Maskless 3D Ablation of Precise Microhole Structures in

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Maskless 3D Ablation of Precise Micro-hole Structures in Plastics Using Femtosecond Laser Pulses Caizhi Liao, Will Anderson, Fiach Antaw, and Matt Trau ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18029 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Applied Materials & Interfaces

Maskless 3D Ablation of Precise Micro-hole Structures in Plastics Using Femtosecond Laser Pulses

Caizhi Liao†, Will Anderson*†, Fiach Antaw†, Matt Trau*†‡

†

Centre for Personalized Nanomedicine, Australian Institute for Bioengineering and

Nanotechnology (AIBN), The University of Queensland, Corner College and Cooper Roads (Bldg 75), Brisbane QLD 4072, Australia

‡School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane QLD 4072, Australia

KEYWORDS: Maskless, 3D Ablation, Micro-hole, Plastics, Femtosecond Laser, Multiphoton Absorption, Resistive-pulse Particle Analysis

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ABSTRACT Femtosecond laser ablation is a robust tool for the fabrication of micro-hole structures. This technique has several advantages compared to other micro-fabrication strategies for reliably preparing micro-hole structures of high quality and low cost. However, few studies have explored the use of femtosecond laser ablation in plastic materials, due to the lack of controllability over the fabrication process in plastics. In particular, the depth profile of microhole structures prepared by conventional laser ablation techniques in plastics cannot be precisely and reproducibly controlled. In this paper, a novel, three-dimensional (3D), femtosecond laser ablation technique was developed, for the rapid fabrication of precise micro-hole structures in multiple plastics in air. Using a three-step fabrication scheme, micro-holes demonstrated extremely clean and sharp geometric features. This new technique also enables the precise creation of arbitrary shaped micro-well structures in plastic substrates through a rapid single-step ablation process, without the need for any masks. As a proof-of-concept for practical application, precise micro-hole structures prepared by this novel femtosecond laser ablation technique were exploited for robust resistive-pulse sensing of micro-particles.

1. INTRODUCTION Precise fabrication of micro-hole structures has a broad range of applications, - from aero-engine turbines and automotive fuel filters, to surgical/biomedical devices and biochemical microfluidic sensing systems. To obtain micro-hole structures, a myriad of machining techniques have been developed in the past few years, such as mechanical ultrasonic drilling, electrochemical drilling (ECD), electrical-discharge machining (EDM) and integrated photolithographic-chemical wet 2 ACS Paragon Plus Environment

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etching1. However, these multi-step micro-hole fabrication techniques are usually time/labour consuming, inflexible, expensive, and can be environmentally hazardous. In comparison to aforementioned approaches, laser ablation methods have emerged as a much more effective and environmentally friendly technique for the preparation of precise micro-hole structures2-5.

Since the mid-1970s, pulsed solid-state lasers have proven to be an efficient ablation tool, which has several advantages over other micro-fabrication techniques for reliably preparing micro-hole structures of high quality and low cost6. This has lead to the development of ultrafast femtosecond laser pulsing that has enabled direct ablation and removal of target substrate with minimal disturbance of the peripheral materials. This control allows the fabrication of precise structures with micrometer to sub-micrometer resolution2, 7-8. As applied to micro-hole fabrication, the femtosecond laser ablation process can be divided into two distinct schemespercussion drilling and trepanning. In percussion drilling, the laser pulse, shaped to a particular sized voxel, is focused on the desired location for a required time to ablate the material with a geometric size that is identical to the size of the voxel9. For trepanation processing, the laser pulses or the target sample is moved in a circular pathway to the desired diameter of micro-hole10. Percussion drilling is more applicable for the fabrication of smaller nano/micro-holes, while trepanning can be employed to create circular micro-hole structures of analysis. To manipulate the size and depth of circular micro-holes, integrated optimization of laser parameters11(e.g., pulse energy, pulse duration and pulse repetition rate) and system operation conditions12-14 (e.g., beam modification, laser pathway and assistant-gas environment) have been widely explored.



