A high performance scanning force microscope head design

Since its inception in 1985, the atomic force microscope’ [also known as the scanning force microscope (SFM)] has held great promise for imaging nonconducting samples with subnanometer resolution. Early attempts to image relatively inelastic samples such as graphite, crystals of ionic salts, and semiconductors yielded atomic resolution data. These data provided reason to believe that SFM might be applied easily to the study of biomaterials. However, as more compliant samples were examined, it became clear that several factors limit resolution on elastic samples. Among these factors are (i) unavoidable forces involved in tip-sample interaction, (ii) tip shape, (iii) sample motion, both scan induced and thermal, and (iv) instrument and detection noise. Progress has been made in all of these areas; for example, imaging with both sample and cantilever immersed in liquid to reduce meniscus forces,’ electron beam deposition of tips to increase tip sharpness,3 imaging with specially prepared substrates to increase sample adhesion during imaging,4 as well as efforts to image at low temperature to eliminate Brownian motion of the sample.’ Other investigators have focused on sources of instrument noise and their reduction.6 This article describes a design to reduce mechanical noise and increase the stability of a SFM stage so that it exceeds the requirements for imaging biomaterials. Establishing the force level acceptable for biomolecular imaging is difficult; it depends on the sample, preparation procedures, imaging environment, and other factors. Theoretical discussions of tolerable forces for biological samples advocate applied forces in the 10-lo-lO-ll N range.7’8 We have used the more conservative lo-” N figure as a design goal for the SFM stage presented. One consequence of this force criterion is that, for stiff micromachined cantilevers (which have spring constants z 1 N/m) motions larger than 0.1 A are unacceptable. We have designed a SFM head that achieves stability andvibrational amplitudes less than this level. The body of this article describes the design of the SFM head and discusses the most important factors considered during the design process.