Application and progress of laser micromachining technology

I. Introduction

Since the first laser was introduced in 1960, laser research and its application in various fields have developed rapidly. Its high coherence has been widely used in high precision measurement, material structure analysis, information storage and communication. The high directivity and high 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 related processes, especially in the past 20 years, 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 the development of microelectromechanical systems (MEMS), which marked the beginning of a new era in the study of micromachines. At present, the manufacturing technologies applied to micromachines mainly include semiconductor processing technology, microlithography electroforming (LIGA) technology, ultra-precision machining technology and special micromachining technology. Among them, the special micro-machining method realizes the removal processing of small molecules by atom or atom by the direct action of processing energy. Special processing is processed by energy such as electric energy, thermal energy, light energy, sound energy, chemical energy, etc. Commonly used methods include: electric discharge machining, ultrasonic machining, electron beam processing, ion beam processing, electrolytic processing, and the like. In recent years, a new method for achieving micromachining has been developed: photoforming, including stereolithography, photomasking, and the like. The use of laser for micromachining shows 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, it is imperative to research and develop high-performance laser sources. As a branch of laser processing, laser micromachining has received extensive attention in the past decade. One of the reasons is due to the emergence of more efficient laser sources. For example, a very high peak power and ultrashort pulse solid-state laser, a two-pole pumped Nd:YAG laser with high beam quality. Another reason is the more accurate and high-speed CNC operating platform. But a more important reason is the growing industrial demand. In microelectronic processing, laser micromachining techniques are used for perforation of semiconductor layers, shearing of registers, and circuit repair. Laser micromachining generally refers to processes that range in size from a few to a few hundred microns. The width of the laser pulse is between femtoseconds (fs) and nanoseconds (ns). The laser wavelength ranges from far infrared to a wide range of X-rays. At present, it is mainly used in three fields of microelectronics, micromachined and microoptical processing. With the development and maturity of laser micromachining technology, it will be promoted and applied in a wider field.

Second, the main application of laser micromachining technology

As electronic products move toward portable and miniaturized, the increase in unit volume information (high density) and the increase in processing speed per unit time (high speed) have placed increasing demands on microelectronic packaging technology. For example, modern mobile phones and digital cameras install approximately 1200 interconnects per square centimeter. The key to improving the level of chip packaging is to preserve the existence of micro vias between different levels of lines, so that the micro vias not only provide a high-speed connection between the surface mount device and the underlying signal panel, but also effectively reduce Package area.

On the other hand, with the development of portable electronic products such as mobile phones, digital cameras and notebook computers in recent years, which are light, thin, short and small, printed circuit boards (PCBs) are gradually exhibiting high-density interconnection technology. The layering and multi-functional features. In order to effectively ensure the electrical connection between the layers and the fixing of external devices, vias have become an important part of the multilayer PCB. At present, the cost of drilling is usually 30%-40% of the cost of PCB board. In high-speed, high-density PCB design, designers always want the smaller the via, the better, so that the board can not only have more wiring space. Moreover, the smaller the via, the more suitable for high speed circuits. The traditional mechanical drilling has a minimum size of only 100 μm, which is obviously not enough. Instead, it is a new type of laser micro-via processing. At present, it is industrially possible to obtain small holes having a via diameter of 30-40 μm or a small hole of about 10 μm by UV laser processing.

Laser micromachining technology can be used for laser cutting, drilling, engraving, scribing, heat penetration, welding, etc. in equipment manufacturing, automotive and aerospace precision manufacturing and various micro-processing industries, such as inkjet printers of more than 20 micrometers. The processing of the ink jet port. Laser surface treatment techniques such as micro-pressing, sanding, etc. are used to process a variety of micro-optical components. The structure can also be changed by amorphization such as laser-filled porous glass, glass-ceramic, and then by modulating external mechanical forces. Micro-optical elements are processed by plasma-assisted micro-forming during the softening phase.


2 Common laser micromachining technology

Laser micromachining technology has the advantages of non-contact, selective processing, small heat-affected area, high precision and high repetition rate, high processing flexibility of part size and shape [1]. In fact, the biggest feature of laser micromachining technology is “direct writing” processing, which simplifies the process and realizes rapid prototyping of micromachines. In addition, this method does not have the problem of environmental pollution caused by methods such as corrosion, and can be described as "green manufacturing". There are two types of laser micromachining techniques used in micromachined manufacturing:
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, etc.