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Although femtosecond laser ablation techniques enable micro-hole fabrication with minimized horizontal peripheral heating effects through optimisation of laser parameter, controlling the hole depth profile in the vertical direction is difficult. Currently, most femtosecond laser created hole structures are funnel-shaped, with resolidified debris anchored on hole sidewalls, and adherent materials at the hole entrance, because of the plasma constraints imposed on the laser ablation process15-16. The degree of the taper effect encountered in direct laser ablation is typically dependent on the thickness of the target materials and is caused by the energy loss of pulses reflected on the sides of the hole wall9, 17. These constraints significantly prohibit the use of femtosecond laser ablation in the fabrication of high-quality micro-holes.

Currently, glass, ceramic and metal films are the most suitable materials for conventional femtosecond laser ablation18-20. Unfortunately for conventional femtosecond laser ablation processes, plastic materials are easily thermally decomposed, which leads to the formation of soot that is hard to remove along the sidewalls of micro-holes21-22. Consequently, no studies have systematically explored the use of femtosecond laser pulses in precise fabrication of microholes in commonly used plastic materials, such as polyethylene terephthalate (PET), polycarbonate (PC) and polyethylene (PE). Due to their low-cost, lightweight, flexibility, manipulability, and a multitude of other desirable properties, plastic materials have attracted increasing attention in micro-fabrication field. The ability to form accurate micro-hole structures within plastic materials would enable remarkable developments in many fields, such as biomedical devices and flexible electronics23-26. Therefore, micromachining technique capable of precise fabrication of micro-holes in varied plastic materials is greatly demanded.


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To overcome the problem of creating precise micro-holes in plastic substrates, we hereby propose a new methodology for 3-dimensional (3D) femtosecond laser ablation, capable of openair operation. The geometric features of micro-holes can be accurately pre-defined using the computer-aided design (CAD) models of the micro-holes, which are then ablated from the target material. Different from that of conventional percussion and trepanning femtosecond laser ablation, virtually any designable 3D micro-hole shapes can be directly achieved by this new technique without the use of masks. In this study, we use optimized ultrafast laser pulses to ablate and vaporize the illuminated volume with a negligible heat affected zone (HAZ) of the peripheral material in plastic substrates, leading to the formation of through micro-hole and nonthrough micro-well structures with extremely sharp and clean geometric features. We demonstrate a three-step 3D direct laser ablation process to create a wide range of perfectly shaped through-cylindrical hole structures (Φ 10µm to Φ 200µm) in many commonly used plastic materials, including PET, medium-density polyethylene (MDPE), poly(methyl methacrylate) (PMMA), polystyrene (PS), polyphenyl ether (PPE) and PC. To accurately prepare these precise micro-hole structures, laser parameters (pulse energy, scanning speed), material aspects (substrate thickness, substrate type) and micro-hole features (hole size, geometric shape), were systematically investigated and are thoroughly discussed. Additionally, this new ablation technique enables the precise fabrication of arbitrary shaped non-through micro-well structures in a rapid single-step ablation process, without the need for any masks. Finally, as a proof-ofconcept, we used precisely fabricated micro-hole structures formed in plastic substrates for resistive-pulse sensing of micro-particles, a technique that plays a crucial role particularly in the fields of biomedical engineering and pharmaceutical development.



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2. EXPERIMENTAL METHODS 2.1 Micro-holes Fabrication in Plastics Micro-hole structures with distinct geometric features (hole diameter, hole depth and hole shape) were designed in the Autodesk AutoCAD program. Depending on specific processing conditions, the diameter and the depth of sketched micro-hole structures ranged from Φ 10 µm to Φ 200 µm and 150 µm to 400 µm, respectively. These AutoCAD generated micro-hole STL files were imported into the DeScribe software (Nanoscribe GmbH, Germany) for slicing (0.3 µm) and hatching (0.4 µm) process. The smallest voxel was set to 0.3 µm in width and 0.4 µm in length (Figure 1, right down), which guarantees sub-micrometer resolution in micro-holes fabrication. The processed General Writing Language (GWL) file was then transferred to the Nanoscribe Photonic Professional GT system (Nanoscribe GmbH, Figure S1) for ablation of micro-hole structures.


