The development prospects of laser micromachining technology

I. Introduction

Since the first laser appeared in 1960, the research and application of lasers have developed rapidly in various fields. It has been widely used in high-precision measurement, material structure analysis, information storage and communication. High-directional and high-brightness lasers can be widely used in processing and manufacturing industries. In the past 20 years, with the continuous innovation and optimization of laser equipment, new stimulated radiation light sources and related processes, laser manufacturing technology has penetrated into many high-tech fields and industries, and has begun to replace or transform some traditional processing industries.

In 1987, American scientists proposed a micro-electromechanical system (MEMS) development plan, marking a new era in human micro-mechanical research. Micro-mechanical manufacturing technology mainly includes semiconductor processing technology, micro-lithography electroforming molding process, ultra-precision mechanical processing technology and special micro-machining technology. Among these micro-machining methods, from small to small, from small to small, they are all through the direct effect of energy processing. Special processing is carried out in the form of electrical energy, thermal energy, light energy, sound energy, chemical energy, etc. Common processing methods include: electric spark, ultrasound, electron beam, ion beam, electrolysis, etc. In recent years, a new type of micro-machining technology has been developed: including stereolithography technology, photomask technology, etc. Micro-machining with laser has great application potential and attractive development prospects.

In order to adapt to the industrialization of high-tech in the 21st century and meet the needs of micro-manufacturing, high-performance laser sources are developed. In the past decade, laser micro-machining technology, as a branch of laser processing technology, has received widespread attention. One of the reasons is the emergence of more and more high-efficiency lasers. For example, solid-state lasers with extremely high peak power and ultrashort pulses, diode-pumped Nd:YAG lasers with high beam quality, etc. Second, the CNC operation platform is fast and accurate. However, a more important reason is the continued growth of industrial demand. Laser micromachining technology suitable for microelectronics processing, semiconductor perforation, register shearing, and circuit repair. Usually laser micromachining is a processing process with an index of up to hundreds of microns. The width of the beam is between femtoseconds (fs) and nanoseconds (ns). A wide frequency range from far infrared to X-rays. The three major fields are currently mainly used in microelectronics, micromechanics, and micro-optics processing. With the continuous maturity of laser micromachining technology, it will surely be promoted and applied in a wider range of fields.

2. Application of laser micromachining:

As electronic products become more and more lightweight and miniaturized, and as the information per unit volume (high density) and the processing speed per unit time (high speed) increase, new demands for microelectronic packaging technology are also increasing. For example, the interconnection distance between modern mobile phones and digital cameras is about 1,200. The key to improving the packaging process is to leave micro-holes between different layers, which can not only achieve high-speed connection between the surface-mounted device and the signal board below, but also effectively reduce the packaging area.

In addition, as global mobile phones, digital cameras, laptops and other portable electronic products tend to develop in the direction of light, thin, short and small in recent years, printed circuit boards (PCBs) have gradually shown multi-functional characteristics of lamination, mainly high-density interconnection technology. Via is an important component in multi-layer PCBs, and its effective performance has become an important component of multi-layer PCBs. Now, the cost of punching generally accounts for 30%-40% of the cost of PCB boards. For high-speed and high-density PCB design, designers always hope that the holes are as small as possible, which can not only leave more wiring space. The smaller the via, the more suitable it is for high-speed circuits. The minimum size of conventional machine drills is only 100渭m, which obviously cannot meet the requirements, and a new method of laser micro-hole processing is adopted. At present, CO2 lasers can obtain micro-holes with a diameter of 30-40 microns in industry, and 10 micron micro-holes can also be processed by ultraviolet laser cutting machines.

It can be applied to cutting, drilling, carving, marking, heat penetration, welding, etc. in equipment, automobile, aviation precision manufacturing and various micro-machining industries. For example, inkjet processing for inkjet printers above 20 microns. Various micro-optical components are processed by laser surface treatment technologies such as micro-pressure and polishing, or porous glass is filled with laser, and the amorphous state of glass ceramics changes its organizational structure, and then the plasma-assisted micro-optical components in the softening stage are softened by mechanical force.

Common processes of laser micro-machining:

This technology has the advantages of non-contact, selective processing, small heat-affected range, high precision, high repetition frequency, and high part size and shape. In fact, the biggest feature of laser micromachining technology is direct writing processing, which simplifies the process and realizes the rapid prototyping and manufacturing of microcomputers. In addition, this method will not cause environmental pollution problems such as corrosion, which can be called green manufacturing. Laser micromachining technology is applied to micro-mechanical manufacturing:

1) Laser direct writing micromachining, laser LIGA and other material micromachining;

2) Laser micro-stereolithography, laser assisted deposition, laser selective sintering and other material accumulation micromachining technologies.

