Abstract Laser machining is an inexpensive and fast alternative to conventional microfabrication techniques that produce complicated three-dimensional, hierarchical structures. One of the major issues plaguing the use of laser micromachining to manufacture commercially usable devices is the formation of debris during cutting and the difficulty in removing these debris efficiently after the machining process. For silicon substrates, this debris can interfere with surrounding components and cause problems during bonding with other substrates by preventing uniform conformal contact. This study summarizes several post-process techniques that can be employed for complete debris removal during etching of Silicon samples using an Nd/YVO 4 pulsed (∼ 1–3 kW) UV laser, detailing the advantages and drawbacks of each approach. A method that was found to be particularly promising to achieve very smooth surfaces with almost complete debris removal was the use of PDMS as a high rigidity protective coating. In the process, a novel technique to strip PDMS from Silicon surface was developed and a study was carried out to optimize the process. The result of this study is very valuable to the microfabrication industry where smooth and clean substrate surfaces are highly desirable. This work could facilitate adoption and significant improvements to the process of using UV lasers to create microstructures for commercial applications as well as in a research environment.

Abstract AM (Additive manufacturing) technology has huge demand from industry for its high-speed fabricating, ability of fabricating complex shapes and low-cost fabrication which does not require expensive equipment such as molds. In particular, resolution of stereolithography technology is reaching to nanometer scale, and it is also expected to be utilized for nanodevice modeling. One reason for stereolithography reaching to nanometer scale high resolution is due to the development of TPP (two-photon polymerization) technology. As TPP requires advanced knowledge of optical, chemical and physical phenomena it is difficult to create a practical numerical method. On the other hand, there is a huge demand for prediction of curing range of resin from these parameters manufacturing of final products. In this research, we discuss on numerical model which calculates curing region of photocurable resin by using short pulsed laser. The calculation will be conducted for line fabrication which will be more useful for practical use.

Abstract As modern electronics continuously exceed their performance limits, there is an urgent need to develop new cooling devices that balance the increasing power demands. To meet this need, cutting-edge cooling devices often utilize microscale structures that facilitate two-phase heat transfer. However, it has been difficult to understand how microstructures trigger enhanced evaporation performances through traditional experimental methods due to low spatial resolution. The previous methods can only provide coarse interpretations on how physical properties such as permeability, thermal conduction, and effective surface areas interact at the microscale to effectively dissipate heat. This motivates researchers to develop new methods to observe and analyze local evaporation phenomena at the microscale. Herein, we present techniques to characterize submicron to macroscale evaporative phenomena of microscale structures using micro laser induced fluorescence (μLIF). We corroborate the use of unsealed temperature-sensitive dyes by systematically investigating their effects on temperature, concentration, and liquid thickness on the fluorescence intensity. Considering these factors, we analyze the evaporative performances of microstructures using two approaches. The first approach characterizes local or overall evaporation rates by measuring the solution drying time. The second method employs an intensity-to-temperature calibration curve to convert temperature-sensitive fluorescence signals to surface temperatures. Then, submicron-level evaporation rates are calculated by employing a species transport equation for vapor at the liquid-vapor interface. Using these methods, we reveal that capillary-assisted liquid feeding dominates evaporation phenomena on microstructured surfaces. This study will enable engineers to decompose the key thermofluidic parameters contributing to the evaporative performance of microscale structures.

The ability to create 3D ICs can significantly increase transistor packing density, reduce chip area and power dissipation leading to possibilities of large-scale on-chip integration of different systems. A promising process for this application is the microscale additive manufacturing (AM) of 3D interconnect structures and capability of writing 3D metal structures with feature sizes of approximately 1 μm on a variety of substrates. Current microscale AM techniques are limited in their capabilities to produce 3D conductive interconnect structures. This paper presents the design and development of a new micro AM technique — microscale selective laser sintering (μ-SLS) — which overcomes many of the limitations of other micro AM processes to achieve true micron sized, electrically conductive features on a variety of substrates. This paper will present preliminary results from set of sintering experiments on copper (Cu) nanoparticle (NP) ink using the continuous wave (CW) laser to be employed in the μ-SLS system which will be compared to Cu NP sintering results produced with other laser sources such as nanosecond (ns) & femtosecond (fs) lasers. This study is important to estimate the optimum working range of fluence/irradiance to be used in the μ-SLS setup depending upon the laser employed. In general, it provides an experimental estimate of the sintering fluence/irradiance range of Cu NPs depending upon the type of laser used and compares their sintering quality based on morphology of sintered spots.

