Because laser has a series of advantages such as good directionality, high energy and good monochromaticity, it has been highly valued in the scientific research field since its advent in the 1960s, and has promoted the rapid development of many fields, especially It is the application of laser in the field of processing. Traditional laser processing machines have been widely used in industrial products, and laser micromachining has also received widespread attention in recent years.
Laser micromachining is of great significance to the production of electronic devices, medical and automotive products with complex structures of small holes or fine grooves. Because the diameters of holes and groove sizes in such products are getting smaller and smaller, and the tolerances on these dimensions are getting tighter. Only lasers can meet all the requirements for micromachined parts from 1μm to 1mm. Laser processing has a small heat action area and can accurately control the processing range and depth, ensuring high repeatability, good edges and wide versatility [1].
In microsystem manufacturing, silicon anisotropic etching and LIGA (Liga) technology are widely used to process various microstructures. The former is suitable for processing two-dimensional structures of silicon and three-dimensional structures with small aspect ratios; the latter is capable of processing precise three-dimensional structures, not only for silicon but also for processing metals, plastics and ceramics. However, the conditions required for this technology are relatively harsh. It requires a synchrotron radiation X-ray source, and the production of the mold is also very complicated, so it is difficult to popularize. It must also be pointed out that the LIGA process is incompatible with IC, which limits its use to a certain extent.
The laser micromachining process developed in the early 1990s can process more complex microstructures, and the required conditions are not so harsh, and it is easier to implement in laboratories and factories [2].
Laser micromachining involves a wide range of applications. This article focuses on the application of laser micromachining in the UV (ultraviolet) band or 532nm and 1.06μm segments. The working state is pulse state, and the processing application The scope is microelectronics and micromachines (MEMS). Other applications of laser beams are not discussed in this article.
2. Pulse laser direct micromachining technology
Pulse laser direct micromachining technology uses high-energy laser pulses to directly process solids, mainly based on the laser ablation process. During the ablation process, laser energy absorbed by the solid material causes the material to be ejected from the machined surface. The ablation effect between laser and solid is closely related to the solid material and pulse laser parameters. Pulse laser parameters mainly include laser wavelength, pulse width and pulse intensity. Under appropriate conditions, almost all solid materials can be processed by pulse laser, and research has now established pulse laser processing parameters for a variety of materials [3].
Figure 1(a) shows the main structure of a common excimer laser processing equipment. The laser beam passes through a series of devices, including shutters, adjustable attenuators, beam shapers and normalizers, and finally hits the mask. In this structure, a beam shaper changes the shape of the beam to make it approximately square, and then a normalizer splits the light into many beams, each of which illuminates the mask from a different direction (Figure 1(b)). This not only improves the uniformity of light exposure, but also introduces off-axis elements. Off-axis light irradiation can complete the processing of vertical structures and even drilled structures, but such structures cannot be processed using traditional planar light irradiation. Some auxiliary equipment is generally required for collimation in the entire system, such as CCD video sensors or independent nonlinear microscopes.
One of the main features of pulsed laser direct micromachining technology is the ability to process complex three-dimensional surface profiles. Multiple exposures of different masks can process stepped multi-level structures, while scanning the mask within the exposure time can complete continuous cutting, or use half-tone masks to directly perform projection ablation to complete continuous cutting [4]. The mask and workpiece are generally installed on a precision moving platform controlled by a stepper motor, and automatic scanning operations are realized through a computer. Other pulsed laser parameters can be changed during processing, such as laser flux and repetition rate. In addition, the maximum viewing angle of off-axis illumination 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 constant laser light flux conditions.
Figure 1 (a) Excimer laser processing equipment block diagram (b) Optical system diagram
Another feature of pulsed laser direct micromachining technology is that it can process a variety of materials [5] , especially suitable for processing polymer materials. Most polymers have strong energy absorption within the laser spectrum, ensuring the energy coupling between the laser and the workpiece, and the relatively low thermal conductivity ensures that heat diffusion during the ablation process and the area affected by heat is very small. Small. In most cases, a good surface finish is achieved with minimal additional losses (melting and chipping), a characteristic not found in many other materials. For example, due to the high reflectivity and thermal conductivity of metal, pulse laser processing has a high ablation threshold, resulting in serious additional losses during the processing. However, if the processing object is a metal film deposited on the surface of a substrate with poor thermal conductivity, good processing results can be obtained with pulse laser.