2.1 Laser direct writing technology.

The excimer laser wavelength, small focused spot diameter and high power density are very suitable for micromachining and semiconductor material processing. Most excimer laser micromachining systems are processed by diaphragm projection, and can also directly use the focused spot to etch parts, combining excimer laser technology with CNC technology. By using the relative movement of the XY platform and the micro-feed in the Z direction, it is possible to directly scan and engrave on the base material or process three-dimensional microstructures. At present, the excimer laser direct writing technology can process high aspect ratio microstructures and line width microstructures. On this basis, the layered scanning three-dimensional micromachining was studied by combining the excimer laser with similar rapid prototyping (RP) manufacturing technology.

2.2 Laser LIGA process.

Replacing carrier lithography with excimer laser deep etching solves the technical problems of high-precision carrier mask production and overlay alignment. At the same time, the application of laser light source in LIGA technology is far superior to synchrotron radiation carrier light source, which greatly reduces the manufacturing cost of LIGA technology. Although the laser LIGA technology has a great difference in the height-to-diameter ratio of high-diameter ratio microcomponents, it can fully accept the processing of microcomponents. In addition, the laser LIGA technology does not require chemical corrosion and development, but directly etches, and is not subject to lateral penetration and corrosion of chemical corrosion. Therefore, the processing edge is steep, the precision is high, and the lithography performance is better than synchronous emission lithography.

2.3 Laser micro-stereolithography technology.

This technology is an advanced rapid prototyping technology used in the field of micro-manufacturing, and it is applied to three-dimensional lithography technology. Micro-three-dimensional lithography technology is called micro-three-dimensional lithography because of its high-precision micro-machining. Compared with other micro-machining technologies, the biggest feature of micro-stereolithography technology is that it is not limited by the shape of micro-devices or system structures. The system can process any three-dimensional structure, including free-form surfaces, and can form different micro-components at one time, saving the micro-assembly link. In addition, the process has the advantages of short processing time, low cost, and process automation, creating favorable conditions for the large-scale production of microcomputers. This method has two limitations.

Low precision:

Based on rapid prototyping, the maximum horizontal precision is about 1mm and the vertical precision is about 3mm. Obviously, this precision cannot be compared with the silicon micromachining technology based on integrated circuits.

Its use is subject to certain restrictions. The existing resin materials have gaps with silicon materials in terms of electrical properties, mechanical properties, thermal properties, etc. In recent years, my country has vigorously researched and developed laser micro-stereolithography. The development direction of improving precision and efficiency is as follows:

1) Use surface exposure instead of point exposure to further shorten the processing time and improve production efficiency;

2) In terms of materials, develop photocurable resins with higher resolution, such as dual-light near-infrared polymer resins, which lay a good foundation for high-precision manufacturing;

3) In terms of process, research and develop processes without support or sacrificial layers, and combine them with planar micromachining technology to further simplify the process and improve processing accuracy and production flexibility.

Laser assisted vapor deposition (LCVD) (LCVD) is a laser-assisted method.

During the curing micromolding process, the solid part is deposited on the substrate surface through a gas phase chemical reaction. This three-dimensional microstructure uses laser-assisted chemical vapor deposition to heat the substrate within a certain range, start and maintain the CVD process, and form a solid structure on the deposited substrate or laser beam. In modeling, there is no limitation of plane projection and plane scanning, and three-dimensional microstructures with complex geometric shapes can be generated. The workpiece stage is moved in a special way so that the speed of the beam is always as fast as the crystal, thus forming the desired microstructure.

2.5 Selective Laser Sintering (SLS)

This is a rapid prototyping technology with a wide range of processable materials and unique advantages in manufacturing any complex three-dimensional shapes. Now, people are using SLS technology to manufacture micro-machines. When the SLS system is implemented, the required three-dimensional CAD model is first completed on the computer, and then the hierarchical software is used to obtain each layer segment in layers. The automatic control technology is used to make the laser selectively sinter the powder corresponding to the cross-section of the computer part, through sintering, melting, cooling and solidification. After the next layer is sintered, the two layers are sintered and connected. Therefore, the sintered part is consistent with the CAD prototype, while the unsintered part is loose powder, which can be used as a support and is easy to clean in the end. The accuracy of the sintering machine mainly depends on factors such as the power, focal diameter, scanning speed, powder particle size, powder anisotropy, and sintering temperature of the sintering machine. Using the SLS process for three-dimensional modeling, various materials can also be integrated into a microstructure to complete a certain function.