This paper presents experimental data of concurrent modulation of a multi-channel transmitter that uses carrier-injection ring modulators at 10Gb/s/channel that is optically driven by a quantum-dot comb laser with 50GHz channel spacing.

From a materials perspective, diamond exhibits properties that are extremely well suited for use in the thermal management of high power and high heat flux electronic devices. While bulk diamond grown via chemical vapor deposition (CVD) has been demonstrated since the 1980s, and people have measured thermal conductivities ranging from 500–2000 W/m-K, these measurements have typically taken place over a large domain that encompasses numerous diamond grains. However, many of these techniques do not reveal the heterogenous nature of the diamond thermal conductivity which arises due to the local grain structure and orientation. The diamond sample investigated in this study contained a high level of boron doping on the order of 10 21 cm −3 , giving rise to a reduced thermal conductivity measured as 714 W/m-K with a laser flash method. Similar bulk CVD diamond samples that are undoped show thermal conductivity values of greater than 1500 W/m-K with the same measurement technique. Through the use of time-domain thermoreflectance (TDTR) we are able to measure the thermal conductivity of bulk CVD diamond at a spatial resolution smaller than the size of the columnar grains. This allows us to examine significant changes in thermal conductivity as a function of spatial location, which is of great significance when the thermal source from electronics is on the size scale of this variation. Using TDTR, we present an approach involving a variation in the laser spot size using multiple focusing objectives to yield the heterogeneous thermal conductivity in bulk CVD diamond. The data show variations in thermal conductivity near 40% over a diameter of 40 μm. Scanning Electron Microscopy (SEM) and electron backscatter diffraction (EBSD) data are presented which also show variation in microstructure over this length scale giving rise to the heterogeneity.

Silicon photonics has emerged as a scalable technology platform for future optotelectronic communication systems. However, the current use of SiO 2 -based silicon-on-insulator (SOI) substrates presents a thermal challenge to integrated active photonic components such as lasers and semiconductor optical amplifiers due to the poor thermal properties of the buried SiO 2 optical cladding layer beneath these devices. To improve the thermal performance of these devices, it has been suggested that SiO 2 be replaced with aluminum nitride (AlN); a dielectric with suitable optical properties to function as an effective optical cladding that, in its crystalline state, demonstrates a high thermal conductivity (∼100× larger than SiO 2 in current SOI substrates). On the other hand, the tuning efficiencies of thermally-controlled optical resonators and phase adjusters, crucial components for widely tunable lasers and modulators, are directly proportional to the thermal resistance of these devices. Therefore, the low thermal conductivity buried SiO 2 layer in the SOI substrate is beneficial. Moreover, to further improve the thermal performance of these devices air trenches have been used to further thermally isolate these devices, resulting in up to ∼10× increase in tuning efficiency. Here, we model the impact of changing the buried insulator on a SOI substrate from SiO 2 to high quality AlN on the thermal performance of a MRR. We map out the thermal performance of the MRR over a wide range of under-etch levels using a thermo-electrical model that incorporates a pseudo-etching approach. The pseudo-etching model is based on the diffusion equation and distinguishes the regions where substrate material is removed during device fabrication. The simulations reveal the extent to which air trenches defined by a simple etch pattern around the MRR device can increase the thermal resistance of the device. We find a critical under-etch below which no benefit is found in terms of the MRR tuning efficiency. Above this critical under-etch, the tuning efficiency increases exponentially. For the SiO 2 -based MRR, the thermal resistance increases by ∼7.7× between the un-etched state up to the most extreme etch state. In the unetched state, the thermal resistance of the AlN-based MRR is only ∼4% of the SiO 2 -based MRR. At the extreme level of under-etch, the thermal resistance of the AlN-based MRR is still only ∼60% of the un-etched SiO 2 -based MRR. Our results suggest the need for a more complex MRR thermal isolation strategy to significantly improve tuning efficiencies if an AlN-based SOI substrate is used.

