Several active and inactive experiments on LDEF were designed to record hypervelocity impacts from manmade orbital debris and natural micrometeorites. Impact craters on these surfaces are being examined by the scientific community with microanalytical techniques in a search for impactor debris or residue. We have carried out an extensive series of such analyses using Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) and Secondary Ion Mass Spectrometry (SIMS) techniques developed for surfaces from two LDEF micrometeorite experiments: Expt. AO201, the Interplanetary Dust Experiment (IDE), and Expt. AO187-1, the Chemistry of Micrometeoroids Experiment (CME). We report here on contamination interferences that were discovered during the course of our studies and describe the recognition and mitigation practices developed to date.
A third LDEF surface type selected for SIMS analysis was the high purity gold (0.9999) plates from the CME that had been mounted on row 3 (trailing side) of LDEF. Horz, et al., have reported results of extensive SEM/EDS analyses of impact sites on these surfaces /3, 4/. The Au plates were mounted in a protective clamshell enclosure (to limit particulate contamination) that opened several days after deployment. Although the clamshell system was designed to close before LDEF retrieval, the extra long stay in orbit resulted in its being open upon retrieval. We attempted to look for sparse impactor debris in craters on these surfaces that would have been undetectable using SEM/EDS.
The Au samples were premounted on SEM stubs and had already been carefully documented with SEM/EDS by the CME team. These samples were placed in the SIMS without further treatment. Optical micrographs of each IDE sensor were taken before and after flight, providing a comparative record of pre-flight and in-flight/post-flight features. Samples were handled in clean rooms or laminar flow hoods during optical examination in order to minimize accumulation of post-flight surface particulate debris. Much of the surface particulate debris was blown off of sample surfaces using a filtered, pre- purified nitrogen stream. This procedure was repeated each time a sample was removed from its protective case, and just prior to insertion into instruments. No solvent rinsing or washing procedures have been used to date. IDE sensor and Ge witness plate surfaces were first scanned with an Olympus stereo optical microscope at 125X in order to locate features for further study. Optical micrographs were taken of some features and/or fiducial marks (scratches) were made in order to assist in relocating the microfeatures during subsequent analyses. Features on IDE sensors were also mapped on the whole-sensor micrographs. SEM/EDS analyses were performed using either a JEOL Model JSM 6400F cold field emission SEM equipped with an Oxford EDS with an ultrathin window, or an Hitachi S-530 scanning electron microscope equipped with thick-window Tracor-Northern TN5500 EDS. Auger Electron Spectroscopy (AES) depth profiles of surface contamination were performed using a JEOL JAMP-30 scanning Auger microprobe. SIMS data was collected with a Cameca IMS 3F using 16O+ or 16O- ion beams. The instrument was used in the ion microscope mode and data was recorded as two-dimensional elemental positive ion maps with lateral resolution of ~2 micrometers. Pixel intensities were used to calculate relative element abundances. The SIMS analytical protocol is detailed in reference /1/.
Fig. 1. (a) SEM micrograph of a pressure induced "blank" discharge on a powered IDE flight sensor. Bits of the ultra-pure Si shard used to cause the discharge are visible in the central portion of the region cleared by the discharge. (b) SIMS ion maps showed that the surface contamination layer was blown clear of the discharge zone.
The Au samples had bulk Cu and Ag contamination levels in the hundreds of ppm range. An interfacial layer of multi-element contamination was found ~1 micrometer below the surface when doing a depth profile of the Au matrix adjacent to an impact crater. The layer was ~ 2 micrometer thick and was rich in C, Na, Mg, Al, Si, K, Ca, Ti, Cr, Fe, Cu, Zn and As. This was traced to the manufacturer's practice of covering the Au plate with a 1 micrometer thick Au leaf in a final processing step in order to produce a smooth surface texture. The presence of As (frequently found with Au ore deposits) in this layer was used as a tracer for the contamination. The 1 micrometer thick Au leaf was clearly visible in micrographs of impact craters, and the exposed contamination layer could be morphologically identified. These two techniques essentially eliminated false identification of impactor debris from this source, but the presence of the contamination severely limited the utility of SIMS in identifying sparse impactor debris that could not be detected with EDS.
