Scanned Probe Microscopes (SPM) by Peter Loos 11/29/98 revised 3/15/04 All SPM models depend on piezoelectric actuators which scan a probe tip in a raster pattern across a solid sample. These actuators are ceramics (PZT or BaTiO3 or PbTiO3 with the Perovskite crystal structure) and are widely available. The actuators should have a full-scale displacement of several microns, so that objects seen on the light microscope can then be found and examined with the SPM. In the plane of the sample, if 100 v gives 10 um displacement (full scale) then 1 mv gives 1 Ang displacement (atomic scale) The piezoelectric actuators are sufficiently linear and can readily be calibrated over this range. A smaller working range may be used normal to the plane of the sample. SPM equipment also includes a coarse mechanical adjustment (for example screws) to bring the sample into the range of the piezoelectric actuators and an optical microscope (perhaps low power binocular microscope with long working distance objective) to facilitate the coarse adjustment. The equipment also requires electrical circuitry to scan the raster pattern and display the results on a CRT, as well as vibration damping components. Scanning Tunneling Microscope (STM) - Invented 1982 by Binnig & Rohrer at IBM, won a Nobel Prize in 1986. - Based on quantum tunneling current ~1nA between probe tip and sample. This can be measured and controlled with good precision. Tunneling current changes by a factor > 2 as separation between tip & sample increases by 1 Ang. - Operates at constant voltage selected by operator (mV to a few V, either + or -). - Feedback circuits move the probe tip up and down as needed to maintain a constant tunneling current. This vertical displacement is the primary signal displayed by the instrument. - Tip must be a good electrical conductor, as well as hard and chemically inert. Tungsten, doped Si and PtIr are common. - Test sample must also be a decent conductor or semiconductor (and solid). Insulating and most organic materials cannot be observed as-is. If atomic resolution is not needed, these samples can be coated with a conductor (metal or carbon) and then examined. - Tip can operate in vacuum, air, water, oil, etc. For best (atomic) resolution, vacuum may be necessary (to avoid monolayers of H20, etc. adsorbed onto the sample). - One can stop the raster scan, hold the tip over an object (atom) of interest and sweep the voltage over a range while holding tunneling current fixed. This is called "spectroscopy" and may be of some use in identifying composition of the object. Atomic Force Microscope (AFM) - Invented 1986, based on the STM - Depends on repulsive forces between electrons in the sample and those in the probe tip. Forces range from ~ 1 nN (.1 ug) which does little damage to most samples to about 100 nN, which will scratch soft films, move atoms around and punch small holes in things (nanoindentation). - Feedback circuits move the probe tip up and down as needed to maintain constant force. As with the STM, this vertical motion is the primary signal displayed by the instrument. - Force is monitored by mounting the probe tip at the end of a small cantilever beam with known stiffness, ~ 1 N/m. Make your own with common Al foil 1 mm by 3 mm by .3 mm. Cantilever deflection is monitored by reflecting a laser off its surface and into a photocell whose output is fed back to control the vertical displacement. - Probe tip is made of diamond, Si or maybe a Buckytube. Make one yourself with a hammer and a small cheap diamond. - Design the cantilever & tip so that the resonant frequency, Fr, of the system is as high as possible, above room vibrations of frequency F ~ 20 Hz. Vibration amplitude is damped by (F/Fr)^2. - Any type of test sample is suitable, conducting or not. No coatings are needed. - It is usually best to move (raster) the sample instead of the probe tip, due to the delicate cantilever and alignment of laser, cantilever and photocell. Magnetic Force Microscope (MFM) - Similar to AFM but uses a magnetized Ni or Fe probe tip. - Due to size of the probe tip, resolution is around 250 Ang, good enough to examine magnetic particles on recording tape and disks as well as magnetized domains in ferromagnetic solids. Scanning Thermal Microscope (SThM) - Similar to MFM but uses a tiny thermocouple probe tip, bimetallic junction (for example Ni-W). - Feedback from the thermocouple is used to drive vertical motion of the probe tip and keep the tip at constant temperature. - Due to size of the probe tip, resolution is around 300 Ang, good enough to find hot spots in IC chips. Scanning Capacitance Microscope (SCM) - Similar to MFM but probe tip has stored electrical charge which deflects when it gets close to charged areas of the test sample. Scanning Electrochemical Microscope (SECM) - Tiny capillary tube / electrode is scanned across the sample. Near-Field Scanning Optical Microscope (NSOM) - Invented 1956 by J. O'Keefe, demonstrated in 1972 by E. Ash. - Light shines through a hole (capillary tube) smaller in diameter than the light's wavelength. Scan this light source across a transparent sample, casting a shadow on a screen behind it. - Resolution is limited by size of hole, not wavelength of light. Bibliography: ============= 1) The Tunneling Microscope: A New Look at the Atomic World by J. Golovchnenko, Science, vol. 232, p. 48, (4 April 1986) 2) Atomic-Resolution Microscopy in Water by R. Sonnenfeld & P. Hansma, Science, vol. 232, p. 211, (11 April 1986) 3) Scanning Tunneling Microscopy and Atomic Force Microscopy: Application to Biology and Technology by P. Hansma et al., Science, vol. 242, p. 209 (14 Oct 1988) 4) Scanned Probe Microscopes by H. Wickramasinghe, Scientific American, Oct 1989, P. 98 5) Atom Tinkerer's Paradise by P. Weiss, Science News Online, www.sciencenews.org/sn_arc98/10_24_98/Bob2.htm, Oct 24, 1998. 6) Machining Oxide Thin Films with an Atomic Force Microscope: Pattern and Object Formation on the Nanometer Scale by Y. Kim and C. Lieber, Science, vol. 257, p. 375 (17 July 1992) 7) Photon Emission at Molecular Resolution Induced by a Scanning Tunneling Microscope by R. Berndt et al., Science vol 262, p. 1425 (26 Nov 1993) 8) Chemical Imaging of Surfaces with the Scanning Electrochemical Microscope by A. Bad et al., Science, vol. 254 (4 Oct 1991) 9) In Situ Scanning Tunneling Microscopy of Corrosion in Silver-Gold Alloys by I. Oppenheim et al., Science, vol. 254, p. 687 (1 Nov 1991)