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Atomic force microscopy on a chip The AFM chip prototype eliminates the he average atomic force microscope (AFM), with its controller, comput- optical readout system and has a fully integrated array of 12 cantilevers. Two er, cables, and wires, engulfs a small lab of the cantilevers at the ends of the bench. But a newly designed AFM, dearray act as references. Four of the reveloped by Andreas Hierlemann and colleagues at the Swiss Federal Institute of Technology, fits comfortably on a fingertip (Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17,011– 17,015). The miniaturized instrument measures only 7 10 mm. It is a stand-alone, monolithic chip that is fabricated by complementary metal oxide semiconductor (CMOS) technology, an industrial process commonly used in the silicon chip industry. Nico de Rooij of the University of Neuchâtel (Switzerland) points out the importance of the achievement. “For the first time, they A new stand-alone, single-chip AFM measures only were [able to] demonstrate they 7 10 mm. could use the industrial process, with additional post-processing steps, to maining 10 cantilevers are used for simake these AFM systems.” The use of a multaneous investigation of a sample. The deflection of the cantilevers is standard industrial method at the redependent on the different thermal exsearch level means potentially fewer pansion coefficients of the cantilevers’ challenges in translating the fabrication silicon and aluminum layers. Each canof the miniaturized AFMs to a mass tilever has its individual thermal actuaproduction scale. tion, piezoresistive detection, control, The most novel feature of the instruamplification, and digital processing ment is the extent of system integration. unit on the chip, so that the movements The mechanical components, analog circuits, and digital computing power are all of each cantilever can be independently monitored. The cantilevers can be moved engineered onto the single CMOS chip. and controlled within a range of 0.5– In comparison, the control electronics in a conventional AFM are housed in a sep- 6.0 µm (at 0.5–6.0-nm resolution). The cantilevers in the AFM chip inarate box the size of a desktop computer terrogate the sample surface in contact tower. mode, in which the tips of the cantilevers In a standard AFM, a single canphysically trace the features on the surface. tilever interrogates a sample at a given Hierlemann says his group is looking to time, and an optical readout system next develop a noncontact mode of operatracks the deflections of the cantilever. 166 A
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tion. In noncontact mode, the cantilevers investigate the sample surface while vibrating at their resonant frequencies, tapping along the surface. This approach has greater measurement sensitivity and causes less damage to samples. The miniaturized AFM can be used in vacuum, air, and liquid environments. However, its current performance cannot rival that of a desktop instrument in terms of sensitivity and resolution. “In this present prototype you won’t be able to see atomic resolution, which you can do with a standard AFM on a good day,” admits Hierlemann. The current vertical resolution of the prototype is
