laser micromachining technology: diverse applications and future prospects

Laser microfabrication technology, with its unique "direct writing" processing method, not only simplifies the manufacturing process, but also achieves efficient and rapid prototyping of micro machinery. This technology has significant advantages, not only avoiding environmental pollution caused by traditional methods such as corrosion, but also known as "green manufacturing". In the field of micro mechanical manufacturing, laser micro machining technology is mainly divided into two categories.
Material removal microfabrication technology, covering various methods such as laser direct writing microfabrication and laser LIGA;
Material stacking microfabrication technology covers various methods such as laser micro stereolithography, laser assisted deposition, and laser selective sintering.

Laser direct writing technology
Excimer lasers perform well in the fields of microfabrication and semiconductor material processing due to their short wavelength, small focused spot diameter, and high power density. In excimer laser microfabrication systems, mask projection processing is usually used, or more flexibly, focused spot etching is directly used to etch the workpiece. By combining numerical control technology, the relative motion between laser beam scanning and X-Y worktable, as well as Z-direction micro feed, can directly scan and engrave fine patterns or process three-dimensional microstructures on the substrate material. At present, high aspect ratio microstructures with line widths of several micrometers can be processed using excimer laser direct writing method. In addition, significant progress has been made in the research of three-dimensional microfabrication using layer by layer scanning method, drawing on rapid prototyping (RP) manufacturing technology.

Laser LIGA technology
Laser LIGA technology has emerged in the field of microfabrication due to its unique feature. It cleverly uses excimer laser for deep etching, thus bypassing the complex process of producing high-precision radiation masks and their alignment. At the same time, the popularity and economy of laser light sources compared to synchrotron radiation carrier light sources have significantly reduced the manufacturing cost of LIGA technology, further promoting the widespread application of LIGA technology. Although laser LIGA technology is slightly inferior to carrier lines in terms of aspect ratio for processing micro components, its performance is already excellent enough for general micro component processing requirements. More importantly, the laser LIGA process abandons the chemical etching and development steps required for ray lithography and adopts the "direct writing" etching technology, effectively avoiding the lateral infiltration problem caused by chemical etching, ensuring the steepness and high precision of the processed edges, and surpassing synchronous ray lithography in terms of lithography performance.

Laser micro stereolithography (mSL) technology
Laser micro stereolithography technology is an innovative processing technique that further applies the stereolithography (SLA) process, which has shown outstanding performance in the field of rapid prototyping, to the field of microfabrication. With its excellent micro fabrication capability and high precision, this technology is named micro stereolithography. Compared to other microfabrication techniques, micro stereolithography has significant advantages. It is not constrained by the shape of microdevices or system structures, and can easily process complex three-dimensional structures including free-form surfaces. It can also form different micro components in one go, thus eliminating the tedious micro assembly process. In addition, this technology also has the advantages of short processing time, low cost, and high degree of automation, providing strong support for the mass production of micro machinery.

However, laser micro stereolithography technology also faces some challenges. On the one hand, its accuracy is currently inferior to that of silicon microfabrication processes based on integrated circuits, with a maximum horizontal accuracy of about 1mm and a vertical accuracy of about 3mm. On the other hand, the resin materials used in this technology still have certain gaps compared to silicon materials in terms of electrical, mechanical, and thermal properties. However, laser micro stereolithography technology is still constantly being researched and developed, especially in improving accuracy and efficiency, and has shown a clear direction of development.
By using surface exposure technology instead of traditional point exposure, laser micro stereolithography technology can significantly shorten processing time and improve production efficiency.

In the field of materials science, researchers are committed to developing photo curable resins with higher resolution. For example, the successful development of dual light near-infrared light polymer resin provides a solid material foundation for high-precision manufacturing.
In terms of process improvement, researchers are committed to exploring new processes that do not require supporting structures or sacrificial layers, and achieving deep integration with planar microfabrication processes. These efforts aim to further simplify the manufacturing process while improving the precision of processing and the flexibility of production.

Laser assisted vapor deposition (LCVD) technology
Plays a crucial role in the chemical vapor deposition (CVD) process. This technology involves the process of depositing solid substances from the gas phase onto the surface of a substrate through chemical reactions. Through laser assistance, we can create three-dimensional microstructures with high resolution. During the deposition process, the laser microbeam heats the substrate locally, initiating and maintaining the CVD process. By moving the substrate or laser beam, we can accurately shape complex three-dimensional microstructures without being limited by planar projection and scanning.

Laser Selective Sintering Technology (SLS)
It is also an important rapid prototyping technology. This technology has a wide range of material applicability and the ability to create any complex three-dimensional shape. Currently, people are trying to use SLS technology for micro mechanical manufacturing. In the SLS process, we first create the required 3D CAD model on the computer, and then use layering software to perform layering processing on it. Next, through automatic control technology, the laser selectively sinter the powder portion corresponding to the cross-section of the parts inside the computer. After sintering is completed, the unsintered loose powder can provide support and be easily cleaned in the final stage.
It is worth noting that the accuracy of the sintering system is affected by various factors, including laser power, laser focal spot diameter, scanning speed, powder particle diameter, powder anisotropy, and temperature control during the sintering process. By controlling these factors reasonably, we can achieve high-precision 3D forming, and even integrate multiple materials within a microstructure to complete specific functions.

Other laser microfabrication technologies
Pulsed laser etching is an emerging research hotspot in the field of laser technology. It combines short wavelength harmonic lasers or picosecond and femtosecond lasers, and is used together with high-precision CNC machine tools to achieve etching and processing of various materials. By using short pulses for fine etching on the surface of materials, combined with material removal techniques, microstructures with much better quality than long pulse processing can be obtained. For example, in 2001, HEIDELBERG INSTRUMENT in Germany used a third harmonic laser (wavelength 354.7nm) to achieve a minimum focused spot size of 5mm and a minimum feature size of 10mm, with an accuracy of up to 1mm. Figure 5 shows the three-dimensional shape processed by pulsed laser on WC/Co material, with a laser focal spot diameter of 5mm and feed rates of 5mm in both x and y directions. After removing 1.3mm from each layer of material, the average surface roughness is only 0.16mm.

In addition, laser micro cutting forming technology is also receiving increasing attention. Its principle is similar to laser etching, using frequency doubled or femtosecond laser as the light source. By finely focusing the beam and precisely controlling the energy input, it achieves fine removal and cutting shaping. This technology has minimal thermal impact and is suitable for fine processing of various materials.