IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005 3005 The Measurement of Sub-Brownian Lever Deflections Mark D. Hammig, David K. Wehe, and John A. Nees Abstract—A micromechanical lever that deflects in response to the impacts of charged particles has previously been proposed as a means of improving upon the capabilities of existing radiation detection technology. The momentum detector offers promise as a highly discriminating, high-resolution tool for ion sensing. Ad- vances required to successfully realize a spectroscopic capability has been completed; specifically, techniques for reproducibly fabricating micromechanical structures have been optimized, and an instrument that measures miniscule deflections has been developed. Even absent substantial refinement efforts, the novel coupled-cavity optical detector can resolve lever motions on the order of 1–10 picometers. A method by which the Brownian motion of the lever can be stilled has been proven which elicits reductions sufficient to measure heavy-ion impact, the deflec- tions from which may be several orders of magnitude below the thermal vibration amplitude. Using active forcing techniques, the Brownian vibration of the microlevers has been reduced from room temperature (288 K) to sub-Kelvin temperatures, for levers vibrating in air. The mechanical factors that limit the noise reduction magnitude are discussed and methods of surmounting those limitations are identified. Index Terms—Microresonators, radiation detectors, stochastic processes, temperature control. I. INTRODUCTION M ICROMECHANICAL structures are increasingly em- ployed to sense environmental variations, in such ap- plications as pressure sensing and acceleration detection. One current focus of micromechanical research is to improve the de- tector sensitivity, which can be achieved by either increasing the sensitivity of the deflecting element or by improving the resolu- tion of the structural-motion detector. In this research, advances in both areas are realized, and necessary, as demanded by the en- visioned application. Regardless of the sensing method or the particulars of the ap- plication, the ability of micromechanical structures to sense in- creasingly small influences is limited by the intrinsic Brownian motion of the lever, the mitigation of which is the focus of this research. One technique is demonstrated, in Section IV, which reduces the effect of the Brownian motion so that influences that normally reside well below the thermal-mechanical noise floor are detected. This effort has been driven by the desire to perform the energy spectroscopy of ionizing radiation via the deflection of pliable microstructures [1]. Upon impact with a pliable structure, an in- cident ion imparts momentum to the body, which then moves as Manuscript received October 14, 2004; revised January 25, 2005. M. D. Hammig and D. K. Wehe are with the Department of Nuclear En- gineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109 USA (e-mail: hammig@umich.edu; dkw@umich.edu). J. A. Nees is with the Department of Electrical Engineering and Com- puter Science, University of Michigan, Ann Arbor, MI 48109 USA (e-mail: nees@umich.edu). Digital Object Identifier 10.1109/TNS.2005.860827 TABLE I CANTILEVER DEFLECTION DUE TO 5 MeV lIGHT (He) AND HEAVY (La, Au) IONS BOTH TRANSMITTED THROUGH AND STOPPED BY THE “TYPICAL” NITRIDE LEVER (DIMENSIONS: m). THE DEFLECTION-TO-THERMAL NOISE (rms) VALUE IS SHOWN IN ITALICS a result. By correlating the subsequent body motion with the im- pact event, the properties of the incident particle can be studied. As shown in [1], the physics of the interaction dictate that the structure be both small and weak. Even in ideal conditions, the deflections induced by radiation impact are subangstrom, and can be on the order of picometers; therefore, a highly sensitive lever-motion sensor must be implemented that minimally per- turbs the deflecting element. The development of that sensor is the subject of Section III. The miniscule deflections and poor signal-to-noise ratios produced by individual ion impact can be assessed by considering both light and heavy ions impacting a typical structure, a bare Si N (nitride) cantilever of di- mensions: m [2]. Table I shows the maximum can- tilever deflection, which occurs at the tip following an impact at the tip, for incident 5 MeV particles. As suggested in the third column of Table I, the largest deflec- tion in response to heavy ions is about ten times bigger than the best response from light ions. More importantly, both responses are more than ten times below the rms thermal noise amplitude at room temperature (300 K), for even an optimized structure [2]. In standard practice, the thermal vibration amplitude is re- duced by coupling the lever to a cold bath, an approach that is ineffective for the envisioned application for the following reason. The heat generated by optically coupling a laser diode to a subsection of the lever cannot be drawn effectively, because the thin ( 100 nm) insulating structure retards conduction. In addition, the alternative method described in this paper holds promise for eliciting greater temperature reductions than those achieved using established techniques. Upon the successful application of the described technique, the mechanical detector, operated in single-particle or flux mode, offers significant promise as an ion sensor or neutron detector, sensing the products of both neutron scattering and 0018-9499/$20.00 © 2005 IEEE