that formed by outgassing from a plastic container, is present. The stylus will push the film ahead of it, then ride over the top of a lump, giving a nonreproducible profile.

PREVENTION OF CONTAMINATION

Prevention of contamination is much better than removing it later. Careful workshop practices are essential to having clean optics. Although most optics shops do not have clean rooms, the inspection areas, cleaning areas, and coating areas normally have clean benches. Shipping of optics in a clean condition is a major problem. Small parts are normally wrapped in lint free lens tissue (which deposits particles on the surfaces), or are placed in plastic boxes that outgas over a period of time and deposit a contamination layer on the optical surface. It used to be common practice to cover finished uncoated optical surfaces with a layer of


strip coat that was supposed to be removed just before the part was used. Fortunately this practice is much less common now since strip coat materials can leave a small amount of residue on a surface4 which is difficult to remove. Some companies leave large finished optics face up on benches in clean rooms. Over a period of time, large particles can settle on a surface even in a Class 100 clean room. It is much preferable to mount a large optic face down close to a clean surface with spacers on the edge to prevent it from touching the surface.

To prevent contamination from forming on optics during shipping, they should be cleaned just prior to shipping and then shipped in plastic containers that have a minimum of outgassing. If storing is necessary, this should be done in a clean area with the optical surfaces covered to prevent particles from falling on them. Shipping containers should be designed so that nothing touches the polished or coated surface. They should be made from plastic that has a very low outgassing rate. If particulate contamination is of concern to the customer, the optics should not be wrapped in lens tissue, even lintless lens tissue. Prevention of contamination is much preferable to removing it later.

CONTAMINATION DETECTION

Contamination, especially particulate contamination, is easy to detect on optical surfaces. If a surface is viewed in a darkened room using a microscope illuminator to direct a beam of light onto the surface, particles light up like little stars. Smudges, fingerprints, dried water droplets, and other cleaning residues are also easy to observe. Finer particulate contamination can be seen in a dark field microscope or in a microscope using differential interference contrast.11 If a laser beam is directed onto an extremely smooth, clean, reflecting surface in a darkened room, the laser spot will not be visible. Although sophisticated optical scatterometers are available to measure total integrated scatter and angle-resolved scatter,12 these are generally not used to inspect an optic for cleanliness. However, as illustrated in Fig. 1, one such measurement was made to show the large increase in scattering caused by particulate contamination. For transparent optics, the technique of total internal reflection microscopy13 can be used to detect surface defects.

Other techniques have also been reported for studying contamination. Song et al.14 have used surface plasmon effects to study surface contamination. Peterson15 describes methods for detecting particles on optical surfaces. Williams and Lockie,16 Thomas,17 and Stover and McGary18 have all measured angle resolved scattering from contaminated surfaces. Facey and Nonnenmacher19 measured total hemispherical emissivity of contaminated mirror surfaces.

Contamination on mirrors in space poses special problems. Jarratt et al.20 describe a self-contained space qualifiable optical contamination monitor. Another prototype instrument was designed to detect contamination in real time on optical samples in space.21 This instrument measured total integrated scattering from the sample at a visible and an infrared laser wavelength. By measuring the ratio of the two scattered light signals and making use of scattering theory,12 it was possible to determine whether sample changes were caused by particulate contamination or by erosion of the entire surface.


CONTAMINATION REMOVAL

As mentioned previously, it is much better to keep optical surfaces clean rather than to remove contamination later. Sometimes this is not possible, and cleaning is necessary. Cleaning of optics comes in many forms depending on the sizes, types, and number of pieces, whether or not they are coated, the type of coating (cleanable or not), and the quality of the optic. If optics are mounted in an instrument, it is much more difficult to clean them than if they are free standing.

Chapter 8 in Ref. 12 contains useful information about cleaning individual unmounted optics. The article, When is a surface clean?22 has additional information about cleaning a variety of optical surfaces that have different types of contamination on them. Mass production cleaning of optics before coating is discussed in a chapter of a book by Pulker.23 Two conference proceedings24,25 deal entirely with Optical System Contamination: Effects, Measurement, Control. A treatise26 and three symposia proceedings,27-29 all edited by Mittal, contain some references to techniques for cleaning optical surfaces. Additional references concerning cleaning special types of optics are given in a recently published reprint book.30

Table I illustrates some cleaning techniques that can be used for cleaning small unmounted optics.12 These techniques are described in detail in Refs. 12 and 22. If a lens or other optical element is dusty, the larger dust particles can be blown off with an air bulb or an aerosol can; the latter should have a filter to prevent liquid droplets from escaping. Particles smaller than ~10 m adhere tenaciously and cannot be removed in this manner. However, they can often be removed by some special laser removal techniques31 which are still in the developmental stage, but could be adapted for production-line cleaning. Alternately, carbon dioxide snow has been shown to be effective under certain conditions.32 Ultrasonic cleaning33-35 has been found to be effective for assembly-line cleaning of optics, for example by an optical coating company. A plasma assisted chemical cleaning process36 has been used successfully for cleaning optical components that are to be used in a high power laser beam.

