High Aspect Ratio Holes Research Articles

The Capabilities of Electrodischarge Microdrilling of High Aspect Ratio Holes in Ceramic Materials

Abstract In the first part of the article the review of ceramic materials drilling possibilities was presented. Among the described methods special attention is paid to electrodischarge drilling. This process have especially been predicted for machining difficult-to-cut electrically conductive materials. The second part consist of the results analysis of electrodischarge microdrilling of siliconized silicon carbide. The experiment involves the impact of current amplitude, discharge voltage and pulse time on the hole depth, side gap, linear tool wear and mean drilling speed. The results shows that electrodischarge drilling is a good alternative when machining inhomogeneous ceramic materials and gives possibility to drill high aspect ratio holes with relatively high efficiency (the drilling speed >2 mm/min).

Burst train generator of high energy femtosecond laser pulses for driving heat accumulation effect during micromachining.

A new method for generating high-repetition-rate (12.7-38.2MHz) burst trains of femtosecond laser pulses has been demonstrated for the purpose of tailoring ultrashort laser interactions in material processing that can harness the heat accumulation effect among pulses separated by a short interval (i.e., 26ns). Computer-controlled time delays were applied to synchronously trigger the high frequency switching of a high voltage Pockels cell to specify distinctive values of polarization rotation for each round-trip of a laser pulse cycling within a passive resonator. Polarization dependent output coupling facilitated the flexible shaping of the burst envelope profile to provide burst trains of up to ∼1 mJ of burst energy divided over a selectable number (1 to 25) of pulses. Individual pulses of variable energy up to 150μJ and with pulse duration tunable over 70fs to 2ps, were applied in burst trains to generate deep and high aspect ratio holes that could not form with low-repetition-rate laser pulses.

Oblique Drilling by Ti:sapphire fs Laser in Silicon

Femtosecond laser ablation results in sharp edges and high aspect ratio holes in case of most solid materials. In this work it is shown that oblique drilling is possible by oblique illumination even in high thermal conductivity materials. Experiments were performed on silicon by varying the number and the angle of the incident laser pulses of a Ti:sapphire laser. Three phases of the formation of the oblique holes were distinguished and described: 1. A cone-like hole is formed in the direction of laser irradiation. 2. The bottom of the hole is widened. 3. The shape of the ablated hole becomes tube-like. It is shown, that reflections from the inner walls have an influence on the shape of the holes. Two models were set up to calculate energy distribution in conical and tube-like holes to describe the hole formation.

Laser drilling of micro-hole arrays in tantalum

X ray collimator optics for space application require an array of high aspect ratio holes of 60:1 with a minimal tantalum (Ta) thickness of ≥2 mm and a very high open area fraction (hole versus wall fraction) of 70% to achieve high collimator efficiency. Each collimator with a drilled area of 110 mm × 70 mm contains several million holes and need a fast drilling process. Laser percussion drilling was performed using an IR pulsed disk laser in a 1 and 2 mm thick Ta plate. A tightly spaced hexagonal closed packed pattern was used to maximize open area fraction with hole-to-hole spacing as small as 80 μm. However, this resulted in a high concentration of debris and a thick recast layer on the remaining walls between the holes. Different process gases were investigated to minimize debris formation and reduce the recast layer thickness. Ramping of pulse energy during the drill cycle was investigated to minimize the adhesion between the substrate and recast layer. Chemical etching was used to remove the debris and recast from the top surface and the inside of the laser-drilled holes. Hole cross sections showed that a high aspect ratio was achieved with a hole diameter of about Ø50 μm in 2 mm thick Ta. To achieve the shortest drilling time of 200 ms per hole, the process parameters were optimized and a hybrid nozzle, with both horizontal and vertical gas flow, was developed and implemented.

J2240104 表面拡散による形態変化を利用したシリコン基板中への多層空洞の形成([J224-01]『マイクロ・ナノ機械の信頼性』のための材料創製・測定・解析技術 ナノ構造加工・プロセス)

We have investigated the formation of multi-layer void structures in Si(001) substrates, applying spontaneous shape changes of high-aspect ratio hole patterns by high temperature annealing. Multi-layer void structures are formed via closure of the hole opening, pinch-off of the vertically elongated closed voids, and coalescence of laterally arranged voids. We have shown that the observe shape transformation is interpreted as surface-diffusion-driven shape evolution, performing numerical simulations based on continuum theory.

