Laser micromachining is of great significance for manufacturing complex holes or slots in electronic devices, medical and automobile products. Because the aperture and groove size of this kind of products are getting smaller and smaller, and the tolerance of these dimensions is getting stricter and stricter. Only laser can meet all requirements of micro-machined parts from 1μm to1mm. Laser processing has a small thermal effect area, which can accurately control the processing range and depth, ensure high repeatability, good edge and wide versatility [1].
Anisotropic etching of silicon and LIGA technology are widely used to process various microstructures in micro-system manufacturing. The former is suitable for processing two-dimensional structure of silicon and three-dimensional structure with small depth-width ratio; The latter can process accurate three-dimensional structures, not only for silicon, but also for metals, plastics and ceramics. However, this technology requires harsh conditions, requires synchrotron radiation X-ray sources, and the production of modules is very complicated, so it is difficult to popularize. It must also be pointed out that LIGA technology is not compatible with IC, which limits its application to some extent.
Laser micromachining technology, which was developed in the early 1990s, can not only process complex microstructures, but also has less stringent requirements on conditions, and can be easily realized in laboratories and factories [2].
Laser micromachining involves a wide range of applications. This paper mainly introduces the application of laser micromachining in ultraviolet band or at 532nm and 1.06μm, which works in pulsed state and is applied to microelectronics and MEMS. Other applications of the laser beam are not detailed here.
2. Pulse laser direct micromachining technology
Pulsed laser direct micromachining technology is the direct machining of solids by high-energy laser pulses, which is mainly based on laser ablation process. During the ablation process, the laser energy absorbed by the solid material causes the material to be ejected from the machined surface. The ablation of laser and solid is closely related to the parameters of solid materials and pulsed laser. Pulse laser parameters mainly include laser wavelength, pulse width and pulse intensity. Under suitable conditions, almost all solid materials can be processed by pulsed laser, and now the pulsed laser processing parameters of many materials have been established through research [3].
Fig. 1 (a) shows the main structure of common excimer laser processing equipment. The laser beam passes through a series of devices, including shutter, adjustable attenuator, beam shaper and normalizer, and finally shines on the mask. In this structure, the beam shaper changes the beam shape to be approximately square, and then the normalizer divides the light into many beams, each of which illuminates the mask from different directions (Figure 1 (b)). This not only improves the uniformity of light irradiation, but also introduces off-axis elements. Off-axis light irradiation can complete the processing of vertical structures or even drill erosion structures, while traditional plane light irradiation can not process such structures. In the whole system, some auxiliary equipment is generally needed for collimation, such as CCD video sensor or independent nonlinear microscope.
One of the main characteristics of pulsed laser direct micromachining technology is that it can process complex three-dimensional surface contours. Multiple exposures of different masks can process stepped multi-level structures, while scanning masks can complete continuous cutting within exposure time, or directly use halftone masks for projection ablation to complete continuous cutting [4]. Masks and workpieces are generally installed on a precision moving platform controlled by stepping motors, and automatic scanning operation is realized by computers. Other pulsed laser parameters, such as laser luminous flux and repetition frequency, can be changed during machining. In addition, the maximum viewing angle of off-axis irradiation can be changed by changing the numerical aperture NA, as shown in figure 1 (b), so that structures with different sidewall angles can be processed under the condition of constant laser luminous flux.
Figure 1 (a) Block diagram of excimer laser processing equipment (b) Optical system diagram
Another feature of pulsed laser direct micromachining technology is that it can process a variety of materials, especially polymer materials. Most polymers have strong energy absorption in the frequency spectrum of laser, which ensures the energy coupling between laser and workpiece, while the relatively low thermal conductivity ensures the small area of thermal diffusion and thermal influence during ablation. In most cases, good surface finish can be obtained, and extra losses (melting and debris) can be minimized, which is a characteristic that many other materials do not have. For example, due to the high reflectivity and thermal conductivity of metals, pulsed laser machining has a high ablation threshold, and there are serious additional losses in the machining process. However, if the processing object is a metal film deposited on the surface of a substrate with poor thermal conductivity, a good processing effect can be obtained by using pulsed laser.