Table I.Cleaning Techniquesa

Particulates

Films

Air bulb

X

Aerosol can

X

Freon-TE spray

X

Drag wiping

X

X

Vapor degreasing

X

Soap and water

X

X

Ultrasonic cleaning

X

X

Strip coatings

X

Glow discharge

X

Sputter etching

X

a. Reproduced from p. 76 of Ref. 12


If an uncoated optic is to be used for a noncritical application and has dust particles, fingerprints, cleaning residue, or other forms of contamination on the surface, it is frequently easiest to clean it with detergent and water. However, drying is a problem since any towel, swab, etc. touching a wet surface can leave residue on the surface. It is much preferable to let the liquid drain off when the piece is held in a vertical position, or else spin the water off with the optic on a spinner such as those that are used for applying photoresist. Cleanable coated surfaces that are dusty can be cleaned by carefully drag wiping them with a piece of lintless lens tissue moistened with methanol or ethanol.12,22 As shown in Fig. 2, the moistened lens tissue is slowly pulled across the surface. The boundary between the wet and dry portions of the tissue traps particulate matter and drags it across the surface and over the edge. Of course, care should be taken to remove large particles that could scratch the surface before drag wiping. Acetone should not be used since it dries too fast and even the cleanest grade can leave a white deposit on the surface. Cleaning a small area on a surface with a swab and solvent is to be avoided since, although the spot will be removed, there will be a residue of liquid remaining on the surface. Careful cleaning is essential since poor cleaning is much worse than no cleaning. An example of cleaning that greatly increased scattering from a group of silicon wafers is given in Ref. 12.

Figure 2.Drag wiping technique for cleaning optics (from ref. 22).

Table II shows examples of various types of contaminants and suggested methods for removing them. If a surface has a light coating of dust from room air, one can try to blow off the dust with spray from an aerosol can. Tiny particles that can only be seen under a microscope will probably not come off. If the surface and/or coatings are hard, drag wiping may remove the tiny dust particles. However, soft materials such as gallium arsenide, aluminum, and copper, or soft coatings will damage if they are drag wiped. Some of the specialized cleaning methods mentioned above can also be used to remove both particles and film-type contamination.

Special cleaning techniques have been developed for cleaning low scatter mirrors that have been exposed to silicone oil37 and mirrors contaminated by carbon in a synchrotron radiation environment.38,39 Feicht et al.40 have studied the mechanism of dust removal from mirrors by analyzing adhesion forces. Breault


et al.41 have described cleaning, coating, and BRDF measurements of a low scatter mirror, and Facey42 has studied particulate contamination on the primary mirror of the Hubble Space Telescope.

Table II.Types of Contaminants and Suggested Methods for Their Removala

Material

Type of Contaminant

Cleaning Process

Uncoated fused silica

Fingerprints, dust

Soap and water; spin dry

Multilayer-coated glass

Dust, cleaning marks

Aerosol can; drag wipe

Aluminum-coated glass

Dust

Aerosol can

Diffraction grating

Dust, cleaning marks

Collodion; strip coat

Silicon wafer

Dust, fingerprints

Organic solvent; spin dry

Gallium arsenide wafer

Dust

Aerosol can

Diamond-turned metal

Oil

Trichloroethylene; drag wipe

a. Reproduced from p. 79 of Ref. 12

Laser polishing43 with a CO2 laser is a technique that has been shown to decrease surface defects and increase the damage threshold of fused silica optics. When a laser beam is rastered over a fused silica surface, a thin surface layer is melted and tiny defects are either removed or buried. Photographs taken using total internal reflection microscopy13 and reproduced in Fig. 3 show that the number of surface defects can be dramatically reduced by using this technique.

Figure 3.Normal polished fused silica surface (left) and part of the same surface after polishing with a CO2 laser (right) (from ref. 43).

CONCLUSION

Contamination on optical surfaces is to be avoided since it is deleterious to the performance of optical systems. Types of contamination have been described


as well as their effects, how they may be detected, and techniques for their removal. It is much better to prevent contamination from occurring on optical surfaces rather than having to remove it later.

ACKNOWLEDGMENT

This work was supported by Independent Research Funding at the Naval Air Warfare Center, Weapons Division.

REFERENCES

1.S. R. Henion, E. A. Johnson, and J. A. Johnson, Particulate debris from pulsed electron bombardment of optical baffle materials, in Stray Light and Contamination in Optical Systems, R. P. Breault, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 967, 314-319 (1988).

2.J. L. Miller, G. Steiner, and E. Dryden, Front surface optic contamination from small rocket plumes, in Stray Light and Contamination in Optical Systems, R. P. Breault, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 967, 320-331 (1988).

