High Aspect Ratio Micro-holes Research Articles

Rapid and High Aspect Ratio Micro-hole Drilling With Multiple Micro-Second Pulses Using a CW Single-Mode Fiber Laser

AbstractLaser drilling is an important industrial process for the production of various sizes of holes. In this paper, we investigate rapid, high aspect ratio microhole drilling using multiple microsecond pulses based on the single pulse drilling technique reported in [17, 18]. It was established that there would be a synergistic effect if a subsequent pulse is irradiated at the target within 100 μs of the previous pulse before the melt solidifies. However, the peak power values of subsequent pulses decrease with higher repetition rates. The results show that the synergistic effect could outweigh the reduction in laser power. Another contributing factor of the synergistic effect is related to the melt ejection efficiency. As the hole deepens, the melt ejection becomes less effective to eject the melt completely out of the hole, resulting in a partially blocked hole. A subsequent laser pulse needs to reopen the hole before the hole can be deepened further. To overcome this hole blocking problem, shooting a subsequent pulse at a higher repetition rate also ensures that the energy absorption is more efficient when a subsequent laser pulse is irradiating at the hole blocking melt which is not yet solidified. This multiple-pulse drilling techniquewas applied for through-hole drilling. It was found that the total drilling times through an 800 μm plate were found to be 634 ms and 21.9 ms at 13 kHz and 20 kHz, respectively. The drilling efficiency at the 20 kHz repetition rate is drastically higher, needing only 428 shots, compared with 8240 shots at the 13 kHz, an improvement of nearly 200 times. It is confirmed that this multiple-pulse drilling technique with microsecond pulses using a 300Wsingle mode fiber laser is a viable technique to produce high aspect ratio through holes with a simple and robust setup for the production environment.

High aspect ratio micro-hole filling employing emulsified supercritical CO2 electrolytes

The feasibility of filling a high aspect-ratio micro-hole with electrodeposited NiP alloy by employing an emulsified supercritical CO2 (sc-CO2) bath was studied. The effects of current density and deposition time on the efficiency of micro-hole filling were also explored. The experimental results showed that void-free NiP alloy could be successfully filled into a micro-hole with a diameter of 50μm and a depth of 400μm under proper electrodeposition conditions, with the advantage of low surface tension characteristics of sc-CO2 in wetting the micro-hole over that of the conventional aqueous bath. The experimental results also showed that the deposition efficiency and the deposit morphology were significantly affected by the deposition time and current density applied. Material characterization indicated that the super-filled NiP exhibited a uniform chemical composition, and an average microhardness value of 730Hv.

A mathematical model to predict overcut during electrochemical discharge machining

Overcut is a major reason for the dimensional deviations, especially during high aspect ratio micromachining in electrochemical discharge machining (ECDM). Since electrolyte concentration plays a vital role in material removal, by understanding the effect of electrolyte concentration on the machined feature, it is possible to reduce the overcut associated with the ECDM process. This reduction in overcut was used in the study to drill high aspect ratio microholes in glass. The effect of concentration on the overcut is explained with an analytical model. The model considers the thermal effects on material removal for ECDM assuming a high-temperature chemical etching mechanism for the material removal. It describes the effect of electrolyte concentration as well as machining time on material removal. Results reveal that at the microscale, tool diameter has minimal effect on overcut for a given machining voltage. Microholes with aspect ratios as high as 12 were produced in glass in this study.

Bioinspired active whisker sensor for geometry detection of high aspect ratio microholes with simultaneous actuation and sensing capability

This paper presents a whisker transducer (WT)—inspired by a rat’s vibrissal tactile location perception—that can be used to detect the geometry of high aspect ratio microholes. The WT acts as both the actuator that generates whisking movement and the sensor that detects the mechanical impedance at its tip. This unique, simultaneous actuation-and-sensing capability is achieved through a 2 × 2 transduction matrix model that characterizes both the forward actuation and backward sensing functions of the transducer. The WT imitates a rat’s vibrissal location perception when driven by a special signal with a low-frequency component and a high-frequency component. The low-frequency component plays the role of the ‘whisking signal,’ encoding the whisker position over time, and the high-frequency component plays the role of the ‘touch signal,’ encoding the onset time of mechanical contact. The combination of these two signals allows the WT to detect the location of an object. The proposed transducer and location detection algorithm are validated using a miniature prototype fabricated through micro-EDM process. The achieved geometry measurement capability is of high linearity (R2 > 0.99) and low measurement uncertainty (200 nm). Its potential application in detecting the taper of high aspect ratio microholes is also demonstrated.

