Introduction
Laser technologies are used in different fields such as instrumentation, micro manufacturing, medicine, imaging and communications. Application of lasers to micro manufacturing is known to have a number of benefits which include capability of remote processing, noncontact processing, no tool wear, automation and the possibility of machining brittle and hard materials. The use of laser processes for specific manufacturing processes has been around for decades. However, the share of micro applications within the whole laser market was a minor one, with laser welding and cutting dominating the laser application sector. With the increase in miniaturization and higher integration of consumer products, laser processes have become more and more significant; with laser micro manufacturing showing a high market increase. The increased use of lasers has been fueled by a strong market pull as well as the availability of high quality lasers that allow the deposition of specific energy on a micrometer scale at high processing speeds. Therefore, lower laser costs, easy installation and maintenance costs have made lasers a versatile tool for industrial micro manufacturing processes. Before exploring the various uses of lasers in micro manufacturing, it is important to understand the technology.
Generation of Laser Light
The word LASER is an acronym that stands for Light Amplification by Stimulated Emission of Radiation. Laser light generation is achieved by an amplification of light intensity through a physical process referred to as stimulated emission. Stimulated emission is a special form of light interaction with material on the atomic scale. Laser technologies have a number of applications in various fields. Laser light has unique properties. First, it is monochromatic meaning that it consists of one wavelength. Two, it is collimated, meaning that the light emitted is highly parallel. Three, it is intense meaning that laser light has a high density of usable photons. Fourth, laser light is coherent meaning that the emitted photons exhibit a fixed phase difference.
General Applications of Laser Technology
In addition to the existing application areas of laser micro processing in fine electronics and mechanics, current megatrends of energy savings, health and global information society have introduced many laser applications. For the production of high efficient solar cells, laser drilling, laser thin film ablation and laser joining are some of the processes which offer low energy deposition and high processing speeds5. For medical implants, the new developed SLM-Technologies enable the generation of patient-specific implants based on real CT-Data. For broadband communication, low temperature laser-based packaging technologies are well suited for high accuracy joining of optical fibers, micro-lasers and related electronic components. Also, the trend toward higher thermal stability in numerous application areas such as sensor and automotive industry has seen these industries change their joining technologies from soldering to welding, where laser offers a versatile tool for the packaging of different materials.
The factor behind the emerging laser applications is the availability of highly flexible laser sources and optical equipment for fast and flexible shaping of the laser light in time and space. Newly developed process variants such as Shadow, Twist, Liftec and beam sources with high beam quality, fast modulation capability and new wavelength regimes, have rendered laser as a universal tool in micro manufacturing2. The wide range of materials that are processed by lasers include micro electronics such as silicon and other semiconductors, hard materials such as tungsten carbide for tool technology, soft materials such as polymers for medical products, ceramics, glass and diamond. Today, laser micro processing is a tool for mass manufacturing with high quality and increased efficiency. They also have lower energy consumption than many other manufacturing technologies. There are a number of lasers and laser processes used in micro manufacturing.
Laser Choices
Modern lasers are of many types, have different wavelengths, pulse durations, repetition rates, beam properties, output powers and configurations. In many micro manufacturing applications, there are two types of lasers which are dominant. These are the excimer gas laser and the diode-pumped solid-state (DPSS) laser. Also, the carbon dioxide laser finds wide application, especially in the cutting of plastics. DPSS lasers commonly operate in the infrared region of the spectrum, but frequency-conversion can be done to make it operate in the visible, near-UV, and UV (Ultra Violet) regions3. These regions are usually known as the 2nd, 3rd and 4th harmonic lasers. Common excimer lasers only operate in the Ultra Violet region, but their properties are completely different from DPSS lasers.
A common factor for most lasers that find application in micro manufacturing is that they have non-continuous or pulsed outputs. Most lasers used in micro machining have pulse repetition rates from a few Hz to few hundreds of kHz, and pulse durations that range from a few picoseconds to few microseconds. Pulsed lasers are significant since they enable the thermal input to the material to be effectively managed to produce high quality, high precision results1. These lasers are used for various laser processes. Some of the common laser processes include drilling, cutting and machining. Laser additive manufacturing (LAM) is widely used and includes processes such as laser cladding and laser metal deposition6. These lasers and laser processes find application in a number of areas which include solar cell manufacturing, micro joining of metals, semiconductors and polymers with laser processes, and micro tooling and wear resistant surface structures.
Laser Processes for Micro Joining of Metals, Polymers and Semiconductors
Elements of laser joining are turning out to be one central factor in assembly processes with respect to the ongoing increment in material diversification. This can be achieved across miniaturized and highly integrated products. Laser beam welding has turned to be one of the common joining techniques for precision engineering. This is because it provides minimal influence on the various functionalities of the distinct parts as well as high levels of manufacturing effectiveness. Selective energy concentration within 10–100 μm provides advantages within laser welding as well as the usage of a large scope of industrial applications. These are all reasons why the use of lasers has become widespread. Different metals that require joining across the non-contact processes can only be made possible with additional joining material components including solders and adhesives. This also addresses the accessibility to the work pieces. Additionally, the scope of laser beam joining continues to be a progressively embraced feature of technology within electronics packaging.
