Study On Microfabrication Of AFM Probes Based On Masked-Maskless Combined Etching Technologies

Since its birth in 1980's, atomic force microscopy (AFM) has become a widely used powerful tool for highly resolved surface image. The most important component in an AFM is the flexible force-sensing cantilever with a sharp probe at the end, also called an AFM probe. The tip of a probe is scanning over the surface of a sample, and forces acting between the probe and sample cause minute deflections of the cantilever which are detected and displayed as an image. Generally, the deflection is measured by the deviation of a reflection beam incidenced on the free end of the cantilever.As a sort of consumed components, the AFM probes used in the scanning applications can be produced by either surface micromachining technology or bulk micromachining technology. The former is disadvantageous for that the low geometric aspect ratio of the pyramid-shaped tip is not suitable for imaging high aspect-ratio nano structures. By using bulk-micromachining technologies, AFM probes can be formed directly by wet anisotropic etching or dry reactive ion etching. Generally, the AFM probe fabrication based on wet anisotropic etching is simple, low-cost and easily implemented, without expensive equipments involved. For fabricating the probing tip with anisotropic etch, normally a cone-shaped tip contour is formed at first, which can be further sharpened by the following oxidation process. But a hard problem of this method is that the finished probes will probably be abraded or knocked in the follow photolithography for defining the cantilevers unless thick photosensitive resist is spun on probes whose height ranges from several microns to twenty microns. But thick resist would degrade the accuracy of transferring the mask of cantilevers to wafers. This has been causing a fabrication-yield problem that in turn increases the cost of the AFM probe product. Still using anisotropic wet etch, in this paper we present a novel method to fabricate silicon AFM probes. Different from the previous works, the tip and the cantilever are simultaneously formed by a masked-maskless combined etching technique.By using the low-cost process, a large range of spring constant of the probes can be designed and formed. The radius of curvature of most of apexes is about 10～30nm. The thickness of the finished cantilevers rangers from 0.3μm to 3.0 μm according to the thickness of initial active layer. Thickness deviation of cantilevers on a whole wafer is less than 0.2 μm. Thanks to the simultaneous formation of thetips and cantilevers, the fabrication yield for 4-inch wafers is as high as about 80%. By implementing the probes in an atomic force microscopy the probes have been demonstrated to provide stable high resolution imaging of nano-grating and DNA sample.