3.R. J. Champetier and R. P. Giguere, Deterioration of superpolished metal mirrors by blue haze, Opt. Eng. 21, 743-750 (1982).

4.J. M. Bennett, L. Mattsson, M. P. Keane, and L. Karlsson, Test of strip coating materials for protecting optics, Appl. Opt. 28, 1018-1026 (1989).

5.R. P. Young, Low-scatter mirror degradation by particle contamination, Opt. Eng. 15, 516-520 (1976).

6.J. Jahanmir, B. G. Haggar, and J. B. Hayes, The scanning probe microscope, Scanning Microscopy 6(3) 625-660 (1992).

7.K. H. Guenther and H. K. Pulker, Electron microscopic investigation of cross sections of optical thin films, Appl. Opt. 15, 2992-2997 (1976).

8.K. H. Guenther, Nonoptical characterization of optical coatings, Appl. Opt. 20, 3487-3502 (1981).

9.K. H. Guenther, Physical and chemical aspects in the application of thin films on optical elements, Appl. Opt. 23, 3612-3632 (1984).

10. K. H. Guenther, Microstructure of vapor-deposited optical coatings, Appl. Opt. 23, 3806-3816 (1984).

11. D. L. Lessor, J. S. Hartman, and R. L. Gordon, Quantitative surface topography determination by Nomarski reflection microscopy. 1. Theory, J. Opt. Soc. Amer. 69, 357-366 (1979).

12. J. M. Bennett and L. Mattsson, Introduction to Surface Roughness and Scattering (Optical Society of America, Washington, D. C., 1989).

13. P. A. Temple, Total internal reflection microscopy: a surface inspection technique, Appl. Opt. 20, 2656-2664 (1981).

14. D. Y. Song, F. S. Zhang, H. A. Macleod, and M. R. Jacobson, Study of surface contamination by surface plasmons, in Optical Thin Films II. New Developments, R. I. Seddon, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 678, 211-218 (1986).

15. R. V. Peterson, Detection methods for particles on optical surfaces, in Stray Radiation V, R. P. Breault, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 675, 37-42 (1986).


16. V. L. Williams and R. T. Lockie, Optical contamination assessment by bidirectional reflectance-distribution function (BRDF) measurement, Opt. Eng. 18, 152-156 (1979).

17. D. A. Thomas, A study of the scatter of visible light from particulate mirror contaminants, in Generation, Measurement, and Control of Stray Radiation III, R. P. Breault, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 384, 50-65 (1983).

18. J. C. Stover and D. E. McGary, Scatter from subsurface defects and contaminants, in Stray Radiation in Optical Systems, R. P. Breault, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1331, 48-53 (1990).

19. T. A. Facey and A. L. Nonnenmacher, Measurement of total hemispherical emissivity of contaminated mirror surfaces, in Stray Light and Contamination in Optical Systems, R. P. Breault, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 967, 308-313 (1988).

20. R. V. Jarratt, Jr., B. K. Flint, R. D. Fancy, and R. C. Linton, Self-contained space qualifiable optical contamination monitor, in Optical System Contamination: Effects, Measurement, Control, A. P. Glassford, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 777, 171-178 (1987).

21. J. L. Pezzaniti, J. B. Hadaway, R. A. Chipman, D. R. Wilkes, L. Hummer, and J. M. Bennett, Total integrated scatter instrument for in-space monitoring of surface degradation, in Optical System Contamination: Effects, Measurement, Control II, A. P. Glassford and D. F. Hall, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 1329, 200-210 (1990).

22. J. M. Bennett, When is a surface clean? Opt. Photonics News 1(6), 29-32 (June 1990).

23. H. K. Pulker, Cleaning of substrate surfaces, in Coatings on Glass, Vol. 6 in Thin Films Science and Technology Series, G. Siddall, ed. (Elsevier, Amsterdam, 1984), Chap. 4, pp. 52-63.

24. A. P. Glassford, ed., Optical System Contamination: Effects, Measurement, Control, Proc. Soc. Photo-Opt. Instrum. Eng. 777 (1987).

25. A. P. Glassford and D. F. Hall, eds., Optical System Contamination: Effects, Measurement, Control II, Proc. Soc. Photo-Opt. Instrum. Eng. 1329 (1990).

26. K. L. Mittal, ed., Treatise on Clean Surface Technology (Plenum, New York, 1987), Vol. 1.

27. K. L. Mittal, ed., Particles on Surfaces 1: Detection, Adhesion, and Removal (Plenum Press, New York, 1988).



30. J. M. Bennett, Surface Finish and Its Measurement (Optical Society of America, Washington, D. C., 1992).

31. S. D. Allen, Laser cleaning techniques for critical surfaces, Opt. Photonics News 3(6), 28-30 (June 1992).

32. R. V. Peterson and C. W. Bowers, Contamination removal by CO2 jet spray, in Optical System Contamination: Effects, Measurement, Control II, A. P. Glassford and D. F. Hall, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 1329, 72-85 (1990).


to the relatively high stresses generated by the adhesion forces, compared to the Youngs modulus of the substrate, that nonlinear elastic effects may be present. This hypothesis is consistent with the lack of any distinct break observed in the curves shown in Figures 5 and 6.

