Microhole Sidewall Research Articles

Precession laser optimized for micro-drilling on aerospace material

Parameters identified to reduce and eliminate micro-hole sidewall tapering in nickel alloy C263.

Extensive Broadband Near-Infrared Emissions from GexSi1-x Alloys on Micro-Hole Patterned Si(001) Substrates.

Broadband near-infrared (NIR) luminescent materials have been continuously pursued as promising candidates for optoelectronic devices crucial for wide applications in night vision, environment monitoring, biological imaging, etc. Here, graded GexSi1−x (x = 0.1–0.3) alloys are grown on micro-hole patterned Si(001) substrates. Barn-like islands and branch-like nanostructures appear at regions in-between micro-holes and the sidewalls of micro-holes, respectively. The former is driven by the efficient strain relation. The latter is induced by the dislocations originating from defects at sidewalls after etching. An extensive broadband photoluminescence (PL) spectrum is observed in the NIR wavelength range of 1200–2200 nm. Moreover, the integrated intensity of the PL can be enhanced by over six times in comparison with that from the reference sample on a flat substrate. Such an extensively broad and strong PL spectrum is attributed to the coupling between the emissions of GeSi alloys and the guided resonant modes in ordered micro-holes and the strain-enhanced decomposition of alloys during growth on the micro-hole patterned substrate. These results demonstrate that the graded GexSi1−x alloys on micro-hole pattered Si substrates may have great potential for the development of innovative broadband NIR optoelectronic devices, particularly to realize entire systems on a Si chip.

Effects of ultrasonic assistance on microhole drilling based on Nd:YAG laser trepanning

The ultrasonic vibration was introduced to assist the Nd:YAG laser trepan drilling for improving drilling quality/performance. Effects of ultrasonic power and pulse energy on geometrical and/or metallurgical quality were deciphered for the ultrasonic-assisted Nd:YAG laser trepanning. The energy dispersive spectroscopy analysis was also carried out to reveal the influence of ultrasonic vibration on the elemental compositions of the areas surrounding the drilled microhole entrance. It was shown that the axial ultrasonic vibration coupled into the Nd:YAG laser trepanning process could effectively improve the hole entrance profile/morphology with less metallurgical defects such as microcracks. Moreover, relatively high ultrasonic power was more beneficial to the improvement of laser trepan drilling quality in terms of smaller hole taper angle, thinner recast layer, and less cracks, etc. However, the oxidation of the microhole sidewall could not be effectively improved by using the ultrasonic assistance.

LIPSS formed on the sidewalls of microholes in stainless steel trepanned by a circularly polarized femtosecond laser

In order to take advantage of microhole fluidynamics, laser-induced periodic surface structures (LIPSS, ripples) orientation should offer the lowest angle γ as possible with respect to hole axis. Investigations have been performed to explore the morphology of LIPSS formed on the sidewalls of microholes by circularly polarized femtosecond laser trepanning. The period of LIPSS on average was smaller than laser wavelength. The energy density of laser beam generally affected the processing effect. Experiments showed that the angle of the LIPSS decreases with increasing single pulse energy. However, increasing trepanning speed led to a decreasing in LIPSS angle.

Effects of microhole sidewall confinement on bubble growth and bubble-generated shock waves

The evolution of bubbles in water may be a critical process in many technologies or applications, including several manufacturing processes. Despite the previous work on cavitations and bubbles, the prior investigations in the literature are not sufficient about the effect of a micro-scale structure (such as a microhole sidewall) confinement on the bubble evolution and on the bubble-generated shock waves. In this paper, this effect has been studied using a physics-based model, which has been verified by comparing its predictions with experimental measurements in the literature on bubble evolutions in water without a micro-scale structure confinement. Under the investigated conditions, it has been found that due to the reflection of the shock waves by the microhole sidewall and the interactions among the bubble and the reflected waves, the peak pressure on the hole bottom wall surface has been significantly enhanced. It is good work in the future to study the implications of the discovery on related manufacturing processes and other applications, and how to intentionally utilize the pressure enhancement effect to benefit the related applications and manufacturing processes.