2.1 Laser direct writing technology

The excimer laser has a short wavelength, a small spot size, and a high power density, making it ideal for micromachining and semiconductor material processing. In the excimer laser micromachining system, most of the mask projection processing is used, or the mask can be directly used to etch the workpiece without using a mask, and the excimer laser technology and the numerical control technology are combined, and the laser beam scanning and the XY table are integrated. Relative motion and micro-feeding in the Z direction can directly scan the matrix material to write fine patterns or process three-dimensional microstructures [2]. Figure 1 shows a miniature gear machined by an excimer laser with a minimum gear diameter of 50 mm. At present, excimer laser direct writing can be used to process high aspect ratio fine structures with a line width of several micrometers. In addition, the use of excimer lasers to adopt similar rapid prototyping (RP) manufacturing techniques, and the use of layer-by-layer scanning for three-dimensional micromachining research has also achieved good results [3].

2.2 Laser LIGA Technology

It uses excimer laser deep etch instead of carrier lithography to avoid the technical problems of high-precision ray-laying mask fabrication and engraving alignment. At the same time, the economical and widely used laser source is much better than synchronous. Radiation-loaded light source greatly reduces the manufacturing cost of the LIGA process and enables the wide application of LIGA technology. Although the laser LIGA technology is inferior to the carrier radiation in processing the high-diameter ratio of the micro-components, it is completely acceptable for general micro-component processing. In addition, the laser LIGA process does not require chemical etching development like carrier lithography, but "direct write" etching, without lateral immersion of chemical corrosion.

Corrosion effect, so the processing edge is steep, high precision, lithography performance is better than synchronous carrier lithography [4].

2.3 Laser micro-stereolithography (mSL) technology

It is a processing technology derived from the advanced rapid prototyping technology (SLA) technology applied to the micro-manufacturing field. Because of its high precision and miniaturization, it is called Microstere-lithography. Olithography or mSL) [5]. Compared with other micromachining technologies, the biggest feature of micro-stereolithography is that it is not limited by the shape of micro devices or system structures. It can process any three-dimensional structure including free-form surfaces, and can form different micro-components at one time. The micro-assembly process is omitted, as shown in Figure 2. In addition, the technology has the advantages of short processing time, low cost, and automatic processing, which creates favorable conditions for mass production of micro-machines. The limitations of this technology are twofold:
1) The precision is low. At present, the precision of the highest level of the micromachining technology based on rapid prototyping is about 1mm, and the vertical direction is about 3mm. Obviously, this precision cannot be compared with the silicon micromachining process based on integrated circuits.
2) The use of materials is subject to certain restrictions. The current resin materials have a certain gap in electrical properties, mechanical properties and thermal properties compared with silicon materials. In recent years, laser micro-stereolithography has been vigorously researched and developed. There are the following development directions in terms of improving accuracy and efficiency:

1) Substituting surface exposure instead of spot exposure to further shorten processing time and increase production efficiency;

2) In terms of materials, research and development of higher resolution photocurable resins, such as the developed double-light near-infrared photopolymerization resin, laid a good foundation for high-precision manufacturing;

3) In terms of process, research and development of processes without any supporting structure or sacrificial layer and integration with planar micromachining process further simplify the process and improve processing precision and production flexibility.

2.4 Laser-assisted vapor deposition (LCVD) technology in chemical vapor deposition (CVD) process


In the two parts of the cured micro-molding, solid matter is deposited on the surface of the substrate by a chemical reaction from the gas phase. Laser-assisted chemical vapor deposition is used to fabricate three-dimensional microstructures by heating the focused micro-beams through a localized substrate to initiate and maintain the CVD process. By moving the substrate or laser beam during deposition, the solid structure is very high. Resolution deposition molding. Shaped geometry is not limited by planar projection and plane scanning, and can produce stereoscopic microstructures with complex geometries. By moving the workpiece table in a specific manner and keeping the laser focal spot motion speed at the same speed as the crystal growth rate, the desired microstructure can be made.