Now consider the glass particles having nominal radii of approximately 103 m. As seen in Figure 4, the contact radius does not increase for these particles over that measured for the 60 m radius particles. Moreover, as evident from Figure 4, there is significantly greater variation in the contact radii associated with these particles than with the smaller ones. As has been indicated, the plane of the substrate was perpendicular to the horizontal for all the samples in this study. Therefore, gravitational forces tend to peel the particles away from the substrate. For most of the particles used in this study, these forces are small compared to the surface forces. However, as indicated by the fact that most of the large particles failed to adhere to the substrate, gravitational forces are not negligible for the 103 m radius particles. Figure 12 shows a typical electron micrograph of a 103 m radius particle in contact with the polyurethane substrate. As can be seen by the different scales shown in this figure, the magnification was increased just above the contact region, thereby permitting both an overview of the particle and a high resolution image of the contact zone in one micrograph. As can be seen, the particle appears to be pulling the substrate. The sharp contact angle makes the appearance of distended substrate distinct from the appearance of the menisci observed with the smaller particles. Similar effects have been reported by Chaudhury.43

The hysteresis associated with the placing and removing of a particle on a surface can also be observed with an atomic force microscope (AFM). For example, Schaefer et al.44 cemented 10 m radius silver coated glass and polystyrenespheres onto the tip of an AFM cantilever and measured the force of adhesion between the particle and highly oriented pyrolitic graphite, polyurethane, and wax coated laserprinter paper substrates as a function of particle displacement. A typical result, in this instance polystyrene spheres on pyrolitic graphite, is shown in Figure 13. As can be seen, during the loading process, the particle initially experiences little or no attractive force. Then, at a sufficiently small displacement, which is a function of the spring constant of the cantilever, the particle jumps to the substrate. This shows as a small, negative force at zero displacement. Upon further loading, a positive force is needed to press the particle into the substrate. Upon unloading, the force initially retraces the loading curve, within the error of the equipment. However, separation does not occur at zero displacement. Rather, an applied force must still be applied until, finally, separation is achieved. Similar observations have been reported by Mizes et al.45

These results suggest that, in the process of separating a particle from a substrate, energy is put into both propagating a crack in the interfacial zone and into creating stress distributions in the materials. This further suggests that, due to the relatively short range nature of surface forces, the placing and removing a particle from a surface may not be a reversible process, even if the materials respond elastically to the adhesion induced stresses. In other words, the loading and unloading process may exhibit a hysteresis effect, even in the absence of plastic or viscoelastic response.


33. K. H. Guenther and H. Enssle, Ultrasonic precision cleaning of optical components prior to and after vacuum coating, in Thin Film Technologies, J. R. Jacobsson, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 652, 33-40 (1986).

34. M. ODonoghue, The ultrasonic cleaning process, Microcontamination 2(5), 62-67 (1984).

35. A. Mayer and S. Shwartzman, Megasonic cleaning: a new cleaning and drying system for use in semiconductor processing, J. Electron. Mater. 8, 855-864 (1979).

36. R. A. Schmell and G. R. Erickson, Low budget plasma assisted chemical cleaning of optical substrates at the LANL optical fabrication laboratory, Precis. Eng. 13, 52-53 (1991).

37. V. L. Williams and G. J. Fleig, A method for cleaning low-scatter mirrors after exposure to silicone oil, in Stray-Light Problems in Optical Systems, J. D. Lytle and H. Morrow, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 107, 170-172 (1977).

38. T. Koide, M. Yanagihara, Y. Aiura, S. Sato, T. Shidara, A. Fujimori, H. Fukutani, M. Niwano, and H. Kato, Resuscitation of carbon-contaminated mirrors and gratings by oxygen-discharge cleaning. 1: Efficiency recovery in the 4-40-eV range, Appl. Opt. 26, 3884-3894 (1987).

39. T. Koide, T. Shidara, M. Yanagihara, and S. Sato, Resuscitation of carbon-contaminated mirrors and gratings by oxygen-discharge cleaning. 2: Efficiency recovery in the 1001000-eV range, Appl. Opt. 26, 4305-4313 (1988).

40. J. R. Feicht, J. R. Blanco, and R. J. Champetier, Dust removal from mirrors: experiments and analysis of adhesion forces, in Stray Light and Contamination in Optical Systems, R. P. Breault, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 967, 19-29 (1988).

41. R. P. Breault, S. R. Lange, and B. B. Fannin, Cleaning, coating and BRDF measurements of a two-meter low-scatter mirror, in Optics in Adverse Environments, M. A. Kahan, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 216, 9-23 (1980).