Nanosecond laser pulse interactions with breakdown plasma in gas medium confined in a microhole

The previous investigations on nanosecond laser pulse interactions with breakdown plasma in a gas medium confined in a microhole have been limited. This kind of plasma has been studied in this paper. Due to the significant measurement difficulty resulted from the very small spatial and temporal scales involved, a physics-based computational model has been employed as the investigation tool. The model is developed by solving gas dynamic equations numerically using the finite difference method based on an essentially non-oscillatory scheme. The gas dynamic equations are coupled with suitable equation of state, where the electron number density for plasma region is calculated through the Saha equation. Using the model, the spatial confinement effects of the microhole sidewall on the plasma evolution under laser radiation have been investigated. It has been found that under the studied conditions the hole sidewall confinement can greatly enhance the plasma temperature, pressure, and thrust (over the same surface area). The enhancement should be due to the sidewall’s restriction on the plasma lateral expansion and the sidewall’s reflection of the pressure wave induced by plasma. This study implies potential advantages of the breakdown plasma confined in a microhole in many relevant applications, such as laser propulsion and laser-induced breakdown spectroscopy. The developed model also provides a useful guiding tool for future fundamental research and practical applications in many areas related to laser interactions with gas breakdown plasma.

Laser-induced periodic surface structures formed on the sidewalls of microholes trepanned by a femtosecond laser

Laser-induced periodic surface structures (LIPSSs) were observed on the sidewalls of 300-μm-diameter holes trepanned on cemented tungsten carbide using femtosecond laser pulses at a wavelength of 800 nm. For a circularly polarized beam, LIPSSs were formed at a period of 300 nm and oriented perpendicularly to the plane of incidence on the sidewalls. For a linearly polarized beam, LIPSS formation was dependent on the relative angle α between the polarization direction and the plane of incidence. For relative angles α from 0° to 70° and from 110° to 180°, LIPSS spacing was 300 nm. However, there were two types of LIPSSs coexisting from 70° to 110°. One had a spacing of 120 nm and the other had a spacing that varied from 500 to 760 nm. It was found that the orientation angle of LIPSSs measured between the LIPSS orientation and the plane of incidence had a nonlinear dependence on α. To understand this dependence, a model was proposed in which LIPSSs are assumed to align perpendicularly to the direction of the absorbed electric field lying in the tangent plane of the sidewall of a drilled hole. The calculated results from this model showed good agreement with the experimental results.

A comparative study of the interaction between microhole sidewall and the plasma generated by nanosecond and femtosecond laser ablation of deep microholes

High-aspect-ratio microholes have many industrial applications, but are difficult to produce. Femtosecond (fs) and nanosecond (ns) laser ablation may produce potential manufacturing solutions. However, the laser-induced plasma–microhole sidewall interaction has not been well understood for laser ablation of deep microholes, which may significantly affect the hole size and/or quality. This interaction has been investigated in this paper. Due to the huge challenges involved in direct experimental observations, a physics-based model is applied as the research tool, which has been verified by measurements from literatures on laser ablation of flat targets (without deep microholes) that are relatively easy to perform. The study shows that under the same laser pulse fluence, the fs laser-induced plasma generates larger transient peak heat flux to the sidewall than the ns laser pulse. However, the high-heat-flux region moves up very rapidly in the hole, and hence the sidewall temperature is not significantly raised and sidewall melting does not occur under the studied conditions. On the other hand, for the ns laser pulse, the induced plasma maintains a relatively high heat flux to the sidewall near the hole bottom for a much longer time, which yields obvious sidewall melting and surface vaporization. The results are consistent with the previously observed sidewall surface morphology for microholes drilled by fs and ns laser pulses in air with the same pulse energy.