Plasma dicing has rapidly gained traction as a viable alternative to conventional blade and laser techniques for wafer singulation. This has been due mostly to the significant benefits plasma dicing delivers in relation to the quality and reliability of devices singulated in this manner. Key to the successful integration of plasma dicing, into the established hierarchy of singulation techniques, is how the ancillary parts of the process flow can be utilized or adapted to accommodate it. More importantly, is the ease at which this can happen and also, how implementation can be achieved in a cost effective manner.

Laser beam shaping techniques are important to optimize a large number of laser-material processing applications and laser-material interaction studies. The authors have developed a novel fluidic laser beam shaper (FLBS) with merits such as flexiblility, versatility and low cost. This work presents a fundamentally new approach for laser beam shaping by using FLBS. A Gaussian beam profile is transformed to a flat top beam and annular beam profile in the focal plane. The shaped laser beam is used for laser drilling to investigate the influence of the laser intensity profile in laser processing. The paper concludes with suggestions for future research and potential applications for further the work.

Laser drilling of silicon carbide (SiC) wafer in air (dry ablation) and underwater by using ns pulsed infrared (1064 nm) Nd: YAG laser is investigated. In order to suggest optimal parameters of via processing in SiC wafer, the effects of pulse number, laser fluence, water film thickness, and focus position are evaluated. As compared with dry ablation vias, decreasing etching rate, increasing via diameter, and generation of cracks in high-energy regime are observed in liquid-assisted processing. However, it is found that it can create vias without debris, HAZ, cracks. Also, optimal parameter set for infrared pulse laser processing under water is found to be the laser fluence of less than 10 J/cm 2 and water thickness of 1mm.

Portable electronics devices such as mobile phone and portable music player become compact and improve their performance. High-density packaging technology such as CSP (Chip Size Package) and Stacked-CSP is used for improving the performance of devices. CSP has a bonded structure composed of materials with different properties. A mismatch of material properties may cause stress singularity, which lead to the failure of bonding part in structures. In the present study, a strain singular field near inter-face edge in three-dimensional joints is investigated using digital image correlation method. A specimen which silicon chip was embedded in resin is used in experiment and tensile load is applied to the specimen. Photograph of specimen surface is taken before and after loadings by laser microscope. Displacement on the surface was evaluated by the digital image correlation method (DICM) using data of surface pattern on the specimen, which the cross correlation coefficients for surface pattern are maximized. Strain on surface of specimen is calculated by using the moving least square method. On the other hand, 3D element free Galerkin method is applied to compute the displacement and strain distribution in a three-dimensional model of the specimen. In the element free Galerkin method, the physical values, i.e., displacement, strain and stress, can be obtained by using the displacement data at node. In this research, strain distribution near the edge of interface is computed based on the element free Galerkin method. Finally, the strain distribution obtained by the digital image correlation method and the moving least square method is compared with that obtained by the element free Galerkin method. The intensity of strain singularity is determined numerically and experimentally.

A numerical model of the thermoreflectance of doped SiC substrate and typical numerical results are presented. The model considers the temporal response of the electron temperature and the number density of the electronic carriers. Calculated results show steep increase of electron temperature and the resulting increase of reflectivity. As a result, the reflected laser pulse by the substrate is compressed in time domain by means of the temporal response of the thermoreflectance characteristics of SiC substrate. Thermal analysis of the electrons reveals the interesting feature of the thermoreflectance response as a function of pulse intensity, pulse width, or doping concentration. The technique can be used for the compression of ultrashort pulse laser light.

This paper examines the thermodynamic and thermal transport properties of the 2D graphene lattice. The interatomic interactions are modeled using the Tersoff interatomic potential and are used to evaluate phonon dispersion curves, density of states and thermodynamic properties of graphene as functions of temperature. Perturbation theory is applied to calculate the transition probabilities for three-phonon scattering. The matrix elements of the perturbing Hamiltonian are calculated using the anharmonic interatomic force constants obtained from the interatomic potential as well. An algorithm to accurately quantify the contours of energy balance for three-phonon scattering events is presented and applied to calculate the net transition probability from a given phonon mode. Under the linear approximation, the Boltzmann transport equation (BTE) is applied to compute the thermal conductivity of graphene, giving spectral and polarization-resolved information. Predictions of thermal conductivity for a wide range of parameters elucidate the behavior of diffusive phonon transport. The complete spectral detail of selection rules, important phonon scattering pathways, and phonon relaxation times in graphene are provided, contrasting graphene with other materials, along with implications for graphene electronics. We also highlight the specific scattering processes that are important in Raman spectroscopy based measurements of graphene thermal conductivity, and provide a plausible explanation for the observed dependence on laser spot size.