2.5 Laser Selective Sintering Technology (SLS)

It is a form of rapid prototyping technology with a unique range of workable materials and the unique advantages of making any complex three-dimensional shape. At present, attempts have been made to fabricate micromachines using the SLS process [6]. In the SLS process, the 3D CAD model that meets the needs is first completed on the computer, and then layered by layering software to obtain the cross section of each layer. The automatic control technology is used to selectively sinter the laser to the cross section of the computer. Corresponding part of the powder, the powder is solidified by sintering and melting. After a layer of sintering is completed, the next layer of sintering is performed, and the two layers are sintered and connected. The layers are sintered and stacked, and as a result, the sintered portion is exactly the same as the CAD prototype, while the unsintered portion is loose powder, which can serve as a support and is easily removed at the end. The accuracy of the sintering system is affected by the following factors: laser power, laser focal spot diameter, scanning speed, powder particle diameter, powder anisotropy, and temperature control during sintering. Three-dimensional forming with the SLS process, it is also possible to integrate a variety of materials into a microstructure to perform certain functions.

3 Other laser micromachining technology

Pulse laser etching is a new research field of laser technology. It uses short-wavelength multi-frequency laser or picosecond, femtosecond laser combined with high-precision CNC machine tools to etch various materials. The surface of these materials is etched with short pulses, and the material is removed. The quality of the microstructure formed on the surface is much higher than that of long pulse processing. In 2001, German HEIDELBERG INSTRUMENTS used a triple frequency (wavelength 354.7nm) to obtain a focused spot with a minimum of 5mm. The minimum processable feature size is 10mm and the precision is 1mm. Figure 5 shows the three-dimensional shape of pulsed laser etching processed on WC/Co. The laser focal spot has a diameter of 5 mm and a feed of 5 mm in the x and y directions. Each layer was removed by 1.3 mm and the average surface roughness was 0.16 mm. Laser micro-cutting, in principle, is the same as laser etching. It is also a multi-frequency or femtosecond laser as the light source. It focuses on the beam, precisely controls the input of energy, and has small thermal influence, and performs fine removal and cutting.

Third, the latest development of ultra-short pulse laser in micro-machining technology

CO2 lasers and YAG lasers are continuous and long-pulse lasers, which mainly rely on focusing to form high energy density, thereby locally generating high temperature to ablate materials, which is basically in the field of thermal processing and has limited processing precision. Excimer lasers rely on their short-wavelength (ultraviolet) to photochemically interact with materials, and their characteristic scale can reach the order of micrometers, but the gas required for excimer lasers is corrosive and difficult to handle, and high-intensity ultraviolet laser processing The optical components of the system are susceptible to damage and their application is thus limited. With in-depth research in the field of lasers, the time-domain width of laser pulses is compressed to be shorter and shorter, from nanoseconds (10-9s) to picoseconds (10-12s) up to femtoseconds (10-l5s). ) magnitude.

The femtosecond pulsed laser has the following two characteristics: (1) The pulse duration is short. The femtosecond pulse can be as short as a few femtoseconds, while light propagates only 0.3μm in 1fs, which is shorter than most cells; (2) peak power is extremely high. The femtosecond laser concentrates the pulse energy in a very short time of a few to several hundred femtoseconds, so its peak power is high. For example, the energy of 1 μJ is concentrated in a few femtoseconds and concentrated into a spot of 10 μm, and its optical power density can reach 1018 W/cm 2 , which is converted into an electric field intensity of 2×10 12 V/m, which is a coulomb in a hydrogen atom. Four times the field strength (5 × 1011 V / m), it is possible to directly separate electrons from the atom.

From the interaction mechanism between laser and transparent material, 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, and the initial electron density in the material is due to the impurity distribution in the material. It varies greatly without unevenness. Therefore, the damage threshold varies greatly. The long pulse laser damage threshold is defined as the laser energy flow density that causes the damage probability to be 50%, that is, the long pulse laser damage threshold is a statistical value. The ultrashort pulse laser has a very high field strength, and the bound electrons can simultaneously absorb n photons to directly transition from the bound energy level to the free energy level. Although the damage caused by the ultrashort pulse laser is also an avalanche ionization process, its electrons are generated by the 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 pulsed laser is significantly reduced as the pulse width decreases.

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