High aspect ratio, high-quality microholes in PMMA: a comparison between femtosecond laser drilling in air and in vacuum

Microholes, especially high aspect ratio, high-quality microholes with small diameters (<100 μm), have broad applications. However, it is very difficult for traditional drilling methods to obtain deep microholes, especially with aspect ratios of over 50:1. Femtosecond lasers provide a promising solution for efficient drilling of deep microholes with high-precision material removal, reduced recast/microcracks, minimized heat-affected zones, and the absence of plasma-shielding effects. In this work, a comparison study of high aspect ratio, high-quality microholes fabricated in a poly(methyl methacrylate) (PMMA) bulk substrate with the ambient pressure adjusted from 105 Pa (air) down to 1 Pa (vacuum) is presented. High aspect ratio (over 100:1) microholes were obtained in a vacuum environment. The contrast between microhole evolution in air and in vacuum was investigated. The results indicate that efficient energy propagation and easy ejection of ablated material/plasma are probably the most important benefits of drilling microholes in vacuum. The dependence of microhole shapes on different fabrication parameters, including pulse energy and ambient pressure, was investigated to quantitatively reveal the underlying mechanisms. The enhanced drilling effect in vacuum was only found in a high pulse energy region (E p > 20 μJ), and it becomes saturated when the ambient pressure was reduced to ~102 Pa at a pulse energy of 50 μJ. Drilling microholes in a vacuum provides a simple and effective way of rapidly fabricating high aspect ratio, high-quality microholes.

Development of a double fiber probe with a single fiber Bragg grating for dimensional measurement of microholes with high aspect ratios.

A fiber Bragg grating (FBG)-based probe is developed by designing a novel double fiber structure to make the probe sensitive to both axial and radial contact displacements. A matched FBG pair interrogation system for the probe is modeled, and the best matched initial condition is found to make the probing system work in the linear and most sensitive mode. Actual measurements of ring gauges and fuel injection nozzles indicate that, for a microhole with an aspect ratio of greater than 14∶1, an axial resolution of 8nm and a radial resolution of 30nm can be achieved with the developed probe. It is therefore concluded that the double fiber probe with a single FBG developed can be used to measure high aspect ratio microholes.

3-D Solar Cells Based on Radial Silicon Heterojunctions Exploiting Microhole Lattices

In this letter, 3-D solar cells based on radial amorphous/crystalline silicon heterojunctions (SHJs), which are formed on 2-D lattices of vertical high aspect-ratio microholes (MHs) etched in crystalline silicon (c-Si) substrates, are realized with conversion efficiency of ~ 3%. Silicon high aspect-ratio MH lattices, with hole diameter of a few micrometers and hole depth of tens micrometers, are produced on n-type c-Si substrates by electrochemical micromachining (ECM) technology with low reflectance ( depending on diameter and depth) in the wavelength range between 400 and 900 nm, which is commonly exploited for solar cell application. The radial SHJ solar cells are obtained by successive quasi-conformal deposition of hydrogenated amorphous silicon (a-Si:H) i-layer and p-layer, respectively, via plasma-enhanced chemical vapor deposition at 200°C, within the MH lattices, and subsequent indium tin oxide and Ag deposition for the formation of electrical front and back contacts, respectively. Experimental conversion efficiencies up to 2.72% are obtained under standard AM1.5 illumination with intensity of 100 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , thus envisaging that ECM technology can be successfully adopted to realize efficient 3-D solar cells based on radial p-n junctions exploiting high aspect-ratio MH lattices, in particular, or other microstructures, in general.