Rapid micromachining of high aspect ratio holes in fused silica glass by high repetition rate picosecond laser

We present multiple methods of high aspect ratio hole drilling in fused silica glass, taking advantage of high power and high repetition rate picosecond lasers and flexible beam delivery methods to excise deep holes with minimal collateral damage. Combinations of static and synchronous scanning of laser focus were explored over a range of laser repetition rates and burst-train profiles that dramatically vary laser plume interaction dynamics, heat-affected zone, and heat accumulation physics. Chemically assisted etching of picosecond laser modification tracks are also presented as an extension from femtosecond laser writing of volume nanograting to form high aspect ratio (77) channels. Processing windows are identified for the various beam delivery methods that optimize the laser exposure over energy, wavelength, and repetition rate to reduce microcracking and deleterious heating effects. The results show the benefits of femtosecond laser interactions in glass extend into the picosecond domain, where the attributes of higher power further yield wide processing windows and significantly faster fabrication speed. High aspect ratio holes of 400 μm depth were formed over widely varying rates of 333 holes per second for mildly cracked holes in static-focal positioning through to one hole per second for low-damage and taper free holes in synchronous scanning.

Shape evolution of high aspect ratio holes on Si(001) during hydrogen annealing

We study the dynamics of void formation through the shape evolution of high-aspect-ratio cylindrical holes in Si(001) substrates under hydrogen annealing. We compare the observed evolution of these holes with numerical simulations based on the continuum theory for surface-diffusion-driven shape evolution. We find that a strong morphological instability arises near the hole opening, regardless of the presence of anisotropy in surface energy. The observed shape evolution of high-aspect-ratio holes during hydrogen annealing is understood as a surface-diffusion-driven evolution subject to the stability of the facets which form the vertical sidewall.

Internal characterization and hole formation mechanism in the laser percussion drilling process

Laser percussion drilling is increasing its relevance in many industrial applications, being used particularly in the aircraft industry in performing the micro-holes in nickel based alloys turbine blades for cooling, or stainless steel medical components drilling, which require small holes size and quality. Laser percussion drilling process presents extremely high speed for high aspect ratio holes. Moreover, the quality and accuracy of the holes can be excellent if the optimal parameters are set.The laser percussion drilling process is usually performed with specific equipment, including lasers that achieve high peak powers of picoseconds duration. These systems are usually dedicated exclusively to laser drilling operation. It is also very common to perform this process using the parameters suggested by the manufacturer of the equipment and without any consideration about the mechanism of formation of the hole. On the other hand, laser percussion drilling is performed by a sequence of pulses on the part surface. Each pulse removes a certain amount of material. The energy and duration of pulses set the amount of removed material by each one.This work discusses the mechanisms of formation of the holes in the laser percussion drilling process of an AISI 304 plate, evaluating the removed material volume in each laser pulse and obtaining the evolution of the hole geometry for the complete pulse sequence. In addition, this experimental analysis has been apply also for the development of a numerical model that can simulate the resulting hole geometry for different pulse sequences.

3D Measurement of TSVs Using Low Numerical Aperture White-Light Scanning Interferometry

We have proposed and demonstrated a low numerical aperture technique to measure the depth of through silicon vias (TSVs) using white-light scanning interferometry. The high aspect ratio hole like TSV's was considered to be impossible to measure using conventional optical methods due to low visibility at the bottom of the hole. We assumed that the limitation of the measurement was caused by reflection attenuation in TSVs. A novel interference theory which takes the structural reflection attenuation into consideration was proposed and simulated. As a result, we figured out that the low visibility in the interference signal was caused by the unbalanced light intensity between the object and the reference mirror. Unbalanced light can be balanced using an aperture at the illumination optics. As a result of simulation and experiment, we figured out that the interference signal can be enhanced using the proposed technique. With the proposed optics, the depth of TSVs having an aspect ratio of 11.2 was measured in 5 seconds. The proposed method is expected to be an alternative method for 3-D inspection of TSVs.

Study on Extending Tool Life of Micro WC Drills by Various Protective Coatings

As the trend of electronic industry is fast moving towards miniaturization, diversity, high efficiency and high throughput, the devices to be mounted onto a PCB are increasingly densely packed so are the holes needed to keep all elements in place. To effectively and economically generate so many holes of small diameter and high aspect ratio, the drilling process has to be done in a high speed manner and tool life has to be kept as long as possible. This study aimed to improve the tool life and the quality of the obtained holes by applying various hard coatings on the drills. The results showed that tool life could be effectively improved from 2500 hits to around 11000 hits at 155Krpm and 3.5m/min when proper hard coating was applied. It was found that wear of drill was resulted mainly from abrasive wear and adhesion wear. It was also showed in the research that while non-coated drills suffered serious deformation after 500 hits those coated with Al2O3 could finish 11000 hits with very limited wear land on the drills.