42. T. A. Facey, A study of surface particulate contamination on the primary mirror of the Hubble Space Telescope, in Measurement and Effects of Surface Defects and Quality of Polish, L. R. Baker and H. E. Bennett, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 525, 140-146 (1985).

43. P. A. Temple, W. H. Lowdermilk, and D. Milam, Carbon dioxide laserpolishing of fused silica surfaces for increased laser-damage resistance at 1064 nm, Appl. Opt. 21, 3249-3255 (1982).


An Advanced Surface Particle and Molecular Contaminant Identification, Removal, and Collection System

Steven P. Hotaling* and Deidra A. Dykeman

USAF Rome Laboratory/OCPA, Griffiss AFB, New York

High performance optical and microelectronic components have ever tightening contamination specifications placed upon them. Contamination is a major reason for the degradation of space based optical systems and failure of high density integrated circuits used throughout industrial and military systems. Contamination is currently controlled by the use of cleanrooms, special process monitors and cleaning techniques which include solvent wipes, strippable coatings, wet-dry processes, ultrasonics and air purges. The major disadvantages of these techniques are their inability to remove submicrometer particles and potential of leaving molecular residues on the cleaned surfaces. Some of these cleaning techniques can be damaging to delicate surfaces and/or have toxic waste products. To address this problem, contamination removal and collection techniques have been developed by Rome Laboratory (RL, formerly RADC). However, some of these removal techniques create a flux of removed contaminants, which then may re-deposit on clean surfaces or be ejected into the environment, for which we present a novel contamination collector capable of collection and containment of contaminants.

BACKGROUND

Contamination control is defined as the detection, measurement, removal and collection of undesired particulate and organic species. As the design and manufacturing of optical and electronic

*Current affiliation: Clarkson University, Potsdam, New York


components becomes more challenging, i.e. pristine low scatter optics and device miniaturization to submicrometer dimensions, contamination control plays an ever increasing role. In 1985, the USAF Rome Laboratory established the need to develop better contamination prevention, detection, cleaning and collection techniques. In 1991, the Rome Laboratory recognized the importance of developing contamination prevention and collection techniques and began a small in-house program to address these areas. Ongoing work in the area of microcellular porous materials resulted in the invention of a collection device with the potential to collect and contain both particulate and organic contaminants. This paper serves as an introduction to the contamination problem and the new RL contamination collection device.

INTRODUCTION

The presence of contamination on optical and microelectronic components degrades their operational performance and lifetimes. It is also the cause of low manufacturing yields. During the production cycle, contamination is controlled by the use of cleanrooms, process monitors, and manual cleaning techniques including solvent wipes, strippable coatings, wet-dry processes, ultrasonics and air purges. The major disadvantages of these techniques are their inability to detect and remove submicrometer particles and they tend to leave molecular residues. Some of the cleaning techniques can be damaging to delicate surfaces and/or have toxic waste products. The next section discusses the phenomenology of the contamination problem.

Once integrated into a system such as a sensor telescope, the optics are no longer accessible to monitor and manually clean. Present space satellites are currently overdesigned to allow for contamination induced degradation while in orbit. Overdesign and redundancy increases the weight of the satellite which increases the material and launch costs. This paper briefly discusses spacecraft optical contamination and control as analyzed and tested in the Rome Laboratory program. A brief summary of contamination detection and removal devices is included in that section.

The new Aerogel Mesh Contamination Collector (AMCC) appears on page 129. This device, invented at RL by the authors, is the first device to employ both active and passive means to collect and contain both particulate and molecular contaminants.


CONTAMINATION PHENOMENOLOGY

Contamination can be either particulate or molecular in nature. There are many sources of these species which can vary throughout the lifetime of a component or system. The impact of contamination is varied, depending on atmospheric conditions, contaminant and component charateristics, deposition mechanism, rate of deposition and aging, etc. Particulate contamination arises from a multitude of sources3. Even ultra-clean environments such as semiconductor processing facilities utilizing high efficency particle absolute (HEPA) filters are not devoid of particles in the air and on exposed surfaces. In such environments, personnel, frictional contact between tools and product, robotic device movements, and equipment processes and vibration create particles. Also, air currents act to redistribute the particles.

Inorganic particles tend to be harder and more spherical in geometry than organic particles.They typically range from less than 1 micrometer to several micrometers in diameter and are generated by friction between surfaces or surface erosion due to atmospheric chemical interactions. Organic particles are softer than inorganics and tend to have a nonspherical geometry. This causes them to have a larger surface area than inorganic particles with similar volume. The combination of softness and larger surface contact area makes organics more difficult to remove. However, submicrometer inorganic particles are just as difficult because they may rest inside the surface roughness profile and are therefore protected from being wiped away.

