Scanning probe microscopy with inherent disturbance suppression using micromechanical systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.Includes bibliographical references (p. 109-116).All scanning probe microscopes (SPMs) are affected by disturbances, or mechanical noise, in their environments which can limit their imaging resolution. This thesis introduces a general approach for suppressing out-of-plane disturbances that is applicable to non-contact and intermittent contact SPM imaging modes. In this approach, two distinct sensors simultaneously measure the probe-sample separation: one sensor measures a spatial average over a large sample area while the other responds locally to topography underneath the nanometer-scale probe. When the localized sensor is used to control the probe-sample separation in feedback, the spatially distributed sensor signal reveals only topography. This technique was implemented on a scanning tunneling microscope (STM) and required a custom micromachined scanning probe with an integrated interferometer for the spatially averaged measurement. The interferometer design is unique to SPM because it measures the probe-sample separation instead of the probe deflection. A robust microfabrication process with a novel breakout scheme was developed and resulted in 100 % device yield. For imaging, an STM setup with optical readout was built and characterized. The suppression improvement over conventional SPM imaging was measured to be 50 dB at 1 Hz, in agreement with predictions from classical feedback theory. Images are presented as acquired with each sensor signal in several environments, and the interferometer images show remarkable clarity when compared with the conventional tunneling images.(cont.) The out-of-plane noise floor with this technique on the home-built microscope was 0.1 i rms. The results of this work suggest that the resolution of STM and other SPM modes, notably tapping mode atomic force microscopy (AFM), can be substantially improved, allowing low noise imaging of nanoscale topography in noisy environments and potentially enabling repeatable atomic scale imaging in ambient conditions.by Andrew William Sparks.Ph.D

Implementation of differential self-mixing interferometry systems for the detection of nanometric vibrations

