White Light Interferometric Microscope Research Articles

High-Transmission Neutron Optical Devices Utilizing Micro-Machined Structures

Neutrons are a powerful probe in material science with unique penetrating abilities. A major challenge stems from the fact that neutron optical devices are limited to refractive indices on the order of n≈1±10−5. By exploiting advances in precision manufacturing, we designed and constructed micro-meter period triangular grating with a high-aspect ratio of 14.3. The manufacturing quality is demonstrated with white-light interferometric data and microscope imaging. Neutron-scattering experiment results are presented, showing agreement with refraction modelling. The capabilities of neutron Fresnel prisms and lenses based on this design are contrasted with existing neutron focusing techniques, and the path separation of a prism-based neutron interferometer is estimated.

Variation of thermal expansion coefficient of freestanding multilayer pristine graphene with temperature and number of layers

The in-plane coefficient of thermal expansion (CTE) of freestanding multilayer pristine graphene was directly measured by the thermal bulge method using a white light interferometric microscope. Graphene ribbons with 3, 5, and 10 layers were prepared by exfoliation of high-quality graphite flakes and then transferred over 60–120 μm wide trenches in a Si substrate. A novel transfer procedure was developed to achieve the freestanding configuration. All the measured CTE values were negative in the temperature range of 25–150 °C, and their absolute values decreased with increasing temperature and number of layers. The CTE of monolayer graphene can be approximated to about −7 × 10−6/°C. It was speculated that a structure with more than 12 layers was graphite rather than graphene.

A quantitative wear measurement method on production engine parts: Effect of DLC thin films on wear

The automotive industry is facing worldwide pressure to reduce carbon emission to meet the Paris Accord target of 2 °C increase in global average temperature by the end of the century. One way to reduce carbon emission is to improve fuel economy of cars and trucks. Recent emergence of ultralow viscosity lubricants has shown significant fuel economy benefits. However, the resulting low oil film may cause increased surface contacts and could lead to increased wear. The ability to measure the initiation phase of wear before it accelerates towards failure is an important issue today. Quantitative measurement of wear in engine parts has always been a challenge. In today’s fuel efficient engines, coatings increasingly being introduced into engine component manufacturing, wear metrology of small amount of wear spread over a large area of rough surfaces presents even a greater challenge.This paper describes the development of a quantitative measurement technique to measure mild wear over a relatively large area of rough production engine surfaces. One of the critical engine sliding components is the piston ring vs cross-hatched cylinder liner contact interface. The availability of a quantitative wear measurement technique would allow comparison of various materials, including coated rings, and liner surface treatments. In this study, production rings and liners were obtained from cooperative engine manufacturers under non-disclosure agreements. A reciprocating ring-liner test rig was used to conduct the wear tests, using a newly developed cyclic wear test procedure, sequence of speed and load cycles were used for simulating sliding motions. Mini-cycles (a constant speed with varying loads from 160 N to 240 N) at 120 °C, and at 6 Hz reciprocating frequency were used. An experimental 0 W-16 lubricant, which has gone through engine testing, was used in the wear tests. A dual white light interferometric microscope (Z resolution 1 Å) was used to measure the liner wear. An analytical balance with 1 µg resolution was used to measure the ring segment weight loss. After the wear measurement procedure was established, diamond-like carbon (DLC) coated rings were also used to assess the effect of the DLC on the wear, and the measurement technique. A digital 3D surface topography subtraction technique was successfully developed to measure quantitative wear of the liner. This technique can be used for other wearing engine parts.

Polymerizing Bone Cement can Roughen a Polyethylene Bearing Surface on Total Hip Replacement A Surface Analysis

The tribological behaviour of the bearing surface in hip arthroplasty is greatly influenced by its surface roughness. During cemented hip arthroplasty, bone cement may contaminate the bearing surface. Hence, the aim of this study was to investigate the effect of cement polymerization on a highly cross-linked polyethylene(HCLP) bearing surface as any process influences the surface roughness of the bearing surface can have potential long term effect on the wear rate. Three new HCLP liners were used. A map of the surface topography of each liner was obtained using a white light interferometric microscope at five different locations in accordance with the standard BSEN ISO 4288:1998. Bone cement was then applied to the bearing surface and allowed to polymerise, then removed and irrigated with saline. The surface roughness measurement procedure was repeated again for all samples. The Ra results showed a 21% increase in mean surface roughness after cement contamination (p=0.01). The average Ra value pre-contamination was 190 nm while post-contamination was 230 nm. There were also changes in the surface topography observed with 3D surface mapping. Our study indicates that bone cement contamination of the HCLP surface can significantly increase its surface roughness and alter its surface topography. Surgeons should be aware of this potentially serious effect on the bearing and be vigilant to avoid bone cement contamination during arthroplasty surgery.