Laser measurement and laser processing techniques have been gaining strong attention from various applications [1,2]. This research aims at the development of a fluidic laser beam shaper, and in order to fulfill the objective, characteristics of the thermal lens effect are studied. This phenomenon has the optical property of a concave lens since the refractive index distribution on the optical axis is formed when the liquid is irradiated. One reason for the refractive index distribution in the liquid is the temperature distribution in the liquid when it is irradiated. In this research, effects of the pump power and propagation distance of the probe beam to probe beam profile are investigated experimentally and theoretically, in order to develop fluidic laser beam shaper. It is indicated that, by controlling some parameters in thermal lens system as pump power (in the regime of linear optics) and absorption coefficient, input Gaussian beam can be converted into flat-top beam profile. The relationship among the distance to obtain a flat-top beam, pump power and absorption coefficient is investigated to show the flexibility of fluidic laser beam shaper in many fields of laser application.

Semiconductor Laser diodes that emit visible light have various interesting applications such as sensing, high density optical storage and projection displays. In any opto-electronic package, the laser diode chips are typically attached or soldered to metal or ceramic substrates that have good thermal conductivity and are well-matched in coefficient of thermal expansion using solder. Some applications require a critical alignment of the front facet of the laser diode to the front edge of the substrate onto which the laser diode chip is attached to. Depending on the application, the alignment precision could be varying from 20 μm to being as stringent as 0.5 to 1 μm. In many of these applications, the cost of packaging is also a very important factor. In such applications, it is essential to develop a laser diode chip bonding process that can meet such stringent die alignments along with a low cost manufacturing process. Therefore, the objective of this research work is to provide a low cost alternative solution for die attach process that can guarantee alignment precision of 0.5 to 1 microns and can be easily adapted to high volume manufacturing. The novel technique proposed in this work uses primarily gravity force for the facet alignments between the two components. In this passive-gravity assisted precision (P-GAP) assembly process, the laser diode (LD) chip is placed on the substrate using a traditional pick and place machine and later the substrate and the chip are tilted such that the chip slides on the substrate due to the gravity and touches a mechanical stop in-front of them. This does not involve any active alignment. In addition, we have provided few ideas to improve the sliding when gravity is used. This technique has been implemented on several samples and the feasibility of achieving the alignment precision to within a micron was demonstrated.

A novel laser ultrasound and interferometer inspection system has been successfully applied to detect solder joint defects including missing, misaligned, open, and cracked solder bumps in flip chips, land grid array packages and chip capacitors. This system uses a pulsed Nd:YAG laser to induce ultrasound in the chip packages in the thermoelastic regime; it then measures the transient out-of-plane displacement response on the package surface using a laser interferometer. The quality of solder bumps is evaluated by analyzing the transient responses. In this paper, the application of this system is expanded to evaluate quality of lead-free solder bumps in ball grid array (BGA) packages; specifically BGA packages with poor wetting are used as test vehicles. Poor wetting not only decreases the mechanical strength of interconnection at the interface between the solder bumps and substrate, but also increases electrical resistance, which is a reliability issue. Causes of poor wetting vary from materials themselves to manufacturing process. Here, poor wetting of solder bumps were intentionally created by using an improper reflow profile. The transient out-of-plane displacement responses from these packages were compared with the responses from defect-free samples. Solder bumps with poor wetting were distinguished from the normal solder bumps by unusual correlation coefficient. Then, laser ultrasound inspection results are also compared with results from X-ray inspection and continuity test. Finally, the cross-section images were used to further confirm the existence of the poor wetting in samples with unusual correlation coefficient. It can be concluded that this laser-ultrasound system is capable of identifying the presence of poor wetting in BGA packages.