The Interactions of Microhole Sidewall With Plasma induced by Femtosecond Laser Ablation in High-Aspect-Ratio Microholes

High-aspect-ratio microholes have many important applications, but their drilling is very challenging. Femtosecond (fs) laser ablation provides a potential solution, but involves many complicated physical processes that have not been well understood, which have hindered its practical application. One of these is that the plasma induced by laser ablation at the hole bottom will transfer some of its energy to the hole sidewall as it expands in the microhole. The plasma–sidewall interaction has been rarely studied in literature, and it is still not clear if or not the energy transferred from the plasma is sufficient to cause significant material removal from the sidewall. Direct time-resolved observations are extremely difficult due to the small temporal/spatial scales and the spatial constraint inside the hole, while the sidewall characterization after laser ablation is difficult to distinguish between the possible material removal due to plasma energy transfer and that due to direct laser energy absorption by the sidewall. In this paper, a physics-based model is applied as the investigation tool to study the plasma–sidewall interaction in fs laser drilling of high-aspect-ratio microholes. It has been found that for the studied conditions the energy transferred from the plasma is not sufficient to cause significant material removal from the sidewall through any thermally induced phase change process.

Characterization of InGaN/GaN light-emitting diodes with micro-hole arrayed indium-tin-oxide layer

We demonstrate that InGaN/GaN multiple quantum well light-emitting diodes (LEDs) with micro-hole arrayed indium-tin-oxide layers exhibit better performance and optoelectrical properties than do conventional LEDs. Under 20 mA injection current operation the room-temperature output power conversion efficiency and external quantum efficiency obtained by employing a micro-hole array on the top surface of the LED structure could be increased by 28.7% and 14.3%, respectively, over that of conventional broad area devices. The room temperature current–voltage characteristics of the LEDs showed the series resistance and leakage current to be related to the hole dimensions and etching depth, respectively. Interestingly, the leakage current of the transparent conductive layer was dominated by the contribution of the micro-hole side-wall, the number of etched micro-holes, and the wet-etching depth. We conclude that a well-designed micro-hole array structure fabricated using the wet etching process can indeed, not only significantly inhibit the leakage current of the indium-tin-oxide transparent conductive layer, but also enhance the external quantum efficiency and extraction efficiency over a broad temperature range.

Experimental study of fabricating a microball tip on an optical fibre

Many side walls of microholes and grooves are not easily measured by current optical or non-contact measuring instruments. Microcontact probes are increasingly demanded in the market. A good ball tip is the basic element for the construction of a contact probe. This paper proposes a low-cost and in-process system to fabricate a microspherical tip on an optical fibre using a commercial fibre fusion splicer. Based on the principles of arc discharging energy absorption and the surface tension phenomenon, a microsphere is formed at the tip of the optical fibre. Experimental results showed that with the selection of proper process parameters, such as the arc power, cleaning arc power offset, and cleaning time, a spherical tip about 300 µm in diameter and with 6 µm roundness error could be produced using a 125 µm diameter single-mode optical fibre. The offset distance between the ball centre and the fibre stylus central line due to the gravity effect could be suppressed to less than 3 µm by rotating the fibre between the discharging cycles. By forming the sphere probe tip directly on an optical fibre, this approach demonstrates an easy and in-process dimensional controlled method to shorten the manufacturing lead-time for making a 3D microprobe. The microprobe can be used for microscale/nanoscale coordinate measuring machines (CMMs) to enhance the measurement resolution and extend the capability for meso- to micro-objects.

The glucose sensor integratable in the microchannel

In this paper, we propose a new structure for a glucose sensor fabricated by micromachining techniques with the working electrode separated from the enzyme membrane. In this sensor, an array of microholes is etched through a silicon membrane. On the sidewalls of these microholes, a working electrode is formed by platinum sputtering. On the top of the microholes, a membrane of glucose oxidase (GOD) is deposited. Owing to the separation of the working electrode and the enzyme membrane, standard MEMS processes can be carried out as the front-end processes during the fabrication. The enzyme membrane will be deposited as the last process step. This avoids the problem of incompatibility between MEMS processes and biomaterials. In this way, the glucose sensor can be easily integrated in the microchannel. The diffusion of hydrogen peroxide in the microholes was simulated with different ratios of depth and diameter. It is shown that when the ratio of diameter and depth of the microholes is 1, most of the produced hydrogen peroxide can be oxidized at the working electrode on the sidewalls of microholes. Laser drilling and magnetron RIE were exploited, respectively, to fabricate the array of microholes. The responses of both the stand-alone sensor and the sensor integrated in the microchannel are shown.