Among the diverse laser joining technologies available, laser welding is established to be the most reliable process that also has one of the largest field applications. This is applicable in the joining of metals and joining of polymers. For the wide range of applications, especially for packaging of electronics, the characteristic geometry within spot welding develops an overlap joint.6 All the joining partners need to be reliably connected even though they will often form the backside of the various parts which need to be molten. The full penetration of lasers will have to be avoided simply due to the functional layers which are situated behind the aesthetic or connection aspects with regards to the visible surfaces. This translates on the need of developing the application to precisely control thermal processes and mechanical stabilities which have to be availed for short joining times as required. Single pulse conventional laser welding processes from heating and melting elements where the materials do not have any impact on the inherent molten material dynamics are evident.1 Therefore, all instabilities within the laser source, material conditions, and beam guiding cause elements of melt expulsions that have subsequent joining errors. Focusing on the copper welding, this becomes a typical phenomenon that was in the past a basic criterion developed to avoid laser welding which was a versatile tool in electronics packaging.
Micro Tooling and Wear Resistant Surface Structures
Laser ablation has become an important tool in the production of micro-scaled products and micro replication tools. It is capable of generating structure sizes that are in the range of 10-100um in steel and hard materials like tungsten carbide and ceramics. Using ultra short pulsed lasers with pulse durations of 10ps in pulse bursts of several pulses with a time spacing of 20ns each and adapted pulse energies, the surface quality of metal micro ablation have been increased significantly and allows the production of tools and parts with ra values less than 0.5um1. In comparison to traditional Electrical Discharge Machining (EDM), laser manufacturing of parts and tools can be performed without additional working tools. Laser ablation technology uses ultra short pulsed lasers in the picosecond range for the manufacture of micro moulding tools4. Accuracies in laser ablation are significantly increases as compared to conventional nanosecond ablation technology. The use of the laser ablation technology enables a new pool of micro processing of tools, thus allowing the manufacture of micro- and nano-scaled functional structures on metal, polymer and glass components.
A machining technology with accuracies of less than 1um is available which can be applied to all kinds of materials using laser ablation picosecond lasers. The development of extremely high repetition rate lasers in the MHz range allows laser micro ablation technology to be applied to large parts thus setting new surface functionalities5. For parts with high demands on wear resistance, laser micro-structured surfaces can provide long lifetimes on the base of oil reservoirs or specific fluid dynamic sliding surfaces. This technique enables the use of low cost materials and the elimination of wear resistance surface coating in the production of motor components, hydraulic services and forming. This technology has the main advantage of eliminating any post processing and free form flexibility of structure size and geometry.
Laser Manufacturing for Solar Cell Solutions
For a while now, laser processes have continued to provide a minimum levels of reference to mechanical and thermal impacts on the processed products resulting from their selective aspects of energy control as well as the high processing speed in general. This way, they have turned to be well established for a wide scope of laser processes that are currently used in manufacturing solar cells. They are also applicable in the highest speed and maximum developments in flexibilities of meeting the demands for the scales of integration as well as functionality.5 Further, the manufacturing of Organic Light-Emitting Diodes (OLED) coupled with displays require selective thin films which are critical in the ablation for material transformation processes that are only provided through new laser sources as well as material adapted laser processes. The various processes need to be used in flexible ways in the customization of the functions as well as properties of the production parts with regards to developing an increment in mass customization requirements.
When such laser sources are used, the high speed functionalism and ablation will embrace the potential for launching new areas which will be under further investigation especially for future cell concepts. This is especially based on high speed laser ablation that is currently one of the most preferred processes for the edge isolation in the manufacture of solar cells. Backside contacting through laser melting is beginning to be one of the aspects that are continuously introduced into industrial manufacturing lines via drilling metal wrap through technologies.2 It is through the use of these technologies that there has been a significant increase of manufacturing productivity as well as efficiencies of solar cells which are already achieved in the past. Manufacturing technologies are needed for enabling the performance of specific processing steps that do not influence the overall material properties and allow ease in the integration of mass manufacturing lines into high production rates.4 For further solar cell concepts and the increment of manufacturing speed as well as efficiency for even more laser processes, they are used for different micro-manufacturing processing steps for developing highly productive processes used in solar cell.
They are also used in module manufacturing based on the reduction of costs as well as increment of production yields. Currently, there are various laser applications that are under investigation for considerable integration. The shift is essentially from the micro manufacturing processes including the micro ablation for doped surface layers and micro drilling for backside contacting. This goes hand in hand with laser soldering within the interconnection of solar cells onto various modules as well as new processes developed from the backside contact formation.1 In the long term, subsequent laser processes that have even more potential for the aspects of efficiency increment will be applied, similar to surface roughening as well as texturing for the improvement of light absorption and transformation to crystalline silicon from amorphous, lasers in micro manufacturing have greatly supported metallization within the improved current forms of management as well as laser supported selective doping.
Conclusion
As demonstrated, lasers have emerged as an extremely versatile tool that is available in modern manufacturing over the past 5 decades. This versatility enables laser technologies to be used in various micro manufacturing processes which include micro-fabrication, micro-machining, micro-marking, and micro-drilling. Therefore, lasers find applications in a wide range of fields such as medicine, instrumentation, imaging and communications. Also, the application of lasers in micro-manufacturing has a number of advantages. They include capabilities of remote processing, noncontact processing, no tool wear, automation, and the possibility of machining brittle and hard materials. Examples of such lasers include the Nd:YAG Lasers and the UV Excimer. The micro-machining capabilities of such lasers enable their application through the various stages of product development which includes prototyping and manufacturing. Therefore, the use of lasers in micro-manufacturing is cost-effective technological achievement.
Works Cited
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- Kunzetal.Applications of lasers in microelectronics and micromechanics: Applied Surface Science, 1994; 80(2): 12-24.
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