Development of a Lens Driving Maglev Actuator for Laser Beam Off-Axis Cutting and Deep Piercing

In laser beam cutting and laser piercing process, the machining speed and quality are very sensitive to the flow of assist gas and laser beam focusing position. In order not only to improve the cutting speed and the removal capability of the molten material and to save the consumption of the gas flow in laser beam cutting, but also to realize high speed piercing of high aspect-ratio holes, a magnetic-levitated (maglev) lens driving actuator was proposed and fabricated. The actuator can drive the lens to achieve real-time positioning control of the relative radial displacement between the lens axis and the assist gas jet nozzle axis (off-axis control) in radial directions in a range of ±1mm within 1.5 μm of tracking error and bandwidths more than 150Hz, and to achieve real-time positioning control of laser beam focusing point in axial direction in a range of ±5mm within 3 μm of tracking error and bandwidth more than 100Hz.

Ultra-short pulse laser deep drilling of C/SiC composites in air

Ultra-short pulse laser machining is an important finishing technology for high hardness materials. In this study, it demonstrated that the ultra-short pulse laser can be used to drill the film cooling holes and square holes in aero-engine turbine blades made of C/SiC composites. Both the edges and bottoms of the drilling holes are covered with small particles. The following factors have a great effect on drilling holes according to this work: (1) circular holes can be processed only at a relative small helical lines spacing. (2) With the increase of laser scanning speed, the depth of holes reduces while the diameter rarely changes. (3) Through the holes of high aspect ratio can be obtained via high processing power.

Optical Properties of Nanohole Arrays in Metal–Dielectric Double Films Prepared by Mask-on-Metal Colloidal Lithography

We present the fabrication and optical characterization of plasmonic nanostructures consisting of nanohole arrays in two thin films, a metal and a dielectric. A novel method called mask-on-metal colloidal lithography is used to prepare high aspect ratio holes, providing efficient mass fabrication of stable structures with close to vertical walls and without the need for an adhesion layer under the metal. Our approach for understanding the transmission properties is based on solving the dispersions of the guided modes supported by the two films and calculating the influence from interference. The methodology is generic and can be extended to multilayered films. In particular, the influence from coupling to waveguide modes is discussed. We show that by rational design of structural dimensions it is possible to study only bonding surface plasmons and the associated hole transmission maximum. Further, numerical simulations with the multiple multipole program provide good agreement with experimental data and enable visualization of the asymmetric near-field distribution in the nanohole arrays, which is focused to the interior of the "nanowells". The refractometric sensitivity is evaluated experimentally both by liquid bulk changes and surface adsorption. We demonstrate how the localized mode provides reasonably good sensitivity in terms of resonance shift to molecular binding inside the voids. Importantly, high resolution sensing can be accomplished also for the surface plasmon mode, despite its extremely low figure of merit. This is accomplished by monitoring the coupling efficiency of light to plasmons instead of conventional sensing which is based on changes in plasmon energy. We suggest that these nanohole structures can be used for studying molecular transport through nanopores and the behavior of molecules confined in volumes of approximately one attoliter.

Development of a maglev lens driving actuator for off-axis control and adjustment of the focal point in laser beam machining

► Develop a compact 6-DOF controlled maglev lens driving actuator. ► Real-time relative displacement control between laser beam and nozzle. ► Real-time focal point adjustment of laser beam. ► Sufficient positioning accuracy under extreme working acceleration. In laser beam machining, the machining speed and quality are very sensitive to the flow of the assist gas and the position of the focal point of the laser beam. In order to reduce the consumption of assist gas and improve the cutting speed and capability to remove molten material, and, in addition, enable the focal point to be adjusted in order to drill high aspect-ratio holes, a six-degree-of-freedom (6-DOF) controlled magnetic-levitated (maglev) lens driving actuator was proposed and fabricated. A novel air core coil type electro-magnetic driving unit was designed to actuate the lens holder. The driving unit can generate both repulsive and attractive forces by utilizing a modified Halbach array on the driven target and the air core coils. The air core coil windings allowed a spatial-cross-arrangement of the components, enabling the actuator to be sufficiently compact for it to be accommodated in the machining head of a laser beam machining tool. The proposed actuator can drive the lens to achieve real-time control of the axial position of the focal point and of the relative radial displacement between the lens axis and the assist gas jet nozzle axis (off-axis control). A range of ±5 mm with a tracking error of less than 12 μm and a bandwidth of more than 100 Hz was achieved in the axial direction, and a radial displacement up to ±1 mm with a tracking error of 1 μm and a bandwidth of more than 150 Hz was also obtained. Furthermore, under half sine wave pulse acceleration with a peak of 40 m/s 2 and duration of 10 ms in the radial direction, sufficient positional accuracy of the lens holder could be maintained.