Process Anatomy for High Aspect Ratio Micro-Hole Drilling with Short Micro-Second Pulses Using a CW Single-Mode Fiber

The objective of this paper is to establish a detailed process anatomy of a micro-hole drilling process using a CW single-mode 300 W fiber laser with short micro-second pulses. For the drilling process with a one micro-second laser pulse, the process started with drastic evaporation and concluded with melt ejection approximately 150 micro-seconds later. It was concluded that two distinct hole drilling mechanisms occurred, one as adiabatic evaporation by the high power initial spike of the laser beam and one as melt ejection by the low power laser energy deposition. The drilling process time line was established based on the measurements by several in-process sensors, such as photodiodes, a high-speed camera, and a spectrometer. The theoretical modeling zoomed in the early stage of the process to investigate the hole formation which could not be observed experimentally. Both experimental and theoretical results were then compared to determine the laser-material interaction mechanisms among three media involved in the process: laser, material, and vapour/plasma. Finally, a series of temporal anatomy diagrams, denoted as process anatomy, were presented to describe the entire drilling process, including process temperature, laser energy deposition, hole formation, and material removal mechanisms.

Investigation on Fabricating High Aspect Ratio Microholes on Silicon by FIB/SEM Milling

The aspect ratio of microholes milled on silicon by FIB/SEM milling was investigated with various beam currents and initial depths of mill. Lower beam current gives finer surface and more accurate structure. The results showed that the highest depth of microhole fabricated using FIB/SEM milling was 17.45µm, and maximum aspect ratio 1:8, without using gas injection system (GIS). The value of depth obtained was less than the initial depth of mill due to re-deposition, influenced by low sputter yield of silicon. Milling time and dwell time play important roles in milling process to get high aspect ratio of microholes.

Numerical Simulation of High-Aspect-Ratio Micro-Hole Electroforming with External Magnetic Field

In this paper, the influences of applied magnetic field on flow state during electroforming of the high-aspect-ratio (HAR) blind micro-hole were numerically analyzed using the Fluent software. The results showed that, when microelectroforming of nickel without external agitation, three vortexes could form due to the magnetohydrodynamic (MHD) effect within the HAR micro-hole with magnetic field in parallel to cathode-electrode surface, and the flow rate in the micro-hole increased with the increase of the magnetic field and current density. The MHD effect helped to enhance mass transfer during the microelectroforming of HAR microstructures.

High aspect ratio micro-hole drilling aided with ultrasonic vibration and planetary movement of electrode by micro-EDM

When a micro-hole is drilled deeply by EDM, the viscous resistance in the narrow discharge gap causes difficulty in the removal of debris and bubbles from the working area, leading to frequent occurrences of abnormal discharges and resulting in extensive electrode wear. This paper presents a new method of drilling high aspect ratio micro-holes by EDM, in which the planetary movement of an electrode, with enhancement from ultrasonic vibration, provides an unevenly distributed gap for the debris and bubbles to escape from the discharge zone easily. Micro-holes with aspect ratio of 29 have been drilled.

Fabrication of High-Aspect Ratio Micro Holes on Hard Brittle Materials -Study on Electrorheological Fluid-Assisted Micro Ultrasonic Machining-

Ultrasonic machining (USM) is an effective method for machining of hard brittle materials. In this process, the slurry is supplied to the gap between the workpiece and the ultrasonic vibrating tool, and the materials are removed by the impacts of the abrasive grains that are pressurized by an ultrasonic vibrating tool. The purpose of this research is to achieve precise and efficient microfabrication on hard brittle materials by USM. However, in the case of microfabrication, chipping which is generally observed around the edges of machined micro holes and grooves, deteriorates the machining accuracy. In addition, there is another problem in that the machining efficiency decreases with the progress of the machining. Electrorheological fluid-assisted USM has been proposed as a countermeasure to these problems. In the present study, the problems and countermeasures associated with the machining of high-aspect ratio micro holes in hard brittle materials by electrorheological fluid-assisted USM are investigated. By positioning an auxiliary electrode under the workpiece, it becomes possible to keep the electric field high even when the machining depth becomes large. As a result, high-precision and high-aspect ratio micro holes can be machined on hard brittle materials.

Resonant-type Micro-probe for Vertical Profiler

We present a characterization method, which is dedicated to the measurement of the inner dimensions of high aspect ratio micro-holes, typically 100μm in diameter and 1mm in depth. Micro-probes with an integrated forcesensitive element are manufactured in silicon technology. Vertical surface profiles can be obtained through the measurement of the deflection at the end of a cantilever beam due to contact force, while scanning over the inner surface of the hole. Two alternative solutions have been developed for force sensing. The first one is based on piezoresistive detection and the other one is based on frequency shift of mechanical resonators. In this paper, the latter sensing method is presented more in detail. The operation principle, modeling and fabrication process are presented.