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Figure 1. Schematic diagram of 3D femtosecond laser ablation process. The plastic material was mounted on a 3D XYZ piezo stage. The ablated plastic material is directly transformed into the gaseous plasma and removed into surrounding environment. The upper right figure shows the Gaussian intensity profile of the used Ti: sapphire femtosecond laser pulse (80 MHz, 100 fs). The lower right figure shows a single ablation unit (voxel): 0.3 µm in diameter (dxy), 0.4 µm in height (lz).

Before laser ablation, the surfaces of plastic substrates (both sides) were thoroughly cleaned by ultrasonic washing (10 mins), followed by a nitrogen drying treatment. The cleaned plastic substrates were mounted onto the Nanoscribe-controlled XYZ piezo stage (Figure S1). The 3D ablation process was performed using the laser pulses generated by a mode-locked Ti: sapphire femtosecond laser system (780 nm central wavelength, 80 MHz repetition rate and 100 fs pulse duration) (Figure 1). The output from the femtosecond laser source had a Gaussian intensity profile that tightly focused within the target material through a 25X objective lens (0.8 NA). To precisely prepare through cylindrical micro-holes, a three-step processing scheme (two FAST ablation steps plus one SLOW polish step) was performed, the processing details are described in Figure S2. The FAST ablation step (Coding file 1 in Supporting Info.) was used to robustly remove the majority of the plastic material in the micro-hole and to define the surface morphology of the prepared micro-hole exits on both sides. While the SLOW ablation step (Coding file 2 in Supporting Info.) was used for the eradication of debris attached to the hole sidewalls, to improve the surface quality of the inner hole wall. For PET substrate material, the optimized laser pulse scanning speed was set at 70 k µm/s in the FAST ablation mode and 8 k µm/s in the SLOW ablation mode. Corresponding suitable scanning speeds of the FAST and



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SLOW modes for other plastic materials have been listed in Supporting Info. (Table S1). For the preparation of complex non-through micro-well structures (triangle, hexagon and donut) with a designed hole depth of 50 µm, a single FAST ablation process was used to create these structures in plastic substrate materials without the aid of masks. No post-treatment process was required for the micro-holes/micro-wells prepared by this new 3D femtosecond laser ablation process.

2.2 Characterization A JEOL IT-300 scanning electron microscope (SEM) was used for the morphological characterization of micro-hole structures. The surfaces of micro-hole structures analyzed by SEM were pre-coated with 20 nm thick platinum film by a JEOL coating system. A Nikon ECLIPSE Ni-U optical system equipped with a Nikon Plan Fluor 20x/0.5 DLL len was employed for the optical imaging of micro-hole/micro-well structures. A DEKTAK stylus profiler was utilized for the measurement of sidewall roughness of micro-hole structures. For the resistive-pulse sensing of 20 µm particles (COULTER cc Standard L20), a qNano system (Izon Science, New Zealand) was utilized with measurements performed at a 0.5 V potential. Buffer solutions used in the upper and lower reservoir chambers were diluted phosphate buffered saline (PBS, 0.01X) electrolyte.

3. RESULTS AND DISCUSSIONS 3.1 Multiphoton Ablation Process Femtosecond laser pulse micromachining tools utilise high-order, multi-photon non-linear absorption processes to ablate target material27-28. To optimise our 3D ablation technique, it is


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essential to understand the fundamental aspects of this technology. Ultrashort femtosecond laser pulses (70 k µm/s), the formulated target plasma cannot be fully removed in time, creating a high density of plasma trapped in ablated area. The rapidly increased plasma density will reduce the input energy of the laser pulse because of pulse reflection as described in introduction, and thus prohibit the material vaporization process induced by multiphoton ionization9. In summary, both laser fluence level and scanning speed can significantly impact the quality of micro-holes prepared by this 3D laser ablation technique.