Molecular contaminants, like particulate contaminants on surfaces, result from a variety of processes including outgassing of polymeric or organic materials, residual molecules from chemicals used in component processing, desorption and/or diffusion, and condensation and adsorption from the environment. Outgassing is a complex physical phenomenon in which molecular species are released by a material by a variety of processes including diffusion, surface desorption or catalysis. The outgassed species are then free to come to rest or condense onto surfaces within a few mean free paths. Condensation is essentially a deposition process involving nucleation, island growth, surface diffusion, and monolayer buildup.

The physical and chemical kinetics of multi-component species deposition onto a surface could give rise to a mixture of amorphous and polycrystalline molecular films. Both the outgassing and the deposition rate processes are temperature and time dependent.


Exposure to ultraviolet radiation can cause photo-induced catalysis of organic molecules. This is especially of concern in spacecraft systems which are subject to vacuum ultraviolet radiation1. Even the low intensities of short wave ultraviolet radiation (1800 Angstrom wavelength) leakage from fluorescent lighting may be a cause of photoinduced conversion of organic vapors into particles2,3. The source of organic vapors in clean rooms is presently suspected to be due to outgassing of plasticizers from vinyl curtains or floor tiles as well as from process chemicals4. Polymerized organics become fixed to surfaces and are extremely hard to remove. The most effective method is reactive etching using oxygen ions. The oxygen ions chemically react with the organic to break it down into volatile species.

Figure 1.The effect of contamination on Solar Absorbance vs. time for 0.002 inch silvered FEP Teflon.

SPACECRAFT CONTAMINATION

General Introduction

Spacecraft are subject to contaminating sources throughout their lifetime from ground processing, launch and deployment, and onorbit operation. If allowed to buildup, contaminants can have devastating effects on the operational performance and lifetime of a satellite. Molecular films from nonmetallic material outgassing5-8,10-16


may adsorb onto sensitive surfaces such as solar panels or thermal control surfaces. Exposure to the UV and vacuum ultra violet (VUV) speeds the deposition process and will polymerize organics creating a nonvolatile covering5-8. As shown by Figure 1, this film causes an increase in the absorbance of the thermal control surfaces and thus spacecraft heatup5. It may also affect power generation capability of the solar arrays5 as shown by Figure 2 .

Figure 2.The effect of contamination on Power Production capability for 5 GPS Block I satellites for 8.5 years on orbit. The solid line is a performance plot.

We have documented molecular and particulate contamination on solar cell coverglass samples from the Long Duration Exposure Facility (LDEF) satellite which was in a low earth orbit for 69 months. Figure 3 illustrates a Nomarski optical micrograph (50x) of a coverslip sample which flew on the LDEF leading edge. Auger analysis revealed the film to be primarially composed of SiOx, which we attribute to outgassed silicones modified by exposure to atomic oxygen and solar ultraviolet. Figure 4 shows an identical solar cell cover glass (at the same magnification and polarization settings) sample which was positioned on the LDEF trailing edge. This sample


Figure 3.Nomarski photomicrograph (50x) of a sample from the LDEF satellite. The interference fringes are indicative of a molecular film. The crater in the center of the figure was from a space debris impact.

shows a much thinner film of the same chemical composition as that of the leading edge sample. Both samples appear to have collected particles of varied size (submicrometer to hundreds of micrometers in spatial extent) and chemical composition (EDX identified metallic and inorganic particles (Na, Zn, K) and fluorescence analysis of the samples revealed organic species). In addition to thin films and particluates, the sample of Figure 3 contains the crater left by a micrometeorite (probably space debris particle) impact. For optical systems, scattered radiation is characterized by the Bidirectional Reflectance Distribution Function (BRDF). The BRDF is a measure of the amount and angular distribution of optical scatter from a surface. Specifically it is the sample radiance divided by the sample irradiance as given by:

Where Pi is the incident power, s is the solid angle subtending the scatter at incidence, and Ps is the scattered power. Thus, the higher


the BRDF, the higher the level of scattered energy from the surface; Ideally, the BRDF should be a delta-function with deviations due to scattered light. The BRDF has been used as a measure for contamination levels on optical surfaces in many studies9-15. Figure 5a is a sketch of the BRDF measurement geometry illustrating the parameters in equation 1. Figure 5b is a schematic of the scatterometer used at RL. It consists of a laser, beam shaping optics, the optic under test and a detector. The detector is mounted on a rotating arm which allows the detector to sweep through a circular arc monitoring scattered irradiance.

To illustrate the application of the BRDF for molecular films, Figure 6a shows a plot of the BRDF curves of a pristine mirror (triangles) which has been contaminated by water cryofilms of thicknesses ranging from 1.06 to 4.00 micrometers . These BRDF measurements were performed using 10.6 micron incident laser radiation generated by a CO2 gas laser. For particles also, the BRDF is a sensitive contamination detection technique. Figure 6b shows

Figure 4.A Nomarski photomicrograph of a sample from the trailing edge of the LDEF satellite. Notice that there is less molecular contamination, but approximately the same amount of particulate debris as appears in the sample of figure 3.


