[en] We present an easy, simple, and inexpensive technique for checking the quality of the collimation of optical beams using the Lau effect combined with moire readout. The experimental arrangement consists of a modified Lau-based interferometer in which a white-light incoherent source illuminates a set of two gratings. A collimating lens is placed between the two gratings such that the self-images of the second grating are formed. The third grating is positioned at one of the self-imaging planes forming moire fringes. The type of the moire fringe demonstrates the quality of collimation of the optical beam. The necessary theoretical background is presented and the results of our experimental investigation are reported. The technique can also be used for accurate determination of the focal length of a collimating lens using low-cost components

[en] A technique for a tilt-angle measurement of reflecting objects based on the Lau interferometry coupled with the moire readout has been proposed. A white-light incoherent source illuminates a set of two gratings, resulting in the generation of the Fresnel image due to the Lau effect. The Fresnel image is projected onto a reflecting object. The image reflected from the object is superimposed onto an identical grating, which results in the formation of a moire fringe pattern. The inclination angle of moire fringes is a function of tilt angle of the object. Theory and experimental arrangement of the proposed technique is presented and results of the investigation are reported

[en] A grating-based shearing interferometeric setup for slope measurement of bent plates has been proposed. The specimen under test is illuminated by a collimated beam from the laser. Light reflected from the specimen passes through two identical holographic gratings placed in tandem. The grating frequency has been so chosen that the diffracted orders from each grating are separated out distinctly. Two first-order beams diffracted from each of the gratings superpose in space. In the resulting interferogram, the fringes due to slope information of the object are visualized. Mathematical formulation for experimental determination of slope values has been undertaken. Validation of the experimental results with theoretical predictions in case of cantilever beam provides good correlation. The main advantage of the technique has been the realization of very compact geometry without the need for spatial filtering arrangement commonly associated with the grating-based techniques used to date.

[en] We demonstrate a simple method for obtaining slope contours of bent plates using Talbot interferometry. The technique has been used to map slope contours of polymethyl methacrylate specimens of different shapes. The Talbot image of a coarse grating is projected onto a specimen such that the self-image is backreflected onto the same grating again. As a Talbot interferometer is basically a grating shearing interferometer, it results in the generation of characteristic slope maps of the specimen under test. Results of the investigation match well with other slope-mapping techniques. Validation of experimental results with theoretical predictions in the case of a cantilever beam specimen has been undertaken. Accuracy of about 4.7% with respect to theoretical predictions is obtained.

[en] PMMA (Polymethyl methacrylate) is commonly used in many microfluidic devices like Lab-on-a-chip devices, bioanalytical devices etc. CO₂ lasers provide easy and cost effective solution for micromachining needs on PMMA. Microchannels are an integral part of most of these microfluidic devices. CO₂ laser beams have been successfully applied by many authors to fabricate microchannels on PMMA substrates. Laser beam power and scanning speed are the most important laser input parameters affecting the output parameters like microchannel depth, width and heat affected zone (HAZ). The effect of these individual parameters on output parameters are well known and already elaborated by many authors. However, these output parameters can more significantly be described by some compound parameters (combination of direct input laser parameters) like laser fluence, specific point energy, interaction time and P/U (power/scanning speed) ratio. The explanation of effect of these compound parameters was not found in earlier researches. In this work, several experiments were carried out to determine the effects of these compound parameters on output parameters i.e. microchannel width, depth and heat affected zone. The effect of pulse overlapping was also determined by performing experiments at different pulse overlaps and with two different energy deposition settings. The concept of actual pulse overlapping has been introduced by considering actual beam spot diameter instead of using theoretical beam diameter. Minimum pulse overlapping was determined experimentally in order to ensure smooth microchannel edges. (paper)

[en] Polymer based microfluidic channels are used in many chemical and biological devices. Polymethylmethacrylate (PMMA) has emerged as a key material for such devices owing to its high optical transparency and mechanical strength. The use of CO_2 laser processing for fabricating microchannels on PMMA has been proved as an efficient and cost effective method. In this work, theoretical models for predicting microchannel profile and depth have been proposed. A model for single-pass laser processing has been proposed based on energy balance. A two-pass laser process for microchannel fabrication produces smoother microchannels with better surface topography and reduced bulging around the microchannel edges. An energy balance based model has also been proposed for two-pass processing. The experimental verification of the proposed models was conducted. Spectroscopic tests were carried out to determine the absorptivity, and simultaneous thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) tests were performed to determine the thermo-physical properties of the PMMA used in the proposed model. The results predicted using the model were found to be in close agreement with the actual values. (paper)

[en] The poor surface finish of CO₂ laser-micromachined microchannel walls is a major limitation of its utilization despite several key advantages, like low fabrication cost and low time consumption. Defocused CO₂ laser beam machining is an effective solution for fabricating smooth microchannel walls on polymer and glass substrates. In this research work, the CO₂ laser microchanneling process on PMMA has been analyzed at different beam defocus positions. Defocused processing has been investigated both theoretically and experimentally, and the depth of focus and beam diameter have been determined experimentally. The effect of beam defocusing on the microchannel width, depth, surface roughness, heat affected zone and microchannel profile were examined. A previously developed analytical model for microchannel depth prediction has been improved by incorporating the threshold energy density factor. A semi-analytical model for predicting the microchannel width at different defocus positions has been developed. A semi-empirical model has also been developed for predicting microchannel widths at different defocusing conditions for lower depth values. The developed models were compared and verified by performing actual experiments. Multi-objective optimization was performed to select the best optimum set of input parameters for achieving the desired surface roughness. (paper)