In this Thesis, we explore Self-mixing interferometry (SMI ), a method capable of producing high resolution optical path related measurements in a simple, compact and cost-effective way. Even with a notably less complex setup than traditional interferometric methods, SMI can produce measurements with a resolution well below the micrometric scale (N'2) which is sufficient for most industrial applications. The SMI effect is produced when a small part of the laser power impacting a target is back-scattered and re-injected into the laser cavity. As a result, the phase and amplitude of the laser wave is modified generating a signature beat, which can be "easily" related to different optical path-related dynamics. The main advantage of this method in relation to other interferometric methods is the simple setup consisting mainly of a single mode laser diode (LO) equipped with a simple electronic system readout a simple optical system may be used to collimate/focus the beam allowing measurements at larger distances. Because of the small amount of reflected optical power required to allow the effect, the technique can produce high resolution measurements even with diffusive targets. While the SMI method has been largely studied in the last three decades, there are still several topics worth the development of further research. One of those topics, how to increase the resolution on displacement measurements, is one of the main topics covered in this work. Classical SMI methods allow the reconstruction of displacement measurement with a resolution of N'2. The use of special processing algorithms can push further this limit reaching values in the order of e.g. N32. In this work, we propose a method to increase even further this limit reach values better than N100.The idea discussed, differential self-mixing interferometry (OSMI) proposes the use of a reference modulation (mechanical or electrical) to be used as a reference for the measurement. Simulated results have shown that under ideal conditions, it may be possible to reach resolutions in the order of N1000. In practice, however, this limit is much smaller (N100) because of LO dynamics, and different practical limitations present in the amplification and readout electronics. Experiments and measurements are presented along the second chapter of this work to present proof of the proposed method. After exploring the basics of OSMI, possible applications for classic SMI and DSMI were pursued. The obtained results are presented in the following sections. First, a review on potential biomedical measurements using SMI is discussed. The obtained results suggest that it is possible to obtain some key values related to biomedical constants (e.g. P.PW) using a displacement SMI measurement. The method, however, may not be reliable enough especially on long time measurements. Moreover, the use of certain wavelengths must be avoided during long exposures as they may prove harmful to the soft tissue due to the requirements of a small laser spot. lt is observed that SNR may lead to difficulties during the signal processing stage which may impact the results of the reconstructed signal. Next, the DSMI method was tested in an AFM-like cantilever system. The results suggest that is possible to follow the motion of a micrometric size cantilever oscillating at low frequencies with a high resolution. Higher frequencies may be achieved by using an electronic reference modulation configuration. The proposed system was able to detect some artefacts on the motion which maybe attributed to possible deflections on the cantilever surface. Possible enhancements to the method are suggested for any researcher who wants to expand the topic.En esta Tesis, se explora la interferometría auto-mezclante, mejor conocida por su nombre en inglés Self-m ixing interferometry (SMI), un método capaz de producir mediciones relativas al cambio del camino óptico en un haz laser. La técnica está caracterizada por su tamaño compacto, bajo coste y alta resolución. Pese a su simplicidad, la resolución alcanzada por sistemas basados en SMI se encuentra por debajo de la escala micrométrica (N2), lo cual es suficiente para la mayoría de las aplicaciones industriales. El efecto SMI se genera cuando una pequeña parte de la potencia óptica del láser es retro reflectada por un blanco y reinyectada en la cavidad láser. Como resultado, se genera una modulación de la amplitud y fase del láser, la cual puede ser "fácilmente" relacionada con diferentes efectos relativos al camino óptico del láser. La principal ventaja del método SMI es la simplicidad del sistema de medición el cual está compuesto de un diodo láser (LO) equipado con una tarjeta de procesamiento electrónico. una lente de enfoque o colimación puede ser utilizada con el fin de regular la reinyección de potencia y la distancia al blanco. Debido a que el SMI se genera con una pequeña cantidad de potencia es posible realizar mediciones incluso en blancos con reflexión difusa. . Si bien el método SMI ha sido estudiado ampliamente durante las 3 últimas décadas, aún existen diversos puntos de interés en su estudio. Uno de estos puntos corresponde a la mejora de resolución en la medida de desplazamiento, el cuál es uno de los temas abordados en el presente trabajo. Los métodos clásicos SMI para la medición de desplazamiento permiten alcanzar una resolución en el orden de A/2. El uso de algoritmos de procesamiento especializados puede permitir mejorar el límite de la técnica alcanzando resoluciones (por ejemplo) en el orden de N32. En este trabajo proponemos un método que teóricamente permitiría alcanzar resoluciones mejores que N1OO. La discusión en este punto se sitúa sobre la técnica differential self-mixing interferometry (DSMI), la cual hace uso de una modulación de referencia (mecánica o electrónica) para realizar la medición. Los resultados de diversas simulaciones sugieren que, en condiciones ideales, la técnica es capaz de producir una resolución superior a N1000. En la práctica, el límite encontrado es menor (N100), lo cual puede ser atribuido a condiciones de ruido y efectos de no linealidad en el láser. Para apoyar la idea propuesta diversas medidas simuladas y experimentales son presentadas a lo largo de esta Tesis. Después de explorar las ideas básicas de DSMI, un grupo de posibles aplicaciones para SMI y DSMI fueron exploradas en este trabajo. Una revisión de posibles aplicaciones biomédicas utilizando SMI fue explorada. Los resultados obtenidos sugieren que es posible obtener valores relacionados con constantes biomédicas de interés (p.e. APW) utilizando medidas de desplazamiento basadas en SMI. El método, sin embargo, no es lo suficientemente fiable como para producir medidas estables en un uso prolongado. El SNR de la señal puede introducir complicaciones durante el procesado SMI que puede derivar en errores de reconstrucción de la señal original. . El método DSMI fue probado en un prototipo de sistema AFM equipado con un cantiléver. Los resultados obtenidos sugieren que la técnica es capaz de medir movimientos producidos por un cantiléver de dimensiones micrométricas con alta resolución en bajas frecuencias. La medición de oscilaciones de mayor frecuencia podría ser alcanzada utilizando una configuración basada en modulación electrónica. El sistema propuesto fue capaz de detectar artefactos en el movimiento que podrían ser atribuidos a deflexiones en el cantiléver. Algunas posibles mejoras a esta implementación son sugeridas como puntos para futuras investigaciones alrededor de este tema.Postprint (published version

Surface and Subsurface Physical and Chemical Characterization of Materials at the Nanoscale