Particulate contamination included all surface particles. These could be traced to pre- and post-flight sources such as clothing fiber, paper fiber, starch grains, pollen, sawdust, spittle, fingerprints, oil droplets, bits of metal, plastic and rubber, etc.; inflight sources from shuttle operations during deployment and retrieval; and self-generated sources from environmental degradation (especially atomic oxygen [AO] erosion) of LDEF surfaces and specimens. The atomic oxygen exposure generated ash on some materials, eroded the polymer from metal backed films leading to the release of bits of thin metal foils, and eroded organic binders in some paints leading to the release of small, inorganic paint particles (pigments). There is also surface debris from the molten ejecta of impacts. A detailed survey of particulate contamination is the subject of several reports from the LDEF community /5-8/.
Some surface particulate contamination features possess the basic symmetry of an impact crater and can be mistakenly identified by inexperienced operators. A particular example of this type of interference is an apparent residue from a droplet of wastewater. As can be seen in Figure 2, the residue is circular and appears to have a central crater. However, stereoscopic examination and line profilometry have verified that these are surface deposits. EDS analyses of the crystalline deposits in these spots reveals a pattern that closely resembles human urine /9/. These features are relatively rare and easily identified by experienced researchers. We have identified less than 10 in optical scans of over 1500 cm2 of highly smooth LDEF surfaces. It should be noted that other LDEF and Solar Max investigators have reported these feature on retrieved surfaces /9, 10/.
Fig. 2. (a) SEM micrograph and (b) EDS spectrum of a suspected wastewater droplet residue found on LDEF.
Interference from particles in or near impact features that were imaged with SIMS were determined by careful SEM/EDS examination of the impact site. Particles that were not associated with the impact event could be recognized by their morphology. If there was no evidence that the particle had undergone melting, it was assumed to be a contaminant. Salt crystals (NaCl and KCl) and bits of metal (Al and stainless steel) were the major particulate interferences. Purely organic based particles were not a significant problem since they could be easily recognized by their composition. In general, the best method of mitigation for particulate interferences was recognition.
The Ge witness plates were contaminated with high levels (~400/cm2) of surface particulates which could not be blown off the surface with nitrogen. Many of these contamination spots, apparently residues from splatters and droplets, were analyzed using SEM/EDS, and a few were analyzed with SIMS. They were found to be alkali-rich silicaceous materials with inclusions (spots) of hydrocarbon based materials. Several other witness plates (Si, quartz and Zirconia) that were mounted concurrently and coincidentally with the Ge plates had surface particulate counts from 2-20/cm2. Thus, it was apparent that the Ge plates had been contaminated before being mounted on LDEF. In addition, the impact features on the Ge witness plates generally had high aspect central crater ratios with very jagged walls and inner spall zones. These jagged central crater walls led to beam shadowing during the SIMS analysis where the primary ion beam is prevented from reaching the bottom of the crater due to the rough contours of the crater walls. Due to this effect, residues at the bottom of the crater could not be extracted at their true levels /1/. The combination of beam shadowing and the high level of surface contamination made it very difficult to study the impact craters on Ge plates with the Cameca 3F SIMS instrument, and work has been suspended on this sample set.
A C/O/Si rich surface contamination layer covers essentially all exposed LDEF surfaces to some degree. Sources for this layer include outgassing products from organic based paint and silicone based room-temperature-vulcanizers (RTV's), and possibly from silizane based waterproofing agents used on shuttle tiles /11, 12/. The thickness of the layer is dependent on the surfaces' position relative to the various outgassing sources, the amount of ultraviolet (UV) light exposure, and the proximity to electric fields. On most IDE sensors there was a dark, arc shaped zone of contamination around the unshielded electrical leads (Au wires) on the upper surface, and there were also some areas of darker contamination near the edges of the MOS wafers. (The MOS wafers were bonded to their Al holders with silicone RTV). As shown in Figure 3, Auger depth profiles of this contamination layer in a dark arc near the electrical leads on one sensor, and in a "light" colored area adjacent to this zone, showed that the layer was twice as thick in the dark area (~700 compared to ~350). Coloration of the layer varies from transparent to dark brown, and is apparently related to thickness and carbon content in a given area. In general, this contamination layer was darker on surfaces that had low exposure to atomic oxygen (AO). SIMS depth profiles of the contaminant layer showed that C was present at concentrations 10 to 100 times higher on sensors from the backside of LDEF (row 3) versus sensors from the front side of the spacecraft (row 9). Thus, it is assumed that the contamination layer color is due to carbon-carbon bond conjugation (C=C). It should be noted that the overall thickness of the layer was aparrently unrelated to AO exposure.
