AFM Investigation of Martian Soil Simulants on Micromachined Si Substrates
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Journal of Microscopy, Vol. 227, Pt 3 September 2007, pp. 236–245 Received 19 July 2006; accepted 16 March 2007 AFM investigation of Martian soil simulants on micromachined Si substrates S. VIJENDRAN, H. SYKULSKA & W. T. PIKE Optical and Semiconductor Devices Group, Electrical and Electronic Engineering, Imperial College London, Exhibition Road, London SW7 2BT, UK Key words. Atomic force microscopy, micromachining, Mars, particles. Summary specifically for the in situ investigations of dust and soil particles from comets/asteroids or on the surface of planetary The micro and nanostructures of Martian soil simulants with bodies. The European Rosetta spacecraft, on the way to particles in the micrometre-size range have been studied Comet 67P/Churyumov-Gerasimenko, carries an AFM as using a combination of optical and atomic force microscopy part of its Micro-Imaging Dust Analysis System. It aims to (AFM) in preparation for the 2007 NASA Phoenix Mars conduct microtextural and statistical analysis of cometary Lander mission. The operation of an atomic force microscope dust particles (Riedler et al., 1998). The Phoenix spacecraft, on samples of micrometre-sized soil particles is a poorly which will land on Mars as part of NASA’s Phoenix mission investigated area where the unwanted interaction between launching in 2007 will also carry an AFM, which will be part the scanning tip and loose particles results in poor image of a microscopy station that includes an optical microscope. quality and tip contamination by the sample. In order to This AFM, dubbed the ‘First AFM on MARS’ or FAMARS, will mitigatetheseeffects,etchedsiliconsubstrateswithavarietyof contribute to the aims of the Phoenix mission by measuring features have been used to facilitate the sorting and gripping the sizes, shapes and surface textures of soil and dust particles of particles. From these experiments, a number of patterns for clues on the history of water at the Northern latitudes were identified that were particularly good at isolating and of Mars. Experiments from previous Mars lander missions immobilizing particles for AFM imaging. This data was used to have indicated that the average size of the airborne particles guide the design of micromachined substrates for the Phoenix at the Martian surface is between 1 and 3 µm (Landis AFM. Both individual particles as well as aggregates were et al., 1996). At this small scale, in situ optical microscopy, successfully imaged, and information on sizes, shapes and such as that being performed by the Microscopic Imagers surface morphologies were obtained. This study highlights (30 µm per pixel) on the two Mars Exploration Rovers is both the strengths and weaknesses of AFM for the potential in insufficient to resolve the individual particles. As such, in situ investigation of Martian soil and dust. Also presented are situ scanning electron microscopes (SEM) or AFMs are the more general findings of the limiting operational constraints better options for interrogating samples at these length scales. that exist when attempting the AFM of high aspect ratio However, miniaturization of an SEM for planetary exploration particles with current technology. The performance of the has thus far been an elusive goal due to the complexity of final designs of the substrates incorporated on Phoenix will such a system. Furthermore, the insulating properties of the be described in a later paper. particles under study would present additional difficulties to electron microscopy. By contrast, the AFM is able to operate 1. Introduction in the ambient environment on Mars, on non-conducting Atomic force microscopy (AFM) has become a versatile samples, and can provide 3D information of a sample. In tool for the study and manipulation of structures at the order to obtain clues about particle transport history, the nanoscale. Applications range from surface characterization dimensions, shapes and surface features of the grains need in material science, to the study of living biological systems to be determined. FAMARS will be able to provide data on in their natural environment, to nanolithography. AFMs these important parameters including particle diameters (up are even finding uses in the field of space exploration, to 40 µm), heights (up to 6 µm) and surface roughness/texture (lateralresolutionof10nm/pixelfora5µmscanarea),thereby Correspondence to: S. Vijendran. Tel: 44 207 5946 242; fax; 44 207 5946 308; addressing length scales unattainable with any other previous + + email: [email protected] or current in situ planetary exploration instrument. Crucially, C ! 2007 The Authors C Journal compilation ! 