We have recently reported the first ever demonstration of active cooling of hot-spots of >1 kW/cm 2 in a packaged electronic chip using thin-film superlattice thermoelectric cooler (TEC) cooling technology [1]. In this paper, we provide a detailed account of both experimental and theoretical aspects of this technological demonstration and progress. We have achieved cooling of as much as 15°C at a location on the chip where the heat-flux is as high as ∼1300 W/cm 2 , with the help of a thin-film TEC integrated into the package. To our knowledge, this is the first demonstration of high heat-flux cooling with a thin-film thermoelectric device made from superlattices when it is fully integrated into a usable electronic package. Our results, which validate the concept of site-specific micro-scale cooling of electronics in general, will have significant potential for thermal management of future generations of microprocessors. Similar active thermal management could also be relevant for high-performance solid-state lasers and power electronic chips.

Network Identification by Deconvolution (NID) method is applied to the analysis of the thermal transient pulsed laser heating. This is the excitation used in many optical experiments such as the Pump-Probe Transient Thermoreflectance experiment. NID method is based on linear RC network theory using Fourier’s law of heat conduction. This approach is used to extract the thermal time constant spectrum of the sample after excitation by either a step or pulsed heat source at one surface. Furthermore, using network theory mathematical transformations, the details of the heat flux path through the sample can be analyzed. This is done by introducing the cumulative and differential structure functions. We show that the conventional NID method can be modified to analyze transient laser heating experiments. The advantage is that the thermal resistance of the top material layers and the major interface thermal resistances can be extracted without the need of assuming a specific multilayer structure. Some of the limitations due to the finite thermal penetration depth of the transient heat pulse will be discussed.

In this paper, we show that improved air circulation above a heat sink is possible using thin winglet-type vortex generators that can be passively retrofitted to an existing unit. By mounting these vortex generators on the leading edge of heat sink fins, pairs of counter-rotating vortices are induced within the interfin spacing. The vortices disturb the boundary layers and serve to mix the air in the interfin channel. The devices we have designed are passive and can be added to existing systems using a simple clip-on mechanism. In this study, several designs are experimentally investigated for the purpose of identifying the optimal configuration that will be most conducive to flow enhancement and, therefore, heat transfer augmentation. Using the typical operational range of air velocities for PCs, routers and servers, an experimental simulation of the interfin channel reveals that certain vortex generators, when placed upstream, can outperform others in their ability to fill the channel with pairs of strong vortices. Multiple pairs can also be generated to further accentuate the heat transfer using dual vortex generators. A description of the specific shapes is furnished here along with particulars of the performance study. By control and manipulation of the vortices, our results suggest the possibility of optimizing the generator design. Experimentation was conducted in two phases. The first phase is a study of the ability to generate and control vortices within the fin channel. This aspect was simulated using a Lexan mock-up of the fin channel that permits introduction of glycerin smoke to visualize the shape, size, strength and structure of the vortices. The clear Lexan permitted viewing of the vortices by passing a red planar laser through the apparatus. The second phase involved using the optimization data gained in the first phase to generate vortices in an actual heat sink fitted with thermocouples to measure the temperatures at various points during heating.

This report describes the quality assessment of Blind Via Holes (BVHs) of Printed Wiring Boards (PWBs) drilled by a CO 2 laser using Cu-direct drilling. In the Cu-direct drilling method, the copper foil and the build-up layer are melted at the same time, and the surface is treated to increase the laser energy absorbed by the copper foil since an untreated copper surface reflects most of the 10.6-μm-wavelength CO 2 laser beam. However, there are few reports dealing with Cu-direct laser drilling of PWBs. In addition, when copper and resin with different processing thresholds are drilled at the same time, occurrences of a defect called overhang have been observed. So, in this report, first we propose a new method using thermography to measure the absorptance of a PWB surface for a CO 2 laser. Moreover, we investigate how surface treatment of the outer copper foil influences the quality of a laser-drilled hole. Then, we observe the circumference of a point irradiated with the CO 2 laser and explain how melting processes are different from surface treatment. Finally, based on the research we establish a method in order to cut down the overhang length as a parameter of drilled-hole quality. We also show that a high absorptance improves BVH quality.