Atomic force microscope using a diamond tip: a tool for micro/nanomachining on single-crystal silicon surface

This paper explores strategies for fabricating and maintaining a sharp atomic force microscope (AFM) tip suitable for AFM data storage applications. To this end, AFM cantilevers have been incorporated into micromachined sled carriers and air-bearing sliders. These supports act to limit the maximum loading force on the AFM tip and allow for improved vibration immunity for the AFM cantilever in comparison to macroscopic loading schemes. Readback from a patterned rotating disk has been demonstrated using these devices. Silicon-carbide AFM cantilevers with diamond tips have been fabricated in a process compatible with that of the sleds and sliders. Molded silicon-nitride tips defined by the intersection of three surfaces have also been fabricated for AFM cantilevers. The molds for plane-plane-concave tips are defined by two silicon {111} planes and a silicon-dioxide curved surface. By geometry, the three surfaces necessarily have a unique intersection point, which may improve the consistency in sharpness of these tips relative to conventional pyramidal tips.

Studying local mechanical properties of metallic alloys and composites and their defect structures such as grain boundaries, cracks, and dislocations allows a better understanding and design of these structural materials. With a nanoindenting atomic force microscope (NI-AFM), which uses diamond tips, such measurements are possible on a nanometer scale. Measurements on the microstructure of superalloys are presented, where different precipitates with sizes in the range of 100 nm are characterized by their different hardness values. In addition, this new technique was used to characterize the nanohardness of the plastic zone and stress fields of cracks and grain boundaries. Therefore, crack tips of in-situ loaded bending specimens of intermetallic alloys (NiAl) were investigated. The measurements show a change in nanohardness and elasticity from the distance of interfaces. In material science the high-resolution capabilities of the atomic force microscopy (AFM) are used for studying metallographic sections, where different phases produce topographic contrasts in the image. A extremly small height difference in the microstructure is sufficient for a height contrast in the image, which allows a quantitative determination of particle size distributions and volume contents of precipitates [1, 2]. The plastic deformation behavior of metals is strongly influenced by the distribution and movement of defects as dislocations, grain boundaries and cracks. In-situ loading experiments in an AFM on specimens loaded in 4-point bending demonstrate the good imaging properties of an AFM for such defects [3]. Mechanical properties such as Young’s modulus of elasticity and hardness values were obtainable in the past by various indentation techniques, where a load displacement curve is recorded during the indentation process. These techniques were mainly applied for studying the mechanical properties of thin films. A combination of a nanoindenting instrument with an AFM now allows the local mechanical properties of different phases and particles in metallic alloys to be measured, since the indents can be positioned very exactly with the AFM. Such a combined nanoindenting AFM (NI-AFM) requires a special force measurement technique. Piezo hysteresis and creep of the used piezo scanner have to be eliminated. In addition, indenting should be performed by a tip which is centrally fixed and not at the end of a bending beam (cantilever) [4], because otherwise the location of the indent changes during loading. These properties are fulfilled by a capacitive transducer, which is used for load generation and measurement instead of the standard laser deflection unit. A diamond tip is used for the indentation experiments and topographic imaging in a conventional AFM mode. The resulting force indentation curves contain two pieces of information: the universal hardness H and the reduced modulus of elasticity Er. The possibilities of the NI-AFM for studying local mechanical properties are demonstrated on two different examples: first the hardness of the superalloy CMSX-6 was characterized; second, local changes of mechanical properties near a crack tip in a NiAl single crystal were examined. 1 Experimental setup and sample preparation The nanoindentation measurements shown in this paper were performed with an add-on-force transducer from Hysitron Inc. This transducer, mounted on a conventional AFM, controls the z movement of the tip (indentation, applied load, and feedback signal) while the piezos of the AFM control the lateral x-y movement. Further technical details on the transducer are described in [5]. Specimens of the superalloy CMSX-6 were examined with a Nanoscope II AFM from Digital Instruments. Since the x-y movement is achieved by scanning the whole specimen, only small specimens with a maximum diameter of 20 mm can be analyzed. Crack tips in NiAl single crystals were investigated with a bending apparatus specially constructed for the Explorer AFM from TopoMetrix. (This bending apparatus is described in [6].) Because the whole transducer assembly is mounted on the x-y scanner, the combined system is more sensitive to mechanical noise. However, large specimens can be examined with

