Yasuhisa Ando

Friction and pull-off forces on submicron-size asperities

Abstract We measured the friction and pull-off forces between an atomic force microscope (AFM) probe and submicron-size asperities of various radii of curvature on a silicon wafer. First, we used a focused ion beam to produce two-types of asperities: two-dimensional arrays in which asperities were arranged at uniform intervals of 240 nm, and independent asperities in which the distance between adjacent peaks was approximately 3 μm. The arrays were made by milling grooves of various depths (3.2 to 49.2 nm) in two perpendicular directions at uniform intervals. Areas in contact with the probe had radii of curvature from 86 to 792 nm, depending on the asperity. The independent asperities were made by milling an area around a point in a step-like form and had radii of curvature from 71 to 941 nm. The tip of the AFM probe had a flat square surface measuring 0.7 × 0.7 μm 2 . On all asperities measured, both the friction and pull-off forces were proportional to the radius of curvature of the tip of the asperity, irrespective of the micro-surface roughness of the asperity. The friction force was proportional to the pull-off force. The radius of curvature of the tip of an independent asperity was determined by approximating the tip as a spherical surface. When this spherical surface was set greater than the contact area, the friction and pull-off forces were proportional to the radius of curvature. This verified that the surface tension due to capillary force around the contact area was the predominant component of the adhesion force.

Friction and pull-off force on silicon surface modified by FIB

Abstract The friction and adhesion forces have been measured for silicon surfaces with various patterns having cyclic asperity. The patterns are created by using a focused ion beam (FIB) to mill ditches and to deposit platinum mounds. To study the effect of cyclic asperity on the forces, the friction and pull-off forces have been measured between the pattern and a flat, square scanning probe (0.7 μm × 0.7 μm) of an atomic force microscope (AFM). Both the friction and pull-off forces decrease as the asperity increases (i.e., as the ditch depth or mound height increases). The friction force is proportional to the pull-off force, which indicates the importance of reducing the adhesion force when devising lubrication methods for micromachines and microelectromechanical systems (MEMS).

The Effect of Asperity Array Geometry on Friction and Pull-Off Force

The friction and pull-off forces between regular asperity arrays with various heights on a silicon wafer and a scanning probe of an atomic force microscope (AFM) were measured. We used two-dimensional periodic asperity arrays. The arrays were created by using a focused ion beam (FIB) to mill patterns on a silicon plate and on a platinum layer deposited on a silicon plate. For both materials, the distance between adjacent peaks was about 240 nm and the groove depth ranged from about 3 to 49 nm. The probe of the AFM was a square flat, 0.7 X 0.7 μm 2 . For the silicon array, the pull-off force decreased with increasing groove depth and was proportional to the radius of curvature of the asperity. The friction force also decreased with asperity height and was proportional to both the asperity curvature and the pull-off force. For the platinum asperity array, although both the pull-off and friction forces also decreased with groove depth, the friction coefficient (calculated by dividing the friction force by the pull-off force) was about half that of the silicon asperity array.

Friction Characteristics and Adhesion Force Under Low Normal Load

The friction coefficient and adhesion force between steel balls and flat test pieces were measured during friction under low normal load in order to examine the tribological characteristics. First, the friction coefficients were measured under a constant normal load of 0.8 to 2350 μN, and the adhesion forces were measured before and after each friction. The result showed that the friction coefficient was highest at low normal loads, while the friction force divided by the sum of the normal load and the mean adhesion force was almost constant over the whole range of loads. Second, when the normal load was reduced gradually during friction, friction still acted when the normal load became negative and a pulling off force was applied to the surface. Thus an adhesion force acts during friction and this adhesion force affects the friction force in the same way as the normal load.

Alkyl Chain Length Effect on Tribological Behavior of Alkanethiol Self-Assembled Monolayers on Au

We have investigated the frictional properties of alkanethiol self-assembled monolayers (SAMs) by pin-on-plate methods. SAMs made from octadecanethiol (C18) with a longer alkylchain exhibited lower friction coefficients (approximately 0.2), while the friction coefficients were higher (approximately 0.4) in the case of the SAMs made from other alkanethiols with short alkylchains. The reasons for the increase in friction were i) the distortion of SAM structures because of the penetration of pins into SAMs and ii) oxidation of molecules as a result of X-ray photoelectron spectroscopy (XPS) measurements. Further friction tests on C18 SAMs for 600 cycles of sliding (1 h) caused the higher friction coefficients (0.6–0.7) and oxidation of alkylchains. It was considered that repeating of slides led to the distortion of SAMs, the oxidation of alkylchains, or adsorption of water or oxygen. The total XPS peak ratios of C(1s)/Au(4f) hardly changed after 1800 cycles of sliding (3 h), indicating that C18 SAM remained even after many repetitions of sliding.

