Laser micromachining is of great significance in the production of electronic devices, medical and automotive products with complex structures of small holes or tiny grooves. Because the diameter of the holes and grooves in these products is getting smaller and smaller, and the tolerances of these dimensions are getting tighter and tighter. Only lasers are able to meet all the requirements placed on micromachined parts from 1 μm to 1 mm. The small area of thermal action of laser machining allows accurate control of the extent and depth of machining, ensuring high repeatability, good edges and wide versatility [1].
In microsystems manufacturing, people widely used silicon anisotropic etching and LIGA (LIGA) technology to process a variety of microstructures. The former is suitable for processing silicon two-dimensional structure and small depth-to-width ratio of three-dimensional structure; the latter can process precision three-dimensional structure, not only for silicon but also suitable for processing metals, plastics and ceramics. However, this technology requires harsh conditions, it needs synchrotron X-ray source, and the production of mold is also very complicated, so it is difficult to popularize. It is also important to note that the LIGA process is not compatible with ICs, which limits its use to some extent.
The laser micromachining process developed in the early 1990s is capable of processing both more complex microstructures and less demanding conditions, making it easier to realize in laboratories and factories [2].
Laser micromachining involves a wide range of applications, this paper focuses on the laser beam in the UV (ultraviolet) band or 532 nm and 1.06 μm section of the laser micromachining applications, the operating state of the pulsed state, the processing of the range of applications for microelectronics and micro-mechanical (MEMS). Other applications of laser beams are not covered in this article.
2. Pulsed laser direct micromachining technology
Pulsed laser direct micromachining technology is the use of high-energy laser pulses on the direct processing of solids, mainly based on the laser ablation process. In the ablation process, the laser energy absorbed by the solid material causes the material to be ejected from the processing surface. The ablation between the laser and the solid is closely related to the solid material as well as to the pulsed laser parameters. Pulsed laser parameters mainly include the wavelength, pulse width and pulse intensity of the laser. Under suitable conditions, almost all solid materials can be processed by pulsed laser, and the pulsed laser processing parameters for a wide range of materials have now been established by research [3].
Figure I(a) shows the main structure of a common excimer laser processing device. The laser beam passes through a series of devices, including a shutter, tunable attenuator, beam shaper and normalizer, and finally irradiates the mask. In this structure, the beam shaper changes the shape of the beam so that it is approximately square, and then the normalizer splits the light into many beams, each of which irradiates the mask from a different direction (Fig. 1(b)). This not only improves the uniformity of light irradiation, but also introduces off-axis elements. Off-axis light irradiation can accomplish vertical structures or even drill-etched structures, which cannot be processed using conventional planar light irradiation. Some auxiliary equipment for collimation is generally required throughout the system, such as a CCD video sensor or a stand-alone nonlinear microscope.
One of the main features of pulsed laser direct micromachining technology is the ability to machine complex three-dimensional surface contours. Multiple exposures to different masks allow for the machining of stepped multilevel structures, while scanning the mask during the exposure time allows for continuous cutting to be accomplished, as well as direct projection ablation with halftone masks to accomplish continuous cutting [4]. The mask and the workpiece are generally mounted on a precision moving platform controlled by stepper motors, and the automatic scanning operation is realized by computer. Other pulsed laser parameters such as laser fluence and repetition frequency can be varied during processing. In addition, the maximum viewing angle for off-axis irradiation can be varied by changing the numerical aperture NA, see Fig. 1(b), which allows structures with different sidewall angles to be processed under constant laser fluence conditions.
Figure I (a) Block diagram of excimer laser processing equipment (b) Optical system diagram
Another feature of the pulsed laser direct micromachining technique is the ability to process a wide range of materials [5], which is particularly suitable for the processing of polymer materials. Most polymers have strong energy absorption within the spectrum of the laser, which ensures energy coupling between the laser and the workpiece, while the relatively low thermal conductivity ensures that the heat diffusion and the area affected by heat during the ablation process are small. In most cases, a very good surface finish can be obtained and additional losses (melting and debris) can be minimized, a characteristic that many other materials do not possess. For example, due to the high reflectivity and thermal conductivity of metals, processing with pulsed lasers has a high ablation threshold and serious additional losses during processing. However, if the object of processing is a thin metal film deposited on the surface of a substrate with poor thermal conductivity, pulsed laser processing can be used to obtain very good results.
The most successful example of direct pulsed laser processing of MEMS devices is the processing of inkjet print heads [6]. In addition, the very high peak power and 3D structuring capability of pulsed lasers can also be applied to the processing of microfluidic chips. Major components in microfluidic chips, like microchannels, microfilters, microstirrers and microreactors require 3D structures (or at least 2.5D). In addition, as a material for microfluidic chips, polymers are more suitable for micromachining with pulsed lasers than materials with silicon substrates.
Examples of direct MEMS machining have also been reported recently, such as the fabrication of dual piezoelectric wafer microactuators on silicon substrates [7] and multilayer magnetic material actuators [8]. In addition, femtosecond laser micromachining technology is also developing rapidly [9]. Since the femtosecond laser has a high energy density, this makes it promising for some aspects of MEMS processing, such as the use of standard transparent materials with the violent interaction of high-energy multiphotons can be used to process microstructures on light-transparent materials.