The most successful example of direct pulse laser processing of MEMS devices is the processing of inkjet print heads [6]. In addition, the high peak power and 3D structure processing capabilities of pulse lasers can also be applied to the processing of microfluidic chips. The main components in microfluidic chips, like microchannels, microfilters, microstirrs, and microreactors all 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 silicon-based materials.
Examples of direct MEMS processing have also been reported recently, such as the production of bimorph microactuators [7] and multi-layer magnetic material actuators [8] on silicon substrates. In addition, femtosecond laser micromachining technology is also developing rapidly [9]. Due to the high energy density of femtosecond laser, it has good application prospects in certain aspects of MEMS processing. For example, the intense interaction between standard transparent materials and high-energy multi-photons can be used to process light-transmitting materials. microstructure.
2.1 Direct processing
The term "direct processing" used here is used to describe the process of using a focused laser beam to process materials. This technology is widely used in micromachining that requires high precision and small size, including drilling of fuel injectors, drilling of gas sensors, characterization of solar cells, and prototyping of MEMS. The workpiece is moved with the beam using a galvano scanner and a movable platform, while being processed with a laser to obtain the desired pattern. The processing speed can reach 10ms-1 by adjusting the galvanometer scanner [10].
Figure 2: (a) Schematic diagram of direct processing using galvano scanner and X-Y movable platform (b) MicrAlater M1000 direct processing laser equipment
2.2 Drilling
p>The machining of a series of holes using a focused laser beam on an X-Y stage or galvanometric scanner is widely used in drilling fuel injectors, gas sensors, tiny circuit boards and detector cards. Figure 3 shows part of the detector card used for IC (integrated circuit) testing. The 100μm holes were drilled using a 355nm ND:YAG laser in a 500μm thick silicon nitride crystal. Use AblataCAM software to convert files directly into laser equipment processing processes. Using this technology, holes of almost any shape can be machined on the detector card.
Figure 3: (a) 100μm hole on the silicon nitrogen crystal detector card for IC testing b) Hole on the hard steel for fuel injection
Engine The need for low losses and better fuel utilization has led to intensive research into smaller holes and thicker walled fuel injectors. Due to the limitations of traditional EMD technology on drilling of diesel injectors, laser processing technology has become a key technology for the next generation of diesel engines. The diameter of the hole is 30-100μm, the tolerance is ±1.5μm, and the taper angle is less than 0.5 degrees. Figure 3(b) shows a hole machined in a diesel engine injector using an Nd:YAG laser at 532nm.
2.3 Solar panel processing
Laser equipment operating at 1.06μm wave, with a typical energy of tens of watts, is widely used in fine processing of the glass bottom layer of thin-film solar panels. Linear engraving. This combination of process and launch technology with BTS enables solar panels to maintain very high precision and accuracy at high speeds. Figure 4(a) is a schematic diagram of the processing process of amorphous silicon thin films under a dual laser system (1.06μm and 532nm). An IR YAG laser beam is used to draw approximately 30 μm wide lines on the ITO layer, followed by α-Si deposition and a visible YAG laser beam passing through the α-Si layer near the disk to process 50 μm diameter interconnects. The ITO layer is not affected by the processing process. Then the aluminum electrode layer is deposited, and a visible YAG laser is used to process a track approximately 25 μm wide to complete the plate processing process. Part of the processing process of the solar panel sample is shown in Figure 4. It takes about 1 minute to process each layer of a 400mm board using 580nm.
Figure 4: (a) Solar panel processed with dual-wavelength laser system
b) Photos of scribing and interconnection on thin-film α-Si solar panels
3. Latest research trends
3.1 UV laser drilling machine for micro processing - Meister 1000DF
MHI has produced the latest DUV266nm laser drilling machine Meister 1000DF, which can Applied to all new solid-state UV-YAG oscillators. Meister 1000DF can be used to perform high-quality micromachining on different materials and working environments. Features: The semiconductor-pumped solid-state laser resonant cavity can achieve a high lifespan and has high reliability, high energy density 266nm UV output, can achieve micro drilling of 50-200μm diameter, high speed and is equipped with a galvanometer scanner [11 ].
Figure 5: Sample diagram of processing application
(a) Through hole: diameter 100μm Polyimide resin: thickness 25μm
(b) Through hole : Diameter 100μm Ceramic: Thickness 250μ