MECA Atomic Force Microscope (AFM)
The MECA instrument suite is a component of the Mars '07 Phoenix investigation, which will also return data from a Thermal and Evolved Gas Analyzer (TEGA), three cameras, and a meteorology suite (MET). Phoenix is motivated by the goals of (1) studying the history of water in all its phases, and (2) searching for habitable zones. Samples of surface and subsurface soil and ice will be delivered to MECA and TEGA from a trench excavated by a Robot Arm (RA), while MECA's Thermal and Electrical Conductivity Probe (TECP) will be deployed in soil and air by the Robot Arm. The Robot Arm Camera (RAC) will document the morphology of the trench walls, while the Surface Stereo Imager (SSI) and the Mars Descent Imager (MARDI) establish a geological context. Throughout the mission, MET will monitor polar weather and local water transport.
The MECA instrument suite is composed of an Atomic Force Microscope (AFM), a Thermal Electrical Conductivity Probe (TECP) and a Wet Chemistry Laboratory (WCL). This data set description catalog file will describe the AFM. A complete description of the AFM can be found in [HECHTETAL2008].
The Atomic Force Microscope (AFM) is part of the MECA Microscopy Station, which comprises a Sample Wheel and Translation Stage (SWTS), an optical microscope (OM), and the AFM. The MECA AFM is located between the OM and the SWTS inside the darkened MECA enclosure on the spacecraft deck. It scans a small region (from 1-65 micron square) on any of 69 substrates, each 3-mm in diameter, positioned along the rim of the SWTS. The chief scientific objectives of the AFM are to analyze small dust and soil particles in terms of their size, size distribution, shape, and texture. The AFM is particularly well suited to analyze particles carried by the wind, which are believed to be in the size range 1-3 microns.
Calibration of the AFM instrument is discussed in the AFM Calibration Report, which can be found in the Calibration folder of the MECA Non-imaging data archive.
Operational Considerations and Operational Modes
Prior to AFM scanning, OM images are acquired to document the substrates and provide context for the AFM scans. OM data is described in the Phoenix camera SIS, along with the RAC and SSI, and is outside the scope of this document. The reader is also referred to that document for more detailed description of the SWTS and its substrates.
The AFM is contributed by a Swiss-led consortium spearheaded by the University of Neuchatel. Run by a dedicated microcontroller, the AFM uses one of an array of eight micromachined cantilevers with sharp tips to obtain topographs (sometimes called 'scans' or 'images') of up to a 65x65 micrometer area of the sample. Within this constraint the scan can be of any size, but the AFM can only address a narrow horizontal stripe of each substrate. Since the sample wheel can be rotated (but not elevated) prior to initiation of scanning, the AFM can access a thin band approximately 1/3 of the way up from the bottom of the corresponding OM image. Note that the x and y axes of the MECA AFM image are rotated by +45 degrees relative to the OM images.
The 69 substrates on the SWTS are divided into ten sets of six (a weak and a strong magnet, two 'microbuckets', a textured substrate, and a sticky silicone pad) and nine utility or calibration targets. Using the SWTS, these can be coated with a thin layer of dust or soil, and then rotated to the vertical scanning position where they can be imaged by the optical microscope. Of the six substrates in each set, two are specifically designed for AFM use in that they resist the tendency for particles to become dislodged and to adhere to the AFM tip. Such particle adhesion can degrade the scans in question and the quality of the tip in general. One of these substrates is a uniform piece of silicone that remains pliant under martian conditions. The second is a custom micro-machined silicon substrate with pits and posts that hold particles of an appropriate scale for AFM scanning. Two of the remaining four substrates are magnets that may, under certain circumstances, be appropriate for AFM scanning. The final two substrates are deep 'buckets' that would not normally be accessible to the AFM.