Figure 4. (a) SEM characterization a Φ 100 µm through-cylindrical micro-hole in 7 Mil thick PET substrate (Top-view). Scale bar: 25µm. (b) Optical image of a Φ 100 µm throughcylindrical micro-hole in 7 Mil thick PET substrate (Top-view). Scale bar: 25µm. (c) Crosssection SEM image of a Φ 100 µm through-cylindrical micro-hole. Scale bar: 30µm. (d) Interfacial SEM characterization of the micro-hole sidewall surface. Scale bar: 10µm.


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3.4 Geometric Design in Laser Ablation In direct laser ablation, the laser pulse becomes less effective at ablating target material as the hole depth increases. The incoming laser pulse will be restricted, both by the absorption of ablation plasma contained in the hole, and the reflection of the laser pulse at the hole sidewall. This creates micro-hole structures with a funnel-like shape (Taper effect)15. Taper effects become more pronounced as hole depth increases9, 16. Although several advanced optical methods have been developed to tailor the incoming laser beams to produce holes with desired depth profiles16, the geometric features of micro-holes generated by conventional femtosecond laser ablation techniques cannot be reproducibly formed, particularly for micro-hole structures prepared using plastic substrates7. Figure 4 demonstrates a precise Φ 100 µm through cylindrical micro-hole prepared by a three-step ablation scheme (Figure S2). The micro-hole has sharp edges and a clean sidewall, without any observable non-defined bulges that would indicate the re-solidification of heat-induced melting debris that form in classical laser ablation processes. Figure 4c demonstrates the cross-section view of a through cylindrical micro-hole. Taper effects that would cause funnel-like hole shapes as seen in standard laser ablation techniques were almost eradicated. Lateral ablation zones that would degrade the surrounding surface texture were also not observed. The hole sidewall has precisely defined features, with an averaged surface roughness around 360nm±50nm (Figure 4d). In some cases with high fluence level laser pulses, micro-cracks would appear on hole sidewalls (Figure S3). The formation of plastically deformed micro-cracks inside the micro-hole was mainly attributed to the tensile stresses developed during the cooling process. Further work is required to investigate the formation and mitigation of these cracks.



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Table 1. Entrance and exit geometric features of the through cylindrical micro-holes with different diameter sizes (Φ 10µm to Φ 200µm) formed in a 7 Mil thick PET plastic substrate. SD: Standard deviation.

Cylinder Hole Diameter (Designed, µm)

10

30

50

100

Frontside Hole Diameter (Experimental, Mean±SD, µm)

9.3±0.6

29.6±0.3

49.8±0.3

99.8±0.2

Frontside Hole Deviation (%) Backside Hole Diameter (Experimental, Mean±SD, µm)

[ -11.2%, +1.5% ]

[ -2.5%, +0.9% ]

[ -1.1%, +0.6% ]

[ -0.8%, +0.3% ]

9.4±0.7

29.8±0.3

49.7±0.4

Backside Hole Deviation (%)

[ -13.9%, +1.3%]

[ -1.6%, +1.1% ]

[ -1.2%, +0.4% ]

150

200

149.8±0.6 199.5±0.8 [ -1.0%, +0.5% ]

[ -1.2%, +0.8% ]

100.1±0.3 150.2±0.5 199.8±1.1 [ -0.5%, +0.5% ]

[ -0.9%, +0.4% ]

[ -1.1%, +0.5% ]