Figure 5a.The geometry of the BRDF measurement. Pin is the input light flux, Pout is the light flux at the angle of reflection, and is the solid angle subtended by the scattered energy.

three BRDF plots for a mirror tested at the Hughes Aircraft Company16. This figure shows the BRDF for a pristine (factory clean) mirror, the same after contamination with Arizona Standard Fine Dust and the BRDF after the mirror has been cleaned with a carbon dioxide gas/solid jet spray. This BRDF curve lies very close to that of the pristine mirror demonstrating the effectiveness of the Jet spray cleaning technique which will be discussed later.


Figure 5b.The Rome Laboratory scatterometer layout.


Figure 7The log of the contact radius as a function of the log of the particle radius for R < 5 m. The observed 3/4 power law dependence (obtained from the slope of the curve) is unexplained but is consistent with previously reported results of indentation experiments on viscoelastic materials (Ref. 42).

In contrast to materials deforming elastically because of the adhesion induced stresses, these stresses can be sufficiently large so as to exceed the elastic limit of at least one of the contacting materials, thereby resulting in plastic deformation. This is extremely important in understanding the adhesion of a particle to a substrate. As previously discussed, much of the energy associated with creating an elastic deformation is recoverable during the removal of the particle. This is not true if the adhesion induced deformation is plastic.

Consider, now, the case of polystyrene spheres, having radii between approximately 1 m and 6 m, on a silicon substrate. The particles are the same as those shown in Figure 9 in contact with a polyurethane substrate. Although it was not possible to directly measure the mechanical properties of the spheres, typical values of the Youngs modulus and yield strengths46 are 3 109 Pa and 9 106 Pa, respectively. Because the particles are much more compliant than the substrate, they, rather than the substrate, would be expected to deform in response to the adhesion induced stresses.


Figure 6a.BRDF for water cryofilms at a laser wavelength of 10.6 microns. Scatter is reduced in cryofilms less than 2 microns thick due to absorption. Thicker cryofilms crack increasing scatter.


Figure 6b.BRDF at a laser wavelength of 514 nm for particulate contamination before and after cleaning with CO2 jet spray.

Spacecraft Contamination Control Technology

To control contamination buildup on spacecraft surfaces, Rome Laboratory developed (under contract) space based contamination detection and removal devices. A cryogenic quartz crystal microbalance (QCM) was developed to detect and measure the amount of molecular contaminants deposited on to an optical surface. The contamination removal devices include: the gas/solid jet spray, electron and ion guns, and lasers.

It is important to recognize the commercial applications of spinoffs of space systems technology. The very nature of satellite


systems implies that any maintenance be autonomous. Thus, the robotic contamination detection, removal, and collection technologies described herein may be applicable to industrial production-line and research laboratories. Examples of this include: semiconductor processing, industrial surface treatments (such as painting and lithography), and cryogenic/vacuum biomedical procedures.

The Cryogenic Quartz Crystal Microbalance (QCM). QCM Research developed the cryogenic QCM(MK-16 QCM) to monitor

Figure 7.The Mark 16 Quartz Crystal Microbalance.

molecular contamination deposition on optical surfaces and to understand contamination effects at cryogenic temperatures. This device along with insitu BRDF scatterometers, provides information to autonomous contamination control systems such as to when to apply cleaning procedures (e.g. on a satellite system). The QCM utilizes two crystals, a sensing crystal which is exposed to contamination and a reference crystal which is protected. The two crystals oscillate at a small beat frequency. As contamination deposits onto the sensing crystal, the beat frequency increases. Using the calibration equation below, the change in frequency can be converted to a change in mass:


TABLE 1MK-16 QCM SPECIFICATIONS*

Sensitivity:

Mass:(5 K)

4.4906 109 g cm2 Hz

(298 K)

4.4878 109 g cm2 Hz

Thickness (= 1 g cm3)

2.2300 Hz A1

Thermal Operating Range:

5 K < T < 298 K

Thermal Sensor Precision:

+/0.25K

Rate of Drift:

< 50 Hz per year

* As quoted by QCM Research

The QCM is extremely sensitive, able to detect monolayers of molecular films and is designed as a flight qualified unit from 5 to 360 degrees Kelvin. The MK 16 has been used successfully at cryogenic and room temperatures13. The MK 16 QCM specifications are summarized in Table 1.

Jet Spray Contamination Removal The jet spray cleaning process is shown in Figure 8. The cleaning mechanism is momentum transfer. A stream comprised of solid particles within a gas is incident upon a contaminated surface. The particles are dislodged from the surface by momentum transfer of the solid snowflakes and both are then carried away from the surface by the gas. The gas/solid interface serves as an insulating layer which protects the surface from damage by the CO2 snow particles. The jet spray has been effective in removing particles from ambient optics and particles in thin cryofilms from cryogenic optics as shown in Figure 9 and Figure 10.


Figure 8.An illustration of the Gas/Solid jet spray cleaning process.

