With Daniel Parrat, Institute of Microtechnology University of Neuchâtel
Scheduled for launch in August 2007, the Phoenix Mars Mission  is designed to study the history of water and habitability potential in the Martian arctic's ice-rich soil. Onboard the Phoenix Lander, the MECA (Microscopy, Electrochemistry and Conductivity Analyzer) instrument suite  assumes, among other tasks, the determination of the size, the distribution, and the shape of dust and soil grains. The involved components, shown in figure 1, are a sample delivery system, an optical microscope and an atomic force microscope (AFM).
Figure 1. Photograph of MECA’s microscopy station. From left to right, we distinguish the optical microscope (A), the FAMARS instrument (B), and the sample wheel (C). Click to enlarge.
The AFM, dubbed FAMARS (“First AFM on MARS”) , was conceived and characterized by IMT  and its partners, Nanosurf AG (Liestal, CH) and the University of Basel (CH). After its implementation in the MECA microscopy station, it was tested in collaboration with the Jet Propulsion Laboratory (Pasadena, USA) and the Imperial College (London, UK).
What is atomic force microscopy?
Atomic force microscopy is a modern method for characterizing surface topography at nanometer resolution. In contrast to optical microscopy, atomic force microscopy employs a local mechanical interaction with the measured sample; atomic-range forces acting between a very sharp sensor tip and the surface . To acquire an image the sensor tip is raster scanned over the sample while measuring these forces. The tip is mounted on a flexible cantilever which bends under the forces exerted to the tip. Detecting this bending as function of tip location allows reconstructing the topography of the sample. The presence of a feedback loop is one of the differences between AFM and older stylus-based instruments such as record players and stylus profilometers. The AFM not only measures the force on the sample but also regulates it by constantly adjusting the vertical position of the probe, allowing acquisition of images at very low, constant forces.
Depending on the application, AFM may operate in different modes. In contact mode, the tip is usually maintained at a constant force, moving the cantilever up and down as it scans, as shown in Figure 2. In this case, the forces acting on the tip will cause it to snap onto the sample, which results in a nanometer-range flattening of the tip, and friction and stiction between the tip and the sample. This can be circumvented by operating the AFM in intermittent non-contact mode (or tapping mode). In this mode, the cantilever is vibrating at its free resonance frequency, and the tip-sample distance is controlled by measuring changes in either the vibration frequency or the vibration amplitude. Thus, the tip enters only temporarily into contact with the sample, reducing the lateral force during a scan. This is especially important for particles measurements, because using intermittent non-contact mode will drastically reduce the risk of loose particles tracked by the tip. This is the major motivation to use this mode for the FAMARS instrument.
Figure 2. AFM working principle described for contact mode with closed loop for constant force imaging. Click to enlarge.
Why was an AFM chosen to characterize Martian soil particles?
In order to characterize Martian soil particles in details, scientists need information from the millimeter scale to the nanometer scale - a scale never examined on Mars so far. The optical microscope of MECA, with its resolution of 4 microns per pixel, allows detection of particles ranging from about 10 micrometers up to the size of the field of view (1 millimeter by 2 millimeters). Large grains can be investigated by stitching several images together Thus, a tool with a resolution of few nanometers was also needed to give detailed information on the shape, the size and the surface texture of the grains.
AFM can easily reach this resolution. In addition, this technique offers several advantages compared to other terrestrial tools having similar resolution, e.g. the scanning electron microscope (SEM). First, AFM does not require a special medium (e.g. vacuum, conductive surface), contrary to SEM. Secondly, the miniaturization of the AFM is possible using microfabrication.
Description of the FAMARS instrument
As for any space mission, volume, weight and power consumption are key design parameters. Moreover, parts and materials must meet shock and vibration, radiation, thermal, and out- or degassing criteria. The Mars AFM consists of an electromagnetic scanner head (see figure 3), a micro-fabricated sensor chip, and a controller board. The total power consumption of the microscope is less than 8.5 W.
The x-y-z scanner measures only 12mm x 18mm x 35mm and weights 15g. By using electromagnetic actuation with voice coils instead of the piezoelectric actuation commonly used in laboratory instruments, the scanner can be operated with low voltages, avoiding electrical arcing.
Figure 3. FAMARS scanner with a chip mounted on it. Click to enlarge.
The electromagnetic forces have to be counter-balance by mechanical forces in order to generate a smooth, controlled motion of the scanner. An innovative system of leaf-springs made of polyimide, shown in figure 4, allowed a good damping of this scan motion, showing moderate temperature dependence over a large range. Moreover, copper leads for connecting the AFM sensor to the controller board could be seamlessly integrated into the springs.
Figure 4. Left) View of the polyimide leaf prior to mounting to the scanner-head. Right) Larger view of a leaf-spring with integrated copper leads. The copper lines on the visible side were designed for compensating the bimorph effect. The electrical connections are established by the lines on the hidden side. Click to enlarge.
The second part of "The FAMARS instrument, an AFM for planetary exploration" will be available Thursday, 12th of July, here at spacEurope.
 Institute of Microtechnology, University of Neuchâtel, Jaquet-Droz 1, 2007 Neuchâtel, Switzerland
 G. Binnig, C. F. Quate, Ch. Gerber, “Atomic Force Microscope”, Physical Review Letters, 56, 930, (1986).