Apr 01, 2020
Since the first laser came out in 1960, the research of laser and its application in various fields have developed rapidly. Its high coherence has been widely used in the fields of high precision measurement, material structure analysis, information storage, and communication. The high directivity and brightness of the laser can be widely used in the manufacturing industry. With the continuous innovation and optimization of laser devices, new stimulated radiation sources, and corresponding processes, especially in the past 20 years, laser manufacturing technology has penetrated into many high-tech fields and industries and began to replace or transform some traditional processing industries.
In 1987, American scientists put forward the development plan of the micro electro mechanical system (MEMS), which marks a new era of human research on micro machinery. At present, the manufacturing technologies used in micromachining mainly include semiconductor processing technology, microlithography electroforming (Liga) technology, ultra-precision machining technology, and special micromachining technology. Among them, the special micromachining method is through the direct effect of processing energy, to achieve the removal of molecules or atoms one by one. Special machining is carried out in the form of electric energy, heat energy, light energy, sound energy, chemical energy, etc. the commonly used methods are EDM, ultrasonic machining, electron beam machining, ion beam machining, electrochemical machining, etc. In recent years, a new method of micromachining has been developed: Photoforming, including stereolithography, photomask, etc. Laser micromachining has great potential in application and development.
2. The main application of laser micromachining technology
With the development of electronic products towards portable and miniaturization, the improvement of unit volume information (high density) and unit time processing speed (high speed) has put forward new requirements for microelectronic packaging technology. For example, modern mobile phones and digital cameras are equipped with about 1200 interconnects per square centimeter. The key to improving the level of chip packaging is to keep the existence of micro vias between the lines of different layers, which not only provides the high-speed connection between the surface mounted devices and the signal panel below but also effectively reduces the packaging area.
On the other hand, with the development of portable electronic products such as mobile phones, digital cameras and laptops to light, thin, short and small in recent years, printed circuit boards (PCBs) gradually show the characteristics of layering and multi-functional with high-density interconnection technology as the main body. In order to effectively ensure the electrical connection between layers and the fixation of external devices, via has become an important part of multi-layer PCB. At present, the cost of drilling usually accounts for 30% - 40% of the cost of PCB manufacturing. In high-speed, high-density PCB design, designers always hope that the smaller the via, the better, so that there is not only more wiring space on the board. And the smaller the via, the more suitable for a high-speed circuit. The minimum size of traditional mechanical drilling is only 100 μ m, which obviously can not meet the requirements. Instead, a new laser micro through-hole processing method is adopted. At present, it is possible to obtain a small hole with a diameter of 30-40 μ m or a small hole with a diameter of about 10 μ m by using a CO2 laser in the industry.
Laser micromachining technology can be used to cut, drill, carve, scribe, heat penetrates, weld and so on in equipment manufacturing, automobile, aviation precision manufacturing and various micro-processing industries, such as the processing of the ink-jet part of the ink-jet printer with the size of more than 20 microns. 威廉希尔官方下载ing laser surface treatment technology, such as micro pressing, polishing and so on, to process a variety of micro-optical elements, or through laser filling porous glass, glass-ceramic amorphization to change the structure, then, by adjusting the external mechanical force, and then in the softening stage, micro-optical elements are processed by plasma-assisted micro forming.
Common laser micromachining technology
Laser micromachining technology has many advantages, such as non-contact, selective machining, small heat affected area, high precision and repetition rate, high machining flexibility of part size and shape. In fact, the biggest characteristic of laser micromachining technology is "direct writing", which simplifies the process and realizes the rapid prototyping of micromachines. In addition, this method has no environmental pollution problems such as corrosion, so it can be called "green manufacturing". There are two types of laser micromachining technologies used in micromachining:
1) Material removal micromachining technology, such as laser direct writing micromachining, laser Liga, etc;
2) Material stacking micromachining technology, such as laser micro stereolithography, laser-assisted deposition, laser selective sintering, and so on.
Other laser micromachining technologies
Pulse laser etching is a new research field of laser technology. It uses short wavelength frequency-doubled laser or picosecond, femtosecond laser combined with high-precision CNC machine tool to etch and process various materials. The quality of the microstructure formed on the surface of these materials is much higher when the materials are etched with a short pulse and then removed. In 2001, Heidelberg instruments in Germany used triple frequency (wavelength 354.7nm) to obtain a focus spot with a minimum of 5mm, a minimum Machinable feature size of 10 mm, and an accuracy of 1 mm. Figure 5 shows the three-dimensional shape of a pulse laser etched on WC / Co. The diameter of the laser focal spot is 5mm, and the feed-in X and Y direction are 5mm. 1.3mm is removed for each layer, and the average surface roughness is 0.16mm. Laser micro cutting is the same as laser etching in principle. It also uses frequency-doubled or a femtosecond laser as the light source to focus the beam precisely and control the input of energy accurately. The thermal effect is small and micro removal cutting is carried out.