OtherLaser Micromachining Technology

This article introduces the latest progress in pulsed laser etching forming technology. Various materials are etched using short-wavelength frequency-doubled lasers or picosecond and femtosecond lasers and high-precision CNC machine tools. Short pulses corrode the surface of these materials and then remove the material. The quality of the microstructure formed on the surface is much higher than that of long pulse processing. In 2001, HEIDELBERGINSTRUMENTS in Germany used triple frequency (wavelength 354.7nm) to obtain a 5mm focused spot, with a processing size of 10mm and an accuracy of 1mm. The diameter of the laser focal spot in the x and y directions is 5 mm. The average surface roughness is 0.16 mm, and the surface roughness is 0.16 mm. Laser micro-cutting, which has the same principle as laser etching, also uses double frequency or femtosecond laser as the light source. The focused beam accurately controls the energy input, has little heat influence, and can accurately cut and shape.

The latest development of micromachining technology in ultra-short pulse lasers

CO2. YAG laser is a continuous. Long pulse laser, which mainly relies on the focus to form high energy density, resulting in local high temperature ablation of materials, basically belongs to the category of thermal processing, and the processing accuracy is limited. Excimer laser can photochemically process materials through the action of short-wave ultraviolet light, and the characteristic scale can reach the micron level, but the corrosion of excimer laser to gas is very serious and difficult to control, and ultraviolet strong laser is easy to be damaged, and the application range is limited. With the in-depth research in the field of lasers, the pulses have gradually become smaller in time, from nanoseconds (10-9s) to picoseconds (10-12s) and then to femtoseconds (10-15s).

Femtosecond pulse lasers have two main characteristics: (1) short pulse width. The duration of a femtosecond pulse is as short as a few femtoseconds, and it can only propagate 0.3 microns in 1fs, which is shorter than the diameter of most cells; 2) extremely high peak power. Femtosecond lasers concentrate the pulse energy in a very short time of a few to hundreds of femtoseconds, so its peak power is very high. For example, if 1渭J of energy is concentrated in a few femtoseconds and gathered into a 10-micron light spot, its optical power density can reach 1018W/cm2, which is converted to 2脳1012V/m/m. This is 4 times the Coulomb field strength of a hydrogen atom (5脳1011V/m), and it is possible to directly remove electrons from atoms.

From the perspective of the action mechanism of laser and light-transmitting materials, the damage mechanism of the material is an avalanche ionization process, which is determined by the initial electron density. The impurities in the material are unevenly distributed, and the initial electron density in the material varies greatly. The results show that the damage threshold varies greatly. The damage threshold of long-pulse laser is 50% of the laser energy flow, that is, the damage threshold of long-pulse laser is a statistical value. Ultrashort pulse laser has a strong field intensity, and bound electrons can absorb n photons and directly jump from bound energy levels to free energy levels. Although the ultrashort pulse laser damage process belongs to the avalanche ionization process, its electrons are generated through the multi-photon ionization process and no longer depend on the original electron density in the material. Therefore, the damage threshold is accurate. When the pulse width decreases, the damage threshold of the pulsed laser decreases significantly. When it reaches the picosecond level, the falling speed slows down, and the femtosecond level remains basically unchanged.

In addition, since ultrashort pulse lasers have a high damage threshold, their energy is equal to or slightly higher than the damage threshold. If it is partial ablation, it can be reduced to diffraction-limited submicron processing. Femtosecond lasers can produce ultra-high light intensity, precise damage threshold, and low heat-affected range, and can accurately process almost all kinds of materials. And it has extremely high processing accuracy and can accurately process submicron sizes.

Lasers are a kind of high-efficiency, low-cost, stable and reliable processing quality with good economic benefits. Due to the unique short pulse width and high peak power characteristics of femtosecond lasers, femtosecond lasers have broken through traditional laser processing methods and created a new field of ultra-fine, heat-free, and three-dimensional space processing. Femtosecond laser processing technology is mainly used in microelectronics, photonic crystal devices, optical fiber communication devices (1Tbit/s), micro-processing, three-dimensional optical storage, micro-medical device production, cell bioengineering technology, etc. It can be seen that laser micro-manufacturing technology will become a rapidly developing high-tech in the 21st century with its irreplaceable advantages.

Finally:

In the era of the Industrial Revolution, countries around the world were proud of producing large machines; in the information age, advanced industrial countries are committed to the research of micro-matter and the manufacture of smaller machines; in the era of nanotechnology, in order to adapt to the development of defense, aerospace, medical, biological and other fields, micro-machining has become one of the most active research directions in the current manufacturing industry, and the development level of micro-mechanical technology has become a symbol of a country's comprehensive strength. In terms of micro-machining technology, laser micro-machining technology has shown unique advantages and has broad development prospects. In order to occupy a place in the future high-tech field, China must develop laser micro-manufacturing technology with independent intellectual property rights.