Boron filling of high aspect ratio holes by chemical vapor deposition for solid-state neutron detector applications

A multiple deposition and etching process has been developed to enable high fill factor boron deposition in high aspect ratio holes fabricated in a (100) silicon substrate. The boron deposition was carried out using low-pressure chemical vapor deposition and the etching was done by inductively coupled plasma reactive ion etching technique. The boron deposition processes were carried out under different conditions in order to find a baseline process condition. The boron etching processes done under different conditions with the photoresist as the mask are also discussed. Finally, the fabricated neutron detector with the highest fill factor was characterized for the thermal neutron detection efficiency.

Precision depth measurement of through silicon vias (TSVs) on 3D semiconductor packaging process

We have proposed and demonstrated a novel method to measure depths of through silicon vias (TSVs) at high speed. TSVs are fine and deep holes fabricated in silicon wafers for 3D semiconductors; they are used for electrical connections between vertically stacked wafers. Because the high-aspect ratio hole of the TSV makes it difficult for light to reach the bottom surface, conventional optical methods using visible lights cannot determine the depth value. By adopting an optical comb of a femtosecond pulse laser in the infra-red range as a light source, the depths of TSVs having aspect ratio of about 7 were measured. This measurement was done at high speed based on spectral resolved interferometry. The proposed method is expected to be an alternative method for depth inspection of TSVs.

Real time ablation rate measurement during high aspect-ratio hole drilling with a 120-ps fiber laser

We report on the instantaneous detection of the ablation rate as a function of depth during ultrafast microdrilling of metal targets. The displacement of the ablation front has been measured with a sub-wavelength resolution using an all-optical sensor based on the laser diode self-mixing interferometry. The time dependence of the laser ablation process within the depth of aluminum and stainless steel targets has been investigated to study the evolution of the material removal rate in high aspect-ratio micromachined holes.

Time dynamics of burst-train filamentation assisted femtosecond laser machining in glasses

Bursts of femtosecond laser pulses with a repetition rate of f = 38.5MHz were created using a purpose-built optical resonator. Single Ti:Sapphire laser pulses, trapped inside a resonator and released into controllable burst profiles by computer generated trigger delays to a fast Pockels cell switch, drove filamentation-assisted laser machining of high aspect ratio holes deep into transparent glasses. The time dynamics of the hole formation and ablation plume physics on 2-ns to 400-ms time scales were examined in time-resolved side-view images recorded with an intensified-CCD camera during the laser machining process. Transient effects of photoluminescence and ablation plume emissions confirm the build-up of heat accumulation effects during the burst train, the formation of laser-generated filaments and plume-shielding effects inside the deeply etched vias. The small time interval between the pulses in the present burst train enabled a more gentle modification in the laser interaction volume that mitigated shock-induced microcracks compared with single pulses.

Electrodischarge drilling performance on parts produced by DMLS

Cobalt-base alloys are generally adopted in applications that require wear, corrosion, and heat resistance. In particular, cobalt–chromium–molybdenum (Co–Cr–Mo) alloy is currently being used in the aerospace and medical field. Both applications require product personalization and take advantage by parts manufacturing through additive technologies, such as direct metal laser sintering (DMLS). This technology still has limitations for the manufacture of small cavities, such as holes with diameters below 0.6 mm and high aspect ratio, which require drilling of the additive manufactured part. This research aims at investigating the effects of electrodischarge drilling (EDD) on surface and subsurface features of CoCrMo samples built by DMLS. High aspect ratio holes are produced with diameter of 0.6 mm and depth 15 times higher. Due to the absence of physical contact between tool and workpiece in EDD, machining forces are negligible and microfeatures can be produced with a high accuracy irrespective of the material hardness and strength. In this study, peak current, pulse-on-time, and duty ratio are adopted as control factors using the Taguchi method. Material removal rate, Electrode wear rate (EWrate), and surface roughness are measured as responses. S/N ratios and analysis of variance analysis lead to identify the significant process parameters and the optimal combination level of machining parameters. From the responses can be determined which ensures high productivity and smooth surface finish, even if tool wear is medium high. All the analyses are supported by the study of material removal mechanisms and surface morphology in the microscale, to account for the macroscopic trends.

Water-Assisted Femtosecond Laser Pulse Ablation of High Aspect Ratio Holes

Two novel techniques to enhance hole drilling in a silicon using a femtosecond laser, is presented. Firstly, the sprayed thin water layer has been used in an ablation region. Sprayed water removes the ablation debris and enables more efficient pulse energy transition into ablated material than in the air ambient. Secondly, the diffractive optical element (DOE) that generates into its far field 5x5 hole matrix at single irradiation, is used. By using the DOE instead of single hole drilling it is possible to ablate several holes simultaneously. Both of these techniques enhance the femtosecond ablation rate significantly.