MECA's AFM comprises three major components, a microfabricated probe-chip, an electro-magnetic scanner-actuator and single board control electronics. The probe-chip features 8 cantilevers, numbered 0-7. The chip is mounted with two orthogonal tilt angles of 10 degrees relative to the sample to ensure that only one tip contacts the sample at a time. In case of contamination or malfunction of this front-most tip, the defective cantilever and its support beam can be cleaved off by a special tool on the sample wheel, after which the next one in the array becomes active. The force constant of the levers varies between 9 and 13 N/m.
Each of the 8 MECA cantilevers features an integrated piezo-electric stress sensor, which is used to measure its pure deflection (static mode) or its vibration amplitude, frequency and phase (dynamic mode). In static mode the deflection signal is proportional to the force, while in dynamic mode the shift of the resonance frequency is a measure of the force gradient. Dynamic mode minimizes the interactions between tip and surface and is less likely to result in particles being moved around or dislodged during the scan. In either mode, these signals are used to regulate the distance between the tip and the sample in the z-direction by means of a proportional-integral feedback loop.
The z-axis servo signal (referred to has height) represents the sample topography as the tip is rastered across the surface in the x (fast) and y (slow) directions. (Though the resulting topograph is sometimes referred to as an image, it bears little resemblance to an optical image until it is transformed and processed.) Imperfect feedback or an out-of-range condition can result in residual bending of the cantilever in static mode, or a phase shift of the oscillation in dynamic mode. This error signal may optionally be recorded in a second data channel. Since each line in the raster scan begins at the same point on the x-axis, both primary and error signals may be recorded either on the forward or the backward legs of the scan (or, typically, both). Thus a single raster scan can produce up to four arrays of data: Forward (height), forward (error), backward (height), and backward (error), each of which can be displayed in image format.
Several of the status values returned in MECA telemetry refer to the configuration and status of the AFM digital feedback loop. The piezoresistors on the cantilevers are addressed by a multiplexer, which links them to a temperature-compensated Wheatsone bridge. For static mode imaging the value of the bridge is directly compared to a setpoint. For dynamic mode imaging a frequency modulation technique is applied that maintains the resonant frequency of the cantilever at a setpoint while stabilizing the amplitude. The measured parameters are the phase-shift between the excitation of the cantilever and the measured signal from the Wheatstone bridge, maintained by a phase locked loop. The array geometry is designed to spread the resonant frequencies of the levers between 30 and 40 kHz in order to avoid cross-talk during dynamic operation (these shift slightly with temperature).
To operate the microscopy station, soil samples are deposited by the RA (or dust from the air) onto a segment of the SWTS ring that has been extended such that exactly one set of 6 substrates protrudes from a horizontal slot in the MECA enclosure. Excess material is removed by passing the substrates under a blade positioned 0.2 mm above the surface, after which the samples are rotated from their horizontal load positions into their vertical imaging positions for imaging by the OM and AFM. The SWTS is also used for focusing and AFM coarse positioning.
Attempting to scan excessively steep or ragged surfaces with the AFM will result in scans that are largely out of range, and could conceivably damage the tip. Further, bandwidth and time constraints severely limit the number of scans that can be acquired and returned to Earth. These considerations dictate a two-day imaging strategy for each set of microscopy samples. On the first day a sample is delivered, then characterized by the optical microscope. AFM calibration scans are also acquired. These images are evaluated on the ground and targets for AFM scanning are selected. On the second day the targeted areas are imaged again with the optical microscope, and then scanned with the AFM.
It must be emphasized that an AFM scan is acquired by rastering a physical tool across a surface. As a result, line-to-line noise and artifacts may be significantly different than point-to-point artifacts along the scan direction. Moreover, outside the range of authority of the cantilever (approximately 65 x 65 micrometers laterally and 13 micrometers in height) the topograph does not go 'out of focus' but simply saturates, while anywhere within its range of authority it is equally 'in focus.' The topograph itself reflects the interaction between a tip of finite size and a non-uniform surface, and therefore convolves physical characteristics of both the probe and the target. Thus, while an AFM topograph may look like an image product, the processing required bears little in common with the processing of an actual optical image.