Supercritical CO2-Pulse Cleaning in Deep Microholes

A novel supercritical cleaning process was proposed for removing contamination in high aspect ratio trenches and microholes of highly integrated semiconductor devices. The supercritical CO2-pulse cleaning with the periodic pressure swing of supercritical fluid between subcritical and supercritical conditions was conducted for removing particles in microholes of fabricated model structures. The microhole depth is 2.0 μm with microhole sizes ranging from 0.2 μm to 1.6 μm. The effects of microhole size on particle removal were investigated by means of non-destructive observation of high aspect ratio model structures with an optical microscope and SEM. The supercritical CO2-pulse cleaning efficiently removed more than 50 nm fine particles from the deep microholes owing to its drastic changes in density near the critical point by depressurization of the supercritical fluid.

Liquid Infiltration Mechanism for Cleaning in Deep Microholes

Liquid infiltration mechanism for cleaning high aspect ratio trenches and microholes of highly integrated semiconductor devices was considered by theoretical and experimental methods. It is theoretically predicted that liquid does not infiltrate into microhole with non-wettable liquid-solid contact surface. Increasing liquid pressure and/or decreasing inside gas pressure overcome the surface tension to infiltrate liquid into non-wettable microholes. On the other hand liquid can infiltrate into wettable microholes independent of microhole size. These theoretical predictions were confirmed by using non-destructive observation of high aspect ratio model structures.

Supercritical CO&lt;sub&gt;2&lt;/sub&gt;-Pulse Cleaning in Deep Microholes

A novel supercritical cleaning process was proposed for removing contamination in high aspect ratio trenches and microholes of highly integrated semiconductor devices. The supercritical CO2-pulse cleaning with the periodic pressure swing of supercritical fluid between subcritical and supercritical conditions was conducted for removing particles in microholes. The effects of microhole size on particle removal were investigated by means of non-destructive observation of high aspect ratio model structures with an optical microscope and SEM.

Drilling Microholes in Hot Tool Steel by Using Micro-Electro Discharge Machining

This study presents an investigation into the drilling of a deep microholes with the depth of 320 μm in tool steel SKD61 by the Micro-EDM process. The electrode with the diameter of 26 μm is machined by the method of wire electrodischarge grinding (WEDG). Optical microscopy, scanning electron microscopy, and confocal laser scanning microscopy techniques are used to determine the influence of the process parameters upon hole enlargement, electrode wear rate, material removal rate, wear ratio, and the observed surface topography. The results of the study reveal the optimum parameter settings for the Micro-EDM machining of a high aspect ratio microhole are as follows: (1) a pulse voltage between 60 and 100 V, (2) a capacitance between 80 pF and 220 pF. Finally when the depth exceeds 200 μm, the shape of the micro hole almost becomes tapered due to the corner wear of the electrode and the secondary discharge along the side of the hole.

306 EDMによる高アスペクト比微細深孔加工に関する研究(精密/微細加工と評価)

306 EDMによる高アスペクト比微細深孔加工に関する研究(精密/微細加工と評価)

Precision glass machining, drilling and profile cutting by short pulse lasers

Demands for producing micro-crack free, high quality and high-aspect ratio microholes and microfeatures in glass substrates have been increasing for a number of applications such as in MEMS device packaging, optical fibre alignment, mini-vision systems and microelectronic packaging. However, due to the poor thermal properties of most glasses the fabrication of finely machined features, e.g. grooves, microholes etc. has been a challenging task. In this study, short pulse solid-state lasers with pulse duration in the ns to fs range were used to process different types of glass materials. The effect of the pulse duration and other process parameters on the machined features was analyzed to reveal the underlying thermal effects and nonlinear processes. Edge quality, circularity, aspect ratio, formation of the redeposit material and machining rate were also studied with respect to the process variables such as focusing optics, laser power, wavelength and repetition rate. Arrays of drilled micro-hole patterns and fabricated microfeatures are demonstrated with discussions on their potential applications.