184 IEEE SENSORS JOURNAL, VOL. 7, NO. 2, FEBRUARY 2007 Surface Electron Spin Resonance Study on Ruby Crystal Using Evanescent Microwave Microscopy Techniques Frank X. Li, Massood Tabib-Azar, Senior Member, IEEE, and J. Adin Mann, Jr. Abstract—As the silicon technology approaches to its physical limit, the future electronic devices will depend on behaviors of a few electrons. This study is to explore the possibility of detecting a single electron spin transition by using nondestructive evanes- cent microwave microscopy (EMM) techniques. To enhance the RF magnetic field and minimize the dielectric losses, the sample is placed at the center of the conventional electron spin resonance (ESR) microwave cavity that does not have scanning image capa- bilities. In this paper, a magnetic dipole probe (MDP) is presented that not only has the advantages of the microwave cavity, but is also capable of surface scanning at high speeds. At present, the min- imum detectable electron spin transitions are 20 000 on the ruby crystal (Cr doped in Al O ) surface, whereas the commercially available ESR microwave cavity has a resolution of minimum detectable spins limit. Three ESR energy absorption spikes were detected at 3.77 and 3.73 GHz with the ruby crystal placed inside and outside of the MDP conductor loop, respectively. The mea- sured ESR energy absorption spectra are consistent with theoret- ical analysis and the conventional ESR experimental results. The current MDP sensor has a 500- m spatial resolution with a 1-mm radius conductor loop made by 150- m copper wires. The nonde- structive and noninvasive natures of the EMM microscopy are suit- able for many biomedical applications, such as DNA sequencing, Alzheimer, and other biological tissue studies. Future efforts will be focused on integration of the MDP on the atomic force microscopy with carbon-nanotube bridges. Index Terms—Atomic force microscopy (AFM), biomedical magnetic resonance imaging, electron spin resonance (ESR), microwave spectroscopy, paramagnetic resonance. I. INTRODUCTION B ASED on the recent observations of the ESR energy spectra of ruby crystal in 550, 136, and 279 mT (milli-Tesla), the MDP with micro stripline resonator is an excellent sensor with high sensitivities to the paramag- netic resonances of free electrons in crystal structures [1], [2]. The purpose of this paper is to summarize the experimental Manuscript received March 14, 2006; revised June 5, 2006; accepted June 11, 2006. This work was supported in part by the Army Research Office under MURI Grant DAAD 19-03-1-0169. This is an expanded paper from the Sensors 2005 Conference. The associate editor coordinating the review of this paper and approving it for publication was Dr. Dwight Wooland. F. X. Li is with the Department of Electrical and Computer Engineering, Youngstown State University, Youngstown, OH 44555 USA (e-mail: xli@ysu. edu). M. Tabib-Azar is with the Electrical and Computer Engineering Department, Case Western Reserve University, Cleveland, OH 44106 USA (e-mail: tabib- azar@case.edu). J. Adin Mann, Jr., is with the Chemical Engineering Department, Case Western Reserve University, Cleveland, OH 44106 USA (e-mail: jam12@case.edu). Digital Object Identifier 10.1109/JSEN.2006.886887 results for both enclosed and surface ESR measurements and to discuss future research. The MDP belongs to the EMM local scanning probe family that is capable of mapping the electromagnetic properties of the sample surfaces and subsurface with highly localized RF elec- tromagnetic fields. The MDP ESR experiment setup is similar to other EMM surface scanning imaging done by our research group [3]–[12]. With the rectangular waveguide, and planar geo- metric probes, we have successfully mapped the surface mi- crowave properties. Other research groups have developed similar EMM probes that contributed to the EMM surface and subsurface imaging techniques [13]–[19]. When the EMM electric dipole probe (EDP) is integrated with the AFM, the spatial resolution of the probe approaches to the atomic level, which is very valuable in surface and subsurface imagining of the biological samples [20]. Although these scanning EMM EDP probes are excellent tools for mapping nonuniformities and defects on the material surface and subsurface, the ESR sensitivity for scanning EDP is around spins at the best [21]. Contrast this sensitivity to the much better sensitivity we predict for the MDP class of probes, which is the subject of this paper. The discovery of the ESR phenomena is dated back to 1940s, just a few years later after the discovery of the nuclear magnetic resonance (NMR). NMR becomes the dominant imaging tools that are widely used in todays medical and biochemical fields. It was not until the early 1980s that the first attempt of ESR spatial imaging was carried out to distinguish the different relaxation times in an inhomogeneous sample [22]. In the last two decades, the ESR imaging was carried out with a well defined magnetic field gradient superimposed on a homogeneous magnetic field, similar to the NMR imaging [23]–[31]. Due to the limitations of the conventional ESR microwave cavity, the spatial locations of the ESR spectra can not be determined directly. In our approach, the spatial location of the electron spin is precisely defined by the EMM MDP tip. Hence, the real-time ESR surface imaging becomes feasible. As the semiconductor manufacturing process approaches the length scale of 10 nm in the next few years, the number of elec- trons that determines the circuit behavior is expected to drop to less than ten. Moreover, the detailed electron distributions in the atomic structures are not fully understood at this moment. Therefore, the drive for surface and subsurface ESR imaging becomes overwhelmingly strong. Suppose a permanent magnet is fixed at the tip of a cantilever in a magnetic field, and in a global RF field. With such an instru- ment, other research groups have detected the single electron 1530-437X/$20.00 © 2006 IEEE