The most successful example of direct processing of MEMS devices by pulsed laser is the processing of inkjet print head [6]. In addition, the high peak power and 3D structure processing ability of pulsed laser can also be applied to the processing of microfluidic chips. The main components in microfluidic chips, such as microchannels, micro-filters, micro-mixers, micro-reactors, etc., all need 3D structures (or at least 2.5D). In addition, as the material of microfluidic chip, polymer is more suitable for pulsed laser micromachining than silicon substrate.
Recently, examples of MEMS direct processing have also been reported, such as manufacturing bimorph microactuators [7] and multilayer magnetic material actuators [8] on silicon substrates. In addition, femtosecond laser micromachining technology has also developed rapidly [9]. Femtosecond laser has a high energy density, which makes it have a good application prospect in some aspects of MEMS processing. For example, by using the strong interaction between standard transparent materials and high-energy multiphotons, microstructures can be processed on transparent materials.
2. 1 direct processing
The term "direct machining" used here is used to describe the process of machining materials with the focal point of a laser beam. This technology is widely used in micromachining which requires high precision and small size, including the drilling of fuel injectors, the drilling of gas sensors, the characterization of solar cells and the prototyping of MEMS. The workpiece is processed by laser while moving along the beam with a galvanometer scanner and a moving platform to obtain a desired pattern. By adjusting the current detector, the machining speed can reach10 ms-1[10].
Fig. 2: (a) schematic diagram of direct processing with current detector and X-Y moving platform (b) laser equipment directly processed with b)MicrAlater M 1000.
2.2 drilling
A series of holes are machined by focused laser beam on X-Y platform or flow detector, which is widely used to drill fuel injectors, gas sensors, miniature circuit boards and detection cards. Fig. 3 shows a part of a test card for IC (integrated circuit) testing. A hole of 100μm was drilled on a 500μm thick silicon nitride crystal with a 355nm ND:YAG laser. Using AblataCAM software, files can be directly converted into the processing process of laser equipment. With this technology, holes of almost any shape can be machined on the test card.
Fig. 3: (a) 100μm hole on the silicon nitride crystal detector card for IC test; B) Fuel injection holes in hard steel.
The demand for low engine loss and better fuel efficiency has led to the in-depth study of smaller holes and thicker fuel injectors. Because of the limitation of traditional EMD technology in drilling injector of diesel engine, laser machining technology has become the key technology of the next generation diesel engine. The diameter of the hole is 30- 100μm, the tolerance is 1.5μ m, and the taper angle is less than 0.5 degrees. Fig. 3 (b) shows a hole machined on the injector of a diesel engine with a 532nm Nd:YAG laser.
2.3 solar panel processing
Laser equipment working at the wavelength of 1.06μm, with a typical energy of several tens of watts, is widely used in fine linear engraving of the glass bottom layer of thin-film solar panels. The combination of this technology and launch technology with BTS can make the solar panel maintain very high precision and accuracy at high speed. Fig. 4 (a) is a schematic diagram of the processing of amorphous silicon thin films under the dual laser system (1.06μm and 532nm). Draw a line with a width of about 30μm on ITO layer with IR YAG laser beam, and then deposit α-Si. Visible YAG laser beam passes through the α-Si layer near the disk, and process the interconnection with a diameter of 50μm m.. The ITO layer is not affected by the treatment process. Then the aluminum electrode layer is deposited, and the track with a width of about 25μm is processed by visible YAG laser to complete the processing process of the plate. Part of the processing process of the solar panel template is shown in Figure 4. It takes about 1 minute to process each layer of 400mm board with 580nm.
Figure 4: (a) Solar panels processed by dual-wavelength laser system.
B) photos of scribing and interconnection on thin-film α -si solar panels.