Abstract The discontinuity in the atomic fabric of materials that defines the transition into a new medium gives rise to intriguing properties. Examples include the electronic tunneling behavior in scanning tunneling microscope or gigantic enhancement in the Raman emission from molecules near the surfaces of noble metals. In modern microscopy, spatial and spectral resolutions are of great importance in tackling questions related to material properties. The emergence of the atomic force microscopy (AFM), which surpasses what can be achieved optically due to the inherent diffraction limit, has opened numerous opportunities for investigating surfaces. However, a contemporary challenge in nanoscience is the non-destructive characterization of materials. The ability to non-invasively explore subsurface domains for presence of inhomogeneities is of tremendous importance. In addition, techniques providing both physical and chemical information are needed to reach a comprehensive understanding of the composition and behavior of complex systems. In order to tackle the subsurface and spectral imaging, here we propose to make use of the nonlinear interaction forces between the atoms of an AFM probe tip and those of a given sample surface. Such forces are known to contain a short range repulsive component and a long range van der Waals attractive contribution. This interfacial force can give rise to a multiple-order nanomechanical coupling between the probe and the sample, offering tremendous potential for obtaining a host of material characteristics. By applying a multi-harmonic mechanical forcing to the probe and another multi-harmonic forcing to the sample, we obtain, via frequency mixing a series of new operational modes. By varying the nature of the excitations, using elastic or photonic coupling, it is possible to obtain physical and chemical signature of a heterogeneous medium with nanoscale resolution. The technique, termed mode synthesizing atomic force microscopy (MSAFM) is therefore described as a generalized multifrequency AFM. We highlight the versatility of MSAFM and its potential to contribute to important problems in material sciences, toxicology and energy research, by presenting three specific studies: 1- imaging buried nanofabricated structures; 2- investigating the presence and distribution of embedded nanoparticles in a cell; and 3- characterizing the complex structures of plant cells

Putting mechanics into quantum mechanics

Nanoelectromechanical structures are starting to approach the ultimate quantum mechanical limits for detecting and exciting motion at the nanoscale. Nonclassical states of a mechanical resonator are also on the horizon

Mechanical resonating devices and their applications in biomolecular studies

To introduce the reader in the subjects of the thesis, Chapter 1 provides an overview on the different aspects of the mechanical sensors. After a brief introduction to NEMS/MEMS, the different approaches of mechanical sensing are provided and the main actuation and detection schemes are described. The chapter ends with an introduction to microfabrication. Chapter 2 deals with experimental details. In first paragraph the advantages of using a pillar instead of common horizontal cantilever are illustrated. Then, the fabrication procedures and the experimental setup for resonance frequencies measurement are described. The concluding paragraph illustrates the technique, known as dip and dry, I used for coupling mechanical detection with biological problems. In Chapter 3, DNA kinetics of adsorption and hybridization efficiency, measured by means of pillar approach, are reported. Chapter 4 gives an overview of the preliminary results of two novel applications of pillar approach. They are the development of a protein chip technology based on pillars and the second is the combination of pillars and nanografting, an AFM based nanolithography. Chapter 5 starts with an introduction about the twin cantilever approach and of the mechanically induced functionalization. Fabrication procedure is described in the second paragraph. Then the chemical functionalizations are described and proved. Cleaved surface analyses and the spectroscopic studies of the mechanically induced functionalization are reported. In Appendix A there is an overview of the physical models that are used in this thesis

Nanomechanical motion transducers for miniaturized mechanical systems

Reliable operation of a miniaturized mechanical system requires that nanomechanical motion be transduced into electrical signals (and vice versa) with high fidelity and in a robust manner. Progress in transducer technologies is expected to impact numerous emerging and future applications of micro- and, especially, nanoelectromechanical systems (MEMS and NEMS); furthermore, high-precision measurements of nanomechanical motion are broadly used to study fundamental phenomena in physics and biology. Therefore, development of nanomechanical motion transducers with high sensitivity and bandwidth has been a central research thrust in the fields of MEMS and NEMS. Here, we will review recent progress in this rapidly-advancing area. © 2017 by the authors