One unique aspect of this laser ablation technique is that the 3D geometric features of microholes, including hole diameter, hole shape and hole depth, can be accurately pre-defined by CAD software, instead of by tuning laser parameters and operating conditions as required in conventional femtosecond laser ablation methods10. Table 1 lists the geometric features of different-sized micro-holes (Φ 10 µm to Φ 200 µm) in PET substrate. Only micro-holes below Φ 10 µm deviated largely from the pre-designed size. This deviation was typically less than 10% of the hole diameter and can be attributed to the fact that smaller holes impede the plasma transportation process from moving ablated material from the target surface into the surrounding environment. As the hole diameter increased above 10 µm, these dimension-induced constraints were dramatically reduced. As shown in Table 1, the ablated holes on both sides have near-


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perfect circularity with a deviation below ±1%, which demonstrates the potential of this new laser ablation technique for the precise fabrication of micro-holes in plastic substrates.

It is important to note that for larger holes ablation, higher laser power is required. To fully remove the target plastic material in desired micro-holes, a laser pulse with sufficient power (near threshold fluence level) is required to generate adequate multiphoton-absorption, to induce the ionization reactions that ablate the material11, 19. In Figure 5a, the blue line demonstrates the relationship between the ablated areas (~D2) and the corresponding minimum required threshold fluence level. For optimized micro-hole fabrication, a higher level of threshold fluence was required as micro-hole size was increased. This can be attributed to the significantly increased amount of target plastic materials that needed to be ablated away, an effect which was even more apparent when the diameter of micro-holes was increased above 100 µm. Additionally, the size of ablated areas also determines the overall ablation processing time (Figure 5a, red line, FAST step). The overall processing time increased substantially as the micro-hole diameters increased from 10 µm (2.2 s) to 200 µm (158.2 s), as shown in Table S2. The increased ablated areas exponentially extend the movement length of the laser voxel along the hatching pathway in each sliced layer, thus correspondingly increase the processing time for larger holes ablation.



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3.5 Material Aspects in Laser Ablation Another essential parameter affecting the laser ablation process in plastics was the thickness of the substrate materials37. Figure 5b illustrates the minimum required threshold fluence level for the optimal preparation of through cylindrical micro-holes in PET plastic substrates with different thicknesses. With a fixed optimal laser pulse scanning speed at 70 k µm/s (FAST step) in the three-step ablation process (Figure S2), the laser fluence level was tuned to effectively ablate Φ 100 µm through cylindrical micro-holes in PET plastic substrates with thicknesses ranging from 5 Mil to 15 Mil. For thin plastic substrates (< 10 Mil), a relatively low threshold fluence level (~0.4 J·cm-2) was adequate to fully transform the solid plastic material directly into gaseous plasma. As the thickness of the ablated plastic material increased (> 10 Mil), the threshold fluence level required to fully remove the material had to be increased because of the increasing hole depth effect. As discussed previously, the increased hole depth will increase the energy reflection phenomenon of the laser pulses, which consequently leads to the reduction of laser pulse energy exerted on the ablated areas and prohibits the laser ablation process9, 17. As a result, to compensate for the reduced laser pulses energy in a thicker plastic substrate, a higher threshold fluence level is required.

The proposed new 3D laser ablation method was shown to be universally robust for the preparation of precise micro-hole structures in multiple other plastic materials, including PE, PMMA, PS, PC and PPE. As shown in Figure 5c, the optical band-gap energy of ablated plastic material (Eg) was found to have a good linear relationship (R2=0.9929) to the optimal fluence level (Fth) for the ionization process induced by multi-photon absorption. For the femtosecond laser ablation process, multi-photon ionization is dominated by the generation of free electrons. 18 ACS Paragon Plus Environment

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Simultaneous nonlinear absorption of m photons when mhv ≥ Eg, where hv is the energy of a single photon and Eg is the optical band-gap energy of the ablated material, will lift the bonded electrons from the ground energy level to the free energy level28 (Figure S4). The low band-gap polymer material PPE-series typically has a band gap energy between 2 eV and 3 eV, which facilitates the multi-photon ionization process and the formation of free electrons (plasma) even under low input laser energy (~0.3 J·cm-2). As the optical band-gap energy of ablated material gradually expanded, the minimum required threshold fluence level to effectively ablate the plastic material increased linearly. To fabricate a precisely controlled Φ 100 µm throughcylindrical micro-hole in a PMMA (Eg ~ 5 eV) plastic substrate, a threshold fluence level around 0.55 J·cm-2 was required to initiate multi-photon ionization, which was nearly two-fold larger than that of micro-holes fabrication using PPE-series plastic material. Promisingly, this result provides new insights into the optimal fluence level for the laser ablation process using different plastics.