Mechanisms of the refractive index change in DO11/PMMA due to photodegradation

Using a white light interferometric microscope (WLIM) we measure the photodamage induced change in the complex index of refraction of disperse orange 11 (DO11) dye-doped (poly)methyl-methacrylate. We find that the change in the imaginary part of the refractive index is consistent with previous measurements of photodamage-induced absorbance change. Additionally, we find that the irreversible change in the real refractive index can be separated into a component due to damage to the dye molecules and a component due to irreversible damage to the polymer. From the features of the polymers contribution to the irreversible change in the real refractive index we conclude that photo-thermal induced chain scission and cross linking are the underlying mechanisms of polymer damage.

Analysis of the misfit of dental implant-supported prostheses made with three manufacturing processes

The microgap between implant components has been associated with complications such as screw loosening or adverse biologic responses. The purpose of this study was to quantify the microroughness of the mating surfaces of implant components manufactured with different processes, to quantify the microgap between implant components, and to determine whether a correlation exists between microroughness and the microgap. Nine dental implants with a standard external connection were paired with 3 milled, 3 cast, and 3 sintered compatible cobalt-chromium alloy abutments. The abutment surface was examined, and the roughness parameter Sz was measured by using a white-light interferometric microscope at ×10 to ×100 magnification. The abutment surface and the microgap of the implant-abutment connection were observed with scanning electron microscopy, and the microgap width was quantified from micrographs made of each implant-abutment pair. The mean and standard deviation of roughness and microgap were evaluated. A 1-way ANOVA (α=.05) was used to assess the influence of the manufacturing process on roughness and microgap. The Pearson correlation was used to check dependence between roughness and microgap. The milled abutments possessed a connection geometry with defined edges and a mean roughness of 29 μm, sintered abutments showed a blurred but functional connection with a roughness of 115 μm, and cast abutments showed a connection with a loss of axial symmetry and a roughness of 98 μm. A strong correlation was found between the roughness values on the mating surfaces and the microgap width. The milled components were smoother than the cast or sintered components. A correlation was found between surface roughness and microgap width.

A white light interferometric microscope for measuring dose-dependent reversible photodegradation

We have developed a white light interferometric microscope (WLIM), which can spatially resolve the change in the complex index of refraction and apply it to study reversible photodegradation of 1,4-Diamino-9,10-anthraquinone doped into (poly)methyl-methacrylate. The measured change in absorbance is consistent with standard spectrometer measurements. The refractive index of a pristine sample measured with the WLIM is also found to be consistent with the spectrum found by independent means [F. Yakuphanoglu and B. Senkal, “Electrical conductivity, photoconductivity, and optical properties of poly(1,4-diaminoanthraquinone) organic semiconductor for optoelectronic applications,” Polym. Adv. Technol. 19, 1193–1198 (2008)]. We report on measurements of the change in refractive index due to photodegradation, which is found to be consistent with Embaye's two-population model [N. Embaye et al., “Mechanisms of reversible photodegradation in disperse orange 11 dye doped in PMMA polymer,” J. Chem. Phys. 129, 054504 (2008)]. We show that the WLIM can be used as a powerful tool to image the complex refractive index of a planar surface and to detect changes in a material's optical properties.

Full-field evaluation of the onset of microplasticity in a steel specimen

This work deals with the study of the local onset of microplasticity in a steel specimen under tensile test through the use of a full-field strain measurement method at the micrometric scale. The kinematic measurements were performed by the so called grid method. In order to produce the gratings, direct interferential photolithography was used. The gratings were digitized through the use of a white-light interferometric microscope. An adapted image stitching algorithm, applied to the phase maps, has been developed and implemented in order to extend the size of the region of interest. With this method, and by paying a particular attention to the conduct of the mechanical tests, the strain resolution obtained ranges from 1 to 2 × 10 −3 for a spatial resolution of about 20 μ m. In this paper, the method was applied to the study of the onset of local elasto-plastic strains in a ferritic steel coupon. The results show that the method is well suited to study small localized strains, for which few alternatives exist. To the best knowledge of the authors, it is the first time that white-light interferometric microscopy is used to measure full-field strains. The technique is reasonably easy to set up although a number of limitations exist. The paper is concluded by some considerations about further work required to extend the range of applicability of the method.

Development of a high cycle fatigue testing system and its application to thin aluminum film

The principal goal of this investigation was to develop a feedback controlled high cycle fatigue testing system and to demonstrate its usefulness by acquiring a preliminary stress-life curve of an aluminum film in a very short period of time. A high cycle fatigue testing system having 2.5 N load capacities was constructed and proved reliable, stable and useful in doing long-run fatigue tests. A set of stress-life curves was collected under a mean stress level with 1.16 μm thick aluminum films under closed-loop load control. The system is compact enough to set it up on subsidiary equipment like an optical microscope. A white light interferometric microscope was utilized to observe a specimen’s surface morphology change with fatigue cycles in-situ. The surface roughness was decreased or remained constant in the beginning but started to increase in the later stage.