2007 The Royal Microscopical Society AFM INVESTIGATION OF MARTIAN SOIL SIMULANTS 237 the AFM images will show surface texture and patterning at with the surface features and so the geometry of the tip has a level of detail not hitherto possible on Mars, which, together a large affect on the accuracy of the imaging. Tip artefact in with other contextual data from the optical microscope the image can greatly distort and mask actual features and and chemistry instruments, will help in determining the make the interpretation of the data difficult. Typical silicon weathering mechanisms and transport history of the particles AFM probes have tips that are about 10 µm in height, and are and could further strengthen the case for past or present water tetrahedrally shaped, when fabricated with the most common on Mars (Kempe et al., 2004). Aeolian and aquatic transport techniques. Although the end of the apex of the tip may have for example, can be differentiated by AFM observation of the a radius of only 10 nm or less, the overall tetrahedral shape microstructureoftheparticlesastheformerproducespittingof limits the aspect ratios to very low values. As such, these the surface whereas the latter results in chemical dissolution tips are excellent for studying topography in the nanometre of minerals along crystal planes. The AFM will also be able height range, but are extremely limited when faced with to measure the sizes of the dust grains suspended in the features in the micrometre range. In fact, there is very little Martian atmosphere which have a big impact on the solar evidence in the literature of work that has been done with radiation balance of Mars and hence are of critical importance AFMs on 1 to 10 µm-high features, except in the area of in climatological models (Gierasch and Goody, 1968; Kahn metrologyofsemiconductordeviceprocessing(photoresistand et al., 1992). Additionally, dust is likely to be one of the major trench profiling) (Ridley et al., 2002). For these applications, hazardsforfuturehumanexplorers,asitcancontaminatesuits special tips with high aspect ratios are required, including and equipment, lead to electrical discharges and interfere with exotic tips such as carbon nanotubes (Bhushan et al., 2004). radio communications. (Horneck et al., 2001). Alternatively, a ‘hopping’ approach to the X-Y scanning of the AFM has been shown to help reduce tip-sample convolution (Hosaka et al., 2002). The Phoenix AFM (Akiyama et al., 1.1. AFM imaging of micrometre-sized particles 2001; Pike et al., 2001) however, being a space-qualified The important difference between the operating conditions instrument, is limited in its range of available tip geometries, that are likely to be experienced by the Rosetta and Phoenix as robustness is a key criteria for space-bound hardware, and AFMs, is that for the Rosetta mission, the cometary particles high-aspect ratio tips tend to be very fragile and more difficult are expected to be in the submicrometre size range (Riedler to use. et al., 1998). By contrast, on Mars, the mean size of the wind- In relation to the study of particle morphology, some borne dust grains is estimated to be 1–3 µm (Landis et al., work has been done on the determination of surface 1996), whereas the soil particles on and below the surface texture/roughness with the AFM (Barkay et al., 2005) and have a distribution of sizes from submicrometres to millimetres also in the search for extraterrestrial fossils in Martian rock or more (Soderblom et al., 2004). In order to be able to (Steeleetal.,1998).However,thesestudieshavetendedtofocus study particles at high spatial resolution, the instrumentation only on very small and relatively smooth (<1 µm roughness) constrains the maximum size of particle that can be imaged: areas of much larger samples. There have been precious little, the optical microscope with a resolution of 4 µm has a depth if any, studies on AFM imaging of individual micrometre-sized of field of 80 µm, whereas the AFM has a maximum height particles in their entirety, outside of the work done by Gautsch range of 14 µm. Hence, the size distribution of the particles et al. (2002) during initial qualification of FAMARS for the delivered to the microscopes by the robotic arm on Phoenix cancelled 2001 Mars Surveyor Lander mission. This work was will be limited by scraping off most of the material from the probably the first to highlight the difficulties in AFM scanning substrates, leaving a layer that is only 200 µm thick. Thus, the of micrometre-sized loose particles, although they were able to maximumparticleheightthatcanbeimagedonaflatsubstrate image a number of <5 µm quartz particles to demonstrate the by the optical and atomic force microscopes will be of the order capabilities of the FAMARS instrument. of 200 µm with both microscopes only able to image a portion Another limitation of AFM when it is used to image loose,