During SIMS analyses, the contamination layer over an impact site, which varied in thickness depending on impact location and age, was sputtered away using the O+ beam while monitoring the concentration of C, Na and Mg (and Si on Ge and Au surfaces). Na and Mg were present in this layer in amounts detectable to SIMS but not to EDS or AES. When the ion signals for these species dropped sharply, the layer was assumed to be essentially gone. However, residual material from this layer can expect to be sputtered more slowly from the valleys and troughs associated with the local surface topography. (This is known as differential sputtering and is an effect of beam shadowing.) Thus, careful interpretation of SIMS data was still required, and the utility of C, O and Si isotopic ratios in suspected impactor debris (not performed by this group) is severely restricted /13/.
Fig. 3. (a) Post-flight photograph of an IDE sensor from the Earth End tray. Area "A" surrounds the sensor's un-shielded electrical leads. (b) AES depth profile of areas "A" and "B". One min. of sputter time equals ~100 Angstroms depth.
[1] The most troublesome interference comes from indigenous contamination of target materials. This can take the form of granular inclusions, subsurface layers and bulk matrix impurities. Careful compositional characterization of target substrates allows SIMS operators to define realistic lower limits of detection, identify tracer ion species or ion ratios associated with specific contaminants, and develop other contaminant recognition criteria.
[2] A myriad of particulate surface contaminants can be found on LDEF surfaces. Small particles that are lodged in or near impact sites will give large signals during SIMS analysis. These interferences are best dealt with through careful SEM/EDS analysis and recognition of morphologies not associated with hypervelocity impact events.
[3] A C/O/Si rich contamination layer covers all LDEF surfaces to varying degrees. This layer is sputtered away before analyzing the impact site for impactor residue. However, differential sputtering of this layer from the highly variable topography associated with impact craters can still interfere with C/O/Si compositional analyses and severely restricts the utility of isotopic ratios of these species in suspected impactor debris.
2. C.G. Simon, J.L. Hunter, D.P. Griffis, V. Misra, D.A. Ricks, J.J. Wortman, and D.E. Brownlee, "Elemental analyses of hypervelocity microparticle impact sites on Interplanetary Dust Experiment sensor surfaces", in press, Second LDEF Post-Retrieval Symposium Proceedings (1992).
3. F. Horz, R.P. Bernhard, J. Warren, T.H. See, D.E. Brownlee, M.R. Laurance, S. Messenger and R.B. Peterson, "Preliminary analysis of LDEF instrument AO187-1 'Chemistry of Micrometeoroids Experiment'", NASA CP 3134, 487- 501 (1991).
4. R. Bernhard and F. Horz, "Compositional analysis of projectile residues on LDEF instrument AO187-1", in press, Second LDEF Post-Retrieval Symposium Proceedings (1992).
5. E. Crutcher, W. Wascher, "Particle types and sources associated with LDEF", NASA CP 3134, 101-120 (1991).
6. E.R. Crutcher, L.S. Nishimura, K.J. Warner and W.W. Wascher, "Migration and generation of contaminants from launch through recovery: LDEF case history", NASA CP 3134, 121-140 (1991).
7. E.R. Crutcher, L.S. Nishimura, K.J. Warner and W.W. Wascher, "Quantification of contaminants associated with LDEF", NASA CP 3134, 141- 154 (1991).
8. E.R. Crutcher and K.J. Warner, "Molecular films associated with LDEF", NASA CP 3134, 155-178 (1991).
9. D. Humes and D. Batchelor, private communication (1992).
10. J.L. Warren, H.A. Zook, J.H. Alton, U.S. Clanton, C.B. Dardano, J.A. Holder, R.R. Marlow, R.A. Schultz, L.A. Watts and S.J. Wentworth, "The detection and observation of meteoroid and space debris impact features on the Solar Max satellite", Proceedings of the 19th Lunar and Planetary Science Conference, 641-657 (1989).
11. G.A. Harvey, "Organic contamination on LDEF" NASA CP 3134, 179-198 (1991).
12. G.A. Harvey, "Silizane to silica", in press, Second LDEF Post-Retrieval Symposium Proceedings (1992).
13. S. Amari, J. Foote, C. Simon, P. Swan, R. Walker and E. Zinner, "SIMS chemical analysis of extended impact features from the trailing edge portion of experiment AO187-2", NASA CP 3134, 503-516 (1991).