Development of a high velocity accessory for atomic force microscopy-based friction measurements

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.

We report parallel topographic imaging and nanolithography using heated microcantilever arrays integrated into a commercial atomic force microscope (AFM). The array has five AFM cantilevers, each of which has an internal resistive heater. The temperatures of the cantilever heaters can be monitored and controlled independently and in parallel. We perform parallel AFM imaging of a region of size 550 μm × 90 μm, where the cantilever heat flow signals provide a measure of the nanometer-scale substrate topography. At a cantilever scan speed of 1134 μm s−1, we acquire a 3.1 million-pixel image in 62 s with noise-limited vertical resolution of 0.6 nm and pixels of size 351 nm × 45 nm. At a scan speed of 4030 μm s−1 we acquire a 26.4 million pixel image in 124 s with vertical resolution of 5.4 nm and pixels of size 44 nm × 43 nm. Finally, we demonstrate parallel nanolithography with the cantilever array, including iterations of measure-write-measure nanofabrication, with each cantilever operating independently.

A commercial atomic force microscope (AFM) was equipped with a hot stage for conducting thermal cycling experiments up to 398 K, as well as a vacuum and purge system to provide a protective environment during heating. Two different hot-stage configurations, one for studying features in the plane of a microelectronic device, and the other for studying features on its cross section, were developed. It is shown that the AFM retains its calibration with no significant introduction of errors at temperatures up to 398 K. Two applications of in situ hot-stage atomic force microscopy, related to microelectronic devices, have been demonstrated. First, the in-plane coefficient of thermal expansion of a low dielectric constant (low-k) thin film dielectric material used in back-end interconnect structures was measured. Second, the equipment was used to conduct in situ studies of deformation of Cu thin film interconnect lines at the back end of silicon chips, under thermo-mechanical loads simulating those imposed on chip-level interconnect structures by a microelectronic package. The design of a bimetallic thermo-mechanical loading stage, which was used for the latter experiments in conjunction with the hot stage, is also discussed.

This paper presents improved design, fabrication, and characterization of a small array of silicon microcantilever heaters with integrated piezoresistors. The fabricated microcantilever arrays are made from single crystal silicon with selective doping such that parasitic bending and electromigration can be suppressed during high temperature operation. Detailed characterization was performed to test the device electrical, thermal, and mechanical properties. The performance and crosstalk between heater and piezoresistor elements was thoroughly tested. The resistive heater can reach temperature higher than 600°C, and its temperature coefficient of electrical resistance was 2.01 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> Ω/Ω-°C. When biased at 2 V in a Wheatstone bridge, the deflection sensitivity of the piezoresistor was 4.25 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> V/V-μm and remarkably, the heater circuit had a non-negligible deflection sensitivity of 7.86 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> V/V-μm. Both the piezoresistor and the resistive heater were interfaced with a commercial atomic force microscope (AFM) to measure their sensitivities during topography sensing of a calibration grating. As expected, the sensitivity of thermal reading was at least one order of magnitude greater than that of piezoresistive reading.

This study presents the design and implementation of a prototype device that interfaces with commercial atomic force microscopy (AFM) instruments to accurately measure ex vivo, regional, passive elasticity measurements of the murine myocardium in three-dimensions (3D). The constructed prototype has a translation stage base allows fine adjustment of planar location at the micrometer level of accuracy. Four degrees of motional freedom are allowed in total, permitting fine, independent rotations of the murine heart (oriented at right-angles with respect to its normal mode of operation) at 2–5° increments, along its two axes of symmetry. Successful mounting of the constructed prototype was realized on the AFM’s base platform, with Zpositioning maintained through the instrument’s electronics. Measurements of normal epicardial tissue elasticity in one excised male C57BL/6 heart led to a mean value (over 15 epicardial locations) of 51.8 ± 5.4 kPa. A flexible, versatile, and low-cost prototype assembly was designed, implemented, and tested to conduct epicardial elasticity measurements in a semi-automatedmanner, using AFM. The epicardial elasticity values are within expected, published ranges for the normal myocardium of 10–55 kPa. Efforts are ongoing to allow 3D regional elasticity measurements at finer resolution with potential usefulness to studies of genetically altered mice.

A vacuum chamber for a commercial atomic force microscope (AFM) is designed and fabricated. Only minimal modifications were made to an existing microscope in an effort to work in a vacuum environment, while most of the available AFM functionalities were kept intact. The optical alignment needed for proper AFM operations including a SLD (superluminescent diode) and a photodiode can be made externally without breaking the vacuum. A vacuum level of 5×10 -3 torr was achieved with a mechanical pump. An enhancement of the quality factor was observed along with a shift in the resonance frequency of a non-contact-mode cantilever in a vacuum. Topographical data of a calibration sample were also obtained in air and in a low vacuum using the non-contact mode and the results were compared.