This article presents the results of experimental studies of local changes in the scattered light intensity and surface morphology in Mn implanted single-crystal silicon samples with electron conductivity and [100] crystalline orientation. The manganese ion energy, implantation dose, and phosphorus concentration in substrate were 40 keV, 5⋅1015÷1⋅1017 ion/cm2, and ~ 9,3⋅1014 cm–3, respectively. Atomic force microscopy (AFM) and Raman spectroscopy, using the backscattering geometry of surface-scattered light, were applied to analyze the surface morphology before and after implantation. AFM micrographs of the surface show characteristic nanometer-sized roughnesses, the shape and size of which strongly depend on the implantation dose. These nanoscale objects are not present on the non-implanted substrate surface. In the Raman spectra of the samples not subjected to implantation, the main Lorentz-type peak is always observed, which is characteristic of single-crystal silicon and centered at 520.0±1.0 cm−1, corresponding to the phonon wave vector. Several peaks are observed in the Raman spectra of manganese ion-implanted silicon samples (184, 291, 373, 468, 659, 798, and 804 cm−1), presumably associated with the formation of radiation defects and nanoscale objects on the surface of single-crystal silicon during ion implantation with the participation of silicon, manganese, phosphorus, and other impurity atoms. These structural defects in the silicon crystal lattice at the surface and near-surface caused by manganese ion bombardment lead to the excitation of new vibrational modes not observed in the initial silicon. These modes are manifested in Raman scattering spectra.

This paper provides a damage mechanism on single crystal silicon wafers by a scratch test. Relationship between normal indent load and scratch force was measured on the major planes of single crystal silicon using scratch test at room temperature. Scratching tracks of the surface of single crystal silicon after the scratching tests under very small loading forces were observed. It appears that relationship between normal indent load and scratch force correlates the critical load of damages of silicon.

This study sought to fabricate a three-dimensional structure on a silicon surface using a combination of tribo-nanolithography (TNL) and wet chemical etching. TNL forms an amorphous phase on a single-crystal silicon surface that has an etch resistance against potassium hydroxide (KOH). A protruding structure can then fabricated with wet chemical etching. When the machined silicon is etched in hydrofluoric acid (HF), the amorphous silicon phase formed by the TNL is selectively etched whereas the non-machined silicon withstands the etching. The etch resistance of the TNL-induced amorphous phase can be controlled by the machining conditions, such as the normal load, overlap ratio, and number of times the area is machined. In this study, the resulting mechanism controlling the etch resistance was examined under different machining conditions. Changes in the etch resistance due to the normal load were caused by the thickness of the amorphous phase, while changes in the resistance due to the overlap ratio or number of times the area was machined were caused by the density of the amorphous phase.

The measurement of small forces by atomic force microscopy (AFM) is of increasing importance in many applications. For example, in analytical applications where individual molecules are probed, or nanoindentation measurements as a source of information about materials properties on a nanometer scale. The fundamentals of AFM force measurement, and some of these applications, are briefly reviewed. In most cases absolute, not relative, measurements of forces are needed for valid comparisons with theory and other measurement techniques (such as optical tweezers). We review methods of AFM force calibration and the major uncertainties involved. The force range considered in this work is roughly from 10 pN to around 500 nN. We describe some issues of the repeatability of force measurements that can be important in common AFM instruments. In most cases the aspect that then limits the accuracy with which forces can be measured is the uncertainty in the stiffness (more specifically the normal force constant) of the atomic force microscope cantilever at the center of the instrument. It is known that commercially available microfabricated atomic force microscope cantilevers have a wide range of force constant, for cantilevers of nominally the same type and even the same production batch. Calibration is necessary, and many methods have been used over the years. We compare the accuracy that can be achieved and the ease of use of these different methods, including theoretical (dimensional), thermal, static and dynamic methods and their variants. A device developed at NPL should help overcome many of the problems of force constant calibration, at least for the most common AFM configurations. This is a microfabricated silicon device, which, because of its very small mass, is not susceptible to vibration as a larger device would be. A new calibration method based on electrical and Doppler measurements allows the calibration of the force constant of this device traceable to the SI newton. It can then be sent to AFM users for straightforward calibration of AFM force constants. We conclude with a brief discussion of the special problems of calibration of lateral forces, such as those obtained in frictional force measurements.

The cantilevers of atomic force microscope (AFM) are mounted under a certain tilt angle, which is commonly assumed to have negligible effect on friction measurements in AFM. We present a theoretical study of the effect of the tilt angle on AFM based friction measurements. A method for correcting the friction coefficient between sample surfaces and AFM tips is also presented to minimize the effects of the tilt. The frictional forces between a silicon tip and a silicon surface at tilt angles ranging from 5 degrees to 25 degrees were measured. The results show that the measured friction coefficient increases with the tilt angle effectively, whereas the variation range of the corrected friction coefficient is within 10%.

Nanowear tests based on atomic force microscopy (AFM) technique have been used to evaluate the wear durability characteristics of various materials and the adhesion characteristics of ultrathin films. However, the motions of wear debris during nanowear tests and the effects of wear debris on topography measurements have not been well clarified. To clarify these items, we observed the scratched surfaces of silicon single crystal by using the both tapping and contact modes at a low load condition in AFM. We have found that the motion of the wear debris of silicon was significantly affected by the observation modes, and the topography measurements of the scratched area were altered by the presence of the wear debris. Moreover, the amount of the wear debris removed from the scratched area depended on the applied load of the cantilever in contact mode AFM.