Applications of a high accuracy optical fiber displacement sensor to vibrometry and profilometry

This paper shows the use of an extrinsic optical fiber displacement sensor as a low cost vibrometer and profilometer. Results obtained with our sensor are compared respectively to those given by a piezoelectric accelerometer and by a white-light interferometric microscope. Calibration step shows that the sensitivity of the optical fiber sensor is 77 μV/nm for the 140–220 μm range over a dynamic bandwidth of 8.5 kHz. The achieved resolution of the sensor is 1 nm/ Hz . The light source feed circuit is designed to reduce drifts. The short time stability of the sensor is 3×10 −5 over 77 s in relative value.

Deformation of blanketed and patterned bilayer thin-film microstructures during post-release and cyclic thermal loading

We study, both experimentally and theoretically, the deformation of blanketed and patterned bilayer thin film microstructures subjected to temperature cycles from room temperature to elevated temperatures following processing by surface micromachining and release from the substrate. While the theoretical treatment is general, the experimental component focuses on beam-like microstructures consisting of a 0.5 /spl mu/m thick gold film on a polysilicon film that is either 1.5 /spl mu/m or 3.5 /spl mu/m thick. For all microstructures the underlying polysilicon film is the same size, but the gold film is patterned into a line that runs the length of the beam. Its width is varied from 0 to 100% of the width of the polysilicon. We experimentally characterize the deformation by measuring the full-field deflection of the gold/polysilicon bilayer beams as a function of temperature using a white-light interferometric microscope. From the deflection, the curvature is determined, and we report the evolution of curvature with the temperature cycling. Qualitatively the behavior is the same regardless of the linewidth. The quantitative differences can be described by a simple model incorporating an inelastic temperature-driven mechanism in addition to linear thermoelastic behavior. We show experimentally and/or analytically, how the parameters in the model vary with linewidth. The results are discussed in the context of the current understanding of microstructural evolution in thin-film metals, and in relation to anticipated thermoelastic response. We show that via a suitable thermal process, the thin film material microstructure can apparently be stabilized over a prescribed temperature range, rendering the subsequent deformation linear thermoelastic. We discuss the implications of these findings in the context of the design and fabrication of high-yield, dimensionally stable MEMS devices utilizing bilayer material systems. Although our measurements are focused on gold/polysilicon bilayer films, the concepts and associated analysis are applicable to other bilayer film systems, particularly ones with metals, although there will surely be quantitative differences.

Fabrication and characterization of metal–oxide–semiconductor capacitor basedon layer-by-layer self-assembled thin films

Metal–oxide–semiconductor-capacitor arrays are fabricated on both P and N typesilicon wafers using layer-by-layer (LbL) self-assembled insulating layers. Thevertical dimension of the self-assembled thin film can be precisely controlled aswell as the molecular order. Unlike the conventional process, the LbLself-assembly allows one to obtain the thin films for a semiconductor device with adramatically lower temperature, lower cost and shorter processing time. Thedeposited thin film is stable and can grow on any substrate other than silicon.The conventional lithographic technique is employed to pattern the self-assembledthin films, resulting in an extremely high reproducibility. This enables thepossibility of industrial applications to fabricate devices with this simplified andversatile technique. A CCD camera was used to produce the image of the pattern,and a white light interferometric microscope was used to measure thedimension and surface roughness of the produced device. The quartz crystalmicrobalance served to monitor the growth of the self-assembled thin films.

Full-field optical coherence tomography with thermal light.

A simple optical coherence tomography system has been developed based on a white-light Linnik interferometric microscope with its reference mirror mounted on a piezoelectric translator. The geometrical extension of the optics allows efficient illumination of this device with a low-power (3-W) light bulb, yielding full-field interferometric images at 50 Hz with a fast CCD camera. Owing to the very broad spectral width of the light source and of the camera response, we achieved axial resolutions equal to 1.1 microm in free space and 0.7 microm through a standard microscope cover plate. Tomographic images of an epithelial cell smear and of an hematological sample are shown.

Explosive change in crater properties during high power nanosecond laser ablation of silicon

Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than ≈2.2×1010 W/cm2. Time-resolved shadowgraph images of the ablated silicon plume were obtained over this irradiance range. The images show that the increase in crater volume and depth at the threshold of 2.2×1010 W/cm2 is accompanied by large size droplets leaving the silicon surface, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). In this case, the estimated thickness of the superheated layer at a delay time of 200–300 ns shows a close agreement with measured crater depths. Induced transparency is demonstrated to play an important role in the formation of a deep superheated liquid layer, with subsequent explosive boiling responsible for large-particulate ejection.