Development of Three-Dimensional Microstages Using Inclined Deep-Reactive Ion Etching

Three-dimensional (3-D) microstages driven by electrostatic comb actuators that provide continuous motion along three axes (x,y , and z ) were designed and fabricated. Each 3-D microstage consisted of sets of traveling tables, suspension systems, and comb actuators. To convert lateral displacement of the comb actuators to vertical motion, one suspension system incorporated leaf springs inclined to a substrate. To efficiently construct the inclined leaf springs, we devised a fabrication technique that uses deep reactive ion etching. Three-dimensional microstages were then fabricated in a 20-mum-thick device layer on a silicon-on-insulator wafer. The maximum vertical (z) displacement of this 3-D microstage was 2.6 mum, and the maximum lateral displacement (x and y) was more than 6 mum in each direction, achieved by using support suspensions to suppress the interference between the comb actuators. A 3-D microstage was then installed in a commercial atomic force microscope, and a 3-D image of a grating was successfully measured without hysteresis using this 3-D microstage as the scanning device.

Wear Tests and Pull-Off Force Measurements of Single Asperities by Using Parallel Leaf Springs Installed on an Atomic Force Microscope

At the micro-scale level, the adhesion force dominates the friction force when the normal load approaches zero. For determining the effects of micro wear on the adhesion (pull-off) force the wear-induced changes in surface topography of asperities and the pull-off force between the asperities and leaf springs were determined. First, single asperities were formed on a single-crystal gold plate and the asperities were rubbed with a silicon leaf spring attached to an AFM (atomic force microscope). A focused ion beam (FIB) system was used to form gold pyramid-shaped asperities on the surface of a single crystal gold plate. The FIB was also used to create the two types of single crystal silicon leaf springs tested here; single and parallel. The single leaf spring was created by flattening the probe-head of a commercially available AFM cantilever for AC mode. The parallel leaf spring was created by removing the central portion of a single-crystal silicon beam (25 μm × 50 μm × 300 μm). For the single leaf spring, the pull-off force no longer increased when the sliding distance exceeded 5 mm at a load of more than 200 nN. On the other hand, for the parallel leaf spring, the pull-off force increased monotonically with sliding distance, showing a more rapid increase at the higher normal load. The worn area of the asperity peak (measured by using an ordinary AFM probe) was proportional to the pull-off force. The wear volume per unit distance (i.e., wear rate) was estimated from the change in pull-off force, and was found to increase monotonically with the external load. There was no effect of adhesion force on the wear volume.

Characteristics of Friction in Small Contact Surface

To determine the friction characteristics of small contact surfaces, the friction coefficient between steel balls of 0.5 to 5 mm radii and block gauges were measured while varying the friction speed, normal load, sliding distance etc. A high friction coefficient was found under a low normal load such as 1 mN, particularly for larger steel balls. However, the friction coefficient under a low normal load increased with sliding distance. Observations by AFM (Atomic Force Microscope) showed that deposited material thought to be wear particles accumulated on the steel ball with sliding distance. The adhesion force between a steel ball of 2 mm radii and a block gauge was measured under friction in order to quantify the adhesion force which affects the friction force under low normal load. The results show that the adhesion force varies greatly, and is sufficiently large to influence the friction coefficient under low normal load.

Development of a microlateral force sensor and its evaluation using lateral force microscopy

A microlateral force sensor (MLFS) was developed and evaluated using atomic force microscopy (AFM). The sensor was attached to a sensing table supported by a suspension system. The lateral motion of the sensing table was activated by a comb actuator. The driving voltage to the comb actuator was controlled to maintain a constant position of the sensing table by detecting the tunneling current at a detector, which consisted of two electrodes where the bias voltage was applied. An AFM was used to apply a lateral force to the sensing table of the sensor. When the probe of a cantilever was pressed against the sensing table and a raster scanning was conducted, the driving voltage of the comb actuator changed to compensate the friction force between the probe and sensing table. AFM measurements of an asperity array on the sensing table were conducted, and a lateral force microscopy image (LFM) was obtained from the change in driving voltage. The image by MLFS was very similar to the LFM image that was conventionally obtained from torsion of the cantilever. The LFM image strongly correlated with the gradient image calculated from the AFM topographic image. The force sensitivity of the MLFS was determined by comparing the LFM image obtained by using the MLFS with the tangential force derived from the gradient of the AFM image.

Friction and Pull-Off Forces on Submicron-Size Asperities Measured in High-Vacuum and in both Dry and Humid Nitrogen at Atmospheric Pressure

Asperities having spherical peaks were fabricated on a silicon substrate using a focused ion beam. Pull-off and friction forces were measured on each asperity using atomic force microscopy (AFM) in a high vacuum (HV) of 2×10-5 Pa and in both dry and humid nitrogen at atmospheric pressure. The radius of curvature of the asperity peaks ranged from 70 to 610 nm. The probe of the AFM cantilever had a flat square tip, approximately 1×1 µm2 in area. The results showed that the pull-off force was roughly proportional to the radius of curvature of the asperity peaks in each atmosphere. The friction force was proportional to the pull-off force. The gradient of the friction force against the pull-off force was slightly higher in the humid nitrogen than in the HV, which suggests the viscous resistance of the capillary is part of the friction force. The friction force in HV increased with lower sliding velocities without heating the substrate, which suggests the capillary also has a lubricating effect that prevents direct solid contact.