[en] CO₂ laser microchanneling has emerged as a potential technique for the fabrication of microfluidic devices on PMMA (Poly-methyl-meth-acrylate). PMMA directly vaporizes when subjected to high intensity focused CO₂ laser beam. This process results in clean cut and acceptable surface finish on microchannel walls. Overall, CO₂ laser microchanneling process is cost effective and easy to implement. While fabricating microchannels on PMMA using a CO₂ laser, the maximum depth of the fabricated microchannel is the key feature. There are few analytical models available to predict the maximum depth of the microchannels and cut channel profile on PMMA substrate using a CO₂ laser. These models depend upon the values of thermophysical properties of PMMA and laser beam parameters. There are a number of variants of transparent PMMA available in the market with different values of thermophysical properties. Therefore, for applying such analytical models, the values of these thermophysical properties are required to be known exactly. Although, the values of laser beam parameters are readily available, extensive experiments are required to be conducted to determine the value of thermophysical properties of PMMA. The unavailability of exact values of these property parameters restrict the proper control over the microchannel dimension for given power and scanning speed of the laser beam. In order to have dimensional control over the maximum depth of fabricated microchannels, it is necessary to have an idea of uncertainty associated with the predicted microchannel depth. In this research work, the uncertainty associated with the maximum depth dimension has been determined using Monte Carlo method (MCM). The propagation of uncertainty with different power and scanning speed has been predicted. The relative impact of each thermophysical property has been determined using sensitivity analysis. (paper)

[en] Highlights: • Interdiffusion in SET-A: [TiN(10 nm)$|$Ti(5 nm)]₁₀ & SET-B: [TiN(5 nm)$|$Ti(10 nm)]₁₀ samples has been studied in this work. • XRR and SIMS study reveal complete interfacial diffusion ($⩾$ 5 nm) for SET-A sample at 973K. • For SET-B sample, it is even pronounced at 773K with faster migration of N atoms. • SXAS study also confirms nitrogen out diffusion for SET-B sample at 973K. • Thickness of TiN layer plays an important role in prevention of interfacial interdiffusion. In pursuit of a systematized interdiffusion study in Ti$|$TiN multilayers at low temperature (300 - 973 K), x-ray reflectivity measurement aided with soft x-ray absorption and secondary ion mass spectroscopy were employed in the present scope of findings. Despite of TiN interfaced with Ti being a well-known diffusion blocking unit, modulation of individual stacking thickness in nm regime in two set of samples: SET – A : [TiN (10 nm) $|$ Ti (5 nm)]${}_{10}$, SET – B : [TiN (5 nm) $|$ Ti (10 nm)]${}_{10}$, out turns higher interfacial mixing for SET – B samples at elevated temperatures as expected due to lower N repository and eventually lesser Ti-N covalent bonds and higher Ti-Ti metal bonds, leading to plausible lower potential barrier for migration of the diffusion species. In turn, interdiffusion across interfaces promotes two distinct distribution profiles — (i) For SET – A samples, as opposed to discrete multilayer stacks, interdiffusion led to uniformly distributed single volume matrix transformation at 973 K, and (ii) For SET – B samples, such transformation is pronounced even at 773 K due to faster migration of N and at 973 K, surprisingly N diffuses out, with some kind of residual interstitial TiN compound residing in the system.

[en] Highlights: • Synthesis and study of TiN(111) thin films using ion beam sputtering is reported. • TiN films were grown at ambient temperature (300 K) without applying any bias. • Tuning of ion energy and a Ti interface has resulted in densest TiN films, hitherto. • TiN films grown at low ion energy on Ti, were densest and smoothest. • SRIM simulations were made to understand the role of sputtered species energy on film growth. This study aims towards a systematic reciprocity of the tunable synthesis parameters - partial pressure of N₂ gas, ion energy (E_i) and Ti interface in TiN thin film samples deposited using ion beam sputtering at ambient temperature (300 K). At the optimum partial pressure of N₂ gas, samples were prepared with or without Ti interface at E_i = 1.0 or 0.5 keV. They were characterized using x-ray reflectivity (XRR) to deduce thickness, roughness and density. The roughness of TiN thin films was found to be below 1 nm, when deposited at the lower E_i of 0.5 keV and when interfaced with a layer of Ti. Under these conditions, the density of TiN sample reaches to 5.80($\pm$0.03) g cm⁻³, a value highest hitherto for any TiN sample. X-ray diffraction and electrical resistivity measurements were performed. It was found that the cumulative effect of the reduction in E_i from 1.0 to 0.5 keV and the addition of Ti interface favors (111) oriented growth leading to dense and smooth TiN films and a substantial reduction in the electrical resistivity. The reduction in E_i has been attributed to the surface kinetics mechanism (simulated using SRIM) where the available energy of the sputtered species ($<$E_sp>) leaving the target at E_i = 0.5 keV is the optimum value favoring the growth of defects free homogeneously distributed films. Secondary ion mass spectroscopy depth profile measurements confirm the uniform distribution of N and Ti across the depth of a sample. The electronic structure of samples was probed using N K-edge and Ti L-edge absorption spectroscopy and the information about the crystal field and spin-orbit splitting confirmed TiN phase formation. In essence, through this work, we demonstrate the role of $<$E_sp> and Ti interface in achieving highly dense and smooth TiN thin films with low resistivity without the need of a high temperature or substrate biasing during the thin film deposition process.