2.1 Direct Processing
The term "direct processing" is used here to describe the process of processing materials by focusing a laser beam. This technique is used in a wide range of micromachining applications where high precision and small dimensions are required, including the drilling of fuel injectors, the drilling of gas sensors, the etching of solar cells, and the prototyping of MEMS. The workpiece is moved along with the beam using a check-flow scanner and a movable stage while being processed with the laser to obtain the desired pattern. The processing speed is up to 10ms-1 by adjusting the flow check scanner [10].
Figure 2: (a) Schematic of direct machining with a detector-flow scanner and an X-Y movable stage (b) MicrAlater M1000 direct machining laser device
2.2 Drilling
Machining of a series of holes using a focused laser beam on an X-Y stage or a detector-flow scanner is used in the drilling of holes in fuel injectors, gas sensors, tiny circuit boards, and detector cards. circuit boards and detector cards. Figure 3 shows a portion of a detector card used for IC (integrated circuit) testing. 100 μm holes were drilled with a 355 nm ND:YAG laser on a 500 μm thick silicon nitride crystal. Using AblataCAM software, the files can be translated directly into the laser machine process. Using this technology it is possible to machine holes of almost any shape on detector cards.
Figure 3: (a) 100 μm hole on silicon nitrogen crystal detector card for IC testing b) hole in hardened steel for fuel injection
The need for lower losses and better fuel utilization in engines has led to intensive research into smaller holes and thicker walled fuel injectors. The limitations of conventional EMD technology for drilling holes in diesel injectors have made laser processing a key technology for the next generation of diesel engines. The holes are drilled with a diameter of 30-100 μm tolerance of ±1.5 μm and a taper angle of less than 0.5 degrees. Figure III (b) shows the hole machined on a diesel injector with a Nd:YAG laser at 532 nm.
2.3 Solar Panel Processing
Laser devices operating on the 1.06-μm wave, with typical energies of a few tens of watts, are widely used for fine linear engraving on the glass substrate of thin-film solar panels. This combination of process and emission technology with BTS enables solar panels to maintain very high precision and accuracy at high speeds. Figure IV(a) shows a schematic of the processing of an amorphous silicon thin film under a dual laser system (1.06 μm and 532 nm).The IR YAG laser beam is used to scribe approximate 30 μm wide lines on the ITO layer, followed by the deposition of the α-Si and the visible YAG laser beam is passed through the α-Si layer in the vicinity of the plate to process 50 μ m diameter interconnects. The ITO layer is not affected by the processing. Next, the aluminum electrode layer is deposited and a visible YAG laser is used to process a track of approximately 25 μm wide to complete the processing of the plate. Part of the processing of the solar panel prototype is shown in Figure IV. It takes about 1 minute to process each layer of a 400mm plate with 580nm.
Figure 4: (a) Solar panel processed with a dual-wavelength laser system
b) Photographs of scribing and interconnection on a thin-film α-Si solar panel
3. Recent Research Developments
3.1 UV Laser Drilling Machine for Micromachining - The Meister 1000DF
MHI has produced its latest DUV266nm laser drilling machine, the Meister 1000DF, which can be used on all new solid-state UV-YAG oscillators. With the Meister 1000DF it is possible to perform high quality micromachining on a wide range of materials and in a variety of operating environments. Features: semiconductor-pumped solid-state laser resonator cavity can achieve a high lifetime and has a high reliability, high energy density 266nm UV output, able to achieve 50-200μm diameter micro-drilling, high speed and equipped with a detector flow scanner [11].
Figure 5: Sample drawing for processing application
(a) Perforated hole: 100 μm diameter Polyimide resin: 25 μm thickness
(b) Perforated hole: 100 μm diameter Ceramics: 250 μm thickness
Figure 6: Structural diagram
3.2 DPSS UV laser
High-pulse 355nm Laser (LD pumped YV04 laser + SHG + THG) air cooled. Summary: This laser is a compact and air-cooled type of high-cycle pulsed DPSS UV laser (355nm). The nonlinear crystal GdYCOB is used in this laser (invented by Osaka University). As a result, high beam quality and stable output can be obtained. It is also very easy to maintain and operate, and is widely used for micromachining, precision measurement, and so on [12].
3.3 DPSS Green Laser
High-pulse 532nm laser (LD-pumped YVO4 laser + SHG) Air-cooled. Summary: This laser is compact and air cooled type of high cycle pulse DPSS green laser (532nm). It has excellent output stability and high beam quality. And it is easy to maintain and operate, and can be widely used in micromachining, measurement and so on.
3.4 DPSS YVO4 Laser
High-pulse 1064nm laser (LD-pumped YVO4 laser) air-cooled. Overview: This is a compact, air-cooled and easy-to-maintain DPSS laser. It is pumped by LD and output by fiber. It can be miniaturized due to the characteristics of high repeatability and thermal tension in the process. Therefore, it can be widely used as a light source for high-speed marking, laser processing, and harmonic generation.
4. Conclusion
Pulsed lasers have unique processing capabilities in terms of the range of materials processed and the flexibility of 3D processing. The combination of pulsed laser processing and other mainstream technologies for microfabrication can provide an important processing tool for the future development of MEMS. The main application areas of pulsed laser processing technology are micro actuators based on functional materials, microfluidic devices and systems. In addition, pulsed lasers have the unique ability to manipulate and connect microcomponents, and therefore will also make a great contribution to the integration and packaging technology of MEMS.
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[11] http://www.mhi.co.jp/kobe/mhikobe-e/products/etc/uvlaser.htm
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