Lowering friction coefficient under low loads by minimizing effects of adhesion force and viscous resistance

The authors provide an introduction to the operation of existing commercial atomic force microscopes (AFMs), present a variety of image data on integrated circuit related objects, and review reliability applications from examples. The AFM offers 3-D surface measurement capability from angstroms to over 100 mu m, the ability to image insulators directly without coating, and minimal sample preparation. These features indicate strong potential for applications in IC failure analysis and reliability, particularly as ICs move toward submicron geometries. Standard commercially available AFMs can create high quality images of a variety of semiconductor process related surfaces. The basic structure and dimensions as well as defects and contaminants can be easily seen. The resolution capability of the AFM will carry these abilities through any foreseeable reduction in geometry. Current AFMs are contact, ultra-low force devices that are limited to small sample sizes. Future developments will include non-contact AFMs and AFMs with the capability to accommodate complete wafers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Understanding the effects of anisotropy and substrate shape on atomic friction force is critically needed for the designed development of nanoscale friction devices. The simulation of atomic force microscope on various substrate shapes using the 2D Prandtl–Tomlinson model is investigated in the framework of three representative surface lattices: $${\hbox {MoS}}_2$$ , NaCl and highly oriented pyrolytic graphite surfaces. The results show that the lateral force map reveals a significant contrast between different surface lattice shapes yielding lattice rows which differ from their neighboring ones. Careful analysis of the friction force during the individual friction scanning revealed that the friction forces over the narrow maxima domains were lower than those over the narrow wells domains. Depending on crystal orientation and the potential shape, variations in the frictional force can also be seen in the simulations. It has been numerically observed that frictional force depends on the crystal orientation as well as on the shape of the substrate potential. Velocity dependence of the kinetic friction force has the form of a power law $$F_{k}-F_{k0}={\hbox {cst}} \, v_{\mathrm{s}}^{2/3}$$ , for small scanning velocities. The effects of the shape parameter r on this law have been shown.

On the conductivity of PEDOT:PSS thin films

A novel constant force feedback mechanism based on fuzzy logic for tapping mode Atomic Force Microscopes (AFM) is proposed in this paper. A mathematical model for characterizing the cantilever-sample interaction subsystem which is nonlinear and contains large uncertainty is first developed. Then, a PID-like fuzzy controller, combing a PD-like fuzzy controller and a PI controller, is designed to regulate the controller efforts and schedule the applied voltage of the Z-axis of the piezoelectric tube scanner to maintain a constant tip-sample interaction force during sample-scanning. Using the PID-like fuzzy controller allows the cantilever tip to track sample surface rapidly and accurately even though the topography of the surface is arbitrary and not given in advance. This rapid tracking response facilitates us to observe samples with high aspect ratio micro structures accurately and quickly. Besides, the overshoot which will result in tip crash in commercial AFMs with a traditional PID controller could be avoided. Additionally, the controller efforts can be intelligently scheduled by using the fuzzy logic. Thus, continuous manual gain-tuning by trial and error such as those in commercial AFMs is alleviated. In final, computer simulations and experimental verifications are provided to demonstrate the effectiveness and confirm the validity of the proposed controller.

The importance of the atomic force microscope (AFM), in nano-technology, demands understanding of factors affecting AFM performance and image quality. Through a simulation study, the impact of scanning parameters on AFM images acquired in contact-mode has been investigated. The performance and limitations of commercial AFM controllers was also discussed. Based on the generated images, it is seen that high speed scans may require a large force set-point to maintain probe-sample contact at the expense of higher surface deformation, contact area, and friction force. Hence, reducing resolution and increasing image distortion. The sensitivity of AFM images to scan and controller parameters, invites the development of a systematic procedure for parameter selection for a given sample and operating environment. This would greatly reduce the current guess work in parameter selection and improve image repeatability. Currently, commercial control systems do limit the capabilities of the AFM in terms of speed and data repeatability. A good controller should be able to compensate for the large uncertainty due to different cantilevers and samples that may be used, nonlinearities, and system resonances. In addition, it should provide a tight tolerance on sample surface tracking both during transient and at steady state operation. This would enable robust high-speed operation and allow small or negative force set-point to be used.

We present 3D‐nonequilibrium molecular dynamics results for the velocity dependence of the frictional force at smooth sliding interfaces for Ta and Al single crystals. For Ta/Al we consider Al(100)/Ta(100) and Al(111)/Ta(110) interfaces sliding along [001] and [110]fcc /[001]bcc respectively. These are compared with Al(111)/Al(100) interfaces at the same loads, corresponding to a pressure of 15 GPa. Both interfacial pairs show similar behavior in the velocity dependence of the frictional force: a low velocity regime with an increasing frictional force followed by a strain induced transformation regime at velocities above approximately 1/10 the transverse sound speed, followed by a fluidized interface at high velocities. For both interfacial pairs, the high velocity dependence of the frictional force exhibits power law behavior, Ft ∝ v−β with β=3/4. We discuss the structural changes that influence dissipation in each of these regimes.

To study the effect of relative humidity (RH) on adhesion behaviors of graphene, adhesion forces between atomic force microscopy (AFM) silica microcantilevers and graphene with different layers were measured. The research shows that the behavior of adhesion force on graphene surface evolves in response to changes in RH, and both the contact history and the number of graphene layers significantly influence the RH dependence of adhesion forces at the silica-graphene interface. Initially, adhesion force of monolayer graphene exhibits no dependence on RH; however, it becomes RH-dependent in the later stage of the experiment. During both stepwise and continuous RH variation, the force remains almost unchanged in the early stage of the experiment, while it shows larger values at high RHs with repeated contacts. When the RH jumps to a certain high level, the force remains stable or even decreases initially; nevertheless, it can exhibit an increasing trend in the later stage of the experiment. In contrast, adhesion force of multilayer graphene demonstrates a strong dependence on RH during the whole experimental stages. The results have the potential to enhance the adhesion properties of graphene and improve the operational efficiency of micro-devices.

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.