3. The latest research trends
3. 1 ultraviolet laser drilling machine for micromachining-Meister 1000DF
MHI has produced the latest DUV266nm laser drilling machine Meister 1000DF, which can be applied to all new solid-state UV-YAG oscillators. Meister 1000DF can be used for high-quality micromachining in different materials and working environments. Features: The semiconductor-pumped solid-state laser resonator can realize long life, high reliability, high energy density of 266nmUV output, micro-drilling with a diameter of 50-200 microns, high speed and current detector [1 1].
Figure 5: Example diagram of processing application
(a) through hole: polyimide resin with a diameter of 100μm and a thickness of 25μ m.
(b) Through hole: ceramic, with a diameter of 100μm and a thickness of 250μ m.
Figure 6: Structure diagram
3.2 DPSS ultraviolet laser
Air cooling of high pulse 355nm laser (LD pumped YV04 laser +SHG+THG). Summary: The laser is a compact air-cooled high-cycle pulsed DPSS ultraviolet laser (355 nm). The nonlinear crystal GdYCOB was applied to the laser (invented by Osaka University). Therefore, high beam quality and stable output can be obtained. Moreover, it is very easy to maintain and operate, and is widely used in micro-machining and accurate measurement [12].
3.3 DPSS green laser
The high pulse 532nm laser (LD pumped yttrium vanadate laser +SHG) is cooled by air. Summary: The laser is a compact air-cooled high-cycle pulsed DPSS green laser (532 nm). It has good output stability and high beam quality. And has a wide range of applications.
3.4 DPSS YVO4 laser
Air cooling of high pulse 1064nm laser (LD pumped YVO4 laser). Overview: This is a compact, air-cooled and easy-to-maintain DPSS laser. LD pump, optical fiber output. It can be miniaturized because of its high repeatability and thermal tension during processing. So it can be widely used in high-speed marking, laser processing and harmonic light source.
4. Conclusion
Pulsed laser has unique processing ability in processing material range and 3D processing flexibility. The combination of pulsed laser processing technology and other mainstream micromachining technologies can provide important processing means for the future development of MEMS. The main application fields of pulsed laser machining technology are micro-actuators, micro-fluidic devices and systems based on functional materials. In addition, pulsed laser has the unique ability to manipulate and connect micro-components, so it will also make great contributions to the integration and packaging technology of MEMS.
References:
[1] Geng, progress of laser micromachining [J]. Laser and infrared,1997,27 (6): 330-332.
[2] Zhang Guangzhao, Ada, micromachining technology. Sensor technology, 1997, 16 (3): 57-60.
Li, Rao Zhijun, Song Xiaohui, Fu Xing, Hu, pulsed laser MEMS processing technology [J]. Micro-nano electronic technology, 2003: 159- 163.
Rizvin. Microstructure processing with excimer laser [J].MST news,1999.1:18-21
Gower M.C. Excimer Laser: Current and Future Applications in Industry and Medicine [A]. In: Reinforced Concrete in Claver, PJ in oakley. Laser processing in manufacturing industry [M]. Chapman & Hall, 1994.
Roman C. Excimer laser drilling precision holes with higher output [J]. Laser Focus on the World, August 1995.
[7] Li J, GK, suresh, ananta. Study on the quality of electro-thermal compatible micro-devices micromachined by excimer laser [J]. 200 1, 1 1; 39-47
Frege ·W·P, et al. Application of laser micromachining in thin film technology [J]. Applied Surface Science, 2000, 154- 155:633-639.
[9] Ostendorf A. Machining dielectric [A] with vacuum ultraviolet wavelength and ultrashort pulse. Continue. Li, 2000, Laser Micro-machining Technology [C].2000, Beijing: Science Press, 200 1
[ 10]http://www . exitech . co . uk/pdfFiles/Thin % 20 films % 20 paper % 20 hjb % 202003 . pdf
[ 1 1]http://www . MHI . co . jp/Kobe/mhikobe-e/products/etc/uv laser . htm
[ 12]http://www.neoark.co.jp/Eng/eng-PDF/YVO4_355.PDF