This paper reports a thermal sensor assembled by two multi-walled carbon nanotubes (MWCNTs) inside a field emission scanning electron microscope (FESEM) which constructed a probe tip in order to detect the local environment of single cell. An atomic force microscope (AFM) cantilever was used for substrate which is composed by Si3N4, and double side was covered by gold layer. Two MWCNTs were assembled to both side of AFM cantilever individually by employing a nanorobotic manipulation. Then another AFM cantilever was used as an end effector to manipulate the MWCNTs to touch, and used electron beam induced deposition (EBID) to bonding the two MWCNTs. The MWCNTs probe thermal sensor was evaluation inside a thermostated container for 30°C to 60°C. The experimental results show the positive characteristics of the temperature coefficient of resistance (TCR). We hope this result will be used rapidly in future. © 2012 IEEE.

Calibration of AFM cantilever stiffness: a microfabricated array of reflective springs

A method to develop DNA fibrils with a length more than a few tens of micrometers, oriented in one direction on the n- and p-type silicon surface is described. A new simple and effective technique is proposed to produce silver nanowires by electrochemical reduction of silver ions bound to DNA using the obtained fibrils as a template, as a result of which DNA molecules fixed on the surface of the n-type silicon single crystal are uniformly covered by silver clusters with a size of about 30 nm. The proposed metallization procedure of DNA on the n-type silicon surface has an advantage in comparison with a similar one for macromolecules fixed on freshly cleaved mica, glass surface, and p-type silicon. n-Type silicon is not only a substrate, but also a source of electrons for silver reduction. The absence of an additional chemical component (reducer) principally distinguishes the proposed method from the others currently known. Atomic force microscopic images of fixed DNA molecules and prepared nanowires are obtained.

A combined atomic force and scanning near-field optical microscope is presented. The critical component of the instrument is a single crystal silicon, microfabricated force-sensing cantilever with an integrated photodiode. Near-field optical images are obtained by monitoring variations in the optical power detected by the photodiode while the cantilever tip is scanned in an evanescent optical field created by illuminating the sample by total internal reflection. Near-field optical power was detected at tip-sample spacings of one-quarter wavelength. Atomic force and scanning near-field optical microscope images of the same samples show corresponding features as small as 25 nm.

This thesis comes within the scope of tribology studies at the nanometer scale. The experimental techniques used in this work are essentially related to atomic force microscopy, which gives a direct access to the topography and forces of the studied systems. Two principal aspects of the nanotribology have kept our attention. They are the nanofriction and the nanomechanics. These two domains divide the thesis in two main parts that remain however intimately bound. The first experimental part of the thesis describes the friction of a nanoscopic tip sliding on hydrophilic surfaces. The dependence of the friction force on the sliding velocity is studied for various applied normal loads and surrounding relative humidity levels. We found a logarithmic dependence of the friction force on both the scanning velocity and the relative humidity. For the first time, a transition from a positive to a negative slope of the friction force versus the logarithm of the sliding velocity has been observed for the very same tip-surface contact, by varying the relative humidity. The role of the relative humidity on the friction force has then been studied more deeply, leading to a two-thirds power law dependence of the capillary force on the normal load. Finally, analytical expressions for the friction phenomenon and the capillary condensation between the asperities of the tip and the surface have been developed to explain the phenomenological behaviors. The second experimental part describes themechanics of carbon nanotubes and tobacco mosaic viruses. In order to stay in the linear elasticity regime, small indentation amplitudes have been applied in the radial direction of multiwalled carbon nanotubes adsorbed on a silicon oxide surface. Using a theory based on the Hertz model, the radial stiffness has been evaluated and compared to molecular dynamics simulations. We found a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30 ± 10 GPa. The tobacco mosaic viruses have been adsorbed on a polyimide porous membrane. Evidence for the softness of the viruses has been obtained by imaging the tubes with the atomic force microscope in non-contact mode. Viruses hanging like ropes over the pores of the surface served as basis to measure their bending Young modulus. Using a model of a clamped beam loaded by a discrete gradient of van der Waals forces, we found a bending Young modulus of 3.1 ± 0.1 MPa.

Current methods to study atomic force microscope (AFM) cantilever dynamics use model simplification or are based on the non-trivial solution of the equation of motion. As an alternative method, transfer function analysis gives a more complete description of system dynamics. In this work a transfer function study of two different AFM configurations, the point force and base driven cantilever, is presented. Exact analytical expressions of the infinite dimensional transfer function are derived for cantilever deflection and slope along the cantilever. Frequency response and transfer function infinite product expansion are obtained for the case where system outputs are set at the free end of the cantilever. The frequency response reflects the full complexity of cantilever dynamics affected by the presence of an infinite number of poles and zeros. An analytical expression for all the zeros and poles of the system is obtained. From the frequency response and pole-zero investigations it is shown how cantilever actuation and output measurement affect AFM operation and cantilever dynamics modelling. Transfer function analysis of AFM cantilevers opens the possibility of model based AFM operation to increase imaging and manipulation performance.

Analytical description of fracture features in single crystal silicon