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Figure 5. (a) The interdependent relationship between ablated area (~D2) and threshold fluence level (blue line), ablated area (~D2) and overall processing time (red line), correspondingly. (b) The interdependent relationship between plastic substrate thickness and threshold fluence level for optimized laser processing. (c) Linear relationship between the optical band-gap energy of plastic materials (Eg) and optimized threshold fluence level (Fth) for precise micro-holes fabrication. (d) Complex micro-well structures prepared in PET substrates: triangular-shaped, hexagon-shaped and donut-shaped micro-wells, with a measured depth (mean) around 48±3 µm.

3.6 Maskless 3D Ablation of Micro-wells Importantly, this proposed 3D ablation technique serves as a new paradigm for the maskless preparation of precise micro-well structures. Non-through micro-wells hold great potential for emerging application scenarios, such as flexible electronics, tissue engineering, cell engineering and pharmaceutical analysis. To prepare non-circular-shaped micro-well structures by conventional femtosecond laser ablation approaches, masks defining the to-be-ablated shapes must be integrated into the laser ablation systems before processing the structures, which significantly increases the complexity of micro-hole fabrication process10, 38. Furthermore, laser pulses passing through these masks cannot be uniformly projected onto the surface of the ablated material, leading to poor controllability over the ablation process. In contrast, the ablation scheme investigated here enables the precise preparation of any desirable-shaped micro-well structures in a straightforward manner without using masks. Desired non-through micro-wells with distinct geometric features can be obtained in seconds via a single-step laser ablation process (Fast step, Figure S2). Figure 5d displays the images of the micro-wells in the shapes of a triangular, hexagon and donut formulated in a PET substrate. The geometric features were 20 ACS Paragon Plus Environment

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accurately controlled, with negligible geometric deviations to the CAD designs. Additionally, the depth profile of these micro-holes is highly manipulable. The averaged depth of these micro-well structures measured by a stylus profiler was around 48±3 µm, while the theoretical depth of CAD designs were 50 µm. Therefore, this new 3D femtosecond laser ablation technique affords the rapid, maskless, precise fabrication of complex-shaped micro-well structures in plastic materials.

3.7 Resistive-pulse Analysis Using Ablated Micro-hole The development of nano/micro-pores for colloidal characterisation has resulted in a renaissance of “Coulter-counter” based techniques in recent years39. This has been driven by new techniques that enable the fabrication of nano/micro-sized holes structures, which when used for resistivepulse sensing (RPS) are powerful sensors that enable high throughput particle-by-particle characterization of colloid suspensions of individual bio-macromolecules40. To validate the performance of the micro-hole structures prepared by this new 3D laser ablation technique, a through cylindrical Φ 100 µm micro-hole in a PET substrate was utilized for resistive-pulse analysis of 20 µm particles (Figure 6 and Figure S5). As shown in Figure 6a, a voltage (0.5 V) was applied across the plastic substrate containing the micro-hole, and the ionic current flowing through the micro-pore was recorded by the Izon qNano particle analysis system. As the particle passes through the pore, there was a measurable increase in resistance caused by the occlusion of electrolyte ions, which in turn leads to a transient decrease in ionic current (Current blockade,



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Figure 6b). For a cylindrical micro-hole, the induced resistance change and ionic current change across the length of micro-hole is given by41:

∆R d 3 = R D2l ∆ i (%) =

∆R / R 1 + ∆R / R

(1)

(2)

where d is the particle diameter, D is the micro-holes diameter (Φ 100 µm) and l is the microholes depth (177.8 µm). As shown in the above Eqns, the magnitude of ionic current changes (~0.4 nA) obtained from experimental results allows the calculation of the mean diameter of an analyzed micro-particle from the magnitude of its resistive pulse. The mean diameter of the characterized standard 20µm latex particle was calculated to be 21±0.35 µm, corresponding to the manufacture size of 20±0.15 µm.