Figure 9.10.6 micron BRDF for jet spray cleaning of particles.


Figure 10.35 mm photographs showing optical samples which were subjected to high velocity particulate contamination before (left) and after(right) treatment with the jet spray.

Laser Contamination RemovalThe laser contamination removal technology7&18 is illustrated in Figures 11 and 12. In Figure 11, a cryofilm contaminated mirror has been partially cleaned by a laser beam. The scatter data is shown on the right side of Figure 11. Notice the increase in mirror scatter due to the opaque cryofilm. The laser was able to return the mirror to its original low scatter level without damage. Figure 12 illustrates the predominant mechanisms for laser cleaning. The laser radiation is either absorbed by the mirror substrate which heats and causes the contaminant film to evaporate, or it can be absorbed by the contaminant film itself. The coupling efficiency between the laser radiation and the contaminant is a function of laser wavelength, polarization, and angle of incidence.


Figure 11.35 mm photograph of a mirror contaminated with a water cryofilm with a small patch of its surface cleaned by the Laser Cleaner (left). To the right is the BRDF for the contaminated and cleaned sections of the mirror.


Figure 12.The two methods of Laser Cleaning: Substrate Absorption (top), and Contaminant film absorption (bottom).

Electron/Ion Contamination RemovalElectron contamination removal can either be done through electronic heating of the substrate with subsequent contaminant vaporization or by electronic


sputtering or some combination of both. The more efficient, i.e. quicker with less potential for substrate damage, method is electronic sputtering with thermal heating19, which is depicted in Figure 13. The incident electrons kinetic energy is transferred to the contaminant atom via the atoms electron cloud as opposed to direct iontocontaminant atom momentum transfer as in ion sputtering. One shortfall of electron cleaning is that it will not remove most organic films or particles. The electron flux has been shown to carbonize organic contaminants thus compounding surface contamination rather than inducing desorption. Low energy oxygen ions will remove films through chemical kinetic interaction with the contaminant film.

Figure 13.Ion and Electron Sputtering Cleaning Techniques Illustrated. The ion sputtering is a direct transfer of kinetic energy from the incoming ion to the contaminant atom nucleus. Electron sputtering involves energy transfer between the incoming electron and the contaminant atom electron cloud. This is a much lower momentum energy transfer process, thus has lower probability for surface damage than the ion sputtering cleaning process.


THE AEROGEL MESH CONTAMINATION COLLECTOR (AMCC)

While the removal devices can successfully clean contaminated optics, there remains the problem of preventing the removed

Figure 14.Direct Ion Etch (left) versus Reactive Ion Etch (right) processes illustrated.

Figure 15.Reflectance data for a clean, contaminated, and oxygen ion cleaned aluminum mirror.

contaminants from redepositing onto the cleaned surfaces. This is where the Rome Laboratory developed Aerogel Mesh Contamination Collector (AMCC) plays a role. Our new method of contamination prevention and collection described herein employs ultra-low density silica aerogel and a highly porous aluminum or composite mesh to capture free floating molecules and particles.


Figure 8The contact radius as a function of particle radius for R < 5 m for glass spheres in contact with a polyurethane substrate. The least squares fit line through the data intersects the origin. These results are similar to those of polystyrene particles having radii less than 6 m on the same substrate (see Figures 10 and 11).

Micrographs of the polystyrene particles having radii of approximately 1 m, 2 m, 4 m and 6 m in contact with a silicon wafer are shown in Figures 14A through 14D, respectively. Another micrograph showing a similar particle with the magnification of the contact region with the silicon wafer increased is shown in Figure 15.

As can be seen from Figure 14, the contact radius appears to increase with increasing particle radius, but at a sublinear rate. Moreover, by comparing the micrographs shown in Figure 9 to those in Figure 14, it can be seen that the contact radii for the relatively rigid polystyrene-silicon system are smaller than are the corresponding radii for the relatively compliant polystyrene- polyurethane system.

For the case of an adhesion induced plastic deformation, in the absence of any applied load, Equation 14 reduces to

Therefore, by determining the power law dependence of the contact radius on particle radius, adhesion induced plastic deformations can be distinguished from elastic ones. This is done using the same methods used previously and, again, the reader is referred to Appendix A for a detailed description of the analysis.


Preliminary Aerogel Mesh Contamination Collector Data

The current state of the art for these getters (devices which perform contaminant capture have been called getters since the early days of vacuum science) includes metal mesh, charged plates (kilovolt charge) and charged dielectric plates. However, these systems have several problems17. The metal mesh (see Figure 16)

Figure 16.Metal mesh with different porosities.

suffers from being brittle and having higher desorption rates and ablation probabilities than are acceptable. The charged plates require kilovolts of potential which is unacceptable on a satellite, due to a high probability of arcing in the space environment. Charged dielectrics have capture radii far too low to be useful in practice (approx. a centimeter). Figure 17 illustrates the poor performance of the charged plates and di

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