Monodisperse micro-drops have been created successfully in T-shaped microchannels fabricated with the 'modified’ excimer laser-LIGA technique. This technique eliminates the surface wetting problems and the occurrence of a deeper ablation path which happened during the joining of different section of the microchannel with the dynamic laser ablation mode. The experimental study on the shear-induced drop formation from T-shaped microchannels shows that the drop size decreases with increasing shear and that a maximum drop productivity was observed depending on the value of the dispersed phase flowrate used.
This paper presents our design and experimental results of nickel microcantilevers, which were fabricated using a laser-LIGA process, based on KrF (248 nm) excimer laser micromachining. A chrome-on-quartz mask, containing the desired mask patterns was prepared for this work. The substrate of copper (30 μm thick) clad printed circuit board (PCB) was laminated with Laminar 5038 photopolymer to be laser patterned. Following laser patterning and laser cleaning, all the samples were electroformed with nickel on top of the copper layer. To release the Ni microcantilevers, the excimer laser was employed again to remove the polymer in the localised area to facilitate Cu selective etching. Here, copper acted as the sacrificial layer as well. The Cu selective etching was carried out with ~ 20 % (wt) aqueous solution of ammonium persulfate. Because the Cu selective etching is isotropic, some undercuts happened next to the anchor area. The samples were characterised using optical microscope, confocal laser scanning microscope and SEM, and some of Ni cantilevers were tested electro-thermally. Their performance was analyzed with respect to the simulation results.
In micromachining, excimer laser ablation was a key process for producing featured by removing parts of a photoresist layer. One hurdle was that the seed layer (e.g., copper), on which the photoresist was spun, was easily attacked or damaged in laser ablation of the photoresist. To overcome this, an acoustic emission transducer (AET) was coupled to the X-Y stage of an excimer laser system to acquire surface acoustic waves (SAWs) arising from pulsed laser-material interaction. The characteristics of such a process could then be investigated by analysing this feedback AE signal. Analysis of the frequency spectrum showed that there was a dominant frequency correlating with the ablation process through one material to another. Specifically, the amplitude of the dominant frequency had an abrupt change when laser beam approached the interface of two layers. The RMS values and the variance values of raw acoustic waves were also indicative of such a process. The exact number of shots machining through one material was indicated by properly calibrating such a correlation at given laser parameters. Furthermore, the etch rate of machined material could be calculated by averaging the thickness of this material with the associated number of shots. Finally, a real-time monitoring scheme of complex laser micromachining process was addressed on the basis of taking SAWs as feedback signals.
This paper describes recent experimental studies on the effect of patterning geometry on the laser machining parameters and electrodeposition rate of nickel microstructures fabricated using laser LIGA. The effect of shape, size and spacing of features has been studied for structures plated into Laminar AX moulds. In contrast to previous work, which has concentrated on low aspect ratio (< 0.1) geometries or on large (> 1 mm) structures, we specifically address here problems relating to aspect ratios in the range 0.14 - 8.75 and feature sizes of ~ 4 micrometers to 200 micrometers . Mould structures and plated features have been examined using optical, scanning electron and laser scanning confocal microscopy. Results show that for features >50 micrometers , the thickness profile of plated shapes varies by approximately +/- 1 micrometers m over most of the surface area with the edges demonstrating corner rounding with a radius ~ 5 micrometers . Below 20 micrometers in size, thickness profiles become peaked towards the center of a feature. A surface roughness (Ra) of ~ 1.0 micrometers is also observed. The reduction in deposition rate over the 3 hour electroforming process has also been explained in terms of an increase in plating area due to the profile of the laser ablated moulds.
A pulsed excimer laser (248 nm) based LIGA-like process is presented for the fabrication of Ni serpentine microstructures, such as those that might be used for micro-heaters. The structures were produced on both Cu (60 micrometers ) clad PCB and on Cu/Ti (up to 4 micrometers /15 nm) sputtered Si (100) substrates. The substrates were coated with a Laminar dry film (35 micrometers ) photoresist, which was then patterned by laser ablation to produce the mould for Ni electroforming. The optimal ablation conditions were identified for laser patterning to prepare the micro polymer mould. Beam fluence (~ 1 J/cm2) and number of shots (~ 60 pulses) for 50 micrometers wide features on this photoresist were established, and it was observed that an increased number of shots and increased fluence were needed for features less than 20 micrometers wide. Additionally, the Cu layer surface was cleaned by the use of 5 -10 laser pulses at the same fluence. Ni electroforming has been carried out using standard Ni sulfamate bath at a current density of ~ 10 mA/cm2. After Ni electroforming, both the Laminar dry film and the Cu layers around the electroformed Ni patterns were removed using a combination of acetone, laser and Cu selective etching. Finally, a series of Ni microstructures were fabricated consisting of up to 50 micrometers wide and 35 micrometers thick serpentine tracks. The devices were measured using a scanning confocal microscope and it was found that using the excimer laser to remove the remaining dry film laminate also smoothed the electroplated Ni surfaces from a pre-laser treated Ra of 1.20 micrometers to 0.19 micrometers . Laser ablation also released the finer features from the substrate.
In this paper we investigate excimer laser micromachining of TiNi shape memory alloy using an image projection system as an alternative to photolithographic patterning. We report on the characteristics of material removal by KrF excimer laser induced ablation at 248 nm and the dependence of material removal rates on laser parameters such as fluence and pulse frequency. Fluences at the workpiece using a 10x projection lens were up to 2.5 J cm-2 with pulse repetition rates up to 100Hz. Conventional chrome-on-quartz masks were used for pattern transfer. Material removal mechanisms and rates of material removal are compared with those observed during excimer laser micromachining of polymers and ceramics and limitations on achievable lateral and depth resolution explored. Data obtained by a variety of characterization methods are correlated to assess the effects of laser induced damage.
This paper presents the results of our investigations on the laser micromachining of structures in a dry film photoresist polymer (Dynachem, Laminar AX dry film) laminated on a copper clad Printed Circuit Board (PCB) and (100) Silicon wafer coated with Ti (15nm)/Cu (100 to 4000 nm) and copper seed layers. This study concentrated on investigating and comparing the effect of laser fluence (0.01 to 2 J/cm2) and number of shots (1 to 1000) on the etch characteristics of the Laminar AX dry film on both substrates. The other important aspects that were studied include the minimum required seed layer thickness for electroplating. The removal of the residual polymer layer at the end of the laser micromachining process and its effect on plating characteristics has been studied. The surface quality and roughness of the laser micromachined sites and their effect on the plated Nickel structures were studied in detail. The laser fluence and the number of shots used at this stage affected the conditions of the seed layer, which in turn influenced the plated film growth kinetics. The seed layers with thicknesses less than or equal to 0.8 mm were completely removed when high fluence (> around 1 J/cm2) was used. The seed layer surface after micromachining was characterised using Scanning Electron Microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and profilometer. High Aspect Ratio Structures (HARS) of Nickel were fabricated using the Laminar AX moulds. The highest aspect ratio achieved so far in this work is 6. The characteristics of these structures are discussed in detail.