The current blockades measured by the qNano system are displayed as Figure 6d. Furthermore, an in-house tool for calibration-free resistive pulse analysis was used to determine the particle size distribution42. As presented in Figure 6e, over 85 % of the micro-particles were located in the size range of 19 µm to 22 µm, with an averaged diameter size around 20.8 µm, which further validated the experimentally calculated result mentioned above (21±0.35 µm). Promisingly, robust resistive-pulse analysis of micro-particles exemplifies the versatile uses of the plastic substrates containing precise micro-hole structure fabricated by this new 3D ablation technique.


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Figure 6. (a) Schematic diagram of the resistive-pulse particle analysis system. (b) Single current blockade induced by a micro-particle passing through the prepared micro-hole structure. (c) Current trace of the resistive-pulse micro-particle analysis. The signal noise is below 30 pA, with a signal/noise ratio (S/N) larger than 10. (d) Size histogram of the current blockade distribution experimentally obtained by q-Nano system. (e) The in-house tool simulated size distribution of 20µm micro-particles. Simulation addressing voltage: 0.5 V.



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4. CONCLUSION In conclusion, we have developed a new 3D femtosecond laser ablation technique, which serves as a robust ablation tool for the precise fabrication of micro-hole structures in multitude plastic materials. The dominating parameters (energy level and scanning speed) of femtosecond laser pulse ablation were systematically investigated, to accurately create micro-holes/micro-wells with extremely sharp and clean geometric features. A wide range of precisely controlled through cylindrical micro-hole structures (Φ 10µm to Φ 200µm) in multiple plastic materials, including PET, MDPE, PMMA, PS, PPE and PC, were successfully realized using our proposed three-step 3D laser ablation process. The results indicated that the optimized laser processing conditions were affected by the target plastic material properties (substrate thickness, substrate type) and pre-designed geometric features (hole size, geometric shape). More importantly, arbitrary-shaped non-through micro-well structures could be rapidly prepared using a single step ablation process without using any masks. Lastly, to demonstrate the practical applications of this new 3D laser ablation technique, robust resistive-pulse analysis of micro-particles using precisely ablated through-cylindrical micro-hole was performed. These findings provide a novel solution to several practical problems faced by precise macro-machining of plastic materials.


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ASSOCIATED CONTENT Supporting Information. Supplemental figures, tables and STL coding file information, including a schematic diagram of the femtosecond laser setting up system and multiphotonabsorption induced ionization process; Description of the three-step laser ablation process for through micro-hole; SEM image of micro-crack on the micro-hole sidewall; A table of optimized scanning speeds for the creation of micro-holes in different types of plastic materials; A table of processing times for the preparation of different-sized micro-hole structures; and STL coding files for the FAST ablation step and SLOW ablation step, respectively.

AUTHOR INFORMATION Corresponding Authors *E-mail：[email protected] (M.T.). *E-mail：[email protected] (W.A.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors would like to acknowledge the Australian Research Council for the financial support (DP160102836) of ongoing research project. C.Z LIAO and F ANTAW would like to acknowledge the financial support from the Australian Government Research Training program (RTP) scholarship. The authors also acknowledge the facilities support from the Australian National Fabrication Facility-Queensland Node (ANFF-Q) in The University of Queensland. 25 ACS Paragon Plus Environment


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GRAPHICAL TABLE OF CONTENT


Maskless 3D Ablation of Precise Microhole Structures in

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