3. The latest development of ultrashort pulse laser in micromachining technology
CO2 laser and YAG laser are continuous and long pulse laser. They are mainly focused to form high energy density, which can generate high temperatures in the local area to ablate materials. They are basically in the field of thermal processing, with limited processing accuracy. The excimer laser relies on its short wavelength (UV) to interact with the material photochemistry, and its characteristic scale can reach the order of a micrometer. However, the gas needed by the excimer laser is corrosive and difficult to control. Moreover, the high-strength UV laser is easy to damage the optical elements of the processing system, so its application is limited. With the further study of the laser field, the time-domain width of the laser pulse is compressed more and more short, from nanosecond (10-9s) to picosecond (10-12s) to femtosecond (10-l5s).
The femtosecond pulse laser has the following two characteristics: (1) the pulse duration is short. The duration of the femtosecond pulse can be as short as a few femtoseconds, and light only propagates 0.3 μ m in 1 FS, which is shorter than the diameter of most cells; (2) the peak power is very high. The Femtosecond laser concentrates the pulse energy in a few to hundreds of femtoseconds, so its peak power is very high. For example, if the energy of L μ J is concentrated in a few femtoseconds and converges into a spot of 10 μ m, its optical power density can reach 1018w / cm2, and its electric field intensity can be converted into 2 × 1012v / m, which is 4 times of the Coulomb field strength (5 × 1011v / M) in the hydrogen atom, it is possible to separate the electron from the atom directly.
From the interaction mechanism of laser and transparent materials, the pulse width is from continuous laser to tens of picoseconds, and the damage mechanism is avalanche ionization process, which depends on the initial electron density, while the initial electron density in materials changes greatly due to the uneven distribution of impurities. Therefore, the damage threshold changes greatly. The damage threshold of long-pulse laser is defined as the laser energy flow density with a damage probability of 50%, that is, the damage threshold of long-pulse laser is a statistical value. The field strength of the ultrashort pulse laser is very high. The bound electron can absorb n photons at the same time and directly transition from the bound level to the free level. Although the damage caused by ultrashort pulse laser is also an avalanche ionization process, its electrons are produced by multiphoton ionization process and no longer depend on the initial electron density in the material. Therefore, the damage threshold is an accurate value. The damage threshold of the pulse laser decreases with the decrease of the pulse width. At the picosecond level, the decrease rate slows down, and at the femtosecond level, it is almost unchanged.
In addition, because the damage threshold of ultrashort pulse laser is very accurate, the laser energy is controlled to be exactly equal to or slightly higher than the damage threshold, then only the part higher than the damage threshold produces ablation, and the submicron processing below the diffraction limit can be carried out. The femtosecond laser can produce ultra-high light intensity, have accurate and low damage threshold, have very small heat affected area, and can process almost all kinds of materials precisely. Moreover, the processing precision is very high and can process submicron size precisely.
Laser micromachining has the advantages of high production efficiency, low cost, stable and reliable processing quality, good economic, and social benefits. The femtosecond laser is breaking the traditional laser processing method with its unique advantages of short pulse duration, high peak power, and creating a new field of ultra-fine materials, non-thermal damage, and 3D space processing and processing. The application of femtosecond laser processing technology includes microelectronics, photonic crystal devices, optical fiber communication devices with high information transmission speed (1tbit / s), micromachining, new three-dimensional optical memory, micro medical device manufacturing and cell bioengineering technology and so on. It can be predicted that laser micromachining technology will become a high-tech in the 21st century with its irreplaceable advantages.
In the era of industrialization, all countries in the world are proud to produce large-scale machines; in the era of information technology, all advanced industrial countries are committed to the research of micro materials and manufacturing increasingly small machines; while in the era of nanotechnology, in order to adapt to the development of national defense, aerospace, medicine and Bioengineering, micro processing is the most active research direction in the manufacturing industry today One is that the development level of micromechanical technology has become one of the standards to measure the comprehensive strength of a country. Laser micromachining technology shows more and more unique advantages in micromachining technology and has broad development prospects. China must develop laser micromachining technology with independent intellectual property rights, in order to occupy a place in the future high-tech field.